## 2 Biodiesel Synthesis Processes
### Criteria for Feedback Selection  
Two requirements should be fulfilled by feedstock for biodiesel: large production scale and low production cost. Vegetable oil contains carboxylic acid and/or triglycerides which take part in chemical reaction of biodiesel synthesis. In addition to this, animal fat can also be used as raw material that reacts with the methanol or ethanol and this reaction is supported by a catalyst. Table 2 summarizes various raw materials for biodiesel synthesis [11]. Biodiesel is synthesized from rapeseed oil globally. In addition to edible and waste vegetable oil, non-edible seed oil has also been explored among which Jatropha has shown a promising potential for biodiesel production. Other feedstocks include bovine tallow, pork fat, fish oil and other animal fats. Bovine tallow is by-product of meat industry and is mostly used to produce biodiesel. It contains a saturated chain of fatty acids with chemical structure just similar to the oil obtained from plant seeds. Amid all types of raw materials, vegetable oil extracted from seeds is the main feedstock for biodiesel production, for instance, is of utmost importance. Oil yielding crops have been reconnoitered for biodiesel production and more than 350 oil producing seed plants have been identified as biodiesel feedstock [12]. While current pursues are being performed to explore novel potential sources that will be promising, natural, and sustainable with respect to food security and environment protection thus favoring the production of high-quality biodiesel. Among plant-based oils, almost 95% biodiesel is prepared from vegetative edible oil. However, it is in competition with the supply of food, and this has led to price hike of both edible oil as well as biodiesel. Since, non-edible seed oils have gained the distinctive attention of researchers. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Edible Oils** & **Non-Edible Seed Oils** & **Animal Fats/Oils** & **Other Oil Sources** \\ \hline Sorghum & Rice bran & Poultry/chicken & Fungi \\ \hline Sunflower & Camelina & Beef tallow & Bacteria \\ \hline Safflower & Jojoba & Fish & Algae \\ \hline Barley & Tobacco & Pork lard & Switchgrass \\ \hline Peanut & Rubber & Grease & Terpenes \\ \hline Oat & Wild Melon & & Latex \\ \hline Coconut & & & Waste cooking oil \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Feedstock for biodiesel production. Figure 2: Schematic steps involved in biodiesel production and characterization. Nonedible oil yielding crops are reasonable in terms of cost for cultivation, can grow best in waste land without exceptional care. Among biodiesel feedstocks, besides vegetable oil, animal fats are also used. These are saturated at room temperature because they have high amount of saturated fatty acids, therefore creating problems for the synthesis of alkyl esters. Moreover, it is more expensive than vegetable oil. Bovine tallow biodiesel can produce cold filter clogging at temperature about 19 \({}^{\circ}\)C. It is a factor for analysis of biodiesel quality and in order to reduce clogging point, it is mixed with vegetable oil. Therefore, the depression in the clogging point occurs at 10 \({}^{\circ}\)C.  
The financial expenses depend on geographical area, price of crude petroleum, base stock, and seasonal variations in crop production. The high expenditure associated with biodiesel production is due to the high cost of feedstock. Non-edible feedstock does not compete with food supply and hence, the crop failure is not at risk and has a lower financial cost. The high budget needed for raw material is also one of the prominent contributing factors in overall price-hike of biodiesel. Therefore, a financially affordable substitute is the use of residual oil or fat as feedstock for biodiesel synthesis process [13]. The collection of residual oil is usually made at large (industrial) as well as small scale (domestic) etc. The advantage of using waste or residual oil is its less aggressiveness to environment via correct disposal.