The by-product glycerol is obtained by transesterification of oil and fat, which contains alkali catalyst, soap produced by side reaction, a small amount of unreacted oil and methanol, as well as trace proteins, hydrocarbons, pigments, precipitates and water. In order to obtain refined glycerol, the above impurities must be purified and separated first.
1.Acid treatment
Solvent and inorganic acid solution were added to crude glycerol sample, pH value of solution was adjusted to acidity, stirring was heated, alkali catalyst was neutralized, and soap was converted into fatty acid to float on the liquid surface and removed. The inorganic acid used could be hydrochloric acid and sulfuric acid. The solvent can be methanol and water. By measuring the glycerol content in the lower solution after acid treatment, the recovery rate of glycerol after acid treatment was calculated.
2.Degumming
Sodium soap is basically converted to fatty acid after acid treatment, and the unreacted sodium soap may still exist in glycerol sample. When flocculant is added, a small amount of colloidal dispersed soap and other charged impurities are neutralized and coagulated under the action of flocculant metal ions. The commonly used degumming reagents are aluminium sulfate and FeCl3.
3.Alkali neutralization
The acidic filtrate obtained by degumming and filtration also contains excessive FeCl3. By neutralizing acid with alkali treatment, the evaporator will be less corroded, and FeCl3 will be converted into Fe (OH) 3 precipitation. At the same time, impurities will be adsorbed and removed by filtration. At the same time, there may be no clean fatty acids in the separation process after acid treatment. By reducing neutralization, the fatty acids can be fixed in the form of soap to prevent the fatty acids from evaporating with glycerol in the distillation process, affecting the quality of glycerol, the amount of alkali liquor added in the process of alkali neutralization, the quality of glycerol, recovery rate and so on. There is a great relationship between evaporation operation. The amount of alkali is too small, glycerol is easy to dehydrate in molecule under acidic conditions to produce irritant substances such as propylene alcohol or acrolein (ketone) intermediate. Fatty acids can not be fixed in the form of soap. During distillation, glycerol is evaporated with glycerol, resulting in loss of glycerol and quality, and alkalinization. Glycerin is easy to polymerized. It is easy to produce foam when evaporating water, which is easy to run, resulting in extra glycerol loss and lower yield.
4.Glycerol Concentration and Filtration Desalination
After alkali neutralization, crude glycerol samples were distilled to 110C under vacuum, evaporated to remove water and filtered to remove precipitated salt.
According to the different uses of glycerol and the different economic consumption in the production process, different refining methods can be used. Generally, distillation, decolorization, distillation, decolorization and ion exchange are used. The glycerol obtained by distillation and decolorization is mainly industrial glycerol. If glycerol is used as a special purpose, such as medicinal, edible and so on, no matter which refining method is used, it must go through the ion exchange process to ensure that glycerol meets the requirements of quality standards.
