Unformatted text preview: of Products 1 and 2. The computed dewpoint
pressure at 130oF = 6.7 psia. This is too low. Therefore, set top pressure to 20 psia with a partial
condenser. Distillate temperature at this pressure = 190oF.
Sequence 2, Column 2: Distillate is Product 1. Therefore, again set pressure to 118 psia with a
partial condenser. Analysis: (continued) Exercise 10.32 (continued) Now, use the Shortcut model in Chemcad to estimate stage and reflux ratios using R =
1.3Rmin. Results are as follows, where recoveries are of the column feed to the distillate:
Sequence 1 Sequence 1 Sequence 2 Sequence 2
Column 1
Column 2
Column 1
Column 2
Pressure, psia
118
20
20
118
Light key, LK
Methane
Benzene
Benzene
Methane
Heavy key, HK
Benzene
Toluene
Toluene
Benzene
Recovery of LK
0.9948
0.9870
0.9870
0.9948
Recovery of HK
0.00495
0.00118
0.00118
0.005012
Minimum stages
2.04
14.37
13.28
2.01
Min. reflux ratio
0.0685
1.188
0.930
5.628
Reflux ratio
0.0822
1.425
1.209
7.317
No. of stages
8.97
32.35
29.23
4.55
Feed stage from top
4.60
23.99
21.34
2.26 Now design the two sequences using the rigorous Tower model of Chemcad. For each column,
the top specification is the flow rate of the LK in the distillate and the bottom specification is the
flow rate of the HK in the bottoms. The stage specifications and estimated reflux and distillate
rates are taken from the above shortcut results, with some modest changes. The rigorous Tower
model then computes the required reflux ratio and distribution of nonkey components. The
results are as follows, where the total stages includes the condenser (total or partial) and the
reboiler. The feed stage is counted from the condenser as stage 1.
Sequence 1 Sequence 1 Sequence 2 Sequence 2
Column 1
Column 2
Column 1
Column 2
Top pressure, psia
118
20
20
118
Pressure drop, psia
2
2
2
2
Total stages
3
30
30
5
Feed stage
2
22
21
2
Reflux ratio
2.2
1.363
1.101
0.0042
Condenser duty, Btu/h
892,100
8,122,800
4,074,800
1,900
Reboiler duty, Btu/h
4,227,300
6,135,800
8,843,100
2,748,100
T of bottoms, oF
347.4
258.8
258.8
404.6
Lbmol/h of toluene in Product 1
0.063
0.0002
Sequence 2 requires an expensive twostage compression system to take the vapor distillate from
Column 1 at 20 psia and increase its pressure to 119 psia to deliver it to Column 2. Note that
Column 2 uses such a small amount of reflux, that it is essentially just a reboiled stripper.
Sequence 1 is simpler and preferred over Sequence 2. However, the first column in Sequence 1
would benefit with more reflux and stages to reduce the loss of toluene. Exercise 10.33
Subject: Separation of propylene from propane by distillation using a rigorous equilibriumstage model
Given: Feed of 360 lbmol/h of propylene and 240 lbmol/h of propane at 125.7oF and 294 psia.
Distillate is to be 99 mol% propylene, and bottoms is to be 95 mol% propane. Column is
equipped with a partial condenser and a partial reboiler. Pressure is 280 psia in the reflux drop.
Condenser pressure drop is 2 psia, and tray pressure drop is 0.1 psi/tray. Tray efficiency is 10...
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 Spring '11
 Levicky
 The Land

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