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Unformatted text preview: d liquid, a partial
reboiler, and 28 equilibrium stages in the column with feed entering stage 14. Distillate rate =
14.08 lbmol/h and reflux ratio = 20. A liquid sidestream of 19.53 lbmol/h is withdrawn at stage
10 from the top, and a vapor sidestream of 24.78 lbmol/h is withdrawn at stage 24 from the top.
PengRobinson equation of state for Kvalues and enthalpies.
Find: Product compositions. Stage temperatures, vapor and liquid flow rates, and
compositions. Reboiler and condenser duties.
Analysis: The calculations were made with the TOWER model (Insideout method) of the
Chemcad program, with the following specifications:
Total condenser
Top pressure = 20 psia
Total stage pressure drop = 5 psia. Therefore, bottom pressure = 25 psia
Number of stages = 30 (includes total condenser and partial reboiler)
Feed stage = 15 from the top
Liquid sidestream from stage 11 from the top, at a flow rate of 19.53 lbmol/h
Vapor sidestream from stage 25 from the top, at a flow rate of 24.78 lbmol/h
At the top, reflux ratio = 20
At the bottom, bottoms rate = B = F  D  S1  S2 = 98.33  14.08  19.53  24.78 = 39.94 lbmol/h
Estimated distillate rate = 14.08 lbmol/h (actual rate)
Estimated reflux rate = 280 lbmol/h (close to actual rate of 281.6 lbmol/h)
Estimated temperature of stage 1 = 120oF
Estimated temperature of bottom stage = 200oF
Estimated temperature of stage 2 = 140oF
Estimated 1st draw rate = 19.53 lbmol/h (actual rate)
Estimated 2nd draw rate = 24.78 lbmol/h (actual rate)
The feed condition at the specified temperature and pressure is computed to be just slightly
greater than the bubblepoint temperature. Based on the feed composition below, the distillate,
sidestreams, and bottoms correspond to the flow rates of the four components in the feed. The
calculations converged rapidly in 4 outerloop iterations with results given below and on the
following pages.
Condenser duty = 2,834,600 Btu/h
Reboiler duty = 3,493,700 Btu/h
o
Top temperature = 47.7 F compared to assumed value of 120oF
Bottom temperature = 289.0oF compared to assumed value of 200oF
Stage 2 temperature = 49.9oF compared to assumed value of 140oF Exercise 10.27 (continued) Analysis: (continued)
A high reflux ratio is needed to minimize the effect of feed composition when attempting
to obtain nearly pure sidestreams. Analysis: (continued) Exercise 10.27 (continued) Exercise 10.28
Subject: Distillation of a hydrocarbon mixture given in Exercise 10.25. Given: Bubblepoint liquid feed of composition given below. Column has a partial condenser
and a partial reboiler, with operation at 120 psia. Recovery of light key, nbutane, to distillate =
98%. Recovery of heavy key, isopentane, to bottoms = 98%. GraysonStreed correlation for Kvalues and RedlichKwong equation of state for enthalpies.
Find: Product compositions, reflux ratio, numbers of equilibrium stages above and below the
feed, stage temperatures, interstage flow rates and compositions, reboiler duty, and condenser
duty.
Analysis: The calculations were made with the TOWER model (Insideout method). Initially,
it w...
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 Spring '11
 Levicky
 The Land

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