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51.10 The example in Perry's Handbook is for a column with a total condenser, while this exercise calls
for a partial condenser. Thus, this exercise has 11 instead of 10 equilibrium stages.
Nevertheless, the solution from Perry's Handbook shows a slightly sharper separation. This is
probably due to the differences in K-values. For example at a mid-column temperature of 210oF,
the Grayson-Streed correlation predicts a relative volatility between n-butane and isopentane of
1.82, while the graphical correlation used in Perry's Handbook predicts a higher value of 1.94. Subject: Exercise 10.26
Distillation of an light alcohol mixture using UNIFAC equation. Given: Feed of 1000 kmol/h of a bubble-point mixture of 60 mol% methanol (M), 20 mol%
ethanol (E), and 20 mol% n-propanol (P) is sent to the middle stage of a column with a total
condenser, 3 equilibrium stages in the column, and a partial reboiler, and operating at 1 atm. The
distillate rate = 600 kmol/h and the reflux rate = 2,000 kmol/h.
Find: Product compositions, stage temperatures and vapor and liquid traffic, reboiler and
condenser duties, and stage compositions for converged solution.
Analysis: The calculations were made with the TOWER model (Inside-out method) of the
Chemcad program, with the following specifications:
Top pressure = 1 atm
No condenser or stage pressure drop. Therefore, bottom pressure also = 1 atm
Number of stages = 5 (includes total condenser and partial reboiler)
Feed stage = 3 from the top
At the top, reflux rate = 2,000 kmol/h
At the bottom, bottoms rate = B = F - D = 1000 - 600 = 400 kmol/h
Estimated distillate rate = 600 kmol/h (actual rate)
Estimated reflux rate = 2000 kmol/h (actual rate)
Estimated temperature of stage 1 = 30oC
Estimated temperature of bottom stage = 100oC
Estimated temperature of stage 2 = 40oC
The feed is given as a bubble-point liquid at 1 atm. The UNIFAC equation gives a bubble-point
temperature of 70.1oC. Based on the feed composition, the distillate rate of 600 kmol/h
corresponds to a desired split between methanol and ethanol. The enthalpies were computed by
the Latent Heat model with a correction for the heat of solution based on liquid-phase activity
coefficients from the UNIFAC equation. The calculations converged in 8 outer-loop iterations
with results given below and on the following pages.
Condenser duty = 93,592 MJ/h
Reboiler duty = 93,841 MJ/h
Top temperature = 70.1oC compared to assumed value of 30oC
Bottom temperature = 79.2oC compared to assumed value of 100oC
Stage 2 temperature = 67.3oC compared to assumed value of 40oC Exercise 10.26 (continued)
Analysis: (continued) Exercise 10.26 (continued)
Analysis: (continued) Exercise 10.27 Subject:
Distillation of a 4-component mixture with two sidestreams in an attempt to
obtain four nearly pure products.
Given: Feed of composition given below at 150oF and 25 psia. Column pressure of 20 psia at
top and 25 psia at the bottom Column has a total condenser giving saturate...
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