Page 124 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 113

AT^ (for top conditions) = Ll/(KiVI)
AB^ (for bottom conditions) = L~/(Kivy)
whereATi = absorption factor for each component at
conditions of top tray
(8- 210) AB^ = absorption factor for each component at
conditions of bottom tray.
Mols off gas leaving top tray 9. Read A, values corresponding to AT^ and AB^ values
= VI = V, + 1 - Mols absorbed (assumed) from Figure 8-59.
10. Read EA values for fraction absorbed from Figure
Mols gas leaving bottom tray No. N 8-58 using the A, values of Step 9 and the fixed or
assumed theoretical trays.
= VN = vx + 1 (Vl/vN +
11. Calculate the mols of each component absorbed by:
(Mol component in inlet rich gas) (E~)
(el) . __ .~
Mols gas leaving Tray No. 2 (from top) = V2 = VI 1/ 3 Suggested tabulation:

Factors
Corn- MolsRich K Absorption &,Frac. Mols
Liquid leaving top tray No. 1 = L1 = Lo + V2 - VI ponent Gash Top Bottom AT AB 4 Absorbed Absorbed
- . .. . . ._ .. .. -~
where V2 = vapor leaving tray No. 2 from top, mols/hr - - - - --- - -
-
Lo = lean oil entering (assumed completely free - - - - --- - -
- ---
of rich gas components), mols/hr -. . - - , - .. -
Ls = liquid lealing bottom tray, mols/hr 12. If the result does not yield the desired amount of the
V, = vapor leaving bottom tray, mols/hr
key component absorbed, then reassume the lean oil
Liquid leaving bottom tray quantity, Lo, and recalculate. Adjustments may have
to be made separately or simultaneously in the
= L4 = LO + Mols absorbed (assumed)
assumed absorption quantity until an acceptable
5. Calculate: At top, LI/VI result is obtained. After two or three trials a plot of
At bottom, LN/VN the key variables will assist in the proper assumptions.
6. Use Horton-Franklin method to estimate tempera-
tures at tower trays: Absorption: Number of Tray for Fixed Recovery of Kg
Component
(8-211)
Here we also consider the more general case when the
lean oil contains some of the components to be absorbed
where To = lean oil temperature, "F from the entering gas. The relationships are most conve-
TN = bottom tray temperature, "F niently written as follows [18], for a given component:
Ti = tray, i, temperature, OF
Ty + 1 = inlet rich gas temperature, "F
VI fslo + (1 - fahn + 1 (8 - 212)
These relations assume constant percent absorp-
tion per tray, and temperature change propor- f, = sen+1 - s, (8- 213)
-
tioned to the vapor contraction per tray. For esti- se=+1 1
mating use only.
Temperature bottom tray = Tx = TN + 1 + fa = A,"+' - A,
(assumed rise) A,"+' - 1 (8- 214)
Temperature top tray
Rearranging 8-213 yields
I VS+l - hay2 \
= Tx - (assume rise) A, -1
vN+l-vl 1-fa = (8 - 215)
A,"+1 - 1
7. Read K values from equilibrium charts for compo- Combining Equations &211, 8-212 and 8-214 results in
nents in feed at temperatures of (a) top tray and (b)
bottom tray.
8. Calculate An and AB^ for each component.