Simulation and Cost Analysis of Distillation and Purification Step in Production of Anhydrous Ethanol from Sweet Sorghum

Growers often evaluate whether it is economical to double crop their land to increase profit. The aim of the present work is to determine if sweet sorghum (an energy crop) is a reasonable secondary crop to grow in Yuma, Arizona on land that is used to grow lettuce in November-February. Fermentation yields for several varieties of sweet sorghum (Dale, T-Sugar, M81E and 350FS) are compared. Because the sweet sorghum fermentation broth has a high water content, the economic analysis focuses on simulating and optimizing the distillation and purification processes involved in the production of anhydrous ethanol. Two different processes are analyzed: extractive distillation using an entrainer and pressure swing adsorption (PSA) with 3 Å molecular sieves. Design and operation parameters of the two models are assessed. Aspen Plus version 11.1 is used as the simulation software and the NRTL model is used to compute the thermodynamic properties. A flow sheet is proposed containing optimized facility sizing, operating conditions and total annualized cost. At the final stage of the modeling process, highly purified 99.8 wt % ethanol is generated. Applying an extractive distillation method results in a cost that is, on average, 12% less than that of PSA using molecular sieves.