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

The effect of temperature on the molecular properties of a "liquid-lignin" phase recovered from Kraft black liquor via CO, 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 degrees 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 content were observed in the new liquid-lignin phase. Because elevated-temperature, CO2 acidification can be carried out continuously via the SLRP process, adjusting residence times in the black liquor liquid lignin phase separator would be an effective method for controlling the molecular weight of the recovered lignin for a given product application.