Page 198 - Separation process engineering

column.

D. Problems

*Answers to problems with an asterisk are at the back of the book.
D1. A continuous, steady-state distillation column with a total condenser and a partial reboiler is
separating methanol from water at one atmosphere (see Table 2-7 for data). The feed rate is 100
kmol/h. The feed is 55 mol% methanol and 45 mol% water. We desire a distillate product that is

90 mol% methanol and a bottoms product that is 5 mol% methanol. Assume CMO.
a. If the external reflux ratio L/D = 1.25 plot the top operating line.

b. If the boilup ratio /B = 2.0 plot the bottom operating line.
c. Step off stages starting at the bottom with the partial reboiler. Use the optimum feed stage.
Report the optimum feed stage and the total number of stages.

d. Plot the feed line. Calculate its slope. Calculate q. What type of feed is this?
D2. We are separating ethanol and water. All percentages are mol%. Find the q values and plot the
feed lines.

a. Feed is 60% ethanol and flashes in the column with V/F = 0.37.
b. Feed is 40% ethanol and is a two-phase mixture with liquid and vapor in equilibrium at a
temperature of 84.1°C.

c. Feed is 40% ethanol and is a liquid at 20°C.

d. Feed is 40% ethanol and is a vapor at 120°C.
e. Feed is 50% ethanol and is a subcooled liquid. One mole of vapor must be condensed to heat
12 moles of feed to their boiling point.

f. Feed is 20% ethanol and 70% is vaporized in a flash distillation system. The products of the
flash system are fed to distillation column. Calculate the two q values and plot the two feed
lines.
D3.*

a. A feed mixture of ethanol and water is 40 mol% ethanol. The feed is at 200°C and is at a high
enough pressure that it is a liquid. It is input into the column through a valve, where it flashes.
2
Column pressure is 1 kg/cm . Find the slope of the feed line.
2
b. We are separating ethanol and water in a distillation column at a pressure of 1 kg/cm . Feed is
50 wt % ethanol, and feed rate is 1 kg/h. The feed is initially a liquid at 250°C and then flashes
when the pressure is dropped as it enters the column. Find q. Data are in Figure 2-4. You may
assume that CMO is valid.

D4.*
a. We have a superheated vapor feed of 60 mol% more volatile component at 350°C. Feed flow
rate is 1000 kmol/h. On the feed plate the temperature is 50°C. For this mixture the average heat
capacities are

C = 50 cal/(mol- °C), C = 25 cal/(mol- °C)
PL
PV
while the latent heat of vaporization is λ = 5000 cal/mol. Plot the feed line for this feed.
b. If a feed to a column is a two-phase feed that is 40 mol% vapor, find the value of q and the
slope of the feed line.

c. If the feed to a column is a superheated vapor and 1 mole of liquid is vaporized on the feed