Hydrodynamic cavitation for process intensification of biodiesel synthesis- a review

Available fossil fuels are decreasing day by day. So the use of renewable sources like alkyl esters (biodiesel) is increasing in CI engines. Alcoholysis is a commonly employed method in the preparation of biodiesel where oil is mixed with suitable CH3OH or C2H5OH and a suitable catalyst. Various techniques are available to carry out alcoholysis reactions. Viz. Mechanical stirring, probe type ultrasonic cavitation, pyrolysis and hydrodynamic cavitation. Hydrodynamic cavitation (HC) is one of the energy-efficient processes for alkyl ester (Biodiesel) synthesis. Cavities are created by reducing the pressure caused by allowing the liquid to flow through constriction. This technique gives a high yield, and it is a time-saving one. The different operating variables Viz. Cavitation number, inlet pressure, and system geometry of hydrodynamic cavitation influence the per cent conversion and the yield of alkyl ester. This review paper broadly covers the basics of the HC technique and its use for biodiesel production by transesterifying various types of vegetable oils. The effect of different types of catalysts (acid, alkaline and heterogeneous) and operating conditions like inlet pressure, oil to alcohol molar ratio, reaction temperature and kinetics have been briefly discussed. Triglyceride conversion decreased beyond the molar ratio of 1:6 because more glycerol was obtained at higher molar ratios. The highest conversion, up to 75%, was achieved with an orifice plate at an optimized temperature of 65 ​°C and upstream pressure of 5 ​bar. The small diameter orifice plate have shown promising results as compared to venturi tubes to for all pressure. The intensity of cavitation was found to be more when the cavitation number was less than at 5 ​bar. The physico chemical characteristics of prepared biodiesel met the requirement as per EN14214 and ASTM D 6751 standards. In the final opinion, biodiesel prepared via this novel and step-up approach was time-saving and energy-coherent, making the process more environmentally friendly. [Display omitted] •The nature and type of catalyst employed play crucial role in ​alcoholysis and the nature and type of catalyst employed play crucial role in ​alcoholysis.•Hydrodynamic cavitation method is more rapid, effective, energy efficient method for alkyl ester (biodiesel) production.•The properties of the synthesized thumba methyl ester met EN 14214 and ASTM D 6751 and were comparable with diesel and Experimental data well fitted seco