Online integrated fractionation-hydrolysis of lignocellulosic biomass using sub- and supercritical water

Abstract

A novel process coupling the fractionation and hydrolysis reactors is presented. Holm oak was used as real lignocellulosic biomass to be treated. In the fractionation reactor, hemicellulose and cellulose were solubilized and partially hydrolyzed in different stages with the aim of feeding the hydrolysis reactor with high C5 or C6 concentrations respectively. The fractionation was performed in two stages: at 180 °C optimizing the hemicellulose extraction and at 260 °C extracting cellulose and hard hemicellulose remaining in the biomass structure. Three water flows were tested: 11, 17 and 26 cm3/min. Sugar yields from 71 to 75% were reached, mainly composed of xylose and glucose oligomers and lower amounts of other chemicals, like retro-aldol products, acetic acid or 5-HMF. The outlet stream from the fractionation reactor was directly mixed with sub or supercritical water at the inlet mixer of a supercritical hydrolysis reactor where the reaction time was precisely controlled. The temperature, pressure and reaction time were modified to get an insight of their effect on the yield of retro-aldol condensation products. Yields of 24% for glycolaldehyde, and pyruvaldehyde were found at 8.3 s, 350 °C and 162 bar (hydrolysis reactor conditions). On other hand, 25% of lactic acid was found at 0.23 s, 396 °C and 245 bar. A discussion based on a known reaction pathway is proposed. Moreover, a kinetic model for the hydrolysis reactor was put forward, being able to reproduce the experimental data with deviations below 10% for sugars and other products extracted. This combined process performs a selective valorization of real lignocellulosic biomass, avoiding the costly process of extreme grinding needed for the fluidization in a continuous hydrothermal process.