MASS TRANSFER AND SEPARATION Question Papers (Regular, November 2007)

SET :1
1. (a) For a binary gaseous mixture of A and B show that DAB = DBA
(b) A mixture of He and N2 gas contained in a pipe at 298 K and 1.01325×105 N/m2 total pressure, which is constant through out. At one end of the pipe at point 1 the partial pressure pA1 of He is 0.60795×105 N/m2 and other end 0.2 m, pA2 = 0.202655×105 N/m2. Calculate the flux of He at steady state if DAB of He-N2 mixture is 0.687×10−4 m2/sec. [6+10]

2. (a) Give the physical significances of the dimensionless groups in mass transfer
(b) In order to measure the gas-phase mass transfer coefficient in a stirred cell at a given stirrer speed, absorption of ammonia diluted with dry air in aqueous sulphuric acid is carried out for a given period of time. The gas in the cell is well-stirred and the cross-sectional area is 31.5 cm2. If the volume of acid taken is 100 ml at 1(N) concentration and at the end of 20 minutes the liquid
is found to contain 0.82 g mole ammonia per litre, calculate the gas-phase mass transfer coefficient. [8+8]

3. In certain equipment used for the absorption of SO2 from air by water, at one section, the gas and liquid phase concentrations of the solute are 10 mole% and 4 mole% respectively. The solution density is 4592 Kg/m3. At the given temperature (40 C) and pressure (10 atm), the distribution of the SO2 between air and water can be approximately described as pA = 25 xA , where pA is the partial pressure of SO2 in the gas phase in atm. The individual mass transfer coefficients are kx = 10 kmol / (h) (m2) ( Δ x) and ky = 8. Calculate the overall coefficient, KG in kmol /(h) (m2) ( Δ p, mm Hg) and xAi and PAi at the gas-liquid interface. [16]

4. The equilibrium partial pressures (p. p) of carbon dioxide over solutions of mono
ethanolamine are as follows:
X 0.058 0.06 0 0.062 0.064 0.66 0.068 0.070
Partial pressure 5.6 13 29 56 99 155 232
of CO2 (mmHg)
A plant for manufacturing dry ice will burn coke in air to produce a flue gas which when cooled will contain 20% CO2, and 80% N2. The gas will be blown into a bubble tray scrubber at 1.2 atm and 250C to be scrubbed with mono ethanolamine solution entering at 250C. The scrubbing liquid which is recycled from a stripper will contain 0.058 mol CO2/mole of solution. The gas leaving the scrubber is to contain 2% CO2. Assume isothermal operation. Determine the number of theoretical trays required for an L/G ratio of 1.2 times the minimum. [16]

5. (a) A charge of 100kmol mixture containing 50 mol % n-heptane and 50 mol % n-octane is to be batch distilled at atmospheric pressure to get 75 kmol of the liquid distilled. Compute the composition of the composite distillate and residue.
X 0.5 0.45 0.40 0.35 0.30 0.25 0.20 0.15
Y* 0.6846 0.6384 0.5846 0.5307 0.4692 0.4153 0.3307 0.2615
(b) Derive modified Rayleighs equation. [8+8]
6. (a) What are the different coordinates by which liquid-liquid equilibrium can be represented?
(b) Discuss cross current multistage extraction operation of insoluble liquids? [8+8]

7. (a) Name five of the most important commercial adsorbents? What is the distinguishing feature of the molecular-sieve zeolites?
(b) What is the BET equation used for? Does it assume physical or chemical adsorption? Does it assume monomolecular or multimolecular adsorption?
(c) What is meant by loading in adsorption? [8+4+4]

8. Describe briefly Micro filtration and ultra filtration and also write their industrial applications. [16]


SET :2

1. (a) What are various method available to get the relation between NA& NB?
(b) Water in the bottom of a narrow tube is held at constant temperature of 293 K. The total pressure of air (assumed dry) is 1.01×105 Pa and the temperature is 293K. Water evaporates and diffuses through air in the tube and the diffusion path is 0.152 m long. Calculate the rate of evaporation at steady state in kg mol/s m2.The diffusivity of water at 293 K and 1 atm is 0.25×10−4 m2 /sec. [6+10]

2. Discuss the correlation between Sherwood, Schmidt and Reynolds for flow insides pipes, flow outside tubes parallel to axis, flow normal to cylinders, flow normal to tube bundle and flow past single spheres. [16]
3. (a) The solubility of A in water is 0.5 kmol/m3 at a partial pressure of 60 mm Hg. If the total pressure is 1.2 atm and the Henry’s law is applicable, calculate the Henry’s law constant.
(b) What are parameters which will effect the equilibrium diagram.
(c) What is Marangoni effect? [6+6+4]

4. (a) Differentiate between absorption and adsorption with suitable example.
(b) Explain Vant Hoff law of equilibrium.
(c) When ideal solution will be formed. [6+6+4]

5. 100 Moles of an acetonitrile ? nitromethane is differentially distilled in a batch still at a pressure of 70kpa .The feed contains 74mole % acetonitrile. Distillation is continued till the liquid left behind in the still contains 32 mole % acetonitrile. The vapor-liquid equilibrium for the system at this pressure is correlated as follows:
Y*=1.05x+0.13 for 0.3 < x < 0.52 and
Y*=0.77x+0.28 for 0.52< 0.80
Where x and y* refer to the mole fraction of acetonitrile in the liquid and equilibrium vapor respectively. Find the average composition of the distillate collected and also calculate quantities of distillate and residue product. [16]

6. (a) Define leaching?
(b) What is lixiviation?
(c) What is decoction?
(d) Write short notes on applications of leaching operation. [4+4+4+4]

7. Derive an expression for the estimation of height of fixed bed adsorber for given solute removal. [16]

8. Describe briefly Micro filtration and ultra filtration and also write their industrial applications. [16]


SET :3

1. (a) Explain briefly various methods for the estimation of liquid diffusivity coefficients.
(b) Calculate the rate of diffusion of trichloroacetic acid across non-diffusing methanol
solution of 0.1 cm thick at 20 C, when the concentrations on the opposite sides of the methanol film are 6% and 2% by weight of acid. The densities of 6% and 2% acid solutions are 1.012 gm/cm3 and 1.003 gm/cm3 respectively. The diffusion coefficient at infinite dilution of trichloro acetic acid in methanol is 1.862×10−5 cm2/s. [8+8]

2. (a) Write short note on Convective Mass Transfer Coefficients.
(b) A large volume of pure gas B at 1 atm pressure is flowing over a surface from which pure A is vaporizing. The liquid A completely wets the surface which is a blotting paper. Hence the partial pressure of A at the surface is the vapor pressure of A at 298 K which is 0.2 atm. The ky has been estimated to be 6.78 ×10−15 kgmol/s.m2. mol fraction. Calculate NA, the vaporization rate
and also the value of ky and kG. [6+10]

3. Describe the diffusion between phases and show how the equilibrium distribution curve varies as a function of temperature and pressure taking a system of liquid water and gaseous mixture ammonia and air? [16]

4. A solvent is to be recovered from a solvent-air mixture by scrubbing with a water in a packed tower at a pressure of 101.3 kN/m2 and at a temperature of 300K. The solvent vapour enters the tower at a rate of 0.1kg/s and has a concentration of 2% by volume. It is necessary to recover 99.9% of the solvent; for the packing selected the optimum gas and water flow rates are known to be 1.3 and 2.0 kg/m2s respectively. Calculate the height and diameter of the tower. The data required is as follows:
Overall mass transfer coefficient KGa = 0.0389 kg/s m2(kN/m2)
Equilibrium diagram Pe = 0.02 x; where Pe equilibrium pressure of the solvent kN/m2 and x the mole fraction of the solvent in water. The molecular weight of the solvent and air are 70 and 29 kg/kmol, respectively. [16]

5. A mixture of acetone and phenol (40 mass % acetone) is flashed consecutively in a cascade consisting two flash drums at 1 atm. The feed enters at a rate of 3000 kg/h and half of it is flashed in the first flash drum. The liquid from the first drum is heated and flashed again in the second drum. What fraction of the feed entering the second drum should be vaporized so that the residue contains not more than 1 mass % acetone? The vapour-liquid equilibrium data of the acetone-phenol system are given below. [16]
X: 0.01 0.04 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 0.96 0.99
Y: 0.67 0.776 0.852 0.91 0.94 0.962 0.98 0.99 0.991 0.994 0.997 0.999 0.9996

6. (a) Why are triangular diagrams useful for liquid-liquid equilibrium calculations with a ternary system? On such a diagram, what are the miscibility boundary, plait point, and tie lines?
(b) Explain briefly solutropic system.
(c) Write the procedure for estimation of number of stages if liquid-liquid extraction is carried out in cross current operation. [6+5+5]

7. (a) What is an adsorption isotherm? How can the heat of adsorption be determined from a series of isotherms?
(b) What is ideal fixed-bed adsorption? What assumptions are necessary for it to apply? What is meant by breakthrough? [8+8]

8. Write the working principle of Ultra filtration. Explain commercial application of ultra filtration. [16]


SET :4

1. Ammonia gas and Nitrogen gas are diffusing in counter currently through diffusion a straight glass tube 0.61 m long with an inside diameter of 24.4 mm at 298 K and 101.32 K Pa. Both ends of the tube are connected to large mixed chambers at 101.32 K Pa. The partial pressure of NH3 in one chamber is constant at 20 K Pa and 6.666 K Pa in the other chamber. The diffusivity at 298K and 101.32 K Pa is 2.30×10−5 m2 /sec.
(a) Calculate the rate of diffusion of NH3 in kg mol/s
(b) Calculate the rate of diffusion diffusion of N2
(c) Calculate the partial pressure at a point 0.305 m in the tube and plot pA, pB and pt verses distance z. [16]

2. (a) Discuss the important and applications of mass transfer analogies.
(b) A plate, 0.5 m square, coated with a layer of benzoic acid, is placed in a stream of water flowing at a velocity of 0.25m/sec at a temperature of 21 C. Calculate the average rate of dissolution of the acid per unit area of the plate and also the mass transfer. The following data are available: solubility of benzoic acid in water at 25 C= 3.01 kg /m3; diffusivity of benzoic acid in water =10 −9 m2 /s and velocity of water at 25 C=8.9×10−4 kg/m.s. [8+8]

3. Describe the diffusion between phases and show how the equilibrium distribution curve varies as a function of temperature and pressure taking a system of liquid water and gaseous mixture ammonia and air? [16]

4. A solvent is to be recovered from a solvent-air mixture by scrubbing with a water in a packed tower at a pressure of 101.3 kN/m2 and at a temperature of 300K. The solvent vapour enters the tower at a rate of 0.1kg/s and has a concentration of 2% by volume. It is necessary to recover 99.9% of the solvent; for the packing selected the optimum gas and water flow rates are known to be 1.3 and 2.0 kg/m2s respectively. Calculate the height and diameter of the tower. The data required is as follows:
Overall mass transfer coefficient KGa = 0.0389 kg/s m2(kN/m2)
Equilibrium diagram Pe = 0.02 x; where Pe equilibrium pressure of the solvent kN/m2 and x the mole fraction of the solvent in water. The molecular weight of the solvent and air are 70 and 29 kg/kmol, respectively. [16]

5. (a) Explain with figure the effect of different feed conditions in a distillation column. Write about the importance of q-line. Also write short notes on feedtray location.
(b) Explain P-x-y diagram. [12+4]

6. Write about the system of three liquids -one pairs partially soluble with the help of an equilibrium diagram and suitable example. Also discuss the effect of temperature on the above system. [16]

7. (a) Draw a graph showing different types of absorption isotherms for vapors and explain.
(b) Explain in detail about single stage adsorption. [8+8]

8. (a) Define flux and separation factor for membranes gas separators and also write the relevant equations.
(b) Explain briefly the effects of process flow patterns on separation and membrane area. [8+8]