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interreboiler. Thus, in Section 2 between the intercondenser and the feed stage, the L/V ratio is
higher than the value in Section 1 between the overhead condenser and the intercondenser. Thus,
the two operating lines above the feed stage have different slopes, but by material balance both
lines pass through the distillate composition on the 45o line.
Correspondingly, in Section 3 between the interreboiler and the feed stage, the L/V ratio
is lower than the value in Section 4 between the reboiler and the interreboiler. Thus, the two
operating lines below the feed stage have different slopes, but by material balance both lines pass
through the bottoms composition on the 45o line. Also note that by material balance, the
operating lines for Sections 1 and 4 intersect on the 45o line, and the operating lines for Sections
2 and 3 intersect on the 45o line. Starting from xD = 0.9974, stages are stepped off from the top
between the operating line for Section 1 and the equilibrium curve for a few stages before switch
to the operating line for Section 2. Then 5 stages are stepped off until the feed stage is reached.
Then, 4 stages are stepped off between the operating line for Section 3 and the equilibrium curve
before switching to the operating line for Section 4.
The switches are made to maintain the same number of total stages, 15, and the same
location for the feed stage. Other combinations of arbitrary operating lines and a q-line can be
used to illustrate the potential of an intercondenser and interreboiler. The important thing to note
is that the addition of an intercondenser and interreboiler increases the distance between the
equilibrium curve and the operating lines for Sections 2 and 3 so that the steps in Sections 2 and
3 accomplish larger composition changes. Exercise 7.38 (continued)
Analysis: Addition (continued) Exercise 7.39
Distillation of a mixture of para-dichlorobenzene (P) and ortho-dichlorobenzene
(O), two close-boiling isomers, using the McCabe-Thiele diagram, with the Kremser equation to
accurately calculate the separations at the two ends of the column.
Given: Feed of 62 mol% P and 38 mol% O that is slightly vaporized with q = 0.9. Distillate is
liquid of 98 mol% P. Bottoms is 96 mol% O. Pressures at top and bottom are 20 psia and 15
psia, respectively. Reflux ratio, R = 1.15 times minimum.
Assumptions: Constant molar overflow. Total condenser and partial reboiler. Average
relative volatility based on Raoult's law.
Find: Number of theoretical stages from McCabe-Thiele diagram, using Kremser supplement
for the two ends of the column.
Analysis: From a simulation program or handbook, the temperatures at the top and bottom of
the column, based on the given pressures and product compositions, are determined to be
approximately 350 and 380oF. From vapor pressure data, e.g. from CHEMCAD, using Eq. (7-1),
α P,O at 350o F = PPs 15.65
P s 2316
= 1167 and α P,O at 380o F = Ps =
PO 19.98 Take the average relati...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
- Spring '11
- The Land