Physical and Chemical Mechanisms of Ultrasound in Biofuel Synthesis

Abstract

Physical and chemical mechanisms ultrasound-assisted processes as related to the synthesis of biofuels are reviewed. Ultrasound and its secondary effectSecondary effect of cavitation have physical and chemical effects on a reaction system, which can contribute to enhancement of the kineticsKinetics and yield. In this chapter, a mechanistic insight into the ultrasound assisted Biofuels biofuels synthesis is given by coupling simulations of cavitation bubble dynamics with experimental data. The physical effect of ultrasound and cavitation is through intense micro-convection in the system that gives marked improvements in the mass transferMass transfer of the system. The chemical effect is through generation of highly reactive radicals through transient cavitation that induce or accelerate chemical reactions. Chemical effects include thermal decomposition of the solvent vapor molecules in the cavitation bubble resulting in generation of smaller molecular species that also affect chemistry of the process. Raising the static pressurePressure of the reaction system above ultrasound pressure amplitudeUltrasound pressure amplitude in the system helps to discriminate between physical and chemical effects of ultrasound and cavitation. Biofuels systems considered in this chapter are the pretreatment of biomass, biodiesel synthesis with acid/base and homogeneous/heterogeneous catalysts, extraction of microalgal lipidsLipids , bioconversion of crude glycerolCrude glycerol from biodiesel industry to value added products and desulfurizationDesulfurization of the fuel. Among the physical effectsPhysical effects of ultrasound and cavitation, micro-streaming by ultrasound has a greater influence on reactions than shock waves generated by cavitation bubbles. In some cases, chemical effects of transient cavitation are revealed to have adverse influence on a reaction. Many biofuels systems are limited by their intrinsic characteristics that restrict the effect of ultrasound and cavitation on the reaction system.