Renewable Sources of Glycerol

Glycerol is present in animal fats and oils in the form of triglycerides. These triglycerides consist of three fatty acids linked to three hydroxyl groups of glycerol through an ester bond. In most industrial applications, glycerol is recovered from the triglyceride molecules by breaking the ester bonds through various chemical or biological processes. Fatty acids present in triglycerides are directly converted into another material such as soap or biodiesel by saponification or transesterification, respectively (Figure 6.1; Yazdani and Gonzalez 2007). Glycerol is obtained from these processes as crude solution with various contaminants.

Biofuels from Agricultural Wastes and Byproducts Edited by Hans P. Blaschek, Thaddeus C. Ezeji and Ju rgen Scheffran 97 © 2010 Blackwell Publishing. ISBN: 978-0-813-80252-7

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Biodiesel industries are the major sources of crude glycerol accumulation. During biodiesel production, fatty acid molecules are detached from the glycerol moiety of triglyceride by the action of a catalyst, and alcohol (methanol or ethanol) binds in its place through transesteri­fication reaction (Figure 6.2). The catalysts commonly used for this purpose are sodium and potassium hydroxide. Reaction could take place at room temperature, albeit slowly. Heating triglyceride at 55°C helps completion of the reaction in 1-3 hours. The mixture is kept at room temperature for 1-2 days for the separation of glycerol and biodiesel to occur. Glycerol is heavier than ester and settles at the bottom of the container (Figure 6.3). For every 100 lb of biodiesel produced by the transesterification of triglycerides, 101b of crude glycerol is generated.

Pure glycerol is primarily used in the production of various foods, beverages, pharmaceu­ticals, cosmetics, and so on. The cost of purifying crude glycerol is high and uneconomical. Pure glycerol is currently sold at a relatively high price, which is in the range of $0.60-$0.90/ lb. The high cost associated with the purification of crude glycerol makes it unattractive for biodiesel companies to invest in the purification process. The number of companies producing biodiesel has grown since the early 1990s due to increased demand for biodiesel fuel. This has led to a huge glut of crude glycerol. Crude glycerol used to be a desired coproduct that

Подпись: Triglyceride

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Подпись: (C'
Подпись: Methanol
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Подпись: R
Подпись: Catalyst Base
Подпись: +
Подпись: OH
Подпись: OH

Glycerol Biodiesel

Figure 6.2. The production of biodiesel. The alcohol reacts with the fatty acid component of triglyc­eride (fat or oil) in the presence of a catalyst such as NaOH, KOH, H2 SO4, and so on to form alkyl ester (biodiesel) and glycerol.

Biodiesel

Glycerol

Figure 6.3. Biodiesel stored in a bottle upon completion of transesterification reaction. Glycerol is heavier than biodiesel and settles at the bottom of the flask.

contributes to the economics of biodiesel production but has now become an unwanted waste stream with disposal cost associated with it.

Companies that produce bioethanol are another well-known source of crude glycerol accu­mulation (Figure 6.12 . During sugar fermentation to ethanol, yeast produces a substantial amount of glycerol in response to external osmotic stress (Figure 6.42 . In a conventional fermentation, 4g of glycerol is generated for every 48 g of ethanol produced and 100 g of reducing sugar consumed (http://www. freepatentsonline. com/5177008.html). There are more than 170 ethanol production facilities in the United States with a total annual capacity of more than 10 billion gallons of ethanol (http://www. ethanol. org/index. php? id=77), resulting in several million gallons of crude glycerol production annually.

The surplus glycerol generated by the biodiesel and bioethanol industries will not only reduce the price of crude glycerol, but its disposal will also become a major issue. The

development of processes that can convert crude glycerol into high-value products is expected to make biofuels economically viable.