The industrial production of glycerin can be divided into two main categories:
- Natural glycerin, which is from natural oils and fats.
- The synthesis of glycerin, which is from propylene.
The production of natural glycerin
glycerin was recovered from the by-products from soap made from animal and vegetable fat before 1984. Up to now, natural oils and fats are still the main raw materials for glycerin production. About 42% of the natural glycerol in the base is the by-product of self-made soap, and 58% is from fatty acid production. Saponification of oils and fats in soap making industry. The product of saponification reaction is divided into two layers: the upper layer is mainly containing sodium fatty acid salt (soap) and a small amount of glycerol, the lower layer is waste alkali solution, containing salt, sodium hydroxide glycerol dilute solution, generally containing 9-16% glycerol, inorganic salt 8-20%. Grease reaction.
The glycerin content of glycerin water (also called sweet water) obtained by hydrolysis of oil is about 14-20% higher than that of waste soap liquor, and the inorganic salt is 0-0.2%. In recent years, the continuous high-pressure hydrolysis method has been widely used, the reaction does not use catalysts, the resulting sweet water generally does not contain inorganic acids, purification method is simpler than the waste lye. The glycerin content in the waste liquor of soap making or the glycerin water obtained by hydrolysis of oil and fat is not high, and it contains various impurities. The production process of natural glycerin includes purification, concentration to crude glycerin, and distillation, decolorization and deodorization of crude glycerin. This process is detailed in some books and periodicals.
Production of synthetic glycerin
The ways to synthesize glycerin from propylene can be classified into two main categories, chlorination and oxidation. Propylene chlorination and propylene oxidation by acetic acid at irregular intervals are still used in industry currently.
- Propylene chlorination is the most important method of glycerin synthesis, which includes four steps: high temperature chlorination of propylene, Chlorpropyl hypochlorite, saponification of dichloropropanol and hydrolysis of epichlorohydrin. Hydrolysis of epichlorohydrin to glycerin was carried out in 10% hydrogen peroxide and 1% sodium carbonate aqueous solution at 150 ℃ and 1.37 MPa CO_2 pressure. The glycerin solution containing 5-20% sodium chloride was obtained by concentration, desalination and distillation. The purity of glycerol was above 98%.
- Propylene oxidation by acetic acid: Propylene is synthesized from propylene oxide with peracetic acid and isomerization of propylene oxide to propylene. The latter reacts with peracetic acid to form epoxy propanol (glycidyl), which is finally hydrolyzed to glycerol. Peracetic acid production does not need catalyst, acetaldehyde and oxygen phase oxidation, in atmospheric pressure, 150-160 (?) C, contact time 24 s, the conversion of acetaldehyde 11%, peracetic acid selectivity 83%. The above two steps are carried out in a reactive distillation column with special structure. After feeding allyl alcohol and ethyl acetate solution containing peracetic acid into the reactor, the reactor was controlled at 60-70 C and 13-20 kPa. At the top of the tower, ethyl acetate solvent and water were evaporated, and the kettle reached glycerol solution. This method has high selectivity and yield. Peracetic acid is used as oxidant and no catalyst is needed. The reaction speed is faster and the process is simplified. The production of 1t glycerol consumes allyl alcohol 1.001t, peracetic acid 1.184t and by-product acetic acid 0.947t. At present, natural glycerol and synthetic glycerol account for almost 50% of the total yield, while propylene chlorination accounts for about 80% of the total yield of Hezhi glycerol. Natural glycerin accounts for more than 90% of total output in China.
Natural glycerine is obtained primarily as a co-product from the production of fatty acid, fatty ester, or soap from oils and fats. Also a lot is produced in recent years as a byproduct in the making of biodiesel from (recycled) cooking oils. Splitting or hydrolysis of oil, carried out under pressure and high temperature, produced fatty acids and sweetwater. The sweetwater contains 10–20% glycerol. Transesterification of oil with methanol, in the presence of a catalyst, yielded methyl esters and glycerine. Since the process does not use water, the glycerol concentration is higher. Saponification of an oil/fat with caustic soda formed soap and soap lye. The soap lye formed contains 4–20% glycerol and is also known as sweetwater or glycerine. Sweetwater or glycerine obtained as a co-product from the above three processes contains impurities and must be made to undergo further purification.
