Comparison of various methods for the estimation of vapor pressure of fatty acid methyl and ethyl esters (FAAE's)

Petroleum fuels are being used at a rapid rate and there is a need for renewable alternative fuels. Biodiesel (mainly a mixture of fatty acid alkyl esters FAAE's) is a potential alternative to petro-diesel. Combustion modeling of biodiesel requires vapor pressure data. The experimental determination of vapor pressure of FAAE's is difficult, therefore the estimation of vapor pressure of FAAE becomes important. In this study, various methods based on group contribution, equation of state (PR EOS) and correlations based were used to estimate the vapor pressure of nine FAME in the carbon chain range of C6 to C18 and four FAEE in the carbon chain range of C8-C13. The FAME's and FAEE's chosen for this study are the main constituents of real world biodiesels. These methods were compared on the basis of their ability to estimate the vapor pressure of FAAE's with accuracy and simplicity in the application of the method. Other than the methods which are already available in literature, two new methods, Othmer and Yu and Peng Robinson equation of state (PR EOS) at zero pressure were used in this study. For FAME's total 244 vapor pressure data points in the temperature range of 306-512 K were used where as for FAEE total 55 data points in the temperature range of 273-462 K were used. Amongst compound specific methods, Yuan and Othmer and Yu gave good prediction for both FAME's and FAEE's. For FAME's the % OARD for Yuan method was 4.83 and for Othmer and Yu it was 2.56. For FAEE's Yuan gave % OARD of 1.30, while Othmer and Yu gave % OARD of 2.85. Amongst generalized methods Ceriani, PR EOS and Othmer and Yu gave good estimation of vapor pressure for FAME's with least % OARD of 7.64 for Othmer and Yu. But, for FAEE's except Othmer and Yu (% OARD 19.19) all other methods gave very high deviation. In the present study strengths, weaknesses and applicability of selected methods have been highlighted.