Question 1 [40 marks] In presence of spherical catalyst pellets, an experimental decomposition rate of gaseous A in a packed bed reactor follows the kinetics: - r’’’A, obs = 1.1 x105 mol/m3cat.hr (a) Derive an equation to evaluate the effects of non-isothermal behavior across the stagnant gas film surrounding spherical pellets (external heat transfer). (10 marks) (b) With the given data, shown below, determine the temperature variations across the stagnant gas film. • Diameter of the pellets, dp = 7 mm • Effective diffusivity, De = 5 x10-5 m3/mcat.hr • Heat transfer coefficient, h = 150 kJ/m2cat.K.hr (for the gas film). • Heat of reaction, ∆Hr = -120 kJ/mol A (exothermic). (10 marks) (c) Which parameters should be optimized to reduce the temperature variations? (5 marks) (d) If the decomposition takes place based on the catalyst volume with the reaction constant: 𝑘𝑘 ′′′ = 5 m3fluid. m-3cat. s-1 Calculate the reaction constant,𝑘𝑘 ′′′′ , based on the reactor volume, given that: • The bulk density of the packed bed is 2100 kg/m3, and • The void is 0.56 m3voids/m3 reactor (15 marks) Page 2/3 Question 2 [60 marks] In a CSTR of 0.25 m3 volume, liquid is being fed at 0.5 m3/hr with 12 mol/m3 concentration of the reactant A according to the catalytic reaction shown below. −𝑟𝑟𝐴𝐴′ = 0.003 𝐶𝐶𝐴𝐴 𝑚𝑚𝑚𝑚𝑚𝑚. 𝑘𝑘𝑘𝑘−1 . ℎ𝑟𝑟 −1 The catalyst bulk density is 1760 kg.m-3 and the particle density is 2000 kg.m-3. Assume external and internal mass transfer resistances are negligible. (a) What is the conversion of the reactant in the CSTR when the catalyst is fresh? (20 marks) (b) Based on a pilot-plant experiment, the catalyst being used has a third order deactivation kinetics: 𝑑𝑑𝑑𝑑 − 𝑑𝑑𝑑𝑑 = 𝑘𝑘𝑑𝑑 𝑎𝑎3 (unit of Kd is day-1) It is found out that the catalyst is deactivating sharply, and that after 7 days, the conversion, XA, has dropped to 60%. What is the deactivation rate constant, kd? (20 marks) (c) Assume you want to regenerate the catalyst when the conversion, XA, drops equal to or below 70%. For how many days should you operate the reactor before regenerating the catalyst? Is it technically feasible? (20 marks) Page 3/3
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