3-12: Technical and economic evaluation of enzyme recycle by re-adsorption for bioethanol production

Increasing demand for energy has spurred research for alternative energy sources such as lignocellulosic ethanol. One of the highest ethanol production costs is the price of enzymes used for enzymatic hydrolysis. Consequently, many different cost reduction strategies such as enzyme recycle have been attempted. Two recycle strategies have been proposed: recycle by ultrafiltration and recycle by re-adsorption of enzymes onto fresh substrate. An economic analysis of ultrafiltration found it to be expensive due to membrane costs. The first economic studies (Zhang, 2011; Tu, 2009) of recycle by re-adsorption method predicted substantial savings, however experimental data to support and refine these analyses is still needed. The first step in the investigation of the technical feasibility of recycle by re-adsorption, was to assess and model the thermal stability of beta-glucosidase.  A new approach to determine the free enzyme concentrations during enzymatic hydrolysis was developed using pure cellulase and beta-glucosidase as calibration standards. The beta-glucosidase model and a model of glucose production were used to assess the effectiveness of enzyme recycling. From the enzyme recycling results, it was determined that a fraction of the cellulases in the commercial cocktail is responsible for the majority of cellulase cocktail activity. In addition to the high activities, these cellulases demonstrated a high recycling potential, indicating an important future research direction. An improved continuous hydrolysis process was designed using the final experimental mass balance and simulated in Aspen Plus. The novel simulation, which allows from easy adjustment of the hydrolysis residence times, supported economic analysis in Icarus.