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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
Ulrich Weise, Jukka Terho <strong>and</strong> Hannu Paulapuro<br />
Chapter 5<br />
Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong><br />
<strong>machine</strong><br />
5.1 Definitions<br />
The following terms are commonly used to specify certain areas <strong>and</strong> <strong>systems</strong> as part <strong>of</strong> <strong>the</strong> entire<br />
<strong>paper</strong> mill <strong>water</strong> system:<br />
Short circulation: The system in which <strong>paper</strong> <strong>machine</strong> wire <strong>water</strong> is separated from <strong>the</strong> stock<br />
in web forming <strong>and</strong> used for dilution <strong>of</strong> <strong>the</strong> thick stock to be delivered to <strong>the</strong> headbox.<br />
Long circulation: The system in which excess white <strong>water</strong> from <strong>the</strong> short circulation <strong>and</strong> o<strong>the</strong>r<br />
<strong>water</strong>s are collected at <strong>the</strong> <strong>paper</strong> <strong>machine</strong> (PM) <strong>and</strong> used for stock dilution <strong>and</strong> o<strong>the</strong>r purposes in<br />
stock preparation. Within <strong>the</strong> long circulation loop, usually fiber recovery <strong>and</strong> <strong>water</strong> cleaning<br />
equipment is installed.<br />
Approach flow system: The system extends from <strong>the</strong> <strong>machine</strong> chest to <strong>the</strong> headbox lip. The<br />
main purpose is to meter <strong>and</strong> dilute <strong>the</strong> stock including blending with o<strong>the</strong>r components like<br />
fillers, chemicals, <strong>and</strong> additives unless not already added in stock preparation. Then, <strong>the</strong><br />
low-consistency stock is pumped <strong>and</strong> screened before feeding to <strong>the</strong> headbox. Stock cleaning by<br />
hydrocyclones <strong>and</strong> deaeration can be included.<br />
Stock preparation: Stock preparation or "stock prep" includes mechanical treatment <strong>of</strong> <strong>the</strong><br />
stock before <strong>the</strong> <strong>machine</strong> chest, proportioning, <strong>and</strong> blending <strong>of</strong> <strong>the</strong> main stock components.<br />
Stock preparation begins with repulping or <strong>the</strong> dilution <strong>of</strong> pulp from integrated mill operations at<br />
<strong>the</strong> pulp storage towers <strong>and</strong> ends at <strong>the</strong> <strong>machine</strong> chest.<br />
5.2 Design principles<br />
5.2.1 Elements <strong>and</strong> operations<br />
The purpose <strong>of</strong> <strong>the</strong> stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> is to supply <strong>the</strong> <strong>paper</strong> <strong>machine</strong> (PM) with stock <strong>and</strong><br />
<strong>water</strong> in such way that<br />
- The quantity <strong>of</strong> supplied stock is sufficient for <strong>the</strong> production capacity <strong>of</strong> <strong>the</strong> PM<br />
- The supply is even <strong>and</strong> <strong>of</strong> such quality in order to reach a high PM productivity<br />
- The product at <strong>the</strong> reel meets <strong>the</strong> given quality parameters.<br />
Additionally, process design <strong>and</strong> operations are carried out in such a way that <strong>the</strong><br />
above-mentioned aims are met at minimum cost. Hence, <strong>the</strong> process design has to be optimized<br />
according to energy consumption <strong>and</strong> stock quality refinement in particular. O<strong>the</strong>r constraints<br />
have to be considered like process <strong>water</strong> quality, especially at low fresh <strong>water</strong> consumption, or<br />
pulp quality characteristics. Finally, <strong>the</strong>se aims have to be met at all possible operation<br />
conditions. The process <strong>and</strong> <strong>the</strong> PM are usually designed so that <strong>the</strong>ir highest efficiencies are<br />
reached with <strong>the</strong> most produced grade, which is preferably in <strong>the</strong> middle <strong>of</strong> <strong>the</strong> range <strong>of</strong> grades<br />
aimed for. A small increase in running time efficiency on <strong>the</strong> main grade is <strong>of</strong>ten more beneficial<br />
than aiming for highest efficiency with all possible grades. The target is to achieve <strong>the</strong> maximum<br />
total production efficiency. This requires a good knowledge <strong>of</strong> <strong>the</strong> product mix to be realized<br />
before building or rebuilding <strong>the</strong> mill. Thus, process <strong>and</strong> installed equipment can be tailored for<br />
<strong>the</strong> specific grade, which gives advantage in specific consumption <strong>of</strong> energy <strong>and</strong> o<strong>the</strong>r resources.<br />
Papermaking Part 1, Stock Preparation <strong>and</strong> Wet End - Page 1
Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
On <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>, producing grades o<strong>the</strong>r than those initially considered is less economical or<br />
even impossible. Generally speaking, <strong>the</strong> larger <strong>the</strong> PM production is, <strong>the</strong> more narrow is <strong>the</strong><br />
window <strong>of</strong> grades which can be produced on this <strong>machine</strong>. Hence, large <strong>and</strong> fast running<br />
<strong>machine</strong>s are usually "single-grade <strong>machine</strong>s."<br />
In order to avoid unfavorable interferences <strong>and</strong> in order to gain steady operation <strong>of</strong> certain<br />
equipment or sub-processes, <strong>the</strong>se units are, at least to some degree, de-coupled from each<br />
o<strong>the</strong>r. This is usually achieved by buffers with sufficient capacity or by <strong>the</strong> possibility to switch or<br />
to connect process streams differently. The higher <strong>the</strong> amount <strong>of</strong> storage or buffer capacity is, <strong>the</strong><br />
larger <strong>the</strong> variation may be, which can be leveled out. However, <strong>the</strong> smoo<strong>the</strong>ning effect <strong>of</strong> large<br />
buffers on very low frequency variation can be limited. On a multigrade <strong>machine</strong>, <strong>the</strong> time needed<br />
for changing grades should be short <strong>and</strong> <strong>the</strong> amount <strong>of</strong> broke produced during <strong>the</strong> grade change<br />
should be small. Hence, stock chests should be relatively small <strong>and</strong> recirculations short. The total<br />
production efficiency on a multigrade <strong>machine</strong> is usually lower than for especially designed<br />
single-grade processes. Table 1 shows typical total efficiencies for modern <strong>paper</strong> mills; <strong>the</strong><br />
efficiency figure considers scheduled <strong>and</strong> unscheduled downtime <strong>and</strong> broke.<br />
Table 1. Typical total efficiencies <strong>of</strong> modern <strong>paper</strong> <strong>machine</strong>s.<br />
Grade produced<br />
Total efficiency<br />
Newsprint<br />
90%<br />
Supercalendered <strong>paper</strong><br />
90%<br />
Lightweight coated <strong>paper</strong><br />
82%<br />
Woodfree <strong>paper</strong><br />
92%<br />
Tissue <strong>paper</strong><br />
85%<br />
Linerboard<br />
85%<br />
Folding boxboard<br />
82%<br />
The sub-processes <strong>of</strong> <strong>the</strong> stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong> are:<br />
- Stock preparation<br />
- Stock approach flow system<br />
- White <strong>water</strong> <strong>and</strong> fiber recovery system<br />
- Broke system.<br />
Figures 1 <strong>and</strong> 2 show stock preparation <strong>and</strong> short circulation as blackbox models. For<br />
process design as well as for troubleshooting in mill practice, it is useful to be aware <strong>of</strong> <strong>the</strong><br />
borders <strong>and</strong> interfaces between <strong>the</strong> sub-processes <strong>and</strong> operations, especially in complex <strong>and</strong><br />
sophisticated <strong>systems</strong>. Addressing <strong>the</strong> criteria, needs, <strong>and</strong> concerns for each process step<br />
makes it evident as to whe<strong>the</strong>r all pieces are suitable to meet <strong>the</strong>ir particular goal. Designing <strong>the</strong><br />
processes accordingly will maximize pr<strong>of</strong>itability. Categorizing <strong>the</strong> activities <strong>and</strong> defining <strong>the</strong><br />
process steps is also known as system analysis. Possible bottlenecks in processes can be<br />
identified. In an interconnected system, <strong>the</strong> entire system is only as good as its weakest part. In<br />
this respect, malfunction or disturbances originating from auxiliary equipment, fittings, piping,<br />
controls, etc., can spoil process performance significantly, even if <strong>the</strong> main equipment is well<br />
chosen <strong>and</strong>, as such, operating perfectly. Separation <strong>of</strong> processes into functional units makes<br />
complex <strong>and</strong> sophisticated <strong>systems</strong> more comprehensible <strong>and</strong> easier to underst<strong>and</strong>, which<br />
means a potential improvement in controllability, system stability, operation safety, etc. Clearly<br />
structured processes also ease troubleshooting because reason <strong>and</strong> cause <strong>of</strong> problems can be<br />
more quickly <strong>and</strong> easily separated <strong>and</strong> identified.<br />
Figure 1. Stock preparation, system principle.<br />
Papermaking Part 1, Stock Preparation <strong>and</strong> Wet End - Page 2
Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
The number <strong>of</strong> used pulp components depends on <strong>the</strong>ir availability <strong>and</strong> on <strong>the</strong> product<br />
properties desired. Accordingly, in stock preparation, <strong>the</strong> fiber furnish is determined by:<br />
- Selection <strong>and</strong> proportion <strong>of</strong> <strong>the</strong> stock components<br />
- Improvement <strong>and</strong> development <strong>of</strong> <strong>the</strong> fibers, i.e., beating.<br />
Chapters 3 <strong>and</strong> 4 describe in detail <strong>the</strong> main operations like slushing, defibration, <strong>and</strong> refining<br />
<strong>of</strong> pulp. The consumption <strong>of</strong> fresh <strong>water</strong> in stock preparation is very low if not zero. Fresh <strong>water</strong> is<br />
used for <strong>the</strong> dilution <strong>of</strong> chemicals <strong>and</strong> eventually as supplement <strong>water</strong> in startup situations. In a<br />
simple case, stock preparation consists <strong>of</strong> dilution <strong>of</strong> a single pulp component <strong>and</strong> mixing it with<br />
recovered fiber <strong>and</strong> broke.<br />
Figure 2 shows a typical short circulation as a blackbox model; <strong>the</strong> streams <strong>of</strong> air from<br />
deaeration <strong>and</strong> filler are optional according to <strong>the</strong> <strong>paper</strong> produced. The "rejects" stream here also<br />
includes <strong>the</strong> end-stage screen accept, if this is fed into <strong>the</strong> couch pit. The heating energy added<br />
to <strong>the</strong> wire pit controls <strong>the</strong> process temperature; its application can be restricted to <strong>the</strong> startup<br />
situation. All fluid streams vary purposely <strong>and</strong> arbitrarily in flow, concentration, <strong>the</strong> composition <strong>of</strong><br />
suspended <strong>and</strong> dissolved materials, <strong>and</strong> temperature to some extent. In this respect, <strong>the</strong><br />
component with <strong>the</strong> most impact on this system is <strong>the</strong> thick stock flow.<br />
Figure 2. Short circulation, system principle.<br />
The main operations in <strong>the</strong> approach flow system are:<br />
- Dilution to headbox consistency<br />
- Removal <strong>of</strong> product <strong>and</strong> production disturbing contaminants (solids <strong>and</strong> air)<br />
- Conditioning with chemicals <strong>and</strong> additives<br />
- Feeding <strong>the</strong> headbox<br />
- Supply <strong>of</strong> additional <strong>water</strong> for PM cross-pr<strong>of</strong>ile control in case <strong>of</strong> a headbox dilution system.<br />
Figure 3 shows <strong>the</strong> approach flow system <strong>of</strong> a printing <strong>paper</strong> mill. The thick stock is pumped<br />
from <strong>the</strong> <strong>machine</strong> chest (1) for dilution at <strong>the</strong> wire pit (2). After <strong>the</strong> thick stock pump a shut-<strong>of</strong>f<br />
valve, a magnetic flowmeter, <strong>and</strong> a connector for <strong>the</strong> consistency measurement are shown. Thus,<br />
<strong>the</strong> basis weight is here controlled by <strong>the</strong> speed <strong>of</strong> <strong>the</strong> thick stock pump. At <strong>the</strong> stock mixing zone<br />
<strong>of</strong> <strong>the</strong> wire pit (2), right before <strong>the</strong> stock fan pump (3), <strong>the</strong> additive dosage points are shown. The<br />
stock fan pump (3) feeds <strong>the</strong> first stage cleaner banks (4), from where <strong>the</strong> accept flows to <strong>the</strong><br />
deaeration tank (5). The air from <strong>the</strong> stock deaeration tank (5) <strong>and</strong> <strong>the</strong> headbox dilution <strong>water</strong><br />
deaeration tank (not shown) passes through an on/<strong>of</strong>f valve <strong>and</strong> <strong>the</strong> condenser to <strong>the</strong> first-stage<br />
vacuum pump (6) followed by a <strong>water</strong> separator. On <strong>the</strong> small branch pointing to <strong>the</strong> right side,<br />
<strong>the</strong> vacuum breaker valve is shown. The second stage vacuum pump followed by a <strong>water</strong><br />
separator is partly hidden. The stock flows from <strong>the</strong> deaeration tank (5) to <strong>the</strong> headbox fan pump<br />
(7). There are connections shown for filler <strong>and</strong> additive dosage right before <strong>the</strong> fan pump (7).<br />
There are two horizontally mounted pressure screens (8), which are equipped each with an<br />
automatic shut-<strong>of</strong>f valve on <strong>the</strong> feed side <strong>and</strong> a manual shut-<strong>of</strong>f valve at <strong>the</strong> accept side. Reject<br />
collection <strong>and</strong> treatment are not shown here. From <strong>the</strong> screens (8), <strong>the</strong> <strong>machine</strong> stock flows to<br />
<strong>the</strong> headbox <strong>of</strong> <strong>the</strong> PM. Retention aid is dosed via a ring pipe header <strong>and</strong> several radial dosage<br />
points (9).<br />
Figure 3. Stock approach flow system <strong>of</strong> a fine <strong>paper</strong> <strong>machine</strong> (refer to text for itemization <strong>of</strong><br />
position numbers).<br />
5.2.2 System stability<br />
Papermaking Part 1, Stock Preparation <strong>and</strong> Wet End - Page 3
Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
Production conditions <strong>and</strong> parameters can be quite different according to <strong>the</strong> wide range <strong>of</strong><br />
different <strong>paper</strong> <strong>and</strong> board grades. Papermaking succeeds even with highly loaded process <strong>water</strong><br />
in closed cycle, i.e., effluent-free, production. However, process conditions have to be stable to<br />
reach <strong>the</strong> required product quality <strong>and</strong> a high production efficiency. Steadiness <strong>and</strong> uniformity <strong>of</strong><br />
flow are vital <strong>and</strong> are determined by <strong>the</strong> design as well as by <strong>the</strong> operation <strong>of</strong> <strong>the</strong> process.<br />
Especially in modern operations with high-speed <strong>machine</strong>s <strong>and</strong> with low fresh <strong>water</strong><br />
consumption, <strong>the</strong> physical system stability is as important as <strong>the</strong> chemical stability <strong>of</strong> <strong>the</strong> <strong>water</strong><br />
<strong>systems</strong> <strong>and</strong> <strong>the</strong> wet end. Attention is paid to <strong>the</strong> variation <strong>of</strong> several parameters, separately<br />
acting or interacting with each o<strong>the</strong>r, when designing or improving <strong>the</strong> operation <strong>of</strong> <strong>the</strong> flow<br />
approach system <strong>and</strong> <strong>the</strong> wet end. Problems can be caused by variation <strong>of</strong>:<br />
- Flow speed<br />
- Pressure (pulsation)<br />
- Temperature<br />
- Flow conditions <strong>and</strong> <strong>the</strong> degree <strong>of</strong> turbulence<br />
- Consistency<br />
- Furnish composition <strong>and</strong> its homogeneity including inorganic solids<br />
- Specific surface <strong>of</strong> fibers, content <strong>of</strong> fines, freeness<br />
- Charge content, cationic dem<strong>and</strong>, pH<br />
- Content <strong>and</strong> distribution <strong>of</strong> chemicals <strong>and</strong> additives<br />
- Content <strong>and</strong> distribution <strong>of</strong> undesired substances <strong>and</strong> contaminants including air.<br />
In <strong>the</strong> stock <strong>and</strong> <strong>water</strong> system <strong>of</strong> <strong>the</strong> PM, <strong>the</strong> importance <strong>of</strong> steadiness <strong>and</strong> uniformity <strong>of</strong> flow<br />
increases when approaching <strong>the</strong> sheet forming. Unevenness in any flow can occur for each <strong>of</strong><br />
<strong>the</strong> above-mentioned parameters:<br />
- In <strong>the</strong> cross-direction <strong>of</strong> <strong>the</strong> flow<br />
- In <strong>the</strong> flow direction.<br />
The latter means a time variation <strong>of</strong> <strong>the</strong> flow, which appears in:<br />
- Short term: in frequencies > 1 Hz, or in wavelengths < 1 s<br />
- Long term: in frequencies < 1 Hz, or in wavelengths > 1 s.<br />
Additionally to <strong>the</strong>se periodical (deterministic) variations, <strong>the</strong>re are r<strong>and</strong>om (stochastic)<br />
variations in <strong>the</strong> short term as well as in long term.<br />
5.2.2.1 Grade changes<br />
Contrary to <strong>the</strong> principle <strong>of</strong> steady operation are grade changes on <strong>the</strong> PM. Carrying out grade<br />
changes has to be considered carefully when designing <strong>the</strong> approach flow system <strong>and</strong> <strong>the</strong> long<br />
circulation <strong>of</strong> a <strong>paper</strong> or board <strong>machine</strong> to ensure stable operation over <strong>the</strong> entire product range.<br />
This applies for so-called "single-grade" as well as for "multigrade" <strong>machine</strong>s; on <strong>the</strong> single-grade<br />
<strong>machine</strong>, <strong>the</strong> difference between <strong>the</strong> produced grades is small. The following points are important<br />
on multigrade <strong>machine</strong>s:<br />
- Quick response upon changes to gain a short transition period when changing to ano<strong>the</strong>r<br />
grade. This means an improvement in production time efficiency <strong>and</strong> less broke. Large storage<br />
capacity dampens <strong>the</strong> response.<br />
- Adaptation without production disturbance. Machine runnability or production should not be<br />
lower right after <strong>the</strong> grade change compared to steady-state operation.<br />
- Fast leveling-out <strong>of</strong> variations, e.g., caused by instability in <strong>the</strong> wet end chemistry. Besides<br />
<strong>the</strong> PM runnability, product quality also should not be on a lower level after <strong>the</strong> grade change<br />
Papermaking Part 1, Stock Preparation <strong>and</strong> Wet End - Page 4
Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
compared to steady-state operation.<br />
- Stable operation <strong>of</strong> <strong>the</strong> system <strong>and</strong> all its components at all operation levels. Flows,<br />
concentrations, <strong>and</strong> chemistry can differ significantly between different grades, <strong>and</strong> this has to be<br />
paid attention to in design <strong>and</strong> operation. Stability is to be achieved hydrodynamically,<br />
mechanically, <strong>and</strong> electrically. Control loops have to be tuned accordingly.<br />
The short circulation shows, according to Norman's simplified short circulation model1, a<br />
t<br />
dynamic response for <strong>the</strong> normalized time<br />
, where V is <strong>the</strong> wire pit volume <strong>and</strong> Q V=Q 2 2 is <strong>the</strong><br />
dilution <strong>water</strong> flow for <strong>the</strong> thick stock (Q 0 , c 0 ) in <strong>the</strong> wire pit. Figure 4 b shows <strong>the</strong> dynamic<br />
response for different retention values R, when starting <strong>the</strong> system in Fig. 4 a with clear <strong>water</strong>.<br />
Consider that each stock fraction has a different retention value. The curves in Fig. 4 b could be,<br />
for example, <strong>the</strong> retention curves for different Bauer McNett fractions <strong>of</strong> a certain stock<br />
composition. Their different response causes variations in <strong>the</strong> basis weight as well as in <strong>the</strong> web<br />
composition as a reaction to quality or quantity changes in <strong>the</strong> thick stock flow. An increase in<br />
thick stock feed leads to a quicker change in <strong>the</strong> long fiber fraction <strong>of</strong> <strong>the</strong> web <strong>and</strong> to a slower<br />
change in <strong>the</strong> fines <strong>and</strong> filler content. Vice versa, when decreasing <strong>the</strong> thick stock flow, fines <strong>and</strong><br />
fillers are over-represented in <strong>the</strong> web, due to <strong>the</strong> less retained material circulating1. This can<br />
cause oscillation in <strong>the</strong> product properties, especially after grade changes, which can reduce <strong>the</strong><br />
total efficiency <strong>of</strong> <strong>the</strong> PM.<br />
If a change between particular grades is not possible without major material losses or<br />
process complications, emptying <strong>and</strong> washing <strong>of</strong> <strong>the</strong> system is necessary. In this case, <strong>the</strong><br />
production must be scheduled accordingly. Such washing periods are carried out, for example, in<br />
colored <strong>paper</strong> production, where <strong>the</strong> dyestuff is changed grade by grade from light to dark<br />
shades. After producing <strong>the</strong> darkest shade, <strong>the</strong> system is cleaned <strong>and</strong> production can be started<br />
again with <strong>the</strong> lightest shade.<br />
Figure 4. (a.) Short circulation model. (b.) Dynamic response for different retention values R as a<br />
function <strong>of</strong> normalized time1.<br />
5.2.2.2 Pressure variations <strong>and</strong> pulsation<br />
Of <strong>the</strong> parameters possibly varying in short-term pressure variations or pulsation are <strong>the</strong> most<br />
critical in <strong>the</strong> short circulation. High-speed, single-layer <strong>machine</strong>s for printing <strong>paper</strong> production<br />
are affected in particular. Changes in pressure alter <strong>the</strong> headbox pressure causing undesired<br />
variations in <strong>the</strong> headbox lip flow <strong>and</strong> in <strong>the</strong> fiber orientation, etc. In <strong>the</strong> PM wet end, flow<br />
variations are experienced as "barring." Pressure pulses can cause segregation <strong>of</strong> fibers, fines,<br />
<strong>and</strong> fillers in <strong>the</strong> suspension, which cause small-scale stock concentration variations <strong>and</strong>, thus,<br />
affect <strong>the</strong> formation <strong>of</strong> <strong>the</strong> sheet. Also fiber spinning, plugging <strong>of</strong> screens, <strong>and</strong> deposits can be<br />
caused. These variations not only reduce <strong>the</strong> flow uniformity in <strong>the</strong> <strong>machine</strong>-direction, but also<br />
make <strong>the</strong> control <strong>of</strong> <strong>the</strong> cross-direction pr<strong>of</strong>ile more difficult. As a result, <strong>the</strong> runnability, thus <strong>the</strong><br />
production efficiency <strong>of</strong> <strong>the</strong> PM is reduced. Besides runnability, also <strong>the</strong> product quality can be<br />
reduced <strong>and</strong>, in many cases, printability in particular.<br />
Couching several plies like in board <strong>machine</strong>s dampens significantly small-scale variation,<br />
typically basis weight variations, which might be critical for a single-layered sheet.<br />
Pulsation analysis<br />
The stock approach system is a collection <strong>of</strong> elements that can<br />
- create,<br />
- transmit,<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
- dampen or amplify<br />
pressure variations. The effect <strong>of</strong> <strong>the</strong>se pulses can be studied from <strong>the</strong> basis weight variations,<br />
ei<strong>the</strong>r on-line or <strong>of</strong>f-line. A more direct measurement <strong>of</strong> pulsation is possible by installing<br />
pressure transducers at pipes or by taking vibration measurements with accelerometers outside<br />
<strong>of</strong> pipes or <strong>machine</strong>s. The occurring frequencies <strong>and</strong> amplitudes characterize <strong>the</strong> pulsation.<br />
Usually, <strong>the</strong> higher <strong>the</strong> amplitude is, <strong>the</strong> greater is <strong>the</strong> disturbance. Signal analysis including <strong>the</strong><br />
Fast Fourier Transformation (FFT) is used to extract frequency components2. Figure 5 shows <strong>the</strong><br />
typical frequency b<strong>and</strong>s <strong>of</strong> sources in which <strong>the</strong> disturbances occur at <strong>the</strong> short circulation.<br />
Figure 5. Frequency b<strong>and</strong>s <strong>of</strong> hydraulic pulsation sources in <strong>the</strong> short circulation.<br />
The detrimental effect <strong>of</strong> mechanical vibrations cannot be underestimated. Sufficient support<br />
<strong>of</strong> pipes <strong>and</strong> solid foundations <strong>of</strong> <strong>machine</strong>ry are important3. Separate foundations <strong>and</strong><br />
substructure <strong>of</strong> <strong>the</strong> headbox <strong>and</strong> wire section reduce vibrations in <strong>the</strong> wet end. In <strong>the</strong> approach<br />
flow system, a well-designed framework <strong>and</strong> proper mounting <strong>of</strong> <strong>the</strong> piping gives enough support<br />
to avoid vibrations but allowing for <strong>the</strong>rmal expansion.<br />
Origin <strong>of</strong> pressure variations<br />
All rotating equipment in contact with stock can contribute to pulses, which affect <strong>the</strong> uniformity <strong>of</strong><br />
<strong>the</strong> headbox lip flow, e.g.:<br />
- Stock <strong>and</strong> headbox fan pump<br />
- Pressure screen(s)<br />
- Headbox rectifier rolls.<br />
If pulses occur at <strong>the</strong> rotational frequency, usually a mechanical problem is indicated like<br />
