REACTANT DEPLETION IN PERIODIC OPERATION OF TRICKLE BED REACTORS

M. ALEJANDRA AYUDE; MIRYAN C. CASSANELLO; OSVALDO M. MARTINEZ; PATRICIA M. HAURE

Rio de Janeiro, Brasil

Congreso; 2nd Mercosur Congress on Chemical Engineering, 4 th Mercosur Congress on Process Systems Engineering; 2005

When a trickle bed reactor is operated under liquid flow modulation, regions of different masstransfer characteristics occur in the external catalyst surface. The reaction rate can be greater or smaller than the rate observed for steady state conditions. Reactor performance is clearly enhanced when the reaction is gas limited and strong external mass transport limitation exists. If an ON-OFF liquid flow modulation is imposed, liquid reactant depletion may occur during the OFF period of the cycle. Moreover, even for the traditional steady state operation, liquid reactant depletion may happen for incomplete external wetting of the catalyst at low liquid velocities. In both cases, this effect should be taken into account when designing andinterpreting the performance of these reactor systems. Hence, the ability to evaluate the overall effectiveness factor for these situations is an important step. In this contribution, the effect of liquid reactant depletion inside the catalyst on the overall effectiveness factor under steady-state and with liquid flow modulation is examined using a phenomenological approach. Explicit finite difference method is used to obtain the solutions of the dynamic mass balances for a partially wetted catalyst particle where a first order reaction with respect to the gaseous reactant and zero order with respect to a liquid non-volatile reactant is taking place, including the following assumptions: (1) spherical catalyst particle (2) isothermal conditions (3) gas limited reaction (4) total internal wetting and variable partial external wetting according to the flowmodulation imposed. (5) negligible gaseous mass transfer resistance once the liquid flow is halted. Overall effectiveness factor was evaluated for both steady state and periodical operation. Results show a significant influence of the ratio of reactants’ concentration on the enhancement that can be attained with periodic operation.