Mainstream strategies to enhance agitation efficiency during transesterification process

Conventionally, biodiesel is produced through the agitation of the reagents, i.e., oil, alcohol (mainly methanol), and catalyst at about 60 ^oC (just below the boiling point of methanol i.e. 64.7 ^oC) for about 1 h.

Currently, the majority of industrial biodiesel production practices worldwide are batch or continuous processes with mechanical agitation. However, since oil and alcohol are not well miscible, mixing efficiency is therefore the main challenge faced.

The most efficient mixing is achieved when the alcohol–oil interfacial area is maximized by decreasing the droplet size of the reactants i.e. alcohol and oil as much as possible. Theoretically, this could be as low as the sizes of the molecules involved in the reaction. Therefore, both the agitation and temperature are indispensable elements required to accomplish a successful transesterification reaction.

Numerous attempts have been made to enhance agitation efficiency including chemical and/or mechanical strategies:

- Chemical strategies used to enhance agitation efficiency, involve the use of a co-solvent in order to achieve a single phase of alcohol-oil. The co-solvents used should:

1) be completely miscible in both the alcohol and oil and

2) have a boiling point close to that of the alcohol used e.g., methanol so that they could be easily , co-distilled and recovered/recycled upon the termination of the reaction. Cyclic ethers such as tetrahydrofuran (THF), 1,4-dioxane, diethyl ether, methyl tertiary butyl ether, and diisopropyl ether, owing to their hydrophilic oxygen atom capable of forming hydrogen bonds with alcohols, and their hydrophobic hydrocarbon portion capable of solubilizing oils, meet the first condition required for an ideal co-solvent. Having included the second condition, THF (boiling point: 66 oC) is regarded as the most ideal co-solvent especially if methanol is used in the transesterification reaction.

- Mechanical strategies used to enhance agitation efficiency fall into three different categories:

1) Improving the conventional impeller agitation systems.

2) Application of non-impeller novel agitation systems in which highly efficient mechanical energy is provided for mixing and initiating the transesterification reaction. These include ultrasound-based agitation systems, e.g., ultrasonic cavitation reactor, high frequency magnetic impulse cavitation reactor, static mixers, oscillatory flow reactors, and spinning tube in tube reactors.

3) application of novel systems in which no agitation is applied but conditions required for a successful transesterification are provided. These include microwave reactors which utilize microwave irradiation to transfer energy directly into reactants and consequently accelerate the rate of reaction and membrane reactors. In fact, the latter integrates reaction and membrane-based separation into a single process and increase the rate of equilibrium-limited transesterification reaction by constantly removing the products i.e. biodiesel from the reactants stream via membranes.

It is worth quoting that the final characteristics of biodiesel could be influenced by the procedure through which the fuel has been produced.