out-<strong>of</strong>-roundness caused by damage or improper installation or misalignment due to worn<br />
bearings. Pulses can also occur at <strong>the</strong> multiples <strong>of</strong> <strong>the</strong> rotational frequency. For example, for <strong>the</strong><br />
pressure screen, <strong>the</strong> possibly occurring frequencies are according to <strong>the</strong> number, <strong>the</strong> geometry,<br />
<strong>and</strong> <strong>the</strong> interaction <strong>of</strong> <strong>the</strong> rotating elements with <strong>the</strong> screen. The amplitude <strong>of</strong> pulses depends on<br />
<strong>the</strong> design, <strong>the</strong> manufacturing precision, <strong>and</strong> <strong>the</strong> degree <strong>of</strong> wear. Besides mechanical problems,<br />
also hydraulic overloading can cause or amplify pulsation. Air in stock has a similar effect.<br />
Stochastic (r<strong>and</strong>om) pressure variations can have different origins, including:<br />
- Air in stock <strong>and</strong> air pockets in <strong>the</strong> system, e.g., at pipe bends<br />
- Mixing <strong>and</strong> dilution sites, e.g., by <strong>the</strong> headbox re-circulation loop<br />
- Controllers <strong>and</strong> faulty measurements.<br />
5.2.2.3 Stock consistency variations<br />
The constant thick stock feed flow to <strong>the</strong> PM has been a major concern since <strong>the</strong> development <strong>of</strong><br />
continuous <strong>paper</strong>making. Long-term thick stock concentration variations can be caused by:<br />
- Large <strong>and</strong> sudden variations in <strong>the</strong> component flows to <strong>the</strong> blend chest<br />
- Problems with consistency measurement <strong>and</strong> control by:<br />
- Faulty measurements- Insufficiently tuned controllers- Strong pressure variations in <strong>the</strong> dilution<br />
<strong>water</strong> header- O<strong>the</strong>r physical problems such as hysteresis <strong>of</strong> valves <strong>and</strong> pressure variations in<br />
control air, etc.<br />
- Wrong design <strong>of</strong> <strong>the</strong> <strong>machine</strong> chest or insufficient agitation.<br />
Figure 6. Stock Sankey diagram <strong>of</strong> supercalendered (SC) <strong>paper</strong> <strong>machine</strong> at design production.<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
Figure 7. Water Sankey diagram <strong>of</strong> supercalendered (SC) <strong>paper</strong> <strong>machine</strong> at design production.<br />
Changes in <strong>the</strong> amount <strong>of</strong> stock circulating in <strong>the</strong> short circulation, i.e., by changes in <strong>the</strong> wire<br />
retention, have an effect on <strong>the</strong> stock concentration in <strong>the</strong> headbox. This includes slow variations<br />
in retention, which might be initiated by changes in <strong>the</strong> amount, size distribution, or charge <strong>of</strong> <strong>the</strong><br />
fillers <strong>and</strong> fines fraction, changes in <strong>the</strong> amount <strong>of</strong> dissolved <strong>and</strong> detrimental substances, or in<br />
cationic charge dem<strong>and</strong>, temperature, pH, etc.<br />
5.2.2.4 Headbox approach flow stability<br />
Pipe bends, joints, <strong>and</strong> installations cause pipe flow disturbances, which appear as turbulence<br />
<strong>and</strong> vortices causing a head loss4,5. Especially immediately before <strong>the</strong> headbox, secondary flows<br />
or vortices have to be avoided as far as possible to ensure <strong>the</strong> most even flow conditions. The<br />
number <strong>of</strong> bends in <strong>the</strong> pipe between <strong>the</strong> <strong>machine</strong> screen(s) <strong>and</strong> headbox should be as small as<br />
possible, also for lessening <strong>the</strong> costs. Figure 3 shows one possible solution. The last bend before<br />
<strong>the</strong> headbox has a radius <strong>of</strong> at least three times <strong>the</strong> pipe diameter. If that is not possible for some<br />
reason, <strong>the</strong> pipe should be carried out as a so-called "hydraulic knee," where <strong>the</strong> pipe diameter is<br />
reduced over <strong>the</strong> bend. This solution is usually more expensive; however, in any case, cavitation<br />
<strong>and</strong> flow separation must be avoided. The bend is followed by a straight section <strong>of</strong> about five<br />
times <strong>the</strong> pipe diameter right before <strong>the</strong> tapered inlet header <strong>of</strong> <strong>the</strong> headbox (if an attenuation unit<br />
or o<strong>the</strong>r special headbox approach is not used)6. No control valves or o<strong>the</strong>r measuring devices<br />
are installed in <strong>the</strong> headbox feed pipe. Properties <strong>of</strong> <strong>the</strong> headbox feed stock are measured after<br />
<strong>the</strong> headbox at <strong>the</strong> recirculation line. See also <strong>the</strong> section about stock transport.<br />
5.2.3 Stock <strong>and</strong> <strong>water</strong> balance<br />
Determining <strong>the</strong> stock <strong>and</strong> <strong>water</strong> balance <strong>of</strong> <strong>the</strong> <strong>paper</strong> mill is <strong>the</strong> starting point for any new<br />
process design as well as for an existing system analysis. The balances can vary substantially for<br />
different situations, e.g., production <strong>of</strong> <strong>the</strong> lowest <strong>and</strong> <strong>the</strong> highest basis weight or in <strong>the</strong> case <strong>of</strong><br />
web breaks. An illustrative manner <strong>of</strong> representing balances is in <strong>the</strong> form <strong>of</strong> a Sankey-diagram<br />
as shown in Figs. 6 <strong>and</strong> 7 for a supercalendered <strong>paper</strong> line in steady operation. Figure 6 shows<br />
<strong>the</strong> flow <strong>of</strong> solids relative to <strong>the</strong> amount <strong>of</strong> <strong>paper</strong> at <strong>the</strong> reel, <strong>and</strong> Fig. 7 shows <strong>the</strong> specific <strong>water</strong><br />
flow for <strong>the</strong> same situation. Note that, for this grade, <strong>the</strong>re is a large amount <strong>of</strong> solids in <strong>the</strong> short<br />
circulation − in this case, at a retention <strong>of</strong> 54%. For clarity reasons, <strong>the</strong> figure shows <strong>the</strong> cleaner<br />
<strong>and</strong> screening system as one block each <strong>and</strong> <strong>the</strong> PM shower <strong>water</strong> system is very simplified.<br />
Collected <strong>water</strong>s are <strong>of</strong> different quality, <strong>and</strong> <strong>the</strong>y are not fed all toge<strong>the</strong>r into <strong>the</strong> white <strong>water</strong><br />
(WW) tank, as shown in Figs. 6 <strong>and</strong> 7. Note also that <strong>the</strong> PM comprises a dilution <strong>water</strong> headbox.<br />
Dilution at <strong>the</strong> second <strong>and</strong> third cleaner stages is done with wire <strong>water</strong>, which is shown by <strong>the</strong><br />
smaller <strong>of</strong> <strong>the</strong> two parallel streams between <strong>the</strong> blocks "wire pit <strong>and</strong> fan pump" <strong>and</strong> "cleaner<br />
system" in Figs. 6 <strong>and</strong> 7. The <strong>paper</strong> mill surplus <strong>water</strong> is usually fed upstream, like in this case,<br />
into <strong>the</strong> mechanical pulp production unit.<br />
5.2.4 Multi-ply <strong>and</strong> multilayer <strong>systems</strong><br />
The number <strong>of</strong> separate stock approach flow <strong>systems</strong> increases according to <strong>the</strong> number <strong>of</strong><br />
different stock components used ei<strong>the</strong>r in multi-ply or multilayer <strong>paper</strong> <strong>and</strong> board production.<br />
Figures 8 through 10 show <strong>the</strong> block diagrams for a printing <strong>paper</strong>-producing <strong>machine</strong>, a<br />
multilayer-producing PM like tissue or linerboard, <strong>and</strong> a multi-ply board or linerboard <strong>machine</strong>.<br />
The multilayered sheet is produced from one multichannel headbox <strong>and</strong> one former; thus,<br />
<strong>the</strong>re is only one wire pit (see Fig. 9). The multichannel headbox combines two or three stock<br />
flows into one jet leaving <strong>the</strong> headbox lip. This flow consists typically <strong>of</strong> two different furnishes,<br />
which means − in <strong>the</strong> case <strong>of</strong> a triple layered flow − that one furnish is used for <strong>the</strong> middle layer<br />
<strong>and</strong> <strong>the</strong> o<strong>the</strong>r one for <strong>the</strong> two outer layers. Both approach flow <strong>systems</strong> have to be dimensioned<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
according to <strong>the</strong> desired sheet structures <strong>and</strong> grades. The requirements on availability <strong>and</strong><br />
trouble-free operation <strong>of</strong> <strong>the</strong> stock approach flow <strong>systems</strong> are as high, if not even higher, as in<br />
single <strong>systems</strong> because <strong>the</strong> failure <strong>of</strong> one approach flow system means that <strong>the</strong> entire <strong>paper</strong><br />
production fails.<br />
Figure 8. Printing <strong>paper</strong> <strong>machine</strong>, block diagram.<br />
On a multi-ply board <strong>machine</strong>, ei<strong>the</strong>r on a multifourdrinier <strong>machine</strong> or on a multiple former<br />
<strong>machine</strong>, <strong>the</strong> wire <strong>water</strong>s are collected separately <strong>and</strong> <strong>the</strong> <strong>water</strong> circuits are usually separated<br />
(see Fig. 10). This is important, if <strong>the</strong> furnishes <strong>of</strong> <strong>the</strong> plies are <strong>of</strong> very different kinds, e.g., a<br />
white top on a brown or gray mid or base layer. Investment costs are higher, due to <strong>the</strong> higher<br />
degree <strong>of</strong> complexity <strong>of</strong> <strong>the</strong> entire stock approach flow system including separate blend <strong>and</strong><br />
<strong>machine</strong> chests for each ply or component. The larger amount <strong>of</strong> equipment also requires more<br />
floor space, electric power, <strong>and</strong> instrumentation <strong>and</strong> control loops. The benefits <strong>of</strong> multi-ply<br />
forming are7:<br />
- Production can be possibly increased,<br />
- Raw material costs can be optimized by using a cheaper furnish <strong>and</strong> still getting <strong>the</strong> same<br />
strength or optical properties,<br />
- Better quality white-lined grades can be produced with lower basis weight <strong>of</strong> white pulp<br />
layer.<br />
Figure 9. Multilayer headbox <strong>machine</strong> with two stock components, block diagram.<br />
Figure 10. Two-ply <strong>machine</strong>, block diagram.<br />
5.2.5 System cleanliness<br />
Cleanliness refers to freedom from dirt <strong>and</strong> contaminants in <strong>the</strong> process <strong>and</strong> in <strong>the</strong> product.<br />
Cleanliness refers also to absence or to a low level in dissolved <strong>and</strong> colloidal material<br />
contamination, which can cause scale or dirt formed by precipitation, coagulation or biological<br />
activity appearing as slime. Slime occurs as lumps or films causing product defects such as<br />
holes, specs, <strong>and</strong> smell, as well as production problems by plugging <strong>and</strong> scaling. The latter leads<br />
to micro-biologically induced corrosion <strong>and</strong> possibly to decreased heat transfer. Micro-organic<br />
activity is unavoidable due to <strong>the</strong> large content <strong>of</strong> nutrients in <strong>paper</strong> mill <strong>water</strong>s <strong>and</strong> <strong>the</strong> usually<br />
favorable temperature. Hence, control <strong>of</strong> biological activity is needed in order to protect <strong>the</strong><br />
production <strong>and</strong> <strong>the</strong> product from disturbing slime.<br />
System cleanliness should be a concern everywhere in <strong>paper</strong> production. Cleanliness is<br />
supported by correct process design. On <strong>the</strong> one h<strong>and</strong>, st<strong>and</strong>ing <strong>water</strong> or low flow speed, dead<br />
ends, edges, corners, rough surfaces, <strong>and</strong> low-quality materials have to be avoided in piping <strong>and</strong><br />
<strong>machine</strong>ry where material can accumulate <strong>and</strong> slime or scaling can build up. Note, for example,<br />
that it is <strong>the</strong> first <strong>and</strong> not <strong>the</strong> last cleaner bank seen in feed flow direction in Fig. 3, which can be<br />
disconnected, <strong>the</strong>reby ensuring full flow without dead ends in <strong>the</strong> distribution feed pipe. Figure 11<br />
shows that a fine finish <strong>of</strong> <strong>the</strong> steel surface hampers <strong>the</strong> growth <strong>of</strong> microbes8. This is important in<br />
<strong>the</strong> stock approach system <strong>and</strong> nearer <strong>the</strong> headbox6, especially at locations where <strong>the</strong> flow<br />
velocity is not high, like in <strong>the</strong> deaeration tank. On <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>, cleaning <strong>of</strong> equipment <strong>and</strong><br />
pipes has to be possible. This includes well-positioned wash fluid connectors <strong>and</strong> drainage<br />
valves to empty pipes during shutdowns. The latter is essential for all thick stock pipes, which<br />
should be inclined <strong>and</strong> equipped with a drain at <strong>the</strong> lowest <strong>and</strong> a vent at <strong>the</strong> highest point. Air<br />
pockets are a prominent place for slime to build up, which <strong>the</strong>n releases as lumps from time to<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
time. Avoiding <strong>the</strong> buildup <strong>of</strong> air pockets is important when positioning valves <strong>and</strong> selecting <strong>the</strong><br />
appropriate type <strong>of</strong> valve. At open surfaces, splashing should be avoided because air is<br />
entrained <strong>and</strong> material builds up on surfaces above <strong>the</strong> <strong>water</strong>line. This <strong>the</strong>n accumulates, dries,<br />
<strong>and</strong> eventually re-enters <strong>the</strong> process as detrimental chunks.<br />
System washings have to be performed during planned maintenance shutdowns, which can<br />
include pressure rinse, high-temperature treatment, <strong>and</strong> washing with chemicals according to<br />
needs9. Timer-controlled washing <strong>systems</strong> <strong>and</strong> showers can be installed on equipment <strong>and</strong><br />
tanks, which are prone to build up dirt or chunks <strong>of</strong> dried stock. Finally, keeping floors <strong>and</strong><br />
<strong>machine</strong>ry clean improves <strong>the</strong> mill operators' safety in general.<br />
Figure 11. Microbe population on metal surfaces with different degree <strong>of</strong> finishing8.<br />
5.3 Stock flow operations<br />
5.3.1 Stock blending<br />
The functional <strong>paper</strong> properties are determined in great part by <strong>the</strong> properties <strong>of</strong> <strong>the</strong> stock<br />
components used. Type, quality, <strong>and</strong> quantity <strong>of</strong> <strong>the</strong> different components are determined by <strong>the</strong><br />
specific recipe for each grade. Therefore, <strong>the</strong> stock is a blend <strong>of</strong> several components in order to<br />
reach <strong>the</strong> desired <strong>paper</strong> properties under <strong>the</strong> most economic circumstances.<br />
Generally speaking, stock blending can take place continuously or in a batch system. In<br />
modern <strong>paper</strong>making, batch blending is used only for specialty <strong>paper</strong>s produced on <strong>machine</strong>s<br />
with small production rates or even in discontinuous operation, applying very special furnish<br />
components, dyes, or chemicals. Figure 12 shows a typical example for a continuous system. All<br />
fiber components are diluted to <strong>the</strong> same pre-set concentration for blending. Each pulp typically<br />
has a separate pulp chest, <strong>the</strong> proportioning chest, to ensure a constant supply at <strong>the</strong> dosage<br />
point. In an integrated mill, pulp is usually picked up at a medium-consistency storage tower by<br />
dilution with <strong>water</strong> from <strong>the</strong> main PM dilution header. The concentration in <strong>the</strong> pulp chest is<br />
usually adjusted to 0.2%−0.3% points higher than in <strong>the</strong> blend chest. The stock is <strong>the</strong>n diluted to<br />
<strong>the</strong> blending concentration <strong>and</strong> pumped to blending via refiners or directly. The components are<br />
proportioned to <strong>the</strong> blend chest by flow metering <strong>and</strong> flow ratio controllers. The setpoints for <strong>the</strong><br />
controllers are given to <strong>the</strong> process control system according to <strong>the</strong> current recipe. The level<br />
controller regulates <strong>the</strong> total amount <strong>of</strong> stock entering <strong>the</strong> blend chest.<br />
Occasionally, <strong>the</strong> blend chest is also called a "mixing chest." Despite <strong>the</strong> name, <strong>the</strong> function<br />
<strong>of</strong> this chest is not only to create complete motion <strong>of</strong> <strong>the</strong> stock, which is referred to as "mixing,"<br />
but also to gain complete stock uniformity, referred to as "blending"10.<br />
There are three or more components mixed in <strong>the</strong> blend chest:<br />
- Primary stock component(s), flow ratio controlled <strong>and</strong> consistency corrected<br />
- Broke, flow ratio controlled <strong>and</strong> consistency corrected<br />
- Recovered fiber from <strong>the</strong> saveall.<br />
Possible consistency differences between <strong>the</strong> stock component flows can be corrected by<br />
calculating <strong>the</strong> mass flow in <strong>the</strong> process control system. The components are typically fed via a<br />
common header pipe to <strong>the</strong> blend chest. The header pipe at <strong>the</strong> side <strong>of</strong> <strong>the</strong> blend chest is also a<br />
possible dosage point for functional chemicals, e.g., dyes. The sweetener stock pump on <strong>the</strong><br />
o<strong>the</strong>r side <strong>of</strong> this header pumps sweetener stock to <strong>the</strong> saveall disc filter. The amount <strong>of</strong> required<br />
sweetener can be large, cf. Fig. 6. The arrangement <strong>of</strong> <strong>the</strong> pipes determines which furnish<br />
component is used predominantly as sweetener (see Fig. 12). The concentration in <strong>the</strong> blend<br />
chest is similar to <strong>the</strong> pre-set consistency <strong>of</strong> <strong>the</strong> blending streams. Under all possible production<br />
conditions, <strong>the</strong> residence time <strong>of</strong> stock in <strong>the</strong> blend chest has to be longer than <strong>the</strong> blending time,<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
i.e., <strong>the</strong> time until a reasonably homogenous mixture is reached. Therefore, agitation has to be<br />
sufficient.<br />
The blended pulp is pumped at a constant rate to <strong>the</strong> <strong>machine</strong> chest. The stock is diluted by<br />
a small concentration decrement, typically about 0.2%−0.3%. All connections to <strong>the</strong> blend <strong>and</strong><br />
<strong>machine</strong> chest are designed in such a way that <strong>the</strong> best possible mixing occurs <strong>and</strong> entrainment<br />
<strong>of</strong> air is low, e.g., by avoiding splashing <strong>and</strong> large vortices in <strong>the</strong> vicinity <strong>of</strong> <strong>the</strong> open surface. In<br />
some installations, additional equipment like refiner or thick-stock screens can be found at <strong>the</strong><br />
position between <strong>the</strong> blend chest <strong>and</strong> <strong>the</strong> <strong>machine</strong> chest. Constant consistency <strong>and</strong> steady<br />
hydraulic load at this position ease <strong>the</strong> operation <strong>of</strong> such equipment. On <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>, it has to<br />
be considered that any extra equipment at this location requires extra maintenance or might<br />
o<strong>the</strong>rwise cause extra downtime, which decreases <strong>the</strong> total PM efficiency. Uneven operation,<br />
e.g., by wear <strong>of</strong> refiner fittings or varying reject flow from <strong>the</strong> screen, can cause variations with<br />
adverse effects on <strong>the</strong> performance <strong>of</strong> <strong>the</strong> stock approach flow system.<br />
Figure 12 shows also how a sampling site can be integrated into <strong>the</strong> blending system.<br />
Constant flow <strong>and</strong> <strong>the</strong> possibility to flush <strong>and</strong> to clean are important in order to collect<br />
representative samples <strong>of</strong> <strong>the</strong> pulps for laboratory analysis. In addition, or instead <strong>of</strong> a station for<br />
manual sampling, automated or robotized pulp analysis equipment can be also installed.<br />
Figure 12. Stock blending <strong>and</strong> <strong>machine</strong> chest including sampling station, an example.<br />
5.3.2 Stock dosage<br />
The basis weight <strong>of</strong> <strong>the</strong> sheet is controlled by <strong>the</strong> amount <strong>of</strong> thick stock from <strong>the</strong> <strong>machine</strong> chest to<br />
<strong>the</strong> PM. It is affected by <strong>the</strong> amount <strong>of</strong> filler added to <strong>the</strong> short circulation. For information on<br />
basis weight control, refer to Volume 14: Process Control <strong>of</strong> this book series. Stock dosage or<br />
metering is commonly carried out in ei<strong>the</strong>r <strong>of</strong> <strong>the</strong> following two alternative ways:<br />
- By a thick stock valve right before dilution at <strong>the</strong> wire pit<br />
- By speed control <strong>of</strong> <strong>the</strong> thick stock feed pump.<br />
In ei<strong>the</strong>r case, <strong>the</strong> thick stock concentration is kept constant. Advantages <strong>of</strong> <strong>the</strong><br />
speed-controlled pump are a lower energy consumption, higher feed flow accuracy, <strong>and</strong> simpler<br />
piping layout. The benefits are worth <strong>the</strong> effort, especially for <strong>machine</strong>s producing a range <strong>of</strong><br />
grades with large variation in <strong>the</strong> basis weight. A stuff box is not recommended in modern <strong>paper</strong><br />
<strong>machine</strong>s in ei<strong>the</strong>r case because it is a source <strong>of</strong> slime problems.<br />
If stock is metered by valve control, <strong>the</strong> basis weight valve is located directly before stock<br />
dilution at <strong>the</strong> wire pit. The pipe at <strong>the</strong> location <strong>of</strong> <strong>the</strong> valve should point upward in order to avoid<br />
<strong>the</strong> accumulation <strong>of</strong> air on <strong>the</strong> downstream side <strong>of</strong> <strong>the</strong> valve. Valve control can be also carried out<br />
with two valves in parallel, i.e., with one valve for coarse metering <strong>and</strong> <strong>the</strong> o<strong>the</strong>r one for fine<br />
control.<br />
5.3.3 Stock dilution<br />
5.3.3.1 Principle<br />
In mill practice, reducing <strong>the</strong> stock consistency means <strong>the</strong> mixing <strong>of</strong> a high-consistency stream<br />
with a low-consistency stream. Hence, consistency variations in <strong>the</strong> blended stream can be<br />
caused by variations in <strong>the</strong> flow <strong>and</strong> in <strong>the</strong> consistency <strong>of</strong> ei<strong>the</strong>r <strong>of</strong> <strong>the</strong> streams to be blended.<br />
Consistency variations in <strong>the</strong> lean stream are usually not significant to <strong>the</strong> diluted stream, except<br />
if <strong>the</strong> rate <strong>of</strong> dilution is high. The flows <strong>and</strong> <strong>the</strong> size <strong>of</strong> <strong>the</strong> mixing volume or <strong>the</strong> mixing zone<br />
determine how much variation can be leveled out according to <strong>the</strong> amplitude <strong>and</strong> wavelength <strong>of</strong><br />
variation. For example, <strong>the</strong> <strong>machine</strong> chest is dimensioned according to this principle.<br />
5.3.3.2 Mixing<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
Mixing is achieved by turbulence created by moving parts, surge on static elements, or<br />
turbulence created when feeding streams toge<strong>the</strong>r. In coaxial pipe arrangements, like in wire pit<br />
stock dilution, <strong>the</strong> thick stock is fed into <strong>the</strong> dilution <strong>water</strong> <strong>and</strong> not vice versa, in order to gain <strong>the</strong><br />
best mixing <strong>and</strong> a stable flow. The streams are mixed by secondary flows, which are created by<br />
<strong>the</strong> speed difference <strong>of</strong> <strong>the</strong> streams to be mixed. In wire pit stock dilution, <strong>the</strong> turbulence created<br />
in <strong>the</strong> fan pump provides good mixing.<br />
Consistency variations can also be filtered by dividing <strong>the</strong> flow into branches, in which <strong>the</strong><br />
stock is retained for varying lengths <strong>of</strong> time before <strong>the</strong> separate flows are recombined. Figure 13<br />
shows schematically an arrangement <strong>of</strong> <strong>the</strong> multiple flow-lag principle through a divided<br />
manifold11. The stock flowing through <strong>the</strong> left-h<strong>and</strong> cleaner <strong>of</strong> <strong>the</strong> parallel cleaners has <strong>the</strong><br />
longest retention time. The principle applies in <strong>the</strong> same manner for pressure screens, deaeration<br />
vessel feed pipes, o<strong>the</strong>r piping, <strong>and</strong> <strong>the</strong> entire PM <strong>water</strong> system.<br />
Figure 13. Multiple flow-lag principle.<br />
5.3.3.3 Machine stock dilution<br />
In a typical stock approach system, <strong>the</strong> thick stock from <strong>the</strong> <strong>machine</strong> chest is diluted at <strong>the</strong><br />
bottom <strong>of</strong> <strong>the</strong> wire pit (see Fig. 14). The diluted stock consistency depends on <strong>the</strong> retention, i.e.,<br />
<strong>the</strong> wire <strong>water</strong> consistency, <strong>and</strong> <strong>the</strong> amount <strong>of</strong> thick stock dosed, which again is adjusted to meet<br />
<strong>the</strong> desired basis weight <strong>of</strong> <strong>the</strong> product. The actual consistency, which is <strong>the</strong> primary cleaner feed<br />
consistency, can differ according to <strong>the</strong> produced grade, its basis weight, <strong>the</strong> current retention,<br />
filler content, etc. The thick stock consistency is kept constant. No consistency control occurs<br />
