The industrial production methods of glycerol can be divided into two categories:
The method of using natural oils and fats as raw materials, the obtained glycerin is commonly called natural glycerin;
In the synthesis method using propylene as a raw material, the obtained glycerin is commonly called synthetic glycerin.
1. Production of Natural Glycerin Prior to 1984, all glycerol was recovered from by-products of animal and vegetable fat soap. Until now, natural oils and fats are still the main raw material for the production of glycerin. About 42% of the natural glycerin in the base is made by the homemade soap by-product, and 58% is derived from the fatty acid production. Saponification of oils and fats in the soap making industry. The saponification reaction product is divided into two layers: the upper layer is mainly composed of fatty acid sodium salt (soap) and a small amount of glycerin, and the lower layer is waste alkali liquor, which is a dilute glycerin solution containing salt and sodium hydroxide, generally containing glycerin 9-16%, inorganic salt. 8-20%. Oil reaction. The glycerin water (also known as sweet water) obtained by hydrolysis of oil and fat has a glycerin content higher than that of the soap waste liquid, and is about 14-20%, and the inorganic salt is 0-0.2%. In recent years, continuous high pressure hydrolysis has been widely used. The reaction does not use a catalyst, and the obtained sweet water generally does not contain inorganic acid, and the purification method is simpler than the waste alkali solution. Whether it is soap waste liquid or glycerin water obtained by hydrolysis of oil, the amount of glycerin is not high, and all contain various impurities. The production process of natural glycerin includes purification, concentration to obtain crude glycerin, and distillation and decolorization of crude glycerin. Deodorization process. This process is described in detail in some books.
2. Production of Synthetic Glycerol The various routes for the synthesis of glycerol from propylene can be grouped into two broad categories, namely chlorination and oxidation. Propylene chlorination and propylene non-scheduled acetic acid oxidation are still used in the industry.
(1) Propylene Chlorination This is the most important production method for the synthesis of glycerol. It comprises four steps, namely high temperature chlorination of propylene, hypochlorochlorination of chloropropene, saponification of dichloropropanol and hydrolysis of epichlorohydrin. Hydrolysis of epichlorohydrin to glycerol is carried out in an aqueous solution of 10% hydrogen peroxide and 1% sodium carbonate at 150 ° C and 1.37 MPa carbon dioxide pressure to form a sodium chloride-containing glycerin aqueous solution having a glycerin content of 5-20%. After concentration, desalting, and distillation, glycerin having a purity of 98% or more is obtained.
(2) Propylene peracetic acid oxidation method Propylene and peracetic acid are used to synthesize propylene oxide, and propylene oxide is isomerized to a olefin. The latter is then reacted with peracetic acid to form glycidol (i.e., glycidol) and finally hydrolyzed to glycerol. The production of peracetic acid does not require a catalyst, and acetaldehyde and oxygen are vapor-phase oxidized. Under normal pressure, 150-160 ° C, and contact time of 24 s, the conversion of acetaldehyde is 11%, and the selectivity of peracetic acid is 83%. The latter two-step reaction described above is continuously carried out in a reaction column of a specific structure. After the raw material allyl alcohol and the ethyl acetate solution containing peracetic acid are sent to the column, the column is controlled at 60-70 ° C and 13-20 kPa. The ethyl acetate solvent and water were distilled off from the top of the column, and the column was stirred to obtain an aqueous glycerin solution. The method has high selectivity and yield, and uses peracetic acid as an oxidant, which can be used without a catalyst, and the reaction speed is fast, which simplifies the process. Production of 1t glycerol consumed 1.001t of allyl alcohol, 1.184t of peracetic acid, and 0.947t of by-product acetic acid. At present, the production of natural glycerin and synthetic glycerol accounts for almost 50% each, while the propylene chlorination method accounts for about 80% of the production of Healing glycerol. China’s natural glycerin accounts for more than 90% of total production.
