Page 201 - Separation process engineering

D14. A distillation column with open steam heating is separating a feed that is 80.0 mol% methanol
and 20.0 mol% water in a steady state operation. The column has 10 stages, a total condenser, and
the feed is on stage 5. Operation is at 1.0 atm. The steam is pure water and is a saturated vapor.
CMO can be assumed to be valid. At 2:16 a.m. 25 days ago the feed and distillate flows were shut
off (D = F = 0), but the steam rate was unchanged and the total condenser is still condensing the
vapor to a saturated liquid. The column has now reached a new steady state operation.

a. What is the current methanol mole fraction in the bottoms?
b. At the new steady state estimate the methanol mole fraction in the liquid leaving the total
condenser.

D15. A partial condenser takes vapor leaving the top of a distillation column and condenses a portion
of it. The vapor portion of mole fraction y is removed as the distillate product. The liquid
D
portion of mole fraction x is returned to the column as reflux. The liquid and vapor leaving
0
partial condensers and partial reboilers can be assumed to be in equilibrium. A distillation
column with a partial condenser and a partial reboiler is separating 300 kmol/h of a mixture that is
30 mol% ethanol and 70 mol% water and is a saturated liquid. We desire a 98% recovery of the
ethanol in the vapor distillate and an 81% recovery of water in the bottoms. If L /D = 2.0 find the
0
optimum feed location and the total number of equilibrium stages.
D16.* Estimate q for Problem 3.D6. Estimate that the feed stage is at same composition as the feed.
D17. A mixture of acetone and ethanol (acetone is more volatile) is fed to an enriching column that has

a liquid side stream withdrawn. The feed flow rate is 100.0 mol/min. Feed is 60.0 mol% acetone
and is a saturated vapor. The liquid side product is withdrawn from the second stage below the
total condenser at a flow rate of S = 15.0 mol/min. The reflux is returned as a saturated liquid.
The distillate should be 90.0 mol% acetone. The external reflux ratio is L/D = 7/2. Column
pressure is 1.0 atm. Column is adiabatic and CMO is valid. Equilibrium data are in Problem
4.D7. Note: Trial and error is not required.

Find the mole fraction of acetone in the sidestream x , the mole fraction of acetone in the bottoms
S
x , and the number of equilibrium stages required.
B
D18. We have a stripping column with two feeds separating acetone and ethanol at 1 atm. Feed F is a
1
saturated liquid and is fed into the top of column (no condenser). Flow rate of F is 100 kgmol/h,
1
and this feed is 60 mol% acetone. Feed F is 40 mol% acetone, it is a two-phase feed that is 80%
2
vapor, and flow rate is 80 kmol/h. We desire a bottoms mole fraction that is 0.04 mole fraction
acetone. The column has a partial reboiler. Equilibrium data are in Problem 4.D7.

a. Calculate ( /B) min .

b. If ( /B) = 1.5, find D and y , the optimum feed stage for feed F , and the total number of stages.
2
D
Please step off stages from the bottom upwards.
D19.* A distillation column is separating acetone and ethanol. The column effectively has six
equilibrium stages plus a partial reboiler. Feed is a two-phase feed that is 40% liquid and 75

mol% acetone. Feed rate is 1000 kmol/h, and the feed stage is fourth from the top. The column is
now operating at a steady state with the bottoms flow valve shut off. However, a distillate product
is drawn off, and the vapor is boiled up in the reboiler. L /D = 2. Reflux is a saturated liquid.
0
CMO can be assumed. p = 1 atm. Equilibrium data are in Problem 4.D7. Find the distillate
composition. If one drop of liquid in the reboiler is withdrawn and analyzed, predict x .
B