Page 286 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Packed Towers 275

in absorbers and scrubbers). Low L/V ratios are D, = vapor density, Ib/ft3
usually associated with distillation, with = D1 = liquid density, Ib/ft3
(approx.) less than 10 gallons of process liq- d = orifice diameter, in.
uid/ (min) (ft2) of tower cross-section area) Q = liquid flow rate, gal/min
(d) Liquid cooling-This is usually better handled in H = differential head at orifice, ft of liquid
tray towers, and it is easier to draw-off liquid from K = discharge coefficient, for punched holes = 0.707
trays for removal from the system, or for external
recirculation. Liquid design height is usually one-half of the riser
(e) Foaming systems-The contact surfaces of pack- height. At minimum rates the depth on the tray can vary
ings promote film action compared to droplets from ?$ in. to about 1 in. below top of riser height for maxi-
from trays for mass and heat transfer. The pack- mum rates. The minimum orifice diameter is recommend-
ings tend to be more resistant to entrainment and ed at % in. diameter to overcome miscellaneous plugging of
induce less foaming. the holes [133]. Experience indicates the holes really
(f) Corrosion-Corrosion problems with some fluid should be M in. to avoid industrial plugging problems.
systems are easier and less costly dealt with by cor- Other useful distributor types have been referred to and
rosion resistant packings than fabricated trays. previously illustrated. For redistribution, the vapor risers
(9) Solids and slurries-Trays will handle solids and may be 12 in.-18 in. tall, and with protective “hats” to pre-
slurry systems better than packings; and if solids vent liquid dropping from the tray/section above. The
build-up does occur washing/flushing treatment space between the cover “hat” on the riser and the bed
will usually “wash” or dissolve the solids attached above should be 18 in. to 12 in. minimum to allow for
to the tmys easier than attempting a thorough proper vapor redistribution entering the packed section
cleaning of packing. above. The importance of a level distributor cannot be
(h) Costs-Other than special needs requiring one overemphasized.
contacting mechanism or another, smalldiameter Any of the available packings will usually perform the
columns of 18-in. diameter or less can be assem- operation of another; the differences being in efficiency
bled less expensively as packed towers. For some of contact, expressed as HTU, HETP or Kga, and pressure
applications of larger diameter columns, the drop for the purtaculurpacking-fluid system. Therefore, sys-
packed tower may still provide the less expensive tem data is very important and helpful in selecting a pack-
choice. This should undergo a cost analysis com- ing. Mrhen it is not available, an effort should be made to
parison. find any analogous system as far as process type, fluids,
physical properties, pressure and temperature conditions,
Chen [133] recommends the following guidelines for etc. If this is not possible, then the best judgment of the
the design of the important distributors of liquid (still designer must be used.
must pass vapor) : Eckert [125] provides some basic guidelines to good
Pun-type distributor: Plate with drilled/punched holes for packing selection for various system performance require-
liquid downflow and vapor risers. ments. Kunesh [ 1261 illustrates the oftendetermined
Vapor riser: 15 to 45% of tower cross-section area, round pressure drop advantage of random packed towers over
risers usually 4 in. or 6 in. diameter, although the round the usual valve tray. See Figure 9-19 [ 1261.
design usually has less free area than a rectangular design. For a preliminary reference and guide to the broad
Usual standard height is 6 in., however, any height can be comparison of packings versus various distillation tray
used as long as it is well above the liquid height on the tray. types Table 9-21 is helpful. Although the table includes the
The pressure drop through the vapor riser should be low: listing for more prominent manufacturers of trays and
packing materials, it is not all inclusive as far as reliable
AP = 0.46 [Dv/D1] C\r/A]*, in. of liquid (9 - 9) manufacturers of either trays or packing. Table 9-22 [ 1231
compares trays, random and structured packing, and
Liquid orifices on tray pan: usually at least 10 or more HETP, where
orifices per square foot of tower area. The orifice diame-
ter can be determined C1331: C-Factor

d = 0.23 [Q/(K)(H)o.’]0.5 (9 - 10) C =1 V, [Dv/(D1 - D,)]o.5, ft/sec (9 - 11)
where AP = pressure drop through risers, in. of liquid Souders-Brown C-Factor
V = vapor rate, ft3/second
A = riser area, ft2 = v, [Pv/(Pl - Pv)lo.5 (9 - 12)