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Unformatted text preview: of methanol (M) from a mixture containing isopropanol (P) and water (W)
by distillation in a sieve-tray column, 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.
Assumptions: Assume both phases are perfectly mixed on each tray. Operation is at about 80%
Find: The separation 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, 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.
Problem converged in 10 iterations. The separation achieved is as follows:
Total: Feed Distillate Bottoms 100.0
150.0 Mole fraction:
0.6662 Exercise 12.10 (continued) Analysis: (continued)
Shown below and on the next page are plots of tray liquid temperatures, vapor and liquid
traffic, liquid composition profile, and mass-transfer rate profile from the Chemsep run. Analysis: (continued) Exercise 12.10 (continued) Exercise 12.10 (continued)
Shown below are the component profiles for the Murphree vapor tray efficiencies. For
methanol, they range mainly from 39.5% to 92.61%, with most efficiencies in the range from 60
to 68%. For isopropanol, the efficiencies range mainly from 60.4% to 69.0%. For water, the
efficiencies range from 65.8% to 73.4%. These values seem reasonable for perfect mixing on the
trays. Exercise 12.11
Subject: Separation of a mixture of acetone (A) and methanol (M) by extractive distillation
with water (W) in a sieve-tray column, using a rate-based method.
Given: A feed of 50 kmol/h of A and 150 kmol/h of M at 1 atm and 60oC is sent to tray 35 from
the top of a column containing 40 trays. 50 kmol/h of W at 1 atm and 65oC is sent to tray 5 from
the top tray. Column operates at 1 atm. Reflux ratio is 10 and bottoms mole flow rate = 200
kmol/h. Use UNIFAC method for liquid-phase activity coefficients and the AIChE method for
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- Spring '11
- The Land