Numerical Modelling of Sonicated, Continuous Transesterification and Evaluation of Reaction Kinetics for Optimizing Biodiesel Reactor Design

Abstract

Biodiesel is an alternative and sustainable fuel that can reduce the dependence on fossil diesel. This commodity has not only been promoted in the developed world but also in Indonesia, Brazil and several developing counties. In a general procedure it is a product of a transesterification reaction of vegetable oils or waste cooking oils with an alcohol, in the presence of an acidic or basic catalyst. It is a slow reaction which is conventionally carried out in a mechanically stirred batch process. An advanced method to achieve high yield quality in less time is sonication of the reaction in an integrated continuous process. Sonication causes micro-cavitation in the reactant mixture. The cavitation bubbles can have an internal pressure and temperature as high as 1000 atm and 5000K, respectively. Violent collapse of these bubbles causes tremendous increase in mass transfer, thereby enhancing the reaction rates [1]. To optimize the design of the reactor, high fidelity modeling assisted design is pursued. This enables the effective integration of the reactant transport and the sonication effect in a coupled acoustic, reactive multiple-specie flow. In this work, a cylindrical reactor is considered in which reactant mixture will be circulated and sonicated by a sonotrode type ultrasound equipment. To simulate the sonication effect the linear, time independent wave equation is solved for the fluid domain, which provides us with the acoustic pressure variation in the fluid. To account for the attenuation of the wave due to cavitation bubbles the modified wave number is used. To account for the chemical reactions, laminar reacting flow is assumed for the reactant mixture for which the Navier-Stokes equations and transport equation for dilute species is solved for the fluid. A logical reaction rate coupling model, which is dependent on the acoustic pressure and flow velocity, is used to evaluate the kinetics of the reaction, which are to be used as a judging factor for the reactor design [2].