Page 240 - Separation process engineering

toluene in the distillate. Pressure is 1 atm. Assume constant relative volatilities α = 2.25, α =
BT
TT
1.0, α = 0.21. Find the optimum feed stages and the total number of equilibrium contacts
CT
required.
D9. A distillation column is separating a feed that is 20 mol% methanol, 50 mol% n-propanol, and 30
mol% n-butanol. Feed is 750 kmol/h. We want a 92 mol% recovery of methanol in the distillate
and 95% recovery of n-propanol in the bottoms. Use L/D = 7. Feed is a saturated liquid. The

column has a total condenser and a partial reboiler. As a guess, assume that n-butanol does not
distribute (it all exits in the bottoms). Do not do iterations to improve this guess. If we choose n-
propanol as the reference, the relative volatilities are methanol = 3.58, n-propanol = 1.0, and n-
butanol = 0.412. Assume relative volatilities are constant. Do not use Aspen Plus. This
calculation is simple enough that it can be done by hand or with a spreadsheet (with or without
Visual Basic for Applications [VBA]) or with MATLAB.
a. Find D, B, x i,dist , and x i,bot .

b. Step off stages using the second stage above the partial reboiler (the third equilibrium contact
from bottom) for feed location and find the total number of stages and the calculated distillate
mole fractions.

D10. What is the bubble-point temperature at 760 mm Hg pressure of a mixture that is 40 mol% n-
pentane and 60 mol% n-hexane? Use Raoult’s law and the Antoine coefficients given in Problem
5.D2.

D11. We have a liquid mixture that is 10 mol% ethane, 35 mol% n-pentane, and 55 mol% n-heptane at
40°C and a high pressure. As the pressure is slowly dropped, at what pressure will the mixture
first start to boil? Use the DePriester chart.
D12. Suppose n-hexane, n-heptane, and n-octane are available so that the desired mole fraction of the
mixture can be changed to any desired value. If the system pressure is 300 kPa,

a. What is the highest possible bubble-point temperature?
b. What is the lowest possible bubble-point temperature?

D13*. Find the bubble-point temperature and vapor mole fractions at the bubble-point for a mixture at
1.0 atm that is 20.0 mol% n-butane, 50.0 mol% n-pentane, and 30.0 mol% n-hexane. Use the
DePriester chart.

E. More Complex Problems

E1. We wish to distill a mixture of ethane, propane, n-butane. The column has a partial reboiler and a
partial condenser and operates at 400 kPa. The feed flow rate is 200 kmol/h. The feed is a
saturated liquid and is 22 mol% ethane, 47 mol% propane, and 31 mol% n-butane. We wish to
recover 97% of the ethane in the distillate and 99% of the propane in the bottoms. The reflux is a
saturated liquid, and the external reflux ratio L /D = 3.0. Find the optimum feed stage and the total
0
number of equilibrium contacts required. Assume constant molal overflow, and use the DePriester
charts or Eq. (2-30) for K values.

H. Computer Spreadsheet Problems

H1. Do part b of this problem with a spreadsheet or with MATLAB. The use of VBA is
recommended if a spreadsheet is used. We have 200 kmol/h of a saturated liquid feed that is 35
mol% methanol, 40 mol% i-propanol, and 25 mol% n-propanol at 1.0 atm. We want 96.1%
recovery of methanol in the distillate and 99.6% recovery of i-propanol in the bottoms. Assume