Vapor-liquid equilibria of triglycerides-methanol mixtures and their influence on the biodiesel synthesis under supercritical conditions of methanol

Abstract

The non-catalytic synthesis of biodiesel (fatty acids methyl esters) from triglycerides and methanol proceeds at elevated pressures above 100 bar and temperatures above 523 K. Kinetic investigations of the system revealed an unusual behavior of the reaction rate constant with increasing temperature and pressure. In order to explain this phenomenon, the phase behavior of the triglycerides-methanol mixture was investigated. The phase equilibria of the binary system sunflower oil-methanol were measured at different temperatures between 473 and 503 K, and a range of pressures between 10 and 56 bar. The experimental data were correlated using the Peng-Robinson, Soave-Redlich-Kwong and Redlich-Kwong-Aspen equations of state and different mixing rules. The best results were obtained with the RK-ASPEN equation of state and the Van der Waals mixing rule (VdW), which were then used to calculate the distribution of the phases at pressures and temperatures usual for the non-catalytic synthesis of biodiesel under high pressures. The obtained data indicated a strong influence of the phase equilibria on the reaction kinetics.

Sinteza biodizela (metil-estara viših masnih kiselina) iz triglicerida i metanola se odvija bez prisustva katalizatora na uslovima povišenog pritiska, iznad 100 bar, i temperature iznad 523 K. Kinetička ispitivanja ove reakcije su ukazala na neobično ponašanje konstante brzine hemijske reakcije sa porastom temperature i pritiska. U cilju objašnjenja ove pojave ispitivana je fazna ravnoteža sistema trigliceridi-metanol na povišenom pritisku i temperaturi. Ravnoteža faza binarnog sistema suncokretovo ulje-metanol je ispitivana na temperaturama od 473 do 523 K i pritiscima od 10 do 56 bar. Eksperimentalni podaci su korelisani korišćenjem Peng-Robinson, Soave-Redlich-Kwong i Redlich-Kwong-Aspen jednačina stanja uz primenu različitih pravila mešanja. Najbolji rezultati korelisanja eksperimentalnih podataka su dobijeni primenom RK-ASPEN jednačine stanja i Van der Waals-ovog pravila mešanja. RK-ASPEN jednačina stanja je zatim iskorišćena za izračunavanje fazne ravnoteže za binarni sistem trigliceridi-metanol na povišenim pritiscima i temperaturama, koji su uobičajeni za reakciju sinteze biodizela bez prisustva katalizatora. Dobijeni rezultati su ukazali na značajnu zavisnost kinetike reakcije od fazne ravnoteže posmatranog binarnog sistema.