Biodiesel acid-catalyzed processes

The problems caused by high free fatty acid content in low quality oils, which may cause interference in biodiesel production using alkali catalysts, could be overcome using an acid-catalyzed process. However, acid catalysis has shown some inconveniences, such aslower yields than thosereached by alkali catalysis, need of high temperatures and more corrosiveness than other processes (Meher et al., 2006 Akoh et al., 2007). 

Zullaikah, S., Lai, C. C., Vali, S. R., Ju, Y. H. A two-step acid-catalyzed process for the production of biodiesel from rice bran oil. Bioresource Technology 96 (2005) 1889-1896. 

In general, acid catalyzed reactions are performed at high alcohol-to-oil molar ratios, low-to-moderate temperatures and pressures, and high acid catalyst concentrations. Table 9 summarizes reactions conditions proposed by Zhang et al. to prepare biodiesel from waste cooking oil using sulfuric acid as the catalyst. A simplified BFD of the acid process is shown in Figure 9. 

In view of the catalytic ability of lipase, the production of biodiesel by direct esterification of fatty acids catalyzed using this enzyme can be utilized as an interesting alternative to decrease the operating costs associated with the conventional process, as well as to overcome the above-mentioned problems. In addition, the use of fatty acid mixtures is possible in this process, usually obtained from vegetable oil refinement with lower cost than triacylglycerides (Vieira et al., 2006). 

Figure 9. A simplified block flow diagram of the heterogeneous acid solid-catalyzed process for the production of biodiesel.

Biodiesel is an alternative to petroleum-based diesel fuel and is most often produced from vegetable oils via an acid- or base-catalyzed process. To be viable, in recent years, a considerable attention has been focused on developing environmentally benign and economically feasible processes to produce alkyl fatty acid methyl esters. Such a process requires to reduce the free fatty acid content to an acceptable level (> 1 wt%) and the ttansesterification of glycerides to fatty acid methyl esters. 

In terms of biodiesel conversion processes, chemical conversion using alkali and acid-based catalysts is stiU the most favorite approach. Various investigations have been carried out to develop novel catalysts and/or novel processes for efficient conversion of TAG to FAME. This part was reviewed in the chapter Production of biodiesel via catalytic upgrading refining of sustainable oleageneous feedstocks. The chapter Biochemical catalytic production of biodiesel introduced a promising alternative way of biodiesel production via enzyme-catalyzed processes. Recentiy, microalgae... 

Biodiesel is a mixture of methyl esters of fatty acids and is produced from vegetable oils by transesterification with methanol (Fig. 10.1). For every three moles of methyl esters one mole of glycerol is produced as a by-product, which is roughly 10 wt.% of the total product. Transesterification is usually catalyzed with base catalysts but there are also processes with acid catalysts. The base catalysts are the hydroxides and alkoxides of alkaline and alkaline earth metals. The acid catalysts are hydrochloride, sulfuric or sulfonic acid. Some metal-based catalysts can also be exploited, such as titanium alcoholates or oxides of tin, magnesium and zinc. All these catalyst acts as homogeneous catalysts and need to be removed from the product [16, 17]. The advantages of biodiesel as fuel are transportability, heat content (80% of diesel fuel), ready availability and renewability. The... 

The transesterification of triglycerides with methanol is a simple and straightforward process. It is commercially practiced worldwide in the production of FAMEs, which have become popular as a replacement for diesel known as biodiesel . The process consists of three separate equilibrium reactions that can be catalyzed by both acids and bases. (4) The overall process is described in Figure 3. Phase separation of the glycerin is the predominant driving force for this process. 

In homogeneous catalysis, the catalyst is in the same phase as the reactants and products. Here we will concentrate on homogeneous catalysis in the liquid phase. In the classic case, the reactant (also called the substrate) molecules and the catalyst are reacted in a solvent. For example, the transesterification of fatty acid triglycerides with methanol (Figure 1.10) is catalyzed by hydroxide (OH-) ions. This is an important process for making fatty acid methyl esters which are then used as biodiesel. 

Several processes for the production of biodiesel fuel have been developed by acid-, alkali-, and enzyme-catalyzed transesterification reactions (7-10). Transesterification, called alcoholysis, is the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis. Transesterification is represented by a number of consecutive and reversible reactions. The reaction step is the conversion of triglycerides to diglycerides, followed by the conversion of diglycerides to monoglycerides and of monoglycerides to glyceride at each step (11,12). 

Biodiesel is produced on the industrial scale by methanolysis of vegetable oUs (usually rape or soybean) or waste fat, particularly using frying oils. Methanolysis proceeds with modest amounts of base catalyst, provided the levels of free fatty acid and water in the oil are low (24, 25). The fatty acid content may be reduced by physical or chemical treatment before methanolysis but for waste fats, alternative processes that do not use base catalysis may be preferred. Lipase catalyzed methanolysis is less sensitive to fatty acid and water in the oil and has been tested in batch (26) and fixed-bed reactor (27) conversion of waste oil and grease to biodiesel. 

Abstract Biodiesel is a fatly acid alkyl ester that can be derived fiom any v etable oil or animal fat via the process of transesterification. It is a renewable, biodegradable, and nontoxic fuel. In this paper, we have evaluated the efficacy of a transesterification process for rapeseed oil with methanol in the presence of an enzyme and tert-butanol, which is added to ameliorate the negative effects associated with excess methanol. The application of Novozym 435 was determined to catalyze the tiansesterification process, and a conversion of 76.1% was achieved under selected conditions (reaction temperature 40 °C, methanol/oil molar ratio 3 1, 5% (w/w) Novozym 435 based on the oil weight, water content 1% (w/w), and reaction time of 24h). It has also been determined that rapeseed oil can be converted to fatty acid methyl ester using this system, and the results of this study contribute to the body of basic data relevant to the development of continuous enzymatic processes. 

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