Process Assessment Of Distillation Using Intermediate Entrainer: Conventional Sequences To The Corresponding Dividing-Wall Columns

In extractive distillation,an intermediate entrainer that interacts differently with the azeotropic components can be introduced.In this work,two conventional extractive distillation sequences and two types of dividing wall column(DWC)were designed and optimized for the separation of methanol-toluene system with triethylamine as the entrainer.First,the conventional extractive distillation separation sequences which include direct sequence(DS)and indirect sequence(IS)were investigated.Compared with IS,DS is more energy saving and has a smaller TAC yet has an obvious remixing effect.IS enjoys a lower remixing effect but twice vaporizations of methanol happens during the process.In order to minish the remixing effect,two types of DWC,one with the wall in the top(DWC-T)and the other with the wall in the bottom(DWC-B)were further explored and investigated.The design of DWC-T which is based on DS was to weaken the remixing effect.However,the result was not that satisfied(-11.30% in energy and-5.93% in TAC).DWC-B which is based on IS leads to a 20.07% energy reduction and a 19.24% TAC saving.The process of DWC-B not only solve the problem of twice vaporizations in IS,but also inherits the advantage of IS in low remixing effect.In terms of DWC,DWC-B which has nearly no remixing effect and much potential in saving energy is more economical and applicable than DWC-T.After steady-state design,a basic control structure was proposed for DWC-B.It is illustrated that the control structure can handle with 10% step changes both in feed flow rate disturbance and composition disturbance as well as enable the adjustment of liquid sidestream split.The purity of the two products returns back quickly to the desired values with tiny offsets in about 2 to 4 hours.