Several techniques are employed to bring down the physical and thermal properties of vegetable oils close to mineral diesel, by which these oils and fats can be used in internal combustion engines as fuel. This mainly requires improvement in viscosity of the vegetable oil. The possible treatments employed to improve the oil viscosity includes dilution with a suitable solvent, microemulsification, pyrolysis, and transesterification [9, 10].
The uses of biodiesel (BD) as a renewable, biodegradable, nontoxic, and eco‐friendly neat diesel fuel or in blends with petroleum‐based fuels are fascinating [11, 12]. “Biodiesel,” termed as the monoalkyl esters of long‐chain FAs, is derived from vegetable oils or animal fats. Numerous types of conventional and nonconventional vegetable oils and animal fats including those of used oils from the frying industry, soybean oil, rapeseed oil, tallow, rubber seed oil, and palm oil have been investigated to produce BD [13–15]. The production of BD involves the conversion of vegetable oils/animal fats using methanol or ethanol and a catalyst to produce fatty acid methyl esters (FAMEs) and crude glycerin as by‐product through a process termed as “transesterification” [16].
The transesterification process is accomplished by reacting vegetable oil with alcohol in the presence of alkaline or acidic catalyst. A catalyst is typically used to accelerate the reaction rate and yield. The stoichiometric equation requires 1 mol of triglyceride and 3 mol of alcohol to form 3 mol of methyl ester and 1 mol of glycerol [17]. Since the reaction is reversible, excess alcohol is used to shift the reaction equilibrium to the product’s side. The most preferred catalysts are sulfuric, sulfonic, and hydrochloric acids as acidic catalysts and sodium hydroxide, sodium methoxide, and potassium hydroxide as alkaline catalysts [18]. The product, fatty esters, have improved viscosity and volatility relative to the triglycerides. A dense, liquid phase rich in glycerol is the coproduct of this process. The separated fatty esters have cetane number and heating value close to that of the conventional diesel. The transesterification process for converting vegetable oils to BD is shown in Figure 1.1.
The “R” groups are the FAs, which are usually 12–22 carbons in length. The large vegetable oil molecule is reduced to about one third of its original size, lowering the viscosity and making it like diesel fuel. The resulting fuel can work like diesel fuel in an engine. The by‐product “glycerin” produced in this process is valuable due to its diverse industrial applications [19].
Technically, BD is a fuel comprising of monoalkyl esters of long‐chain FAs derived from vegetable oils or animal fat, which meets current EN 14214 and ASTM D 6751 BD standards of Europe and the United States, respectively. These standards are frequently employed as references to evaluate and compare the properties of other fuels.
Presently, the BD is commonly produced using a base‐catalyzed transesterification reaction because it involves low temperature and pressure processing, high conversions, no intermediate steps, and lower costs of processing materials [20]. Alkoxides and hydroxides of potassium and sodium are often used as catalysts in the transesterification of refined oils and low FA greases and fats. However, acid esterification followed by transesterification of high free fatty acid (FFA) fats and oils is also applicable. The base catalysts have better efficiency than the acid catalysts [21]. The base‐catalyzed transesterification reaction can be carried out at lower temperature, yet at room temperature, with the base catalysts, whereas acid catalysis required higher temperature (100 °C) and longer reaction time. During the process, basic catalyst breaks the FAs from the glycerin one by one. When a methanol molecule contacts an FA molecule, it will bond and form BD molecule. The hydroxyl group from the catalyst alleviates the glycerol formation. The resulting product named as methyl esters (BD) has appreciably lower viscosity and increased volatility relative to the triglycerides present in vegetable oils [22–24].
Figure 1.1 General reaction for transesterification of vegetable oil.
The second usual method of producing BD involves the use of an acid as a substitute of a base catalyst. Any mineral acid can be employed to catalyze the process; the most used acids are sulfuric acid and sulfonic acid. Although yield is high, the acids, being corrosive, may cause damage to the equipment, and the reaction rate is also observed to be relatively low [9, 21]. Oil feedstocks containing more than 4% FFAs must pass through an acid esterification process to increase the BD yield [25]. Such feedstocks are filtered and preprocessed to remove water and contaminants and then fed to the acid esterification process. The catalyst (sulfuric acid) is dissolved in methanol and then mixed with the pretreated oil [26].
The alcohols employed in the transesterification are generally short‐chain alcohols such as methanol, ethanol, propanol, and butanol producing esters named as methyl‐, ethyl‐, propyl‐, and butyl‐esters, respectively [9, 10]. It is reported that when transesterification of soybean oil using methanol, ethanol, and butanol was performed, 96–98% of ester’s yield could be obtained after an hour of reaction [27]. Though utilizing different alcohols presents little differences with regard to the kinetic of reaction, the final yield of esters remains unchanged. Thus, assortment of the alcohol is based on cost and performance consideration. Generally, reaction temperature is set at near the boiling point of the alcohol used [28].
Due to the reality that many vegetable oils, including soybean, canola (rapeseed) oil, and rice bran oil, have a major number of FAs with double bonds, oxidative stability is a problem, particularly when storing BD for longer period of time [29, 30]. This problem becomes severe due to improper storage conditions, which may include exposure to air and/or light, temperatures above ambient, and presence of extraneous materials (contaminants) with catalytic effect on oxidation. Some additives such as antioxidants might control the oxidation.
Characterization of BD fuel properties and evaluation of its quality are the matters of great concern for the successful commercialization of this fuel. A high fuel value with no operational problems is a condition for market acceptance of BD. Accordingly, the analysis of BD and the monitoring of the transesterification reaction have been the subject of numerous publications [31, 32]. The constraints, which are used to define the quality of BD, can be divided in two groups [33]. One of them is also used for mineral diesel, and the second illustrates the composition and purity of fatty esters. The former includes, for example, density, viscosity, flash point, sulfur percentage, carbon residue, sulfated ash percentage, cetane number, and acid number. The latter comprises, for example, methanol, free glycerol, total glycerol, phosphorus contents, water, and esters content. Chromatography and spectroscopy are the mainly used analytical methods for BD analyses, but procedures based on physical properties are also available [34]. Furthermore, it is important to mention that in most chromatographic analyses, mainly gas chromatography (GC) has been applied to methyl and not to ethyl esters [29].
As the demand for vegetable oils for food has increased tremendously in recent years, hence, the contribution of nonedible oils such as jatropha, Moringa oleifera, rice bran oils, etc. can play an important role for BD production. In view of the limited petro‐oil resources and rapidly growing energy demands of the world, there is an extensive need to take immediate initiatives for exploring alternative energy sources to meet the domestic needs and reduce the dependence on imported fossil