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Unformatted text preview: in the bottoms compared to a specification of 0.4 lbmol/h Analysis: (d) (continued) Exercise 10.30 (continued) Analysis: (d) (continued) Exercise 10.30 (continued) Analysis: (d) (continued) Exercise 10.30 (continued) For the rigorous TOWER model of chemcad, now use the following specifications for the case of
a reflux ratio = 1.5 times minimum, with 15 theoretical plates and feed to plate 9 (assume from
Top pressure = 314 psia
No condenser pressure drop. Column pressure drop = 2 psi
Therefore, bottom pressure = 316 psia
Number of stages = 17 (includes partial condenser and partial reboiler)
Feed stage = 10 from the top
At the top, reflux ratio = 1.5(0.87) = 1.305
At the bottom, bottoms flow rate = 40.1 lbmol/h
Estimated distillate rate = 59.9 lbmol/h (actual rate)
Estimated reflux rate = 59.9(1.305) = 78.17 lbmol/h (actual rate)
Estimated temperature of stage 1 = 73oF
Estimated temperature of bottom stage = 302oF
Estimated temperature of stage 2 = 83oF
The TOWER model converged in 3 iterations, with the following results for the products:
Note that the desired split of the two key components is not achieved:
0.6269 lbmol/h of nC4 (HK) in the distillate compared to a specification of 0.3 lbmol/h
0.7256 lbmol/h of C3 (LK) in the bottoms compared to a specification of 0.4 lbmol/h Exercise 10.30 (continued) Analysis: (e) (continued)
The calculations with the TOWER model of Chemcad are repeated here for the case of a
reflux ratio = 1.3 times the minimum, with 24 total stages (including the partial condenser and
partial reboiler) and a variable feed stage in an attempt to make the best separation. We compare
the results on the basis of the lbmol/h of nC4 (HK) in the distillate product as follows:
15 lbmol/h of nC4
0.559 From these results, it is seen that the optimal feed location is stage 11 (stage 10 in the column),
with stage 10 giving almost the same separation.
For the case of a reflux ratio of 1.5 time the minimum, the desired separation could not be
achieved for any feed stage location. Exercise 10.31
Subject: Separation of toluene from biphenyl (diphenyl) by distillation
Given: The following feed at 264oF and 37.1 psia, and the desired products:
Feed Distillate Bottoms
Reflux ratio = 1.3 times minimum with total condenser and partial reboiler. Pressure at top = 36
psia. Pressure at bottom = 38.2 psia.
Assumptions: SRK equation of state for K-values and enthalpies.
Find: (a) Actual reflux, number of equilibrium stages, and feed stage location.
(b) Separation for a D/F = 86.9/93.1 = 0.9334
(c) Specified separation by adjusting reflux ratio in Part (b)
Analysis: (a) The recovery of the LK, toluene, to the distillate = 82.5/84.6 = 0.9752. The
recovery of the HK, biphenyl, to the distillate = 1.0/5.1 = 0.1980. Using the Shortcut (FUG)
model of Chemcad, the following results ar...
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