The quality problems of glycerol products are mainly manifested in coke taste, yellow color, unsatisfactory saponification equivalent, excessive acrolein and reducing substances, excessive chloride, ash and carbide-prone substances, etc. The reasons for these problems are as follows:
1.Oils and fats: Low-grade oils and fats generally refer to the oxidative deterioration of organic components caused by improper storage of sour and deteriorating oils or oils with more impurities, as well as various kinds of recovered oils and fats, because they contain many oxidized fatty acids, low-carbon fatty acids and impurities such as aldehydes, ketones and proteins, or because of improper processing of saponification waste liquor, long storage time, and unclean containers. These substances are dissolved in saponification waste liquor and are not easily removed in the treatment process. As a result, the crude glycerol is dark and heavy, and the distilled essential glycerol has odor and yellowing color. Therefore, in the operation of distillation, it is necessary to avoid excessive residence time, excessive liquid level, excessive fluctuation of liquid level and too low vacuum. Low-grade grease should be properly pretreated or separately treated.
If medicinal grade glycerol and explosive glycerol are produced, the variety and quality of oil and fat must be selected and controlled. Experience has proved that besides the saponification waste liquor of castor oil, fish oil, Litsea cubeba seed oil and silkworm chrysalis oil, rice bran oil and cottonseed oil should also be used cautiously.
2.Low Molecular Fatty Acids (LMFAs): When saponified waste liquor or or sweet water is purified, low molecular fatty acids (salts) enter the purified water and then into crude glycerol. During distillation, organic salts are decomposed and free fatty acids are esterified by glycerol vapor in the gas phase, resulting in an increase in the ester content of essential glycerol. Therefore, the alkalinity of crude glycerol can be controlled appropriately (0.1% – 0.2%) during distillation. Conversely, it also shows that when purifying saponification waste liquor or or sweet water, the fatty acid can be removed as much as possible.
3.Operating conditions, low vacuum and high temperature or local overheating under vacuum can increase the amount of propanal, overflow the liquid level, too large vacuum fluctuation, entrainment of mist into the condenser due to imperfect gas separation device, resulting in unqualified indicators such as chloride of essential glycerol, ash, carbide and so on. In addition, leakage of equipment should be prevented during operation.
4.The steamed glycerol from roasted feet is dark in color and strong in smell. It should be separated from the normal steamed glycerol and should be re-steamed with crude glycerol.