after <strong>the</strong> <strong>machine</strong> chest.<br />
The geometry <strong>of</strong> <strong>the</strong> bottom as well as <strong>of</strong> <strong>the</strong> outlet <strong>of</strong> <strong>the</strong> wire pit is very important to ensure<br />
stable hydraulic conditions <strong>and</strong> a good mixing <strong>of</strong> <strong>the</strong> different components. A fan pump, which is<br />
feeding <strong>the</strong> primary cleaner stage in most <strong>systems</strong>, is directly connected to <strong>the</strong> asymmetrically<br />
tapered mixing zone. Additives like dyes <strong>and</strong> possibly filler are dosed at <strong>the</strong> topside <strong>of</strong> <strong>the</strong><br />
fan-pump suction piece. The coaxial flow <strong>of</strong> <strong>the</strong> main components is steady, if <strong>the</strong> flow velocity<br />
difference between <strong>the</strong> components is large enough under all possible operation conditions:<br />
V 1 > V 2 > V 3 > V 4<br />
where V 1 −V 4 are <strong>the</strong> flow speeds as shown in Table 2 <strong>and</strong> in Fig. 14. The outer concentric feed<br />
pipe is <strong>the</strong> end <strong>of</strong> <strong>the</strong> st<strong>and</strong>pipe collecting circulation flows o<strong>the</strong>r than <strong>the</strong> wire <strong>water</strong>, possibly:<br />
- Second- <strong>and</strong> third-stage cleaner accept<br />
- Second- <strong>and</strong> third-stage <strong>machine</strong> screen accept<br />
- Headbox recirculation, especially if <strong>the</strong>re is no deaeration tank<br />
- Deaeration overflow<br />
- Overflow <strong>and</strong> circulation from <strong>the</strong> headbox dilution system.<br />
Figure 14. PM wire pit with single stock dilution.<br />
Feeding <strong>the</strong> thick stock <strong>and</strong> o<strong>the</strong>r circulated stock top-down at <strong>the</strong> fan-pump suction side,<br />
thus, at <strong>the</strong> position which is today exclusively used for additives (see Fig. 14), is an outdated<br />
solution. The disadvantages <strong>of</strong> this arrangement are, on <strong>the</strong> one h<strong>and</strong>, poor mixing due to <strong>the</strong><br />
type <strong>of</strong> arrangement itself <strong>and</strong> to low velocity differences. On <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>, feed pipes that are<br />
arranged side-by-side can cause hydraulic interaction between <strong>the</strong> flows12.<br />
Table 2. Common wire pit flow velocities as shown in Fig. 14.<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
Flow<br />
velocity [m/s]<br />
Thick stock V 1 2.0 ± 0.5<br />
Circulation V 2 1.5 ± 0.5<br />
Wire <strong>water</strong> V 3 1.0 ± 0.5<br />
Wire <strong>water</strong> V 4 0.08−0.15<br />
If a low headbox consistency is required, a two-stage dilution system might be needed. In <strong>the</strong><br />
first stage, <strong>the</strong> stock is diluted to <strong>the</strong> cleaner feed consistency <strong>and</strong>, in <strong>the</strong> second stage, <strong>the</strong><br />
cleaner accept is diluted fur<strong>the</strong>r to reach <strong>the</strong> desired headbox consistency after screening.<br />
Hence, cleaner feed accept consistency <strong>and</strong> screen feed consistency are de-coupled. As an<br />
advantage, <strong>the</strong> cleaner plant can be operated constantly at <strong>the</strong> optimal consistency, while <strong>the</strong><br />
headbox consistency may change according to <strong>the</strong> grade produced. Two-stage <strong>systems</strong> are<br />
usually considered for multiple grade producing <strong>machine</strong>s with stock cleaning, if <strong>the</strong> headbox<br />
consistency is about 0.7% or lower. On <strong>the</strong> contrary, tissue <strong>paper</strong> <strong>machine</strong>s without cleaners <strong>and</strong><br />
little product variations <strong>of</strong>ten have one-stage dilution, even though <strong>the</strong> consistency can be as low<br />
as 0.15%. Similar to a multichannel headbox system with two approach flows, <strong>the</strong> wire pit also<br />
has for a traditional two-stage dilution two separate dilution zones, which are usually located<br />
opposite to each o<strong>the</strong>r. Ano<strong>the</strong>r design consists <strong>of</strong> connected parallel wire <strong>water</strong> tanks each with<br />
a dilution zone. Such a system <strong>of</strong> connected wire <strong>water</strong> tanks has been applied in, e.g., multiple<br />
layer production with two-stage dilution. However, <strong>the</strong> two-stage dilution system operates only<br />
<strong>the</strong>n stable, if enough wire <strong>water</strong> is consumed at <strong>the</strong> second dilution stage.<br />
5.3.3.4 Headbox dilution system<br />
The aim <strong>of</strong> dilution at <strong>the</strong> headbox is to control <strong>the</strong> basis weight cross-pr<strong>of</strong>ile at <strong>the</strong> PM (see<br />
Chapter 6). The amount <strong>of</strong> dilution <strong>water</strong> ranges from a few percent <strong>of</strong> <strong>the</strong> headbox lip flow up to<br />
over 20%. Basis weight control is more efficient <strong>the</strong> lower <strong>the</strong> solids content is <strong>of</strong> <strong>the</strong> dilution<br />
<strong>water</strong>13. To ensure efficient control, a certain dilution flow is needed at all dilution positions all <strong>the</strong><br />
time. According to <strong>the</strong> design <strong>and</strong> recommendations by <strong>the</strong> headbox manufacturer, <strong>the</strong> amount <strong>of</strong><br />
required dilution <strong>water</strong> can be quite high, even if <strong>the</strong>re is a high one-pass retention like on board<br />
<strong>machine</strong>s. In order to avoid an increase in hydraulic load on <strong>the</strong> saveall disc filter, <strong>the</strong> headbox<br />
dilution <strong>water</strong> is usually taken directly from <strong>the</strong> wire pit. For products with characteristically low<br />
retention, <strong>the</strong> required <strong>water</strong> for dilution has to be increased accordingly. The approach flow<br />
system for <strong>the</strong> headbox dilution <strong>water</strong> consists <strong>of</strong> similar elements as <strong>the</strong> system for <strong>the</strong> diluted<br />
stock main flow:<br />
- A speed-controlled fan pump<br />
- A fine screen<br />
- Deaeration equipment similar to <strong>the</strong> stock system<br />
- Overflow from <strong>the</strong> headbox dilution header, usually similar to <strong>the</strong> stock flow.<br />
The dilution <strong>water</strong> is deaerated in a separate unit or in a separate compartment, which is<br />
integrated into <strong>the</strong> stock deaeration tank. The same design criteria are applied as for <strong>the</strong> stock<br />
approach flow system <strong>and</strong> its equipment concerning low pulsation <strong>and</strong> <strong>the</strong> proper design <strong>of</strong><br />
surfaces <strong>and</strong> materials.<br />
5.3.3.5 Medium- <strong>and</strong> high-consistency stock dilution<br />
In integrated mills, <strong>the</strong> prepared pulp is <strong>of</strong>ten collected from medium-consistency (MC) storage<br />
towers, with a storage consistency <strong>of</strong> typically 8%−12%. Dilution to a pumpable suspension takes<br />
place in <strong>the</strong> bottom part <strong>of</strong> <strong>the</strong> storage tower by injecting dilution <strong>water</strong>, which is filtrate or white<br />
<strong>water</strong> taken from <strong>the</strong> main header <strong>of</strong> <strong>the</strong> PM <strong>water</strong> system. Figure 15 shows a<br />
medium-consistency storage tower. The dilution <strong>water</strong> is fed to <strong>the</strong> suction zone <strong>of</strong> <strong>the</strong> agitator or<br />
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injected along <strong>the</strong> propeller shaft directly into <strong>the</strong> zone <strong>of</strong> highest turbulence at <strong>the</strong> agitator14. Its<br />
amount is usually controlled by <strong>the</strong> diluted stock flow. The fine adjustment <strong>of</strong> <strong>the</strong> diluted stock<br />
consistency is by injection <strong>of</strong> dilution <strong>water</strong> at <strong>the</strong> suction side <strong>of</strong> <strong>the</strong> pump according to <strong>the</strong><br />
consistency measurement, as shown in Fig. 15.<br />
For long distance transport <strong>of</strong> pulp in high consistency <strong>of</strong> 20%−50%, ei<strong>the</strong>r lorries or belt/tube<br />
conveyors are used. High-consistency pulp is transported over short distances by screw<br />
conveyors. Stock from lorries is usually slushed in pulpers, while <strong>the</strong> use <strong>of</strong> a dilution screw is<br />
possible for diluting <strong>the</strong> pulp, which is continuously fed by <strong>the</strong> conveyor. The pulp is diluted in <strong>the</strong><br />
screw to storage consistency <strong>and</strong> drops into a storage tower from where pulp proceeds as<br />
described above.<br />
The higher <strong>the</strong> transfer consistency is from integrated pulping to <strong>the</strong> <strong>paper</strong> <strong>machine</strong> system,<br />
<strong>the</strong> better is <strong>the</strong> separation <strong>of</strong> <strong>the</strong> <strong>water</strong> circuits <strong>of</strong> <strong>the</strong> <strong>systems</strong>. Thus, <strong>the</strong> less detrimental<br />
substances are carried over into PM <strong>water</strong>, when <strong>the</strong> white <strong>water</strong> exchange is made at highest<br />
stock consistency.<br />
Figure 15. Medium-consistency storage tower <strong>and</strong> stock dilution.<br />
5.3.4 Cleaning <strong>and</strong> screening<br />
Chapter 9 <strong>of</strong> Volume 5: Mechanical Pulping <strong>of</strong> this book series describes <strong>the</strong> principles <strong>of</strong><br />
cleaning <strong>and</strong> screening <strong>and</strong> related equipment. Depending on <strong>the</strong> stock components, pulp has<br />
already been subjected to various classification processes like cleaning <strong>and</strong> screening upstream.<br />
Hence, <strong>the</strong> amount <strong>of</strong> foreign material to be removed is small; thus, <strong>the</strong> reject stream should be<br />
small. Therefore end-stage cleaners are <strong>of</strong> special design, <strong>and</strong> screens are <strong>of</strong>ten not discharging<br />
reject continuously in an effort to reduce <strong>the</strong> loss <strong>of</strong> fibers. The main purpose <strong>of</strong> cleaning <strong>and</strong><br />
screening is to ensure clean stock by removing bundles, flakes, <strong>and</strong> occasional debris <strong>and</strong><br />
strings, which are partly created within <strong>the</strong> system; refer to <strong>the</strong> above section about system<br />
cleanliness.<br />
5.3.4.1 Hydrocyclone cleaning<br />
Centrifugal cleaners are used to remove dense debris <strong>of</strong> fiber size or smaller from <strong>the</strong> diluted<br />
stock within <strong>the</strong> short circulation15,16. This debris can be s<strong>and</strong>, grit, shives, pitch, or o<strong>the</strong>r dense<br />
particles. Practically all low basis weight <strong>and</strong> printing <strong>paper</strong> <strong>machine</strong>s have a multistage cleaner<br />
cascade system, which can be attached to a deaeration tank.<br />
In <strong>the</strong> hydrocyclone, <strong>the</strong> suspension path involves a double vortex with <strong>the</strong> suspension<br />
spiraling downward at <strong>the</strong> outside <strong>and</strong> upward at <strong>the</strong> inside. At <strong>the</strong> beginning <strong>of</strong> <strong>the</strong> conical part <strong>of</strong><br />
<strong>the</strong> cyclone, <strong>the</strong> stream velocity undergoes a redistribution so that <strong>the</strong> tangential component <strong>of</strong><br />
velocity increases with decreasing radius. The spiral velocity in <strong>the</strong> cyclone might reach a value<br />
several times <strong>the</strong> inlet velocity. The separation <strong>of</strong> accepted <strong>and</strong> rejected particles depends on <strong>the</strong><br />
velocity pr<strong>of</strong>ile <strong>and</strong> <strong>the</strong> location <strong>of</strong> <strong>the</strong> layer <strong>of</strong> zero-vertical velocity. The smaller <strong>the</strong> main<br />
diameter <strong>of</strong> <strong>the</strong> cyclone is, <strong>the</strong> more efficient is <strong>the</strong> separation <strong>of</strong> debris but <strong>the</strong> smaller is <strong>the</strong><br />
hydraulic capacity. At small diameter, <strong>the</strong> risk <strong>of</strong> plugging is higher. In <strong>the</strong> PM cleaner cascade,<br />
however, plugging is a minor problem due to dilution after every stage <strong>and</strong> due to oversized<br />
particle removal already performed during stock preparation by screening or high-density<br />
cleaners.<br />
No cut<strong>of</strong>f size or critical particle diameter exists for cyclone separation. Centrifugal <strong>and</strong> shear<br />
forces determine <strong>the</strong> separation or fractionation17−19. The flow pattern in <strong>the</strong> cyclone is very<br />
complex, <strong>and</strong> <strong>the</strong> separation efficiency curve is unique for a given cleaner geometry. Figure 16<br />
shows <strong>the</strong> flow pattern schematically. Hence, <strong>the</strong> debris removal efficiency must be determined<br />
experimentally. The conical part <strong>of</strong> <strong>the</strong> cleaner can contain baffles <strong>and</strong> helical guides to modify or<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
direct <strong>the</strong> flow. These tend to increase <strong>the</strong> hydraulic capacity <strong>and</strong> to reduce <strong>the</strong> fiber reject rate<br />
<strong>and</strong> pressure drop, but <strong>the</strong>y also reduce <strong>the</strong> cleaning efficiency to some degree15. The removal<br />
<strong>of</strong> vapor <strong>and</strong> air from <strong>the</strong> cleaner can also stabilize cleaner flow. Air accumulates along <strong>the</strong><br />
longitudinal axis <strong>of</strong> <strong>the</strong> cleaner, thus in <strong>the</strong> core <strong>of</strong> <strong>the</strong> vortex. Besides <strong>the</strong> cleaner geometry <strong>and</strong><br />
design, particle separation is determined by <strong>the</strong> specific gravity difference between fluid <strong>and</strong><br />
particle, by <strong>the</strong> specific surface <strong>of</strong> <strong>the</strong> particle, by <strong>the</strong> stock viscosity, which varies with <strong>the</strong> stock<br />
temperature <strong>and</strong> <strong>the</strong> air content, <strong>and</strong> by <strong>the</strong> velocity field in general. The operational cleaner<br />
parameters are:<br />
- Stock flow rate <strong>and</strong> feed pressure<br />
- Ratio <strong>of</strong> underflow <strong>and</strong> overflow, i.e., <strong>the</strong> reject ratio<br />
- Feed consistency<br />
- Back pressure on <strong>the</strong> reject side.<br />
Depending on <strong>the</strong>se parameters <strong>and</strong> on <strong>the</strong> cleaner design, a certain pressure drop <strong>and</strong><br />
reject thickening occurs.<br />
Figure 16. Flow pattern in <strong>the</strong> hydrocyclone (forward cleaner).<br />
Hydrocyclones can remove heavy debris, if designed as forward cleaners, or light debris, as<br />
reverse or through-flow cleaners. In <strong>the</strong> stock approach flow system <strong>of</strong> <strong>the</strong> PM, only forward<br />
cleaners are used, <strong>of</strong> which up to seven stages are commonly connected in cascade fashion.<br />
The overall debris removal efficiency is best in <strong>the</strong> cascade system. The higher <strong>the</strong> number <strong>of</strong><br />
stages, <strong>the</strong> higher is <strong>the</strong> debris concentration in <strong>the</strong> reject <strong>and</strong> <strong>the</strong> smaller <strong>the</strong> reject stream.<br />
Mounting <strong>of</strong> PM cleaners is ei<strong>the</strong>r linearly in banks or racks, or radially, e.g., in canisters, <strong>of</strong>ten<br />
with <strong>the</strong> reject pipe at <strong>the</strong> center. Cleaners can operate in a horizontal as well as in a vertical<br />
position. The advantage <strong>of</strong> a linear arrangement is good accessibility <strong>of</strong> individual cleaners for<br />
service, while canisters usually require less floor space. Accept <strong>and</strong> reject lines are equipped with<br />
pressure transmitters for monitoring <strong>the</strong> cleaner operation. The filler <strong>and</strong> abrasive pigment<br />
content <strong>of</strong> <strong>the</strong> stock increases toward <strong>the</strong> higher stages <strong>of</strong> <strong>the</strong> cascade, which can cause<br />
excessive wear <strong>and</strong> reduce <strong>the</strong> cleaner efficiency <strong>and</strong> ultimately cause <strong>the</strong> cleaner to burst. The<br />
highest wear is at <strong>the</strong> narrow side <strong>of</strong> <strong>the</strong> cone, where flow velocity as well as consistency are<br />
highest. Filtrate is <strong>of</strong>ten used instead <strong>of</strong> white <strong>water</strong> for dilution at later cleaner cascade stages to<br />
improve cleaning efficiency <strong>and</strong> to reduce fiber loss.<br />
End-stage cleaner rejects are usually rich in pigments or filler. Due to <strong>the</strong> separation principle<br />
<strong>of</strong> cleaners, especially coarse filler particles <strong>and</strong> agglomerated pigments from coated broke are<br />
rejected. It may prove to be feasible to recover <strong>the</strong>se minerals by dispersion <strong>and</strong> to feed <strong>the</strong>m<br />
back to <strong>the</strong> PM. Thereby, <strong>the</strong> disposed amount <strong>of</strong> reject is reduced. In such a filler recovery<br />
process, fractionation <strong>of</strong> <strong>the</strong> filler containing rejects takes place at first, possibly including coating<br />
kitchen wash <strong>water</strong>. The fine fraction contains most <strong>of</strong> <strong>the</strong> filler, which is again classified<br />
according to particle size. The coarse mineral particles are concentrated to 30%−50% solids<br />
content, treated by a disperser <strong>and</strong> looped back to previous classification. Undispersable, coarse<br />
particles are classified along with <strong>the</strong> coarse fraction from fractionation <strong>and</strong> <strong>the</strong> reject from initial<br />
classification. The accepts <strong>of</strong> both <strong>of</strong> <strong>the</strong>se classification stages are recovered20.<br />
5.3.4.2 Screening<br />
In nearly all <strong>paper</strong>making operations, <strong>the</strong> installation <strong>of</strong> at least one pressure screen is m<strong>and</strong>atory<br />
right before <strong>the</strong> headbox. Exceptions could be, for example, for <strong>the</strong> lowest quality level <strong>of</strong> bogus<br />
board, or if using extremely long specialty fibers, which would become wrapped up in a screen.<br />
The main functions particular to <strong>the</strong> PM screen are:<br />
- Protecting <strong>the</strong> wet end from occasional coarse foreign material which could damage forming<br />
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fabrics, i.e., <strong>the</strong> function <strong>of</strong> a police filter<br />
- Removing debris <strong>and</strong> dirt<br />
- Deflocculation <strong>of</strong> <strong>the</strong> stock <strong>and</strong> improvement <strong>of</strong> formation.<br />
Even if all stock components including broke were previously screened with a finer screening<br />
medium, <strong>the</strong> flakes, bundles, <strong>and</strong> lumps can be created, e.g., by deposit on <strong>the</strong> walls <strong>of</strong> chests<br />
<strong>and</strong> tanks. Secondary stickies <strong>and</strong> pitch particles can also form in <strong>the</strong> PM system.<br />
The PM screen is located directly before <strong>the</strong> headbox without any control valves or o<strong>the</strong>r<br />
installations on <strong>the</strong> accept side o<strong>the</strong>r than retention aid dosage nozzles. Due to its particular<br />
position in <strong>the</strong> process, this screen has to fulfill <strong>the</strong> following special characteristics:<br />
- Very low pulsation generating operation<br />
- Polished surfaces<br />
- Metal-to-metal flanged connections, especially on <strong>the</strong> accept side21<br />
- Highest possible availability, i.e., virtually trouble-free operation<br />
- Dimensioning according to simplicity in layout, <strong>of</strong>ten use <strong>of</strong> a single screen is preferred<br />
- Optimized design to prevent air pockets.<br />
Because <strong>of</strong> <strong>the</strong>se special features, <strong>the</strong> PM screen typically has a lower screening efficiency in<br />
comparison to pressure screens in stock preparation22. On <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>, <strong>the</strong> higher <strong>the</strong><br />
screening efficiency is, <strong>the</strong> better <strong>the</strong> runnability <strong>of</strong> <strong>the</strong> PM usually is. In order to improve<br />
screening efficiency, <strong>the</strong> use <strong>of</strong> slotted screen plates has become common. The slot size <strong>of</strong> <strong>the</strong><br />
<strong>machine</strong> screen is a compromise between high screening efficiency <strong>and</strong> a large open area. The<br />
finer <strong>the</strong> slotting is, <strong>the</strong> higher is <strong>the</strong> flow velocity at <strong>the</strong> slot or <strong>the</strong> larger is <strong>the</strong> screen or <strong>the</strong><br />
number <strong>of</strong> screens. High slot velocity increases <strong>the</strong> risk <strong>of</strong> plugging <strong>and</strong> pressure pulses, which<br />
are in contrary to <strong>the</strong> above-mentioned requirements. Generally speaking, <strong>the</strong>re is also a higher<br />
risk <strong>of</strong> stringing or fiber spinning for low-consistency screening with slots in comparison to holes.<br />
An alternative system for high screening efficiency is a combination <strong>of</strong> thick stock screening<br />
with narrow slot width <strong>and</strong> <strong>machine</strong> screening with wider slots or holes. In thick stock screening<br />
outside <strong>the</strong> stock approach system, no special requirements exist concerning <strong>the</strong> screen<br />
surfaces, <strong>the</strong> generation <strong>of</strong> pulsation, or o<strong>the</strong>r system design features compared to a location<br />
right before <strong>the</strong> headbox. In this respect, it is possible to install a thick stock screening system<br />
between blend chest <strong>and</strong> <strong>machine</strong> chest23,24. If several stock components are not well screened,<br />
a decrease in PM breaks <strong>and</strong> a product quality increase by fewer spots <strong>and</strong> holes in <strong>the</strong> product<br />
can be achieved with a smaller number <strong>of</strong> screens, if installed at this position. This alternative is<br />
particularly to be considered for rebuilds, where available floorspace is restricted. However, <strong>the</strong><br />
load <strong>of</strong> debris reaching <strong>the</strong> short circulation can <strong>of</strong>ten be reduced in existing mills by installation<br />
<strong>of</strong> a new broke cleaning <strong>and</strong> screening system or by improving <strong>the</strong> existing one.<br />
The end-stage <strong>machine</strong> screen reject flow is <strong>of</strong>ten discontinuous with a timer-controlled<br />
flushing system. Thereby, <strong>the</strong> retention time within <strong>the</strong> screen is increased, <strong>and</strong> more time is<br />
given to separate valuable fiber from discharged debris.<br />
5.3.5 Deaeration<br />
5.3.5.1 Air in stock <strong>and</strong> <strong>water</strong><br />
Gases in stock, in <strong>the</strong> following called "air," refer to dispersed air, i.e., air bubbles, <strong>and</strong> dissolved<br />
gas in <strong>water</strong>. Additionally, gases can be created by chemical or biological reactions within <strong>the</strong><br />
<strong>water</strong> system. Deaeration, or more precisely degasification, refers usually to mechanical vacuum<br />
treatment <strong>of</strong> <strong>the</strong> stock suspension in order to reduce its air content significantly. Free air consists<br />
<strong>of</strong> air bubbles, which rise to <strong>the</strong> surface if not interfered by flow. The remaining dispersed air is<br />
<strong>of</strong>ten called "bound" or "residual" air.<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
Water temperature, surface tension, <strong>and</strong> pressure affect <strong>the</strong> adsorption <strong>and</strong> desorption <strong>of</strong> air,<br />
<strong>the</strong> bubble stability, <strong>the</strong> ability <strong>of</strong> bubbles to coalesce, <strong>water</strong> viscosity, <strong>and</strong> thus <strong>the</strong> bubble<br />
retention in <strong>the</strong> fluid phase25. Average bubble diameters usually range from 50 to 120 μm. With<br />
increased temperature, <strong>the</strong> amount <strong>of</strong> dispersed air <strong>and</strong> <strong>the</strong> air solubility in <strong>water</strong> decreases <strong>and</strong><br />
<strong>the</strong> weighted average <strong>of</strong> <strong>the</strong> bubble size decreases. On <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>, air solubility in <strong>water</strong><br />
increases linearly with pressure according to Henry's law. With a pressure increase <strong>of</strong> about,<br />
e.g., 400−500 kPa after a fan pump or even higher after <strong>the</strong> headbox dilution <strong>water</strong> pump, most<br />
<strong>of</strong> <strong>the</strong> dispersed air is dissolved. With <strong>the</strong> pressure decrease to ambient pressure at <strong>the</strong> headbox<br />
lip, many small-sized bubbles are created by desorption. This redispersed air remains partly with<br />
<strong>the</strong> drained <strong>water</strong> because small bubbles do not coalesce <strong>and</strong>, hence, are able to accumulate in<br />
<strong>the</strong> short circulation25. Thus, <strong>the</strong> mean size <strong>of</strong> <strong>the</strong> bubbles is in <strong>the</strong> circulation <strong>water</strong> smaller than<br />
at <strong>the</strong> point where air is mechanically entrained by splashing.<br />
5.3.5.2 Effect <strong>of</strong> air on <strong>paper</strong>making<br />
Air in <strong>paper</strong>making stock has adverse implications on <strong>the</strong> production process as well as on<br />
product quality. The critical amount <strong>of</strong> air leading to noticeable problems depends on <strong>the</strong> type <strong>of</strong><br />
PM, <strong>the</strong> <strong>paper</strong> grade, <strong>and</strong> <strong>the</strong> kind <strong>of</strong> furnish being used26. Consider as an illustrative example a<br />
headbox lip flow concentration <strong>of</strong> 1% by volume dispersed air for a PM without mechanical<br />
