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

14-80 EQUIPMENT FOR DISTILLATION, GAS ABSORPTION, PHASE DISPERSION, AND PHASE SEPARATION
component was soluble in both liquid phases, and HETP was about 50
percent above normal. Harrison argued that a second liquid phase
leads to lower efficiency only when it impairs diffusion of the key
species. On this basis, Harrison expects efficiency loss also when an
“inert” liquid or vapor represents a large fraction of the liquid or vapor
phase. Meier et al. recommend obtaining efficiencies by scaling up
laboratory-scale data using a similar type of packing.
Both Harrison and Meier et al. emphasize adequately distributing
each liquid phase to the packing. Harrison noted that a well-designed
ladder pipe distributor can maintain high velocities and low residence
times that provide good mixing. With trough distributors that separate
the phases and then distribute each to the packing, a light-to-heavy
phase maldistribution may occur, especially when the phase ratio and
separation vary. Meier et al. noted the existence of a cloudy two-liquid
layer between the clear light and heavy liquid and recommend an
additional middle distribution point for this layer. They also noticed
that phase separation unevenness can have a large influence on the
phase ratio irrigated to the packing.
High Viscosity and Surface Tension Bravo (Paper presented
at the AIChE Spring National Meeting, Houston, Tex., 1995) studied
a system that had 425-cP viscosity, 350 mN/m surface tension, and a
high foaming tendency. He found that efficiencies were liquid-phasecontrolled
and could be estimated from theoretical HTU models.
Capacity was less than predicted by conventional methods which do
not account for the high viscosity. Design equations for orifice distributors
extended well to the system once the orifice coefficient was calculated
as a function of the low Reynolds number and the surface
tension head was taken into account.
OTHER TOPICS FOR DISTILLATION AND GAS ABSORPTION EQUIPMENT
COMPARING TRAYS AND PACKINGS
Most separations can be performed either with trays or with packings.
The factors below represent economic pros and cons that favor each
and may be overridden. For instance, column complexity is a factor
favoring trays, but gas plant demethanizers that often use one or more
interreboilers are traditionally packed.
Factors Favoring Packings
Vacuum systems. Packing pressure drop is much lower than that of
trays because the packing open area approaches the tower crosssectional
area, while the tray’s open area is only 8 to 15 percent of
the tower cross-sectional area. Also, the tray liquid head, which
incurs substantial pressure drop (typically about 50 mm of the
liquid per tray), is absent in packing. Typically, tray pressure drop
is of the order of 10 mbar per theoretical stage, compared to 3 to
4 mbar per theoretical stage with random packings and about
one-half of that with structured packings.
Consider a vacuum column with 10 theoretical stages, operating at
70-mbar top pressure. The bottom pressure will be 170 mbar with
trays, but only 90 to 110 mbar with packings. The packed tower will
have a much better relative volatility in the lower parts, thus reducing
reflux and reboil requirements and bottom temperature. These
translate to less product degradation, greater capacity, and smaller
energy consumption, giving packings a major advantage.
Lower-pressure-drop applications. When the gas is moved by a fan
through the tower, or when the tower is in the suction of a compressor,
the smaller packing pressure drop is often a controlling
consideration. This is particularly true for towers operating close to
atmospheric pressure. Here excessive pressure drop in the tower
increases the size of the fan or compressor (new plant), bottlenecks
them (existing plant), and largely increases power consumption.
Due to the compression ratio, pressure drop at the compressor discharge
is far less important and seldom a controlling consideration.
Revamps. The pressure drop advantage is invaluable in vacuum column
revamps, can be translated to a capacity gain, an energy
gain, a separation improvement, or various combinations of these
benefits. Likewise, for towers in the suction of compressors,
replacing trays by packings reduces the compression ratio and
helps debottleneck the compressor.
Packings also offer an easy tradeoff between capacity and separation.
In the loaded sections of the tower, larger packings can
overcome capacity bottlenecks at the expense of loss in separation.
The separation loss can often be regained by retrofitting
with smaller packings in sections of the tower that are not highly
loaded. In tray towers, changing tray spacing gives similar results,
but is more difficult to do.
Foaming (and emulsion). The low gas and liquid velocities in packing
suppress foam formation. The large open area of the larger
random packing promotes foam dispersal. Both attributes make
random packing excellent for handling foams. In many cases
recurrent foaming was alleviated by replacing trays by random
packing, especially when tray downcomers were poorly designed.
Switching from trays to structured packing can aggravate foaming.
While the low gas and liquid velocities help, the solid walls
restrict lateral movement of foams and give support to the foams.
Small-diameter columns. Columns with diameter less than 1 m
(3 ft) are difficult to access from inside to install and maintain the
trays. “Cartridge” trays or an oversized diameter are often used.
Either option is expensive. Cartridge trays also run into problems
with sealing to the tower wall and matching tower to tray hardware
[Sands, Chem. Eng., p. 86 (April 2006)]. Packing is normally
a cheaper and more desirable alternative.
Corrosive systems. The practical range of packing materials is
wider. Ceramic and plastic packings are cheap and effective.
Trays can be manufactured in nonmetals, but packing is usually a
cheaper and more desirable alternative.
Low liquid holdup. Packings have lower liquid holdup than do
trays. This is often advantageous for reducing polymerization,
degradation, or the inventory of hazardous materials.
Batch distillation. Because of the smaller liquid holdup of packing, a
higher percentage of the liquid can be recovered as top product.
Factors Favoring Trays
Solids. Trays handle solids much more easily than packing. Both gas
and liquid velocities on trays are often an order of magnitude
higher than through packing, providing a sweeping action that
keeps tray openings clear. Solids tend to accumulate in packing
voids. There are fewer locations on trays where solids can be
deposited. Plugging in liquid distributors has been a common trouble
spot. Cleaning trays is much easier than cleaning packings.
Not all trays are fouling-resistant. Floats on moving valve trays
tend to “stick” to deposits on the tray deck. Fouling-resistant trays
have large sieve holes or large fixed valves, and these should be
used when plugging and fouling are the primary considerations.
There is much that can be done to alleviate plugging with random
packing. Large, open packing with minimal pockets offers
good plugging resistance. Distributors that resist plugging have
large holes (> 13-mm diameter). Such large holes are readily
applied with high liquid flow rates, but often not practical for
small liquid flow rates.
Maldistribution. The sensitivity of packing to liquid and gas maldistribution
has been a common cause of failures in packed towers.
Maldistribution issues are most severe in large-diameter towers,
long beds, small liquid flow rates, and smaller packing. Structured
packing is generally more prone to maldistribution than random
packing. While good distributor design, water testing, and inspection
can eliminate most maldistribution issues, it only takes a few
small details that fall through the cracks to turn success into failure.
Due to maldistribution, there are far more failures experienced