Vapour-liquid equilibrium measurements and extractive distillation process design for separation of azeotropic mixture (dimethyl carbonate + ethanol)

Highlights

• The entrainers p-xylene, butyl propionate and isobutyl acetate were selected.

• The VLE data for (DMC + entrainers) and (ethanol + isobutyl acetate) were measured.

• The measured VLE data were correlated by the NRTL, UNIQUAC and Wilson models.

• The extractive distillation process for separating DMC and ethanol was proposed.

Abstract

Dimethyl carbonate and ethanol can form an azeotrope with the minimum boiling point. To separate the azeotrope (dimethyl carbonate + ethanol) by extractive distillation, p-xylene, butyl propionate, and isobutyl acetate were chosen as entrainers, and the isobaric vapour-liquid equilibrium (VLE) data for the binary systems of (dimethyl carbonate + p-xylene), (dimethyl carbonate + butyl propionate), (dimethyl carbonate + isobutyl acetate) and (ethanol + isobutyl acetate) were measured at 101.3 kPa using a modified Rose type recirculating still. The thermodynamic consistency of the VLE experimental data for the four binary mixtures was tested by the Herington and van Ness methods. Furthermore, the measured VLE data were correlated by the NRTL, UNIQUAC and Wilson thermodynamic models and the binary interaction parameters of the three models were regressed. Based above, an extractive distillation process with the entrainer of p-xylene was proposed and simulated to separate the azeotrope (dimethyl carbonate + ethanol).

Introduction

Dimethyl carbonate (DMC) is an environmentally benign chemical product [1], which is widely used in production of pesticides, medicines [2] and polymer synthesis, and can be used as fuel additives [3] and solvents [4]. Meanwhile, DMC is also an important raw material for preparation of a variety of high value-added special fine chemicals [5]. For synthesis of diethyl carbonate or methyl ethyl carbonate by transesterification, dimethyl carbonate and ethanol are used as raw materials [6]. However, since the reaction is reversible, the reactive distillation technology is required to improve the reaction yield. Therefore, the efficient separation of DMC and ethanol is involved in the process. DMC and ethanol can form the lowest azeotrope, which is difficult to separate by conventional distillation. Thus, special distillations are considered to separate such azeotropic mixture, such as extractive distillation [7], reactive distillation [8], azeotropic distillation [9]. In this work, the extractive distillation was applied for separation of the azeotropic mixture of DMC and ethanol.

For separation of the azeotrope of DMC and ethanol by extractive distillation, a suitable solvent is needed to enhance the relative volatility [10] of DMC to ethanol. According to the standard for the selection of entrainers proposed by Gmehling and Möllmann [11], p-xylene, isobutyl acetate and butyl propionate were considered as the entrainer candidates for separation of the azeotropic mixture (DMC + ethanol). In the literatures, few researchers have reported the VLE data for the system of (DMC + ethanol). Oh et al. [12] measured the isothermal VLE data for (DMC + ethanol) at 333.15 K by headspace gas chromatography (HSGC). Fukano et al. [13] reported the boiling point data of three binary systems (methanol + ethanol + DMC) under different pressure. Zhao and Yang [14] determined the VLE data for (DMC + ethanol) at 100 kPa using a double cycle vapour-liquid equilibrium still. And the vapour-liquid equilibrium data for the binary system (DMC + ethanol) at 101.3 kPa were reported by Luo et. al. [15]. However, the isobaric VLE data for the binary systems of DMC with p-xylene, isobutyl acetate, butyl propionate and ethanol with isobutyl acetate have not been found in the NIST [16] and Dortmund Data Bank (DDB) [17].

The main contents of this work are as follows: (1) the isobaric VLE data for four binary systems (DMC + p-xylene), (DMC + butyl propionate), (DMC + isobutyl acetate) and (ethanol + isobutyl acetate) under pressure of 101.3 kPa were determined using a modified Rose type recirculating still; (2) two thermodynamic consistency tests of Herington [18] and van Ness [19] were adopted to check the consistency of the measured VLE data; (3) the VLE data were correlated by the NRTL [20], UNIQUAC [21] and Wilson [22] activity coefficient models and the binary interaction parameters of the three models were regressed; (4) the excess Gibbs energy of the four binary systems were calculated based on the measured VLE data and (5) an extractive distillation process for separating the azeotrope (DMC + ethanol) was developed.