CL 351                                   Homework # 8

 

Due date: 7 November 2005

 

1.       In the production of styrene from toluene and hydrogen, it is required to recover most of the benzene from a saturated liquid stream containing 5 mol% benzene, 20 mol% toluene, 5 mol% ethylbenzene, 20 mol% styrene, 15 mol% diethylbenzene, 5 mol% triethylbenzene and 30 mol% diphenyl.  Design a distillation column to recover 98% of benzene in the distillate and 99.9% toluene in the bottoms stream.   Use Fenske-Underwood-Gilliland method with operating reflux ratio 20% larger than the minimum.  Assume that the nonkeys do not distribute.  Approximate the VLE behaviour with constant relative volatility model using the geometric mean of the volatilities at the top and bottom of the column.  The Antoine constants for the components are tabulated below.  The form of the equation is

lnP = A – B/(T-C) with P in Pa and T in K.

 

Component

A

B

C

Benzene

20.793

2788

52.4

Toluene

20.893

3096

53.7

Ethylbenzene

20.893

3279

60

Styrene

20.893

3328

63.7

Diethylbenzene

20.993

3657

71.2

Triethylbenzene

21.093

4000

81

Diphenyl

21.593

4602

70.4

 

1.       A batch still is loaded with 100 kmol of a liquid containing a binary mixture of 50 mol% benzene in toluene.  As a function of time, make plots of a) still temperature, b) instantaneous vapour composition, c) still composition and d) average total distillate composition.  Assume a constant boilup rate of 10 kmol/hr and a constant relative volatility of 2.41 at a pressure of 101.13 kPa.  T-x-y data for benzene-toluene system at 1 atm is given below

 

x

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.95

y

0.21

0.37

0.51

0.61

0.71

0.79

0.86

0.91

0.96

0.98

T, oC

105.3

101.5

98

95.1

92.3

89.7

87.3

85

82.7

81.4

 

2.       Consider separation of 200 kmol/hr mixture of 55 mol% n-hexane and 45 mol% p-xylene in a distillation column operating with a reflux ratio of 1.0.  The mixture contains equal amounts of vapour and liquid, that is, it is a partially vapourized feed.  The distillate and bottoms purities are 95% hexane and 97% p-xylene.  Boiling points of hexane and p-xylene are 68.7oC and 138.3 oC and, their densities are 659 kg/m3 and 861 kg/m3.  Find the diameter of the top and bottom sections of the distillation column if the sections are operated at 60% flooding.  If the diameters are different, what do you recommend?  Redo the calculations for a saturated liquid feed.