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CHEN 20007 One and a half hours THE UNIVERSITY OF MANCHESTER
CHEN 20007 One and a half hours THE UNIVERSITY OF MANCHESTER Distillation and Absorption Thursday 29th January 2009â¨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 shouldâ¨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 withoutâ¨restriction. Graph paper is available. If used, write your name or registration number on itâ¨and insert inside your answer book. Professor. C. Webbâ¨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 mixturesâ¨demonstrate the following generalisations: i) Mixtures with high relative volatility are easier to separate than mixtures withâ¨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 beâ¨distilled to remove 95% of the butane. How much pentane will be lost and whatâ¨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 andâ¨vapour with compositions xf = 0.4 and yf = 0.6. What would be the minimum refluxâ¨ratio for which it would be possible to separate the binary mixture to produce aâ¨distillate with composition, xd = 0.9? d) Figure Q1(d) shows enthalpy-composition data for the hexane-octane binaryâ¨system and includes tie-lines representing the equilibrium compositions on platesâ¨in a continuous distillation column. Assuming the column is fed with liquid at itsâ¨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, gasâ¨phase-based mass transfer coefficient (Ky) can be related to the individual filmâ¨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 aâ¨sulphuric acid plant by reducing the mole ratio of SO2 to air from 0.01 to 0.001.â¨This is to be achieved by absorption into water at 1 atm, for which equilibriumâ¨data are given in Figure Q1(f). The water is supplied free of SO2 and is targetedâ¨to leave the absorber with a mole ratio (S02:water) of no more than 0.0004. If theâ¨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 actâ¨as a solvent for the light oil. The light oil may be considered to be essentiallyâ¨benzene and the washing oil is non-volatile. The equilibrium between benzeneâ¨vapour and the washing oil is described by y = 0.275 x, where x and y are moleâ¨fractions. (a) Define the Absorption Factor (A) and show that for a system that followsâ¨Henry's law, the number of theoretical stages (N) required to achieve aâ¨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 equilibriumâ¨stages required in a counter-current tower if 30 kmol h"1 of a gas mixtureâ¨containing 2% light oil vapour is to be contacted with 13 kmol h"1 of washing oilâ¨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. Theâ¨feed, of composition 35 mole% methanol, will be at a temperature of 20Â°C. Theâ¨composition of the top product is to be at least 90% methanol and that of the bottomâ¨product, exactly 5.0% methanol. The column will be fitted with a total condenser andâ¨partial reboiler, and will operate with a reflux ratio of unity (i.e. R = 1). It is estimatedâ¨that the Murphree plate efficiencies in the rectifying and stripping sections will beâ¨75% and 50% respectively. Using the data below, determine the actual number ofâ¨plates required in the column and the optimal location of the feed. DATA [30 marks] Molar heat capacity of liquid 77.5 kJ kmoP1 Kâ¨Molar heat of vaporization 39713 kJ kmolâ¨Bubble point temperature 76.7Â°C -1 An x-y diagram for the system methanol-water is provided in Figure Q3 andâ¨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 â¢r O) 1-^ 11000 CO 1- o T'*~ 7"~ r >> 9000 Q, 1 J CO 5 7000 7^" c 4 LU ii 5000 t- âi- * 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 1â¨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 â¢1 iEiff 0.8 i 0.7 >% r^t 3 Â£ o "ifis; a 4- a 0.6 > Â£ i- o :)-i" c ttff 0.5 Â£ L" a> E c .2 0.4 It *A- O 2 4â o 0.3 E 'm 0.2 xm- R !:1 tÂ±r i 0.1 T? T :lÂ£ 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