Sorptive recovery of dilute ethanol from distillation column bottoms stream

Abstract

Modern ethanol distillation processes are designed to ensure removal of all ethanol from the column bottoms, i.e., to levels <100 ppm ethanol, and utilize substantial stripping steam to achieve this result. An alternate approach using sorption was attempted as a means to reduce energy requirements in the stripping section, and thereby reduce cost. Adsorbents tested for use in such an application showed that carbonaceous supports, in particular Ambersorb XEN 572, gave alcohol-free water as effluent when a 1% (w/w) starting ethanol concentration was passed downflow at 1 bed vol/h over a fixed-bed adsorber at 70°C. Regeneration was readily achieved at 70–90°C using hot air, vacuum, superheated steam, or hot water to strip the ethanol from the column, and yielded ethanol streams containing a maximum of 5.9% alcohol, with average concentrations of 2.5–3.5% depending on the regeneration method used. These experimentally determined operating conditions combined with distillation energy calculations have enabled development of a process concept for sorptive concentration of dilute ethanol which is more energy efficient than distillation alone. The combination of existing distillation and corn grit drying technologies, with sorptive recovery of dilute ethanol (from the column bottoms) shows promise of recovering a fuel grade, 99.4% ethanol product from a 4.5% ethanol broth with an energy requirement of 23,100 BTU/gal. The potential energy saving of 3600 BTU/gal over distillation alone corresponds to 1.8¢/gal, and provides motivation for further examination of this approach in reducing costs of ethanol production from biomass.