Unformatted text preview: istillate and bottoms compositions:
Component
Flow, kmol/h:
Methanol
Isopropanol
Water
Total: Feed Distillate Bottoms 100.0
50.0
100.0
250.0 90.09
8.89
0.02
100.00 8.91
41.11
99.98
150.00 Mole fraction:
Methanol
0.4000
Isopropanol
0.2000
Water
0.4000 0.9109
0.0889
0.0002 0.0594
0.2740
0.6666 Backcalculated values of HETP ranged from about 1.3 to 2 ft for M and P, and 1.1 to 1.4 ft for
W. Column diameter was computed to be 5.1 ft with a column pressure drop of 0.43 psi. Exercise 12.14
Subject: Separation of a mixture of methanol (M) from a mixture containing isopropanol (P)
and water (W) by distillation in a sievetray column for various conditions of % flood, weir
height, and % hole area, using a ratebased method.
Given: A bubblepoint feed of 100 kmol/h of M, 50 kmol/h of P, and 100 kmol/h of W at 1 atm
is sent to tray 25 from the top tray of a column containing 40 trays, operating at 1 atm. Reflux
ratio is 5 and bottoms mole flow rate = 150 kmol/h. Use UNIFAC method for liquidphase
activity coefficients and the ChanFair method for masstransfer coefficients and interfacial area.
Consider combinations of:
80%, 60%, and 40% of flooding.
3, 2, and 1 inch weir heights.
14%, 10%, and 6% hole area.
Assumptions: Assume both phases are perfectly mixed on each tray.
Find: The separations achieved.
Analysis: The Chemsep method is applied with the Simple Distillation option, a total
condenser, a partial reboiler, and 42 stages (counting the condenser and the reboiler). Thus, the
feed is sent to stage 26 from the top. To estimate Kvalues from the UNIFAC method, the ideal
gas law is assumed with DIPPR vapor pressure, and excess enthalpy for the liquid phase.
For physical properties necessary to size the column diameter and compute masstransfer
coefficients and interfacial area, the following options were chosen:
Rackett equation for liquid density.
Wilke and DIPPR for vapor viscosity.
Molar averaging of DIPPR pure component liquid viscosity, vapor thermal conductivity, liquid
thermal conductivity, and surface tension. Binary liquid diffusivities from WilkeChang
method.
Neglect pressure drop.
Automatic initialization.
Newton's method, but maximum change of liquid or vapor flow rate of only 10% and maximum
temperature change of only 2oC.
It was found that the % hole area had a negligible effect on the purity of methanol in the
distillate. Therefore, only combinations of % flood and weir height are reported here. The
separations achieved are given below in terms of the methanol purity in the distillate in mole%.
% Flood
80
80
80
60
60
60
40
40
40
Weir height, in.
1
2
3
1
2
3
1
2
3
Mole % M in distillate 90.54 90.76 90.86 90.03 90.34 90.71 89.32 89.81 90.17
For this problem, the separation achieved is not significantly affected by the % flood or weir
height. Exercise 12.15
Subject: Effect of % flood on the Murphree efficiency for oxygen and the separation of air in
the upper column of an air separation system, using a ratebased method.
Given: A sievetray column with 48 plates, no condenser, and a split reboiler that gives 50
mol% liquid bottoms and 50 mol% vapor bottoms. A feed at 80 K, 131.7 kPa, and 1349 lbmol/h,
with a composition of 97.868 mol% N2, 0.365 mol% A, and 1.767 mol% O2, entering the top
tray. A feed at 83 K, 131.7 kPa, and 1832 lbmol/h,...
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 Spring '11
 Levicky
 The Land

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