| Summary: | Ethanol produced from lignocellulose is one of the most promising biofuels. However, the technology to produce lignocellulosic ethanol is still under development and needs to be improved to become economically viable. Enzymatic hydrolysis is one of the most expensive process stages, primarily due to high enzyme costs. Consequently, two cost reduction strategies were studied: optimization of hydrolysis conditions and enzyme recycle by adsorption.
To optimize enzymatic hydrolysis, the changes in concentration of cellulases during the reaction must be determined. Protein concentration changes under hydrolysis conditions for Celluclast 1.5L and Novozyme 188, were studied in the absence of substrate. Novozyme 188 protein concentration decreased by 55 to 64% at 50°C after 92 h. A model describing Novozyme 188 protein concentration changes was developed and used to determine free and adsorbed cellulases concentrations. Glucose and xylose yields (58 to 89% conversion) during enzymatic hydrolysis were modeled as a function of enzyme loading, time, lignin content and solids concentration. The proposed model successfully describes hydrolysis of substrates with different lignin contents, linking pretreatment and hydrolysis. The effect of lignin content, enzyme loading and hydrolysis time on enzyme recovery was evaluated, achieving 0 to 35% cellulases recycled. A mass balance of the enzyme recovery process was built and used to achieve a uniform production of sugar.
Based on experimental data and the proposed models, the production of ethanol with and without enzyme recycling was simulated in AspenPlus. The ethanol production process at different operating conditions was economically evaluated. The economic analysis showed that raw material expenses determine production costs, where biomass, caustic and enzyme expenses are the major contributors to the operating cost. The lowest production costs ($1.86 and $2.13/ kg ethanol) were obtained at low enzyme loadings and mild pretreatment conditions. Sugar losses at severe pretreatment conditions have a significant negative effect on production costs: severe conditions increased production cost by 18 to 23%. Therefore, optimal hydrolysis conditions must be determined considering the entire process. The implementation of the enzyme recycling process decreased production costs up to 14% depending on operating conditions, demonstrating the potential benefits of the enzyme recycling technology.
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