the reactants are the feedstock and an alcohol, which in the presence of a catalyst are converted into their esters, producing either water or glycerol as by‐products. Depending on the reaction conditions (based on the approach used), catalysts may not be required, although a multitude of catalysts have been developed and tested with varying degrees of efficiency. Such catalysts range from the simplest mineral acids, enzymes, or bases, which are added for achieving a homogeneous system and discarded with every use to simple heterogeneous catalysts that rely on solid metal oxides or the use of inert carbonaceous or siliceous biomass doped with the required catalytic groups (including transition metals) or enzymes, as well as nanocatalysts that have increased efficiency (when compared with inert microporous support‐based catalysts), while specially designed catalysts based on resin supports or metal organic frameworks have also been developed and can be very efficient but may be difficult to commercialize due to high development costs and unavoidable losses in each cycle of use. Strangely, processes such as supercritical fluid technology or superheated vapor technology can function reliably even without the use of catalysts, although the use of catalysts can augment the process, which may require a cost‐to‐benefit analysis before commercialization.
The process of biodiesel commercialization does not simply end at its production, since there are many stages that need to be considered for downstream processing as well as the consideration for treatment of hazardous materials generated (such as biodiesel wastewater that contains spent catalyst or leached ions) and the recovery of spent alcohol and the valorization of generated glycerol. Additionally, the produced fuel must have an acceptably long shelf life, and since biodiesel is prone to auto‐oxidation (it contains high oxygen content that helps in reducing pollution due to complete fuel combustion), such additives are essential for storage. Such processes generally increase the cost of available fuel, which has made it necessary to consider these hurdles that are yet to be overcome before the complete utilization of biodiesel is feasible as an environment‐friendly and affordable alternative to petrodiesel.
Editors: Samuel Lalthazuala Rokhum, Gopinath Halder, Kanokwan Ngaosuwan, Suttichai Assabumrungrat
1 Advances in Production of Biodiesel from Vegetable Oils and Animal Fats
Umer Rashid and Balkis Hazmi
Institute of Nanoscience and Nanotechnology (ION2), Universiti Putra Malaysia, Serdang, Selangor, Malaysia
1.1 Introduction
Currently, the energy requirements of the world are mainly met through fossil fuel resources, such as gasoline, petroleum‐based diesel, and natural gas. Such fossil‐derived resources are too limited to fulfill the future energy demands and meet the challenges of rapid human population growth coupled with technological developments [1]. Presently, research is progressively more directed toward exploration of alternative renewable fuels. Several types of biofuels, such as vegetable oil/animal fat (raw, processed, or used), methyl esters from vegetable oil/animal fat, and ethanol or liquid fuels from biomass (bioethanol and biomethanol), have been investigated as a replacement for gasoline and petrodiesel [2].
At present over 197.97 million metric tons of 10 major vegetable oils are produced worldwide [3]. Vegetable oils are commonly derived from various oilseed crops. In a vegetable oil, almost 90–95% is glycerides, which are basically esters of glycerol and fatty acids (FAs) [4]. The vegetable oils can be considered as a feasible alternative for diesel fuel as the heating value of vegetable oils is comparable to that of diesel fuel [5, 6]. However, the uses of vegetable oils in direct injection diesel engines are restricted due to some unfavorable physical properties, particularly the viscosity. The viscosity of vegetable oil is roughly 10 times higher than the diesel fuel. Therefore, the use of vegetable oil in direct injection diesel engines creates poor fuel atomization, incomplete combustion, and carbon deposition on the injector [7, 8].
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].
