Hydrothermal treatment and organosolv pulping of softwood assisted by carbon dioxide

Abstract

A profitability requirement for biorefineries is the integral use of raw materials through efficient and optimized fractionation processes. Supercritical carbon dioxide has been stated as a promising green solvent which advantages have been previously suggested in the literature. This work evaluates the effect of subcritical and supercritical CO2 in both the hydrothermal treatment and the organosolv delignification, either as individual stages or as sequential processes, using pine sawdust, chips, and milled chips as raw materials. Both, the hydrothermal and the organosolv trials were first performed according to simple factorial designs, followed by a set of extra experiments for further understanding of the pine behavior. Carbohydrates content in the hydrothermal liquors and structural carbohydrates and lignin in the final pulps were analyzed. The yield (percentage of final biomass over original biomass) of the hydrothermal trials ranged from 76.2%–99.2%. CO2 effect on yield and hemicelluloses removal is more noticeable at shorter periods and low (120 °C) and mild temperatures (160 °C), and it weakens as reaction time and temperature increase. The delignification rate during organosolv pulping ranged between 10.3 % and 70.1 %. For industrial sawdust, the presence of CO2 during organosolv pulping significantly decreased delignification. The effect of hemicelluloses removal before organosolv pulping varies depending on the raw material, but in general, has a deleterious effect on delignification of pulps.