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recirculating packed column reactor using 1M BS‐30 as the stationary phase and ethanol and restaurant grease as the substrates, without solvent addition. The bioreactor was operated at temperatures (40–60 °C), flow rates (5–50 ml min⁻¹), and times (8–48 h) to optimize ester production. Under optimum operating conditions (flow rate, 30 ml min⁻¹; temperature, 50 °C; mole ratio of substrates, 4 : 1, ethanol:grease; reaction time, 48 h), the ester yields were >96%.

      Nasaruddin et al. devised a two‐step enzymatic protocol for the conversion of acid oils, a mixture of FFA and partial glycerides obtained after acid dilution of soap stock, to fatty esters. In the first step, the lipids in the acid oil were hydrolyzed using Caulerpa cylindracea lipase. In the second step, the high acid oils were esterified to short‐ and long‐chain esters using an immobilized Mucor miehei lipase [155].

Another important aspect of lipase‐catalyzed transesterifications is whether or not to use an organic lipophilic solvent. In general, alcoholysis with long‐chain or branched alcohols proceeds efficiently even in a solvent‐free medium, whereas solvent‐free methanolysis tends to give low ester yields. This may be attributed to the poor solubility of methanol in fats and oils [152]. Depending on the type of lipase used, various solvents for fatty material and methanol have been suggested, including petroleum ether [156], hexane [147], isooctane [142], commercial fossil diesel fuel [157], 1,4‐dioxan [158], and supercritical carbon dioxide [159]. From an economic viewpoint, however, the use of organic solvents is hardly useful [137], even more so as these have to be removed from the ester product by evaporation. Moreover, the toxicity and inflammability of organic solvents is also another issue to be considered. As a consequence, considerable effort has been directed toward conducting lipase‐catalyzed methanolysis reactions in a solvent‐free medium.

      Zhang et al. reported regenerating enzyme preparations by using them with 2‐butanol or tert‐butanol [154], which proved successful for mobilized C. antarctica lipase. A recommendation for further treatment with 1‐propanol for immobilized Thermomyces iamgmosa lipase [160]. If the enzyme chosen for transesterification turns out to be particularly sensitive to glycerol released by ester formation, it might make sense to use methyl acetate instead of methanol [161]. The authors claim that triacetylglycerol, which is produced instead of glycerol in this process, has no negative effects on the enzyme activity of immobilized C. antarctica lipase and does not affect the quality of the resulting fuel either.

      Finally, BD producers can choose between several methods of preventing enzyme inactivation, which is a phenomenon frequently reported for lipase‐catalyzed methanolysis. Enzymes are easily inactivated by compounds contained in the oil or fat. Quayson et al. found that phospholipids present in crude soybean oil efficiently inhibit methanolysis, as these bind to the immobilized enzyme and interfere in the interaction of lipase and substrate [162]. They concluded that for enzymatic methanolysis, vegetable oils have to be degummed. The enzyme‐catalyzed reactions have the following disadvantages: (i) lose some initial activity due to volume of the oil molecule; (ii) number of support enzyme is not uniform; (iii) biocatalyst is more expensive than the natural enzyme; (iv) inactivation by acyl acceptors, such as methanol, and inactivation by minor components in the crude oil and waste oils; and (v) desorption from immobilization support and fouling in packed bed bioreactors. Due to said disadvantages, the enzymatic catalyzed transesterification reactions are not in common practice for commercial scale BD production.

1.9 Fuel Properties and Quality Specifications for Biodiesel

While BD is produced in reasonably differently scaled plants from vegetable oils of varying origin and quality, it was essential to install a standardization of fuel quality for assurance of engine performance exclusive of any difficulties. Austria was the first country in the world to define and approve the standards for rapeseed oil methyl esters as diesel fuel. As standardization is a prerequisite for successful market introduction and penetration of BD, standards or guidelines for the quality of BD have also been defined in other countries like Germany, Italy, France, the Czech Republic, and in the United States.

      Quality standards are prerequisites for the commercial use of any fuel product. They serve as guidelines for the production process, guarantee customers that they are buying high quality fuels, and provide authorities with approved tools for the assessment of safety risks and environmental pollution [163]. Moreover, engine and automobile manufacturers rely on fuel standards for releasing warranties for their vehicles to be operated on BD.

In 1997, the European Committee for Standardization was mandated to develop a uniform standard for FAME fuels and come up with respective measurement procedures. The resulting standard EN 14214, which has come into picture in 2004, is valid for all member states of the European Union and thus replaces the respective national specifications. Apart from Australia and the United States of America, which have already drawn up BD quality norms, a number of countries worldwide (e.g. Brazil, Canada, Japan) are currently working on their introduction, having released drafts or preliminary specifications. Then, ASTM D 6751 and EN 14214 conditions as well as their analysis methods for BD are illustrated in Table 1.3.

      The constraints that are utilized to describe the quality of BD can be divided in two groups [164]. One of them is also used for mineral diesel, and the second describes the composition and purity of fatty esters. The former includes, for example, density, viscosity, flash point, sulfur percentage, Conradson carbon residue, sulfate ash percentage, cetane number, and acid number. The latter comprises, for example, methanol, free glycerol, total glycerol, phosphorus contents, water and esters content, and other properties described in Table 1.3. Thus, ASTM D 6751 and EN 14214 specifications methods for BD are illustrated in Table 1.3.

1.10 Conclusion

      Currently, the uses of BD as an eco‐friendly alternative to petrodiesel are gaining much recognition. The production of BD from nonconventional oils may simultaneously reduce dependence on imported fossil fuels and help alleviate the food versus fuel dilemma that plagues rapeseed, soybean, palm, and other oilseed crops that are also traditional oil sources. As a result of development of local BD industry and market, opportunities would be raised for the farmers to grow new oilseed crops and increase production of traditional and nonconventional oils, generating profit and income for all the stakeholders. Moreover, the establishment of local BD industry not only will generate opportunities for employment and personnel training but also might help reduce the dependence on imported petroleum and fuel derived from it, which continues to decrease in availability and affordability. More research and advancements in BD technology coupled with large‐scale cultivation of oilseed crops, especially the nontraditional crops, additional subsidies, and the relevant technological sector, may lead to further reduction of the cost of this renewable fuel. Furthermore, there is a real need to appraise the environmental benefits of producing BD and to consider such attributes while determining the cost incurred in the production of such green fuels.

Table 1.3 Biodiesel specifications according to ASTM D6751 and EN 14214 standards.

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Property	ASTM D 6751	EN 14214
Test method	Limits	Test method	Limits
Density (15 °C)	—	—	EN ISO 3675	860–900 kg m−3
Kinematic viscosity (40 °C)	ASTM D 445	1.9–6.0 mm2 s−1