Page 237 - Separation process engineering

1. The column is hottest at:
a. The condenser.

b. The feed plate.

c. The reboiler.
2. In the rectifying (enriching) section:
a. Liquid flow rate > vapor flow rate

b. Liquid flow rate = vapor flow rate

c. Liquid Flow rate < vapor flow rate
3. Comparing the stripping section to the rectifying section:

a. Liquid flow rate in stripping section > liquid flow rate in rectifying section.
b. Liquid flow rate in stripping section = liquid flow rate in rectifying section.

c. Liquid flow rate in stripping section < liquid flow rate in rectifying section.
4. The HK is:

a. Methanol.
b. Ethanol.

c. n-propanol.
d. n-butanol.

5. If you were going to do external mass balances around the column to find B and D, the best
assumption to make is:
a. All of the methanol and n-propanol are in the distillate.

b. All of the methanol is in the distillate and all of the n-butanol is in the bottoms.
c. All of the ethanol is in the distillate and all of the n-propanol is in the bottoms.

d. All of the n-propanol and n-butanol are in the bottoms.
A8. Show for the problem illustrated in Figure 5-2 that L/V is more constant than either L or V when

CMO is not valid. Explain why this is so.
A9. It has often been suggested that when there is the corresponding NK component present, key
components should be withdrawn as sidestreams at the location where their concentration
maximum occurs. If there is an LNK, can a pure LK be withdrawn? Why or why not? Can a pure
LNK be obtained at the top of the column? Why or why not?

A10. Develop a key relations chart for this chapter. You will probably want to include some sketches.
A11. In Figure 5-4, a 99% recovery of benzene does not give a high benzene purity. Why not? What
would you change to also achieve a high benzene purity in the distillate?

A12. Explain Figure 5-5 in terms of the distillation of binary pairs.
A13. In Figure 5-4, the HNK and HK concentrations cross near the bottom of the columns, and in
Figure 5-5 the LK and LNK concentrations do not cross near the top of the column. Explain when
the concentrations of HK and HNK and LK and LNK pairs will and will not cross.

A14. Figure 5-6 shows the distillation of a four-component mixture. What would you expect the
profiles to look like if xylene were the LK and cumene the HK?

A15. a to e. Determine whether the following multicomponent distillation problems can or cannot be
solved with a stage-by-stage calculation and if a stage-by-stage calculation can be used indicate
the direction one should step off stages.