Experimental and theoretical analysis of ultrasonic separation in bioethanol purification

The application of ultrasonic atomization on aqueous ethanol dewatering/dehydration was investigated using a single transducer device. This study predicted the ultrasonic properties in the aqueous ethanol solutions according to temperature and bulk solution composition. The predicted ultrasonic properties include speed, near-field length, last minimum length, impedance, reflection, transmission, and attenuation. The effects of solution thickness, solution temperature, and carrier gas flow rate on the ethanol ultrasonic separation were studied. Both ultrasonic separation efficiency and collection rates increased with solution temperatures. The collection rates increased with carrier gas flow rates while the ultrasonic separation efficiency decreased. Solution thickness below the last minimum length or above the near-field length can result in decreased ultrasonic separation efficiency. Additionally, based on the surface enrichment with the atomization droplet sizes, a thermodynamic model was developed to predict the mist composition. This model is more accurate than previous models because of considering the effects of temperatures and composition on the droplet size and using a more accurate model, modified Myers-Scott model, to predict the surface tension precisely. The role of carrier gas flow rate on the selectivity of droplet/bubble was analyzed through force analysis. The force analysis indicated the coexistence of pure ethanol nanodroplets and solution microdroplets during the ultrasonic separation. An instability characteristic time was applied to evaluate the atomization threshold related to wavelength, density, surface tension, and viscosity. The optimal operations prefer high temperature, low carrier gas flow rate, maximum thickness between near field length and last minimum length, and atmospheric pressure. Since ultrasonic separation uses electricity, replacing the current bioethanol refining process can result in the potential CO 2 reduction up to 2.3 kg CO 2 equivalent/kg ethanol if renewable electricity is used. The future replacement of bioethanol refining in the United States can cut over 100 million metric tons of CO 2 yearly. • Determine the primary sound properties in ethanol-water solution. • Develop an accurate thermodynamic model for separation based on surface enrichment. • Ultrasonic separation can potentially reduce 2.3 equivalent kg CO 2 /kg Ethanol. • Carrier gas affects the separation by choosing the different sizes of bubbles. • Instability time characterizes the effect of temperature on collected rates.