Ultrasound-assisted biodiesel synthesis by in–situ transesterification of microalgal biomass: Optimization and kinetic analysis

Abstract

In-situ transesterification of microalgal biomass has emerged as efficient technique for biodiesel synthesis, as it eliminates the step of lipid extraction. In this paper, we report new insights of in-situ transesterification with application of sonication. The major objectives of our study were: (1) optimization of biomass pretreatment, (2) statistical optimization of transesterification parameters, (3) kinetic and Arrhenius analysis, and (4) determination of fuel properties of biodiesel. Lyophilized microalgal biomass with base catalyst NaOH resulted in the highest biodiesel yield of 31.6% w/w dry biomass. The statistical optimization resulted in biodiesel yield of 33.0 ± 1.1% w/w for parameters: catalyst loading = 1.7% (w/w biomass), methanol to biomass ratio = 30 (v/w), temperature = 50 °C and reaction time = 50 min. Application of sonication during in-situ transesterification showed 60–80% rise in kinetic constants with reduction in activation energy from 26.55 to 21.52 kJ/mol and rise in biodiesel yield from 33.0% to 39.9% w/w dry biomass. These results are attributed to intense micro-convection generated by sonication that not only enhances lipid extraction from biomass but also causes fine emulsification of reaction mixture that enhances the interfacial area. Using fatty acid methyl ester profile of biodiesel, the fuel properties were determined as: Higher heating value = 39.474, cetane number = 54.232, iodine value = 84.306, kinematic viscosity = 3.696, density = 0.875, saponification value = 202.295. These properties are satisfactory are within the prescribed American standards for automotive fuels.