Design and operation of an integrated membrane reactor for enzymatic cellulose hydrolysis

Abstract

A purposely built membrane reactor with combined reaction and separation zone in a single piece of equipment was tested for heterogeneous enzymatic hydrolysis of a hardwood derived cellulosic material. The design and operation features a process of simultaneous reaction and in situ removal of products (reducing sugars) in order to reduce the effect of product inhibition on the cellulase enzyme. The integrated reactor system differs from the hybrid integration of reaction-separation system where the reaction takes place in a stirred reaction vessel and the separation of the product is carried out in a separate cross-flow membrane filtration unit. Various operating strategies of semi-continuous and continuous product removal and substrate feeding were examined and optimised in respect to their effect on reducing product inhibition and increasing reactor productivity and product yield. The effect of total enzyme concentration and possible enzyme deactivation by shear stress were also investigated. The findings from examining the performance of a well understood enzymatic system in a new integrated reactor system would facilitate the understanding and development of system integration and process intensification in combined reaction-separation process which may alleviate common problems (product inhibition, inefficient interfacial mass transfer, slow kinetics and low reactor productivity) in heterogeneous enzyme catalysis systems. The cellulose substrate conversion ratio was 53% in the continuous operation of reaction with simultaneous product removal. This compares favourably with 35% substrate conversion in traditional batch operations. The improvement in overall reaction rate through adopting an in situ product removal strategy was rather limited, mainly because of the inherent slow reaction kinetics of the heterogeneous cellulose hydrolysis and the complex interfacial interactions between the cellulase enzymes and the solid cellulose substrate.