Abstract
Biodiesel is a promising renewable and sustainable fuel that can replace fossil fuels in some applications. Among the different techniques used to produce biodiesel, the transesterification process is currently the preferred method. The conventional transesterification process is based on acid-base catalysis, but this technique has many drawbacks including a requirement for high-purity feedstocks, and costly pre-treatment and downstream processes. An alternative process, using a supercritical alcohol (preferably methanol) without a catalyst, may offer advantages [1]. This process can utilise a wide range of potential feedstocks (especially wastes), shows high production efficiency, and requires only simple post-processing. However, this technique requires conditions of high temperature and high pressure that increase the utility costs and may restrict the economic feasibility and sustainability of the process. In order to overcome these issues and improve the process, better understanding of the phase behaviour of the mixtures (fatty acids, esters and co-solvent like CO2) involved in the biodiesel process under various pressures (medium and high) and over a wide range of temperatures is needed.
In this work we report phase equilibrium measurements on the systems (butanoic acid + carbon dioxide), (propanoic acid methyl ester + carbon dioxide) and the ternary system (propanoic acid methyl ester + propanoic acid + carbon dioxide) carried out with a high-pressure quasi-static-analytical apparatus [2]. The measurements have been obtained along eight isotherms for (butanoic acid + carbon dioxide), five isotherms for (propanoic acid methyl ester + carbon dioxide) and five isotherms for the ternary system (propanoic acid methyl ester + propanoic acid + carbon dioxide) at temperatures from (323.15 to 423.5) K and at pressures up to the critical pressure at each temperature. Vapour-liquid equilibrium (VLE) data obtained for these mixtures have been compared with the predictions of the Statistical Associating Fluid Theory coupled with the Mie potential and a group-contribution approach (SAFT-γ Mie) in which the functional group parameters were fitted to pure-component and binary-mixture data. The group interaction parameters for COOH-CO2 and COO-CO2 have been obtained in this work by fitting to our experimental VLE data. The SAFT predictions after fitting the parameters were found to be in good agreement with the measured data for both the liquid and vapour phases. We have also modelled the data using the Peng-Robinson equation of state (PR-EoS) [3] combined with classical one-fluid mixing rules incorporating a temperature-dependent binary interaction parameter. The results show that the PR-EoS can also describe the experimental VLE data of both systems with acceptable accuracy, except in the critical region. The present work is expected to contribute to optimization of biodiesel production processes.
References
[1. Saka, S. and D. Kusdiana, Biodiesel fuel from rapeseed oil as prepared in supercritical methanol. Fuel, 2001. 80(2): p. 225-231.
2. Al Ghafri, S.Z., et al., Experimental and modeling study of the phase behavior of (methane + CO2 + water) mixtures. J Phys Chem B, 2014. 118(49): p. 14461-14478.
3. Peng, D.Y. and D.B. Robinson, A New Two-Constant Equation of State. Ind. Eng. Chem. Fundam., 1976. 15: p. 59-64.
