Glycerol, also known as glycerin, is a simple polyol compound, recognized for being a colorless, odorless, sweet-tasting, viscous liquid. This compound forms the backbone for triglycerides, found in natural fats and oils. It has numerous applications across various industries.
Glycerol from Natural Fats and Oils
Historically, glycerol was a co-product of saponification, the process of converting natural fats and oils into soap. This method reacts triglycerides, which are the main components of animal fats and vegetable oils, with a strong alkali like sodium or potassium hydroxide. The reaction breaks down triglycerides, releasing fatty acid salts (soap) and glycerol.
Another traditional method is hydrolysis, or fat splitting. Here, triglycerides are broken down using water under high pressure and temperature, typically around 200°C, yielding fatty acids and glycerol. While once primary sources, these methods are now practiced on a smaller scale compared to modern routes.
Glycerol from Biodiesel Production
The most significant modern source of glycerol is as a co-product of biodiesel manufacturing. Biodiesel is produced through transesterification, a chemical reaction where vegetable oils or animal fats react with an alcohol, usually methanol, in the presence of a catalyst. This converts triglycerides into fatty acid methyl esters, forming biodiesel, while simultaneously generating crude glycerol.
Approximately 1 kilogram of crude glycerol is formed for every 10 kilograms of biodiesel produced. The substantial increase in global biodiesel production has led to a surplus of natural glycerol. This has made transesterification the dominant method for glycerol production, driven by the rising demand for renewable fuels.
Synthetic Glycerol Production
While natural sources now dominate, glycerol can also be synthesized from petroleum-derived feedstocks, primarily propylene. Synthetic routes involve multi-step chemical processes like chlorination, oxidation, and hydrolysis. One common pathway converts propylene into allyl chloride, which then undergoes further reactions to yield glycerol.
Synthetic production methods were more prevalent in the mid-20th century, peaking in the 1960s and 1970s. These routes are often more expensive than obtaining glycerol as a byproduct from natural processes. However, synthetic methods can produce high-purity grades or ensure supply when natural sources are insufficient.
Refining Glycerol for Use
Crude glycerol from any production method contains various impurities, such as water, salts, methanol, soaps, and unreacted fatty acids. Purification is necessary before glycerol can be used in most applications.
The refining process involves several stages to remove contaminants and achieve desired purity. Initial steps include pre-treatment to remove methanol and water, followed by neutralization to adjust pH and precipitate salts. Further purification involves de-colorization with activated carbon, deodorization, and multiple distillation steps. Filtration also removes solid particles. These processes yield different grades of glycerol, from technical to highly pure pharmaceutical grade, depending on the intended application.