Glycerol biodiesel transesterification

Fig. 4.3 Routes of glycerol production, (i) Production of biodiesel and glycerol from transesterification reaction (ii) Industrial route for production of glycerol from propene synthesis...

Glycerol is a marketable co-product of the biodiesel transesterification process. Therefore, mass allocation can be Justified. Due to similar prices for biodiesel and glycerol, economic allocation would only yield a slightly changed picture. 

In order to convert the raw oils into useful material, transesterification technology is used. The oil is reacted with a low molecular weight alcohol, commonly methanol, in the presence of a catalyst to form the fatty acid ester and glycerol (Scheme 6.1). The ester is subsequently separated from the glycerol and used as biodiesel, the glycerol being used as a raw material for fine chemicals production. Although the chemistry is simple, in order to make biodiesel commercially viable the process must be... 

Biomass is a renewable resource from which various useful chemicals and fuels can be produced. Glycerol, obtained as a co-product of the transesterification of vegetable oils to produce biodiesel, is a potential building block to be processed in biorefineries (1,2). Attention has been recently paid to the conversion of glycerol to chemicals, such as propanediols (3, 4), acrolein (5, 6), or glyceric acid (7, 8). 

Biodiesel (fatty acid methyl ester (FAME)) production is based on transesterification of vegetable oils and fats through the addition of methanol (or other alcohols) and a catalyst, giving glycerol as a by-product (which can be used for cosmetics, medicines and food). Oil-seed crops include rapeseeds, sunflower seeds, soy beans and palm oil seeds, from which the oil is extracted chemically or mechanically. Biodiesel can be used in 5%-20% blends with conventional diesel, or even in pure form, which requires slight modifications in the vehicle. 

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... 

Current biodiesel can not be considered as a 100% biomass-based fuel as long as methanol is derived from petrochemical resources. A clean way to solve the biorelated problem is the conversion of glycerol waste from the transesterification process into syngas. In this context, glycerol reforming is a suitable target reaction worthy of study. 

Keywords Triacetin, transesterification, glycerol, triglyceride, biodiesel... 

A byproduct of vegetable oil transesterification to make biodiesel fuel, glycerol (GO) has captured our attention for several reasons, in aqueous medium, GO itself can be converted to value-added commodity products such as propylene glycol (PG), lactic acid (LA) and ethylene glycol (EG) in the presence of a metal catalyst at mild conditions (2-7). An array of metals deposited on various supports have been examined as catalysts for the above reaction (6, 7). 

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. 

The components of biodiesel are vegetable oils composed of glycerol esters of fatty acids. In the process of transesterification, the glycerol components of the triglyceride molecules are exchanged for methanol. The products are fatty-acid methyl esters consisting of straight saturated and unsaturated hydrocarbon chains, as described under chemical processes. 

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. 

In the past, glycerol was produced mainly from propene via allyl chloride and epi-chlorohydrin, a process developed by I. G. Farben and in operation since 1943. Today, glycerol is obtained almost completely as a coproduct in oleochemistry (fat splitting) and biodiesel production (transesterification) with 110 kg crude glycerol or 100 kg pure glycerol per ton of biodiesel [37]. With the rise in biodiesel production, the availability increased while the price decreased drastically by approximately 66% within 15 years in the United States [38]. 

Scheme 7.1. Base-catalyzed transesterification of triacylglycerols (TAGs) to produce fatty acid esters (biodiesel). Methyl esters (shown) are the most common but others, such as ethyl esters, can be produced depending on the alcohol used in the reaction. Ri, R2 and R3 represent unique fatty acids attached to the glycerol backbone of the TAG.

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