Enhancing chemical recovery in biomass processing : an approach using membrane technology

Abstract

This thesis investigated the separation of chemical components from black liquors derived from soda pulping of eucalyptus and straw biomass without chemical pre-treatments. Black liquors were produced under cooking conditions: eucalyptus with an H-factor of 1800 and 23% effective alkali (EA), and straw with an H-factor of 400 and 16% EA. Membrane filtration experiments were done with polyethersulfone (PES) membranes with molecular weight cut-offs (MWCO) of 20kDa, 10kDa, 4kDa, and 1kDa at controlled pressures and temperatures with dead-end type filtrations.

Performance evaluation of the membranes was based on permeate fluxes, retentions, and the effectiveness of membrane washing. Eucalyptus black liquor exhibited three times higher initial permeate fluxes, indicating a higher proportion of small molecular components. Flux recovery after washing was also significantly higher for eucalyptus in most cases, suggesting cake-layer type fouling. Straw black liquor demonstrated more persistent pore-blocking type fouling, likely due to a broader molecular size distribution and sugar molecular interactions with other components.

Lignin retentions were comparable across membranes, with the 4kDa membrane achieving the highest retention (82% for eucalyptus and 81% for straw). Straw black liquor contained 53% more total sugars than eucalyptus, with significant sugar retention observed for straw using 10kDa and 4kDa membranes (73% and 82%, respectively) whereas no retention were observed with eucalyptus black liquor due to small molecular size of the sugars. Sodium retention was higher with eucalyptus due to higher initial concentrations. Silica retention with eucalyptus showed consistently higher retentions despite much lower initial concentrations than with straw black liquor.