Page 110 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 99

(text continuedfiam page 95) Stripping Volatile Organic Chemicals (VOC) fmm Water
with Air
ical. Actual trays at an estimated 65% tray efficiency =
6/0.65 = 9.23 or use 10 actual trays in the column itself. Li and Hsiao [143] provide a useful approach to the
environmental problem of removing (by stripping)
Example 8-31: Multicomponent F ' tion of Reflux volatile organics from solution in a contaminated water
Ratio and Distillate to Feed Ratio stream by using fresh air as the stripping medium. It
should be noted that a number of industrial firms per-
The detailed calculations of Figure 8-54 present an form this stripping with steam. The mass balance on the
example of the excellent performance analysis informa- VOC component around the column (trayed or packed)
tion that can be developed by an orderly or systematic as shown in Figure 8-55 uses the symbols of Reference 143.
study of the variables in a multicomponent system. There
are other variables to be studied as well. -= Yi -Yn+l (slope of operating line) (8- 189)
This design is targeted to produce 99.5 weight % propy- v Xo-XN
lene overhead while not allowing more than l weight % in
the bottoms. where xo = VOC mol fraction (ratio of number of mols of a
Note that in a high purity condition as is represented in specific VOC component in water solution to the
this example, the system is quite sensitive to the overhead total mols of all contaminants contained in the
water)
withdrawal rate (product from the system). This system is XN = mol fraction of VOC component in the stripped
for the purification of propylene from a feed high in propy- water
lene, with lessor amounts of propane, butane, and ethane. N = number of trays (theoretical) or transfer units for
Without a digital computer the detail of Figure &54 a packed tower
would be practically impossible and cost prohibitive in yi = mol fraction VOC component in exiting VOC
terms of time involved. contaminated air

100.0

c
0
3
2 99.8
a
a,
-
c
a,
2-
p.
99.6
a
*
0
2- 99.5,
.-
Y
L
S
a
g 99.4
99.2

.2 .3 .4 .5 .6 .7 .8 .9 1.0 2 3 4 5 6 7 8 910
Wt. % Propylene in Bottoms
Figure 8-54. Effect of reflux ratio and distillate feed ratios on propylene content of product and bottoms for Example 8-29.