It mainly comes from the by-products of soap production and oil cracking: before 1948, glycerol was completely recovered from the by-products of animal and vegetable oil soap production. Up to now, natural oils and fats are still the main raw materials for glycerol production. About 42% of natural glycerol comes from by-products of soap making and 58% from fatty acid production.
Because this method takes natural oil as raw material and glycerol is a by-product, Chinese chemical workers envisage that it can be used in waste water treatment and utilization of oil foot, which not only plays an environmental role, but also has certain economic effects. For example, Ge Wenguang recovered glycerin from the waste water produced by the production of fatty acids from cottonseed oil. Feng Liwen of Anhui Institute of Applied Technology has published technical papers on the production of oleic acid, glycerin, stearic acid and poly amide resin from animal and vegetable oils and oil legs. Wang Xisheng of the National Fatty Acid Technology Research and Promotion Center and others introduced the new technology of producing oleic acid, glycerin and stearic acid from hogwash oil and waste animal and vegetable oil.
1.1 Saponified glycerol
The saponification reaction products are divided into two layers: the upper layer is mainly composed of sodium fatty acid salt (soap) and a small amount of glycerol; the lower layer is waste alkali liquor, which is a dilute glycerol solution containing salt and sodium hydroxide, and generally contains 9% – 16% glycerol, especially 8% – 20% organic salt.
At present, the traditional distillation method is mainly used to extract glycerol from the saponification waste liquor, which is clarified and treated to obtain the refined waste liquor, then concentrated to 40% glycerol, after recovering salt, concentrated to 80% glycerol, then vacuum distillation, activated carbon treatment and pressure filtration to obtain glycerol products. This method has the shortcomings of low product quality and high energy consumption, resulting in excess of low-grade glycerol in our country. More than 98% of saponified or medicinal-grade glycerol, especially 99.9% of high-purity glycerol, mainly depends on imports.
1.2 Oil hydrolysis glycerin
Oil glycerin refers to glycerol produced by hydrolysis of oils. The main products of oil hydrolysis are stearic acid, oleic acid and other oil products. Glycerol is a by-product. The glycerol content of glycerol water obtained by hydrolysis of oils and fats is higher than that of soap-making waste liquor, the mass fraction is about 14%-20%, and the mass fraction of inorganic salts is 0%-0.2%. In recent years, continuous high-pressure hydrolysis has been widely used. The reaction does not use catalysts. The glycerol obtained is generally free of inorganic acids. The purification method is simpler than the treatment of waste alkali liquor. The production capacity of glycerol in China has exceeded 10000 tons.
Whether it is the waste liquor of soap making or the glycerol water obtained by hydrolysis of oil and fat, the glycerol content is not high (about 10% in mass), and all kinds of impurities are contained. Therefore, crude glycerol needs to be purified and concentrated first, then distilled, decolorization and deodorization of crude glycerol to obtain natural glycerol.