CL 351                                   Homework # 9

 

No due date but solve for end-semester exam

 

1.       As part of applying paint, you produce a large amount of methanol vapour.  You recover the methanol by absorbing the vapour in water, and then distilling the water-methanol mixture in an old six-stage column plus reboiler.  You have an old description of the column which says that the Murphree efficiency of the top three stages is 50%, but that of the bottom three and the reboiler is 100%.  The man who wrote the report died last year, only eleven months after retiring.  Company policy dictates a reflux 1.5 times the minimum.  You need to process 1200 kg/hr of a 32 mole% methanol feed.  You want a distillate that is 90% methanol.  What is concentration of the bottoms?  Equilibrium data is given below.

 

x(methanol)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

y(methanol)

0

0.42

0.58

0.67

0.73

0.78

0.83

0.87

0.92

0.96

1.0

 

 

2.       Lemon grass is a major source of lemon flavouring.  You are extracting the lemon grass with three well-mixed, counter-current stages, using dilute bisulphate solution as an extraction solvent.  The solvent enters with no lemon flavour.  The extraction factor for this extraction is 0.38.  If you extract 85% of the flavour, what is the stage efficiency? (Chapter 11, Cussler)

 

3.       An extraction column is used to remove an impurity P from toluene with water as the solvent.  The toluene and water may be considered to be completely immiscible.  Toluene enters the column with 20 wt% of P, and it should leave with 1.0 wt% P.  Water enters the column with no P.  Over the range of operation, equilibrium data can be satisfactorily represented by

 

Wt. frac of P in water = 0.75 (wt. frac of P in toluene) + 10.0 (wt. frac of P in toluene)2

 

            Find the number of equilibrium stages required.

 

4.       We are extracting acetic acid from water with isopropyl ether at 20oC and 1 atm pressure.  Equilibrium data is given below.  The column has three equilibrium stages.  The entering feed rate is 1000 kg/hr.  The feed is 40 wt% acetic acid and 60 wt% water.  The exiting extract stream has a flow rate of 2500 kg/hr and is 20 wt% acetic acid.  The entering extract stream (which is not pure isopropyl ether) contains no water.  Find: a) the exit raffinate concentration, b) the required entering extract stream concentration and c) flow rates of exiting raffinate and entering extract streams.

 

 

Equilibrium data for water - acetic acid – isopropyl ether at 20oC and 1 atm pressure.

 

 

Acetic acid, xA

Water, xD

Isopropyl ether, xs

Acetic acid, yA

Water, yD

Isopropyl ether, yS

0.69

98.1

1.2

0.18

0.5

99.3

1.41

97.1

1.5

0.37

0.7

98.9

2.89

95.5

1.6

0.79

0.8

98.4

6.42

91.7

1.9

1.93

1.0

97.1

13.3

84.4

2.3

4.82

1.9

93.3

25.5

71.1

3.4

11.4

3.9

84.7

36.7

58.9

4.4

21.6

6.9

71.5

44.3

45.1

10.6

31.1

10.8

58.1

46.4

37.1

16.5

36.2

15.1

48.7