Reaction Kinetics of the Catalytic Esterification of Citric Acid with Ethanol

Reaction kinetics are presented for the reversible esterification reaction of citric acid with ethanol to form tri-ethyl citrate via mono-ethyl and di-ethyl citrates. The reaction was studied in batch isothermal experiments, self-catalyzed homogeneously by citric acid and the formed mono- and di-ethyl citrates, and heterogeneously catalyzed by macroporous Amberlyst-15 ion-exchange resin catalyst. Experimental data were obtained between 78 and 120 °C at different mole ratios of ethanol to citric acid and catalyst concentrations up to 5 wt % ion-exchange resin. The kinetics of ethanol etherification to form di-ethyl ether were included in the investigation. Kinetic modeling was performed using a pseudo-homogeneous UNIQUAC-based activity model, taking into consideration the rate of self-catalyzed esterification and the side reaction to form diethyl ether. The activity coefficients for the tri-ethyl citrate−ethanol and tri-ethyl citrate−water binary pairs were obtained from experimental vapor−liquid equilibrium data. Kinetics of the di-ethyl citrate to tri-ethyl citrate reaction limit the overall tri-ethyl citrate formation rate, as citric acid and mono-ethyl citrate are esterified rapidly to their equilibrium compositions. Higher temperatures lead to faster reaction kinetics but significantly increase the production of the undesired byproduct di-ethyl ether. The kinetic model developed is useful for the design and simulation of processes such as reactive distillation for tri-ethyl citrate formation.