Simulation and Preliminary Design of a Reactive Distillation Column for Removal of Water From Ethanol-Water Mixtures

Anhydrous ethanol, also known as absolute ethanol, is an excellent alternative clean-burning fuel to conventional hydrocarbon-based fuels such as gasoline, etc. Driven by several world energy crises that have taken place since the 1973 oil crisis, demand for anhydrous ethanol continues to be high. Because of the minimum-boiling azeotrope of ethanol (95.6 wt %) and water (4.4 wt %), special processes have to be employed to remove the remaining water from the mixture. Azeotropic distillation and extractive distillation are two widely used processes, in both of which a third component is introduced into the mixture to purify the ethanol. Besides, technologies of pervaporation and adsorption are also commercially used. However, no matter distillation or non-distillation technologies, all of the above processes involve either considerable energy consumptions or huge capital investments.

This work proposes a novel approach to remove water from a near-azeotropic ethanol-water mixture by reaction with ethylene oxide in a reactive distillation (RD) column. RD has been proved to be a very high efficient process with enhanced conversion, improved selectivity, and the overcoming of azeotropic limitations, since it combines the chemical reaction and separation into one unit. The proposed approach exploits the advantages of RD by forcing the water in the mixture to react with ethylene oxide, and simultaneously separating purified ethanol from the reaction products. The feasibility is verified by simulation studies. Sensitivity analyses are performed on key operating parameters, such as the column pressure, water-to-ethylene oxide ratio, distillate rate, reflux ratio, holdups, feed location, etc., – which are essential to the performance of the system. Appropriate variables are selected with the purpose to gain higher conversion of reactants, selectivity of ethylene glycol, yield of ethanol and desired product purity. By implementing selected parameters (i.e. operating pressure is 0.8 MPa; the molar feed ratio of ethylene oxide to water is 35:20; the reflux ratio is 9; the number of stages is 22), a high-grade anhydrous ethanol (99.6 wt %) can be produced continuously, plus a byproduct of the mixture of ethylene glycol, diethylene glycol, etc., which can be further used as a solvent for nitrocellulose, resins, dyes, or other applications. Especially, the heat released by the exothermic reactions is directly utilized by the distillation. Compared to the conventional processes, the energy consumption can be reduced by 30-70% in addition to the savings in the capital investment and added values of the worthwhile byproduct. In conclusion, the proposed approach shows its promising applications for removing water from a near-azeotropic ethanol-water mixture with great potentials for reduced energy and capital cost over traditional approaches.