Exploring carbon dioxide + propane as nonflammable solvent mixtures for extraction and dehydration of ethanol

ESTEBAN A. BRIGNOLE; FRANCISCO A. SÁNCHEZ

Campinas

Conferencia; X Iberoamerican Conference on Phase Equilibria and Fluid Properties for Process Design (EQUIFASE 2022); 2022

Universidad Estatal de Campinas (UNICAMP)

Conventionalliquid extraction is not a suitable separation method for therecovery and dehydration of ethanol and other light alcohols fromaqueous solutions. Condensed non-polar gases like propane and CO2looked as promising solvents because of their high selectivity andlow solvent losses. The present work develops a process scheme and astrategy to select favorable and nonflammable conditions forextraction and dehydration of ethanol using solvent mixtures of CO2and propane that exhibit (1) good alcohol distribution coefficient inthe high-pressure extractor and (2) the water entrainment effect inthe solvent recovery column.. LiquidCO2 was one of the first solvent choices because has good selectivityand low heat of vaporization. In addition, there is no azeotropeformation for the binary ethanol + CO2. However, the ethanolconcentration obtained in pilot plant studies, in the bottom of thesolvent recovery column was always below the ethanol-water azeotropiccomposition, because CO2 lacks the water entrainment effect. Anotherproblem with CO2 was the low critical temperature, that hinders theuse of higher temperatures in the high-pressure extractor. Highertemperatures increase the alcohol distribution coefficient, becausein the aqueous phase, the activity coefficient of the alcoholincreases with temperature and the opposite effect takes place in thenon-polar condensed phase. The combination of both effects results ina significant increase of the distribution coefficient withtemperature, as long as the solvent density remains liquid-like.Light hydrocarbons like propane and butanes with critical temperaturein the range of 350–450 K allows the extraction at highertemperaturesInthe case of propane, the water/propane relative volatility is alwaysgreater than one and this is the basis of the water entrainmenteffect of propane and other light hydrocarbons like the butanes andpentanes. The afore-mentioned properties are the basis of the hotpropane process for ethanol extraction and dehydration.Inthe present work the phase scenario of the carbon dioxide + propanesolvent mixtures are studied using the General Phase EquilibriumCalculation (GPEC) software for automatic generation of the phasediagrams of binary mixtures. With this program the phase scenario ofCO2 – propane mixtures are investigated to generate densities ofvapor and liquid at saturation of both components and at the criticalline of the binary mixtures. From this information the isopleths ofnon-flammable mixtures are investigated at isochoric conditions fromtheir critical points in search of favorable conditions of pressureand temperature of operation of the high pressure extractor. A heatintegrated process scheme is presented where the conditions of highsolvent power in the extractor and alcohol dehydration on the solventrecovery column are obtained.