Biodiesel production transesterification reaction

In this communication a study of the catalytic behavior of the immobilized Rhizomucor miehei lipase in the transesterification reaction to biodiesel production has been reported. The main drawbacks associated to the current biodiesel production by basic homogeneous catalysis could be overcome by using immobilized lipases. Immobilization by adsorption and entrapment have been used as methods to prepare the heterogeneous biocatalyst. Zeolites and related materials have been used as inorganic lipase supports. To promote the enzyme adsorption, the surface of the supports have been functionalized by synthesis procedures or by post-treatments. While, the enzyme entrapping procedure has been carried out by sol-gel method in order to obtain the biocatalyst protected by a mesoporous matrix and to reduce its leaching after several catalytic uses. 

Figure 7 depicts a simplified block flow diagram (BFD) for a typical biodiesel production process using base catalysis. In the first step, methanol and catalyst (NaOH) are mixed with the aim to create the active methoxide ions (Figure 4, step 1(b)). Then, the oil and the methanol-catalyst solution are transferred to the main reactor where the transesterification reaction occurs. Once the reaction has finished, two distinct phases are formed with the less dense (top) phase containing the ester products and unreacted oil as well as some residual methanol, glycerol, and catalyst. The denser (bottom) layer is mainly composed of glycerin and methanol, but ester residues as well as most of the catalyst, water, and soap can also be found in this layer. 

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). 

Generally, alkali-catalyzed transesterification is performed near the boiling point of the alcohol, but several researchers have reported high conversion yield at room temperature (8,14). Low reaction temperature was desirable, since reaction temperature was closely related to the energy cost of the biodiesel production process. 

Reaction temperature and time were significant operating parameters, which are closely related to the energy costs, of the biodiesel production process. Figure 7 shows the effect of reaction time on the transesterification of rapeseed oil at a catalyst concentration of 1%, molar ratio of 1 6, and 60°C. Within 5 min, the reaction was rapid. Rapeseed oil was converted to above 85% within 5 min and reached equilibrium state after about 10 min. Several researchers reported that the conversion of vegetable oils to FAME was achieved above 80% within 5 min with a sufficient molar ratio (8,11). For a reaction time of 60 min, linoleic acid methyl ester was produced at a low conversion rate, whereas oleic and linolenic methyl ester were rapidly produced. 

FAME production of rapeseed oil by alkali-catalyzed transesterification reaction was investigated. To obtain optimum conversion yield, anhydrous methanol and rapeseed oil with a free fatty acid content of <0.5% were used. The optimum conditions for alkali-catalyzed transesterification using KOH were determined as follows an oil to methanol molar ratio of 1 8 to 1 10 KOH, 1.0% (w/w) on the basis of oil weight, as catalyst a reaction temperature of 60°C and reaction time of 30 min. At these conditions, the FAME conversion yield was approx above 98%. From the refined FAME product (biodiesel), the FAME purity was obtained above 99% through posttreatment such as washing and centrifugation. 

Biodiesel was prepared in various supercritical alcohol treatments with methanol, ethanol, 1-propanol, 1-butanol, or 1-octanol to study transesterification of rapeseed oil and alkyl esterification of fatty acid at temperatures of 300 and 350°C. The results showed that in transesterification, the reactivity was greatly correlated to the alcohol the longer the alkyl chain of alcohol, the longer the reaction treatment. In alkyl esterification of fatty acids, the conversion did not depend on the alcohol type because they had a similar reactivity. Therefore, the selection of alcohol in biodiesel production should be based on consideration of its performance of properties and economics. 

Fatty acids are obtained by fat splitting using water (hydrolysis), methanol (metha-nolysis), and base (saponification) of amines (aminolysis). Splitting with water or methanol can be considered transesterification because glycerol is liberated. The methanolysis is the reaction taking place in biodiesel production as the resulting product is called fatty acid methyl ester. 

Li, L., Du, W., Liu, D., Wang, L., and Li, Z. 2006. Lipase-catalyzed transesterification of rapeseed oils for biodiesel production with a novel organic solvent as the reaction medium. /. Mol. Catal. B Enzym., 43, 58-62. 

Waste oils, from restaurants and household disposals and being creating serious problems of environmental control and food safety, have been considered as good raw material for biodiesel production. Immobilized Candida antarctica lipase was found to be effective for the methanolysis of waste oil. A three-step methanolysis protocol could be used to protect lipase from inactivation by methanol. Compared with one-step reaction, it needs a longer time to reach the reaction equilibrium. So, efforts should be made to increase enzymatic reaction rate. Reports on the enhancement of the activity of certain enzymes by applying ultrasonic irradiation on the enzymes led us to investigate its effects on the enzymatic transesterification of waste oil to biodiesel in a solvent free system. 

Du et al. (6) have recently reported that methyl acetate was an effective acyl acceptor for biodiesel production. To the best of our knowledge, the biodiesel production from waste edible oil with methyl acetate as the acyl acceptor has not yet been reported. Therefore, the transesterification of different kinds of waste edible oil to biodiesel with methyl acetate in a solvent free system was explored in this paper and the major influential factor on the reaction was further investigated. 

A comparative study of biodiesel production with WDO-2 using three-step methanolysis and one-step transesterification with methyl acetate was also conducted here. As can be seen in Figure 5, the ME yield of the three-step methanolysis after reaction for 72 h was 69.1%, which was much lower than those of the one-step transesterification with or without addition of organic base depicted in Figure 4, suggesting that the enzymatic transesterification with methyl acetate was more effective than the enzymatic methanolysis in solvent free system for biodiesel production. 

The alkali process for biodiesel production can achieve high purity and yield of biodiesel in a short time. However, vegetable oils high in free fatty acids result in the production of soap and the loss of catalyst in the alkali process. To overcome this, the free tatty acids should be removed before the transesterification reaction. Because a homogeneous acid catalyst like sulfuric acid can not be recovered and is toxic, a heterogeneous acid catalyst can be used for the esterification of free fatty acids. Solid catalysts can be easily recovered after the reaction and reused [10-12]. 

In this study, we have attempted to evaluate the efficacy of a technique for the production of the methyl ester of rapeseed oil via enzyme-catalyzed transesterifications using tert-butanol, a moderately polar organic solvent. We conducted experiments involving the alteration of several reaction conditions, including reaction temperature, methanol/oil molar ratio, enzyme amount, water content, and reaction time. The selected conditions for biodiesel production were as follows reaction temperature 40 °C, Novozym 435 5% (w/w), methanol/oil molar ratio 3 1, water content 1% (w/w), and 24h of reaction time. Under these reaction conditions, a conversion of approximately 76.1% was achieved. Further studies are currently underway to determine a method by which the cost of fatty acid methyl ester production might be lowered, via the development of enzyme-catalyzed methanolysis protocols involving a continuous bioprocess. 

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