Application of Ultrasonic Separation in Bioethanol Purification Process

Bioethanol is a key renewable energy source that is now used as a gasoline additive and viewed as a promising feedstock in synthesizing renewable gasoline/diesel or sustainable aviation fuel. Annual U.S. production exceeds 16 billion gallons, mainly produced from the fermentation of starch or lignocellulose, with ethanol (EtOH) mass fractions in the broth ranging from 1-15% depending on feedstock, enzymes, and operating conditions. Water, as the major broth impurity, must be removed, and the significant energy consumption in purification becomes a major economic hurdle. State-of-the-art purification technology requires 30-60 % of the 29 MJ/kg EtOH combustion heat, depending on the EtOH broth concentration. Here, we describe the development of a novel fully electrified liquid/vapor ultrasound separation based on surface enrichment. The electrically-driven ultrasound transducer generates cavitation bubbles in the liquid, which rise in an ultrasonically-generated fountain, and shatter into fine droplets upon reaching the interface between the bulk liquid and headspace. Compared to distillation, no energy is consumed in effecting a liquid-to-vapor phase change, and second-law losses associated with heat transfer are greatly reduced. This approach also avoids the azeotropic bottleneck of atmospheric pressure distillation and the separate steps necessary to overcome it. Ultrasonic separation is influenced by bulk liquid temperature, carrier gas flow rate and inlet pressure, and transducer voltage. With the partial recycling of large drops (which are less enriched in EtOH than small droplets), the ultrasonic separation was significantly improved. The strategy of single and multi-stage separation investigated the feasibility of replacing the beer column and rectifier used in conventional distillation. Energy analysis indicates that ultrasound separation in bioethanol purification can significantly reduce energy consumption in dewatering. The potential annual energy reduction is about 0.5 quadrillion BTU for the U.S. and 0.7 quadrillion BTU globally. A carbon-balance analysis shows a potential annual carbon reduction of about 200 million tons.