Carbon molecular sieve and ceramic membranes for black liquor concentration

Abstract

The concentration of Kraft black liquor is an energy-intensive process requiring 6-7 multiple effect evaporators. Membranes have the potential to replace the first 1-2 of these evaporator effects (stages) by concentrating black liquor to 30-35%, resulting in significant energy savings. There are no membranes in industrial use for this purpose today, though both polymeric and ceramic (metal oxide) have been studied in the laboratory and on a pilot scale. These membranes face issues of sufficient stability (polymers) and larger costs and pore sizes (ceramics), resulting in inadequate performance. This work addresses improvements in the performance of ceramic (α-alumina and γ-alumina) membranes by adding coatings with smaller pore sizes onto supports with larger pores (for high fluxes). These coated supports are able to serve as both prefilters and as adequate supports for the coating of Matrimid-based carbon molecular sieves (CMSs) to achieve reverse osmosis-quality filtrates. Ceramic coatings achieved a maximum lignin rejection of 60%, which is inadequate for good filtration based on the state-of the art commercial membranes. However, for CMS coatings, the alumina-coated layer yielded a CMS membrane with flux and ~68% lignin rejection. Though defects in the CMS layer are present, the initial results are promising. Future work would address the mitigation of defects and incorporate sacrificial nanoparticles into the CMS layer to improve liquid flux performance.