Minimizing fouling in spiral heat exchangers at a BCTMP mill
Minimizing fouling in spiral heat exchangers at a BCTMP mill Minimizing fouling in spiral heat exchangers at a BCTMP mill
T74scalingum oxalate in the scale is estimated by thefollowing calculation.CaC 2 O 4 (s)—— CaO(s) + CO 2 (g) + CO(g)(Overall decomposition reaction)Mol. Mass (g/mol) 128 56 44 28Using a basis of 1 mole (128gm) of sample,Theoretical mass loss % during totaldecomposition:Mass of Gas Evaporated—————————— 100% =Total Mass of Sample(44 + 28)gm—————— 100% = 56%128gmActual mass loss % from thermal analysisexperiment, Fig. 11: Step 2 + Step 3:FIG. 6. Positions of sample scale collection.FIG. 7. Pulp mill effluent.Mass of Gas Evaporated—————————— 100% = 49%Total Mass of SamplePercentage of calcium oxalate =49—— 100% = 87.5%56Calcium oxalate has three differenthydrated forms: monohydrate, dihydrateand trihydrate. Monohydrate is the moststable and has the lowest solubility [2].Thus it can be speculated that calciumoxalate monohydrate (CaC 2 O 4 .H 2 O) isthe main component (87.5%) of the heatexchanger deposit. The study by Zhang etal. also showed that the pulp mill disc-filtershower and reject press had calciumoxalate deposition [3]. This suggests thatthe mill has been experiencing calciumoxalate scaling problems elsewhere in thesystem and thus it is likely that in the heatexchangers, the scale type is indeed calciumoxalate.Determining Fouling RateOnce the type of scale has been determined,the next step involves quantifyingfouling rate based on a few simplifiedassumptions. The flow rate trends over aone-year period through HEX 3 & 5 andthrough HEX 4 are plotted in Figs. 12and 13.The flow rate trend shows the extent ofthe fouling problem in the pulp mill.Maximum flow is achieved immediatelyafter high pressure cleaning. Then thescaling material and foulants begin todeposit and slowly reduce the flow rate,eventually plugging up the heat exchanger.The deposit build-up reduces the flowchannel area. This increases the pressuredrop across each heat exchanger andreduces the flow rate through the system.The change in flow rate over time isthus a function of fouling rate. A fasterfouling rate leads to a steep decrease inflow. Subsequently a slower fouling ratetakes longer to affect flow. The change inflow rate over time, the slope in Figs. 12and 13, can thus be used to estimate foulingrate.FIG. 8. Schematic of procedure to measure level of saturation of effluent solution.d(flowrate)—————— = f(Fouling rate)dtThe following assumptions are madein order to quantify the fouling rate.• The spiral heat exchanger surface isflattened into a rectangular cross section,Fig. 14• Effluent velocity remains constant overtime• The fouling rate is constant over timeFor a constant velocity, the volumetricflow rate is defined asVolumetric Flowrate =Velocity Unblocked AreaThe maximum flow occurs at time = 0,when the deposit thickness (t) = 0 andchannel spacing (b) = 15.875mm (heatexchanger inner channel). As time increases,the t is increased by a constant value(fouling rate) until the channel completelyblocks up. The fouling rate that bestmatches the actual flow rate profile withthe theoretical profile, shown in Fig. 15, isselected as the correct estimated foulingrate. The same treatment is applied to analyzeindividual slopes for all data. The averageresults are presented in Table II.It is seen that the series arrangementof HEX 3 and 5 foul up faster than HEX4. This agrees with mill observations thatthey have to clean HEX 5 more often. It isalso seen that the fouling rate throughHEX 3 & 5 increased after April 2005 andthe average maximum flow after scaleremoval decreased after April 2005, notethe boxed area in Fig. 12.Both observations suggest that the heatexchanger cleanup procedure waschanged in April 2005, inversely affectingthe heat exchanger performance. Thepulp mill confirmed that, before April, anexternal company cleaned the heatexchangers with a