GLYCEROL CHEMICAL PROPERTIES BASIC INFORMATION

WHAT ARE GLYCEROL?


Glycerol (glycerin, melting point: 18oC, boiling point: 290oC, density: 1.2620, flash point: 177oC) is a clear, nearly colorless liquid having a sweet taste but no odor.

Glycerol may be produced by a number of different methods, such as:

1. The saponification of glycerides (oils and fats) to produce soap.
2. The recovery of glycerin from the hydrolysis, or splitting, of fats and oils to produce fatty acids.
3. The chlorination and hydrolysis of propylene and other reactions from petrochemical hydrocarbons.

Natural glycerol is produced as a coproduct of the direct hydrolysis of triglycerides from natural fats and oils in large continuous reactors at elevated temperatures and pressures with a catalyst. Water flows countercurrent to the fatty acid and extracts glycerol from the fatty phase.

The sweet water from the hydrolyzer column contains about 12% glycerol. Evaporation of the sweet water from the hydrolyzer is a much easier operation than with evaporation of spent soap lye glycerin in the kettle process.

The high salt content of soap lye glycerin requires frequent soap removal from the evaporators. Hydrolyzer glycerin contains practically no salt and is readily concentrated.

The sweet water is fed to a triple-effect evaporator where the concentration is increased from 12% to 75 to 80% glycerol. After concentration of the sweet water to hydrolyzer crude, the crude is settled for 48 hours at elevated temperatures to reduce fatty impurities that could interfere with subsequent processing. Settled hydrolyzer crude contains approximately 78% glycerol and 22% water.

The settled crude is distilled under vacuum at approximately 200oC. A small amount of caustic is usually added to the still feed to saponify fatty impurities and reduce the possibility of codistillation with the glycerol.

The distilled glycerin is condensed in three stages at decreasing temperatures. The first stage yields the purest glycerin, usually 99% glycerol and lower-quality grades of glycerin are collected in the second and third condensers. Final purification of glycerin is accomplished by carbon bleaching, followed by filtration or ion exchange.

There are several synthetic methods for the manufacture of glycerol. One process involves chlorination of propylene at 510oC (950oF) to produce allyl chloride in seconds in amounts greater than 85 percent of theory (based on the propylene). Vinyl chloride, some disubstituted olefins, and some 1,2 and 1,3-dichloropropanes are also formed.

Treatment of the allyl chloride with hypochlorous acid at 38oC (100oF) produces glycerin dichlorohydrin (CH2ClCHClCH2OH), which can be hydrolyzed by caustic soda in a 6% Na2CO3 solution at 96oC. The glycerin dichlorohydrin can be hydrolyzed directly to glycerin, but this takes two molecules of caustic soda; hence a more economical procedure is to react with the cheaper calcium hydroxide, taking off the epichlorohydrin as an overhead in a stripping column. The epichlorohydrin is easily hydrated to monochlorohydrin and then hydrated to glycerin with caustic soda.

CH3CH=CH2 + C12 → CH2ClCH=CH2 + HCl
CH2ClCH=CH2 + HOCl → CH2ClCHClCH2OH
CH2ClCHClCH2OH + 2NaOH → CH2OHCHOHCH2OH + 2NaCl