Page 280 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Packed Towers 269

poor efficiency due to concentration gradients brought essentially in vertical streams. However, most packed tower
about by liquid maldistribution. The “spreading coeffi- services do not use stacked packing.
cients” of structured packing are larger than for random Physically the redistributions may be a simple and rela-
packing. The spreading for structured packing occurs in a tively inefficient side wiper as in Figure 9-12 or 9-13; a con-
somewhat different manner due to the layering of the ventional support grid or plate plus regular distribution
structured packing. plate as used at the top; a combination unit similar to Prym
The entrance of a liquid-flashing vapor mixture into the support and distributor; or a support plate as shown in Fig-
distillation column feed location requires a specially ures 9-14 and 9-7D and 7E, a circular plate with holes.
designed distribution tray to separate the vapors from the The possibility of causing flooding in the tower at the
liquid, which must drop onto the packing bed for that sec- redistribution point must not be overlooked, as too much
tion in a uniform pattern and rate. restriction by a wall wiper, or by packing on a plate can be
Structured packing requires specially designed distribu- the focal point for poor tower performance. The velocity
tors recommended by the respective manufacturers to conditions should be checked for the smallest cross-section.
ensure the same important uniform liquid distribution
across any bed of this type of packing. Wall Wipers or Side Wipers

Redistributors The wall wiper liquid collector/redistributor, Figure
9-12, is most useful in reducing the by-passing effects of
The liquid coming down through the packing and on liquid running down the walls of small towers. They do not
the wall of the tower should be redistributed after a bed truly take the place of a redistributor system placed peri-
depth of approximately 3 tower diameters for Raschig odically in the tower. For larger towers of 4 ft diameter and
rings and 5-10 tower diameters for saddle packings. As a greater, they are not as useful because they collect a small-
guide, Raschig rings usually have a maximum of 10-15 ft er portion of the total tower liquid, and cannot effectively
of packing per section, while saddle packing can use redistribute it throughout the tower cross-section as dis-
12-20 ft. This redistribution brings the liquid off the wall cussed in the paragraphs under “Distributors.” They do
and outer portions of the tower and directs it toward the serve a useful purpose for the smaller towers, through
center area of the tower for a new start at distribution and about 18 in-20 in.-30 in. They can restrict vapor flow up,
contact in the next lower section. as well as inhibit redistributing the collected liquid uni-
formly across the tower.
The redistributor must be sealed against the tower wall
to collect all of the liquid coming down the tower from the
packed section above (Figure 9-11). Then it must be capa- Hold-down Grids
ble either singly or in conjunction with a distributor
placed below it of redistributing the collected liquid from To reduce ceramic or carbon packing breakage and
an upper packed section to the top of the next lower sec- blowing out of light weight plastic packing when a tower
tion in an efficient manner. (See discussion in previous surges due to gas pockets, uneven loading, etc., it is some-
paragraph on distributors.) The gas/vapor riser opening times helpful to have heavy hardware cloth or other stiff
must be so covered by design as to avoid liquid dropping but open grid resting (floating) at the top of the tower and
directly through the gas risers and onto the packed section on the top of the packing (Figures 9-15A and 15B).
below. This vapor flow area must be relatively large to This grating or grid must be heavy enough to hold
avoid localized development of high pressure drop and down about the top five feet of packing, yet must be able
upsetting the performance. to move down as the packing settles, always resting at the
top of the packing. If the packing is restricted in upward
The height of packing before redistribution is a func-
tion of the liquid flow pattern through the packing, and movement, it usually will not be crushed. If the packing
this is a function of the size and type of packing. Some tow- does break and crush, the bed settles and its characteris-
ers have 20-30 ft of packing with no redistribution; how- tics change considerably.
ever, the reasons may be economic as well as operational. Bed-limiters are usually lighter weight and must be bolt-
The exact amount of performance efficiency sacrificed is ed in place, not resting on the packing.
subject to question, although with 2&35% of the liquid They are used with metal and plastic packings to pre-
flowing down the walls after 10 ft of ring packing depth, it vent the bed lifting, or the entrainment of individual
appears reasonable to consider that performance is lost pieces of packing from being carried out of the tower.
for most of this liquid. Redistribution is usually not neces- These packings usually do not break, and as long as the
sary for stacked bed packings because the liquid flows bed temperature is below the softening or deflection point