Page 200 - Separation process engineering

the distillate flow rate is D = 80 kmol/h. We desire a distillate mole fraction of x = 0.66 mole
D
fraction ethanol and a bottoms that is x = 0.04 mole fraction ethanol. You can assume that CMO
B
is valid. Equilibrium data are in Table 2-1.
a. Find the flow rates F and B.
1
b. Find the liquid and vapor flow rates in the middle section, L′ and V′.

c. Determine and plot the operating lines. Be neat.
d. Find both optimum feed locations (above partial reboiler) and the total number of equilibrium
stages needed. Step off stages from the bottom up. Be neat.

D10.* A distillation column is separating phenol from p-cresol at 1 atm pressure. The distillate
composition desired is 0.96 mole fraction phenol. An external reflux ratio of L/D = 4 is used, and
the reflux is returned to the column as a saturated liquid. The equilibrium data can be represented
by a constant relative volatility, α phenol−cresol = 1.76 (Perry et al., 1963, pp. 13–3). CMO can be
assumed.

a. What is the vapor composition leaving the third equilibrium stage below the total condenser?
Solve this by an analytical stage-by-stage calculation alternating between the operating equation
and the equilibrium equation.

b. What is the liquid composition leaving the sixth equilibrium stage below the total condenser?
Solve this problem graphically using a McCabe-Thiele diagram plotted for α p−c = 1.76.

D11. A mixture of methanol and water is being separated in a distillation column with open steam. The
feed is 100.0 kmol/h. Feed is 60.0 mol% methanol and is at 40°C. The column is at 1.0 atm. The
steam is pure steam (y = 0) and is a saturated vapor. The distillate product is 99.0 mol%
M
methanol and leaves as a saturated liquid. The bottoms is 2.0 mol% methanol and since it leaves
an equilibrium stage must be a saturated liquid. The column is adiabatic. The column has a total
condenser. External reflux ratio is L/D = 2.3. Assume CMO is valid. Equilibrium data are in
Table 2-7. Data for water and methanol are available in Problem 3.E1.

a. Estimate q.
b. Find optimum feed stage and total number of equilibrium stages (step off stages from top
down). Use a McCabe-Thiele diagram.

c. Find (L/D) min . Use a McCabe-Thiele diagram.

D12. Solve Problem 3.E2 for the optimum feed location and the total number of stages. Assume CMO
and use a McCabe-Thiele diagram. Expand the portions of the diagram near the distillate and
bottoms to be accurate.
D13. A distillation column is separating a 30% methanol–70% water feed. The feed rate is 237 kmol/h

and is a saturated liquid. The column has a partial reboiler and a partial condenser. We desire a
distillate mole fraction of y D,M = 0.95 and a bottoms mole fraction of x B,M = 0.025. Assume CMO
is valid. Data are in Table 2-7 and Problem 3.E1.

a. Find N min .
b. Find (L/V) min and (L/D) min .

c. If L/D = 2.0 (L/D) min , find the optimum feed plate location and the total number of equilibrium

stages required.
d. Determine the boilup ratio used.