Liquid–liquid equlibria of the system dimethyl carbonate + methanol + water at different temperatures
Introduction
In the past decade, the number of papers and patents related to dimethyl carbonate (DMC) have increased considerably, as it is an environmentally benign and biodegradable chemical with the LD50 value close to ethyl/butyl acetates [1], and it is able to substitute phosgene as a carbonylation intermediate for manufacturing polycarbonate and isocyanate [2], [3]. The most intriguing market opportunities of DMC are as gasoline additive [1], and painting solvent [4]. Recently, methyl tert-buthyl ether (MTBE), a widely used gasoline additive for octane and oxygen enhancement, was found polluting the groundwater and being not biodegradable [5], as a result, the research activity related to DMC has increased greatly.
DMC can be prepared, for example, by reacting carbon monoxide, methanol and an acid by using copper chloride; by trans-esterifying a cyclic carbonate, such as ethylene/propylene carbonate, with methanol in the presence of a catalyst, and by vapour phase reaction of carbon monoxide and nitrite in the presence of a catalyst [1].
However, in any of these methods, dimethyl carbonate is obtained as a mixture of DMC, methanol and water so the separation of these components is indispensable for purifying dimethyl carbonate [1]. Dimethyl carbonate and methanol form an azeotropic mixture at a composition ratio of 30:70 (weight ratio), and thus it is difficult to separate the mixture by distillation at atmospheric pressure.
It has been reported that DMC and water form another azeotropic mixture [1], but experimental data for this system are not been reported in literature till present. Finally, to the best of our knowledge, also no experimental data for the ternary system (DMC + methanol + water) are available in literature.
Many investigations have been carried out on the method to separate DMC from the mixture of both DMC and methanol, and various proposals have been made, including a method to obtain a crystalline product enriched in dimethyl carbonate by cooling [6], a method of separation of the mixture by distillation by breaking the azeotrope with pressurization [7], a method of separation by distillation by adding a hydrocarbon [8], a method of using extraction and distillation with water [9] or organic solvents [10], [11], and a method of pervaporation using crosslinked chitosan membranes [12], [13].
Unfortunately, the study of the viability of some of these techniques is limited by the lack of data on the thermodynamic behavior of systems containing DMC, water and methanol. Examination of the literature shows that the binary system DMC + methanol has been widely studied. ENIChem has a German patent showing that the percentage methanol in the binary DMC + methanol azeotrope increases with pressure [14]. The thermodynamic properties of the binary methanol + dimethyl carbonate under atmosphere pressure have been reported, as well as the variation of the azeotrope temperature with pressure [7], [15]. Yunhai et al. measured the isothermal VLE of DMC + methanol at elevated pressure [16]. Other authors measured VLE of this binary system at atmospheric pressure [17], [18], [19], [20].
This work tries to solve the lack of information in the literature on the ternary system DMC + methanol + water. The LLE for this system have been measured from 283 to 333 K. On the other hand, LLE and LVE for the binary system dimethyl carbonate + water have been measured. The equilibrium data presented are correlated using the NRTL and UNIQUAC equations. Finally, the reliability of these models is tested by comparison with experimental results.
Section snippets
Chemicals
Water from NANO-pure (Wasserlab) was used. Methanol (Aldrich) and dimethyl carbonate (Aldrich) had the normal purities of >99.9 and >99 mass%, respectively. Prior to the measurements, chemical purities were checked by gas chromatography. The purities, densities and refractive indices of all chemicals used in this study are presented in Table 1.
Equilibrium measurements
LLE measurements were made at six temperatures for the ternary system. Equilibrium data were obtained by preparing mixtures of known overall composition
Experimental data
The liquid–liquid equilibrium (LLE) data of the ternary system DMC (1) + methanol (2) + water (3) at 283.15, 293.15, 303.15, 313.15, 323.15 and 333.15 K and atmospheric pressure together with the selectivity are presented in Table 2. All concentrations are expressed in mole fractions. As an example, in Fig. 1, Fig. 2 have been plotted the experimental data at 283.15 and 333.15 K respectively, together with the tie-lines and binodal curve calculated using UNIQUAC model. The plots for the other
Conclusions
Liquid-liquid equilibrium of the DMC + methanol + water system has been measured at different temperatures. The LLE data were correlated using the NRTL and UNIQUAC activity coefficient models. The correlation with the UNIQUAC equation gives better results than the NRTL equation and fits the experimental data satisfactorily. The simultaneous correlation of the six isothermal data sets gives a unique set of parameters in the range of the temperature considered. Finally, the VLE for the system DMC +
Acknowledgments
Financial support from the Ministerio de Ciencia y Tecnología of Spain, through project CTQ2004-04477/PPQ, FEDER European Program and the Conselleria de Cultura, Educació i Esport (Generalitat Valenciana) of Valencia (Spain) are gratefully acknowledged.
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