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.