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Unformatted text preview: = 140oF
Based on the feed compositions below, the distillate rate of 36 lbmol/h corresponds to a desired
split between propane and nbutane. The calculations converged rapidly in 3 outerloop
iterations with results given below and on the following pages. The separation with feed stages
at 12 and 15 from the top is much better than the separation with feed stages at 17 and 26. For
the better stage locations:
Top temperature = 118.3oF compared to assumed value of 120oF
Bottom temperature = 252.8oF compared to assumed value of 300oF Analysis: (continued) Exercise 10.24 (continued) Exercise 10.25
Subject: Distillation of a hydrocarbon mixture and comparison with solution in Perry's
Handbook.
Given: Bubblepoint liquid feed of composition given below. Column has partial condenser,
partial reboiler, and 9 equilibrium stages in the column, with operation at 120 psia. Reflux rate =
126.1 lbmol/h and vapor distillate rate = 48.9 lbmol/h. GraysonStreed correlation for Kvalues
and RedlichKwong equation of state for enthalpies.
Find: Product compositions, stage temperatures, interstage flow rates and compositions,
reboiler duty, and condenser duty.
Analysis: The calculations were made with the TOWER model (Insideout method) of the
Chemcad program, with the following specifications:
Partial condenser
Top pressure = 120 psia
No condenser or stage pressure drop. Therefore, bottom pressure also = 120 psia
Number of stages = 11 (includes partial condenser and partial reboiler)
Feed stage = 6 from the top
At the top, reflux rate = 126.1 lbmol/h
At the bottom, bottoms rate = B = F  D = 100  48.9 = 51.1 lbmol/h
Estimated distillate rate = 48.9 lbmol/h (actual rate)
Estimated reflux rate = 126.1 lbmol/h (actual rate)
Estimated temperature of stage 1 = 120oF
Estimated temperature of bottom stage = 250oF
Estimated temperature of stage 2 = 140oF
The feed is given as a bubblepoint liquid at 120 psia. Using the GraysonStreed correlation, the
temperature is computed to be 180.8oF. Based on the feed composition below, the distillate rate
of 48.9 lbmol/h corresponds to a desired split between nbutane and isopentane. The calculations
converged rapidly in 3 outerloop iterations with results given below and on the following pages.
Condenser duty = 1,002,600 Btu/h
Reboiler duty = 1,427,000 Btu/h
Top temperature = 161.4oF compared to assumed value of 120oF
Bottom temperature = 230.0oF compared to assumed value of 250oF
Stage 2 temperature = 173.9oF compared to assumed value of 140oF Analysis: (continued) Exercise 10.25 (continued) Analysis: (continued) Exercise 10.25 (continued) Exercise 10.25 (continued)
Analysis: (continued)
The above results for the product compositions are compared to the results in Perry's
Handbook as follows: Component
Propane
Isobutane
nButane
Isopentane
nPentane
Total: Feed,
lbmol/h
5.00
15.00
25.00
20.00
35.00
100.00 Perry's Handbook:
Distillate, Bottoms,
lbmol/h
lbmol/h
5.00
0.00
14.82
0.18
23.60
1.40
3.21
16.79
2.30
32.70
48.93
51.07 This exercise:
Distillate, Bottoms,
lbmol/h
lbmol/h...
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 Spring '11
 Levicky
 The Land

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