Unformatted text preview: istillate and bottoms compositions:
Total: Feed Distillate Bottoms 100.0
150.00 Mole fraction:
0.6666 Back-calculated 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 sieve-tray column for various conditions of % flood, weir
height, and % hole area, using a rate-based method.
Given: A bubble-point 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 liquid-phase
activity coefficients and the Chan-Fair method for mass-transfer 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 K-values 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 mass-transfer
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 Wilke-Chang
Neglect pressure drop.
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%.
Weir height, in.
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 rate-based method.
Given: A sieve-tray 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,...
View Full Document
- Spring '11
- The Land