Energy Optimization of Bioethanol Production via Hydrolysis of Switchgrass

In this work, we propose the optimal flowsheet for the production of bioethanol from switchgrass, via hydrolysis. A superstructure embedding a number of alternatives is proposed. Two technologies are considered for switchgrass pretreatment, dilute acid and ammonia fibre explosion (AFEX) so that the structure of the grass is broken down. Next, enzymatic hydrolysis follows any of the pretreaments to obtain fermentable sugars, mainly xylose and glucose. Ethanol is obtained by fermentation of the sugars. In order to obtain fuel quality ethanol, water must be removed from the water-ethanol mixture. A number of dehydration technologies is considered including rectification, adsorption in corn grits, molecular sieves and pervaporation. The problem is formulated as an MINLP. The superstructure is optimized by decomposing the MINLP for each of the pretreaments. Then, multieffect columns and heat integration are used to reduce the energy consumption and cooling needs. Finally, an economic evaluation is performed. The optimal flowsheet consists of using dilute acid hydrolysis followed by molecular sieves as dehydration technology, which requires less energy and cooling and yielding a promising production price of 0.8 $/gal.