The Current Process of Biodiesel Production

Recently (Chhetri and Islam 2008b) detailed the process involved in biodiesel production. Conventionally, biodiesel is produced either in a single-stage or a double-stage batch process or by a continuous flow type transesterification process. These are either acid catalyzed or base catalyzed processes. The acids generally used are sulfonic acid and sulfuric acid. These acids give very high yields in alkyl esters, but these reactions are slow, requiring high temperatures above 100°C and more than three hours to complete the conversion (Schuchardt 1998). Alkali catalyzed transesterification is much faster than acid catalyzed transesterification, and all commercial biodiesel producers prefer to use this process (Ma and Hanna 1999). The alkalis generally used in the process include NaOH, KOH, and sodium methoxide. For an alkali-catalyzed transesterification, the glycerides and alcohol must be anhydrous because water changes the reaction, causing saponification. The soap lowers the yield of esters and makes the separation of biodiesel and glycerin complicated.

No catalysts are used in the case of supercritical methanol methods, where a methanol and oil mixture is superheated to more than 350°C, and the reaction completes in 3-5 minutes to form esters and glycerol. Saka and Kudsiana (2001) carried out a series of experiments to study the effects of the reaction temperature, pressure, and molar ratio of methanol to glycosides in methyl ester formation. Their results revealed that supercritical treatment of 350°C, 30 MPa, and 240 seconds with a molar ratio of 42 in methanol is the best condition for transesterification of rapeseed oil for biodiesel production. However, the use of methanol from fossil fuel still makes the biodiesel production process unsustainable and produces toxic by-products. Since this process uses high heat as catalysts, then the use of electricity to heat the reactants will increase the fossil fuel input in the system. The direct heating of waste cooking oil by solar energy using concentrators will help to reduce the fossil fuel consumption, and the process will be a sustainable option.

The major objectives of transesterification are to breakdown the long chain of fatty acid molecules into simple molecules and reduce the viscosity considerably in order to increase the lubricity of the fuel. The transesterification process is the reaction of a triglyceride (fat/oil) with an alcohol, using a catalyst to form esters and glycerol. A triglyceride has a glycerin molecule as its base with three long chain fatty acids attached. During the transesterification process, the triglyceride is broken down with alcohol in the presence of a catalyst, which is usually a strong alkaline like sodium hydroxide. The alcohol reacts with the fatty acids to form the mono-alkyl ester, or biodiesel and crude glycerol. In most production, methanol or ethanol is the alcohol used, and it is base catalysed by either potassium or sodium hydroxide. After the completion of the transesterification reaction, the glycerin and biodiesel are separated (gravity separation). The glycerin is either re-used as feedstock for methane production or refined and used in pharmaceutical products. A typical transesterification process is given in Figure 16.1.

CHj—OCOR3 CH2OH R3COOCH3

Triglyceride Methanol Glycerol Methyl ester;

Figure 16.1 Typical equation for transesterification, where R1, R2, R1 are the different hydrocarbon chains.

Interestingly, the current biodiesel production uses fossil fuel at various stages such as agriculture, crushing, transportation, and the process itself (Carraretto et al. 2004). Figure 16.2 shows the share of energy use at different stages from farming to biodiesel production. Approximately 35% of the primary energy is consumed during the life cycle from agriculture farming to biodiesel production. This energy basically comes from fossil fuel. To make the biodiesel completely green, this portion of energy also has to be derived from renewable sources. For energy conversion and crushing, direct solar energy can be effectively used while renewable biofuels can be used for transportation and agriculture.

16.2.3.1 Pathways of Petrodiesel and Biodiesel

Figure 16.3 below shows the pathways of additives for the production of conventional petrodiesel and biodiesel. Highly toxic catalysts, chemicals, and excessive heat are subjected to crude oil in the oil refineries. Chemicals such sulfuric acid (H2S04), hydrofluoric acid (HF), aluminium chloride (A1CI,), aluminium oxide (A120,), and

Figure 16.3 below shows the pathways of additives for the production of conventional petrodiesel and biodiesel. Highly toxic catalysts, chemicals, and excessive heat are subjected to crude oil in the oil refineries. Chemicals such sulfuric acid (H2S04), hydrofluoric acid (HF), aluminium chloride (A1CI,), aluminium oxide (A120,), and

Transport,

Agriculture,

Crushing,

Energy required at various stages in biodiesel production

Conversion, 7%

Energy required at various stages in biodiesel production

Figure 16.2 Share of energy at different stages of biodiesel production.

Crushing,

Transport,

Agriculture,

catalysts such as platinum and high heat are applied for oil refining. These all are highly toxic chemicals and catalysts. The crude oil that is originally non-toxic, yields a more toxic product after refining. Conventional biodiesel production follows the similar pathway. Methanol (CH3OH) made from natural gas, a highly toxic chemical that kills receptor nerves even without feeling pan, is used for the alcoholysis of vegetable oil. Sulfuric acid or hydroxides of potassium or sodium, which are highly toxic and caustic, are added to the natural vegetable oils.

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