Investigation On The Vapor-Liquid-Liquid Ternary Phase Distillation In Azeotropic Distillation Column

Azeotropic distillation is a kind of special rectification method for the separationof liquid mixtures, widely used in petroleum, chemical, food, pharmaceutical andother industries. Vapor-liquid-liquid three-phase distillation process in the distillationcolumn is a complex mass transfer separation process, often appear in theheterogeneous azeotropic distillation column, the column separation efficiency andcan have a very important influence, many problems need to further research andfurther understandingIn this paper, homogeneous and heterogeneous azeotropicdistillation of ethanol-water-benzene (cyclohexane) and acetic acid-water-n-butylacetate ternary systems was studied using ASPEN PLUS. Mathematical model forternary system was set up, ternary VLL equilibrium phase diagrams of ethanol-water-benzene (cyclohexane) and acetic acid-water-n-butyl acetate was analyzed,meanwhile, distillation zones for homogeneous and heterogeneous distillation wasanalyzed, the residue curve was used to determine the sequences of the azeotropicdistillation towers. Since the system is strongly non-ideal, the vapor-liquidequilibrium model for ethanol system was NRTL, and acetic acid system wasNRTL-HOC model. Gibbs free energy minimization method was adopted for liquidphase stability tests in azeotropic distillation towersand decanters.By fixing the total reflux rate of organic and aqueous flow in ethanol dehydrationsystem, and changing the ratio of the organic phase and aqueous phase, the ratio oforganic flow and aqueous flow was calculated for homogeneous and heterogeneousazeotropic distillation using ASPEN PLUS. Distillation curves, distribution ofentrainer, reflux rate and energy consumption of homogeneous and heterogeneousazeotropic distillation for benzene and cyclohexane, separately, were analyzed andcompared.For acetic acid dehydration system, the feed position of azeotropic distillationtower had great influence on the distillation process, azeotropic distillation status wasstudied by fixing reflux flow and changing the feed position. It is found that bylowering with the feed position, azeotropic distillation changed from heterogeneous tohomogeneous. Comparison was made on the distillation curve, distribution of liquidphase concentration, temperature and reboiler energy consumption for heterogeneousand homogeneous azeotropic distillation with minimum reflux rate. By comparing different circumstances in azeotropic distillation towers forethanol dehydration process using benzene (cyclohexane) as the entrainer and aceticacid dehydration process using n-butyl acetate as the entrainer, the results showed thatthe influence of homogeneous and heterogeneous situations to azeotropic distillationwhen benzene was the entrainer for ethanol dehydration process and n-butyl acetatewas the entrainer for acetic acid dehydration process. Considering the vapor-liquidmass transfer efficiency and energy consumption, the azeotropic distillation columnshould adopt homogeneous azeotropic distillation operating conditions to avoidliquid-liquid phase split conditions; when cyclohexane was the entrainer for ethanoldehydration process, heterogeneous azeotropic distillation could be selected as theoperating condition, liquid-liquid phase split could happen in order to reduce energyconsumption. So for different systems, whether we should avoid liquid-liquid phasesplit in the tower, should be based on the characteristics of the system for a rationalchoice.