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 called natural glycerin;
In the synthesis method using propylene as a raw material, the obtained glycerin is called synthetic glycerin.
Natural glycerin
Prior to 1984, all glycerol was recovered from by-products of animal and vegetable fat soaps. So far, natural oils and fats are still the main raw material for the production of glycerin, of which about 42% of natural glycerin is made by-product soap and 58% is derived from fatty acid production. The saponification reaction 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 fats and oils has a higher glycerin content than the soap-making 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 contained in the oil 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.
Synthesis
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.
Propylene chlorination
This is the most important production method for the synthesis of glycerol. It consists of four steps, namely high temperature chlorination of propylene, hypochlorochlorination of chloropropene, saponification of dichloropropanol and hydrolysis of epichlorohydrin. The hydrolysis of epichlorohydrin to glycerol is carried out in an aqueous solution of 10% sodium hydroxide and 1% sodium carbonate at 150 ° C and 1.37 MPa carbon dioxide pressure to produce a sodium chloride-containing glycerin aqueous solution having a glycerin content of 5-20%. It is concentrated, desalted, and distilled to obtain glycerin having a purity of 98% or more.
Propylene peracetic acid oxidation
Propylene reacts with peracetic acid to synthesize propylene oxide, and propylene oxide isomerized to allyl alcohol. 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 steps are carried out continuously 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. Currently, natural glycerin and synthetic glycerol yield almost 50% each, while propylene chlorination accounts for about 80% of synthetic glycerol production. China’s natural glycerin accounts for more than 90% of total production.
Industrial grade glycerin
The amount of industrial grade glycerin is diluted with 1/2 amount of distilled water. After stirring well, activated carbon is added and heated to 60-70 ° C for decolorization, and then vacuum filtered to ensure clear and transparent filtrate. The dropping rate is controlled, and the filtrate is added to a column of a previously prepared 732 type strong acid cation resin and a 717 type strong base yin and yang resin to adsorb and remove the electrolyte and aldehydes, pigments, esters and the like in the glycerin. The glycerin solution after removing impurities is subjected to vacuum distillation to control the degree of vacuum of 93326 Pa or more, the pot temperature is 106 to 108 ° C, and most of the water is distilled off, and then the temperature of the pot is raised to 120 ° C to rapidly dehydrate, and the heating is stopped when no water is discharged. The material in the kettle is the finished product
