Page 122 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 111

Sometimes the last term on right can be neglected. At lean end, Yi = 0 (or nearly so in most cases); if
7. Calculate number of theoretical trays, M. not, plot accordingly.

(8 - 205) Stripping-Determine Stripping-Medium Rate For Fixed
Recovery [18]

(8 - 206) 1. The composition and quantity of rich oil, and per-
cent recovery of a specified key component are
known, also column pressure and temperature.
8. Actual trays at operating reflux: 2. Using Figure 8-58, assume a value for theoretical
plates, read S, corresponding to specified value of
M
M. =A (8 - 207) recovery E,i for key component, since:
dCt
EO
9. Calc-date for each component corrected amount E s- - (SM+l - S)/(S'+l - 1)
stripped: Note that with this procedure, the effect of the num-
For each component: ber of theoretical plates available can be determined.
In an existing column where the number of trays are
(8 - 208) fmed, the theoretical trays can be obtained by evalu-
ating an efficiency for the system.
3. The value of S, = Si for key component obtained in
(Vo/Lo)oper = fixed in Step 5. Step 2 is equal to

(1 + mi) and (1 + ZXi) come from Step 6.
10. From Figure 8-58, read (SM + ' - S)/S'+ ' - 1) = Esi
for each component at the fured theoretical required
trays and at individual Si values. Using key component:
11. For final detail, recalculate mols stripped per hour
from new E,i values and the total quantities of each
component in the incoming rich oil. If values do not
check exactly, adjustments can be made in steam
rate and XYi to give exact values. In many cases this Set up table: use I& for each component to calcu-
accuracy is not justified since the method is subject late Column 4.
to some deviation from theoretically correct values.
.
.
12. A graphical solution is presented by Edmister [18] Corn- Mols/hr QatCol. { Vo (1 + ZYi)) Mols/hr
and handled like stepwise distillation. ponent inRichOd Cond. I$ b(l+m &i Stripped
Equilibrium line: starts at origin of X-Y plot. - - ,... ~ - - -
~
For assumed X values, calculate Y corresponding - - - - - -
-
for key component from - - - - - -
z z
~
From values of Si calculated (=S,) , read E,i values
At lean oil end of tower: ZZi = 0 and XYi = 0. from Figure 8-58 at the number of theoretical trays
Slope of equilibrium line is Y/X = K, assumed in Step 2. Note that the S, corresponds to
At rich oil end of tower: the number of trays selected, hence will give a value
for performance of the system under these particular
conditions.
-
(8 209)
4. Calculate the mols of each component stripped/hr
Where Xi, XXi and XYi are known. R = rich end.
Operating line:
5. Calculate, V,, mols/hr. of stripping medium required
Mols lean oil leaving stripper (steam or gas)
Slope = Lo /Vo =
Mols stripping steam (or gas) entering