Part A (20 x 2 = 40 Marks)

- Define the term diffusion
- What is the unit for mass diffusivity?
- Define eddy diffusion
- Draw a graph showing concentration gradient for equimolal diffusion
- What is Sherwood number?
- Give the relationship between mass transfer coefficient and diffusivity
- What is the effect of pressure and temperature on the diffusion coefficient in gases?
- Define 'Absorption factor'
- Define humidity
- What is meant by percentage humidity?
- How the cooling effect in a cooling tower can be increased?
- Define Dew point
- Define the term Equilibrium moisture and free moisture content of solid
- What is freeze-drying?
- Which drier is suitable for handling fragile crystals?
- During drying operation, critical moisture content varies with _________.
- Write down the sequence of stages in the evolution of a crystal
- Write down the three methods used to produce super-saturation
- What is magma?
- What is the purpose of agitator in a crystallizer?
- (a) (i) Distinguish between molecular and eddy diffusion. (4)
- (a) Explain what do you understand by j
_{D}and j_{H}factors. - (a) (i) Describe the methods available for estimating humidity of a sample of air.
- (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.
- (a) (i) Discuss the working of a continuous vacuum crystallizer with the help of a neat sketch.

Part B (5 x 12 = 60 Marks)

(ii) Derive from fundamentals the expression for steady state equimolal counter diffusion of gas A through another gas B. (8)

(b) Methane diffuses at steady state through a tube containing helium. At point 1, the partial pressure of methane is P_{A1} = 55 kPa and at point 2, 0.03 m apart P_{A2} = 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} m^{2}/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.

(b) Write short notes on:

(i) Film theory

(ii) Surface renewal theory

(iii) Penetration theory

(ii) What are the different types of cooling towers used in industries? Briefly explain them.

(b) The temperature of air in a room is 40.2^{o}C and the total pressure is 101.3 kPa abs. The air contains water vapor with a partial pressure P_{A} = 3.74 kPa. Calculate

(i) the humidity

(ii) the saturation humidity and percentage humidity

(iii) the percentage relative humidity

(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/m^{2}. 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.

(ii) What are the parameters controlling the crystal size distribution in a crystallizer? Explain them briefly.

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

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