high-pressure waterhose. Since April, the cleaning has beendone internally with a lower pressure hoseto reduce operating expenses. The effectof inefficient cleaning thus had significantimpact on fouling rate. This is discussedin detail in a later section.Calcium Oxalate FormationOnce it is confirmed that there is excesscalcium oxalate passing through the heatrecovery system, it is important to determinethe source of the calcium andoxalate ions in the BCTMP process.Zhang et al. have done significant workon the formation of oxalate in a BCTMPmill with a similar configuration to ourpulp mill [3]. They have investigated theBCTMP process and identified 3 sourcesof oxalate: input with wood, refining stageand bleaching stage.38 • 108:4 (2007) • PULP & PAPER CANADA
peer-reviewedT75FIG. 9. Data points available around the Heat RecoverySystem.FIG. 10. Thermograph for decomposition of pure calciumoxalate monohydrate.FIG. 11. Thermograph for decomposition of scale sampledried at 85°C from HEX5. FIG. 12. Flow trend through HEX 3 & 5.Their work shows that the input withwood and the refining stage contributedsmall amounts of oxalate. However, a significantamount was formed during peroxidebleaching. Other researchers havealso confirmed peroxide bleaching as amajor oxalate source in BCTMP mills[9,12]. The mechanism of formation hasbeen described as an attack of hydrogenperoxide (H 2 O 2 ) on oxalate precursors(OP) present with fiber in the pulp solution(Fiber-OP). Fiber-OP exists as Fiber-OP-H + , which is in equilibrium with Ca 2+ions to form Fiber-OP-Ca 2+ . Ca 2+ ionsenter the BCTMP process with hard waterand wood. The oxidation of Fiber-OP withH 2 O 2 produces oxalates. The reactionchain is summarized in Fig. 16 [3].Effect of TemperatureIt is well known that crystals formed in onepart of a system and carried elsewhere areless adherent than those crystals formedon site. Therefore, a change of operatingcondition that leads to precipitation withinthe heat exchangers is what causes stablescale formation. For a heat exchanger,temperature is most likely the key factorthat would influence scaling.Figure 17 shows the solubility curve forcalcium oxalate. The solubility values areobtained from various sources compiledin the solubility handbook [10]. The datapoints can be fit with a straight line, suggestinga linear relationship between temperatureand solubility. Thus, contact witha lower temperature surface would reducethe solubility of calcium oxalate and thenprecipitate solids, causing build up ofscale. The equation of the solubility-temperaturegraph is as follows.Solubility (g/L) = 0.0001Temp(°C)+0.0051The solubility graph can be used todetermine the precipitation amountbased on the temperature difference. Thedata for temperature difference is presentedin Fig. 18.Table III displays the precipitationrates formed across each heat exchanger.Using the temperature-solubility relationship,the temperature difference acrosseach heat exchanger is converted to a solubilitydifference value. Maximum flowrate is assumed to be 2100L/min, fromthe average maximum flows achieved,Figs. 12 and 13.Precipitation Rate = Solubility FlowrateFrom the last column in Table III, itcan be seen that HEX 5 has the highestprecipitation rate. This once again agreeswith the mill observation that HEX 5needs to be cleaned more frequently. Toconfirm if the effect of temperature is significant,a theoretical fouling rate is calculated,based on estimates of foulingrates calculated earlier.Fouling rate (kg/day) = Exposed Surface AreaFouling Rate (mm/day) Foulant DensityExposed Surface Area = 90.9 m 2Fouling Rate = 0.07 mm/day, from TableII, HEX 4Density is assumed to be 1500 kg/m 3based on the visible structure of deposits,which appears to have the same structureas clay. Typical clay density is 1500 kg/m 3 .Using the above data, theoretical foulingrate for HEX 4 is found to be 9.54 kg/day.HEX 4 was considered because it is theonly HEX in its line. The series