Page 290 - Separation process engineering

n-butane, and 10 mol% n-pentane.
A 99.7% recovery of propane is desired in the distillate. A 99.8% recovery of n-butane is desired
in the bottoms.

a. Find N min from the Fenske equation (hand calculation).

b. Find (L/D) min from the Underwood equation (hand calculation).

c. Use L/D = 1.15 (L/D) min and estimate N and N Feed from Gilliland correlation (hand
calculation).

Use DePriester charts. Assume relative volatility is constant at value calculated at the bubble-
point temperature of the feed. For bubble-point calculation, choose propane as the reference
component. Choose n-butane (the heavy key) as the reference component for other calculations.
D3.* We have designed a special column that acts as exactly three equilibrium stages. Operating at
total reflux, we measure vapor composition leaving the top stage and the liquid composition
leaving the bottom stage. The column is separating phenol from o-cresol. We measure a phenol

liquid mole fraction leaving the bottom stage of 0.36 and a phenol vapor mole fraction leaving the
top stage of 0.545. What is the relative volatility of phenol with respect to o-cresol?
D4. We desire to separate 1,2 dichloroethane from 1,1,2 trichloroethane at one atmosphere. We
desire 99.15 mol% dichloroethane in the distillate and 1.773% dichloroethane in the bottoms. The
feed is a saturated liquid and is 60.0 mol% 1,2 dichloroethane. Assume the relative volatility is
approximately constant, α = 2.4.

a. Find the minimum number of stages using the Fenske equation.
b. Calculate L/D min .

c. Estimate the actual number of stages for L/D = 2.2286 using the Gilliland correlation.

d. A detailed simulation gave 99.15 mol% dichloroethane in the distillate and 1.773%
dichloroethane in the bottoms for L/D = 2.2286, N =25 equilibrium contacts, optimum feed
location is 16 equilibrium contacts from the top of the column. Compare this N with part c and
calculate the % error in the Gilliland prediction.

D5.* A column with 29 equilibrium stages and a partial reboiler is being operated at total reflux to
separate a mixture of ethylene dibromide and propylene dibromide. Ethylene dibromide is more
volatile, and the relative volatility is constant at a value of 1.30. We are measuring a distillate
concentration that is 98.4 mol% ethylene dibromide. The column has a total condenser and
saturated liquid reflux, and CMO can be assumed. Use the Fenske equation to predict the bottoms
composition.

D6.* We are separating 1,000 mol/h of a 40% benzene, 60% toluene feed in a distillation column with
a total condenser and a partial reboiler. Feed is a saturated liquid. CMO is valid. A distillate that
is 99.3% benzene and a bottoms that is 1% benzene are desired. Use the Fenske equation to find
the number of stages required at total reflux, a McCabe-Thiele diagram to find (L/D) min , and the
Gilliland correlation to estimate the number of stages required if L/D = 1.15(L/D) min . Estimate
that the relative volatility is constant at α = 2.4. Check your results with a McCabe-Thiele
BT
diagram.

D7. We are separating a mixture of ethane, propane, n-butane and n-pentane in a distillation column
operating at 5.0 atm. The column has a total condenser and a partial reboiler. The feed flow rate is
1000.0 kmol/h. The feed is a saturated liquid. Feed is 8.0 mol% ethane, 33.0 mol% propane, 49.0
mol% n-butane and 10.0 mol% n-pentane. A 98.0% recovery of propane is desired in the