deaeration <strong>and</strong> a headbox solids concentration <strong>of</strong> 0.7%27. In this case, <strong>the</strong> volume <strong>of</strong> dispersed<br />
air is about as large as <strong>the</strong> volume <strong>of</strong> <strong>the</strong> solids. It is <strong>the</strong>refore well underst<strong>and</strong>able that air in<br />
stock reduces <strong>the</strong> production performance, e.g., by:<br />
- Pumping efficiency decrease<br />
- Screening efficiency decrease<br />
- Reduction <strong>of</strong> drainage<br />
- Foam problems at open surfaces, which can also cause accumulation <strong>of</strong> hydrophobic <strong>and</strong><br />
sticky material similar to froth flotation<br />
- Increase in microbiological activity leading to slime problems<br />
- Pressure <strong>and</strong> flow velocity variations due to air pockets within <strong>the</strong> system<br />
- Instability <strong>and</strong> noise by cavitation.<br />
Air in stock <strong>and</strong> <strong>water</strong> adversely affects <strong>the</strong> product quality28−30 in <strong>the</strong> form <strong>of</strong>:<br />
- Pinholes <strong>and</strong> holes<br />
- Spots, specks, <strong>and</strong> lumps in <strong>the</strong> <strong>paper</strong>, due to foam or accumulation at <strong>the</strong> surface <strong>of</strong> larger<br />
air bubbles<br />
- Decreased wet-web strength<br />
- Decrease in smoothness <strong>and</strong> tensile strength due to deteriorated formation.<br />
On <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>, finely dispersed air bubbles can also improve formation in some cases31.<br />
A higher level <strong>of</strong> bulk due to <strong>the</strong> presence <strong>of</strong> small air bubbles can be a beneficial side effect.<br />
In addition to <strong>paper</strong> quality improvement, <strong>the</strong> investment into a mechanical deaeration system<br />
is <strong>of</strong>ten justified due to increases in:<br />
- PM runnability <strong>and</strong> more stable production<br />
- Possible speed increase <strong>of</strong> <strong>the</strong> PM<br />
- Savings on defoaming agents<br />
- Savings due to decreased energy losses in pumps.<br />
5.3.5.3 Sources <strong>of</strong> air<br />
From <strong>the</strong> hydrodynamics st<strong>and</strong>point, dispersions <strong>of</strong> gas bubbles in liquids are basically unstable.<br />
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Forming bubbles means an expansion <strong>of</strong> <strong>the</strong> normal surface area <strong>of</strong> <strong>the</strong> liquid <strong>and</strong>, thus, requires<br />
a work input. Dissolved substances can stabilize bubbles <strong>and</strong> retard <strong>the</strong>ir collapse. Bubbles can<br />
also attach to hydrophobic sites in <strong>the</strong> stock or get trapped between fibers. Air is ei<strong>the</strong>r brought<br />
with <strong>the</strong> feed flows into <strong>the</strong> system, or it is brought in, ei<strong>the</strong>r due to inherent process operation or<br />
by action, which could be avoidable. For example, <strong>the</strong> splashing occurring in PM wire de<strong>water</strong>ing<br />
is unavoidable. Possibly avoidable sources <strong>of</strong> air are:<br />
- Air via incoming stock <strong>and</strong> <strong>water</strong> flows<br />
- Splashing in open cleaner reject discharge<br />
- Faulty operation <strong>of</strong> <strong>the</strong> deaeration unit, e.g., due to insufficient vacuum generation<br />
- Liberation <strong>of</strong> gas by chemical reactions; e.g., acidic process <strong>water</strong> or acidic additives in<br />
combination with calcium carbonate pigments cause a release <strong>of</strong> CO 2<br />
- Leakage<br />
- Biological activity.<br />
As already mentioned above, pressure, temperature, <strong>and</strong> flow speed differences can desorb<br />
dissolved air into dispersed air. The release <strong>of</strong> carbon dioxide from carbonate pigments in acidic<br />
conditions causes distinct problems, due to <strong>the</strong> higher solubility <strong>of</strong> CO 2 compared to air, <strong>and</strong> due<br />
to resistant scaling by calcium oxalate32.<br />
5.3.5.4 Chemical deaeration<br />
The principle <strong>of</strong> defoaming chemicals is to increase <strong>the</strong> rate <strong>of</strong> bubble coalescence. This means<br />
that two colliding bubbles have a higher probability <strong>of</strong> joining <strong>and</strong> forming a bigger bubble in <strong>the</strong><br />
presence <strong>of</strong> a surface-active defoaming agent, commonly a product on mineral oil basis.<br />
Defoaming agents usually affect to little extent, or not at all, <strong>the</strong> amount <strong>of</strong> dissolved gases in <strong>the</strong><br />
<strong>paper</strong> stock, which can be up to 2%−3% by volume, even if <strong>the</strong> amount <strong>of</strong> dissolved air is low.<br />
Hence, <strong>the</strong> ratio <strong>of</strong> dissolved gas to total gas is increased with <strong>the</strong> addition <strong>of</strong> a defoaming agent.<br />
If a low level <strong>of</strong> dispersed air in stock is required, <strong>the</strong> exclusive use <strong>of</strong> defoaming chemicals<br />
becomes expensive <strong>and</strong> thus <strong>the</strong> installation <strong>of</strong> a mecha-nical deaeration unit is feasible. The<br />
addition <strong>of</strong> defoaming agents in large amounts <strong>of</strong>ten causes a decrease in system cleanliness,<br />
deposition <strong>of</strong> dirt on fabrics <strong>and</strong> felts, a decrease in retention, or an increased need <strong>of</strong> retention<br />
aids respectively. Over-dosage <strong>of</strong> some defoaming agents can even cause <strong>the</strong> reverse effect <strong>and</strong><br />
stabilize bubbles.<br />
Defoaming chemicals are also used in combination with mechanical deaeration units for a<br />
maximum in air removal. The mechanically deaerated stock is "hungry" <strong>and</strong> has a high capacity<br />
to dissolve more gas into <strong>water</strong>27. Some types <strong>of</strong> defoaming agents can prevent <strong>the</strong> pulp<br />
suspension from reabsorbing significant amounts <strong>of</strong> air when exposed to it after deaeration<br />
treatment29.<br />
5.3.5.5 Deaeration tanks<br />
Deaeration tanks remove dissolved <strong>and</strong> dispersed air very efficiently. Air is desorbed above <strong>the</strong><br />
boiling point <strong>and</strong> effectively driven out, when creating a large fluid surface. The typical deaeration<br />
tank is designed to treat <strong>the</strong> headbox stock or headbox dilution <strong>water</strong> by:<br />
- Spraying<br />
- Impingement<br />
- Boiling.<br />
With spraying <strong>and</strong> by impingement against <strong>the</strong> interior surface <strong>of</strong> <strong>the</strong> tank, a large fluid<br />
surface is created <strong>and</strong> trapped or bound bubbles are released <strong>and</strong> removed. The air-containing<br />
suspension enters upward into <strong>the</strong> deaeration tank from coaxially connected centrifugal cleaners<br />
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or feed nozzles through pipes in helical flow pattern. The helical flow causes <strong>the</strong> desired<br />
spraying. The headbox circulation flow is usually fed back to <strong>the</strong> deaeration tank, but without<br />
spraying. Boiling takes place at a vacuum, which is high enough so that no additional heating <strong>of</strong><br />
<strong>the</strong> suspension is required. The minimum required vacuum depends <strong>the</strong>refore on <strong>the</strong> feed<br />
temperature. The system temperature is controlled by heat or steam injection into <strong>the</strong> wire pit. In<br />
practice, <strong>the</strong> absolute pressure should be about 1 kPa lower than <strong>the</strong> boiling point at <strong>the</strong> given<br />
temperature. To support <strong>the</strong> required amount <strong>of</strong> vacuum, <strong>the</strong> deaeration tank is placed in an<br />
elevated location, <strong>and</strong> all flows from <strong>the</strong> unit are barometric drop-leg lines; this includes also <strong>the</strong><br />
reject lines, in case cleaners are directly connected to <strong>the</strong> deaeration. Likewise, <strong>the</strong> primary<br />
cleaner feeding fan pump is provided by sufficient back-pressure. Besides deaeration, positive<br />
side effects by tank deaeration are as follows:<br />
- Constant weir overflow within <strong>the</strong> tank is maintained by controlling <strong>the</strong> fan pump speed.<br />
- Excellent ability to level out short-term stock consistency variations, especially if <strong>the</strong> flows<br />
are connected obeying <strong>the</strong> multi-lag principle as shown in Fig. 13.<br />
- Good mixing is obtained by <strong>the</strong> stock spraying.<br />
- Upstream pressure pulses are well dampened.<br />
The vacuum is generated by a one- or two-stage <strong>water</strong> ring pump system. Figure 3 shows a<br />
two-stage system. The removed gases are cooled by cooling <strong>water</strong> in a pre-condenser, ei<strong>the</strong>r by<br />
direct contact or in a noncontact condenser. The more expensive <strong>and</strong> more cooling<br />
<strong>water</strong>-consuming indirect condenser has <strong>the</strong> advantage that <strong>the</strong> cooling liquor does not get in<br />
contact with <strong>the</strong> exhaust; thus, it cannot become contaminated. The use <strong>of</strong> a steam ejector to<br />
discharge <strong>the</strong> deaeration tank receiver33 causes heat loss <strong>and</strong> loads <strong>the</strong> condenser <strong>the</strong>rmally.<br />
The use <strong>of</strong> a sufficiently large vacuum pump or two connected in series usually proves to be<br />
more economical. Each vacuum pump is followed by a <strong>water</strong> separator. Condensed <strong>and</strong><br />
separated <strong>water</strong> is collected via drop-legs at <strong>the</strong> deaeration seal tank in <strong>the</strong> cellar <strong>and</strong> typically<br />
added to <strong>the</strong> PM vacuum pump system. The removed air is exhausted via a silencer.<br />
Two different deaeration <strong>and</strong> process design alternatives are commonly used:<br />
- All cleaner stages are on <strong>the</strong> PM floor level <strong>and</strong>, thus, separated from <strong>the</strong> deaeration tank.<br />
- One to three cleaner stages are connected directly to <strong>the</strong> deaeration tank. For mid-sized or<br />
large PMs, <strong>the</strong> number <strong>of</strong> required cleaners is large; thus, <strong>the</strong> deaeration tank is extended by<br />
wings to collect <strong>the</strong> cleaner accepts. This arrangement is called a "flying wing" system.<br />
Table 3 shows <strong>the</strong> advantages <strong>of</strong> both alternatives. In several cases, <strong>the</strong> deaeration tank is<br />
supplied with only <strong>the</strong> first stage cleaner accept. If also <strong>the</strong> second- <strong>and</strong> third-stage accepts are<br />
fed into <strong>the</strong> deaeration receiver, <strong>the</strong> discharge is into <strong>the</strong> weir overflow, which means that <strong>the</strong><br />
third-stage accept is fed forward. The last stages are located on <strong>the</strong> PM floor level <strong>and</strong> connected<br />
in cascade fashion.<br />
High requirements for air-free headbox feed flow <strong>and</strong> operational constraints <strong>of</strong>ten make<br />
deaeration necessary for <strong>the</strong> headbox dilution <strong>water</strong> also, if such a PM cross-direction control<br />
system is applied. The deaeration <strong>of</strong> this dilution <strong>water</strong> can be ei<strong>the</strong>r integrated into <strong>the</strong> stock<br />
deaeration tank, or it can occur in a separate vessel. The combined stock <strong>and</strong> headbox dilution<br />
<strong>water</strong> deaeration tank consists <strong>of</strong> two compartments located opposite to each o<strong>the</strong>r with facing<br />
weirs discharging ei<strong>the</strong>r into <strong>the</strong> same or into different overflow discharging pipes. The separate<br />
headbox dilution <strong>water</strong> deaeration tank is connected to <strong>the</strong> vacuum pump system <strong>of</strong> <strong>the</strong> stock<br />
deaeration. Principle <strong>and</strong> operation are similar to <strong>the</strong> stock deaeration system. The headbox<br />
dilution header circulation can also be fed back to <strong>the</strong> dilution deaerator similarly to <strong>the</strong> headbox<br />
stock circulation.<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
Table 3. Advantages <strong>of</strong> <strong>the</strong> two most common deaeration tank design solutions.<br />
Deaeration tank with cleaners connected<br />
Deaeration tank <strong>and</strong> cleaners separately<br />
High cleaner efficiency<br />
St<strong>and</strong>ard cleaners can be used<br />
Low pressure drop cleaners with low reject<br />
Lower investment costs for <strong>the</strong> deaeration<br />
thickening<br />
receiver tank<br />
Larger reject orifice diameter due to air core in<br />
Smaller volume to exhaust, which may save a<br />
cleaner causing less plugging <strong>and</strong> less wear<br />
second stage vacuum pump<br />
More space on <strong>the</strong> <strong>machine</strong> floor level<br />
All cleaner stages operators on PM level,<br />
regular inspection by operators is easier<br />
Lower energy consumption<br />
More suitable to add in rebuild design<br />
5.3.5.6 Deaeration by o<strong>the</strong>r equipment<br />
Dispersed air can leave <strong>the</strong> system by bubbling out at any open surface, i.e., <strong>the</strong> wire pit in <strong>the</strong><br />
short circulation. To allow bubbles to rise, <strong>the</strong> downward flow velocity in <strong>the</strong> wire pit must not<br />
exceed 0.15 m/s. In order to lower this velocity, <strong>the</strong> diameter <strong>of</strong> <strong>the</strong> wire pit <strong>and</strong>, thus, <strong>the</strong> amount<br />
<strong>of</strong> <strong>water</strong> in <strong>the</strong> short circulation has to be increased. However, only <strong>the</strong> larger bubbles <strong>of</strong> about 1<br />
mm diameter are removed from <strong>the</strong> wire pit27, <strong>and</strong> its deaerating capacity remains poor, even<br />
with <strong>the</strong> addition <strong>of</strong> defoaming agents.<br />
Air is removed from stock suspension to some extent when exposing to gravitational field like,<br />
e.g., in centrifugal cleaners. Cleaners with light reject removal <strong>the</strong>refore have a slight deaerating<br />
effect. The principle <strong>of</strong> deaeration by centrifugation is applied in a special type <strong>of</strong> pump, which is<br />
connected to a vortex chamber (see Fig. 17). By means <strong>of</strong> this deaeration pump, it is possible to<br />
redesign <strong>the</strong> conventional PM short circulation, as mentioned below in <strong>the</strong> section about novel<br />
approaches. Finally, deaeration outlets from pressure screens or o<strong>the</strong>r equipment keep <strong>the</strong><br />
device free from air pockets formed by accumulation <strong>of</strong> single air bubbles. Instead <strong>of</strong> active<br />
deaeration, <strong>the</strong> redispersion <strong>of</strong> air is avoided <strong>and</strong> <strong>the</strong> buildup <strong>of</strong> dirt <strong>and</strong> slime is prevented.<br />
Figure 17. Stock pump with deaeration by centrifugation, courtesy <strong>of</strong> POM Technology.<br />
5.3.6 Chemical conditioning<br />
5.3.6.1 Wet end chemistry<br />
The wet end chemistry <strong>of</strong> each PM is unique. A plethora <strong>of</strong> different <strong>and</strong> partly unknown<br />
substances governs <strong>the</strong> chemistry <strong>of</strong> all <strong>paper</strong>making process <strong>water</strong>s. These substances are in<br />
interaction, which is influenced by a large number <strong>of</strong> parameters. In practice, process behavior<br />
depends on a complex balance, <strong>and</strong> predicting a reaction upon process changes or upon<br />
addition <strong>of</strong> a chemical can be difficult, if not sometimes impossible. While principles <strong>of</strong> wet end<br />
chemistry are explained in Volume 3: Forest Products Chemistry <strong>and</strong> Volume 4: Papermaking<br />
Chemistry <strong>of</strong> this book series, some implications by <strong>the</strong> process design are stated in <strong>the</strong><br />
following.<br />
Chemicals are added ei<strong>the</strong>r to <strong>the</strong> stock or to <strong>the</strong> process <strong>water</strong> for <strong>the</strong> following reasons:<br />
- To improve <strong>the</strong> product properties, e.g., fillers, strength enforcing additives, sizing agents,<br />
dyes, etc.<br />
- To support <strong>and</strong> to maintain efficiency <strong>of</strong> <strong>the</strong> product properties improving additives, e.g.,<br />
pH-control, retention aids, fixatives, etc.<br />
- To maintain system stability <strong>and</strong> cleanliness, e.g., defoaming agents, biocides, fixatives,<br />
pitch dispersants, washing detergents, etc.<br />
Ano<strong>the</strong>r way <strong>of</strong> classification is according to functional <strong>and</strong> process additives34. However,<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
some <strong>of</strong> <strong>the</strong> process additives are added in order to support <strong>the</strong> functional <strong>paper</strong> chemicals<br />
because <strong>the</strong> performance <strong>of</strong> <strong>the</strong> functional aid is measured only from that part, which remains<br />
with <strong>the</strong> <strong>paper</strong> product35. The dosed chemical reacts with <strong>the</strong> dispersed fibers <strong>and</strong> pigments as<br />
well as with <strong>the</strong> dissolved material <strong>and</strong> potentially with previously added chemicals circulating<br />
with <strong>the</strong> process <strong>water</strong>. The less process <strong>water</strong> is discharged <strong>and</strong>, <strong>the</strong>refore, <strong>the</strong> higher <strong>the</strong> rate<br />
<strong>of</strong> recirculation, <strong>the</strong> higher is <strong>the</strong> threat <strong>of</strong> unfavorable effects in <strong>the</strong> process caused by<br />
accumulating substances. Hence, process chemicals should only be used when absolutely<br />
necessary. Overdosing is to be avoided for cost reasons <strong>and</strong> because <strong>of</strong> unnecessary loading <strong>of</strong><br />
<strong>the</strong> process <strong>water</strong>. In mill practice, <strong>the</strong> addition <strong>of</strong> each wet end chemical should be checked on a<br />
regular basis. Practice has shown that changes in <strong>the</strong> process or in its chemistry can make <strong>the</strong><br />
use <strong>of</strong> some chemicals obsolete, which should be consequently turned <strong>of</strong>f. This tends to be<br />
"forgotten" because <strong>the</strong>ir exact effect on <strong>the</strong> system is <strong>of</strong>ten unknown.<br />
5.3.6.2 Dosage points<br />
The right dosage point for a particular chemical can differ from system to system. At <strong>the</strong> dosage<br />
point, <strong>the</strong>re is <strong>of</strong>ten a trade-<strong>of</strong>f between <strong>the</strong> best possible mixing with <strong>the</strong> targeted reaction<br />
components <strong>and</strong> undesired reaction with o<strong>the</strong>r components or process disturbance. The flow<br />
conditions <strong>and</strong> <strong>the</strong> flow speed difference at <strong>the</strong> dosage point have to be known to ensure proper<br />
mixing with <strong>the</strong> stock or <strong>water</strong> stream. The order <strong>of</strong> addition <strong>of</strong> some additives matters, for<br />
example, in a two-component retention system. Possible dosage sites are:<br />
- In stock preparation, e.g., before or after refining<br />
- To single stock components before blending<br />
- At <strong>the</strong> blend chest, <strong>the</strong> <strong>machine</strong> chest, or in between<br />
- Before <strong>the</strong> thick stock feed pump<br />
- Before thick stock dilution<br />
- At stock dilution, before <strong>the</strong> primary fan pump<br />
- Before <strong>the</strong> headbox fan pump<br />
- Before <strong>the</strong> <strong>machine</strong> screen<br />
- Before <strong>the</strong> headbox<br />
- To <strong>the</strong> wire pit.<br />
5.3.6.3 Dosing<br />
In <strong>the</strong> <strong>paper</strong> mill, dosage <strong>of</strong> chemicals means blending a stream <strong>of</strong> usually small volumetric flow<br />
but high concentration with a large flow <strong>of</strong> stock <strong>water</strong>. In order to ensure good mixing with <strong>the</strong><br />
main stream, <strong>the</strong> additive is <strong>of</strong>ten diluted with <strong>water</strong>, which has to be in most cases <strong>of</strong> purified<br />
fresh <strong>water</strong> quality. The chemical is dosed directly into <strong>the</strong> mixing zone or into <strong>the</strong> middle <strong>of</strong> <strong>the</strong><br />
stream. A multiple radial injector feed system is used typically for high-molecular retention<br />
polymer or bentonite proportioning before <strong>the</strong> headbox (see Fig. 18). In this particular case,<br />
mixing is supported by <strong>the</strong> turbulence created at <strong>the</strong> joint <strong>of</strong> <strong>the</strong> accepts <strong>of</strong> <strong>the</strong> two parallel<br />
<strong>machine</strong> screens. The location between <strong>the</strong> PM screens <strong>and</strong> headbox can also be seen in Fig. 3.<br />
The multiple injection ensures better distribution <strong>of</strong> <strong>the</strong> chemical component compared to a single<br />
nozzle; this is needed, if consecutive mixing with high shear forces is to be avoided.<br />
Figure 18. Bentonite dosage at <strong>the</strong> headbox feed pipe after pressure screens.<br />
5.3.6.4 Measurements<br />
Various sensors <strong>and</strong> measuring devices are installed for control <strong>and</strong> to supply <strong>the</strong> operators with<br />
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necessary information. Besides <strong>the</strong> basic process parameters like flow, consistency,<br />
temperature, <strong>and</strong> pressure, o<strong>the</strong>r data more specific to <strong>the</strong> <strong>paper</strong>making process can be<br />
obtained, as shown in Table 4. Reliable metering <strong>and</strong> measuring is most important for good<br />
control <strong>of</strong> stock operations <strong>and</strong> high process performance.<br />
Table 4. Parameters for wet end chemistry monitoring <strong>and</strong> control36.<br />
Important parameters:<br />
Often useful parameters:<br />
Temperature<br />
Dissolved organic compounds, COD<br />
pH<br />
Cationic dem<strong>and</strong><br />
Conductivity<br />
Inorganics (Ca+, Ca2+, Al3+, SiO 2 )<br />
Consistency<br />
Charge<br />
Freeness, fines, <strong>and</strong> filler content<br />
Turbidity<br />
Flow<br />
Hole <strong>and</strong> specks count<br />
Air content<br />
According to <strong>the</strong> type <strong>of</strong> measuring device, <strong>the</strong> installation is ei<strong>the</strong>r at <strong>the</strong> main pipe or at a<br />
measurement line or at a bypass pipe. Advanced, timer-controlled operating stock or wet end<br />
chemistry measuring devices can also be grouped toge<strong>the</strong>r in an analysis center. Sampling pipes<br />
<strong>and</strong> bypass lines are needed if <strong>the</strong> sensor has to be disconnected occasionally or regularly for<br />
cleaning or calibration. Sampling <strong>and</strong> bypass pipes have to be designed <strong>and</strong> placed at <strong>the</strong> main<br />
pipe so that no process disturbances like dirt accumulation occur, which might adversely affect<br />
<strong>the</strong> process or distort <strong>the</strong> measured values or even harm <strong>the</strong> device. Flow conditions in particular<br />
in bypass lines must be steady, <strong>and</strong> <strong>the</strong> buildup <strong>of</strong> air pockets must be avoided.<br />
All sensors <strong>and</strong> advanced measurement devices have to be installed in such a way that <strong>the</strong><br />
measured values are reliable <strong>and</strong> consistent. Therefore <strong>the</strong> following points have to be<br />
considered:<br />
- The sensor is processwise positioned at a suitable location. The sensor is usually installed<br />
prior to <strong>the</strong> control valve, if connected in series. In some instances in chemical conditioning, <strong>the</strong><br />
sensor is situated downstream, for example, in retention control.<br />
- The sensor has to be installed at a suitable location in order to gain reliable measurement<br />
results <strong>and</strong> according to <strong>the</strong> requirements <strong>of</strong> connected controls, e.g., to avoid unnecessary dead<br />
time.<br />
- At all times, sensors have to be in full contact with <strong>the</strong> medium to be measured.<br />
- Good accessibility to operators <strong>and</strong> maintenance people, especially if tuning, regular<br />
calibration, or o<strong>the</strong>r maintenance is required.<br />
- Maintenance <strong>and</strong> cleaning is carried out regularly according to a schedule.<br />
- Flushing or scavenging media have to be suitable <strong>and</strong> sufficient.<br />
- External sources <strong>of</strong> error <strong>and</strong> disturbance should be avoided, e.g., magnetic field or<br />
vibration.<br />
5.3.7 Stock transport<br />
5.3.7.1 Piping<br />
According to <strong>the</strong> flow rate calculated during <strong>the</strong> process design phase, a stream velocity is<br />
chosen, which determines <strong>the</strong> diameter <strong>of</strong> <strong>the</strong> pipe. In practice, <strong>the</strong> stream velocity is determined<br />
among o<strong>the</strong>r things by <strong>the</strong> following factors:<br />
- Investment cost<br />
- Operating costs<br />
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- Properties <strong>of</strong> <strong>the</strong> fluid<br />
- Pipe erosion<br />
- Vibrations<br />
- Pressure shocks<br />
- Noise (at valves)<br />
- Cleanliness<br />
- Potential for production increase<br />
- Functional location <strong>of</strong> <strong>the</strong> pipe<br />
- Pressure or position on a suction side<br />
- Function as main- or sideline.<br />
The price <strong>of</strong> <strong>the</strong> pipe <strong>and</strong> costs resulting from <strong>the</strong> needed parts, valves, insulation, <strong>and</strong><br />
support, <strong>the</strong> capital cost, <strong>and</strong> <strong>the</strong> pay-back period affect <strong>the</strong> total investment cost. The operating<br />
costs arise from <strong>the</strong> energy consumed for fluid transport <strong>and</strong> from maintenance costs. The piping<br />
head loss is proportional to <strong>the</strong> square <strong>of</strong> <strong>the</strong> flow velocity <strong>of</strong> <strong>water</strong> <strong>and</strong> <strong>of</strong> pulp slurry within<br />
