Читать книгу Biodiesel Production - Группа авторов - Страница 21

The direct use of a vegetable oil in diesel engines is problematic because of its high viscosity (about 11–17 times higher than petrodiesel fuel), which reduces the fuel atomization leading to high engine deposits, thickening of lubricating oil, and lower volatilities that cause the formation of deposits in engines due to incomplete combustion [78, 79]. The extremely high flash points of vegetable oils and their tendency for thermal or oxidative polymerization aggravate the situation, leading to the formation of deposits on the injector nozzles, a gradual dilution and degradation of the lubricating oil, and the sticking of piston rings. As a consequence, long‐term operation on neat plant oils or on mixtures of plant oils and fossil diesel fuel inevitably results in engine breakdown [80].

Chemically, the vegetable oils/animal fats consist of triglyceride molecules of three long‐chain FAs that are ester bonded to a single glycerol molecule. These FAs differ by the length of carbon chains and by the number, orientation, and position of double bonds in these chains. Thus, BD refers to alkyl esters of long‐chain FAs, which are synthesized either by transesterification with alcohols or by esterification of FAs. The latter strategy aimed at modifying plant oils by various technologies to produce fuels that approximate the properties and performance of fossil diesel. Four methods to decrease the high viscosity of vegetable oils to enable their use in common diesel engines without operational problems such as engine deposits have been investigated: blending with petrodiesel, pyrolysis, microemulsification, and transesterification [81]. Transesterification is by far the most common method that leads to the products commonly known as BD, which are alkyl esters of vegetable oils or animal fats.

Pyrolysis denotes thermal decomposition reactions, usually brought about in the absence of oxygen. Pyrolysis of vegetable and fish oils, optionally in the presence of metallic salts as catalysts, was conducted as a means of producing emergency fuels during the Second World War, as various Chinese, Japanese, or Brazilian publications show [13]. In addition, the technology has occasionally found entry into the more recent literature as well [82, 83]. This treatment results in a mixture of alkanes, alkenes, alkadienes, aromatics, and carboxylic acids, which are similar to hydrocarbon‐based diesel fuels in many respects. The cetane number of plant oils is increased by pyrolysis, and the concentrations of sulfur, water, and sediment for the resulting products are acceptable. However, according to modern standards, the viscosity of the fuels is considered as too high, ash and carbon residue far exceed the values for fossil diesel, and the cold flow properties of pyrolyzed vegetable oils are poor [81]. Moreover, it is argued that the removal of oxygen during thermal decomposition eliminates one of the main ecological benefits of oxygenated fuels, namely, more complete combustion due to higher oxygen availability in the combustion chamber [22].

Microemulsification is the formation of thermodynamically stable dispersions of two usually not miscible liquids, brought about by one or more surfactants. Drop diameters in microemulsions typically range from 100 to 1000 Å [84]. Various investigators have studied the microemulsification of vegetable oils with methanol, ethanol, or 1‐butanol [85, 86]. They arrived at the conclusion that microemulsions of vegetable oils and alcohols cannot be recommended for long‐term use in diesel engines for similar reasons as applicable to neat vegetable oils. Moreover, microemulsions display considerably lower volumetric heating values as compared to hydrocarbon‐based diesel fuel due to their high alcohol contents [84], and these have also been assessed insufficient in terms of cetane number and cold temperature behavior [87].

Transesterification with lower alcohols, however, has emerged to be an ideal modification, so that the term “biodiesel” is now only used to denote products obtained by this process. The reaction between triglycerides and lower alcohols, yielding free glycerol and the FA esters of the respective alcohol, was first described in 1852 [88]. In the 1930s and 1940s, this reaction was frequently applied in the fat and soap industry. The Belgian patent on the production of palm oil ethyl esters by acid‐catalyzed transesterification describes the first use of a fuel, which would now be referred to as “biodiesel” [7].

Transesterification is one of the reversible reactions and proceeds essentially by mixing the animal fat/vegetable oil with an alcohol (usually methanol). However, the presence of a catalyst (usually a base) accelerates the conversion to produce the corresponding alkyl esters (or for methanol, the methyl esters) of the FA mixture that is found in the parent vegetable oil or animal fat [79]. The general scheme of the transesterification reaction is given in Figure 1.1.

The production of BD by transesterification has been the focus of many research studies. Several reviews on the production of BD by transesterification have been published [7189–92]. Usually, transesterification can proceed by base or acid catalysis. However, in homogeneous catalysis, alkali catalysis (sodium or potassium hydroxide; or the corresponding alkoxides) is a much more rapid process than acid catalysis [92, 93].