Separation of lignocellulosic hydrolyzate components using ceramic microfilters

Among the biorefinery processes based on wood resources that are under current development, hemicellulose extraction using aqueous media is one of the prominent ones. The hemicelluloses in wood extracts need to be separated from inhibitory compounds. We examined the separation of sugar maple (Acer saccharum) wood extracts by ceramic microfilters of two different pore sizes: 0.2 μm and 0.01 μm as a function of membrane fouling. This was the first separation step intended to prepare the permeate for subsequent nanofiltration where components will be separated. Cross-flow permeation fluxes were determined for different trans-membrane pressures and cross-flow velocities. Large declines in the permeate fluxes were observed, indicating extensive fouling. The kinetics of the flux decay appears to indicate pore blocking and the development of external fouling layers as the cause. Colloidal and particulate materials were separated from the extracts with turbidity reductions of 94 to close to 100% in most cases. No particulates were detected in the permeates. Significant separation of sugars assayed in xylose form was observed and could be correlated with the extent of fouling of the membranes. Since the membrane pores are much larger than the sizes of xylo-oligomers, fouling layers built up during filtration seem to affect oligomeric separations. Further, the conditions that favor build-up of thicker fouling layers (higher pore size, higher TMP, and longer time into filtration) seem to also aid the retention of more hemicellulose sugars. Under the most severe case of fouling, xylose concentrations in the permeate dropped to less than one-fifth of the feed values. Cleaning the membranes by soaking in alkali followed by alkali and de-ionized water rinse and back-flushing were effective in reversing most of the membrane fouling.