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Seat No.:
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Enrolment No.
GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER-VI(NEW) - EXAMINATION - SUMMER 2019
Subject Code:2163609
Subject Name: Basic of Mass Transfer
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Time:10:30 AM TO 01:00 PM
Date:10/05/2019
Instructions:
- Attempt all questions.
- Make suitable assumptions wherever necessary.
- Figures to the right indicate full marks.
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Q.1
(a) Define following terms: MARKS
- Diffusivity
- Mass transfer coefficient
- Absorption Factor
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(b) Explain direct contact of two Immiscible Phases. 04
(c) A layer of benzene 3 mm deep lies at the bottom of an open tank 5 m 07 in diameter. The tank temperature is 295 k and the diffusivity of benzene in air is 8 X 10-6 m2/s at this temperature. If the vapor pressure of benzene in the tank is 13.3 KN/m2 and diffusion may be assumed to take place through stagnant air film of 3 mm thick. How long (in sec) will it take for the benzene to evaporate? The density of the benzene is 880 Kg/m3. Molecular weight of benzene is 78 kg/Kmol.
Q.2
(a) Prove that DAB = DBA with proper assumptions. 03
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(b) Derive an equation of flux NA For diffusion from a sphere in to the 04 infinite air medium.
(c) An ethanol (A) Water (B) solution in the form of a stagnant film 2.0 07 mm thick at 293 K is in contact at one surface with an organic solvent in which ethanol is soluble and water is insoluble. Hence, NB = 0. At point 1 the concentration of ethanol is 16.8 wt% and the solution density is ρ1 = 972.8 Kg/m3. At point 2 the concentration of ethanol is 6.8 wt% and ρ2 = 988.1 Kg/m3. The diffusivity of ethanol is 0.740 x 10-9 m2/s. Calculate the Steady State flux NA. Take the molecular weights of ethanol and water as 46.05 and 18.02 respectively.
OR
Q.2
(a) Derive the equation of flux NA for A diffusing through non diffusing 07 B; assume the ideal gases.
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(b) Differentiate J and N flux. 03
(c) Explain Film theory of Mass Transfer. 04
Q.3
(a) In a laboratory experiment, the solute A is being absorbed from a 07 mixture with an insoluble gas in a falling film of water at 30 °C and pressure of 1.45 bar. The gas phase mass transfer coefficient kG = 90.3 Kmol/h(m2) (Kmol/m3). It is known that 13.6 % of the total mass transfer resistance lies in gas phase. At a particular section of the apparatus the mole fraction of the solute in the liquid is known to be xi =0.00201. The equilibrium solubility of the gas in water at the given temperature is p = 3.318 X 104 x*, where p is partial pressure of gas in mm Hg and x* is the solubility of A in water in mole fraction.
Calculate: a) The overall liquid phase mass transfer coefficient
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b) The individual gas phase driving force Δp and Δy
Total Marks: 70
OR
Q.3
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(a) Discuss different types of Diffusivity. 03
(b) Discuss application of the molecular diffusion. 04
(c) Derive the equation of overall resistance using two phase mass transfer 07 theory, from both the liquid and gas phase.
Q.4
(a) Briefly explain the difference between Absorption, Adsorption and 03 Extraction.
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(b) Write a short note on a choice of solvent for Absorption. 04
(c) A coal gas is to be freed of its light oil by scrubbing with wash oil as 07 an absorbent and the light oil recovered by stripping the resulting solution with steam. Absorber: Gas in 0.250 m3/s at 26°, Pt = 1.07 x 105 N/m2 Containing 2.0% by volume of light oil vapors. The light oil will be assumed to be entirely benzene and a 95% removal is required. The wash oil is to enter at 26°C, containing 0.005 mole fraction benzene and has an average molecular weight is 260. An oil circulation rate of 1.5 times the minimum is to be used. Wash oil-benzene solutions are ideal. The temperature will be constant at 26°C. At 26°C, the vapor pressure of benzene is 13330 N/m2. Assume the system as a counter current packed tower. Compute the oil circulation rate required.
OR
Q.4
(a) Define different moistures, (a) unbound (b) bound (c) equilibrium 03
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(b) Discuss Minimum Liquid-Gas Ratio theory for absorbers. 04
(c) A gas mixture containing 15 mole % ‘A’ and 85 mole % inerts is fed to 07 an absorption tower where it is contacted with liquid solvent ‘B’ which absorbs ‘A’. The mole ratio of solvent to gas entering the tower is 1.5:1. The gas leaving the absorber contains 2.5 % A’, 1.5 % ‘B’ and rest is inert on mole basis.
Calculate:
- the percentage of the original solute A’ that remains un recovered.
- the fraction of solvent ‘B’ fed to the tower lost in gas leaving the column.
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Q.5
(a) Discuss minimum reflux ratio theory. 03
(b) Explain role of Raoult’s law in distillation and derive the following 04 equation y = αx / 1 + (α -1)x
(c) A binary feed has q = 0.25 is separated in a staged distillation column. 07 The mole fraction of the more volatile component in the distillate product is 0.95. The molar flow rate of the distillate is 50% of the feed flow rate and the McCabe-Thiele method can be used to analyze the column. The q-line intersects the operating line of the enriching section at (0.35, 0.5) on the x-y diagram. Calculate the slope of the Stripping section operating line.
OR
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Q.5
(a) Explain positive and negative deviation from ideality. 03
(b) Explain Drying Rate Curve. 04
(c) Calculate the time (in hours) required to reduce the moisture contents of a 07 solid from 0.66 to 0.25 Kg moisture/Kg dry solid. The rate of drying N (Kg Water evaporated/m2 s) is given as following,
N =0.0015 X for X <0.3
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=0.00045 for X=>0.3
where X = Kg moisture/Kg dry solid. The drying surface is 0.025 m2/Kg dry solid
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