Production of Water-Soluble Carbohydrates from Aspen Wood Flour with Hydrogen Chloride Gas

The overall aim of this study was to optimize the reaction conditions for concentrated acid hydrolysis of aspen wood flour by employing anhydrous hydrogen chloride gas to produce fermentable sugars. Gas hydrolysis with HCl was conducted both with and without temperature control during the hydrolysis under relatively low pressure of 1 bar. Process parameters for HCl gas hydrolysis included the moisture content of aspen wood flour (0.7-50 %) and reaction time under pressure (30 minutes to 24 hours). Temperature control during gas hydrolysis was used to prevent excess degradation of C5-sugars during the gas application phase via cooling in ice bath and to speed up the hydrolysis reaction during the last 10 minutes via heating to 50 °C. This enabled the hydrolysis to proceed in comparably mild temperature conditions, resulting in higher yields of water-soluble carbohydrates. In addition, liquid-phase hydrolysis with concentrated hydrochloric acid was conducted in concentrations of 32-42 % and 15 minutes to 24 hours reaction times for comparison with gas-phase process. Highest yield for water-soluble carbohydrates from aspen wood flour (92 % of available glucan and 91 % of available xylan) was achieved with temperature-controlled gas hydrolysis using 50 % moisture content and 2 h total reaction time, which is in line with previous research and comparable to hydrolysis with concentrated (42 %) hydrochloric acid.