The prediction of critical parameters for triolein, diolein, monoolein and methyl esters

Prediction and preliminary modelling of equilibrium data represents the best tool for obtaining the real picture of reaction system when the experimental data are not available or can not be easily measured. The complex reaction mixture under supercritical condition is one such system and for its modelling two main problems usually exists. The first is the approximation of real data at critical point, and the second is the asymmetry of the mixtures due to large differences in both size and attractive forces between the supercritical solvent and solute molecules. Triolein, one of the typical triglycerides, is the most abundant component in many plant oils, such as sunflower, canola, soybean, rapeseed, which are mostly used as feed in synthesis of fatty methyl esters (FAME or biodiesel). Nowadays the biodiesel synthesis starts to be very important in the case of vegetable oil (triolein) alcoholysis under supercritical condition of methanol. The reaction is performed in three steps including several intermediary products /reactants (diolein, monoolein) which are reactants for obtaining the methyl oleate and glycerol as the final products. In this study the critical parameters, as well as acentric factor of triolein, diolein, monoolein and methyl oleate were estimated by using Aspen software. The group contribution method suggested by Constantinou and Gani [1, 2] had been used to predict the critical parameters. The method is based on molecular structure and represents addition of group contribution influence in the molecule (the first order and second order group). This method is very useful for prediction the normal boiling point of pure compound which are one of the most important parameter for estimation other critical parameters. However, the normal boiling point of triolein cannot be determined experimentally due to its thermal decomposition at temperatures even below its boiling point. Some thermodynamical data of analyzed components are given in the literature, and exists in well prepared data base, e.g. in Aspen or DIPRR data base. Estimations of critical parameters realized in this study were compared to available data in Aspen and DIPRR base. The importance of valid value of critical parameters and their use in the process of Aspen simulation of biodiesel production under supercritical condition was pointed out in this study. INTRODUCTION In process simulation, reliable and accurate property estimation methods play an important role in the solution of various simulation problems where convergence is often traced to failures in the reliable predictions of physical and thermodynamic properties. In the literature there are a several papers with this subject [1-11] and they proposed, for the estimation of physical and thermodynamic properties of pure compounds, the group contribution methods as the most accurate one. According to these methods, the property of a compound could be estimated as a summation of the contributions of firstand second-order groups which are defined by specific molecular structure. They provide the important advantage of quick estimates without requiring substantial computational resources. This method is very important for calculating the normal boiling point (for many organic compounds are not available in literature) which is essential in separation processes and for the estimation of the critical temperature. Also, the molecular structure is often oversimplified, making isomers indistinguishable. In this paper the group contribution method was used [1, 2] which is based on estimation the influence of the firstand second-order groups of complex molecule structure. The structure of a functional second-order group should have adjacent first-order groups as building blocks. The performance of second-order groups is independent of the molecule in which the groups occur, satisfying a fundamental group-contribution principle. The method was applied to the following physical and thermodynamic properties of pure compounds estimation: the normal boiling point, the normal melting point, the critical pressure, the critical temperature, the critical volume, the standard enthalpy of vaporization at 298 K, the standard Gibbs energy, the standard enthalpy of formation at 298 K, the acentric factor and liquid molar volume at 298 K. Estimations of critical parameters realized in this study was compared to corresponding available data proposed by Aspen and DIPRR base. MATERIAL AND METHODS Triolein, one of the typical triglyceride, is the most abundant component in many plant oils, such as canola, soybean, sunflower seed, etc. In this study, the physical and thermodynamic properties of pure triolein as well as of diolein, monoolein and methyl oleate were estimated. The parameters of these compounds which are main representative of biodiesel could be found in Aspen and DIPPR data bases. The base equation for estimating the thermodynamic and physical parameters for triolein, diolein, monoolein, and methyl esters can be described as a simple function of the property X,: