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2003 Course Chemical Reaction Eng. - II

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Total No. of Questions : 12] P1012 [Total No. of Pages : 4 [3664]-273 B.E. (Chemical) CHEMICAL REACTION ENGINEERING - II Time : 3 Hours] [Max. Marks : 100 Instructions to the candidates: 1) Answer 3 questions from Section-I and 3 questions from Section-II. 2) Answers to the two sections should be written in separate books. 3) Neat diagrams must be drawn wherever necessary. 4) Figures to the right indicate full marks. 5) Use of logarithmic tables, slide rule, Mollier charts, electronic pocket calculator and steam tables is allowed. 6) Assume suitable data, if necessary. SECTION - I Q1) a) In case of shrinking core model derive the rate expression for the case of chemical reaction resistance is rate controlling step for solid as reactant. [9] b) Spherical particles of zinc blende of size R=1 mm are roasted in an 8% oxygen stream at 900 C and 1 atm. The stoichiometry of the reaction is: 2ZnS + 3O2 2ZnO + 2SO2 Assuming that the reaction proceeds by the shrinking-core model calculate the time needed for complete conversion of a particle and the relative resistance of ash layer diffusion during the operation. Data: Density of solids = 4.13 gm/cm3 = 0.0425 mol/cm3. Reaction rate constant k = 2cm/sec, For gases in the ZnO layer, De = 0.08 cm2/sec. Note that film resistance can safely be neglected as long as growing ash layer is present. [8] OR Q2) Uniform sized spherical particles of UO3 are reduced to UO2 in a uniform environment with the following results: t, hr 0.180 0.347 0.453 0.567 0.733 XB 0.45 0.68 0.8 0.95 0.98 If the reaction follows shrinking core model, find the rate controlling mechanism and a rate equation to represent this reaction. [17] P.T.O. Q3) Air with gaseous A bubbles through a tank containing aqueous B. Reaction occurs as follows: [17] A (g 1) + 2B (1) ------- R(1) , -rA= kCACB2 k = 106 m6/mol2.hr For this system kAg.a = 0.01 mol/hr.m3.Pa, f1= 0.98, kA1.a = 20 hr -1, HA= 105 Pa.m3/mol (very low solubility), DA1= DB1= 10-6 m2/hr, a = 20m2/m3 For a point in the absorber reactor where PA = 5*103Pa and CB= 100 mol/m3, a) locate the resistance to reaction (what % is in the gas film, in the liquid film, in the main body of liquid) b) locate the reaction zone c) determine the behavior in the liquid film (whether pseudo first-order reaction, instantaneous, physical transport, etc) d) calculate the rate of reaction (mol/m3.hr) OR Q4) a) In case of fluid-fluid reactions derive rate expression for fast reactions. [8] b) In case of fluid- fluid reaction taking place in the tower reactor the inlet partial pressure of reactant is 0.002 atm and outlet partial pressure is 0.003 atm. The concentration of liquid phase reactant entering is 132 mol/m 3. Following operating parameters have been determined. kAg.a = 32000 mol/hr.m3 atm, k1A.a = 0.1/hr HA = 125*10-6 atm m3/ mol, G = 1*105 mol/hr m2, CT= 56000 mol/m3, L = 7*105 mol/hr.m2, = 1 atm. Determine the height of the tower. [9] Q5) a) Compare physical and chemical adsorption. [6] b) An 8.01 g sample of Glaucosil is studied with N 2 a dsorption at 195.8 C. The following data are obtained. The vapour pressure of N2 at 195.8 C is 1 atm. Estimate the surface area of the sample. [10] P mm Hg 6 V cm3 61 0 C, 1atm [3664]-273 25 140 230 285 320 430 505 127 170 197 215 230 277 335 -2- OR Q6) a) Write short note on catalyst poisoning. [6] b) Low temperature ( 195.8 C) nitrogen adsorption data were obtained for an FeAl2O3 ammonia catalyst. The result for 50.4 g sample were: P mm Hg 8 30 50 102 130 148 233 258 330 442 480 550 V cm3 103 116 130 148 159 163 188 198 221 270 294 365 0 C,1atm Estimate the surface area for this catalyst. [10] SECTION - II Q7) An experimental rate measurement on the decomposition of A is made with a particular catalyst. Is it likely that film resistance t mass transfer influences the rate? Could this run have been made in the regime of strong pore diffusion? Would you expect to have temperature variations within the pellet or across the gas film? The data is as given below: Data: For the spherical particle: dp = 2.4 mm, L=R/3=0.4mm, De=5*10-5 m3/hr.m cat, keff=1.6 kJ/hr.m cat.K For gas film surrounding the pellet: h = 160 kJ/hr.m2.cat.K, kg= 300 m3/hr.m2 cat For the reaction: Hr = 160 kJ/mol A, CAg = 20 mol/m3, -rAobs =105 mol/hr.m3 cat Assume that the reaction is of first order. [18] OR Q8) Derive the expression for effectiveness factor in case of cylindrical pore. State clearly the assumptions made if any. Explain the Thiele s modulus and its importance. [18] Q9) a) Explain steps involved in solid catalyzed reaction with neat diagram. [6] b) For the following reaction mechanism derive the rate expression for the case of surface reaction as rate controlling step. [10] A+S A.S B+S B.S A.S+B.S C.S+S C.S C + S [3664]-273 -3- OR Q10) a) In case of fluid-fluid reaction taking place in the tower reactor the inlet partial pressure of reactant is 0.01 atm and outlet partial pressure is 1*10-6 atm. The concentration of liquid phase reactant entering is 0.35 mol/1. Following operating parameters have been determined. kAg.a = 6*10-5 mol/sec.cm3 atm, k1A.a = 0.03/sec, kIB.a = 0.02/sec, HA= 115 atm.cm3/mol, G = 3*10-3 mol/sec.cm2, CT = 56000 mol/m3, L = 6.6*10-3 mol/sec.cm2, = 1 atm. Determine the height of the tower. [10] b) Explain the procedure to determine rate controlling step in case of solid catalyzed reactions. [6] Q11) a) Substrate A and enzyme E flow through a mixed flow reactor (V=6 liter). From the entering and leaving concentrations and flow rate find a rate equation to represent the action of enzyme on substrate. [8] CE0 mol/lit CA0 mol/lit CA mol/lit v lit/hr 0.02 0.2 0.04 3 0.01 0.3 0.15 4 0.001 0.69 0.60 1.2 b) Discuss design of fermentor design with emphasis on air velocity and agitation speed. [8] OR Q12) a) Explain Michaelis-Menten kinetics. Discuss how to determine the parameters of the model. [8] b) Compare relative merits and demerits of fluidized and packed bed reactors. [8] [3664]-273 -4-

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