Page 205 - Separation process engineering

every 4 moles of feed. The column has a partial condenser and uses open steam heating. We
desire a distillate product y = 0.75 and a bottoms product x = 0.10. CMO is valid. The steam
B
D
used is pure saturated water vapor. Data are in Table 2-1 and Figure 2-2.
a. Find the minimum external reflux ratio.

b. Use L/D = 2.0(L/D) min , and find the number of real stages and the real optimum feed location if
the Murphree vapor efficiency is 2/3 for all stages.
c. Find the steam flow rate used.

D34.* A distillation column with two equilibrium stages and a partial reboiler (three equilibrium
contacts) is separating methanol and water. The column has a total condenser. Feed, a 45 mol%
methanol mixture, enters the column on the second stage below the condenser. Feed rate is 150
mol/h. The feed is a subcooled liquid. To heat 2 moles of feed to the saturated liquid temperature,
1 mole of vapor must condense at the feed stage. A distillate concentration of 80 mol% methanol
is desired. Reflux is a saturated liquid, and CMO can be assumed. An external reflux ratio of L/D
= 2.0 is used. Find the resulting bottoms concentration x . Data are in Table 2-7.
B

E. More Complex Problems

E1. A system known as a pump-around is shown below. Saturated liquid is withdrawn from stage 2
above the partial reboiler, and the liquid is returned to stage 3 (assume it is still a saturated
liquid). Pump-around rate is P = 40.0 kmol/h. The column is separating methanol and water at
101.3 kPa. The feed flow rate is 100.0 kmol/h. The feed is 60.0 mol% methanol and 40.0 mol%
water. The feed is saturated liquid. We desire a bottoms product that is 2.5 mol% methanol. The
distillate product should be 95.0 mol% methanol. The column has a total condenser and the reflux
is a saturated liquid. Assume CMO. Use (L/D) = 2.0 x (L/D) . Data are given in Table 2-7. Find
min
x , the optimum feed stage and total number of equilibrium stages required.
p

E2.* A distillation column is separating methanol and water. The column has open (direct) steam
heating and a total of five stages. A liquid side stream is withdrawn from the second plate above
the bottom of the column. The feed is 30 mol% methanol and is a subcooled liquid. One mole of
vapor is condensed to heat 2 moles of feed to the saturated liquid temperature on the feed plate.
Feed rate is 1000 mol/h. A bottoms concentration of 1.5 mol% is desired. The steam used is pure
saturated water vapor and the steam flow rate is adjusted so that Steam flow rate/Bottoms flow
rate = 0.833. The side stream is removed as a saturated liquid. The side-stream flow rate is
adjusted so that Side-stream flow rate/Bottoms flow rate = 0.4. A total condenser is used. Reflux