# UPTU Previous Question Papers

## Mass Transfer Operation-II

Note : Attempt all questions.

1. Attempt any four parts of the following :

(a) Figure 1 depicts steady one-dimensional diffusion of water vapour from the surface of water taken in a conical flask at room temperature. Derive the equation for determining the concentration profile of water vapour in the gas medium. Neglect change of level of water due to condensation. The temperatures of the gas and the liquid media are identical and constant.

(b) State Fick’s law. Explain film theory of diffusion

(c) Derive an expression, based on the film theory, for the overall mass-transfer coefficient for the absorption of gas in liquid in terms of both liquid-side and gas-side mass-transfer coefficients and Henry’s constant.

(d) A stream of air is flowing at 45 m/s over a 1.44 m2 (square type) thin plate of solid naphthalene. The air is at 298K and 1.013xl05 Pa pressure and the plate is at 298K. Determine the rate of sublimation from the plate. The diffusivity of naphthalene in air at 298K and1.013x 105 Pa is 5.14 x 10′6 m2/s and the vapour pressure of naphthalene at 298K is 25 Pa. The density of air can be taken as 1.14 kg/m3 and viscosity as 1.8xl0-5 kg/ms. Sh = 2 + 0.552 Re053 Sc1/3.

(e) Differentiate between molecular diffusion and eddy diffusion.

(f)  Water is being evaporated into a countercurrent air steam at atmospheric pressure flowing at a rate of 1 kg/min in a vertical wetted wall column 0.03m in diameter. If the water flow rate is about 0.1 kg/min, estimate the evaporative flux at a point in the column where the water temperature is 40°C and the mole fraction of H20 in the bulk gas is 0.01. Vapour pressure of water at 40°C = 55 mm Hg Mass transfer coefficient = 3.3 xl0“3 sm

2. Attempt any two parts of the following:

(a) If mass-transfer resistance is essentially all in the gas phase, derive the following equation :

f mG\fl-x).,, HTUqq = htug + — htul lM xM

(b) A waste air stream containing 2 mol% of impurity A is purified to 0.02 mol % by scrubbing with pure water in a countercurrent packed column. The column provides 7 overall gas phase transfer units in a height of 1.75 m. The equilibrium mole fractions of A in the vapour and liquid are related according to yA = 1.75 xA Find HTYog, HTUol, NTVol and HETP for this system. How many theoretical stages would be required to achieve the same separation in a plate column?

(c) With the help of neat diagram describe the construction and working of packed absorption column.

3. Attempt any two parts of the following :

(a) A dryer requires 1.5 m3/s of air at 65°C, 20% humidity. This is prepared from air of dry-bulb temperature of 27°C and wet bulb temperature of 18°C by direct injection of steam into the air stream followed by passage of the air over steam-heated finned tube heat exchanger. The available steam is saturated at 35000 N/m2 gauge. Compute the kilograms of steam per second required :

(i) for direct injection

(ii) for the heat exchanger

(b) Define wet bulb temperature. Derive the following equation : tG~tw = Jh^)iyw~Y)where tQ, tyy = dry-bulb and wet bulb temperatures Y’ = absolute humidity at Iq Y^y = absolute humidity at tyy ‘kyy = latent heat at tyy hg Iky = phychrometric ratio

(c)             0.08 m3/s of air at 32°C and 60% relative humidity is to be cooled at 2°C. By the use of psychrometric chart calculate the amount of heat to be removed for each 10°C internal of cooling process. What total mass of moisture will be deposited? What is the humid heat of the air at the beginning and end of the process?

4. Attempt any two parts of the following :

(a) With the help of a typical drying curve, explain the following terms:

(i) Constant and falling rate periods

(ii) Free moisture content

(iii) Equilibrium moisture content

(iv) Critical moisture content

(v) Bound moisture

(b) A wet solid is dried from 35% to 10% moisture under constant drying conditions in 5 hours. If the equilibrium moisture content is 4% and the critical moisture content is 14%, how long will it take to dry from 30% to 6% moisture under the same conditions. All percentages are on wet basis. Assume linear relation between rate of drying and moisture content during falling rate period.

(c) What is meant by cross-circulation drying and through circulation drying.

5. Attempt any four parts of the following :

(a) What is meant by supersaturation? Give the various methods for generating supersaturation.

(b) CuS04H20 containing 3.5 per cent of a soluble impurity is dissolved continuously in sufficient water and recycled mother liquor to make a saturated solution at 80°C. The solution is then cooled to 25°C and crystals of CuS04 5H20 are obtained. These crystals carry 10 per cent of their dry weight as adhering mother liquor. The crystals are then dried to zero free water(CuS04 5H20). The allowable impurity in the product is 0.6 per cent. Calculate the percentage recovery of copper sulphate, assuming that the mother liquir not recycled is discarded. The solubility of CuS04 5H20 at 80°C is 120 kg per kg of free H20 and at 25°C it is 40 kg per 100 kg of free H20.

(c) Differentiate between homogeneous and heterogeneous nucleation.

(d) Define and explain eutectic temperature and eutectic composition.

(e) Describe the working of draft tube-baffle (DTB) crystallizer.

(f)  What is meant by fractional crystallization?