Page 119 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Part 2: Hydrocarbon Absorption and Stripping

(With Contributions by Dr. P. A. Bryant) assumed number of actual trays (or an existing col-
umn with trays).
Many operations in petrochemical plants require the (a) Theoretical trays, N = (tray efficiency, Eo,) (no.
absorption of components from gas streams into “lean” actual trays)
oils or solvents. The resultant “rich” oil is then stripped or (b) Fraction absorbed
denuded of the absorbed materials. The greatest use of
this operation utilizes hydrocarbon materials, but the (8-195)
basic principles are applicable to other systems provided
adequate equilibrium data are available.
Several methods [17, 18, 29, 40, 62, 67, 2231 for han- where Yo* is often considered zero or very small.
dling this design have been offered and each has intre Solve using Ai values.
duced a concept to improve some feature. An approxima- 7. Mols each component absorbed/hr.
tion method combination of Kremser-Brown [40,67] and
a more complete method of Edmister [18] will be sum- = (VJ (n + 1)i) (Ea9
marized. Figure 8-57 summarizes the system and termi- 8. Mols each component absorbed/ (mol inlet lean oil)
nology. The accepted nomenclature for absorption and (hr) = X~R
stripping is located on page 121. 9. Mols of each component in gas out top of absorber:
= (mols component in inlet gas)-(mols component
Kremser-Brown-Sherwood Method- absorbed)
No Heat of Absorption [18] 10. Mols of component in gas out top of absorber/mol
of inlet rich gas:
This method gives reasonably good results for systems
involving relatively lean gas and small quantities being
absorbed. For rich gases the error can be considerable Ei = yN+l -yl
yN+l -yo *
(more than 50% for some components). It has given gen-
erally good results on natural gas and related systems. Solve for Y1
11. Correct values from first calculation, Steps 1
Absorption-Determine Component Absorption in Fixed through 10, using the ZX~R values of Step 8, as fol-
Tray Tower (Adapted in part from Ref. 18).
lows.
1. Calculate the total mols of gas inlet to the absorber 12. Calculate Ai:
identifying the quantities of individual components.
2. Assuming the tower pressure as set and an average of (8 - 196)
top and bottom temperatures can be selected (these
may become variables for study), read equilibrium &
values from charts for each component in gas. 13. Calculate absorption efficiency, Ei, using new Ai
3. Assume or fix a lean oil rate. value
4. Calculate
-A.
A~N+~
Mols/
L,= hr lean oil in E,i = Ai N+1 - 1 , read Figure 8 - 58
(8-194)
VN+I Mols/ hr rich gas in
14. Calculate mols absorbed/hr:
Assume this value constant for tower design. = (E~) (mols component in inlet rich gas)
5. Calculate absorption factor, Ai = Lo/(V~ + 1) (b),
using values of (2) and (4) above for each component. 15.Mols of each component in gas out top of
6. Calculate fraction absorbed for each component, absorber/hr = (mols component in)-(mols com-
assuming a fixed overall tray efficiency for an ponent absorbed)

108