Experimental determination of the critical locus of binary systems containing CO2 and an ethyl ester

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Introduction
Progressive depletion of world oil resources, combined to increasing energy consumption as well as the negative environmental impact of fossil fuel use, led to a shift toward alternative renewable sources of energy. Biodiesel is among the most viable liquid transportation fuels for the foreseeable future, with the potential of contributing significantly to sustainable development in terms of socioeconomic and environmental concerns. Indeed, biodiesel is a mixture of alkyl esters obtained by transesterification (alcoholysis) of triglycerides from vegetable oils or animal fats with an alcohol (methanol or ethanol). The transesterification reaction is commonly carried out in the presence of a catalyst the main drawback of which is to be water sensitive, preventing the use of non-conventional feedstocks (such as waste cooking oils or algal biomass) [1]. Emerging noncatalytic alcoholysis methods based on supercritical fluids allow solving this problem. Nevertheless, their industrial scale application is limited because of the severe operating conditions required (high temperature, high pressure, and high alcohol to oil molar ratio) which can be successfully reduced by addition of a co-solvent, such as CO 2 [2][3][4]. To optimize the supercritical process via simulation, the phase behaviour under high temperatures and pressures for systems containing CO 2 and components involved in biodiesel production must be firstly investigated.
In this work, the critical properties of nine binary mixtures containing CO 2 and an ethyl ester are investigated. A synthetic-dynamic apparatus capable of measuring the critical points of pure compounds and mixtures is employed using the dynamic method. The critical points can be determined by visually observing the critical opalescence phenomenon in the view cell. Regarding the system CO 2 + ethyl acetate, the experimental results in this work are compared to those available in the open literature. Such a comparison is not possible for the eight other systems which are measured for the first time in this study.

EXPERIMENTAL
All compounds were purchased from commercial sources and used without any future purification. Their purity and the suppliers are listed in Table 1.

MATEC Web of Conferences
In this work, the measurements were carried out using the dynamic method in which circulation of fluid ensures an efficient stirring.

Figure 2
The transition of a fluid from the subcritical state to the critical state on temperature increase. From left to right: classical liquid-vapour mixture (the two phases are perfectly separated by a thin meniscus); cloudy subcritical state (the meniscus is not thin any more, it is coloured in orange which characterizes the critical opalescence); a thick and cloudy subcritical state (the meniscus becomes thicker and thicker); disappearance of the liquid-vapour interface (the meniscus occupies the entire view cell which is orange: we are at the critical point). in the open literature. The experimental setup used in this work is the same as that described in one of our previous studies [7]. The uncertainties obtained in the critical temperature and critical pressure for pure compounds and mixtures at different mole fractions were ≤ 0.14 K and ≤ 0.0135 MPa, respectively. Likewise, a mean value uncertainty in the mole fraction, u(x i ) = ±0.00006 was obtained.

RESULTS
In this work, the vapour-liquid critical points for nine binary systems containing CO 2   As can be seen in Fig. 3a, the critical points of the first six systems (i.e. from the system CO 2 + ethyl acetate to the system CO 2 + ethyl heptanoate) could be determined throughout the entire range of composition. Such systems exhibit a continuous vapour-liquid critical locus. For the three other systems, the temperature and pressure upper limits of the apparatus prevented some critical points from being measured. Furthermore, six overall compositions (0.92 ≤ x 1 ≤ 1.00) of the CO 2 (1) + ethyl nonanoate (2) and CO 2 (1) + decanoate (2) systems were prepared and introduced in the view cell but no critical state could be found. As a consequence, these two systems are suspected to exhibit a discontinuous vapourliquid critical locus. Additionally, the experimentally determined critical points of the CO 2 (1) + ethyl acetate (2) can be compared to literature data [9] whereas the critical loci of the eight other binary systems are measured for the first time in this study. Fig 3b and 3c illustrate that, at a same composition, the critical pressures and temperatures measured in this study are a bit higher than those from the collection of Byun et al. These authors used the Peng-Robinson equation of state to correlate the critical locus of the CO 2 (1) + ethyl acetate (2) system. Although they concluded to an accurate correlation, it is noticeable that the calculated values are higher than their experimental data which were measured using a statictype experimental apparatus. Such an apparatus enables to accurately determine bubble and dew points but is often said not to be accurate enough for critical point measurements due to an iterative process.

Conclusions
In this work, the critical properties of nine binary systems containing CO 2 and an ethyl ester were studied. The measurements were carried out using a syntheticdynamic apparatus which was employed in the dynamic mode. For six binary mixtures, the critical loci were experimentally determined over the whole composition range. For the three other systems, the critical loci were only determined in a smaller composition range due to the cell's capacity. The experimental results were compared to literature data. Some variations were found owning to different experimental methods used.