certain limits37. The total pipe friction loss curve is non-linear <strong>and</strong> consists <strong>of</strong> several areas <strong>of</strong><br />
different flow regimes. These can be simplified into a plug flow, a transition flow, <strong>and</strong> a turbulent<br />
flow region38. The actual stock flow conditions depend mainly on:<br />
- Velocity<br />
- Consistency<br />
- Pipe diameter<br />
- Temperature<br />
- Pipe roughness<br />
- Type <strong>of</strong> pulp<br />
- Pigment or filler content<br />
- Pretreatment <strong>of</strong> <strong>the</strong> pulp, like drying <strong>and</strong> beating.<br />
In practice, <strong>the</strong> stream velocity is chosen according to generally agreed upon empirical<br />
values that have been proven sound in practice. When pumping stock over long distances or<br />
wherever else possible, it might be useful to calculate <strong>the</strong> optimum consistency for pumping pulp<br />
<strong>and</strong> adjusting accordingly39.<br />
5.3.7.2 Pumping<br />
The following types <strong>of</strong> pumps are usually used in <strong>paper</strong> mills:<br />
- Centrifugal pumps<br />
- Process pumps for <strong>water</strong> <strong>and</strong> stock up to 5% consistency- Fan pumps: cleaner feed pump <strong>and</strong><br />
headbox pump- Medium-consistency (MC) pumps<br />
- Displacement pumps<br />
- Screw pumps for sludge, pigment slurry, coating color, etc.- Plunger pumps <strong>and</strong> membrane<br />
pumps for chemicals <strong>and</strong> additive dosage- Water ring pumps for vacuum generation.<br />
The main parameters for selecting a centrifugal pump are:<br />
- Capacity, Q<br />
- Head, H<br />
- Solids concentration<br />
- Temperature.<br />
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At a given speed, each pump has a characteristic curve H P (Q). Also <strong>the</strong> system response is<br />
described by a characteristic curve H S (Q), which consists <strong>of</strong> a static part <strong>and</strong> an operational part<br />
according to factors such as <strong>the</strong> downstream load characteristics, pipe friction, <strong>and</strong> valve<br />
performance. Typically, <strong>the</strong> head changes as a square <strong>of</strong> <strong>the</strong> flow rate. The operating point <strong>of</strong> <strong>the</strong><br />
pump is determined by <strong>the</strong> point, where <strong>the</strong> characteristic curve <strong>of</strong> <strong>the</strong> pump H P (Q) meets <strong>the</strong><br />
characteristic curve <strong>of</strong> <strong>the</strong> system H S (Q). In process design, <strong>the</strong> desired pump capacity is usually<br />
given <strong>and</strong> <strong>the</strong> required pumping head is calculated. The flow through <strong>the</strong> system can be altered,<br />
by <strong>the</strong> following means:<br />
- Speed control <strong>of</strong> <strong>the</strong> pump, which changes <strong>the</strong> characteristic curve H P (Q)<br />
- Control by a throttling valve, which creates additional system friction <strong>and</strong> thus changes <strong>the</strong><br />
characteristic curve H S (Q)<br />
- Control by bypass circulation<br />
- Irreversible, mechanical reduction <strong>of</strong> <strong>the</strong> impeller diameter by rotary cut, or replacement by<br />
an impeller <strong>of</strong> a different size.<br />
Speed control is <strong>of</strong>ten feasible for larger pumps operating at variant conditions because <strong>of</strong> <strong>the</strong><br />
energy savings compared to throttling valve control, which has <strong>the</strong> lowest investment cost. The<br />
more throttling <strong>the</strong>re is, <strong>the</strong> higher are <strong>the</strong> losses <strong>and</strong> <strong>the</strong> less economic is <strong>the</strong> type <strong>of</strong> flow control<br />
compared to pump speed control. Throttling losses are however less if <strong>the</strong> characteristic curve <strong>of</strong><br />
<strong>the</strong> pump H P (Q) is flat, which means a moderate decrease in head, H, with increased flow, Q.<br />
Fur<strong>the</strong>rmore, <strong>the</strong> type <strong>of</strong> solids <strong>and</strong> <strong>the</strong> ions contained in <strong>the</strong> fluid determine <strong>the</strong> material<br />
requirements for <strong>the</strong> impeller <strong>and</strong> <strong>the</strong> pump casting in order to avoid erosion <strong>and</strong> corrosion during<br />
long-term operation. Stock pumps are especially designed to avoid plugging <strong>and</strong> fiber spinning<br />
as well as excessive wear. The air content <strong>of</strong> <strong>the</strong> fluid also has to be considered because air<br />
accumulates around <strong>the</strong> rotation axis on <strong>the</strong> suction side <strong>of</strong> <strong>the</strong> impeller <strong>and</strong> lowers <strong>the</strong><br />
characteristic curve <strong>of</strong> <strong>the</strong> pump <strong>and</strong> reduces pumping efficiency.<br />
The largest pumps in <strong>the</strong> <strong>paper</strong> mill are <strong>the</strong> fan pumps feeding <strong>the</strong> first cleaner stage <strong>and</strong> <strong>the</strong><br />
headbox. Especially <strong>the</strong> headbox pump has to operate at lowest possible pulsation, which is<br />
achieved by design <strong>and</strong> manufacturing precision, for example, by a staggered position <strong>of</strong> vanes<br />
between <strong>the</strong> two suction sides. The characteristic curve <strong>of</strong> <strong>the</strong> headbox pump H P (Q) should be<br />
steep in order to keep <strong>the</strong> transformation <strong>of</strong> pressure variations into flow variations as low as<br />
possible12.<br />
5.4 Broke system<br />
5.4.1 Introduction<br />
Broke is <strong>paper</strong> that is discarded at any point <strong>of</strong> <strong>the</strong> manufacturing <strong>and</strong> finishing processes inside<br />
<strong>the</strong> <strong>paper</strong> mill. Broke occurs on a continuous basis as trims from <strong>the</strong> wire <strong>and</strong> from winders, <strong>and</strong><br />
broke occurs occasionally as, e.g., reel slab-<strong>of</strong>fs, in <strong>the</strong> finishing room, or during breaks. Usually,<br />
all broke is repulped, cleaned, <strong>and</strong> stored in <strong>the</strong> broke system. The processed broke is blended<br />
with o<strong>the</strong>r components at <strong>the</strong> blend chest <strong>and</strong> thus fed back into <strong>the</strong> production process. The<br />
amount <strong>of</strong> broke dosed to <strong>the</strong> furnish depends on web breaks <strong>and</strong> <strong>the</strong> broke line capacity.<br />
Pulps are irreversibly altered during <strong>the</strong> first drying, which impairs some <strong>paper</strong> properties as<br />
known from <strong>paper</strong> made <strong>of</strong> recycled fibers. Hence, <strong>the</strong> properties <strong>of</strong> dry-broke pulp is different<br />
when compared to <strong>the</strong> fresh pulp used. There is no specific quality difference between dry broke<br />
<strong>and</strong> clean, unprocessed <strong>paper</strong>, e.g., unprinted printing shop waste, which is called "waste<strong>paper</strong>"<br />
or "recovered <strong>paper</strong>" by definition.<br />
In specialty <strong>paper</strong> or dyed <strong>paper</strong> production, <strong>the</strong> reuse <strong>of</strong> broke might be somehow limited by<br />
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<strong>the</strong> required product quality or due to o<strong>the</strong>r reason. Depending on <strong>the</strong> <strong>paper</strong> grade <strong>and</strong> <strong>the</strong><br />
degree <strong>of</strong> processing, <strong>the</strong> broke might be pulped <strong>and</strong> used at ano<strong>the</strong>r time or elsewhere.<br />
However, when this broke leaves <strong>the</strong> mill, it becomes per definition "recovered <strong>paper</strong>" or<br />
secondary fibers.<br />
One way to classify broke h<strong>and</strong>ling toge<strong>the</strong>r with o<strong>the</strong>r processes connected to <strong>the</strong> PM is to<br />
consider it as a system <strong>of</strong> its own. Ano<strong>the</strong>r way is to consider <strong>the</strong> broke system as part <strong>of</strong> stock<br />
preparation because <strong>the</strong> same operations <strong>of</strong> slushing <strong>and</strong> defibration are performed. Finally,<br />
broke h<strong>and</strong>ling means that fibers are recovered for use on <strong>the</strong> PM. However, <strong>the</strong> broke system<br />
should not be integrated into <strong>the</strong> fiber recovery or saveall operation because it can lead to system<br />
instability. During steady operation <strong>of</strong> <strong>the</strong> PM, a steady flow <strong>of</strong> white <strong>water</strong> with constant solids<br />
content is h<strong>and</strong>led in <strong>the</strong> saveall, while broke occurs at an inconstant rate. The broke system<br />
should <strong>the</strong>refore be decoupled from <strong>the</strong> white <strong>water</strong> treatment. Also <strong>the</strong> use <strong>of</strong> broke as<br />
sweetener <strong>of</strong> <strong>the</strong> disc filter is not favorable. The broke thickener filtrate, however, is normally fed<br />
into <strong>the</strong> fiber recovery, while <strong>the</strong> dilution <strong>water</strong> to <strong>the</strong> broke pulpers is taken from <strong>the</strong> white <strong>water</strong><br />
or clear <strong>water</strong> tower, hence, from <strong>water</strong> after fiber recovery.<br />
5.4.2 Broke system requirements<br />
Uniform <strong>and</strong> good stock quality is <strong>the</strong> main requirement to a properly operating broke system. If<br />
breaks are long-lasting or occur repeatedly, <strong>the</strong> amount <strong>of</strong> broke in <strong>the</strong> stock has to be increased.<br />
Poor performance <strong>of</strong> <strong>the</strong> broke system due to malfunctions or faulty design can cause fur<strong>the</strong>r<br />
breaks, which in turn raises pressure to increase <strong>the</strong> proportion <strong>of</strong> broke in <strong>the</strong> stock. Hence,<br />
proper design <strong>and</strong> sufficient storage capacity <strong>of</strong> <strong>the</strong> broke system are essential. The total broke<br />
storage capacity is commonly equivalent to 2−4 hours <strong>of</strong> net production, depending on <strong>the</strong> <strong>paper</strong><br />
grade; complex <strong>machine</strong>s with coating stations require large broke storages like an on-line<br />
lightweight-coated (LWC) <strong>paper</strong> <strong>machine</strong>.<br />
Broke h<strong>and</strong>ling is determined by <strong>the</strong> following functional steps:<br />
- Broke transport<br />
- Pulping<br />
- Storage<br />
- Cleaning <strong>and</strong> homogenizing<br />
- Dosage.<br />
The capacity <strong>of</strong> all <strong>machine</strong> pulpers <strong>and</strong> equipment <strong>of</strong> <strong>the</strong> broke system has to be sufficient to<br />
h<strong>and</strong>le <strong>the</strong> amount <strong>of</strong> <strong>paper</strong> produced at maximum gross production. On <strong>the</strong> o<strong>the</strong>r end, <strong>the</strong> broke<br />
system also has to function properly from <strong>the</strong> broke storage tower to broke dosage, if no breaks<br />
occur over a long period <strong>of</strong> production. In this case, <strong>the</strong> broke consists only <strong>of</strong> trims <strong>and</strong> slab-<strong>of</strong>fs<br />
<strong>and</strong> eventually occurring broke from rejected rolls, which accounts toge<strong>the</strong>r for only a few percent<br />
<strong>of</strong> <strong>the</strong> maximum capacity. Therefore, broke can be circulated to assure proper functioning <strong>of</strong> <strong>the</strong><br />
broke screens <strong>and</strong> deflakers.<br />
On multigrade <strong>machine</strong>s, grade changes can create a special problem if furnishes <strong>of</strong> two<br />
grades are not compatible. A running grade change in such a situation is not possible, but a<br />
time-consuming cleanup <strong>of</strong> <strong>the</strong> system is necessary. Therefore, it is not usually feasible to use a<br />
large single broke storage tower on a multigrade <strong>machine</strong>. At <strong>the</strong> next grade change, <strong>the</strong> stored<br />
broke might not be suitable for <strong>the</strong> new grade <strong>and</strong> would have to be discarded. For this reason,<br />
wet broke is used immediately. At least on small <strong>machine</strong>s, dry broke can be stored <strong>and</strong> added<br />
toge<strong>the</strong>r with o<strong>the</strong>r raw material components at <strong>the</strong> beginning <strong>of</strong> <strong>the</strong> stock system. The flexibility<br />
required by a multigrade <strong>machine</strong> is achieved by sacrificing <strong>the</strong> stability <strong>of</strong> <strong>the</strong> system, especially<br />
during web breaks. During a web break, <strong>the</strong> proportion <strong>of</strong> broke in <strong>the</strong> stock pumped to <strong>the</strong> PM<br />
increases, which can easily cause new breaks. This is one reason among several o<strong>the</strong>rs causing<br />
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usually a lower operation speed <strong>of</strong> a multigrade <strong>machine</strong> than <strong>the</strong> speed <strong>of</strong> a single-grade<br />
<strong>machine</strong> operating under similar conditions.<br />
5.4.3 Broke h<strong>and</strong>ling<br />
5.4.3.1 Transport <strong>and</strong> auxiliary equipment<br />
The location <strong>and</strong> <strong>the</strong> number <strong>of</strong> broke pulpers is chosen for each <strong>machine</strong> as a compromise<br />
between <strong>the</strong> minimum transport effort <strong>of</strong> broke <strong>and</strong> <strong>the</strong> cost for installation <strong>and</strong> operation. Right<br />
after a break begins, <strong>the</strong> web is cut at <strong>the</strong> next pulper before <strong>the</strong> break location to avoid<br />
accumulation <strong>of</strong> broke in <strong>the</strong> cellar or on a conveyor. Showers spray some <strong>of</strong> <strong>the</strong> pulper dilution<br />
<strong>water</strong> in order to ensure proper transport <strong>of</strong> <strong>the</strong> web into <strong>the</strong> pulper. Broke pulpers are equipped<br />
with an exhaust fan to avoid moisture entering <strong>the</strong> <strong>machine</strong> hall <strong>and</strong> to ensure <strong>the</strong> <strong>paper</strong> web<br />
feed into <strong>the</strong> pulper. Loose bits <strong>of</strong> dry broke are transported under <strong>the</strong> PM drying section on<br />
dem<strong>and</strong> by conveyor belts <strong>of</strong> full <strong>machine</strong>-width to <strong>the</strong> pulpers. Older <strong>and</strong> slower running<br />
<strong>machine</strong>s may lack such conveyor <strong>systems</strong>; thus, broke is collected in <strong>the</strong> cellar <strong>and</strong> fed into<br />
pulpers manually. Winder trims are <strong>of</strong>ten carried through a pneumatic system over longer<br />
distances to a dry-broke pulper or to a separate trim-pulper. The conveying air is separated in a<br />
cyclone separator, which can be integrated into <strong>the</strong> trim-pulper. Spray showers are installed,<br />
which might also be used to de-dust exhaust from certain dust removal sites, like <strong>the</strong> slitting<br />
stations. Especially for wide <strong>machine</strong>s, a continuously operating trim pulper with a single bottom<br />
rotor is usually more practical than running a pulper <strong>of</strong> <strong>machine</strong>-width, e.g., <strong>the</strong> winder pulper,<br />
continuously. Rejected rolls are ei<strong>the</strong>r fed into <strong>the</strong> PM dry-end pulper, into some o<strong>the</strong>r finishing<br />
room pulper, or into a separate broke roll pulper with a single bottom rotor. The rejected rolls are<br />
opened by a guillotine cutter, which can be equipped with two conveyor belts, one on each side<br />
<strong>of</strong> <strong>the</strong> cutter. Large rolls are moved back <strong>and</strong> forth for cutting into small segments, which are<br />
repulped bit by bit to avoid upsetting <strong>the</strong> broke system.<br />
5.4.3.2 Pulpers<br />
Old <strong>and</strong> very slowly running PMs might have no broke pulper or just one located beside <strong>the</strong><br />
<strong>machine</strong>. Also for a modern tissue <strong>machine</strong> with a Yankee dryer <strong>and</strong> especially with one wire/one<br />
felt design, one broke pulper at <strong>the</strong> dry end is sufficient. In contrast, for large board <strong>machine</strong>s <strong>and</strong><br />
modern high-speed <strong>machine</strong>s for graphic <strong>paper</strong> production, <strong>the</strong> number <strong>of</strong> broke pulpers can be<br />
large. A couch pit, press, <strong>and</strong> dry end pulper are needed at least. The total number <strong>of</strong> pulpers<br />
depends on <strong>the</strong> <strong>paper</strong> grade <strong>and</strong> particular process. For example, a modern uncoated wood-free<br />
<strong>paper</strong> <strong>machine</strong> could have <strong>the</strong> following pulpers:<br />
- Couch pit<br />
- Press pulper(s)<br />
- Size-press pulper<br />
- Calender pulper<br />
- Reel pulper<br />
- Winder pulper<br />
- Winder trim pulper<br />
- Finishing room pulper<br />
- Finishing room trim pulper<br />
- Broke roll pulper.<br />
The broke pulpers under <strong>the</strong> PM are dimensioned according to <strong>the</strong> PM width. The<br />
construction is ei<strong>the</strong>r concrete or steel. On wide <strong>machine</strong>s, usually two rotors are located next to<br />
each o<strong>the</strong>r at <strong>the</strong> long side <strong>of</strong> <strong>the</strong> pulper. The direction <strong>of</strong> rotation is opposite, so that <strong>the</strong> web is<br />
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pulled into <strong>the</strong> vat. The pulped stock leaves <strong>the</strong> vat through a screen plate behind <strong>the</strong> rotor. Also<br />
cross-shaft agitated pulpers are used with several impellers on <strong>the</strong> shaft. There, <strong>the</strong> discharge is<br />
from <strong>the</strong> short side <strong>of</strong> <strong>the</strong> pulper. Figure 19 shows <strong>the</strong> principle <strong>of</strong> a discontinuously operating<br />
one-pump broke pulping system. In <strong>the</strong> case <strong>of</strong> a break, <strong>the</strong> pulper is started <strong>and</strong> dilution <strong>water</strong> is<br />
added immediately. This is commonly initiated by break automatics. The pulping consistency<br />
ranges usually between 3% <strong>and</strong> 6%. The pulped broke is pumped into <strong>the</strong> couch pit, which<br />
operates continuously. Two pumps with different capacity discharge <strong>the</strong> couch pit <strong>and</strong> possibly<br />
o<strong>the</strong>r pulpers with continuous broke feed like, e.g., trims. During normal running condition, <strong>the</strong><br />
smaller pump runs <strong>and</strong> <strong>the</strong> level is controlled similarly to that shown in Fig. 19. In a break<br />
situation, <strong>the</strong> level rises quickly <strong>and</strong> <strong>the</strong> larger pump starts. In case trims at <strong>the</strong> PM dry end are<br />
not pulped in a separate trim pulper, <strong>the</strong> receiving broke pulper is also equipped with a two-pump<br />
system. Finally, finishing room broke pulping can operate in some cases at high consistency in a<br />
separate system.<br />
Figure 19. Broke pulping system.<br />
Coated broke is not fed to <strong>the</strong> couch pit, but collected separately. The pigments <strong>of</strong> <strong>the</strong> coating<br />
color appear in coated broke like filler. In order to control <strong>the</strong> amount <strong>of</strong> coated broke, thus <strong>the</strong><br />
amount <strong>of</strong> pigment added to <strong>the</strong> <strong>paper</strong> stock, collection, thickening, <strong>and</strong> storage are separated<br />
from <strong>the</strong> uncoated broke system. The separate system has its own broke storage, while cleaning<br />
<strong>and</strong> o<strong>the</strong>r broke treatment can be toge<strong>the</strong>r.<br />
Wet-strength broke requires sometimes chemicals <strong>and</strong> elevated temperature by <strong>the</strong> addition<br />
<strong>of</strong> steam in order to reduce pulping time. Steam or chemicals are added to pulpers, in particular,<br />
to finishing room or broke roll pulpers in order to slush broke, which has been dry for some time.<br />
5.4.3.3 Slushed broke-h<strong>and</strong>ling system<br />
The degree <strong>of</strong> sophistication <strong>of</strong> a broke system can vary from plain recirculation <strong>of</strong> pulped broke<br />
to <strong>the</strong> blend chest without any treatment up to complex <strong>systems</strong> with multiple cleaning stages.<br />
The following equipment can be part <strong>of</strong> a broke-h<strong>and</strong>ling system:<br />
- Broke storage tower<br />
- Broke thickener<br />
- Pressure screen, possibly in multiple stages<br />
- High-density cleaner<br />
- Open screen, like vibrating screen or scalping screen<br />
- Deflaker.<br />
Broke storage towers are usually operated in combination with a thickener, e.g., a gravity<br />
decker or sometimes inclined screens. The thickener increases <strong>the</strong> consistency <strong>of</strong> <strong>the</strong> stored<br />
pulp, <strong>and</strong> <strong>water</strong> can be moved back to <strong>the</strong> clear <strong>water</strong> system. Consistency fluctuations are<br />
<strong>the</strong>reby reduced. The thickened pulp is collected in a chest, from where part <strong>of</strong> <strong>the</strong> broke is<br />
circulated to <strong>the</strong> tower. In a few special cases <strong>of</strong> dyed <strong>paper</strong>, broke bleaching chemicals are<br />
added to <strong>the</strong> broke storage. If <strong>the</strong> saveall is employed also as a broke thickener, which is<br />
sometimes done for wire pit trims, <strong>the</strong> saveall is loaded with fines. Especially at PM web breaks,<br />
variations in <strong>the</strong> saveall operation affect <strong>the</strong> white <strong>water</strong> system <strong>and</strong> possibly <strong>the</strong>reby cause<br />
system instability. A clear separation between broke h<strong>and</strong>ling <strong>and</strong> <strong>the</strong> white <strong>water</strong> system is<br />
preferred. For coated <strong>paper</strong> production, double broke lines are used for thickening <strong>and</strong> storage,<br />
one for uncoated broke <strong>and</strong> one for coated broke.<br />
The degree <strong>of</strong> sophistication <strong>of</strong> broke cleaning <strong>systems</strong> is determined by <strong>the</strong> dem<strong>and</strong>s in<br />
quality <strong>and</strong> in quality constancy <strong>of</strong> <strong>the</strong> stock. For example, a high-speed PM for graphical <strong>paper</strong><br />
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production can have multiple-stage pressure screening. To reduce fiber losses, <strong>the</strong> end-stage<br />
screen is <strong>of</strong>ten operated with sequential flushing. Also open screening devices, <strong>of</strong> which <strong>the</strong><br />
vibrating screen is most common, have been used at <strong>the</strong> last stage. However, modern broke<br />
cleaning <strong>systems</strong> are operated at <strong>the</strong> same consistency as <strong>the</strong> entire broke system, i.e.,<br />
2.5%−3.5%. High-density cleaners can be installed to remove, e.g., s<strong>and</strong> <strong>and</strong> o<strong>the</strong>r heavy dirt.<br />
Coated <strong>and</strong> uncoated broke are <strong>of</strong>ten treated toge<strong>the</strong>r in a single system, in order to operate <strong>the</strong><br />
pressure screens under desirably constant load. Thus, <strong>the</strong> proportioning <strong>of</strong> coated <strong>and</strong> uncoated<br />
broke is done at <strong>the</strong> broke screening chest.<br />
More efficient than <strong>the</strong> removal <strong>of</strong> dirt from <strong>the</strong> stock is to avoid contamination by dirt entering<br />
<strong>the</strong> broke pulpers. The way broke is collected <strong>and</strong> transported (see above) affects its dirt content.<br />
The cleanliness <strong>of</strong> areas where broke falls onto <strong>the</strong> ground determines <strong>the</strong> dirt entry into <strong>the</strong><br />
stock system.<br />
The deflaker (see Chapter 3) creates high hydraulic forces when passing <strong>the</strong> stock through<br />
<strong>the</strong> gaps between static <strong>and</strong> rotating plates, which are ei<strong>the</strong>r perforated or equipped with bars.<br />
Flocks, flakes, <strong>and</strong> fiber bundles are sheared due to high acceleration <strong>and</strong> deceleration <strong>of</strong> <strong>the</strong><br />
stock <strong>and</strong> by impact <strong>of</strong> surfaces. The content <strong>of</strong> flakes in broke is usually high due to <strong>the</strong> short<br />
dwelling <strong>and</strong> repulping time. Deflakers are needed in particular for coated broke <strong>and</strong> wet-strength<br />
<strong>paper</strong> broke. Possible arrangements for deflakers are:<br />
- Deflaking <strong>of</strong> whole broke before dosage<br />
- Deflaking <strong>of</strong> first-stage screen reject only<br />
- Deflaking <strong>of</strong> coated or dry broke only<br />
- No deflaking.<br />
If needed, deflakers are connected in parallel. In special cases or for certain specialty <strong>paper</strong>s,<br />
broke h<strong>and</strong>ling arrangements can be different.<br />
5.4.4 Broke dosage<br />
Broke is usually added to <strong>the</strong> stock at <strong>the</strong> blend chest. It should be considered that <strong>the</strong><br />
composition <strong>and</strong> <strong>the</strong> quality <strong>of</strong> broke might be different from <strong>the</strong> fresh furnish components. Broke<br />
contains fillers <strong>and</strong> o<strong>the</strong>r dispersed <strong>and</strong> dissolved material according to <strong>the</strong> additives <strong>and</strong><br />
materials applied to <strong>the</strong> stock or to <strong>the</strong> <strong>paper</strong> surface in <strong>the</strong> PM. Fluctuations in <strong>the</strong> broke dosage,<br />
especially those <strong>of</strong> coated broke, can disturb or even upset <strong>the</strong> wet end chemistry. Consider, for<br />
example, a sudden increase <strong>of</strong> broke added to <strong>the</strong> furnish, which causes an increase in fine <strong>and</strong><br />
filler material resulting in an increased cationic dem<strong>and</strong>. In consequence, wire retention drops<br />
which affects production <strong>and</strong> <strong>the</strong> dem<strong>and</strong> for retention aids.<br />
Quality variations can also originate from changes in <strong>the</strong> proportion <strong>of</strong> wet <strong>and</strong> dry broke. In<br />
some cases, separate storage <strong>systems</strong> for wet <strong>and</strong> dry broke are applied. This might be<br />
