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### M A S S T R A N S F E R O P E R A T I O N S I J N T U M O D E L P A P E R{Www

1. 1. www.studentyogi.com www.studentyogi.com Code No: R05310804 Set No. 1 III B.Tech I Semester Regular Examinations, November 2007 MASS TRANSFER OPERATIONS-I (Chemical Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks 1. Explain in detail the classi cation of mass transfer operations based on the phases involved. [16] 2. Ammonia is di using through a stagnant mixture containing 1/3 N2 and 2/3 H2 by volume. The total pressure is 206kN/m2 and temperature of 55 0C. Calculate the rate of di usion of ammonia through a lm of 0.5 mm thick when the concentrations of ammonia on either side of the lm are 10% and 5% by volume. Di usivity of NH3 in N2 at 25 0C is 1.32×10-5 m2 /sec Di usivity of NH3 in H2 at 250C is 4.1×10-5 m2 /sec. [16] 3. Discuss the correlation between Sherwood, Schmidt and Reynolds for ow insides pipes, ow outside tubes parallel to axis, ow normal to cylinders, ow normal to tube bundle and ow past single spheres. [16] 4. (a) With the help of gure explain what do you mean gas phase controlled mass transfer coe cient and liquid phase mass transfer coe cient. What is their relation to local overall mass transfer coe cient? Further, what is the signi - cance of overall mass transfer coe cient? (b) Explain the terms operating line, driving force line and equilibrium curve. [10+6] 5. (a) How is a tall tower supported? What problems may occur if a tall tray or packed tower has tilted by 5 degrees, say? (b) Explain how to liquid misdistribution in a packed bed a ects its performance. Can liquid misdistribution lead to misdistribution of gas too? [8+8] 6. (a) A mixture of benzene and p-xylene is batch distilled at atmospheric pressure. The rate of heat input to the still is 4000 kcal/h. Individual heats of vapor- ization are: benzene = 100 kcal/kg, p-xylene = 85 kcal /kg, and the relative volatility of benzene in the mixture is 5.6. Calculate the instantaneous rate of vaporization of p-xylene when the liquid in the still has 40 mole% benzene in it. (b) Derive Rayleigh’s equation and mention assumptions made. [6+10] 7. A continuous fractionating column operates with re ux ratio 3.5 to separate equimo- lar mixture of feed of 13600 kg/hr. to get a distillate containing 97% Benzene and bottoms containing 2% benzene (all wt%). Calculate moles of distillate and residue
2. 2. www.studentyogi.com www.studentyogi.com Code No: R05310804 Set No. 1 if feed is at its boiling point and number of ideal stages required. The equilibrium data is as follows. [16] Mol. Fraction of benzene x 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Mol. Fraction of benzene y 0.21 0.35 0.51 0.64 0.72 0.79 0.86 0.91 0.96 8. (a) Explain brie y choice of the solvent in extractive distillation. (b) How operating and initial costs changes as re ux ratio increase? Explain.[8+8]
3. 3. www.studentyogi.com www.studentyogi.com Code No: R05310804 Set No. 2 III B.Tech I Semester Regular Examinations, November 2007 MASS TRANSFER OPERATIONS-I (Chemical Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks 1. (a) Explain brie y various pressure driven membrane separation process. (b) Explain brie y separation process where surface phenomena is important with suitable examples. [10+6] 2. Ammonia gas (A) di uses through nitrogen gas (B) under steady state conditions with nitrogen non di using. The total pressure is 1.013×105 pa and the temperature is 298 K the di usion path (z2-z1) is 0.15 m the partial pressure of NH3 at one point is 1.5×104 Pa the DAB of the mixture at 1.013×105 pa and 298 K is 2.3×10-5 m2 /sec. Calculate (a) the ux of NH3 and (b) the equimolar counter di usion ux, assuming that N2 also di uses. [8+8] 3. (a) Describe the basic di erences between Film Theory, Penetration Theory and Film-surface renewal theory. (b) Write short notes on analogies between heat and mass transfer. [10+6] 4. Absorption of oxygen from an air bubble o ccurs in ‘pure’ water. Calculate the overall coe cients Ky and Kx for the following cases: (a) The two- lm model is applicable. The air- lm thickness is 2 mm, and the