Modelling of the Gas-Liquid Transesterification Reaction to Produce Biodiesel

Abstract

The most widely used process technology for biodiesel manufacture is the basecatalysed liquid-liquid transesterification reaction. Due to the immiscibility between the reactants – alcohol (usually methanol) and a triglycerides source (refined vegetable oil or animal fat), this reaction is slow and limited by mass transfer. The reaction temperature is also limited by the boiling point of the alcohol (69oC for methanol). These resulted in overall slow reaction rates. In addition, reaction kinetics previously reported for the transesterification reaction may be inaccurate due to the effects of mass transfer. Presence of such mass transfer effects are acknowledged but have never been isolated or quantified in the literature for biodiesel research. The gas-liquid transesterification reaction carried out in a spray reactor is an innovative method to produce biodiesel developed by Behzadi and Farid [1]. This method uses a gas-liquid arrangement for biodiesel processing for the first time in biodiesel research. Mass transfer resistances are significantly reduced due to large interfacial surface area per volume of the small oil droplets, while high reaction temperatures can be achieved because they are no longer limited by the boiling point of the alcohol. As such, up to 97 – 99% conversion to biodiesel is achieved in about 8 seconds as opposed to 60 minutes using a liquid-liquid arrangement. However, the kinetics of this rapid process is yet to be explored. Therefore, the main objective of this thesis is to analyse and model the kinetics of the gas-liquid transesterification reaction between oil droplets and vapour alcohol. The mathematical model is derived based on the process of gas absorption followed by a second-order chemical reaction. The model has independent mass transfer and reaction kinetics terms. This enables the examination of the contribution of each term towards the overall process. In other words, the mass transfer effect on the transesterification reaction can now be isolated and quantified for the first time. In addition, the model in combination with experimental measurements also enables the calculation of the “true” reaction rate constant which does not incorporate any mass transfer effects. The experimental work of this thesis involves the fabrication of a simple droplet reactor capable of producing soybean oil droplets of 400 – 500 μm diameters. This facilitates the study of gas absorption followed by chemical reaction. Soybean oil is chosen for use in this thesis because it is a common feedstock for biodiesel production. Vapour methanol and 1-propanol are both used as the alcohol to investigate the effect of the gas reactant solubility on the overall process. Methanol has significantly lower solubility in soybean oil when compared to 1-propanol. The respective sodium alkoxides are used as the base catalyst. The “true” reaction rate constant, activation energy and Arrhenius constant are reported for the gas-liquid transesterification reaction. Simulation of the model is also performed to investigate the effect of temperature, oil droplet size, catalyst loading and alcohol solubility on the gas-liquid transesterification reaction. The performance of the model is validated by simulating the conditions within the spray reactor of Behzadi and Farid [2], and comparing their experimental measurements of oil conversion to the predictions of the present model. In addition, the model is also extended to simulate other gas-liquid reactions and reactors to demonstrate its application to cases other than those of biodiesel and spray reactors. The mathematical model can be used to quantify the mass transfer and reaction kinetics contributions towards the transesterification reaction which is not previously investigated. It is found that both mass transfer and reaction kinetics contributes towards the gas-liquid transesterification process, and the extent of these contributions changes with reaction conditions like temperature and catalyst concentration; pr effectively the Hatta modulus (MH). During the gas-liquid transesterification process, the liquid phase reaction is slow enough for the gaseous reactant to diffuse into the entire liquid droplet while reacting. However, the liquid film resistance is not negligible. This provided insights into the rapid gas-liquid transesterification reaction which will be useful in scale-up procedures. The mathematical model has also been shown to be versatile and can be applied for other gasliquid reactions and reactors other than for biodiesel and spray reactor applications.