Solid-liquid separation of hydrolysates obtained from enzymatic hydrolysis of recycled fibre

Production of bioethanol from lignocellulosic raw materials is likely to be increased in the near future because of the increasing demand on transportation fuels and the goverments' attempts to reduce the production of greenhouse gases. Enzymatic hydrolysis provides an environmentally friendly pathway from cellulosic biomasses to sugars that can be fermented to ethanol. The hydrolysis step has a marked influence on the overall efficiency of the process. As the cellulosic substrate can not be completely cleaved to sugars, there always remains a solid residue after the hydrolysis. Solid-liquid separation performed at this stage can improve the membrane filtration e.g. for recycling the enzymes, or concentrating the sugars prior to ethanol fermentation. The fermentation process can also be more readily controlled, if the cellulosic residue is removed after the hydrolysis. The objective of the study was to separate non-degraded solids from enzymatically produced hydrolysates by using a laboratory-scale pressure filter. Recycled fiber, mainly shredded cardboard, was chosen as the cellulosic raw material. The influence of process-related variables, such as enzyme dosage, pre-milling, and the initial substrate concentration were investigated by determining the concentrations of glucose and xylose during the 72 hours of hydrolysis. Filtration tests were carried out at three different pressures immediately after each batch of hydrolysis. The results show that some of these variables have unexpected effects on the filtration characteristics. Conversion of the cellulosic substrate to glucose and xylose was also affected by the investigated process variables. There are, however, still several technical and economical difficulties to overcome.