Bio-additives from glycerol acetylation with acetic acid: chemical equilibrium model

Abstract

In this work, the chemical equilibrium of glycerol (G) acetylation with acetic acid (AA) to form mono- (MAG), di- (DAG) and tri- (TAG) acetylglycerols has been studied. These compounds are biodegradable and renewable options as high-quality bio-additives to improve the antiknock properties and the viscosity of fuels and biofuels. Due to the absence of thermodynamic data, the physicochemical and thermodynamic properties of the compounds were determined, such as the specific heat, and the enthalpy and entropy of formation, by employing a second-order group-additivity predictive method. The values obtained were validated with few experimental data available in the literature (298 K, 101.325 kPa), showing differences in the range 0.2–8.9%. The compositions at equilibrium were calculated by minimizing the total Gibbs free energy of the system and considering the non-ideality of the liquid phase. For this purpose, different temperatures (350–500 K), reactant molar ratios (1–12) and initial water contents (0 and 40 wt%) were studied. The results revealed the global exothermicity of the system, showing that total glycerol conversion (~100%) and high yields to TAG (>90%) can be achieved in the 350–500 K range by employing AA:G molar ratios between 9 and 12. As the presence of water in the glycerol solution produces a decrease of the glycerol conversion and selectivity to TAG, its removal from the reaction medium should be considered. A comparison between our results with the reported data based on different catalytic systems indicates that this model could successfully describe the chemical equilibrium of the system.