Unformatted text preview: e obtained:
Minimum equilibrium stages = 2.5
Therefore, assume that a rectifying section is not needed. Try a reboiled stripper with 4
stages plus a partial reboiler.
(b) For the rigorous TOWER model of chemcad, use the following specifications:
Top pressure = 36 psia
No condenser pressure drop. Column pressure drop = 2.2 psi
Therefore, bottom pressure = 38.2 psia
Number of stages = 5 (includes partial reboiler)
Feed stage = top stage
At the bottom, bottoms flow rate = 6.2 lbmol/h (corresponds to the specified D/F ratio.
Estimated distillate rate = 86.9 lbmol/h (desired rate)
Estimated reflux rate = feed rate = 93.1 lbmol/h)
Estimated temperature of stage 1 = 300oF
Estimated temperature of bottom stage = 400oF
Estimated temperature of stage 2 = 320oF
The TOWER model converged in 5 iterations, with the following results: Analysis: (b) (continued) Exercise 10.31 (continued) Reboiler duty = 1,379,300 Btu/h Note that the desired split of the two key components is better than desired with:
1.204 lbmol/h of toluene (LK) in the bottoms compared to a specification of 2.1 lbmol/h
0.106 lbmol/h of diphenyl (HK) in the distillate compared to a specification of 1.0 lbmol/h
(c) Because the column has no reflux, the flow rate of the toluene in the bottoms was specified
on the basis that the reboiler duty would be adjusted to obtained that flow rate. The TOWER
model converged in 4 iterations with the following results, which did give the desired toluene
rate in the bottoms, but much less than desired diphenyl in the distillate:
Reboiler duty = 1,354,500 Btu/h Exercise 10.32
Subject: Comparison of two sequences of two distillation columns each for the separation of a
mixture into three products.
Given: Feed at 100oF and 480 psia and desired products with the following compositions in
component flow rates:
Feed Product 1
22.0 Product 2
258.3 Product 3
93.1 In Sequence 1, the keys in Column 1 are methane and benzene, and the keys in Column 2 are
benzene and toluene. For Sequence 2, the keys in Column 1 are benzene and toluene, and the
keys in Column 2 are methane and benzene. Use partial condensers when the bulk of the
methane is taken overhead and set condenser pressures to at least 20 psia, with distillate
temperatures of about 130oF. Assumptions: SRK for K-values and enthalpies. Isobaric operation of columns.
Find: Column pressures, type condensers, and reflux and stage requirements for columns in
each sequence, using a reflux ratio of 1.3 times minimum reflux ratio.
Analysis: First determine column pressures and type condensers, using Chemcad.
Sequence 1, Column 1: Distillate is Product 1.
The computed dew-point pressure at 130oF = 118 psia. Set top pressure to 118 psia with a
Sequence 1, Column 2: Distillate is Product 2. The computed bubble-point pressure at 130oF =
8.4 psia. This is too low. Therefore, set top pressure to 20 psia with a total condenser.
Distillate temperature at this pressure = 187oF.
Sequence 2, Column 1: Distillate is the sum...
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