Dewatering black liquor and lignin with supercritical CO2

Abstract

Black liquor, the residue remaining from pulping wood chips for papermaking, is used as fuel to power the mill. For energy efficiency, black liquor must be concentrated to over 65% solids prior to combustion. Supercritical CO2 achieves this by precipitating lignin by dropping the pH below the pKa of the phenolic groups in lignin, thereby depositing solids and creating a porous liquid/solid matrix. It then displaces the water from the matrix in a manner analogous to the dewatering of materials such as biomass, coal, and sludge. Thus, the supercritical CO2 performs a dual role of generating a porous matrix from the black liquor and then dewatering it. The amount of water removed far exceeds its solubility in supercritical CO2 and is proportional to the boiling point rise of black liquor. This opens the possibility of replacing one or more stages of a multi-effect evaporator with a supercritical CO2 dewatering unit. A similar approach applies to lignin deposited from black liquor for use as biofuel or as a chemical feedstock. Porosity appears to control displacement efficiency, with the process being more facile for open structures.