arrangementon the other line makes it difficultto relate flow rate and fouling rate.The underlying assumption in estimatingfouling rate based on flow rate gradientsis that deposit builds up uniformlyacross the entire heat exchanger surface.The fouling rate estimated from the flowrate gradient ignores the fact that depositedsolids at a single point along the effluentflow channel are sufficient to constrictthe flow. In reality, the deposit formationis not uniform. This is visible in Fig. 19.The real fouling rate is thus significantlylower than 9.54 kg/day. The overestimatedfouling rate of 9.54 kg/day howeverprovides a frame of reference for comparisonwith the precipitation rate due totemperature difference across HEX 4,which is 4.35 kg/day.From previous calculations, Table III, itwas found that 4.35 kg/day of calciumoxalate precipitated due to the change inPULP & PAPER CANADA • 108:4 (2007) • 39
- Page 1 and 2: peer-reviewedT71Minimizing fouling
- Page 6 and 7: T76scalingFIG. 13. Flow trend throu
- Page 8 and 9: T78scalingFIG. 21. Proposed calcium
- Page 10: T80scalingrate (>2000L/min). This i
peer-reviewedT75FIG. 9. D<strong>at</strong>a po<strong>in</strong>ts available around the He<strong>at</strong> RecoverySystem.FIG. 10. Thermograph for decomposition of pure calciumoxal<strong>at</strong>e monohydr<strong>at</strong>e.FIG. 11. Thermograph for decomposition of scale sampledried <strong>at</strong> 85°C from HEX5. FIG. 12. Flow trend through HEX 3 & 5.Their work shows th<strong>at</strong> the <strong>in</strong>put withwood and the ref<strong>in</strong><strong>in</strong>g stage contributedsmall amounts of oxal<strong>at</strong>e. However, a significantamount was formed dur<strong>in</strong>g peroxidebleach<strong>in</strong>g. Other researchers havealso confirmed peroxide bleach<strong>in</strong>g as amajor oxal<strong>at</strong>e source <strong>in</strong> <strong>BCTMP</strong> <strong>mill</strong>s[9,12]. The mechanism of form<strong>at</strong>ion hasbeen described as an <strong>at</strong>tack of hydrogenperoxide (H 2 O 2 ) on oxal<strong>at</strong>e precursors(OP) present with fiber <strong>in</strong> the pulp solution(Fiber-OP). Fiber-OP exists as Fiber-OP-H + , which is <strong>in</strong> equilibrium with Ca 2+ions to form Fiber-OP-Ca 2+ . Ca 2+ ionsenter the <strong>BCTMP</strong> process with hard w<strong>at</strong>erand wood. The oxid<strong>at</strong>ion of Fiber-OP withH 2 O 2 produces oxal<strong>at</strong>es. The reactioncha<strong>in</strong> is summarized <strong>in</strong> Fig. 16 [3].Effect of Temper<strong>at</strong>ureIt is well known th<strong>at</strong> crystals formed <strong>in</strong> onepart of a system and carried elsewhere areless adherent than those crystals formedon site. Therefore, a change of oper<strong>at</strong><strong>in</strong>gcondition th<strong>at</strong> leads to precipit<strong>at</strong>ion with<strong>in</strong>the <strong>he<strong>at</strong></strong> <strong>exchangers</strong> is wh<strong>at</strong> causes stablescale form<strong>at</strong>ion. For a <strong>he<strong>at</strong></strong> exchanger,temper<strong>at</strong>ure is most likely the key factorth<strong>at</strong> would <strong>in</strong>fluence scal<strong>in</strong>g.Figure 17 shows the solubility curve forcalcium oxal<strong>at</strong>e. The solubility values areobta<strong>in</strong>ed from various sources compiled<strong>in</strong> the solubility handbook [10]. The d<strong>at</strong>apo<strong>in</strong>ts can be fit with a straight l<strong>in</strong>e, suggest<strong>in</strong>ga l<strong>in</strong>ear rel<strong>at</strong>ionship between temper<strong>at</strong>ureand solubility. Thus, contact witha lower temper<strong>at</strong>ure surface would reducethe solubility of calcium oxal<strong>at</strong>e and thenprecipit<strong>at</strong>e solids, caus<strong>in</strong>g build up ofscale. The equ<strong>at</strong>ion of the