## 2 Biodiesel Synthesis Processes  
Biodiesel is an outstanding substitute for petroleum based diesel fuel that can be directly used in compression ignition engine with little or no modifications thus securing cleaner emissions. Biodiesel from potential feedstock is produced by using various techniques, for example, pyrolysis, micro-emulsification, dilution and transesterification using both catalytic and superfluid method (Table 1). Moreover, for the process intensification, microwave technology, ultrasonication assisted methods are used. 1. **Pyrolysis or catalytic cracking** is the breakdown of vegetable oil or animal fats in the absence of air using a catalyst. The required catalyst enhances the thermal decomposition of biodiesel feedstock, as a result long chain hydrocarbons are broken down into short chain hydrocarbons. The biofuel obtained from pyrolysis process is acidic in nature and encompass different hydrocarbons and moisture contents. Therefore, a pretreatment process is required to remove moisture content and use it as alternative fuel;
2. In **micro-emulsification**, crude oil is mixed with emulsifying agent, this can be any type of alcohol to make emulsions. These emulsions cause accumulation of carbon in the engine and inappropriate burning;
3. In the **dilution** process, a required amount of crude oil and diesel are mixed together. Dilution is achieved by enhancing the solvent and lowering the solute concentration resulting in decrement in viscosity and density. Conventionally, diesel and ethanol are used as solvents. The dilution process is easy, however, due to high FFA content, viscosity and acid value makes this fuel unsuitable to be used directly in diesel engine yet some problems associated to it, for example, incomplete burning and carbon deposition. 4. **Transesterification** is the most utilized technology in biodiesel synthesis as it is favorable from economic point of view at industrial scale production. It involves the reaction between oil/fat obtained from a feedstock and alcohol in presence of suitable catalyst. Intensification strategies such as microwave assisted technology and ultrasonic cavitation are used to promote mass transport effect in overcoming the drawback of the high immiscibility of the solid catalyst with the oil and the alcohol liquid phases in the normal mixing system of the transesterification reaction. Process intensification through the use of active catalyst, pressure reactor, high temperature and high stirring rate often subjects to drawbacks with respect to energy consumption, product quality and reactants cost [6]. Process intensification is achieved by using microwave and ultrasonication technologies. Microwave technology offers a fast rate of reaction and less heat loss. However, after the process catalyst is removed and conversion process is influenced by catalyst activity. Ultrasonication or acoustic cavitation is considered as a promising, ecofriendly, and efficient method for biodiesel production. It involves utilization of cavity collapse to generate highly disruptive forces that rupture the layer between alcohol and oil and reactant get uniformly distributed due to the microlevel intermixing hence the reaction yield and rate is enhanced [7]. When transesterification is used for biodiesel production, one of two routes are followed: supercritical methanol transesterification or catalytic transesterification. 1. Supercritical fluids (SCF) are used for biodiesel synthesis in the absence of catalysts. It results in fast rate of reaction, high efficiency of conversion; there is no requirement of a catalyst but it requires energy and has a high installation cost. The supercritical transesterification involves single phase mixture, produced at 340 \({}^{\circ}\)C temperature and 43 MPa pressure with reduction in dielectric constant of alcohol. The reaction time is 2-4 min at supercritical stage. Though product purification is comparatively easy due to the absence of a catalyst, the production cost is high. 2.