The role of dielectric properties in the microwave glycerol acetylation and gaseous emissions of blended biodiesel

Abstract

Microwave assisted glycerol acetylation reactions in a monomode pilot reactor were performed in order to obtain glycerol derivatives as potential biodiesel additives. The reactions were carried out with acetic acid and acetic anhydride using sulfuric acid, pyridine and triethylamine as catalysts. The acetylation reactions yielded a mixture of mono, di and triacetin respectively with short irradiation time and high selectivity to triacetin. Pyridine exhibits 100% of selectivity to triacetin in 30 min of microwave heating with 0.88% (w/w_T) of catalyst concentration. Microwave-heating technology has been demonstrated as an alternative to reach a green chemistry and to this end becomes essential the knowledge of the dielectric properties of the materials involved in microwave heating in order to operate under optimal conditions. Dielectric properties of the pure reagents and during the glycerol acetylation under conventional heating were measured. High loss tangent were obtained for reactions mixtures employing H₂SO₄ and triethylamine and dielectric heating is dominated by ionic conductivity whereas with pyridine the dielectric heating of the reaction mixture is governed by dielectric relaxation process. Physical–chemical analysis of blended biodiesel with triacetin show values for viscosity, flash point, water content, density and acid number in accordance with current international standards. Gaseous emissions analyses of blended biodiesel showed significant reduction of CO emission (50%), CO₂ (25%) and 30% reduction in unburned hydrocarbon (UBHC) and 50% of NOx emissions. The best values were observed in the samples containing 5 and 10% of triacetin.

Graphical abstract