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Unformatted text preview: denser with a refrigerant for removing heat at 16oF. Exercise 9.3
Subject:
Selection of type condenser and operating pressure for the distillation of a mixture
of methane, ethane, benzene, and toluene by the direct sequence in two columns.
Given: Feed and product conditions as follows from Fig. 9.20:
kmol/h:
Component
Feed Product 1 Product 2
Methane
20
20.000
0.000
Ethane
5
4.995
0.005
Benzene
500
5.000
485.000
Toluene
100
0.000
0.500
Total:
625
29.995
485.505 Product 3
0.0
0.0
10.0
99.5
109.5 Find: Type condenser and operating pressure in each column.
Analysis: Use Chemcad or other simulator to make the bubblepoint and dewpoint
calculations, with the SRK equation of state. Use Fig. 7.16 to determine pressure and type
condenser.
Column 1:
Distillate is Product 1. Bubble point on Product 1 at 120oF gives an impossible very high
pressure because the critical temperatures of methane and ethane are less than 120oF. A dew
point on Product 1 at 120oF gives 31.4 psia because of the presence of benzene. Therefore, can
use a partial condenser with cooling water. No problems occur at the bottom.
Column 2:
Distillate is Product 2. Bubble point on Product at 120oF gives 5.1 psia. No need to
operate at a vacuum. Raise condenser pressure to 30 psia or just above atmospheric pressure and
use a total condenser, possibly with subcooled reflux. No problems at the column bottom. Subject: Exercise 9.4
Number of equilibrium stages for a deethanizer. Given: Deethanizer with feed and average relative volatilities as follows from Fig. 9.21:
Component
C1
C2
C3
nC 4
nC 5 Feed, kmol/h Avg. Relative volatility
160
8.22
370
2.42
240
1.00
25
0.378
5
0.150 Distillate is to contain 2 kmol/h of C3. Bottoms is to contain 2 kmol/h of C2. Therefore, LK is
C2 and HK is C3.
Assumptions: Relative volatilities are relative to C3.
Find: Number of equilibrium stages for 2.5 times minimum number of stages.
Analysis: To compute the minimum number of stages, use component flow rate form of the
Fenske equation given by (912): log
N min = d C2 bC3 dC3 bC 2 log α C 2 ,C3 log
= 370 − 2 240 − 2
2
2
log 2.42 N = 2.5Nmin = 2.5(11.3) = 28.3 = 11.3 Exercise 9.5
Subject:
Determination of the minimum number of stages by the Fenske equation for each
of three sections in a distillation column with a vapor sidestream
Given: The following feed and product compositions for a distillation column:
kmol/h:
Component
Feed Distillate
Vapor sidestream
Bottoms
Benzene, B
260
257.0
3.0
0.0
Toluene, T
80
0.1
79.4
0.5
Biphenyl, BP
5
0.0
0.2
4.8
Pressure, kPa
165
130
180
200
Assumptions: Raoult's law for Kvalues.
Find: Using the Fenske equation, the minimum number of stages for sections between:
(a) Distillate and feed.
(b) Feed and vapor sidestream.
(c) Sidestream and bottoms.
Analysis: (a) For this section, take the LK as benzene and the HK as toluene. At the top, have
almost pure benzene at 130 kPa = 19 psia. Therefore, from Fig. 2.4, the distillate temperature =
boiling point of benzene at 19 psia = 195oF. At the feed po...
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 Spring '11
 Levicky
 The Land

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