(782b) Thermo-Enzymatic Conversion of Lignocellulosic Biomass in a High Pressure Fixed Bed Reactor

Upcoming biorefinery processes for lignocellulosic biomass mainly focus on one end product. As cellulose (C6‑oligomer), hemicellulose (C5-oligomer) and lignin (phenolic biopolymer) make up the major part of lignocellulosic biomass, an effective separation of these fractions is needed to ensure a comprehensive usage. As the use of supplementary chemicals should be limited as much as possible, pressure can be a tool for changing the properties of water in the direction to develop valuable products out of lignocellulose resources without generating additional waste streams.

It is known that pressurized hot water can extract the hemicellulose and short chained lignin from lignocellulosic biomass in flow-through fixed-bed reactors. After that enzymes can degrade the remaining sugar oligomers to monomers. Hereby high pressure can increase the stability of enzymes towards higher temperatures and therefore accelerate the reaction rates. After the enzymatic treatment two solid lignin fractions (low and high M_w) and two liquid sugar solutions (C5/C6) remain. Depending on the reaction conditions, the optimum slightly varies depending on the desired product. Therefore a deep knowledge of the process is needed for planning and further changes in scale.

This work focuses on the high pressure assisted degradation of lignocellulosic biomass to monomeric sugars (glucose and xylose) and lignin in a 4 liter fixed bed reactor by a consecutive thermal and enzymatic hydrolysis. Different sets of experiments were conducted: (1) variation of the hot water extraction parameters (flow rate, temperature, time and biomass loading) leading to optimal conditions by means of C5-oligomer yield, enzymatic degradability and lignin extraction; (2) enzymatic hydrolysis in fixed conditions following the optimal hot water treatment conditions from the step (1) with a variation of flow rate, pressure, temperature and enzyme activity; (3) optimization of enzymatic degradation of LHW-hydrolysates (pretreatment, enzyme, pH). The products were analyzed for a variety of properties including the amount of sugars, lignin content, pH and density.

From this analysis a set of process parameters for both steps could be identified allowing high enzymatic degradability of the solid residue with high glucose yields, high hemicellulose recovery in the liquid hydrolysate at low generation of further degradation products and the generation of a lignin product in the liquid phase. Pressure showed a significant influence on the yield of the enzymatic hydrolysis of cellulose. The process was further improved by including and testing of different operation modes to purify the product streams (recycling streams, direct evaporation and membrane techniques).