Page 331 - Applied Process Design For Chemical And Petrochemical Plants Volume II

320 Applied Process Design for Chemical and Petrochemical Plants

Packing Wetted Area For vaporization in packed beds of Raschig rings and
Berl saddles [ 661 :
Wetted packing area may differ considerably from the
physical area of a packing. This is of particular importance
in comparing the effectiveness of different packings. (9 - 60)
It is only recently that a coordinated group of data
became available for wetted areas in Raschig ring and Berl
saddle packing [651. (9 - 61)
Figure 9-43 represents the water-air and ammonia-water
data for Berl saddles by [65, 671 : where the subscripts vap and abs represent conditions of
vaporization and absorption respectively, and subscript w
fa = - = 0.35 [ $1 0.20 (9 - 58) represents a water system.
a’
at Entrainment From Packing Surface
and for Raschig rings by There is not much data available on this point. Opem-
fa = - = 0.24 [ $1 0.25 tional experience plus qualitative tests indicate that entrain-
a’
ment is negligible until the packing reaches the flooding
at (9 - 59) condition. See discussion under distillation section.
where fa = fraction of total packing area, a,, that is wetted Example 9-6: Operation at Low Rate, Liquid Hold-up
a’ = wetted packing surface not necessarily same as effec-
tive interfacial surface area for contact, fi2/ft3 A sulfuric acid drying tower uses 98% acid for drying an
at = total packing surface, ft2/ft3 incoming air stream. The pilot plant tests show that 15 ft
L’ = superficial liquid rate, Ib/hr (ftz) of 1-in. ceramic Intalox packing will do this job. The plant
G = superficial gas rate, lb/hr (ftz) scale rates are:

The fraction wetted area immediately indicates the Air = 500 scfm at 90°F and 2 psig
effectiveness of contact for the liquid system in the pack- Acid = 6 gpm at 90°F and sp gr = 1.81
ing. This packing area contact efficiency must be consid-
ered in some design problems. Determine (1) the tower diameter (2) pressure drop
(3) liquid hold-up
Effective Interfacial Area
500 29 460+60 14.7+2
The effective interfacial area is used in mass transfer airrate=359(G) (Gzij) (F)
studies as an undivided part of individual and overall coef-
ficients when it is difficult to separate and determine the = 0.725 lbs/(sec)
effective area. The work of Shulman et.al.,65 presents a
well organized evaluation of other work in addition to
their own. One of the difficulties in correlating tower
packing performance lies in obtaining the correct values Assume fwjmt trial:
for the effective interfacial areas of the packing on which
the actual absorption, desorption, chemical reaction, etc. Inside tower diameter = 12 in.
are completed. Figures 9-47 A, B, C, D, E, F, G present a
(112
correlation for water flow based on the ammonia-water Cross-section area = - 0.785 ft2
=
data of Fellinger [27] and are valid for absorption work. 4
There are differences between wetted and effective area 0 725
=
as discussed by Shulman [65] : (1) wetted-areas increase as then : air rate, G = - 0.923 lb/ (sec) (ft2 )
0.789
packing size decreases; (2) effective area is smallest for the
smallest packings; (3) the effective area seems to go = 3340 lb/ (hr) (ft2 )
through a maximum for the 1-inch size packing although
the larger packings have almost as much area. This is bet- 0 461
=
ter understood in terms of the hold-up data for these liquid rate, L = - 0.388 lb/ (sec) (ft2 )
0.785
packings. = 21201b/(hr) (ft2)