Page 404 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

In the production of diethyl ether, it is necessary to purify a stream of equimolar diethyl ether and
water that is available at 1500 kPa. Suggest a method for achieving this separation. Use the
15. UNIQUAC model for the K-value. Would your answer be different if, for example, only 99 mole %
purity were needed instead of 99.9 mole %? What do you learn and how does your answer change if
the four models for K-values in Problem 14 are used? What do you learn from this comparison?

In the production of diethyl ether, assume that it is necessary to purify a stream at 1500 kPa containing
75 mole % diethyl ether, 20 mole % ethanol, and 5 mole % dimethyl ether. Assume that the
UNIQUAC model adequately predicts the thermodynamics of this system.

a. Use the residue curve plotting routine on your simulator to plot the residue curves.

16. b. Suggest a method for separating this mixture into three relatively pure components.

c. How would your answer to Part (b) change if the stream contained 85 mole % diethyl ether, 10
mole % ethanol, and 5 mole % dimethyl ether?

d. Examine the effect of changing the pressure of the distillation columns. Suggest a much simpler
method for achieving the necessary separation.

When your parents or grandparents were in college, it was not uncommon to “borrow” some “pure”
ethanol (grain alcohol) from the university to add to a party punch. However, because of the
azeotrope between ethanol and water, “pure” ethanol is not easy to manufacture. At that time, it was
not uncommon for benzene to be added as an entrainer to break the azeotrope. Of course, this means
17.
that all “pure” ethanol contained trace amounts of benzene, which was later identified as a
carcinogen. By plotting both residue curves and TPxy diagrams for the ethanol-water system, suggest
a method for purifying ethanol by adding benzene as an entrainer. Assume that the UNIQUAC model
applies and that the feed stream of ethanol and water is equimolar and at atmospheric pressure.

Does Equation (12.1) give the number of alternative sequences for separation processes other than
18.
distillation? If so, give two examples. If not, why not?

What constraints on the separation process and on the sequence are assumed in the derivation of
19.
Equation (12.1)?

Figure 12.1(d) shows a nonsimple separation unit. When one says that the minimum number of simple
separation units is (N–1), these nonsimple units are not considered. Identify four other examples of
nonsimple separation units. For each, discuss how it might affect

20. a. Capital cost

b. Operability and safety

c. Operating costs

21. Does Equation (12.1) apply to batch or semibatch separation processes? Why or why not?