(257ar) Evaluation of RK-PR Equation of State in the Correlation of VLE of Binary Systems Related to Biodiesel Production

New energy sources are constantly being researched and the biodiesel is one of most remarkable with enormous potential to be the main source of clean energy [1]. Biodiesel (BD) is understood as a mixture of fatty acid alkyl esters which can obtained by the transesterification reaction between triglycerides from vegetable oils or animal fats with a short chain alcohol. In this reaction glycerol is also obtained as a side product [2].

The non-catalytic production process with supercritical alcohols (SCA) using vegetable oils has been considered as a potential method for BD production without suffering some inconveniences like catalyst removal, pretreatment, water sensibility etc. [3]. However, SCA process demands a high energy to achieve the supercritical state and large excess of alcohol. To decrease these reaction conditions, the use of co-solvents such as carbon dioxide has been proposed since it is able to increase the miscibility of oil and alcohol in the mixture [4].

To design this type of process it is very important to predict the phase behavior of reaction mixtures in order to avoid mass transfer limitation caused by heterogeneity of phases and to optimize product separation. Some authors have been studying the phase equilibrium of some systems present in biodiesel production with relative simple cubic equations of state (EOS) like Soave-Redlich-Kwong (SRK) and Peng-Robinson (PR) [5-8].

Despite of large use of SRK and PR, these equations have some limitations and this fact is consequence of their two-parameter density dependence rather than of their empiric parameter. In ref. [9] the authors presented a parametrization procedure to use the three parameter generalized Redlich-Kwong-Peng-Robinson (RK-PR) equation. The results presented in [9-10] indicated that this equation of state [EOS] presents a better representation of the phase behavior of asymmetric mixtures compared to the classical PR and SRK EOS. However, there is no study in the literature presenting its application to mixtures involving triglycerides and esters.

In the present work the three parameter generalized RK-PR EOS was evaluate in the representation of VLE of some mixtures present in biodiesel production. The results obtained were compared with experimental results, obtained from literature, and also with those obtained by the classical PR and SRK EOS. The parameters like critical temperature and pressure and acentric factor of the triglycerides, esters and glycerol were predicted by different methods. To critical temperature and pressure the methods evaluated were: Marrero and Gani (MGM), Joback and Reid (JRM) and Constantinou and Gani (CGM) methods. To the acentric factor the methods were the Pitzer’s and Kesler and Lee’s. The effect of the estimated parameters was evaluated the in the performance of the EOS. The classic van der Waals mixing rule was used. The effect of the inclusion of one adjustable parameter (k_ij) in the mixing rule was also investigated.

The ELV at high pressure of several systems were evaluated. The binary mixtures include alcohol+ triglyceride, alcohol+ester, glycerol+ alcohol, carbon dioxide+ triglyceride, carbon dioxide+ester and carbon dioxide+glycerol.

Before the calculations of VLE, liquid densities of the species involved in the VLE were calculated and compared with experimental values obtained in a data bank. The comparison between RK-PR, SRK and PR EOS's showed that the inclusion of the third parameter improves the prediction of density, but the improvement is so small that it does not justify the increment in the complexity of equation.

Considering the prediction and correlation of VLE, the results show that, in general, the RK-PR represents better the behavior in the whole range of composition compared to the classical SRK and PR EOS's. The PR represents better than SRK the region closer to pure bigger molecules (triglycerides, esters and glycerol) and the opposite is observed to the small molecules. These results are very similar that it was presented in ref. [9].

Acknowledgements

The authors wish to acknowledge CAPES, CNPq and FAPEMIG for financial support

References

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