Page 121 - Applied Process Design For Chemical And Petrochemical Plants Volume II

110 Applied Process Design for Chei mica1 and Petrochemical Plants

3. From a fixed percentage of recovery for key compo-
nent (= E,i for key component), mols component
(8 - 202)
stripped/hr = Gmi 0 (L, + 1) (Xm + 1) (Ed
4. Estimate stripping efficiency for components other
a limiting value of unity. than the key by:

Stripping-Determine Theoretical Trays and Stripping (8 203)
-
Steam or Gas Rate For a Component Recovery [ 181
The rich gas from the absorption operation is usually
stripped of the desirable components and recycled back to
the absorber (Figure 8-57). The stripping medium may be
steam or a dry or inert gas (methane, nitrogen, carbon
oxides-hydrogen, etc.). This depends upon the process
application of the various components.

1.The rich oil flow rate and absorbed component
compositions (this is the only composition of con-
cern, not the oil composition, unless reaction or
change takes place under the system conditions) are
known. From the temperature levels of the available
condensing fluids (water, refrigerant, etc.) , deter-
mine a column operating pressure which will allow
proper condensation of the desirable components
at the selected temperature, allowing for proper At Note that no recovery can be greater than 1-00, so
for efficient heat transfer. The condensing pressure any value so calculated is recorded as 1.00, indicat-
(and column operating pressure) may be dictated ing that the component is completely stripped from
by the available steam pressure used in stripping or the rich oil. Calculate mols stripped per hour for
the pressure on the inert stripping gas. each component as in Step 2.
2. From K charts, determine I(4 values for each compo- 5.The minimum stripping medium (steam or gas)
nent at the column temperature and pressure. lean oiI ratio is estimated by a trial and error proce-
dure based on key component:
By assuming several values of V,, plot V,/L, versus
Esw (1 + ZXi) /KRey (1 + mi). The point where they
are equal gives the minimum value for V,/L,. This
calculation can be thought of as assuming equilibri-
um at the gas outlet end and being slightly conserv-
ative by including the (1 + ZXi) term. Operation at
this point requires infinite plates; therefore, values
larger than the minimum should be used. For eco-
nomical as well as reasonable operation several val-
ues of (V0/L,,),, should be tried and correspond-
ing plates evaluated.
V, (operating) = (assumed (V,/Lo),per) (Lo inlet),
mols/hr
6. Calculate Si for the key component, using the value
of (1 + ZXi) calculated in Step 5. Calculate

-
Figure 8-57. Flow diagram of absorption-stripping for hydrocarbon (8 204)
recovery from gaseous mixture. Used by permission, Edmister, W. C.,
Petroleum Engr., Sept. (1 947') to January (1 948).