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CHEN 20007 One and a half hours THE UNIVERSITY OF MANCHESTER by asafwewe

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CHEN 20007 One and a half hours THE UNIVERSITY OF MANCHESTER

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									CHEN 20007
One and a half hours
THE UNIVERSITY OF MANCHESTER
Distillation and Absorption
Thursday 29th January 2009
09:45-11:15
Answer Question 1 (compulsory) and ONE other question.
Note: Question 1 accounts for 60 of the total 90 marks and you should
therefore spend an appropriate amount of time on this question.
Electronic calculators may be used, provided that they cannot store text.
All numerical working must be shown.
Tables of mathematical formulae are available and may be used without
restriction.
Graph paper is available. If used, write your name or registration number on it
and insert inside your answer book.
Professor. C. Webb
Ext. 64379
P.T.O.
Page 1 of 7
CHEN 20007
DISTILLATION AND ABSORPTION
Answer Question 1 (compulsory) and ONE other question
1. Attempt ALL parts of this question and for each part explain any calculations.
a) Using sketches of temperature versus composition, for binary liquid mixtures
demonstrate the following generalisations:
i) Mixtures with high relative volatility are easier to separate than mixtures with
low relative volatility.
[5 marks]
ii) Separation increases as operating pressure is decreased.
[5 marks]
iii) Operating temperature is lower for plates higher up the column. [3 marks]
iv) Azeotropic mixtures cannot easily be separated by distillation.
[2 marks]
b)	A 150 kmol batch of crude pentane (containing 0.12 mole fraction Butane) is to be
distilled to remove 95% of the butane. How much pentane will be lost and what
will be the purity of the pentane remaining in the still? The average relative
[10 marks]
c)	The feed to a continuous distillation column is an equilibrium mixture of liquid and
vapour with compositions xf = 0.4 and yf = 0.6. What would be the minimum reflux
ratio for which it would be possible to separate the binary mixture to produce a
distillate with composition, xd = 0.9?
d)	Figure Q1(d) shows enthalpy-composition data for the hexane-octane binary
system and includes tie-lines representing the equilibrium compositions on plates
in a continuous distillation column. Assuming the column is fed with liquid at its
bubble point, determine the compositions of:
i) Bottom product
volatility of butane to pentane is 3.5.
[6 marks]
[3 marks]
[3 marks]
ii) Distillate product
[3 marks]
iii) Feed
Figure Q1(d) should be returned with your answer.
e) Show, for the situation where the equilibrium line is straight, that the overall, gas
phase-based mass transfer coefficient (Ky) can be related to the individual film
mass transfer coefficients (ky and kx) by the following equation (where m is the
[10 marks]
gradient of the equilibrium line):
1 _ 1 m
Page 2 of 7
CHEN 20007
f) A packed column absorber is to be designed to scrub the gaseous effluent from a
sulphuric acid plant by reducing the mole ratio of SO2 to air from 0.01 to 0.001.
This is to be achieved by absorption into water at 1 atm, for which equilibrium
data are given in Figure Q1(f). The water is supplied free of SO2 and is targeted
to leave the absorber with a mole ratio (S02:water) of no more than 0.0004. If the
height of a transfer unit for the packing is 1.8m, determine the total height of
[10 marks]
packing required for the absorber.
Figure Q1(f) should be returned with your answer.
Page 3 of 7
CHEN 20007
Answer ONE of the following two questions
2. Answer BOTH parts of this question
Coal gas containing a light oil vapour is to be scrubbed with a washing oil that will act
as a solvent for the light oil. The light oil may be considered to be essentially
benzene and the washing oil is non-volatile. The equilibrium between benzene
vapour and the washing oil is described by y = 0.275 x, where x and y are mole
fractions.
(a)
Define the Absorption Factor (A) and show that for a system that follows
Henry's law, the number of theoretical stages (N) required to achieve a
particular separation can be estimated from the following equation: [15 marks]
To-To
In
y-r-y-r
N =
In .4
(b)
Use the Absorption Factor method to determine the number of equilibrium
stages required in a counter-current tower if 30 kmol h"1 of a gas mixture
containing 2% light oil vapour is to be contacted with 13 kmol h"1 of washing oil
in order to remove 90% of the light oil. The washing oil initially contains 0.005
[15 marks]
mole fraction benzene.
3. A mixture of methanol and water is to be separated by continuous distillation. The
feed, of composition 35 mole% methanol, will be at a temperature of 20°C. The
composition of the top product is to be at least 90% methanol and that of the bottom
product, exactly 5.0% methanol. The column will be fitted with a total condenser and
partial reboiler, and will operate with a reflux ratio of unity (i.e. R = 1). It is estimated
that the Murphree plate efficiencies in the rectifying and stripping sections will be
75% and 50% respectively. Using the data below, determine the actual number of
plates required in the column and the optimal location of the feed.
DATA
[30 marks]
Molar heat capacity of liquid 77.5 kJ kmoP1 K
Molar heat of vaporization 39713 kJ kmol
Bubble point temperature 76.7°C
-1
An x-y diagram for the system methanol-water is provided in Figure Q3 and
should be returned with your answer.
END OF EXAMINATION PAPER
Page 4 of 7
CHEN 20007
Registration number.
Figure Q1(d): Hexane-octane VLE diagrams
25000
23000
21000
19000
17000
15000
o
E 13000
•r
O)
1-^
11000
CO
1-
o
T'*~
7"~
r
>> 9000
Q,
1
J
CO
5 7000
7^"
c
		 4
LU
ii
5000
t- —i-
*
3000
1000
-1000
+-
-3000
j
-5000
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
y or x, mol fraction hexane
Page 5 of 7
CHEN 20007
Registration number
Figure Q1(f)
Equilibrium line for S02/air/water at P=1 atm.
0.011
0.01
0.009
0.008
0.007
.5=
5
O 0.006
V)
.2
i: o.oo5
o
E
0.004
i-
0.003
0.002
0.001
L
0
0.0005
0.0002
0.0003
0 0004
0.0001
0
mol ratio SOj/water
Page 6 of 7
CHEN 20007
Registration number,
Figure Q3: Equilibrium data for Methanol-water
1
Ri
0.9
•1
iEiff
0.8
i
0.7
>%
r^t
3
£
o
"ifis;
a
4-
a
0.6
>
£
i-
o
:)-i"
c
ttff
0.5
£
L"
a>
E
c
.2 0.4
It
*A-
O
2
4—
o 0.3
E
'm
0.2
xm-
R
!:1
t±r
i
0.1
T?
T
:l£ W
:fe
ffi-
-T
0
0.6
0.7
0.8
0.9
1
0.3
0.4
0.5
0.1
0.2
0
mole fraction methanol in liquid (x)
Page 7 of 7

								
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