Phase behaviour of high molecular mass methyl esters in supercritical ethane
Graphical abstract
Highlights
► Phase equilibria of long chain methyl ester in supercritical ethane. ► Increase in temperature leads to linear increase in phase transition pressure. ► Increase in molecular mass of methyl ester leads to linear increase in phase transition pressure. ► Peng Robinson and simplified PC-SAFT equation of state not able to model systems accurately.
Introduction
Long chain aliphatic compounds with functional groups have attracted a lot of interest in recent years. These compounds, particularly fatty acids, occur in many natural oil products and are highly sought after. However, processing with traditional methods is difficult due to the low thermal stability and high boiling points of the long chain aliphatic compounds. Supercritical fluid extraction is an attractive alternative method for the fractionation of high molecular mass compounds. However, for high molecular mass saturated acids supercritical fluid extraction is not necessary realistic due to the low solubility of the acids [1], [2], [3], [4] in supercritical solvents. In order to overcome the high phase transition pressures (i.e. low solubility) of high molecular mass fatty acids in supercritical fluids, the acids are often transesterified to methyl or ethyl esters, which usually have a higher solubility (i.e. lower operating pressures) than the corresponding acid [3] and are thermally more stable.
A number of studies have been conducted on the phase behaviour/solubility of long chain saturated acids [5] and their corresponding methyl [6], [7] and ethyl esters [2], [8] in supercritical carbon dioxide. The data shows total solubility of methyl and ethyl esters at moderate pressures while pressures in excess of 25 MPa are usually required for total solubility of the acids. However, the pressures for the esters are still reasonably high. Rovetto et al. [9] and Schwarz et al. [4] studied the phase behaviour of methyl hexadecanoate (C15COOCH3) and high molecular mass acids in high pressure propane. Their results indicate that the solubility of both the methyl esters and acids is considerably higher in propane than in carbon dioxide. However, the higher solubility (i.e. lower phase transition pressure) in propane compared to carbon dioxide is often at the cost of selectivity and the use of higher operating temperatures. Ethane has been suggested as an alternative supercritical solvent to carbon dioxide for supercritical processes. The critical temperature of ethane (305.4 K) is very similar to that of carbon dioxide (304.1 K) [10] and studies comparing supercritical ethane with carbon dioxide for the fractionation of paraffin wax [11] and separation of detergent range (nC10–nC16) alkanes and alcohols [12] have indicated that ethane has similar if not superior selectivity while at the same time allowing for lower operating pressures. Ethane thus shows promise for the processing of products containing high molecular mass esters and acids. However, no phase behaviour studies for any of these compounds in supercritical ethane have been published to date.
The aim of this paper is to investigate some of the aforementioned phase behaviour by considering the phase behaviour of a range of methyl esters in supercritical ethane. The measured phase behaviour, together with a future study on the phase behaviour of long chain acids in ethane, will indicate whether supercritical ethane may be a viable alternative to supercritical carbon dioxide for the processing of long chain esters and acids from natural sources. The phase equilibria of methyl esters (varying from methyl decanoate (C9COOCH3) to methyl docosanoate (C21COOCH3)) in supercritical ethane were measured at temperatures between 312 and 355 K (Tr = 1.02 to Tr = 1.16) for methyl ester mass fractions between 0.65 and 0.018.
In addition, two methods used to predict the phase behaviour are investigated. Firstly, an empirical method, based on the concept of the principle of congruence, is applied. This method uses the fact that, at constant temperature and mass fraction methyl ester, there exists a linear relationship between the number of carbon atoms in the hydrocarbon backbone and the phase transition pressure. The method has previously been applied to similar homologous series [4], [13], [14] and should thus also be successful in predicting the homologous series ethane–methyl esters.
Secondly, thermodynamic modelling using two existing equations of state (EOSs) is conducted. The aim of the thermodynamic modelling is to use existing models and not to develop new and/or modify current models, as such an exercise is a study on its own. The Peng Robinson (PR) equation of state (EOS) [15] and simplified perturbed chain statistical associating fluid theory (sPC-SAFT) EOS [16] were selected as these two models are typically available in process simulators, such as Aspen Plus®. The PR EOS is one of the most widely used cubic EOSs and therefore represents this family of EOSs. On the other hand, the statistical associating fluid theory (SAFT) approach [17], [18] is one of the state of the art approaches and the sPC-SAFT approach balances the accuracy of the SAFT approach with mathematical simplicity.
Section snippets
Experimental procedure, set-up and accuracy
A static synthetic method, using two previously constructed setups, each consisting of a variable volume high pressure view cell, was used to measure the phase equilibria data. The view cells have been described in detail in previous publications [13], [19] and are very similar, the main difference being the volume: 45 cm3 [19] versus 80 cm3 [13]. The view cells are used interchangeably and measurements can be conducted on either. Generally, higher methyl ester concentration measurements were
Experimental results
Phase equilibrium measurements were conducted for C9COOCH3, methyl dodecanoate (C11COOCH3), methyl tetradecanoate (C13COOCH3), C15COOCH3, methyl octadecanoate (C17COOCH3) and C21COOCH3 in supercritical ethane between 312 and 355 K. For low C9COOCH3 mass fractions (<0.05) no measurements were conducted above 338 K due to total solubility and/or a steep pressure-composition gradient (at constant temperature) in this region. When the pressure-composition gradient becomes too steep, a small change in
Experimental observations
The experimental measurements indicate that in all cases an increase in temperature leads to an increase in the phase transition pressure. No three phase regions or temperature inversions (decrease in phase transition pressure with an increase in temperature) were observed and neither did the experimental measurements indicate the presence of either of these phenomena in or near the temperature range of the measurements. A comparison of the phase behaviour for the various methyl esters in
Conclusions and further work
High pressure phase equilibria measurements for a range of methyl esters (C9COOCH3 through C21COOCH3) have been conducted in supercritical ethane at temperatures between 312 and 355 K for methyl ester mass fractions between 0.018 and 0.65. In this range no three-phase regions or temperature inversions were observed and the data showed a generally linear relationship between the phase transition pressure and temperature at constant composition. The experimental results allowed for the
Acknowledgements
The financial assistance of Sasol (Pty) Ltd and the Department of Trade and Industry (DTI) of South Africa through the Technology and Human Resources for Industry Programme (THRIP) towards this research is hereby acknowledged. Opinions expressed and conclusions arrived at are those of the authors and are not necessarily to be attributed to the sponsors. The assistance of Mr. J. Batt with some of the experimental measurements is hereby acknowledged.
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2015, Journal of Supercritical FluidsCitation Excerpt :However, SC CO2 is not such a good solvent for high molecular mass aliphatic components such as acids [10] and esters [13–15], and thus high pressures are required for significant solubility. Ethane has been considered as an alternative solvent to CO2 as it has a similar critical temperature (305.32 K [12]) and improved solubility for both acids and esters [11,16,17]. However, ethane is flammable and rather costly and, while improved solubility is obtained, especially for higher molecular mass components, high pressures (>20 MPa) are at times still required.