Page 202 - Separation process engineering

D20.* A distillation column with a total condenser and a partial reboiler is separating ethanol and
2
water at 1 kg/cm pressure. Feed is 0.32 mole fraction ethanol and is at 30° C. Feed flow rate is
100 kmol/h. The distillate product is a saturated liquid, and the distillate is 80 mol% ethanol. The
condenser removes 2,065,113 kcal/h. The bottoms is 0.04 mole fraction ethanol. Assume that
CMO is valid. Find the number of stages and the optimum feed stage location.

Data: C PL EtOH = 24.65 cal/(mol- °C) at 0 °C

The enthalpy-composition diagram is given in Figure 2-4, and the y-x diagram is in Figure 2-2.
Note: Watch your units.
D21. An enricher column has two feeds. Feed F (input at the bottom) is a saturated vapor. Flow rate
1
F = 100.0 kmol/h. This feed is 20.0 mol% methanol and 80.0 mol% water. Feed F (input part
1
2
way up the column) is a two-phase mixture that is 90 % liquid. Flow rate F = 80.0 kmol/h. Feed
2
F is 45.0 moles % methanol and 55.0 mol% water. We desire a distillate that is 95.0 mol%
2
methanol. Reflux is returned as a saturated liquid. Pressure is one atmosphere. L/D = 1.375.
Assume CMO. Data are available in Table 2-7.

Find: D, B, x , optimum feed location and number of equilibrium stages required.
B
D22.* When water is the more volatile component we do not need a condenser but can use direct
cooling with boiling water. This was shown in Problem 3.D3. We set y = 0.92, x = 0.04, z =
D
B
0.4 (all mole fractions water), feed is a saturated vapor, feed rate is 1000 kmol/h, p = 1 atm,
CMO is valid, the entering cooling water (C) is a saturated liquid and is pure water, and C/D =
3/4. Derive and plot the top operating line. Note that external balances (that is, balances around
the entire column) are not required.

D23.* When water is the more volatile component we do not need a condenser but can use direct
cooling. This was illustrated in Problem 3.D3. We set y = 0.999, x = 0.04, z = 0.4 (all mole
B
D
fractions water), feed is a saturated liquid, feed rate is 1000 kmol/h, p = 1 atm, CMO is valid, the
entering cooling water (C) is pure water and C/D = 3/4. The entering cooling water is at 100 °F
while its boiling temperature is 212 °F.

Find the slope of the top operating line, L/V.

Note: Equilibrium data are not needed.
D24. We have a distillation column with a partial condenser and a total reboiler separating a feed of
200.0 kmol/h. The feed is 40.0 mol% acetone and 60.0 mol% ethanol. The feed is a two-phase
mixture that is 80% liquid. We desire a distillate vapor that is 85.0 mol% acetone and a bottoms

that is 5.0 mol% acetone. Column pressure is one atmosphere. Reflux is returned as a saturated
liquid and L/D = 3.25. Assume CMO. Equilibrium data are in Problem 4.D7.
a. Find the optimum feed location and the total number of equilibrium stages.

b. What is the minimum external reflux ratio?
c. What is the minimum number of stages (total reflux)?

D25. We are separating methanol and water. Calculate the internal reflux ratio inside the column,
L /V , for the following cases. The column is at 101.3 kPa. Data are available in Table 2-7 and in
2
1
Problem 3.E1.
a. Distillate product is 99.9 mol% methanol. External reflux ratio is L /D = 1.2. Reflux is cooled
0