The quality problems of glycerol mainly including the smell of burnt, yellow color unqualified saponification equivalent,acrolein and reducing substances exceed the standard,chlorides, ash and carboniferous substances exceed regulations.There are several reasons for these problems as below.
- Oil: low grade oil generally refers to thespoiled oil or oil with impurities due to improper storage and all kinds of recovered oil.The organic components are oxidized and deteriorated due to high amount of oxidized fatty acids, low carbon fatty acids, aldehyde, ketone, protein and other impurities, or due to the processing of saponified waste liquid, long-term storage time and dirty container.These substances are dissolved in saponified waste liquid and not easily removed during treatment.As a result, the crude glycerine has a dark color and heavy taste, distilled refined glycerol has a smell, yellow color or unstable color.For this reason, during the distillation operation, the material should not stay in the distillation kettle for a long time, the level should not be too high, the level fluctuation should not be too big, the vacuum degree should not be too low.Low grade oil should be properly pretreated or treated separately.For the production of pharmaceutical grade glycerol and explosive glycerol, the variety and quality of oil must be selected and controlled.Experience has shown that, in addition to castor oil, fish oil, ash seed oil, silkworm pupa oil saponification waste liquid should not produce explosive glycerin, rice bran oil, cottonseed oil should also be used with caution.
- Low molecular fatty acid: when saponified waste liquid or sweet water is purified, low molecular fatty acid (salt) enters purified water and then enters crude glycerol.During distillation, organic salts are broken down and free fatty acids are esterified by glycerol vapour in the gas phase, resulting in high esters of refined glycerol.In addition, the presence of protein and magnesium salts in crude glycerol may increase the ester content of refined glycerol.Therefore, a certain alkalinity of crude glycerol can be properly controlled during distillation.(0.1%~0.2%)The other way around, when cleaning saponified waste liquid or sweet water, the fatty acids should be removed as much as possible.
- Operating conditions: low vacuum high temperature or local overheating under vacuum can increase the amount of acrolein,the liquid level overflows and the vacuum fluctuates too much,entrainment of fog and foam enters into condenser due to the imperfect vapor phase separation device,which result in the index of fine glycerol chloride, ash, readily carbonizable substance to be unqualified.In addition, equipment leakage should be prevented during operation.
- The dark color and strong smell of the stewed glycerine should be separated from the normal stewed glycerine, which should be combined with crude glycerol.