considered in particular for an integrated mill using never-dried chemical pulp or for specialty<br />
<strong>paper</strong>s. The fiber properties <strong>and</strong> <strong>the</strong> needed amount <strong>of</strong> drying energy change at most when<br />
chemical pulp fibers are dried for <strong>the</strong> first time.<br />
In multi-ply board production or at multilayer headboxes, broke can be exclusively dosed to<br />
certain layers or plies <strong>of</strong> <strong>the</strong> sheet, e.g., in order to hide dirt or specks in <strong>the</strong> middle layer.<br />
5.4.5 Coated broke <strong>systems</strong><br />
In particular applications, like special coating colors with high binder content <strong>and</strong> for high-end<br />
dem<strong>and</strong>s, a disperser or a kneader can be installed to crush coat particles. Platelets or grainy,<br />
insufficiently broken-up coat particles can cause problems in forming <strong>and</strong> streaks especially in<br />
low-weight coat application. The coarse particles are also entering <strong>the</strong> cleaner system <strong>of</strong> <strong>the</strong><br />
stock approach system <strong>and</strong> cause an increased loss <strong>of</strong> pigments, if <strong>the</strong>y are not recovered from<br />
<strong>the</strong> cleaner rejects. A de<strong>water</strong>ing press is usually required because <strong>the</strong> disperser or kneader runs<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
at a consistency <strong>of</strong> 30% or higher. Recycling <strong>the</strong> press filtrate back to coated broke pulping ends<br />
up in a separate <strong>water</strong> circulation, which reduces <strong>the</strong> amount <strong>of</strong> detrimental substances passed<br />
to <strong>the</strong> PM. Treatment <strong>of</strong> this circulating <strong>water</strong> by, e.g., dissolved air flotation (DAF) can also<br />
reduce <strong>the</strong> amount <strong>of</strong> hydrophobic substances ("white pitch") significantly.<br />
5.5 Fiber recovery <strong>and</strong> <strong>water</strong> clarification<br />
Equipment installed in <strong>the</strong> white <strong>water</strong> system to separate suspended solids <strong>and</strong> <strong>water</strong> has a truly<br />
tw<strong>of</strong>old function. On <strong>the</strong> one h<strong>and</strong>, stock components are recovered <strong>and</strong>, on <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>,<br />
process <strong>water</strong> is clarified for fur<strong>the</strong>r use in <strong>the</strong> mill. If separation <strong>machine</strong>ry is installed in a series,<br />
<strong>the</strong> fiber recovery unit, or synonymously <strong>the</strong> saveall, is located in <strong>the</strong> first place, followed by<br />
possible steps <strong>of</strong> more advanced <strong>water</strong> purification for special uses, like for high pressure<br />
showers. The equipment at each stage is optimized for its particular purpose. Generally, <strong>the</strong><br />
separation <strong>of</strong> suspended solids <strong>and</strong> <strong>water</strong> is achieved by40:<br />
- Filtration<br />
- Flotation<br />
- Fractionation<br />
- Sedimentation.<br />
All <strong>of</strong> <strong>the</strong>se techniques can also be applied in combination. Hydraulic capacity <strong>and</strong><br />
purification performance are <strong>the</strong> key parameters. O<strong>the</strong>r parameters that have to be considered<br />
include: concentration <strong>of</strong> suspended solids in <strong>the</strong> feed, presence <strong>of</strong> colloidal <strong>and</strong> dissolved<br />
substances, chemicals needed, available space, power requirement, all-over energy<br />
consumption, need for auxiliary equipment, etc. As is applicable for every type <strong>of</strong> <strong>machine</strong> or<br />
system, <strong>the</strong> most important factor is cost, which includes <strong>the</strong> cost for <strong>the</strong> equipment <strong>and</strong> its<br />
installation, operational cost, <strong>and</strong> maintenance. Cost effectiveness is gained by a reduced<br />
amount <strong>of</strong> raw material losses, increased process performance, <strong>and</strong> <strong>the</strong> benefits due to stable<br />
<strong>and</strong> efficient <strong>paper</strong> production.<br />
Fiber recovery feed <strong>water</strong> consists <strong>of</strong> <strong>water</strong> from:<br />
- The wire pit overflow<br />
- Water separators in <strong>the</strong> PM vacuum system, i.e., <strong>the</strong> <strong>water</strong> removed from <strong>the</strong> web by<br />
vacuum at <strong>the</strong> wet end <strong>and</strong> by wet pressing<br />
- PM wet end tray<br />
- Broke thickener.<br />
The process connected to <strong>the</strong> saveall has to be designed so that a constant feed flow is<br />
maintained. Despite that, variations in <strong>the</strong> saveall load, ei<strong>the</strong>r in solids content or in flow, originate<br />
from:<br />
- Variations in wire retention<br />
- Variations in sweetener quality <strong>and</strong> quantity<br />
- PM grade changes<br />
- Changes in fresh <strong>water</strong> supply.<br />
If <strong>the</strong> fiber recovery system is designed correctly, no variations originate from web breaks on<br />
<strong>the</strong> PM. Besides a constant filtrate quality, <strong>the</strong> amount <strong>of</strong> recovered fines <strong>and</strong> filler should be as<br />
constant as possible.<br />
5.5.1 Filtration<br />
5.5.1.1 Disc filter<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
The disc filter is a unit where multiple discs rotate in a vat. Each <strong>of</strong> <strong>the</strong> 30 or more discs consists<br />
<strong>of</strong> several segments covered with fine wire <strong>and</strong> rotates over <strong>the</strong> stationary filtration zones, as<br />
shown in Fig. 20. During filtration, a fiber mat builds up with <strong>the</strong> aid <strong>of</strong> pre-coating pulp, <strong>the</strong><br />
so-called "sweetener," which is added to <strong>the</strong> white <strong>water</strong> feed flow. The thicker <strong>and</strong> denser this<br />
filter cake becomes during vacuum filtration, <strong>the</strong> less solids are passed through. Hence, <strong>the</strong><br />
filtrate consistency is getting lower, until <strong>the</strong> filtration process is interrupted, when <strong>the</strong> filter mat<br />
emerges from <strong>the</strong> filled vat. The filtrate <strong>of</strong> <strong>the</strong> different phases is <strong>the</strong>refore collected separately as<br />
richer cloudy filtrate, leaner clear filtrate, <strong>and</strong> <strong>the</strong> optionally leanest super-clear filtrate. Table 5<br />
shows <strong>the</strong> typical filtrate properties41. The disc segments <strong>of</strong> <strong>the</strong> same radial shaft position<br />
discharge into <strong>the</strong> same discharge channel in <strong>the</strong> shaft, i.e., <strong>the</strong> channel to which <strong>the</strong>se segments<br />
are firmly connected. The different filtrate qualities are collected at one end <strong>of</strong> <strong>the</strong> shaft, where<br />
<strong>the</strong> pipes or drop legs to <strong>the</strong> filtrate tanks are attached. The split ratio <strong>of</strong> <strong>the</strong> 2−3 filtrates is ei<strong>the</strong>r<br />
adjustable by altering <strong>the</strong> access to <strong>the</strong> shaft channels <strong>of</strong> <strong>the</strong> disc filter, or it is determined by a<br />
fixed design <strong>of</strong> <strong>the</strong> rotary filtrate valve at <strong>the</strong> shaft. Cloudy filtrate is discharged under<br />
atmospheric conditions via a free-fall pipe. Clear filtrate <strong>and</strong> super-clear filtrate are collected in<br />
two separate drop legs, each discharging <strong>the</strong> filtrates into a seal chamber in <strong>the</strong> filtrate tanks (see<br />
Fig. 21). The height difference between disc filter center-shaft <strong>and</strong> <strong>the</strong> level in <strong>the</strong> filtrate tank is<br />
7−8 m for creating <strong>the</strong> required vacuum generated by <strong>the</strong> virtue <strong>of</strong> a sufficient velocity in <strong>the</strong> drop<br />
legs. Vacuum can be also generated by a vacuum pump, especially if a high dryness <strong>of</strong> <strong>the</strong> filter<br />
cake is desired. The filter cake maintains <strong>the</strong> vacuum in <strong>the</strong> super-clear filtrate line, even at <strong>the</strong><br />
end <strong>of</strong> filtration when it emerges above <strong>the</strong> <strong>water</strong>line. After that, vacuum is released from <strong>the</strong> mat,<br />
which is <strong>the</strong>n removed from <strong>the</strong> wire by <strong>the</strong> knock-<strong>of</strong>f shower or by an air jet. Ano<strong>the</strong>r shower<br />
keeps <strong>the</strong> trough clean from bits <strong>of</strong> discharged pulp. An oscillating shower cleans <strong>the</strong> filter cloth.<br />
The <strong>water</strong> from this shower enters into <strong>the</strong> cloudy filtrate tank, or it is collected separately in a<br />
discharge channel according to <strong>the</strong> design <strong>of</strong> <strong>the</strong> disc filter (see Fig. 20). All filter shower <strong>water</strong>s<br />
are usually taken directly from <strong>the</strong> clear filtrate tank. The cloudy filtrate is <strong>of</strong>ten circulated as<br />
shown in Fig. 21.<br />
Table 5. Typical filtrate properties41.<br />
Property White <strong>water</strong> Cloudy filtrate Clear filtrate Super-clear<br />
filtrate<br />
Consistency,<br />
3 500<br />
400<br />
50<br />
5<br />
mg/L<br />
2 500<br />
200<br />
30<br />
10<br />
Consistency<br />
1<br />
2−3<br />
4<br />
5<br />
variation, mg/L<br />
Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
clear <strong>and</strong> super-clear filtrate is gained. The rotational speed <strong>of</strong> <strong>the</strong> disc filter determines <strong>the</strong> time<br />
given to <strong>the</strong> fibers to build up a mat. The typical speed in <strong>the</strong> range <strong>of</strong> 0.2 to 1.5 rpm can be<br />
controlled by <strong>the</strong> level signal <strong>of</strong> <strong>the</strong> filter vat. Ano<strong>the</strong>r mode <strong>of</strong> operation is to run with a selected<br />
speed <strong>and</strong> to maintain <strong>the</strong> vat level with cloudy filtrate circulation, as shown in Fig. 21. Variations<br />
in <strong>the</strong> following feed flow parameters cause changes in <strong>the</strong> filter mat properties <strong>and</strong>, hence,<br />
fluctuation in <strong>the</strong> hydraulic capacity <strong>and</strong> <strong>the</strong> quality <strong>of</strong> <strong>the</strong> filtrates:<br />
- Fines <strong>and</strong> filler content<br />
- Feed flow<br />
- Freeness<br />
- Type <strong>and</strong> properties <strong>of</strong> <strong>the</strong> sweetener.<br />
If <strong>the</strong>re is for example a jump in freeness, <strong>the</strong> level in <strong>the</strong> disc filter vat can drop even at low<br />
disc speed. In such a case, <strong>the</strong> hydraulic load can be increased by adding white <strong>water</strong> from <strong>the</strong><br />
header to <strong>the</strong> cloudy filtrate circulation loop, as shown in Fig. 21. The sweetener is dosed at a<br />
certain ratio to <strong>the</strong> white <strong>water</strong> flow to <strong>the</strong> saveall. The dilution <strong>of</strong> <strong>the</strong> recovered solids at <strong>the</strong><br />
discharge screw <strong>of</strong> <strong>the</strong> disc filter is proportional to <strong>the</strong> amount <strong>of</strong> sweetener supplied. Fur<strong>the</strong>r on,<br />
an exhaust can be connected to <strong>the</strong> filter hood, especially if <strong>the</strong> white <strong>water</strong> temperature is high.<br />
Instead <strong>of</strong> vacuum-driven filtration, pressure can be applied to increase <strong>the</strong> filtration rate by<br />
an increased pressure difference. Air is blown from <strong>the</strong> filtration tank into <strong>the</strong> disc filter to create a<br />
certain pressure under <strong>the</strong> hood. If no drop legs are needed, it is not necessary to install <strong>the</strong><br />
pressure disc filter at an elevated location. Pressure disc filters are used, e.g., in mechanical<br />
pulping, where <strong>the</strong> <strong>water</strong> temperature might be high. In <strong>the</strong> PM <strong>water</strong> system, low-pressure disc<br />
filters are sometimes used to post-treat <strong>the</strong> saveall clear filtrate to produce a super-clear filtrate<br />
for PM showers <strong>and</strong> similar purposes.<br />
The <strong>paper</strong> mill discharge point is at <strong>the</strong> clear filtrate tank where <strong>water</strong> leaves <strong>the</strong> system; this<br />
is <strong>the</strong> point where <strong>the</strong> <strong>paper</strong> mill effluent occurs regularly. This effluent stream is flow-controlled,<br />
for example, according to <strong>the</strong> level in <strong>the</strong> filtrate tower. This surplus <strong>water</strong> is fed upstream into <strong>the</strong><br />
<strong>water</strong> system <strong>of</strong> <strong>the</strong> pulping or <strong>the</strong> recycled fiber processing plant. If no such integrated operation<br />
exists, <strong>the</strong> surplus filtrate is let into <strong>the</strong> sewer.<br />
5.5.1.2 Sweetener<br />
The fiber content <strong>of</strong> <strong>the</strong> white <strong>water</strong> entering <strong>the</strong> filter vat is increased by adding pulp, <strong>the</strong><br />
so-called "sweetener," or precoating pulp. The vat concentration is in a similar range as known<br />
for PM headboxes. Sweetener is needed to form a sufficient fiber mat. The function <strong>of</strong> <strong>the</strong> filter<br />
fabric is to support this filter cake, which acts as <strong>the</strong> filter medium. In cake filtration, <strong>the</strong> fineness<br />
<strong>of</strong> <strong>the</strong> fabric has ra<strong>the</strong>r little significance on <strong>the</strong> filtrate quality, if chosen within a range <strong>of</strong><br />
reasonable mesh size. The sweetener acts as filter medium; hence, its quality determines:<br />
- Filtrate quality<br />
- Constant <strong>and</strong> high filtration rates to reach <strong>the</strong> desired filtrate quality<br />
- Reliable filter cake discharge.<br />
Sweetener fibers are not circulating in <strong>the</strong> saveall system; thus, sweetener fibers are always<br />
used once <strong>and</strong> <strong>the</strong>n blended with <strong>the</strong> PM stock. The fines content <strong>of</strong> <strong>the</strong> sweetener pulp is<br />
preferably low <strong>and</strong> constant. Thus, long-fiber chemical s<strong>of</strong>twood pulp is <strong>the</strong> best possible<br />
sweetener. The following are less preferable sweeteners in descending order: TMP, GW, mixed<br />
recovered <strong>paper</strong> stock, <strong>and</strong> broke42. The sweetener stock is taken from <strong>the</strong> stock feed header to<br />
<strong>the</strong> blend chest (see also Fig. 12). The recovered fibers are fed back to <strong>the</strong> blend chest. Due to<br />
<strong>the</strong> increased fines content, <strong>the</strong> recovered stock should not be forwarded into <strong>the</strong> broke system in<br />
order to avoid fluctuation <strong>of</strong> fines content entering stock blending.<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
In addition to <strong>the</strong> sweetener, <strong>the</strong> filtration result can be influenced by <strong>the</strong> dosage <strong>of</strong> charged<br />
polymers as flocculating agent. If no optimal sweetener is available − for example, in <strong>paper</strong><br />
production with 100% recycled fiber furnish, addition <strong>of</strong> a chemical aid can benefit <strong>the</strong> filtrate<br />
quality. Polymer addition reduces <strong>the</strong> amount <strong>of</strong> required filter area; hence, it might be a suitable<br />
approach in de-bottlenecking a disc filter.<br />
5.5.1.3 Drum filter<br />
A drum filter is a filter fabric-covered drum rotating in a vat with filtration toward <strong>the</strong> inside <strong>of</strong> <strong>the</strong><br />
drum, i.e., similar to <strong>the</strong> disc filter. Filtration can be supported by vacuum created in drop legs,<br />
similar to <strong>the</strong> disc filter. Drum filters with a low-pressure differential are ei<strong>the</strong>r gravity deckers or<br />
valveless drum filters. Gravity deckers are <strong>the</strong> most simple drum filters, where filtration is driven<br />
by <strong>the</strong> head difference between <strong>the</strong> vat level <strong>and</strong> <strong>the</strong> filtrate level inside <strong>the</strong> drum. The<br />
low-pressure difference causes a gentle flow through <strong>the</strong> filter medium, which results in a fairly<br />
good filtrate quality. A valveless drum filter contains tubes inside <strong>the</strong> drum, which are arranged in<br />
a helical pattern. One end <strong>of</strong> <strong>the</strong> tubes is connected to a longitudinal channel under <strong>the</strong> drum<br />
surface, <strong>and</strong> <strong>the</strong> o<strong>the</strong>r opening is inside <strong>the</strong> drum. As <strong>the</strong> longitudinal chamber emerges from <strong>the</strong><br />
stock, <strong>the</strong> open end <strong>of</strong> <strong>the</strong> connected tube is still immersed in <strong>the</strong> filtrate inside <strong>the</strong> drum, which<br />
creates a siphon for vacuum40.<br />
Drum filters can be used for high freeness long fiber pulp, which quickly builds up thick pulp<br />
mats <strong>and</strong> which does not discharge cleanly from disc filter elements43. However, for most<br />
applications, <strong>the</strong> hydraulic capacity <strong>of</strong> drum filters is too low <strong>and</strong> only one filtrate quality is<br />
produced. This usually makes <strong>the</strong>m unsuitable for a saveall application. Drum filters are <strong>of</strong>ten<br />
used as simple but reliable thickeners, e.g., in <strong>the</strong> broke-h<strong>and</strong>ling system.<br />
5.5.1.4 O<strong>the</strong>r filters<br />
- Special pressure filters or some bag filters are used for post-treatment <strong>of</strong> clear or<br />
super-clear filtrate for some shower applications.<br />
- Sludge <strong>and</strong> wet rejects can be de<strong>water</strong>ed by gravity tables <strong>and</strong> screw or wire filter presses.<br />
For low-quality board grades, fiber-containing sludge or even effluent treatment sludge can be<br />
returned into <strong>the</strong> <strong>paper</strong>making process. The same applies for <strong>the</strong> pressate, which may be<br />
returned into <strong>the</strong> process <strong>water</strong> system. In various instances, some rejects − sludge from flotation<br />
<strong>and</strong> from o<strong>the</strong>r locations in <strong>the</strong> integrated mill − are de<strong>water</strong>ed <strong>and</strong> treated toge<strong>the</strong>r. In such a<br />
case, <strong>water</strong> should not be returned to <strong>the</strong> process without advanced treatment.<br />
- S<strong>and</strong> filters are <strong>of</strong>ten used in fresh <strong>water</strong> preparation. S<strong>and</strong> filtration has been applied in <strong>the</strong><br />
special case <strong>of</strong> total effluent-free <strong>paper</strong> production to post-treat process <strong>water</strong>44. Also if effluent<br />
from <strong>the</strong> effluent treatment plant is recycled, it can be s<strong>and</strong> filtered before reintroduction into <strong>the</strong><br />
<strong>paper</strong> mill process. In some devices, flotation <strong>and</strong> s<strong>and</strong> filtration are combined into one unit. S<strong>and</strong><br />
filters are periodically cleaned by backwashing.<br />
- Classifying filters are dicussed below.<br />
5.5.2 Flotation<br />
Dissolved air flotation (DAF) units are used in <strong>the</strong> <strong>paper</strong> mill to clarify fresh <strong>water</strong>, process <strong>water</strong>,<br />
or effluent. Due to <strong>the</strong> size <strong>of</strong> <strong>the</strong> created air bubbles, DAF is also called micr<strong>of</strong>lotation. Unlike <strong>the</strong><br />
o<strong>the</strong>r <strong>water</strong> clarification techniques, <strong>the</strong> physical treatment is usually preceded by addition <strong>of</strong><br />
chemicals, which can also reduce <strong>the</strong> amount <strong>of</strong> dissolved organic substances in process <strong>water</strong><br />
to some degree. The removal <strong>of</strong> colloidal material by DAF is typically in a range <strong>of</strong> 10%−40% if<br />
chemicals for precipitation <strong>and</strong> fixing are added. Inorganic salts, similar to <strong>the</strong> o<strong>the</strong>r techniques<br />
discussed in this section, are not reduced. Usually, DAF does well in removing hydrophobic<br />
particles <strong>and</strong> colloids, which are prone to form stickies <strong>and</strong> pitch45. In a process with little effluent<br />
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discharge, colloidal <strong>and</strong> potentially sticky material accumulates, especially when recycled fiber<br />
furnish is used or coated broke occurs. In <strong>the</strong>se cases, DAF improves process <strong>water</strong> <strong>and</strong> thus<br />
<strong>the</strong> PM runnability efficiently. The brightness <strong>of</strong> graphical <strong>paper</strong> made from recycled fiber furnish<br />
is improved when applying DAF to grayed process <strong>water</strong> filtrates. In <strong>the</strong>se cases, <strong>the</strong> flotation<br />
sludge must be discarded <strong>and</strong> not reintroduced into <strong>the</strong> process.<br />
The removal <strong>of</strong> dispersed microscopic <strong>and</strong> fine material is also very good. According to <strong>the</strong><br />
solids content <strong>of</strong> <strong>the</strong> feed flow <strong>and</strong> <strong>the</strong> type <strong>and</strong> amounts <strong>of</strong> chemical aids dosed, <strong>the</strong> removal<br />
efficiency is usually between 85% <strong>and</strong> 98%. Typically, a coagulation aid, fixative, or alternatively<br />
bentonite is added at first. The dosage point is at <strong>the</strong> suction side <strong>of</strong> <strong>the</strong> feed pump or at ano<strong>the</strong>r<br />
location with high turbulence to ensure good mixing. Then, a long-chained polymer is added to<br />
<strong>the</strong> feed flow via a chemical injector to flocculate suspended solids. Right after this, <strong>the</strong><br />
microscopic air bubbles are introduced, which make <strong>the</strong> created flocs buoyant (see Fig. 22).<br />
Figure 22. Dissolved air flotation unit for process <strong>water</strong> treatment.<br />
The air bubbles are created according to Henry's law when releasing pressure from <strong>water</strong><br />
saturated by dissolved air. The over-saturation with dissolved air by <strong>the</strong> sudden pressure<br />
decrease is <strong>the</strong> so-called "soda-bottle effect." The generation <strong>of</strong> large amounts <strong>of</strong> microscopic<br />
bubbles is crucial for <strong>the</strong> operation <strong>of</strong> DAF.<br />
Air is dissolved into <strong>water</strong> at <strong>the</strong> air saturation reactor, where <strong>water</strong> is mixed at high<br />
turbulence with compressed air at an absolute pressure usually <strong>of</strong> 500−700 kPa. The reactor can<br />
contain turbulence-creating <strong>and</strong> flow-guiding elements; also two-stage <strong>systems</strong> are available. Air<br />
is supplied by a separate compressor system or from <strong>the</strong> mill's compressed air network, if <strong>the</strong><br />
supplied pressure is sufficient. Ei<strong>the</strong>r cleared <strong>water</strong> or feed <strong>water</strong> is delivered to <strong>the</strong> air saturation<br />
reactor by <strong>the</strong> booster pump. There are three system modes for flow through <strong>the</strong> reactor:<br />
- Partial flow mode<br />
- Full flow mode<br />
- Recycle flow mode.<br />
In recycle flow mode, a part <strong>of</strong> <strong>the</strong> clear <strong>water</strong> is used for aeration <strong>and</strong> mixed with <strong>the</strong> feed<br />
flow. Figure 22 displays this most commonly used mode in operation while <strong>the</strong> valve for partial<br />
flow operation is closed. In recycle flow mode, <strong>the</strong> hydraulic load is increased by about <strong>the</strong><br />
amount <strong>of</strong> circulating <strong>water</strong> <strong>and</strong> <strong>the</strong> flotation unit has to be dimensioned accordingly. In partial<br />
flow mode, part <strong>of</strong> <strong>the</strong> feed passes <strong>the</strong> aeration system. In full flow mode, all feed <strong>water</strong> has to<br />
pass aeration <strong>and</strong> <strong>the</strong> reactor as well as <strong>the</strong> compressed air system has to be increased in size<br />
accordingly. The advantages <strong>of</strong> recycling vs. partial flow mode are:<br />
- Slightly lower chemicals consumption because <strong>the</strong> effect <strong>of</strong> <strong>the</strong> fixative may be reduced for<br />
<strong>the</strong> partial flow passing <strong>the</strong> air saturation system<br />
- No plugging <strong>and</strong> less dirt accumulation in <strong>the</strong> air saturation system<br />
The pressure <strong>of</strong> <strong>the</strong> aerated <strong>water</strong> is released by a decompression valve or by a turbine right<br />
before mixing with <strong>the</strong> feed stream (see Fig. 22). Pressure can also be released at<br />
decompression nozzles feeding <strong>the</strong> air-saturated stream into <strong>the</strong> feed pipe. The microbubbles are<br />
created immediately at <strong>the</strong> pressure drop <strong>and</strong> attach to <strong>the</strong> fibers <strong>and</strong> flocs to create buoyant<br />
sludge forming a blanket at <strong>the</strong> surface <strong>of</strong> <strong>the</strong> flotation tank. The bubble size is typically 40−100<br />
μm. Heavy particles are removed from <strong>the</strong> bottom <strong>of</strong> <strong>the</strong> flotation tank, typically via a<br />
discontinuously operating trash trap. The flow conditions inside <strong>the</strong> flotation tank are important for<br />
<strong>the</strong> separation performance. In round tanks, <strong>the</strong> surface is large <strong>and</strong> <strong>the</strong> radial flow velocity<br />
toward <strong>the</strong> clear <strong>water</strong> side is low. In rectangular tanks, parallel guiding plates installed inside <strong>the</strong><br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
tank control <strong>the</strong> flow pr<strong>of</strong>ile <strong>and</strong> <strong>the</strong> bubble rise. Particularly in older devices, <strong>the</strong> feed <strong>and</strong> <strong>the</strong><br />
aerated stream are fed separately to <strong>the</strong> flotation tank.<br />
The floatage is removed by one or by a combination <strong>of</strong> <strong>the</strong> following devices:<br />
- Scoop or skimmer<br />
