Modelling of the intensified esterification using ozone-rich microbubbles for biodiesel production

The aim of this thesis is to model the intensified esterification in order to improve the pretreatment stage of biodiesel production, where the free fatty acids found in vegetable oils are converted to fatty acid methyl esters. The intensified esterification considers the use of a microbubble reactive distillation as an alternative to the acid pretreatment. The proposed set of reactions based on a free-radical mechanism would favour the process towards completion achieving a yield higher than 90%. This is achieved due to the respective water stripping and removal, leading to a higher efficiency of the process and avoiding inhibition caused by products. Both the 0-D irreversible and reversible model are built in order to portray the relevance of the reverse reaction, since it is known that esterification is a reversible reaction of second order. The rate constants obtained in these models are fed into the 2-D model, where the reaction kinetics, mass and heat transfer and surface reactions in the gas-liquid interface are studied. Some of the results obtained in the 2-D model for the reversible esterification are described below. A higher FAME concentration is obtained due to the free-radical direct injection into microbubbles with plasma and the water removal (Le Chatelier’s push and pull). An enhanced reaction kinetics is found with shorter residence times. An increase in temperature would mean an increase in both forward and reverse rate constants, favouring the forward rate constant (Esterification is endothermic). Decreasing the bubble size results in an increase of the FAME production due to the enhanced gas-liquid ratio at the interface and the increased vaporisation and stripping of water. Increasing the concentration of the O· radical results in an increase in the FAME concentration in the liquid domain. A higher bubble temperature results in a higher water concentration inside the bubble, leading to a higher reaction rate and water stripping. These findings are used in order to propose an esterification reversible model using J. platyphylla, which accounts shorter residence times lower than 1x10-4 s, in other words (τres<1x10-4 s), when the maximum water concentration in the bubble is reached before it reaches the chemical equilibrium.