Semi-continuous liquid hot water pretreatment of rye straw

This article is based upon the findings of our recent publication dealing with the liquid hot water (LHW) hydrolysis of lignocellulosic materials (LCM) in different reactor types [T. Rogalinski, T. Ingram, G. Brunner, Hydrolysis of lignocellulosic biomass in water under elevated temperatures and pressures, J. Supercrit. Fluids (2008), doi:10.1016/j.supflu.2008.05.003.]. As an advancement of these results, semi-continuous fixed-bed reactors were constructed aiming at the efficient hydrolysis of rye straw at elevated temperatures as a pretreatment process for bioethanol production. The temperature range between 170 °C and 230 °C was found to be most suitable for the thermal pretreatment of rye straw. The resulting liquid hydrolyzates as well as the solid residues were enzymatically converted to monomeric sugars (xylose/glucose) using cellulases and xylanases from Penicillium janthinellum. Ninety-five percentage of the initial xylan and 92% of the glucan was converted using a pretreatment temperature of 200–215 °C. The solubilization of the biomass in the fixed-bed reactor could be described by a reaction model following first-order kinetics. For that reason, the λmax-factor was introduced, which describes the maximum ratio of biomass that can be hydrolyzed at a certain temperature, related to the initial mass. The usage of a fixed-bed reactor combines several advantages compared to other reactor types, namely high solid-to-water-ratios, the prevention of degradation product formation, and considerable energy savings since no biomass comminution is necessary.