Packed Bed flooding.pdf - Youngstown State University's Personal ...

with packing than in trays, and it takes more trials “to get it right”
than with trays. This makes trays more robust.
Complex towers. Interreboilers, intercondensers, cooling coils, and
side drawoffs are more easily incorporated in trays than in packed
towers. In packed towers, every complexity requires additional
distribution and/or liquid collection equipment.
Feed composition variation. One way of allowing for design uncertainties
and feedstock variation is by installing alternate feed
points. In packed towers, every alternate feed point requires
expensive distribution equipment.
Performance prediction. Due to their sensitivity to maldistribution
there is greater uncertainty in predicting packed column performance.
Chemical reaction, absorption. Here the much higher liquid
holdup on trays provides greater residence time for absorption or
chemical reaction than does packing.
Turndown. Moving valve and bubble-cap trays normally give better
turndown than packings. Unless very expensive distributors are
used, packed tower turndown is usually limited by distributor
turndown.
Weight. Tray towers usually weigh less than packed towers, saving
on the cost of foundations, supports, and column shell.
Trays vs. Random Packings The following factors generally
favor trays compared to random packings, but not compared to structured
packings.
Low liquid rates. With the aid of serrated weirs, splash baffles,
reverse-flow trays, and bubble-cap trays, low liquid rates can be
handled better in trays. Random packings suffer from liquid
dewetting and maldistribution sensitivity at low liquid rates.
Process surges. Random packings are usually more troublesome
than trays in services prone to process surges (e.g., those caused
by slugs of water entering a hot oil tower, relief valve lifting, compressor
surges, or instability of liquid seal loops). Structured
packings are usually less troublesome than trays in such services.
Trays vs. Structured Packings The following factors generally
favor trays compared to structured packings, but not compared to random
packings.
Packing fires. The thin sheets of structured packing (typically 0.1
mm) poorly dissipate heat away from hot spots. Also, cleaning,
cooling, and washing can be difficult, especially when distributors
or packing plug up. Many incidents of packing fires during
turnarounds (while towers with structured packings were open
to atmosphere) have been reported. Most of these fires were
initiated by pyrophoric deposits, hot work (e.g., welding) above
the packing, opening the tower while hot organics were still
present, and packing metallurgy that was not fire-resistant.
Detailed discussion can be found in Fractionation Research
Inc. (FRI) Design Practices Committee, “Causes and Prevention
of Packing Fires,” Chem. Eng., July 2007.
Materials of construction. Due to the thin sheets of structured
packings, their materials of construction need to have better
resistance to oxidation or corrosion. For a service in which carbon
steel is usually satisfactory with trays, stainless steel is usually
required with structured packings.
Column wall inspection. Due to their snug fit, structured packings
are easily damaged during removal. This makes it difficult to
inspect the column wall (e.g., for corrosion).
Washing and purging. Thorough removal of residual liquid, wash
water, air, or process gas trapped in structured packings at startup
and shutdown is more difficult than with trays. Inadequate
removal of these fluids may be hazardous.
High liquid rates. Multipass trays effectively lower the liquid load
“seen” by each part of the tray. A similar trick cannot be applied
with packings. The capacity of structured packings tends to
rapidly fall off at high liquid rates.
Capacity and Efficiency Comparison Kister et al. [Chem. Eng.
Progr., 90(2), 23 (1994)] reported a study of the relative capacity and
efficiency of conventional trays, modern random packings, and conventional
structured packings. They found that, for each device optimally
designed for the design requirements, a rough guide could be
developed on the basis of flow parameter L/G (ρ G/ρ L) 0.5 (abcissa in
OTHER TOPICS FOR DISTILLATION AND GAS ABSORPTION EQUIPMENT 14-81
Figs. 14-31, 14-55, and 14-56) and the following tentative conclusions
could be drawn:
Flow Parameter 0.02–0.1
1. Trays and random packings have much the same efficiency and
capacity.
2. Structured packing efficiency is about 1.5 times that of trays or
random packing.
3. At a parameter of 0.02, the structured packing has a 1.3–1.4
capacity advantage over random packing and trays. This advantage
disappears as the parameter approaches 0.1.
Flow Parameter 0.1–0.3
1. Trays and random packings have about the same efficiency and
capacity.
2. Structured packing has about the same capacity as trays and random
packings.
3. The efficiency advantage of structured packing over random
packings and trays decreases from 1.5 to 1.2 as the parameter
increases from 0.1 to 0.3.
Flow Parameter 0.3–0.5
1. The loss of capacity of structured packing is greatest in this
range.
2. The random packing appears to have the highest capacity and
efficiency with conventional trays just slightly behind. Structured
packing has the least capacity and efficiency.
Experience indicates that use of structured packings has capacity/
efficiency disadvantages in the higher-pressure (higher-flow-parameter)
region.
Zuiderweg and Nutter [IChemE Symp. Ser. 128, A481 (1992)]
explain the loss of capacity/efficiency by a large degree of backmixing
and vapor recycle at high flow parameters, promoted by the solid walls
of the corrugated packing layers.
SYSTEM LIMIT: THE ULTIMATE CAPACITY
OF FRACTIONATORS
Liquid drops of various sizes form in the gas-liquid contact zones of
tray or packed towers. Small drops are easily entrained upward, but
their volume is usually too small to initiate excessive liquid accumulation
(flooding). When the gas velocity is high enough to initiate a
massive carryover of the larger drops to the tray above, or upward
in a packed bed, liquid accumulation (entrainment flooding) takes
place. This flood can be alleviated by increasing the tray spacing or
using more hole areas on trays or by using larger, more open packings.
Upon further increase of gas velocity, a limit is reached when the
superficial gas velocity in the gas-liquid contact zone exceeds the settling
velocity of large liquid drops. At gas velocities higher than this,
ascending gas lifts and carries over much of the tray or packing liquid,
causing the tower to flood. This flood is termed system limit or ultimate
capacity. This flood cannot be debottlenecked by improving
packing size or shape, tray hole area, or tray spacing. The system limit
gas velocity is a function only of physical properties and liquid flow
rate. Once this limit is reached, the liquid will be blown upward. This
is analogous to spraying water against a strong wind and getting
drenched (Yangai, Chem. Eng., p. 120, November 1990). The system
limit represents the ultimate capacity of the vast majority of existing
trays and packings. In some applications, where very open packings
(or trays) are used, such as in refinery vacuum towers, the system limit
is the actual capacity limit.
The original work of Souders and Brown [Ind. Eng. Chem. 26(1),
98 (1934), Eq. (14-80)] related the capacity of fractionators due to
entrainment flooding to the settling velocity of drops. The concept of
system limit was advanced by Fractionation Research Inc. (FRI),
whose measurements and model have recently been published (Fitz
and Kunesh, Distillation 2001: Proceedings of Topical Conference,
AIChE Spring National Meeting, Houston, Tex., 2001; Stupin, FRI
Topical Report 34, 1965 available through Special Collection Section,
Oklahoma State University Library, Stillwater, Okla.). Figure 14-75 is
a plot of FRI system limit data (most derived from tests with dual-flow
trays with 29 percent hole area and 1.2- to 2.4-m tray spacing) against
liquid superficial velocity for a variety of systems (Stupin, loc. cit.,