Temperature Effects on the Molecular Properties of Liquid Lignin Recovered from Kraft Black Liquor

Abstract

The effect of temperature on the molecular properties of a “liquid-lignin” phase recovered from Kraft black liquor via CO2 acidification at elevated temperatures was investigated. For softwood black liquors, up to a 4-fold increase in the molecular weight (mol wt) of the recovered liquid-lignin phase was observed as the acidification temperature was increased from 100 to 150 °C. Thus, experiments were carried out in which the CO2-acidification steps of preheating, acidification, and phase separation were decoupled from each other in order to elucidate the mechanisms for the observed changes in molecular weight. Results indicate that some lignin cleavage and a corresponding decrease in molecular weight occur during the black-liquor preheating step. However, a significant net increase in molecular weight occurs after the lignin has phase-separated from the black liquor in the form of a new, denser liquid phase, pointing to condensation reactions as the most likely cause. Minimal changes in lignin aromatic c...