Page 292 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Packed Towers 281

trates the effects of iron present in these systems on pack- Table 9-25
ing and fouling of distributors. Packing Wetting Rates Related to Packing Material
Fouled packing can significantly reduce the perfor- Surface
mance of the system and is one condition that should be ..... __ ..... ._ .
examined when packed tower systems deteriorate. For Reasonable
some systems, an acid or solvent flush (or wash) will MWRT Minimum
accomplish the needed thorough cleaning. The need for Gpm/ Wetting rate
such a supplemental maintenance during various stages Surface .. - ft2 ft?/f$hr m3/m% Materials
.
._
should be recognized at the time of initial system design. Unglazed 0.187 1.5 0.5 Chemical
Also for water soluble systems, the use of pH adjustment, ceramic stoneware
pretreating with sequestering agents or biocides, ozone Oxidized 0.27 2.2 0.7 Cabon steel,
treatment and other steps specific to the system's prob metal copper
lems [ 1351 may be necessary or helpful. Surface treated 0.40 3.2 1.0 Etched stainless
metal steel
Glazed ceramic 0.80 6.4 2.0
Minimwm Liquid Wetting Rates
Glass 1.00 8.0 2.5
Brightmetal 1.20 9.6 3.0 Stainless steel,
To feed enough liquid into the tower to effectively place tantalum
a wet film of liquid over all the packing, a minimum wet- WGCPVC 1.43 11.6 3.5 *8
ting rate (W) has been evaluated for guidance in oper- Polypropylene 1.60 12.8 4.0 **
ation and design. Morris and Jackson [32] recommended pTFE/FEP 2.00 16.0 5.0 **
-
......
.. -
. --
the MWR shown in Table 9-24. *Modified by author from Glitsch literature. .
A minimum liquid rate for any tower packing is used by **Requires proper surface wetting treatment. Important that surface
some designers as 1,500 lb liquid/(hr) (ft2 tower) refer- wetting be tested and treatment applied if necessary
encing to liquid of properties of water. Values based on > 43 fi2/ft? specific area of packing. By permission of
Mass Transfer, Inc., Bull. TF'/US/Bl (1978) and Glitsch Bull. No. 345.
Lmin 3 (MW (at) (9 - 14)
where Lmin = liquid rate, ft3/(hr) (ft2 cross-section) surface tension. Most plastic and some metal packings
MWR = value of minimum wetting rate from Table 9-24
require surface treatment before the packing particle will
The minimum wetting rate is a function of the packing wet uniformly, or even will wet at all. Without film forming
material surface (Table 9-25) and the physical properties characteristics on the surface area, the contact of the liq-
of the liquid involved, particularly the viscosity and the uid-vapor will be poor and the tower performance effi-
ciency can be expected to fall off. The packing should be
tested for wettability in the service before completing the
tower design and packing selection.
Table 9-24 Kister [go] has evaluated Schmidt's E921 somewhat com-
Minimum Wetting Rate* plicated equation for minimum wetting rate and proposes:
.__ ... .- __
Liquid, ft3/(hr) (ft' cross MMXG = (MWRT using Table 9-25 in gpm/f$) (60/at)0.j (9 - 15)
sect.)
MWR Rate, .... ..... where M m - minimum wetting rate, gpm/ft2, generalized
Packingsurfaceareapertower for other packings using Kister's evaluation
Packing volume, ftZ/fts MWRT = minimum wetting rate, gpm/ft2 from Table 9-25
...
.... ..... ..... - .. ... . at = specific surface area of packing, fG/ft3
Rings (Raschig, Lessing,
etc.), thru sin. &a. 0.85 Another expression of Veasonable minimum wetting
Grid type (wooden, etc.)
(pitch 2 in.) 0.85 rate" [48] is given in Table 9-25. The surface characteris-
All packings larger than 3 in. 1.3 tics of the packing material are important in the type of
Polished metal packings 1.3, estimate to 2.5 liquid film (or droplets) that flow across, around, and drip
and poor wetting surtkces Preferably etch surfaces off of the surface. The better the specific liquid wets the
(some plastics, glazed to reduce problem. packing surface and forms a moving film the more effi-
porcelain, etc.) cient will be the packing for distillation, absorption, etc. In
.- ...... . ........ ... .
*Compiled by permission from Morris and Jackson, Absorption Tmm, general, from the table it can be noted that the surfaces
Butteworth Scientific Pub. (1953) London and Imperial Chemical that tend to wet easily have the lower minimum wetting
Industries, Ltd., Ref. 32. rates. The data given in Table 9-25 do not agree too well