Review and comparison of the methodology adopted for biodiesel production

Biodiesel is a versatile energy source synthesized by trans esterifying various edible or nonedible oils using catalysts. It is preferable to diesel because of its higher flash points, reduced sulphur content, and biodegradability. Biodiesel synthesis by esterification or transesterification methods involves conventional homogeneous or heterogeneous, enzymatic, supercritical, ultrasound, and microwave techniques. Since the operating conditions and mechanisms in each method differ, a comprehensive evaluation is necessary. This manuscript examines and covers a comprehensive summary of conventional heating, homogeneous and heterogeneous catalytic systems. A review of enzymatic, supercritical, microwave, electrolysis, and ultrasound-assisted biodiesel synthesis techniques is also included. The comparative study of a microwave with a conventional system shows that it is superior to the latter due to inverse temperature gradient, high thermal efficiency, and reduction in activation energy, resulting in improved product purity and operating time. It performs better than slower enzymatic processes that involve product inhibition. It outperforms supercritical transesterification, which involves high operating conditions (temperature 200 to 300 °C, pressure 20 to 30 MPa) and product deterioration. When compared to alternative approaches, microwave-aided transesterification significantly reduces response time and outperforms other methods. Techno-economic study and green chemistry principles are also favors in microwave-assisted biodiesel synthesis. Use of oleaginous microorganisms and microalgae as a feedstock, and process integration using valorization of waste glycerol, improved the sustainability of biodiesel synthesis.