Thermodynamic model for process design, simulation and optimization in the production of Biodiesel

N. COTABARREN; P. HEGEL; S. PEREDA; E.A. BRIGNOLE

Puerto Iguazú

Congreso; 13 th International Conference on Properties and Phase Equilibria for Products and Process Design; 2013

PLAPIQUI - CONICET

The biodiesel production industry requires thermodynamic models able to correlate and predict the phase equilibria of the process mixtures. Two main technologies have been studied: i) the conventional transesterification with methanol using basic catalysts and ii) the supercritical non-catalytic route using supercritical methanol. In both cases, the molecular nature of the reacting mixture indicates the existence of strong hydrogen bonding interactions between the alcohol, the vegetable oil and fatty derivatives. These highly non-ideal mixtures of large molecular size asymmetry are a challenging problem of phase equilibrium correlation and prediction, over the wide ranges of pressure, temperature and composition covered by both technologies. The supercritical route allows the use of low quality oils that are contaminated with fatty acids and water. Also water is used in the purification of the biodiesel product. From a phase equilibrium point of view we have three immiscible pairs: methanol + vegetable oil (triglyceride), fatty ester (biodiesel) + glycerin and fatty ester + water. Moreover, very high accuracy is required due to the sensitivity of the components distributions on product specifications; for example in the glycerol washing unit. In this work, the Group Contribution Equation of State (GCA-EoS) is applied to model several systems of importance in this field: methanol + vegetable oil, methanol + mono and diglycerides, methanol + fatty esters, glycerine + fatty esters, water + fatty esters and the multicomponent mixtures of these components. The mixtures evaluated can be highly non-ideal, especially in the last purification steps where polar compounds are diluted in biodiesel. Experimental VLE and LLE data were satisfactorily correlated and predicted with a single set of interaction parameters, using a simplified version of the association contribution of the model. The model has been successfully applied to several key biodiesel plant sections: i) recovery of methanol in a high-pressure three-phase flash of fatty ester + glycerin + methanol mixture; ii) water stripping of methanol from the reaction products in a liquid-liquid-vapor distillation column; iii) liquid-liquid equilibria for the removal of glycerin and methanol from biodiesel; iv) correlation of the homogeneous region boundaries in the supercritical transesterification of vegetable oils. GCA-EoS is a group contribution model; therefore, biodiesel production from different raw materials (i.e soy, palm or canola oil) can be predicted without requiring new parameterization.