Phase behavior for carbon dioxide + ethanol system: Experimental measurements and modeling with a cubic equation of state
Graphical abstract
Vapor–liquid equilibria (VLE) data for the carbon dioxide + ethanol at 293.15, 303.15, 313.15, 333.15, and 353.15 K up to 11.08 MPa systems are reported. The experimental method used in this work was a static-analytical method with liquid and vapor phase sampling. The new experimental results are discussed and compared with available literature data. Measured VLE data and literature data for carbon dioxide + ethanol system were modeled with a general cubic equations of state (GEOS) using classical van der Waals (two parameters conventional mixing rule—2PCMR) mixing rules. A single set of interaction parameters was used to calculate the global phase behavior in the binary mixture carbon dioxide + ethanol in a wide range of temperatures (283.3–453.15 K).
Introduction
Supercritical fluid processes are merging as important alternative to conventional methods in many of fields, such as extraction, particle micronization, material processing, chromatography or crystallization/purification [1]. Equilibrium and volumetric properties of binary mixtures containing organic solvent and supercritical fluids (especially carbon dioxide) play a fundamental role in determining the success of many of these applications.
High-pressure vapor–liquid equilibrium measurements of carbon dioxide + alcohol systems are of interest due to their importance in the supercritical extraction of thermal labile compounds, dehydration of alcohols using supercritical carbon dioxide, and extraction of natural products using near critical solvents [2].
The low molecular weight alcohols are among the most important compounds in separation processes. They are often used as entrainers to control the polarity of a supercritical fluid (SFC) solvent in extraction applications and are also used as modifiers in SFC chromatography. Carbon dioxide has shown to be the most important supercritical fluid for these processes, because it is cheap, nontoxic, nonflammable, and has a low critical temperatures of 304.25 K.
The carbon dioxide + ethanol mixture is very important industrially and the system has received much attention. The experimental data and the methods used to measure solubilities or phase equilibria were discussed extensively by Fornari et al. [3], Jennings et al. [4], Staby and Mollerup [5], Dohrn and Brunner [6], Christov and Dohrn [2]. A complete list of references of carbon dioxide + alcohols systems is covered in these reviews (from 1855 to 1999). In addition, a manual search of papers in the years uncovered by the mentioned reviews of a number of journals has been carried out. We recorded in our vapor–liquid equilibria database available experimental data, but some works have not been included (doctoral thesis, habilitationsschrift, conference proceedings, papers written in Japanese, Chinese, Russian), which were difficult to obtain. Numerous articles presenting only solubilities of carbon dioxide in ethanol or VLE data at low pressures (1 bar) were also not included. Therefore, in this work we considered many papers investigating the carbon dioxide + ethanol binary system [1], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], but only few report VLE data measured from low pressures (5 bar) to the critical point of mixture. In addition, there are significant deviations among the data from different sources [4], [30].
The goals of this work were to add new experimental data and to represent the global phase behavior of this system with a simple model, using a single set of interaction parameters.
Therefore, in this work we made new measurements using a static-analytical method, in a high-pressure visual cell, for carbon dioxide + ethanol at 293.15, 303.15, 313.15, 333.15, and 353.15 K and pressures between 5.2 and 110.8 bar.
Polishuk et al. [35] have used a single set of parameters for the Peng–Robinson (PR), Trebble–Bishnoi–Salim (TBS) and a four-parameter equation of state, called C4EOS [36]. All these equations of state are particular cases of the general cubic equation of state (GEOS) [37], [38], [39], [40]. Therefore, in this work, the global phase behavior of the system was modeled with the GEOS [37], [38], [39], [40] coupled with classical van der Waals mixing rules (2PCMR). This cubic equation is a generalized form with four parameters for all cubic equations of state with two, three and four parameters [39].
A single set of interaction parameters, representing well the critical pressure maximum (CPM) and avoiding a false upper critical endpoint (UCEP) at high temperatures, was used to model the global phase behavior of the carbon dioxide + ethanol system. The prediction of the critical line and sub-critical phase behavior in this binary mixture was done in a wide range of temperatures. The calculations results were compared to the new data reported in this work, and to all available literature data. The results show a satisfactory agreement between the model and the experimental data.
Section snippets
Materials
Carbon dioxide (mass fraction purity >0.997) was provided by Linde Gaz Romania, and ethanol (mass fraction purity >0.998) was a Sigma product. The chemicals were used without further purification, except for careful degassing of ethanol.
Apparatus and procedure
A detailed description of the experimental apparatus was presented in earlier papers [41], [42]. The apparatus used in this work is based on the static-analytical method with liquid and vapor phase sampling. The procedure is the same as in our previous work [41]
Modeling
The modeling of phase behavior of this system was made with the GEOS equation [37], [38], [39], [40] coupled with classical van der Waals mixing rules (2PCMR). The GEOS [37] equation of state iswith the classical van der Waals mixing rules:with “+” for ci, cj > 0 and “−” for ci, cj < 0. Generally, negative values are common for the c parameter of pure components.
The four parameters a
Results and discussion
The equilibrium compositions for the carbon dioxide + ethanol binary system were measured at 293.15, 303.15, 313.15, 333.15, and 353.15 K and the results are given in Table 1. The values are typically averages of two or three measurements. For the vapor–liquid equilibria (VLE) measurements, the uncertainty of the mole fraction is typically 0.001 and always <0.003. As usually in the literature, we calculated the mole fractions with four decimal places. The critical points for our measured isotherms
Conclusions
New VLE experimental data for the binary system carbon dioxide + ethanol were measured at 293.15, 303.15, 313.15, 333.15, and 353.15 K and pressures between 5.2 and 110.8 bar, with a high-pressure static apparatus. Measured and literature VLE data for carbon dioxide + ethanol system were modeled with a cubic equations of state (GEOS) using classical van der Waals (two parameters conventional mixing rule—2PCMR) mixing rules. One set of interaction parameters was used to predict the critical and
Acknowledgement
The authors are grateful to National Council for Scientific Research of Romania, for financial support.
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