Optimal simultaneous production of i-butene and ethanol from switchgrass

In this work, we propose the optimization of a flowsheet for the production of i-butene from switchgrass. A superstructure embedding a number of alternatives is proposed. Two technologies are considered for switchgrass pretreatment, dilute acid and ammonia fiber explosion (AFEX) so that the structure of the grass is broken down. Surface response models are used to predict the yield. Next, enzymatic hydrolysis follows any of the pretreatments to obtain fermentable sugars, mainly xylose and glucose. i-Butene is obtained by fermentation of the sugars. Next it is separated mainly from CO2 for which PSA or membrane separation are considered. However, xylose cannot be easily converted, and thus we also evaluate the possibility of using it to produce ethanol. The problem is formulated as an MINLP with simultaneous optimization and heat integration. Finally, an economic evaluation is performed. The most promising process involves the use of dilute acid pretreatment and membrane purification of the i-butene. However, the decision related to the production of i-butene alone or the simultaneous production of i-butene and ethanol depends on the prices for ethanol and for switchgrass.