Transesterification of fats and oils produces by-product glycerol, which contains alkali catalysts, soaps produced by side reactions, small amounts of unreacted oils and un-exhausted methanol, and trace amounts of proteins, hydrocarbons, pigments, precipitates and water. The refined glycerin must first be purified to separate the above impurities.
(1) Acid treatment
Adding a solvent and a mineral acid solution to the crude glycerol sample, adjusting the pH of the solution to acidity, heating and stirring, neutralizing the alkali catalyst, and simultaneously converting the soap into a fatty acid, and floating it on the liquid surface to remove it. The inorganic acid used may be hydrochloric acid. sulfuric acid. The solvent can be methanol or water. The acid-treated glycerin recovery rate was calculated by measuring the glycerin content in the lower layer solution after the acid treatment.
(2) Degumming
After the acid treatment, the sodium soap is basically converted into fatty acid and the layer is removed. The unreacted sodium soap may also be present in the glycerol sample, the flocculant is added, a small amount of colloidally dispersed soap and other charged impurities are in the flocculant metal ion. Under the action of electricity, it produces electricity neutralization and coagulation. Common degumming agents are aluminum sulfate and FeCl3.
(3) alkali neutralization
The acidic filtrate obtained after degumming filtration also contains an excess of FeCl3, neutralizing the acid by alkali treatment, reducing corrosion to the evaporator, and converting FeCl3 into Fe(OH)3 precipitate while adsorbing impurities and removing by filtration. At the same time, after the acid treatment, there may be no separated fatty acids in the separation process. By reducing the neutralization, it can be fixed in the form of soap to prevent the fatty acid from being distilled out together with the glycerol during the distillation, which affects the quality of the glycerol. And the addition of lye in the process, has a great relationship with the quality of glycerol, recovery and evaporation. The amount of alkali is too small. Under acidic conditions, glycerol is easily dehydrated in the molecule to form propenol or acrolein (ketone) intermediates. Stimulating substances, fatty acids can not be fixed in the form of soap, distilled together with glycerin during distillation, resulting in loss of glycerin and quality, excessive addition of alkali, glycerol is easy to polymerize, easy to produce foam when evaporating water, easy to run, resulting in Additional glycerin loss, reduced yield.
(4) Concentration of glycerol and filtration and desalting
After neutralization by a base, crude glycerol was distilled under reduced pressure to 110 ° C, water was evaporated, and the precipitated salt was removed by filtration.
(5) Refinement of glycerin
Different refining methods can be used depending on the use of glycerin and the economic consumption in the production process.
In general, distillation, decolorization, rectification, decolorization, and ion exchange are employed. The glycerin obtained by distillation and decolorization is mainly industrial glycerin. If glycerol is used for special purposes, such as medicinal, edible, etc., no matter which purification method is used, it must undergo an ion exchange process to ensure that glycerol meets the quality standards.
