ANNA UNIVERSITY, VI – SEMESTER, B.TECH. CHEMICAL ENGINEERING
CHEMICAL REACTION ENGINEERING – I
Answer all Questions
PART – A (10 x 2 = 20 Marks)
1. Define F-type and K-type mass transfer coefficients and give its importance.
2. Explain the term flooding and loading in gas-liquid equipment where liquid is the dispersed phase.
3. Define Sherwood, Stanton and Schmidt number.
4. Explain the relationship between NTP and NTU?
5. State the basic assumptions made in film and penetration theory.
6. Define the term number of overall enthalpy transfer unit and height of enthalpy transfer unit.
7. What do you understand by the term fractional crystallization?
8. Define the term bound and unbound moisture content.
9. Define the super-saturation and state any four methods of achieving super-saturation.
10. Discuss briefly the term unsaturated surface drying.
PART – B (5 x 16 = 80 Marks)
11. Water is to be cooled from 45 to 27°C in a counter-current cooling tower under conditions such HTU is 2.8m. Air enters at the bottom of the tower at 24°C and a wet-bulb temperature of 21°C. Find the height if 1.6 times the minimum air flow rate is used. Neglect the heat transfer resistance of the liquid phase.
12.a) An cylindrical tank of 6.0m in diameter contains benzene at 25°C which is exposed to the atmosphere in such a manner that the liquid is covered with a stagnant air film having an estimated thickness of 5mm. The concentration of benzene beyond the stagnant film is negligible. The vapour pressure of benzene 25°C is 100mm Hg. If the cost of benzene is Rs. 50 per litre what is the value of benzene lost from this tank per day. The diffusivity of benzene is 7.71 x 10-6 m2/s and the density of benzene at 25°C is 880 kg/m3.
12.b) An ethanol (A)-water (B) solution in the form of a stagnant film 2.0 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, Ng = 0. At point 1 the concentration of ethanol is 16.8 wt% and the solution density is ?1 = 972.8 kg/m3. At point 2 the concentration of ethanol is 6.8 wt% and ?2 = 988.1 kg/m3. The diffusivity of ethanol is 0.74 x 10-9 m2/s. Calculate the steady state flux NA.
13.a) Naphthalene is submitted from a cylinder of naphthalene of 0.075m diameter and 0.5m long into a stream of pure carbon dioxide flowing at a velocity of 6 m/s and 1 atm. and 100°C. Calculate the mass transfer coefficient F. The vapor pressure of naphthalene at 100°C is 1320 N/m2.
Viscosity of gas 0.018 x 10-3 kg/ms
Density 1.43 kg/m3
Diffusivity 0.08 x 10-4 m2/s
JD = 334.6 x 10-4 (Re)1-0.2
Where Reynolds number is based on the length of cylinder
13.b)i) State the difference between stage wise and differential contact gas-liquid equipment used in mass transfer operations.
ii) Derive the relationship between the overall and individual mass transfer coefficients.
14.a) A continuous countercurrent dryer is being used to dry 453.6 kg dry solid /h containing 0.04 kg total moisture/kg dry solid to a value of 0.002 kg total moisture/kg dry solid. The granular solid enters at 26.7°C and is to be discharged at 62.8°C. The dry solid has a heat capacity of 1.465 kJ/kgK, which is assumed constant. Heating air enters at 93.3°C, having a humidity of 0.01 kg H2O / kg dry air and is to leave at 37.8°C, Calculate the air flow rate and the outlet humidity, assuming no heat losses in the dryer.
14.b) A porous solid under constant drying conditions takes 6.5 hours to reduce the moisture content from 30 to 10%. The critical moisture content is 14% and the equilibrium moisture content is 2.5%. If the rate of drying in the falling rate period is proportional to the free moisture content, how long will it take to dry a sample of the same solid 40 to 5%?
15.a)i) Explain briefly how crystal growth is controlled in a continuous crystallizer.
ii) Explain in detail the various factors governing the nucleation and crystal growth.
15.b) Explain in detail the working of Swenson-Walker Continuous crystallizer with a neat diagram.