- Scraper or sometimes a blower<br />
- Flow over a weir.<br />
The solids content <strong>of</strong> <strong>the</strong> sludge depends on various parameters, which determine, e.g., <strong>the</strong><br />
floc size <strong>and</strong> density. When treating PM white <strong>water</strong> <strong>and</strong> filtrates, <strong>the</strong> solids content <strong>of</strong> <strong>the</strong><br />
flotation sludge is typically in a range <strong>of</strong> 3%−5%. Similarly to <strong>the</strong> saveall principle, <strong>the</strong> recovered<br />
solids can be returned to <strong>the</strong> process; o<strong>the</strong>rwise <strong>the</strong> sludge is disposed. In <strong>the</strong> latter case, <strong>the</strong><br />
sludge is ei<strong>the</strong>r de<strong>water</strong>ed <strong>and</strong> post-treated, e.g., toge<strong>the</strong>r with o<strong>the</strong>r mill sludges <strong>and</strong> wet rejects,<br />
or it is pumped into an effluent treatment plant. In some instances, <strong>the</strong> solids content <strong>of</strong> <strong>the</strong><br />
flotation sludge is controlled by <strong>the</strong> speed <strong>of</strong> <strong>the</strong> scoop or scraper.<br />
Finally, by applying special measurements to determine, for example, charge or turbidity, <strong>the</strong><br />
chemicals dosage <strong>and</strong> <strong>the</strong> operation <strong>and</strong> efficiency <strong>of</strong> <strong>the</strong> DAF unit can be controlled <strong>and</strong><br />
improved.<br />
5.5.3 Classification<br />
Screening or fractionation <strong>of</strong> white <strong>water</strong> in order to recover fibers <strong>and</strong> fines is not feasible due to<br />
<strong>the</strong> large flow to be treated <strong>and</strong> due to relatively high dem<strong>and</strong>s in fines recovery.<br />
Inclined screens, i.e., curved or bow screens, which are sometimes called scalping screens,<br />
are used for various applications including fiber recovery, thickening, or protecting downstream<br />
operations from fibers or debris. Inclined screens have no moving parts, except an oscillating<br />
cleaning shower, which removes plugging material <strong>and</strong> which prevents buildup <strong>and</strong> blinding by<br />
slime or scaling. Washing is <strong>of</strong>ten realized by backspraying, flushing nozzles, or by a combination<br />
<strong>of</strong> both. Screen plate design <strong>and</strong> slot size depend on <strong>the</strong> application. Inclined screens require a<br />
constant feed flow, which can also be pressurized, <strong>and</strong> have a limited hydraulic capacity.<br />
Microscreens <strong>and</strong> shower <strong>water</strong> screens are used to post-treat clear <strong>and</strong> super-clear disc<br />
filtrate for mid- <strong>and</strong> high-pressure shower applications. Microscreens are sometimes called<br />
"polishing filters." In contrast to drum or disc filters, <strong>the</strong> type <strong>of</strong> medium determines <strong>the</strong> separation<br />
<strong>of</strong> <strong>the</strong> solid <strong>and</strong> <strong>the</strong> filtrate phases, although <strong>the</strong> design <strong>of</strong> <strong>the</strong> screen can be ra<strong>the</strong>r similar to<br />
filtration.<br />
If contaminated white <strong>water</strong> is fed to a microscreen, inclined screens are <strong>of</strong>ten installed<br />
upsteam to retain coarse particles from entering in order to avoid damage <strong>of</strong> <strong>the</strong> filter medium.<br />
Even at a low solids concentration, as low as 0.001%, long fibers can be still present in <strong>the</strong><br />
filtrate44, which can plug needle jets or high-pressure spray showers. Showers particularly<br />
sensitive to coarse particles are <strong>the</strong>refore protected by a screen, which in this respect is called a<br />
"police filter."<br />
Membrane filtration, in particular ultra- <strong>and</strong> nano-filtration, is discussed in <strong>the</strong> section about<br />
advanced <strong>water</strong> treatment.<br />
5.5.4 Sedimentation<br />
Settling chests <strong>and</strong> hopper-bottomed tanks were commonly used before saveall equipment like<br />
disc filters was introduced. The clarification <strong>of</strong> PM white <strong>water</strong> by sedimentation has several<br />
disadvantages, which make its application as process <strong>water</strong> purification unit unpractical. Some <strong>of</strong><br />
<strong>the</strong>se disadvantages are:<br />
- Long retention time<br />
- Low concentration <strong>of</strong> recovered fibers<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
- Fouling <strong>and</strong> microbiological activity causing slime <strong>and</strong> deposits<br />
- Heat loss<br />
- Sensitivity to changes in hydraulic load<br />
- Requiring a larger amount <strong>of</strong> <strong>water</strong> circulating in <strong>the</strong> process.<br />
Sedimentation units are used in effluent treatment.<br />
The use <strong>of</strong> flocculating agents in DAF can form aggregates which are too dense to float <strong>and</strong><br />
thus settle in <strong>the</strong> flotation tank. Therefore, sediment removal is needed from DAF units.<br />
O<strong>the</strong>rwise, sedimentation is not used for PM process <strong>water</strong> treatment in modern mills.<br />
5.6 Mill <strong>water</strong> <strong>systems</strong><br />
5.6.1 Paper mill <strong>water</strong> household<br />
The function <strong>of</strong> <strong>water</strong> in <strong>paper</strong>making is to transport <strong>and</strong> to distribute <strong>the</strong> fibers <strong>and</strong> to consolidate<br />
<strong>the</strong> sheet when <strong>water</strong> is removed from <strong>the</strong> web. In mill operation, <strong>water</strong> is needed to also fulfill<br />
various o<strong>the</strong>r functions, as listed in Fig. 24 under <strong>the</strong> topic "purposes."<br />
In <strong>the</strong> following, only <strong>the</strong> process <strong>water</strong>s are considered, i.e., which is <strong>the</strong> <strong>water</strong> in contact<br />
with <strong>the</strong> stock. Figure 23 shows <strong>the</strong> <strong>paper</strong> mill <strong>water</strong> household in principle. Water enters <strong>the</strong><br />
system as fresh <strong>water</strong> <strong>and</strong> via stock from integrated pulping, which is usually stored at a<br />
concentration <strong>of</strong> 4%−12%. There are also o<strong>the</strong>r minor <strong>water</strong> sources. A small amount <strong>of</strong> <strong>water</strong><br />
enters in <strong>the</strong> form <strong>of</strong> steam, as indicated in Fig. 23, which comprises steam added to <strong>the</strong> wire pit<br />
for temperature control <strong>and</strong> possibly used for <strong>the</strong> preparation <strong>of</strong> additives. In <strong>the</strong> modern<br />
integrated mill, <strong>the</strong> effluent from <strong>the</strong> PM to <strong>the</strong> sewer is small if not zero. This means that most, if<br />
not all, surplus <strong>water</strong> is possibly used upstream in integrated mechanical pulping or in deinking.<br />
Fresh <strong>water</strong> is entering <strong>the</strong> integrated mill mainly at <strong>the</strong> PM; see also Fig. 25.<br />
Figure 23. Paper mill <strong>water</strong> household, system principle.<br />
Matching supply <strong>and</strong> dem<strong>and</strong> <strong>of</strong> <strong>water</strong> reduces purging <strong>and</strong> process <strong>water</strong> supplement by<br />
fresh <strong>water</strong>; hence, it reduces <strong>water</strong> consumption as well as <strong>the</strong> amount <strong>of</strong> effluent. Figure 24<br />
shows <strong>the</strong> principle to improve <strong>water</strong> usage. In a <strong>water</strong> conservation program, <strong>the</strong> first step is to<br />
determine <strong>the</strong> minimum <strong>water</strong> quality required for each particular purpose. O<strong>the</strong>r <strong>water</strong> than<br />
process <strong>water</strong> is kept in separate <strong>water</strong> <strong>systems</strong>, e.g., cooling <strong>water</strong> in <strong>the</strong> cooling <strong>water</strong> system,<br />
which can also circulate in <strong>the</strong>re, via a cooling tower if needed. Uncontaminated <strong>water</strong> can be<br />
discharged from <strong>the</strong> mill without fur<strong>the</strong>r treatment, hence, without loading <strong>the</strong> effluent treatment<br />
plant hydraulically. Instead <strong>of</strong> discharging <strong>the</strong> uncontaminated nonprocess <strong>water</strong>, it can be added<br />
to <strong>the</strong> process as fresh <strong>water</strong>, which decreases <strong>the</strong> total <strong>water</strong> consumption <strong>of</strong> <strong>the</strong> mill.<br />
Figure 24. Working cycle for segregated <strong>and</strong> optimized internal mill <strong>water</strong> use.<br />
Process <strong>water</strong> is stored as clear <strong>water</strong> or as white <strong>water</strong>. In <strong>the</strong> normal operation <strong>of</strong> <strong>the</strong> PM,<br />
all white <strong>water</strong> passes <strong>the</strong> saveall <strong>and</strong> only clear filtrate is stored. If two process <strong>water</strong> storage<br />
towers are available, one is used for filtrate exclusively, while <strong>the</strong> o<strong>the</strong>r one might also contain<br />
white <strong>water</strong>. In this way, <strong>the</strong> best separation <strong>of</strong> <strong>water</strong>s <strong>of</strong> different quality is achieved. However,<br />
<strong>the</strong> coupling <strong>of</strong> <strong>the</strong> towers <strong>and</strong> <strong>the</strong> filtrate tanks should allow flexibility. Process <strong>water</strong> storage<br />
towers are always equipped with agitators, <strong>and</strong> <strong>the</strong>y are <strong>of</strong>ten insulated. A rule <strong>of</strong> thumb says that<br />
<strong>the</strong> process <strong>water</strong> storage should have <strong>the</strong> same volume as <strong>the</strong> total pulp storage. Ano<strong>the</strong>r rule<br />
<strong>of</strong> thumb determines <strong>the</strong> broke storage as 1.5 times <strong>the</strong> longest sustained trouble period that is<br />
acceptable without taking in fresh <strong>water</strong>46.<br />
A lack <strong>of</strong> sufficient process <strong>water</strong> storage means that a large amount <strong>of</strong> fresh <strong>water</strong> has to be<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
added during startups <strong>and</strong> web breaks compared to normal PM operation. This can cause<br />
process instability. Fresh <strong>water</strong> can be adjusted in temperature <strong>and</strong> pH, but <strong>the</strong> content <strong>of</strong><br />
suspended <strong>and</strong> dissolved solids is different. This can imbalance wet end chemistry; variations<br />
imposed on <strong>the</strong> effluent treatment plant are <strong>of</strong>ten increased. The fresh <strong>water</strong> to supplement<br />
lacking process <strong>water</strong> is heated to <strong>the</strong> process <strong>water</strong> temperature <strong>and</strong> added to <strong>the</strong> process at<br />
one location only, i.e., <strong>the</strong> white <strong>water</strong> tank. This location is processwise distant from <strong>the</strong><br />
headbox; thus, a possible impact on <strong>the</strong> wet end chemistry is attenuated. Similarly, consistency<br />
variations in <strong>the</strong> dilution <strong>water</strong> header are avoided.<br />
A supply tower for mechanically <strong>and</strong>/or chemically pretreated fresh <strong>water</strong> is useful, for<br />
example, if varying amounts <strong>of</strong> fresh <strong>water</strong> are needed in <strong>the</strong> cold season <strong>of</strong> <strong>the</strong> year. In this<br />
case, fresh <strong>water</strong> circulates from <strong>the</strong> tower through a heating loop utilizing recovered heat from<br />
<strong>the</strong> PM hood exhaust or from <strong>the</strong> mechanical pulping heat recovery system, if available.<br />
O<strong>the</strong>rwise, steam is injected. For many grades, <strong>the</strong> optimum stock temperature is found within a<br />
range <strong>of</strong> 46°C to 54°C (115°F to 130°F)47. Higher temperatures <strong>of</strong> up to over 60°C (140°F) limit<br />
microbiological growth significantly, while fungal growth stops already when exceeding 50°C<br />
(122°F)48.<br />
5.6.2 Fresh <strong>water</strong> use<br />
As shown in Table 6, <strong>the</strong> specific fresh <strong>water</strong> consumption <strong>of</strong> modern mills is in a range <strong>of</strong> 2−20<br />
L/kg <strong>paper</strong>49,50 with <strong>the</strong> exception <strong>of</strong> technical or specialty <strong>paper</strong> mills which might consume up<br />
to 100 L/kg or more. In older mills, <strong>the</strong> fresh <strong>water</strong> consumption can be higher due to suboptimal<br />
process design. Table 6 shows average consumption figures in 197151 to indicate <strong>the</strong> progress in<br />
<strong>the</strong> reduction <strong>of</strong> fresh <strong>water</strong> consumption. The large reduction in <strong>the</strong> last decades was possible<br />
due to <strong>the</strong> introduction <strong>of</strong> saveall equipment, <strong>the</strong> installation <strong>of</strong> sufficient process <strong>water</strong> storage,<br />
<strong>the</strong> use <strong>of</strong> more efficient <strong>machine</strong>ry, <strong>the</strong> separation <strong>of</strong> streams <strong>of</strong> different <strong>water</strong> quality, <strong>and</strong> an<br />
all-over improved process design.<br />
Table 6. Typical specific fresh <strong>water</strong> consumption <strong>of</strong> modern <strong>paper</strong> mills in comparison to<br />
<strong>the</strong> situation in 197151.<br />
Paper grade Today Today 1971a 1971a<br />
L/kg gal/lb L/kg gal/lb<br />
Newsprint<br />
5−15<br />
0.6−1.8<br />
85<br />
10<br />
Wood-free fine<br />
5−10<br />
0.6−1.2<br />
180<br />
22<br />
<strong>paper</strong><br />
10−15<br />
1.2−1.8<br />
120b<br />
14b<br />
Supercalendered<br />
10−20<br />
1.2−2.4<br />
−<br />
−<br />
(SC) <strong>paper</strong><br />
5−15<br />
0.6−1.8<br />
290<br />
35<br />
Lightweight<br />
2−10<br />
0.25−1.2<br />
40−85<br />
5−10<br />
coated (LWC)<br />
8−15<br />
1−1.8<br />
130<br />
16<br />
<strong>paper</strong><br />
Tissue<br />
Liner <strong>and</strong> fluting<br />
Multiply board<br />
a Mills in Sweden<br />
b "Magazine <strong>paper</strong>" 51<br />
The driving forces to reduce <strong>the</strong> fresh <strong>water</strong> consumption are:<br />
- Legislation <strong>and</strong> permissions in respect to ei<strong>the</strong>r <strong>the</strong> fresh <strong>water</strong> consumption or <strong>the</strong> effluent<br />
discharge<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
- Cost:<br />
- Fresh <strong>water</strong> <strong>and</strong> its treatment- Effluent treatment <strong>and</strong> possibly effluent discharge cost- Material<br />
savings: fibers, fines, <strong>and</strong> filler- Energy savings<br />
- Fresh <strong>water</strong> availability<br />
- Higher process stability, if fresh <strong>water</strong> reduction means a reduction in process <strong>water</strong><br />
supplement.<br />
A mill is called closed if no process <strong>water</strong> is discharged. Due to process inherent evaporation,<br />
mainly in <strong>the</strong> drying section <strong>of</strong> <strong>the</strong> PM, <strong>and</strong> due to <strong>the</strong> discharge <strong>of</strong> <strong>water</strong> contained in cleaning<br />
<strong>and</strong> screening rejects, a certain input <strong>of</strong> fresh <strong>water</strong> is always needed. This amount is smaller <strong>the</strong><br />
more <strong>water</strong> enters with moist raw materials, pigment slurry, chemicals, etc., cf. Fig. 23. If a ra<strong>the</strong>r<br />
dry raw material like recycled fiber is used, <strong>the</strong>n <strong>the</strong> specific fresh <strong>water</strong> consumption is about 2<br />
L/kg in a closed mill operation. In this case, <strong>the</strong> amount <strong>of</strong> <strong>water</strong> evaporated at <strong>the</strong> PM drying<br />
section is about 50% <strong>of</strong> <strong>the</strong> fresh <strong>water</strong> intake. Effluent-free <strong>paper</strong>making is possible, but <strong>the</strong><br />
system stability <strong>and</strong> PM runnability are likely reduced <strong>and</strong> quality problems can occur if process<br />
design <strong>and</strong> internal <strong>water</strong> purification are inadequate. Fresh <strong>water</strong> is used for selected shower<br />
applications at <strong>the</strong> PM, <strong>and</strong> no fresh <strong>water</strong> is consumed to supplement process <strong>water</strong>.<br />
Table 7 shows <strong>the</strong> shower <strong>water</strong> need <strong>of</strong> a wide PM52. Fresh <strong>water</strong> can be replaced by<br />
cleaned process <strong>water</strong>, if <strong>the</strong> content <strong>of</strong> suspended solids is sufficiently low, <strong>and</strong> if detrimental<br />
substances do not deposit or precipitate, which can plug <strong>the</strong> orifices <strong>of</strong> showers, cf. Table 853.<br />
High-pressure showers are operated with fresh <strong>water</strong> for that reason. Fresh <strong>water</strong> is also used at<br />
o<strong>the</strong>r locations, where dissolved organic <strong>and</strong> inorganic compounds can cause problems or where<br />
scaling is harmful. As already mentioned, up to 10%−15% <strong>of</strong> fresh <strong>water</strong> is needed for<br />
preparation <strong>and</strong> dilution <strong>of</strong> chemicals.<br />
Table 7. Shower <strong>water</strong> dem<strong>and</strong> <strong>of</strong> a modern 9-m-wide <strong>paper</strong> <strong>machine</strong>52.<br />
Wire section<br />
Warm fresh <strong>water</strong><br />
300 kPa<br />
3 500 kPa<br />
55 L/s (14.5 gal/min)<br />
30 L/s (7.9 gal/min)<br />
25 L/s (6.6 gal/min)<br />
In total, <strong>of</strong> which:<br />
Washing <strong>and</strong> lubrication<br />
High-pressure showers<br />
Post-treated clear filtrate<br />
300 kPa<br />
1 200 kPa<br />
80 L/s (21 gal/min)<br />
70 L/s (18.5 gal/min)<br />
10 L/s (2.5 gal/min)<br />
In total, <strong>of</strong> which:<br />
Former <strong>and</strong> roll showers<br />
Trim knock-<strong>of</strong>f <strong>and</strong> edge<br />
showers<br />
Additionally during breaks 60 L/s (15.9 gal/min) Knock-<strong>of</strong>f<br />
Condensate 1 L/s (0.3 gal/min) Trim squirt, roll edge<br />
moistening<br />
Press section<br />
Warm fresh <strong>water</strong><br />
300 kPa<br />
3 500 kPa<br />
35 L/s (9.2 gal/min)<br />
25 L/s (6.6 gal/min)<br />
10 L/s (2.5 gal/min)<br />
In total, <strong>of</strong> which:<br />
Washing <strong>and</strong> lubrication<br />
High-pressure showers<br />
Post-treated clear filtrate<br />
300 kPa<br />
55 L/s (14.5 gal/min)<br />
55 L/s (14.5 gal/min)<br />
In total, <strong>of</strong> which:<br />
Internal roll washing<br />
Table 8. Shower <strong>water</strong> quality dem<strong>and</strong>s53.<br />
Solids load<br />
< 50 ppm<br />
50−75 ppm<br />
75−100 ppm<br />
Possible application <strong>of</strong> <strong>water</strong><br />
Equivalent to filtered fresh <strong>water</strong><br />
Usable in ≥1 mm orifice<br />
Usable in ≥1.5 mm orifice<br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
100−200 ppm<br />
Usable in ≥3 mm orifice<br />
200−500 ppm<br />
Brush type shower recommended<br />
> 500 ppm<br />
Purgable shower recommended<br />
5.6.2.1 Dissolved <strong>and</strong> detrimental substances<br />
Detrimental substances are non-ionic <strong>and</strong> anionic dissolved <strong>and</strong> colloidal substances54,55.<br />
Anionic trash, which can be determined by <strong>the</strong> cationic dem<strong>and</strong>, is <strong>the</strong>refore a subgroup <strong>of</strong><br />
detrimental substances; it has been shown that non-ionic substances can also be detrimental.<br />
Anionic trash especially consumes retention aids <strong>and</strong> thus decreases <strong>the</strong> PM wire retention.<br />
Detrimental substances can adsorb or precipitate onto <strong>the</strong> surfaces <strong>of</strong> fibers, fillers, <strong>and</strong> fines,<br />
which adversely affects fiber-to-fiber bonding, brightness, <strong>and</strong> <strong>the</strong> accessibility <strong>of</strong> process<br />
chemicals55. Temperature has important influence on <strong>the</strong>se sorption kinetics. Dissolved <strong>and</strong><br />
chemically active substances:<br />
- Enter with all raw materials, including fresh <strong>water</strong><br />
- Are created by operations in integrated pulping, e.g., by bleaching or deinking<br />
- Are created by <strong>paper</strong> surface treatment entering via <strong>the</strong> broke system.<br />
Table 9 lists <strong>the</strong> composition <strong>of</strong> detrimental substances according to <strong>the</strong>ir origin55.<br />
Table 9. Composition <strong>and</strong> origin <strong>of</strong> detrimental substances55.<br />
Chemical compound(s)<br />
Origin<br />
Sodium silicate<br />
Peroxide bleaching, deinking, recovered <strong>paper</strong><br />
Polyphosphate<br />
Filler dispersing agent<br />
Polyacrylate<br />
Filler dispersing agent<br />
Organic acids<br />
Pitch dispersing agent<br />
Carboxymethylcellulose<br />
Coated broke<br />
Starch<br />
Recovered <strong>paper</strong>, broke, streng<strong>the</strong>ning agents<br />
Humic acids<br />
Fresh <strong>water</strong><br />
Lignin derivatives<br />
Kraft pulp, mechanical pulp<br />
Lignosulfonates<br />
Sulfite <strong>and</strong> NSSC pulp, CTMP<br />
Hemicelluloses<br />
Mechanical pulp<br />
Fatty acids<br />
Mechanical pulp<br />
Organic dissolved <strong>and</strong> colloidal substances (DCS) are an excellent nutrient for microbe<br />
populations to propagate fast at usually favorable process <strong>water</strong> temperature. According to <strong>the</strong><br />
level <strong>of</strong> dissolved oxygen in process <strong>water</strong>, ei<strong>the</strong>r aerobe or anaerobe microbiological activity<br />
causes a decrease in <strong>the</strong> system cleanliness by slime <strong>and</strong> smell. Slime deposits by anaerobic<br />
micro-organisms propagate corrosion in particular. This can also occur with <strong>the</strong> presence <strong>of</strong><br />
oxygen because anaerobic conditions are reached on <strong>the</strong> metal surface underneath deposits <strong>of</strong><br />
aerobic micro-organisms56.<br />
Inorganic dissolved substances, namely salts, are also deteriorating <strong>the</strong> process performance<br />
<strong>and</strong> potentially <strong>the</strong> product properties. A high content <strong>of</strong> electrolytes, in particular chloride,<br />
increases <strong>the</strong> potential for corrosion. According to <strong>the</strong> osmotic equilibrium, <strong>the</strong> swelling <strong>of</strong> fibers<br />
decreases with an increased content <strong>of</strong> electrolytes in <strong>the</strong> process <strong>water</strong>; hence, beatability <strong>and</strong><br />
<strong>the</strong> behavior <strong>of</strong> fibers in web consolidation deteriorate to some extent.<br />
In mill practice, a rough estimate <strong>of</strong> <strong>the</strong> amount <strong>of</strong> detrimental substances is obtained by <strong>the</strong><br />
chemical oxygen dem<strong>and</strong> (COD). O<strong>the</strong>r values, like <strong>the</strong> amount <strong>of</strong> dissolved organic compounds<br />
(DOC) can also be determined on-line. Particle charge on-line titration can be applied to samples<br />
from <strong>the</strong> thick stock flow as well as to low concentration stock. Sensors measuring <strong>the</strong><br />
zeta-potential, turbidity, <strong>and</strong> <strong>the</strong> amount <strong>of</strong> total organic compounds (TOC) can give additional<br />
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information. The effect <strong>of</strong> salts in <strong>the</strong> process <strong>water</strong> can be monitored on-line by measuring <strong>the</strong><br />
conductivity <strong>of</strong> <strong>the</strong> fluid. Specific metal cations <strong>and</strong> some anions are usually determined in <strong>the</strong><br />
laboratory; only a few are measured on-line, for example, calcium, aluminum, <strong>and</strong> silicate. See<br />
also Table 4.<br />
5.6.2.2 Accumulation mechanisms<br />
Continuous feed or generation <strong>of</strong> a substance within a process with internal circulation causes<br />
this substance to accumulate. This means that <strong>the</strong> concentration in <strong>the</strong> circulation loop increases<br />
reaching levels <strong>the</strong> higher <strong>the</strong> less effluent is discharged, which is sometimes described by <strong>the</strong><br />
(misleading) term "degree <strong>of</strong> closure"57. Each organic <strong>and</strong> inorganic dissolved compound<br />
eventually reaches equilibrium. The equilibrium appears earlier <strong>and</strong> a lower concentration level is<br />
reached if part <strong>of</strong> <strong>the</strong> substance is consumed in chemical reactions by a phase change like<br />
precipitation, by flocculation, by microbiological degradation, or in some o<strong>the</strong>r way. The particular<br />
equilibrium <strong>of</strong> each substance is determined by <strong>the</strong> process conditions, which can be monitored<br />
by measuring, for example temperature, pH, <strong>and</strong> charge density. If a substance is not consumed,<br />
a higher level <strong>of</strong> concentration is reached, which is governed by <strong>the</strong> streams discharged from <strong>the</strong><br />
process.<br />
In order to show trends <strong>and</strong> possible levels <strong>of</strong> accumulation, so-called "enrichment" or<br />
"accumulation" curves are plotted, which are <strong>of</strong>ten based on short circulation balance<br />
calculations. For example, <strong>the</strong> term "enrichment factor" has been defined as <strong>the</strong> ratio <strong>of</strong> <strong>the</strong><br />
headbox concentration <strong>of</strong> a given substance at a certain degree <strong>of</strong> white <strong>water</strong> recycling to that at<br />
zero-recycling58. To study <strong>the</strong> accumulation <strong>of</strong> detrimental or o<strong>the</strong>r substances, <strong>the</strong> simple short<br />
circulation model has to be extended to <strong>the</strong> entire mill <strong>water</strong> system, as shown below in Fig. 26.<br />
In order to gain a better underst<strong>and</strong>ing <strong>of</strong> mill <strong>water</strong> <strong>systems</strong>, it can be useful to incorporate<br />
adsorption parameters into model calculations to study, e.g., wire retention or <strong>the</strong> concentration<br />
<strong>of</strong> chemical aids59, or pursuit accumulation mechanisms by dynamic simulation60. Table 10<br />
summarizes <strong>the</strong> disadvantages <strong>of</strong> decreased fresh <strong>water</strong> use.<br />
Table 10. Disadvantages <strong>of</strong> decreased fresh <strong>water</strong> consumption61.<br />
Increase in suspended<br />
Increase in dissolved<br />
