Anhydrous ethanol recovery from wet air in TSA systems - Equilibrium and column studies

An experimental and theoretical study of two combined temperature swing adsorption (TSA) systems for the recovery of anhydrous ethanol from wet air using activated carbon Sorbonorit 4 (ACS4) and zeolite molecular sieve 3A (ZMS3A) are developed. Adsorption isotherms of ethanol and water vapors on ACS4 and ZMS3A in the temperature range of 20-140 °C and water from liquid ethanol on ZMS3A at temperatures of 20-60 °C were measured. The data were analyzed using multi-temperature Toth, Sips, hybrid Langmuir-Sips, inhomogeneous DA and the excess surface work models. The results demonstrate selective water adsorption on ZMS3A with higher adsorption capacity in the liquid (28.02 mol/kg) than in the gas phase (10.67 mol/kg), and higher adsorption of ethanol (7.21 mol/kg) than water vapor on ACS4 up to relative pressure 0.6. The influence of the inlet concentration in the gas (2.8-8.4 g/m3 for ethanol) and liquid phase (45-141 kg/m3 for water-ethanol mixture) on the adsorption dynamic was studied in two fixed-bed columns with ACS4 and ZMS3A. The characteristic parameters of breakthrough curves and maximum temperature rise in the column were determined. The Thomas model was successfully employed to predict the breakthrough curves for both gas and liquid phase. The energy requirement for ethanol electrothermal desorption from ACS4 bed (151-235 kJ/mol) was lower than for hot purge gas regeneration of wet ZMS3A bed (423 kJ/mol). These values were several times larger than the isosteric heat of adsorption (13-71 kJ/mol) which represents minimum energy requirement for desorption.