New vapour-liquid equilibrium data on the ternary system carbon dioxide – methanol – dimethyl sulphoxide
Introduction
Although supercritical carbon dioxide has already been applied on an industrial level, the possibilities of finding new applications are still intensively researched [1]. Its popularity originates from its variable physico-chemical properties that are mostly temperature and pressure dependent but also can be altered by adding an organic co-solvent in low quantity. Various applications make use of these tuneable properties like supercritical fluid extraction [2,3], several precipitation methods [[4], [5], [6]] or for example supercritical fluid chromatography. A common property of these is that they necessitate knowledge about the phase equilibrium behaviour of the mixtures that are present. For example, antisolvent fractionation processes involve an extractive washing step during which, in most of the equipments, a two phase system containing a solid and a fluid phase has to be achieved. Otherwise the supercritical carbon dioxide stream used as the extraction solvent does not sufficiently mix with the more polar phase in the precipitation vessel. Then, the organic solvent and other dissolved components are not completely extracted. Fractionation remains unsuccessful and a wet product is obtained [[7], [8], [9]].
Several methods are known to experimentally investigate phase equilibria, depending on what phase transition is expected. The exhaustive review of Fonseca et al. divides methods to two basic groups: those based on the investigation of the phase behaviour of a mixture having an accurately known composition (synthetic methods) and those based on analysing the equilibrium phases (analytical methods) [10].
Methods can also be classified by being static or dynamic. Dynamic methods involve the mixing of flowing components [[11], [12], [13]]. Equilibration is carried out then effluents from both phases are collected and analysed. However, it is necessary to ensure that the exiting phases are really in equilibrium with each other. Static methods make use of an equilibrium cell in which thermodynamic equilibrium is reached without a flowing mixture. The vessel is usually stirred, shaken or capable of circulating the mixture. Equilibrium properties can be measured by taking samples [14] – then the problem of changing pressure has to be solved but the overall concentration of the mixture does not have to be exactly known – or by visually observing phase separation or single phase formation [15]. In this latter case, the quantity of the components has to be known exactly. In the present study, a static synthetic method is used to gather measurement data on the behaviour of the system carbon dioxide – methanol – dimethyl sulphoxide. Binary systems of the components have already been investigated [[16], [17], [18], [19], [20], [21], [22], [23]], but the description of the equilibrium behaviour of this ternary mixture has not been found in the literature. A similar ternary system has already been described in the literature and the Peng-Robinson equation of state has been shown to be capable of predicting its behaviour [24]. The aim of our study was to collect and model measurement data in a range that could directly be applied in antisolvent precipitation.
Organic solvents and their mixtures increase the polarity of the non-polar supercritical fluid, being useful as solvents of increased extraction efficiency for the regeneration of heterogeneous catalysts [25] or as tuneable mobile phases [26]. Optimised dissolving power (solubility parameter) is also critical in case of any antisolvent process, where DMSO and its mixed solvents are favoured [[27], [28], [29], [30], [31], [32], [33]]. Mixtures of organic solvents, including DMSO and methanol are frequently used in mixture with carbon dioxide; in any of these applications phase equilibrium is a key knowledge.
Section snippets
Materials
Carbon dioxide was supplied by Linde Gas Hungary Ltd. Its purity is appropriate for food industry applications. The carbon dioxide content of the cylinders is over 99.5%. Our laboratory equipment makes it possible to use carbon dioxide freshly distilled (by drawing from the gas phase of a cylinder then condensing the gas in a piston pump). This probably results in a slightly higher purity, although no further measurement was carried out regarding the purity of the gas. Methanol was ordered from
Results and discussion
Raw measurement data is given in the Supplement, while a shorter sample is shown in Table 2 and Table 3. These datasets are divided based on the methanol – DMSO molar ratio. Temperature, cloud point pressure and redissolution pressure values are averages measured at one composition and temperature setting. Each row represents at least three repetitions. Table 2 gives the averages of the cloud point data gathered. stands for the mole fraction of the components marked in indices. stands for
Conclusion
The phase behaviour of the carbon dioxide – methanol – dimethyl sulphoxide system was investigated between 308.15 K and 323.15 K, in the mole fraction ranges of ; ; and . Measurements were done in a high-pressure view-cell by observing cloud points and redissolution points. Linear regression could be performed on the obtained data. No hysteresis was observed between the phase separation and single phase formation. Also, prediction using the
Acknowledgement
Supported by the ÚNKP-18-3 New National Excellence Program of the Ministry of Human Capacities under the personal reference numbers ÚNKP-18-3-IV-BME-260 and ÚNKP-18-3-I-BME-267.
We would like to thank Fanni Vitkóczi for her valuable help during the editing of the manuscript.
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