Increase in temperature<br />
solids<br />
material<br />
Blocking <strong>of</strong> showers<br />
Scale deposits<br />
Sizing problems<br />
Increase <strong>of</strong> fines <strong>and</strong> change<br />
Alteration <strong>of</strong> wet end<br />
Reduction in vacuum pump<br />
in retention<br />
chemistry<br />
capacity<br />
Spots <strong>and</strong> dirt in <strong>the</strong> product<br />
Increase <strong>of</strong> biological activity<br />
Increase <strong>of</strong> biological activity<br />
Deposit formation<br />
Deposit formation<br />
(at low temperature)<br />
Abrasion<br />
Corrosion<br />
Reduction <strong>of</strong> wire life<br />
Color, smell<br />
The PM headbox should always be <strong>the</strong> reference point for evaluating changes in <strong>water</strong><br />
quality <strong>and</strong> in <strong>the</strong> load by detrimental substances62. The following points have relevance in<br />
respect to accumulation <strong>of</strong> detrimental substances <strong>and</strong> system stability:<br />
- Correct choice <strong>of</strong> source for <strong>water</strong> to discharge from <strong>the</strong> mill as effluent<br />
- Correct selection <strong>of</strong> <strong>water</strong> to be treated by advanced purification<br />
- Suitable application <strong>of</strong> <strong>the</strong> purified <strong>water</strong> in <strong>the</strong> process.<br />
While savealls improve process <strong>water</strong> reuse, advanced <strong>water</strong> purification techniques are<br />
needed just as well in order to remove dissolved material, if <strong>the</strong> fresh <strong>water</strong> consumption <strong>and</strong> <strong>the</strong><br />
discharge <strong>of</strong> effluent should be reduced fur<strong>the</strong>r.<br />
5.6.2.3 Vertically integrated mills<br />
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Many <strong>paper</strong> mills are connected to a pulp plant, producing chemical or mecha--nical pulp or<br />
processing recovered <strong>paper</strong>. The interface to <strong>the</strong> <strong>paper</strong> mill is usually a medium- or sometimes a<br />
high-consistency storage, from where <strong>the</strong> pulp is picked up by dilution <strong>water</strong> from <strong>the</strong> <strong>paper</strong> mill.<br />
The reduction in fresh <strong>water</strong> consumption in an existing process has to be accompanied by<br />
two means in order to keep <strong>the</strong> load <strong>of</strong> detrimental substances low in <strong>the</strong> headbox. On one h<strong>and</strong>,<br />
<strong>the</strong> increase <strong>of</strong> <strong>the</strong> transfer consistency at <strong>the</strong> interface between pulping <strong>and</strong> <strong>paper</strong> mill<br />
decreases <strong>the</strong> carryover <strong>of</strong> detrimental substances that are created in pulping <strong>and</strong> bleaching. On<br />
<strong>the</strong> o<strong>the</strong>r h<strong>and</strong>, fresh <strong>water</strong> is used in high-pressure showers <strong>and</strong> o<strong>the</strong>r selected locations at <strong>the</strong><br />
PM <strong>and</strong> if needed for preparation <strong>and</strong> dilution <strong>of</strong> chemicals. Fur<strong>the</strong>r on, effluent is not discharged<br />
from <strong>the</strong> PM system, but surplus PM filtrate is added to <strong>the</strong> white <strong>water</strong> system in <strong>the</strong> pulping<br />
plant. Figure 25 shows this principle. The separation <strong>of</strong> <strong>the</strong> pulping <strong>and</strong> <strong>paper</strong> mill <strong>water</strong> <strong>systems</strong><br />
leads to a higher dissolved matter concentration in <strong>the</strong> upstream <strong>water</strong> loops, from where all<br />
effluent <strong>of</strong> <strong>the</strong> integrated mill is discharged.<br />
Figure 25. Vertically integrated mill <strong>water</strong> household, system principle.<br />
Designing <strong>the</strong> integrated mill <strong>water</strong> system according to this counter-current flow principle, or<br />
as a cascade <strong>of</strong> mill <strong>water</strong> loops, results in:<br />
- Decreased load <strong>of</strong> detrimental substances in <strong>the</strong> PM <strong>water</strong><br />
- Reduced effluent flow <strong>and</strong> lower fresh <strong>water</strong> dem<strong>and</strong><br />
- Possibly improved efficiency in pulping, e.g., in deinking63.<br />
The separation <strong>of</strong> <strong>the</strong> <strong>water</strong> <strong>systems</strong> is enhanced fur<strong>the</strong>r, when introducing a "washing" loop<br />
by installing a second de<strong>water</strong>ing press at <strong>the</strong> interface between pulping <strong>and</strong> <strong>the</strong> PM. This is<br />
particularly useful to reduce detrimental substances caused by <strong>the</strong> high alkalinity in peroxide<br />
bleaching <strong>of</strong> deinked or mechanical pulp, which is <strong>of</strong>ten carried out as a final step before<br />
transferring <strong>the</strong> pulp to <strong>the</strong> PM. In addition to organic dissolved <strong>and</strong> colloidal material, supporting<br />
chemicals − like sodium silicate used in deinking <strong>and</strong> in peroxide bleaching − also can be<br />
detrimental. After <strong>the</strong> bleaching tower, pulp is diluted to a lower consistency <strong>and</strong> pumped to a<br />
second de<strong>water</strong>ing press prior to <strong>the</strong> pulp storage. The pressate is fed upstream. Figure 26<br />
shows <strong>the</strong> lower level in detrimental substances at <strong>the</strong> headbox <strong>of</strong> <strong>the</strong> two-press system against<br />
<strong>the</strong> effluent discharge from <strong>the</strong> integrated mill. For comparison, a situation without press<br />
de<strong>water</strong>ing is also shown, assuming a 12% solids content after thickening compared to 35% after<br />
press de<strong>water</strong>ing. The specific load <strong>of</strong> detrimental substances is plotted relative to <strong>the</strong><br />
concentration at <strong>the</strong> kink point <strong>of</strong> <strong>the</strong> two-press curve. At this point, <strong>the</strong> amount <strong>of</strong> surplus PM<br />
filtrate meets exactly <strong>the</strong> amount <strong>of</strong> dilution <strong>water</strong> required to reach <strong>the</strong> desired consistencies at<br />
bleaching <strong>and</strong> pulp storage towers. Reducing fur<strong>the</strong>r <strong>the</strong> amount <strong>of</strong> <strong>the</strong> fresh <strong>water</strong> added to <strong>the</strong><br />
<strong>paper</strong> mill would create a lack <strong>of</strong> PM filtrate in this particular example; hence, it would require<br />
supplement by highly contaminated pulping filtrate. Figure 26 shows clearly that this must not be<br />
done in mill practice without applying advanced <strong>water</strong> purification treatment to this recycle <strong>water</strong>.<br />
On <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>, applying advanced treatment to this stream is most feasible, when closing <strong>the</strong><br />
<strong>water</strong> system <strong>of</strong> <strong>the</strong> integrated mill62.<br />
Figure 26. Accumulation <strong>of</strong> detrimental substances in <strong>the</strong> PM headbox <strong>water</strong> at reduced effluent<br />
discharge for an integrated <strong>paper</strong> mill with different degrees <strong>of</strong> process <strong>water</strong> separation.<br />
5.6.2.4 Horizontally integrated mills<br />
Horizontal integration here means parallel <strong>paper</strong> production lines. Even if parallel <strong>paper</strong><br />
<strong>machine</strong>s were identical, or if <strong>the</strong> same product is produced on both lines, <strong>the</strong> amount <strong>of</strong><br />
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Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
dissolved organic <strong>and</strong> inorganic substances can vary significantly. In most cases, it is feasible to<br />
separate <strong>the</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> neighboring <strong>machine</strong>s, especially if <strong>the</strong> fresh <strong>water</strong> consumption is<br />
low or different for both lines. This requires separated stock preparation, wet broke <strong>systems</strong>,<br />
savealls, <strong>and</strong> process <strong>water</strong> storage for each line. Hence, <strong>the</strong> independent <strong>and</strong> trouble-free<br />
operation <strong>of</strong> ei<strong>the</strong>r line is guaranteed regardless to whe<strong>the</strong>r changes occur on <strong>the</strong> o<strong>the</strong>r line,<br />
ei<strong>the</strong>r in wet end chemistry, in production state, or due to grade change. A case in which<br />
collected mill effluent is re-introduced as process <strong>water</strong> requires sufficient treatment by advanced<br />
<strong>water</strong> purification.<br />
5.6.3 Advanced <strong>water</strong> purification techniques<br />
Advanced <strong>water</strong> purification techniques are applied to <strong>paper</strong> mill process <strong>water</strong> in order to reduce<br />
<strong>the</strong> amount <strong>of</strong> dissolved <strong>and</strong> colloidal substances circulating in <strong>the</strong> process. Advanced<br />
techniques might become feasible or necessary when reducing <strong>the</strong> fresh <strong>water</strong> consumption<br />
below a certain level as described above. Investment <strong>and</strong> operational costs <strong>of</strong> advanced<br />
equipment are reduced by <strong>the</strong> savings from decreased flows through fresh <strong>and</strong> waste<strong>water</strong><br />
treatment.<br />
From <strong>the</strong> many known techniques for advanced <strong>water</strong> purification, <strong>the</strong> following have been<br />
successfully applied to <strong>paper</strong> mill process <strong>water</strong> in industrial scale:<br />
- Biological treatment<br />
- Membrane filtration<br />
- Evaporation.<br />
The selection <strong>of</strong> <strong>the</strong> suitable technique is determined by parameters similar to those for<br />
choosing equipment to remove suspended solids, namely:<br />
- Desired quality <strong>of</strong> purified <strong>water</strong><br />
- Cost:<br />
- Investment cost- Operational cost- Spare parts <strong>and</strong> replacement media; maintenance<br />
- Energy dem<strong>and</strong>, power, <strong>and</strong> steam consumption<br />
- Reliability, availability, <strong>and</strong> responsiveness<br />
- Properties <strong>and</strong> reachable concentration <strong>of</strong> concentrate<br />
- Required feed flow properties <strong>and</strong> pre-treatment<br />
- Uniformity requirements on <strong>the</strong> flow rate <strong>and</strong> properties <strong>of</strong> <strong>the</strong> feed<br />
- Floor space.<br />
The reject stream from advanced treatment consists ei<strong>the</strong>r <strong>of</strong> a concentrate with increased<br />
viscosity or <strong>of</strong> sludge. Required post-treatment <strong>of</strong> this reject stream <strong>and</strong> costs resulting from it are<br />
<strong>of</strong>ten crucial for <strong>the</strong> feasibility <strong>of</strong> a certain technique. Possible fur<strong>the</strong>r uses or treatments <strong>of</strong> <strong>the</strong><br />
concentrate or sludge are:<br />
- Drying <strong>and</strong>/or combustion<br />
- Trading or use elsewhere<br />
- Disposal, if allowed<br />
- Treatment in <strong>the</strong> waste<strong>water</strong> treatment plant<br />
- Re-use within <strong>the</strong> process.<br />
The most frequently applied method for fluid concentrate h<strong>and</strong>ling is combustion64.<br />
5.6.3.1 Membrane filtration<br />
Membrane filtration is used in <strong>the</strong> <strong>paper</strong> industry to purify process <strong>water</strong> or to recover valuable<br />
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material like pigments or latex from coating kitchen effluents. The membrane works as a<br />
molecular screen; thus, <strong>the</strong> properties <strong>of</strong> <strong>the</strong> membrane determine <strong>the</strong> quality <strong>of</strong> <strong>the</strong> cleaned<br />
stream. Table 11 shows classification <strong>of</strong> membrane filtration according to <strong>the</strong> particle cut-<strong>of</strong>f size<br />
in separation. The separation sizes <strong>and</strong> operating pressures in Table 11 are guideline figures.<br />
Table 11. Membrane filtration techniques.<br />
Method Separation size Operating pressure Membrane type<br />
Reverse osmosis<br />
< 1.5 nm<br />
3−6 MPa<br />
Non-porous<br />
(RO)<br />
0.5 nm−7 nm<br />
1−4 MPa<br />
Micro-porous<br />
Nano-filtration (NF)<br />
3 nm−0.1 μm<br />
0.2−1 MPa<br />
Micro-porous<br />
Ultra-filtration (UF)<br />
50 nm−5 μm<br />
0.1−0.4 MPa<br />
Porous<br />
Micr<strong>of</strong>iltration (MF)<br />
Membranes can be shaped in various module types such as tubes, plates or frames, hollow<br />
fibers, monoliths, or spiral wound. Membrane material can also be charged when applied to UF<br />
or RO. The cross-flow principle is generally applied to <strong>paper</strong> mill process <strong>water</strong> to avoid<br />
excessive fouling. Membrane fouling means a decrease in flux due to adsorption, blockage <strong>of</strong><br />
pores, or deposition on <strong>the</strong> membrane surface by gel or layer formation. Periodical or continuous<br />
cleaning is necessary. Good pretreatment <strong>of</strong> <strong>the</strong> feed, like micr<strong>of</strong>iltration, is required. NF <strong>and</strong><br />
especially RO usually post-treat ultra-filtrate. Despite pretreatment <strong>and</strong> cleaning, membranes<br />
have to be replaced every 1−5 years. The specific energy consumption is 0.5−5 kWh/m3(65). The<br />
filtrate quality as well as <strong>the</strong> flux depend on <strong>the</strong> type <strong>of</strong> membrane used <strong>and</strong> on <strong>the</strong> feed. The<br />
reduction in COD depends on <strong>the</strong> contributing amount <strong>of</strong> large molecules <strong>and</strong> colloids. The COD<br />
reduction from PM filtrates by UF is usually in <strong>the</strong> range <strong>of</strong> 20%−40%66,67, but by NF possibly up<br />
to 90%68. NF or RO reduces <strong>the</strong> content <strong>of</strong> ions to some degree.<br />
5.6.3.2 Evaporation<br />
Evaporation is <strong>the</strong> most effective technique to concentrate all nonvolatile substances for removal<br />
from <strong>the</strong> process. The quality <strong>of</strong> <strong>the</strong> condensate is equivalent to fresh <strong>water</strong> quality or better69.<br />
For <strong>paper</strong> mill process <strong>water</strong>, multi-effect (ME) <strong>and</strong> mechanical vapor recompression (MVR)<br />
evaporation are used at mill scale, both employing falling film technology <strong>and</strong> vacuum<br />
evaporation70−72. The feed <strong>water</strong> is usually pretreated by filtration <strong>and</strong>/or dissolved air flotation.<br />
In <strong>the</strong> ME cascade, <strong>the</strong> evaporated vapor is used as heating steam in <strong>the</strong> next lower pressure<br />
effect. The vapor from <strong>the</strong> final effect is condensed by <strong>water</strong> cooling. Waste heat in form <strong>of</strong><br />
secondary or low-pressure steam is used for heating. According to <strong>the</strong> number <strong>of</strong> effects <strong>and</strong> <strong>the</strong><br />
amount <strong>of</strong> heating, a final contaminant concentration <strong>of</strong> 50% or higher is achieved. In MVR, a fan<br />
or compressor to be used as <strong>the</strong> heating medium recompresses <strong>the</strong> vapor. No steam or cooling<br />
<strong>water</strong> is required. By using polymeric film as heat-exchanging material, a large heating surface is<br />
provided at low investment cost <strong>and</strong> relatively low power requirements. For <strong>the</strong> latter, <strong>the</strong> specific<br />
energy consumption is 8−10 kWh/m3(70), <strong>and</strong> <strong>the</strong> concentrate flow is about 10% <strong>of</strong> <strong>the</strong> feed flow.<br />
Post-treatment <strong>of</strong> <strong>the</strong> MVR condensate by ME evaporation or direct steam concentration is<br />
needed to reach a higher concentration level.<br />
The COD removal efficiency by evaporation is about 95%, <strong>and</strong> <strong>the</strong> removal <strong>of</strong> electrolytes is<br />
even better. The removal <strong>of</strong> organic material can be improved by stripping volatile organic<br />
compounds at <strong>the</strong> evaporator <strong>and</strong> by treating <strong>the</strong> foul condensate, e.g., in an anaerobic<br />
reactor71. Evaporation <strong>of</strong> volatile low-molecular organic acids can be reduced by increasing <strong>the</strong><br />
pH.<br />
5.6.3.3 Biological in-process treatment<br />
Paper process <strong>water</strong> is loaded with non-toxic substances, <strong>the</strong> majority <strong>of</strong> which are<br />
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carbohydrates. It is <strong>the</strong>refore well suited for biological degradation. The type <strong>of</strong> treatment<br />
required does not differ appreciably from that used for ordinary <strong>paper</strong> mill effluent treatment.<br />
Contrary to waste<strong>water</strong> treatment, <strong>the</strong> biological treatment becomes part <strong>of</strong> <strong>the</strong> manufacturing<br />
process, which requires <strong>the</strong> process design optimized for reliability <strong>and</strong> responsiveness.<br />
Especially for mills without effluent discharge, anaerobic treatment is feasible to degrade <strong>the</strong><br />
highly concentrated dissolved organic substances in <strong>the</strong> process <strong>water</strong>. To avoid dissipation <strong>of</strong><br />
anaerobic conditions into <strong>the</strong> <strong>paper</strong> production process, an aeration tank follows <strong>the</strong> anaerobic<br />
reactor. The <strong>water</strong> cleaned in a clarification tank after aeration can be purified fur<strong>the</strong>r by s<strong>and</strong><br />
filtration or directly by membrane filtration. Biological <strong>water</strong> treatment is not efficient to remove<br />
colored substances from <strong>water</strong>, especially from mechanical pulping filtrates. An integrated<br />
post-treatment by membrane filtration73 can be useful to remove color <strong>and</strong> to detain<br />
microorganisms from entering into <strong>the</strong> <strong>paper</strong> process if required74. Sludge can be recycled back<br />
into <strong>the</strong> process as furnish used in brown or gray liner <strong>and</strong> board production75,76.<br />
Compared to <strong>the</strong> o<strong>the</strong>r techniques, biological treatment does not require prior solids removal.<br />
Anaerobic micro-organisms are sensitive against unsuitable temperature <strong>and</strong> pH, as well as<br />
against major variations in nutrition feed. The advantages compared to aerobic cultures are a<br />
higher degradation efficiency <strong>of</strong> organic material if fed at high concentration, about ten times<br />
lower generation <strong>of</strong> sludge, <strong>and</strong> possible energy recovery from produced biogas. The reduction in<br />
BOD, especially in a combined aerobic <strong>and</strong> anaerobic system, is naturally high, typically up to<br />
95%−99%. Depending on <strong>the</strong> amount <strong>of</strong> biodegradable material contributing to <strong>the</strong> COD, a<br />
reduction <strong>of</strong> 90% in COD is also reachable. Precipitation occurring in biological treatment<br />
reduces <strong>the</strong> content <strong>of</strong> dissolved inorganic material like calcium <strong>and</strong> sulfate to some extent.<br />
5.7 Novel approaches <strong>and</strong> possible future<br />
The major challenges for improving <strong>the</strong> PM stock <strong>and</strong> <strong>water</strong> system are good control <strong>of</strong> <strong>the</strong> wet<br />
end chemistry <strong>and</strong> prediction <strong>and</strong> management <strong>of</strong> circulating fines <strong>and</strong> filler. Variations in <strong>the</strong><br />
form <strong>of</strong> responses upon changes are overlaying due to different propagation times through <strong>the</strong><br />
system. Response times are in <strong>the</strong> range <strong>of</strong> minutes in short circulation <strong>and</strong> <strong>of</strong> several tens <strong>of</strong><br />
minutes, up to hours, in long circulation. An increase in tank sizes cannot well attenuate such low<br />
frequency variations. Contrary to that, reducing <strong>the</strong> amount <strong>of</strong> circulating stock <strong>and</strong> <strong>water</strong> in <strong>the</strong><br />
approach flow system will improve attenuation by shorter circulation times <strong>and</strong>, thus, reduce <strong>the</strong><br />
system reaction time. This allows faster grade changes <strong>and</strong> reduces <strong>the</strong> amount <strong>of</strong> broke, which<br />
is <strong>the</strong> more important <strong>the</strong> faster <strong>the</strong> <strong>machine</strong> runs.<br />
The volume <strong>of</strong> stock <strong>and</strong> <strong>water</strong> should be reduced in <strong>the</strong> short circulation to improve system<br />
reaction upon changes because sufficient process <strong>water</strong> storage capacity is needed in <strong>the</strong> long<br />
circulation to achieve a low fresh <strong>water</strong> consumption. The novel approaches77−80 require air-free<br />
dilution <strong>water</strong>. Accurate on-line measurements <strong>of</strong> consistency are also necessary <strong>and</strong>,<br />
additionally, on-line measurements are required <strong>of</strong> fines <strong>and</strong> filler content <strong>and</strong> <strong>of</strong> fiber properties<br />
in order to allow reliable feed-forward control81. By in-pipe blending <strong>and</strong> dilution <strong>of</strong> well-controlled<br />
<strong>and</strong> air-free streams, chests might become obsolete in <strong>the</strong> stock approach flow system, namely<br />
blend chest, <strong>machine</strong> chest, <strong>and</strong> wire pit. Air-free dilution <strong>water</strong> is provided by using ei<strong>the</strong>r a<br />
centrifugal deaeration pump (see Fig. 17) or a deaeration tank for <strong>the</strong> dilution <strong>water</strong>. The use <strong>of</strong><br />
deaerated dilution <strong>water</strong> instead <strong>of</strong> deaerating <strong>the</strong> entire <strong>machine</strong> stock has <strong>the</strong> following<br />
additional advantages:<br />
- Increased <strong>and</strong> stabilized cleaner consistency, due to possible air-free post-dilution<br />
- Improved attenuation <strong>of</strong> variations due to applied flow-lag principle, cf. Fig. 13, by<br />
post-dilution<br />
- Improved system cleanliness <strong>and</strong> pumping efficiency in an air-free dilution <strong>water</strong> system.<br />
Papermaking Part 1, Stock Preparation <strong>and</strong> Wet End - Page 42
Chapter 5 Stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> <strong>paper</strong> <strong>machine</strong><br />
The removal <strong>of</strong> <strong>the</strong> blend <strong>and</strong> <strong>machine</strong> chests from <strong>the</strong> mill layout requires a constant level in<br />
all stock proportioning chests <strong>and</strong> consistence constancy. This can be achieved by stock<br />
circulation between <strong>the</strong> proportioning chest <strong>and</strong> <strong>the</strong> dilution zone <strong>of</strong> <strong>the</strong> pulp storage tower79.<br />
Operation without wire pit <strong>and</strong> short circulation with a centrifugal deaeration pump showed on a<br />
pilot <strong>machine</strong> an about 10 times improved stabilization time <strong>and</strong> 2−4 times faster basis weight<br />
adjustment on a grade change82.<br />
Examples <strong>of</strong> stock <strong>and</strong> <strong>water</strong> <strong>systems</strong> <strong>of</strong> modern <strong>machine</strong>s producing different <strong>paper</strong> <strong>and</strong><br />
board grades are given in Chapter 1.<br />
References<br />
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Eds.), European Commission, Luxembourg, 1997, Chap. 2.<br />
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64. Lilja, K. <strong>and</strong> Ullman, P., "Evaporation in pulp <strong>and</strong> <strong>paper</strong> mill <strong>water</strong> pollution control −<br />
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Papermaking Part 1, Stock Preparation <strong>and</strong> Wet End - Page 46
Figure 1. Stock preparation, system principle.<br />
Figure 2. Short circulation, system principle.
Figure 3. Stock approach flow system <strong>of</strong> a fine <strong>paper</strong> <strong>machine</strong> (refer to text<br />
for itemization <strong>of</strong> position numbers).<br />
Figure 4. (a.) Short circulation model. (b.) Dynamic response for different<br />
retention values R as a function <strong>of</strong> normalized time 1 .
Figure 5. Frequency b<strong>and</strong>s <strong>of</strong> hydraulic pulsation sources in <strong>the</strong> short<br />
circulation.
Figure 6. Stock Sankey diagram <strong>of</strong> supercalendered (SC) <strong>paper</strong> <strong>machine</strong> at<br />
design production.
Figure 7. Water Sankey diagram <strong>of</strong> supercalendered (SC) <strong>paper</strong> <strong>machine</strong> at<br />
design production.
Figure 8. Printing <strong>paper</strong> <strong>machine</strong>, block diagram.
Figure 9. Multilayer headbox <strong>machine</strong> with two stock components, block<br />
diagram.
Figure 10. Two-ply <strong>machine</strong>, block diagram.
Figure 11. Microbe population on metal surfaces with different degree <strong>of</strong><br />
finishing 8 .<br />
Figure 12. Stock blending <strong>and</strong> <strong>machine</strong> chest including sampling station, an<br />
example.
Figure 13. Multiple flow-lag principle.<br />
Figure 14. PM wire pit with single stock dilution.
Figure 15. Medium-consistency storage tower <strong>and</strong> stock dilution.
Figure 16. Flow pattern in <strong>the</strong> hydrocyclone (forward cleaner).
Figure 17. Stock pump with deaeration by centrifugation, courtesy <strong>of</strong> POM<br />
Technology.<br />
Figure 18. Bentonite dosage at <strong>the</strong> headbox feed pipe after pressure<br />
screens.
Figure 19. Broke pulping system.
Figure 20. Filtration principle <strong>of</strong> <strong>the</strong> disc filter.<br />
Figure 21. Disc filter saveall system.
Figure 22. Dissolved air flotation unit for process <strong>water</strong> treatment.<br />
Figure 23. Paper mill <strong>water</strong> household, system principle.
Figure 24. Working cycle for segregated <strong>and</strong> optimized internal mill <strong>water</strong><br />
use.<br />
Figure 25. Vertically integrated mill <strong>water</strong> household, system principle.
Figure 26. Accumulation <strong>of</strong> detrimental substances in <strong>the</strong> PM headbox<br />
<strong>water</strong> at reduced effluent discharge for an integrated <strong>paper</strong> mill with<br />
different degrees <strong>of</strong> process <strong>water</strong> separation.