A theoretical and experimental analysis of a membrane bioreactor performance in recycle configuration

Abstract The behavior of membrane bioreactors operating in recycle configuration is characterized from both a theoretical and an experimental point of view. The theoretical model is based on the unsteady-state balance equations governing momentum and mass transfer within the regions that can be identified in a membrane reactor with immobilized enzyme, coupled to the unsteady-state mass balance on the components in feed tank. The resulting system of non-linear partial differential equations has been numerically solved by Finite Elements Method (FEM), implemented by a commercial package called FEMLAB. The available CFD code allowed determining the time evolution of both velocity and concentration profiles without resorting to considerable simplification hypotheses. As far as the reaction term in the mass balance equations is concerned, reference has been made to lactose hydrolysis catalyzed by immobilized β-galactosidase, also carried out in the experimental part of this work. The chemical reaction is supposed to take place solely in the membrane dense and spongy layers where biocatalyst is actually confined and distributed according to its concentration ratio E3/E2. A set of intrinsic kinetic constants, obtained from the literature, has been used to evaluate the actual reaction rate thus evaluating the effects of immobilization on enzyme activity. The contribution of the most significant transport mechanisms affecting reactor performance has been determined analyzing the dynamic evolution of three representative parameters, i.e. the effectiveness factor, the degree of conversion within the membrane and the bioreactor performance index, as functions of the operating conditions. The proposed transport model is very general as it may describe the behavior of different types of heterogeneous reacting systems by just adapting either the reaction rate expression or the set of intrinsic kinetic constants. An experimental analysis of system behavior has validated the theoretical predictions, showing a good agreement between experimental and theoretical results.