CHE 318 Midterm Tests and solution

— October 4, 2013 Time: 12:10 pm — 3:00 pm Answer all questions Closed book, formula sheets allowed 1. A granular polymeric material is to be dried by blowing air through the material in a fixed bed. The thickness of the bed is 7 cm. The bed porosity is 0.60, and the specific area of the solid particles in the bed is 185 m2/m3. The particle average diameter is 0.013 m and the dry solid bulk density is 640 kg/m3. The dry solid density is 1600 kg/m3. Air at 55°C and a wet bulb temperature of 25 °C is used to dry the wet material. Air mass flux is 2592 kg/m2.h. Average air viscosity is 2.15x10-5Pa.s. The latent heat of evaporation of water at the wet bulb temperature is 2442 KJ/kg. a) Estimate the required drying time to reduce the total moisture content of the material from 0.70 kg water/kg of dry solid to 0.10 kg water/kg of dry solid. The equilibrium moisture content of the material under the given conditions is 0.010 kg water/kg of dry solid. The critical free moisture content is 0.080 kg water/kg of dry solid. (30 marks) b) If humidity of the inlet air is increased to 0.0127 kg water vapour/kg dry air (same other conditions), would the drying time increase or decrease and by how much (10 marks)? c) If the gas mass flow rate is reduced to 50% of the original flow rate, what would the required drying time be? (10 marks)
2. An air stream containing 4% ammonia (by volume) is to be introduced to the bottom of a packed column, which is filled with 2-inch ceramic Berl saddles, at a molar flux of 20 lbmol/ h. ft2. Pure water is fed to the top of the column at 1020 Ibmol/h.ft2. 95% of ammonia in the gas is to be removed by the packed column. The tower is operated at 68°F and 1 atm. Under the operational conditions in the column, the film mass transfer coefficients are: lc, a = 510 lbmol/h.ft3 ky a = 12.5 lbmol/h.ft3 Gas density = 0.073 lb/ft3; molecular of gas is 28.9 lb/lbmol Liquid density = 62.4 lb/ft3; molecular weight of liquid is 18 lb/lbmol At 68 °F and the total pressure of 1 atm, it was observed that at the mole fraction of ammonia in water of 0.0309, the equilibrium mole fraction of ammonia in air (gas phase) was 0.0239. In addition, the equilibrium of ammonia/water system can be considered linear over the small range of ammonia concentrations in the present operation. The operating line can also be assumed to be linear. a) Estimate the packing height needed. (30 marks) b) Given that the column already exists as sized in part a), if there is an upset upstream of the packed column resulting a reduction of the ammonia concentration to 3% while the flow rates of liquid and gas streams remain unchanged, provide an estimation/calculation procedure that can be taken to determine the new ammonia concentration in the gas stream exiting the column (note that the mass transfer coefficients are unchanged since there is no change in gas and liquid flow rates) (10 marks). c) Suppose a further reduction of the mole fraction of ammonia in the gas stream leaving the tower to 0.0010 is required. If the gas molar flux and its composition at the tower inlet remain the same, and the same packing height as estimated in part a) is to be used, what could be done to the operation of the tower to meet the new requirement? (procedure and qualitative solution is sufficient) (10 marks)