Product removal strategy and fouling mechanisms for cellulose hydrolysis in an enyzmatic membrane reactor

Abstract

Enzymatic cellulose hydrolysis from lignocellulose biomass has been extensively studied as the product from the hydrolysis can be used to convert into renewable biochemical such as bioethanol. Cellulose hydrolysis were traditionally carried out in a batch reactor. However, cellulose hydrolysis in batch reactor leads to product inhibition which results in low yield of glucose. Kinetic study of cellulose hydrolysis in batch reactor was performed, and showed that cellulase was inhibited by glucose and cellobiose in a competitive manner, with Ki of 2.58 g/L and 2.24 g/L respectively. Therefore, it is necessary to separate glucose from the hydrolysis reactor in order to minimize product inhibition. In this study, enzymatic membrane reactor (EMR) was used to reduce the amount of enzyme used and to prevent product inhibition. The filtration technique used was ultrafiltration (UF) in a crossflow mode. Before performing cellulose hydrolysis in an EMR, a membrane screening was done to select a suitable membrane to be used in the EMR. Results had shown that HFK-131 membrane is the most suitable membrane as it has the lowest contact angle and the highest permeability. Cellulose hydrolysis was then carried out in an EMR with different substrate concentrations (5 g/L to 20 g/L) and different product removal strategies in order to study their effect on the product yield, membrane performance, and fouling mechanisms. The PES membrane showed almost 95% and above rejection of cellulase as the cellulase molecular weight (MW) was larger than molecular weight cut off (MWCO) of the membrane. Hermia’s pore blocking model was applied to determine the predominant fouling mechanism of the membrane filtration. From the results, intermittent product removal at 24 hours interval was better as the cellulose conversion could achieve more than 80% and the membrane flux decline is less severe than the product removal at 4 hours interval. For the effect of substrate concentrations, the cellulose conversion decreased from 88.48% to 61.43% with increasing substrate concentration. The flux also declined from 23.92 L/m2.h to 15.15 L/m2.h as the substrate concentrations were increased resulting in more cellulose to be deposited on the membrane surface, and leads to a more severe membrane fouling. It was also observed that the cake layer model was the predominant fouling mechanisms at 5 g/L and 10 g/L of substrate concentration, whereas 20 g/L has a combination of complete pore blocking and cake layer model. This result was further proved by SEM images, where the fouled membrane at 20 g/L appeared to have the most fouling layer on the membrane surface. Besides that, the membrane surface roughness increased with increasing substrate concentration, with the highest at 38.50 nm at 20 g/L. Results demonstrate the potential of using EMR for the production of reducing sugars and enzyme recovery in cellulose hydrolysis. With known fouling mechanism of cellulose hydrolysis in EMR, further improvement of the EMR operation at high substrate concentration could be done to minimize fouling.