Phase behavior of (CO2 + methanol + lauric acid) system

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Abstract

In this study the phase equilibrium behaviors of the binary system (CO2 + lauric acid) and the ternary system (CO2 + methanol + lauric acid) were determined. The static synthetic method, using a variable-volume view cell, was employed to obtain the experimental data in the temperature range of (293 to 343) K and pressures up to 24 MPa. The mole fractions of carbon dioxide were varied according to the systems as follows: (0.7524 to 0.9955) for the binary system (CO2 + lauric acid); (0.4616 to 0.9895) for the ternary system (CO2 + methanol + lauric acid) with a methanol to lauric acid molar ratio of (2:1); and (0.3414 to 0.9182) for the system (CO2 + methanol + lauric acid) with a methanol to lauric acid molar ratio of (6:1). For these systems (vapor + liquid), (liquid + liquid), (vapor + liquid + liquid), and (solid + fluid) transitions were observed. The phase equilibrium data obtained for the systems were modeled using the Peng–Robinson equation of state with the classical van der Waals mixing rule with a satisfactory correlation between experimental and calculated values.

Highlights

► We measured SVL, LLE and VLE for the binary system {lauric acid + methanol + CO2}. ► Bubble point and dew point were measured at high pressures. ► The experimental data were modeled using the Peng–Robinson equation of state with the classical van der Waals mixing rule.

Introduction

In last decade, there has been a great interest in finding alternative routes for production and purification of biodiesel. The use of heterogeneous catalysts for biodiesel production gained particular attention due the advantages related to costs reduction and simplification of products purification. Recent investigations have shown that esterification and transesterification reactions can be carried out with relative success using layered (lamellar) carboxylates catalysts, such as zinc laurate in the presence of methanol. In particular promising results for esterification of lauric acid to biodiesel production were reported [1], [2]. However, relative low yields were obtained even at high molar ratio methanol to oil. A common explanation given in the literature is that the catalyst is not effective for triacylglycerols systems. These reactions are commonly carried out at high temperature and high molar ratio of methanol to oil and pressures closes to the methanol vapor pressure [1]. From kinetics point of view, coexisting two phase systems, can lead to mass transfer limitation of reactants and a subsequent termination of the reaction. To perform the mass transfer in the reaction media, an alternative way is the use of supercritical dioxide carbon as diluents or solvent. There are a great variety of potential applications of supercritical carbon dioxide in the industry processing of fat oils and derivatives, and this subject has been extensively studied over the last three decades [3], [4], [5], [6]. On the other hand, supercritical fluid extraction is an interesting separation process, because the removal of the solvent is accomplished without exposing the triacylglycerols and fatty acids to high temperatures, avoiding the thermal degradation of these compounds [6], [7]. In this case, the use of supercritical CO2 is attractive because it is inexpensive, nontoxic, nonflammable, inert, naturally abundant and has a relatively low critical temperature (304.1 K) [8], [9], [10], [11].

Several studies report the phase behavior of systems involving triacylglycerols, fatty acids and carbon dioxide [11], [12], [13], [14], [15], [16], [17], [18], [19]. These studies showed that triacylglycerols and fatty acids are poorly soluble in CO2, due to its non-polarity. However, the addition of third component commonly known as cossolvent can improve this solubility [20], [21].

Knowledge of the phase behavior is of great importance for the modeling, design and optimization of separation processes controlled by equilibrium [9]. However, a lack of experimental data and reliable thermodynamic models which can aid the technical-economic evaluation of processes has limited the industrial use of supercritical fluids [10].

Some studies reporting the phase behavior of such systems in the presence of cosolvents are available in the literature [21], [22], [23], [24], [25], [26], [27], [28]. However, none of these studies reported phase equilibrium data for systems with methanol as the co-solvent. The use of methanol instead ethanol is mainly due to the fact that in the methanol process, reaction are faster and purification is easier, especially when water is used, so that most of the actual biodiesel plant are designed for methanol use. Understanding phase behavior of these systems in the presence of methanol is the first and fundamental step for development and optimization of esterification process using the lamellar catalysts.

In this context, the aim of this work was to investigate the phase behavior at high pressures of binary and ternary systems involving CO2, methanol and lauric acid, by means of the obtainment of experimental data and thermodynamic modeling.

The phase equilibrium of (lauric acid + carbon dioxide) has been previously studied by Bamberger et al. [11], Maheshwari et al. [13], Yau et al. [14] and Bharath et al. [16]. However, no experimental data have been reported on the phase equilibrium of the system (lauric acid + carbon dioxide + methanol). Therefore, the present study aims to fill this gap.

Section snippets

Materials

Lauric acid (purity of 99.2 wt%) was purchased from Vetec Química Fina (Duque de Caxias/RJ/Brazil), methanol (99.8%) was supplied by Qeel (São Paulo/SP/Brazil) and carbon dioxide (99.9 wt% in the liquid phase) was purchased from White Martins S.A (Osasco/SP/Brazil). All chemicals were used without further purification.

Phase equilibrium apparatus and experimental procedure

Phase equilibrium experiments were conducted by employing the static synthetic method in a high-pressure variable-volume view cell. The experimental apparatus and procedure have

Modeling

In order to model the phase equilibrium for the systems studied the isofugacity criterion was used [30], [33];fˆkV-fˆkL=0(k=1,,nc),fˆncS-fˆncF=0,where “nc” is the number of components in the system, and in this study nc = 3 (the last component in the system is the solid component), the superscript “S” is the solid phase and “V” and “L” are the vapor and liquid phases, respectively. The fugacity of all components in the mixture of fluid phases (V and L) is calculated using equation (3);fˆkF=xiϕˆiF

Conclusions

In this study, the static synthetic method was used to obtain experimental data on the phase equilibrium of the binary system (CO2 + lauric acid) and for the ternary system (CO2 + methanol + lauric acid) at high pressure. The experimental measurements were conducted in a temperature range of (303 to 343) K, covering a wide range of mixtures, and the transition pressures measured were between (5.7 and 23.8) MPa. These systems showed a complex phase behavior with the presence of different types of

Acknowledgements

The authors thank CNPq, CAPES and PRH24/ANP for the financial support and scholarships.

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