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Unformatted text preview: al reboiler.
Absorbent oil enters stage 1 (top stage). Feed enters stage 9. Interreboiler with a heat duty of 1.0
MMBtu/h at stage13. Column operates at 230 psia. Overhead flow rate = 103 lbmol/h.
Assumptions: SRK for Kvalues and enthalpies.
Find: Product rates and compositions, stage temperatures, and interstage vapor and liquid flow
rates and compositions. Reboiler duty with and without the interreboiler. Whether an
interreboiler is worthwhile and if an intercooler should be considered.
Analysis: The calculations were made with the Tower model (Insideout method) of Chemcad,
with the following specifications:
No condenser
Top pressure = 230 psia
Column pressure drop = 0 psi.
Number of stages = 15 plus a partial reboiler
Absorbent oil (1st feed) to stage 1
Feed (2nd feed) to stage 9
Interreboiler (3rd feed) to stage 13
At the top, no condenser
At the bottom, bottoms flow rate = 459  103 = 356 lbmol/h
Estimated distillate rate = 103 lbmol/h (specified rate)
Estimated reflux rate = 40 lbmol/h
Estimated temperature of stage 1 = 80oF
Estimated temperature of bottom stage = 300oF
Estimated temperature of stage 2 = 90oF
Note that the interreboiler is treated as the 3rd feed with a specification as a stream of 1.00
MMBtu/h. Convergence was achieved in 7 iterations. The results are shown on the following
two pages. The column feed is 14.2% vaporized. The distillate rate is set to send the methane
and ethane overhead, while a portion of the propane and all heavier components to the bottoms.
The temperature of the interreboiler stage is 202.6oF compared to a bottoms temperature of
302.4oF. The heat duty of the reboiler is 3.164 MMBtu/h. Thus, almost 25% of the total
required heat duty can be supplied by waste heat..
Results without the interreboiler, on a subsequent page, show only a small effect on the
separation, with the major effect being 17% more ethane leaving in the overhead vapor and a
reboiler duty of 4.177 MMBtu/h. If an intercooler of a heat duty of 100,000 MMBtu/h is added
at stage 4, along with the interreboiler at stage 13, as shown below, less ethane leaves in the
overhead vapor. Thus, the use of an intercooler is not desirable. Exercise 10.40 (continued)
Analysis: (continued) With interreboiler Exercise 10.40 (continued)
Analysis: (continued) With interreboiler Exercise 10.40 (continued)
Analysis: (continued) Without interreboiler Exercise 10.40 (continued)
Analysis: (continued) With interreboiler and intercooler Exercise 10.41
Subject: Design of a reboiled stripper.
Given: Feed of composition below at 39.2oF and 150 psia. Column with 7 equilibrium stages
plus a partial reboiler, operating at 150 psia. Bottoms flow rate of 99.33 lbmol/h.
Assumptions: SRK for Kvalues and enthalpies.
Find: Product compositions, stage temperatures, interstage flow rates and compositions, and
reboiler duty.
Analysis: : The calculations were made with the Tower model (Insideout method) of
Chemcad, with the following specifications:
No condenser
Top pressure = 230 psia
Column pressure drop =...
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 Spring '11
 Levicky
 The Land

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