Production of triacetin from industrially derived purified glycerol: Experimental proof of concept, kinetic model derivation and validation

This study illustrates the purification of end-of-life crude glycerol from a real biodiesel production process and its conversion into triacetin via esterification with acetic acid. Crude glycerol of 50.9 wt.% purity was physicochemically treated to 74.3 wt.% reducing the content of ashes (16.1 to 9.8 wt.%), matter organic non-glycerol (25.6 to 9.8 wt.%) and water (7.4 to 6.1 wt.%). For esterification, catalyst screening with commercial ion exchange resins and zeolites showed that Amberlyst 36 attained a triacetin yield of 30.1% and stability after 5 cycles. An increase of temperature, molar ratio, and catalyst loading positively affected the reaction rates and equilibrium, whereas increasing water content in the feedstock only showed effect on the equilibrium leading to lower triacetin yields. After dismissing mass transfer limitations, a thorough study on the reaction kinetics analysed the effect of temperature (100-130°C), molar excess of acetic acid (6-12) and catalyst loading (2.5-7.5 wt.%) on the concentration profiles. Kinetic models based on Langmuir-Hinshelwood-Hougen-Watson and Eley-Rideal equations were formulated to fit to the experimental data, where an Eley-Rideal model accounting only for water adsorption fitted best. Deactivation was also contemplated considering the ash content in the substrate, yet the model does not correlate with a loss of catalytic activity. The activation energies for the forward and backward reactions of the in-series esterification reactions are 38.2 – 45.8 kJ mol^-1 and 47 – 50.3 kJ mol^-1, respectively, and the enthalpy for water adsorption is 28.2 kJ mol^-1.