solubility-temper<strong>at</strong>uregraph is as follows.Solubility (g/L) = 0.0001Temp(°C)+0.0051The solubility graph can be used todeterm<strong>in</strong>e the precipit<strong>at</strong>ion amountbased on the temper<strong>at</strong>ure difference. Thed<strong>at</strong>a for temper<strong>at</strong>ure difference is presented<strong>in</strong> Fig. 18.Table III displays the precipit<strong>at</strong>ionr<strong>at</strong>es formed across each <strong>he<strong>at</strong></strong> exchanger.Us<strong>in</strong>g the temper<strong>at</strong>ure-solubility rel<strong>at</strong>ionship,the temper<strong>at</strong>ure difference acrosseach <strong>he<strong>at</strong></strong> exchanger is converted to a solubilitydifference value. Maximum flowr<strong>at</strong>e is assumed to be 2100L/m<strong>in</strong>, fromthe average maximum flows achieved,Figs. 12 and 13.Precipit<strong>at</strong>ion R<strong>at</strong>e = Solubility Flowr<strong>at</strong>eFrom the last column <strong>in</strong> Table III, itcan be seen th<strong>at</strong> HEX 5 has the highestprecipit<strong>at</strong>ion r<strong>at</strong>e. This once aga<strong>in</strong> agreeswith the <strong>mill</strong> observ<strong>at</strong>ion th<strong>at</strong> HEX 5needs to be cleaned more frequently. Toconfirm if the effect of temper<strong>at</strong>ure is significant,a theoretical <strong>foul<strong>in</strong>g</strong> r<strong>at</strong>e is calcul<strong>at</strong>ed,based on estim<strong>at</strong>es of <strong>foul<strong>in</strong>g</strong>r<strong>at</strong>es calcul<strong>at</strong>ed earlier.Foul<strong>in</strong>g r<strong>at</strong>e (kg/day) = Exposed Surface AreaFoul<strong>in</strong>g R<strong>at</strong>e (mm/day) Foulant DensityExposed Surface Area = 90.9 m 2Foul<strong>in</strong>g R<strong>at</strong>e = 0.07 mm/day, from TableII, HEX 4Density is assumed to be 1500 kg/m 3based on the visible structure of deposits,which appears to have the same structureas clay. Typical clay density is 1500 kg/m 3 .Us<strong>in</strong>g the above d<strong>at</strong>a, theoretical <strong>foul<strong>in</strong>g</strong>r<strong>at</strong>e for HEX 4 is found to be 9.54 kg/day.HEX 4 was considered because it is theonly HEX <strong>in</strong> its l<strong>in</strong>e. The series arrangementon the other l<strong>in</strong>e makes it difficultto rel<strong>at</strong>e flow r<strong>at</strong>e and <strong>foul<strong>in</strong>g</strong> r<strong>at</strong>e.The underly<strong>in</strong>g assumption <strong>in</strong> estim<strong>at</strong><strong>in</strong>g<strong>foul<strong>in</strong>g</strong> r<strong>at</strong>e based on flow r<strong>at</strong>e gradientsis th<strong>at</strong> deposit builds up uniformlyacross the entire <strong>he<strong>at</strong></strong> exchanger surface.The <strong>foul<strong>in</strong>g</strong> r<strong>at</strong>e estim<strong>at</strong>ed from the flowr<strong>at</strong>e gradient ignores the fact th<strong>at</strong> depositedsolids <strong>at</strong> a s<strong>in</strong>gle po<strong>in</strong>t along the effluentflow channel are sufficient to constrictthe flow. In reality, the deposit form<strong>at</strong>ionis not uniform. This is visible <strong>in</strong> Fig. 19.The real <strong>foul<strong>in</strong>g</strong> r<strong>at</strong>e is thus significantlylower than 9.54 kg/day. The overestim<strong>at</strong>ed<strong>foul<strong>in</strong>g</strong> r<strong>at</strong>e of 9.54 kg/day howeverprovides a frame of reference for comparisonwith the precipit<strong>at</strong>ion r<strong>at</strong>e due totemper<strong>at</strong>ure difference across HEX 4,which is 4.35 kg/day.From previous calcul<strong>at</strong>ions, Table III, itwas found th<strong>at</strong> 4.35 kg/day of calciumoxal<strong>at</strong>e precipit<strong>at</strong>ed due to the change <strong>in</strong>PULP & PAPER CANADA • 108:4 (2007) • 39
Change language