Optimal Economic Design of an Extractive Distillation Process for Bioethanol Dehydration



The large-scale production of bioethanol fuel requires energy-demanding distillation steps to concentrate the diluted product streams from the fermentation step and to overcome the azeotropic behavior of the ethanol-water mixture. The conventional separation sequence consists of three distillation columns performing several tasks with high energy penalties: preconcentration of ethanol (PDC), extractive distillation (EDC) and solvent recovery (SRC). It is remarkable that almost all papers on this topic focus on the azeotropic separation only, neglecting the preconcentration step. The energy usage in the PDC increases as the distillate composition gets closer to the azeotrope, and the energy requirements in the EDC and SRC units decrease as the input stream to the EDC becomes richer in ethanol and vice versa. In this study we optimize the economics and energy consumption of a PDC as a function of the distillate composition, a fundamental issue that was not investigated before. Simulations were conducted using Aspen Plus to investigate how this parameter affects the energy usage and investment costs of the complete system. This procedure applies as well to other processes that contain a preconcentration column (e.g., extractive and azeotropic distillation). The optimal economic conditions are reached at a distillate concentration of 91 wt % (80 mol %) ethanol, for which the specific energy consumption is 2.11 kWh (7596 kJ) per kg ethanol.