Thermodynamic modeling of urea inclusion fractionation
Abstract
The thermal onset temperature (cloud point) of biodiesel limits its utilization scope and current methods have drawbacks in improve the low temperature of biodiesel. A unique technology, urea inclusion fractionation, is used to reducing the cloud points. Urea inclusion fractionation is a unique heterogeneous phase equilibrium due to the formation of urea inclusion compounds. A thermodynamic model is proposed to predict the urea inclusion fractionation according to the principles of heterogeneous phase equilibrium. For the non-ideal behavior of the system, the activity coefficients of the components in the mixture are calculated according to the modified UNIFAC model. In the thermodynamic model, the unknown decomposition enthalpies and entropies are calculated through the group contribution methods. The unknown decomposition temperatures of urea inclusion compounds formed by saturated FAME are calculated according to the relationship between the melting points of FAME and decomposition temperature of urea inclusion compounds formed by saturated FAME. The decomposition temperature of urea inclusion compounds formed by unsaturated FAME are calculated according to HDec=TD SDec. The unknown group interaction parameters in the modified UNIFAC model are obtained from the nonlinear regression of the solubility of urea and urea inclusion compounds in alcohols. The effects of the ratios of urea to solvent to FAME, type of solvent, compositions of FAME, operation temperature and operation pressure on the yields, compositions and cloud points of FAME in liquid phase in cooling/evaporative urea inclusion fractionation are studied and compared with the predictions from the proposed thermodynamic model. The proposed thermodynamic model can accurately predict the urea inclusion fractionation. Through urea inclusion fractionation, the fractions of saturated FAME in liquid phase decrease and the cloud points decrease. The cloud points are related to the compositions and show the heterogeneous phase equilibrium. According to the principles of heterogeneous phase equilibrium, a thermodynamic model is proposed to predict the cloud points according to the composition. The unknown fusion enthalpies are predicted from the group contribution model. The activity coefficients in the model are calculated from the modified UNIFAC model. The cloud points and compositions of binary, ternary and multiply components mixtures are measured. These cloud points are compared with the predictions from the proposed thermodynamic model. These results show the proposed thermodynamic model can accurately predict the cloud points according to the composition. The predicted cloud points and the measured cloud points are linearly related.
Degree
Ph.D.
Advisors
Tao, Purdue University.
Subject Area
Alternative Energy|Chemical engineering|Petroleum engineering
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