water- lm thickness is 0.02 mm. (b) The lm model is applicable for the gas-phase. The contact time of a liquid element with the gas is 1 s. Given: di usivity of O2 in air = 0.176×10-4 m2/s, in water = 2.1×10-9 m2/s; the equilibrium relation is p = 4.36×104x, p = partial pressure of O2 in atm; temperature = 298 K; total pressure = 1 atm. [16] 5. (a) State the various characteristics of tower packings. (b) What are the two ma jor types of packings? Explain each with neat diagram. [8+8] 6. Ethanol forms a nearly ideal solution with iso-butanol and has a relative volatility 2.2. A heated feed containing 40 mole% ethanol and 60 mole% iso-butanol enters a ash drum at rate of 50 kmol/h. prepare the plot of the fractional yield of the distillate versus its composition. What fraction of the feed should be vaporized in order to have a bottom product containing not more than 10% ethanol? [16]
4. 4. www.studentyogi.com www.studentyogi.com Code No: R05310804 Set No. 2 7. An equimolar binary mixture of saturated liquid is to be separated in a packed distillation column operated at atmospheric pressure. The relative volatility is 1.2. The distillate product to be obtained should contain 95 mole% of the more volatile component bottom product contains 5 mole% of the more volatile compo- nent. Compute for this case, the number of transfer units required if the column is operated at total re ux. [16] 8. (a) In extractive distillation, why is a large concentration of solvent required in the liquid phase? Why doesn’t the solvent enter the column at the top tray? In azeotropic distillation, what is the entrainer? (b) Can a distillation column be operated at total re ux? Explain (c) Under what conditions should extractive distillation be considered? [8+4+4]
5. 5. www.studentyogi.com www.studentyogi.com Code No: R05310804 Set No. 3 III B.Tech I Semester Regular Examinations, November 2007 MASS TRANSFER OPERATIONS-I (Chemical Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks 1. (a) Mention minimum 4 examples for separation processes where mass in sepa- rating agent. (b) Explain brie y the Separation process where contact of solid-liquid is involved with suitable examples. (c) Explain brie y the Thermal di usion. [4+8+4] 2. Estimate the di usivity of n-Butanol vapor (A) (C2H5CH2CH2OH) through air at 1 atm. Pressure and 59 0C. Data: Molecular weight n-butanol=74; Molecular weight of air=29. For air B/ =97, rB = 0.3617 nm. Group contribution of atomic volume (m3/K.atom) C=0.0148 O=0.0074 H=0.0037 and rA = 1.18 1/3. Normal boiling point of n-butanol=117 0C. Collision function data: [16] KT/ f(KT/ ) 1 0.72 1.5 0.6 2 0.54 3. Discuss the following Mass Transfer Theories: (a) Penetration Theory (b) Film-surface renewal theory. [16] 4. (a) Jasmone is a high-value product used in perfumery and cosmetic industries. A suspension of jasmine owers in water is contacted with an organic solvent to recover the compound. During extraction, the compound di uses from the aqueous phase to the dispersed organic phase. The individual mass transfer coe cients are 3.5×10-6 cm/s (organic phase) and 2.7×10-5 cm/s (aqueous phase). The solute is 175 times more soluble in the organic phase (y = 175x; y = mole fraction of the solute in the organic phase). The aqueous solution is dilute and the total molar concentration of the organic phase is 10 kmol/m3. Calculate the overall mass transfer coe cient Ky. Which phase resistance controls? (b) Explain brie y two lm theory with neat sketch. [8+8] 5. Brie y explain the design and importance of tray spacing, tower diameter, down spot, weirs in tray tower. [16] 6. (a) An ethanol-water mixture containing 36% by weight of ethanol is di erentially
6. 6. www.studentyogi.com www.studentyogi.com Code No: R05310804 Set No. 3 concentration of 6 mole%. Determine the quantity and composition of the distillate. Molecular weight of ethanol = 46. The VLE data are: Mole frac.of Ethanol in Liquid(x) 0.18 0.16 0.14 0.12 0.10 0.08 0.06 Mole frac. of Ethanol in Vapour(y) 0.517 0.502 0.485 0.464 0.438 0.405 0.353 (b) Write short notes on applications of distillation. [12+4] 7. A mixture of 60 mole%A and 40 mole% B is separated in a column to yield 96 mole% A as the top product. Also, 97% of A entering the tower must be recovered in the distillate. The feed is a saturated vapour. The following concentrations have been measured by analyzing the liquid and vapour samples around the 6 th tray x6 = 0.55, y6 = 0.72, y7 = 0.63. (a) What is the local value of the relative volatility of A with respect to B? (b) How many ideal trays do es the column have? Assume that the relative volatil- ity remains constant. [16] 8. A feed containing 40 mole percent n-hexane and 60 percent n-octane is fed to a distillation column as saturated liquid. A re ux ratio of 1.2 is maintained. The overhead product is 95 percent hexane and the bottoms 10 percent hexane. Draw H-X-Y diagram from the given data. Also calculate the number of stages if re ux ratio of 1.4 times minimum is used Equilibrium data is given below. [16] VLE Data, Mole Fraction Hexane, 1 atm x (liquid) 0.0 0.1 0.3 0.5 0.55 0.7 1.0 y (vapor) 0.0 0.36 0.70 0.85 0.90 0.95 1.0 Enthalpy-Concentration Data Mole Fraction Hexane Enthalpy cal/gmol in the liquid Sat. Liquid Sat. Vapor 0.0 7000 15,700 0.1 6300 15,400 0.3 5000 14,700 0.5 4100 13,900 0.7 3400 12,900 0.9 3100 11,600 1.0 3000 10,000
7. 7. www.studentyogi.com www.studentyogi.com Code No: R05310804 Set No. 4 III B.Tech I Semester Regular Examinations, November 2007 MASS TRANSFER OPERATIONS-I (Chemical Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks 1. Explain brie y various factors that should be considered in the design of plant involving the di usional operations. [16] 2. (a) A narrow tube is partially lled with a liquid and maintained at constant temperature. A gentle stream of gas is passing across the open end of the tube. As the liquid evaporates the level drops slowly. At a given time t, the level is z from the top. Derive an expression to calculate the value of the di usivity of the liquid vapor in the gas. (b) An Arnold cell is used to measure the di usivity of acetone in air at 20 0C and 100 kPa pressures. At time t=0, the liquid acetone surface is 11 mm from the top of the tube and after 8 hours of operation the liquid surface drops to 20.5 mm. If the concentration of acetone in air that ows over the tube is zero, what is the di usivity of acetone in air? At 20 0C the vapor pressure of acetone is 24 kPa and the density of acetone is 790kg/m3. [8+8] 3. Water is being evaporated into a counter-current stream of air in a wetted-wall column, 30 mm in diameter. Estimate the evaporative ux of water at a point in the column where the water temperature is 305 k and the mole fraction of water vapour in the bulk air is 0.0098. The following data and information are supplied: air rate (dry basis) =1 Kg/min; air temperature at the lo cation =38 0c; air viscosity (at 38 0c)=12×10-5 kg/m-s; total pressure =1atm. Vapour pressure of water at the given temperature is 4.5 kN/m2 the gas phase mass transfer coe cient can be calculated using the Gilliland and Sherwoo d correlation given by Sh=0.023(Re)0.8 (Sc)0.33 The Reynolds number of air may be calculated at the bulk condition. [16] 4. (a) Derive an expression to nd local overall gas phase mass transfer co e cient. (b) What is the necessity of introducing the overall mass transfer coe cient? [12+4] 5. (a) Explain the phenomena of liquid distribution and redistribution in packed towers. (b) What are the factors to be considered for choice between tray towers and packed towers? [8+8] 6. (a) With the help of the boiling point diagrams explain what is a minimum boiling azeotrope and maximum boiling azeotrope.
8. 8. www.studentyogi.com www.studentyogi.com Code No: R05310804 Set No. 4 (b) What is the relative volatility? How is it computed for ideal systems? [10+6] 7. (a) For distillation of an equimolar binary mixture of A and B, the equations of the operating lines are: Rectifying section: y = 0.663x + 0.32. Stripping section: y = 1.329x - 0.01317. What is the condition of the feed? (b) Write short notes on partial condensers. [8+8] 8. (a) Represent use of open steam in distillation graphically. (b) Derive the equation to calculate the amount of liquid vaporized upon entering of superheated steam. [8+8]