Page 308 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Packed Towers 297

1. As packing factor, F, becomes larger by selection of For liquids of viscosity of 30 centipoise and lower, effect
smaller sized packing; gas capacity for the column on capacity is small. For high viscosity select larger pack-
is reduced; and pressure drop will increase for a ing to reduce pressure drop, and also consult packing
fixed gas flow. manufacturer.
2. Some packings are sized by general dimensions in Robbins’ [96] correlation for pressure drop in random
inches, while some shapes are identified by num- particle packed towers is based on a “dry packing factor,”
bers, #1, #2, #3 for increasing size. Fpd, whereas most of manufacturer’s published values have
3. Not all packings are manufactured in all materials determined F (packing factor) from wet and dumped data,
of construction, i.e. ceramic, various plastics, vari- and is that used in Figures 9-21A-F, 9-21H. Referenced to
ous metals. the tables in Robbins’ presentation indicates that the dif-
4. Packing size versus tower diameter recommenda- ferences ma): be small between the dry, Fpd, and the wet
tions; general guides not mandatory, base selec- and dumped, F (such as Table 9-26A), being from 0 to 10
tion on performance. points, averaging about 2-3 points lower. The packing
manufacturer should be consulted for dry packing factors
Tower Diam., ft Nominal Packing Size, in. to use in Robbins’ method. The “dry” data simply means
d.0 <1 that there is no liquid (but gas) flowing. Robbins [96] lists
1.0-3.0 1-1% dues of Fpd for metal, plastic and ceramic packings.
>3.0 2-3 Dry bed pressure-drop [96] :
Table 9-27 shows what Strigle [82] recommends
for the maximum liquid loading as related to pack- AP = C, pg V? = C, FS2 = C,G2/pg (9 - 31A)
ing size.
E. Referring to Figure 9-21B or 21C, read up from the Values of C, come from Leva [41].
abscissa to the pressure drop line selected, and read Robbins’ new equation for generalized pressure drop
across to the ordinate (note differences): for random tower packings:
+
AP = C3G? 10-*i[w) 0.4 [1+/20,000]0~0’
Ordinate No. = c2~poJ , (Figure9-21C) (9-2‘7) x [C3@ (lo2.’ IO-5 CLf])l4 (9 - 31B)
PG (PL - PG gc

(9 - 28)

or, C, F0.5 uo.O5, (Figure 9 - 21H) (9 - 29) The method as described by Robbins [96] :

Note units change for Figure 9-21G,
1. For operating pressures above 1 atm, multiply Gf
ft/sec
where C, = Vg [p,/(p~ - p~)]~.~, (Equation 9-31C) by (pg).
Y = kinematic liquid viscosity, centistokes (9 - 30) 2. For small packings with F of 200 or greater, substi-
P.d
~
Substitute F and the other knowns into the equa- tute ~ 0 in .previous equation for ~0.1.
tion and solve for G, the gas mass flow rate, lbs/ft2 3. For large packings with Fpd below 15, use (20/Fpd)0.5
sec, or Gg, lb/hr-ft2, as applicable. in place of [Fpd/20]o.5 in previous equation for Lf,
Then, determine the required tower cross-section Equation 9-31D.
area and diameter: 4. Dry bed pressuredrop:

Gas rate, lb/sec, G“ FPd = 278 (A Pdb)/F? (9 - 31E)
Diameter, ft = 1.1283 lii2 Dry bed pressure drop values usually run 0.1 to 0.5 in.
G, lb/sec- ft2 water/ft of packing [96]. Use Equation 9-31B when Lf is

Effects of Physical Properties below 20,000. Packings operate essentially dry when Lf is
below 1,500 (about 3 gpm/ft2) at Fp = 20. Pressure drop at
For nonfoaming liquids, capacity of packing is indepen- flooding is suggested to be predicted by Kister and Gill’s
dent of surface tension. Foaming conditions reduce capac- relationship [93] presented in this text.
ity significantly and design should recognize by selecting Robbins [96] suggested random packed column design
operating pressure drop at only 50% of normal non-foam- is similar to others presented in this text, but high-lighted
ing liquid. to determine diameter of packed column: