### Mass Transfer I - Semester Exam Question

6HC - Mass Transfer I - April'1998

Part A (20 x 2 = 40 Marks)

1. Define the term diffusion
2. What is the unit for mass diffusivity?
3. Define eddy diffusion
4. Draw a graph showing concentration gradient for equimolal diffusion
5. What is Sherwood number?
6. Give the relationship between mass transfer coefficient and diffusivity
7. What is the effect of pressure and temperature on the diffusion coefficient in gases?
8. Define 'Absorption factor'
9. Define humidity
10. What is meant by percentage humidity?
11. How the cooling effect in a cooling tower can be increased?
12. Define Dew point
13. Define the term Equilibrium moisture and free moisture content of solid
14. What is freeze-drying?
15. Which drier is suitable for handling fragile crystals?
16. During drying operation, critical moisture content varies with _________.
17. Write down the sequence of stages in the evolution of a crystal
18. Write down the three methods used to produce super-saturation
19. What is magma?
20. What is the purpose of agitator in a crystallizer?
21. Part B (5 x 12 = 60 Marks)

22. (a) (i) Distinguish between molecular and eddy diffusion. (4)
23. (ii) Derive from fundamentals the expression for steady state equimolal counter diffusion of gas A through another gas B. (8)

Or

(b) Methane diffuses at steady state through a tube containing helium. At point 1, the partial pressure of methane is PA1 = 55 kPa and at point 2, 0.03 m apart PA2 = 15 kPa. The total pressure is 101.32 kPa and the temperature is 298 K. At this pressure and temperature, the value of diffusivity is 6.75 x 10-5 m2/sec.

(i) Calculate the flux of CH4 at steady state for equimolal counter-diffusion.

(ii) Calculate the partial pressure at a point 0.02 m apart from point 1.

24. (a) Explain what do you understand by jD and jH factors.
25. Or

(b) Write short notes on:

(i) Film theory

(ii) Surface renewal theory

(iii) Penetration theory

26. (a) (i) Describe the methods available for estimating humidity of a sample of air.
27. (ii) What are the different types of cooling towers used in industries? Briefly explain them.

Or

(b) The temperature of air in a room is 40.2oC and the total pressure is 101.3 kPa abs. The air contains water vapor with a partial pressure PA = 3.74 kPa. Calculate

(i) the humidity

(ii) the saturation humidity and percentage humidity

(iii) the percentage relative humidity

28. (a) Find an expression for the determination of total time of drying of a wet solid material under constant drying conditions to a final moisture content well below the critical moisture content.
29. Or

(b) In a textile mill, wet cloth passes through a hot air drier. The cloth enters with 90% moisture regain and leaves at 6% moisture regain at a speed of 1.15 m/sec. The width of the cloth is 120 cm and its density on bone-dry basis is 0.095 kg/m2. The temperature of the cloth leaving the drier is 368 K. The ambient air enters the dryer at 303 K DB and 298 K WB while the hot air leaves the dryer at 393 K DB and 328 K WB. Calulate:

(i) the bone-dry production of the dryer

(ii) the evaporation taking place in the dryer and

(iii) the air circulation rate.

30. (a) (i) Discuss the working of a continuous vacuum crystallizer with the help of a neat sketch.
31. (ii) What are the parameters controlling the crystal size distribution in a crystallizer? Explain them briefly.

Or

(b) A solution of sodium sulfate in water is saturated at a temperature of 40oC. Calculate the weight of crystals and the percentage yield obtained by cooling this solution to a temperature of 5oC.The solubilities are as follows:

at 40oC: 32.6% Na2SO4
at 5oC: 5.75% Na2SO4
Note: At a temperature of 5oC the decahydrate will be the stable crystalline form.

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