Page 111 - Applied Process Design For Chemical And Petrochemical Plants Volume II

100 Applied Process Design for Chemical and Petrochemical Plants

y~ + 1 = mol fraction VOC component in the incoming The Henry’s Law constant, H, can be substituted for the
fresh air, equals zero for fresh air equilibrium constant, K, when the system operates at or
L = volumetric flow rate for incoming contaminated very close to atmospheric pressure:
water
V = volumetric flow rate for incoming fresh air H = p*/x* (8-193)
Vmin = minimum fresh air flow required based on slope
of operating line L/V on x-y diagram where p* = the partial pressure, atm, of the contaminant in
xx = mol fraction VOC contaminant in exiting water equilibrium with x*
stream, usually aimed at meeting the environmen-
tal regulations Tables 8-8 and 8-9 provide values for selected Henry’s
S~,, minimum stripping factor at minimum flow rate Law Constants respectively [ 1431.
=
for air
Sop, = optimum stripping factor, where treatment costs The optimum stripping factor, Sopt, is expressed as a
are a minimum, referenced to costs of utilities, percent of residue, (100) (XK/X,), for water rates of 30
maintenance, depreciation, labor. As economic gpm, 300 gpm, and 3,000 gpm.
conditions change one may need to adjust Sopt,
see Reference 143. Sopt = 1 + aHb

The concentrations of most of the VOC compounds in Constants a and b were determined from a linear
the contaminated water are usually expressed in pprn as regression for XN/X, = 4.75% and XN and x, = 0.05% for
are the remainder residue compounds in the water exiting the packed and tray towers. The optimum stripping factor
the tower. These are usually small values. As an approxi- decreases as the Henry’s Law constant decreases. Due to
mation: the complex relationship between cost and performance,
the authors [143] recommend caution in attempting to
extrapolate from the water flowrate ranges shown.

where K = equilibrium constant (varies for each component) Example 8-32: Stripping Dissolved Organics from Water
K = y*/x* in a Packed Tower Using Method of Li and Hsiao [143]
y* = equilibrium molar fraction of VOC components in
air
x* = equilibrium molar fractions of VOC components in Using a packed tower, remove hexachloroethane
water (HCE) concentration of 110 pprn in water to 0.05 pprn
using fresh air operating at essentially atmospheric pres-
Minimum stripping factor at corresponding minimum sure using a fan/blower putting up 14 in. water pressure.
air flowrate: The concentration of propylene dichloride (PDC) in the
contaminated water is 90 ppm, and is to be reduced to
Smin = K/(L/Vmjn) = 1.0 (8-190) 0.05 pprn in the exiting water. The water flowrate = 300
gpm. The required packing (or trays) must be determined
Vmin = L/K by using a vapor-liquid equilibrium plot, setting slope L/V
and stepping off the number of stages or transfer units.
The component with the lowest equilibrium constant is See Figure 8-55.
called the key component in the stripping process, From Table 8-9 (Packed Tower) :
because it yields the largest value of Vmin. This largest
value is the “true” minimum air flowrate, whereas the actu- Hexachloroethane: Henry’s Law constant = 547.7 atm
al air flowrate should be selected at 1.20 to 2.0 times the Propylene dichloride: Henry’s Law constant = 156.8 atm
minimum. This becomes a balance between fewer theo-
retical stages at actual air flowrate, yet requires a larger 1. For hexachloroethane: XN/X, = 0.05 ppm/100 pprn
diameter column to carry out the operation. = 0.05%
It can be important to examine the problem and evalu- For propylene dichloride: XN/X, = 0.05 ppm/100
ate the optimum stripping factor based on related costs, pprn = 0.05%
thus: 2. Sopt = 6.0 for HCE, and 3.9 for PDC.

3. For HCE:
Sop = K (L/Vopt) (8-191)
Vmin = L/K = (300) (8.33) (359 scf/mol) / (18 lb/mol)
(547.7) = 91.1 scf/minute