Crude glycerin distillation

The boiling point of glycerin at atmospheric pressure is 290 ℃, and at 204 ℃ it begins to decompose and polymerize, so crude glycerol distillation needs to be carried out under decompression. For example, under the pressure of 20mmHg, the boiling point of glycerol is 181.34℃, and it is 116.11 ℃ at 10mmHg pressure. In order to further reduce the distillation temperature of glycerol distillation, steam distillation under decompression can be used in industry.

The distillation process of crude glycerol is to turn glycerol into glycerol vapor at high temperature and distill it with water, thus separating it from other components that cannot be distilled at this temperature; Then the glycerin vapor and water vapor are condensed to separate glycerin from water. Under good controlled conditions, glycerin products with more than 99% content can be obtained.

According to the principle of distillation, each component of the mixed liquid produces steam when heated. The vapor pressure on the liquid surface of the mixed liquid equals the sum of the vapor partial pressures of each component at this temperature. If the steam pressure is equal to the external pressure, the mixture begins to boil. If water vapor is introduced into the mixed liquid, the vapor pressure on the liquid surface is composed of the vapor pressure of the water vapor and the vapor pressure of the distilled liquid. At this point, the mixed liquid is boiling and evaporating at a lower temperature. When crude glycerin is distilled, glycerin is boiling at 190.87℃when the external pressure is 30mmHg. If water vapor is introduced, the vapor pressure of water on the surface of crude glycerin is 15 mm Hg, and the vapor pressure of glycerin is 30-15 = 15 mm Hg, that is, glycerin boils and evaporates at 174.86℃. The reduction of distillation temperature can reduce the decomposition and polymerization of glycerol and improve the distillation yield and product quality of glycerol. So the lower the distillation temperature is, the better the distillation temperature is below 170 C.

During distillation of crude glycerol from waste liquor of chemical industry, with the distillation of glycerol, salt in crude glycerol gradually precipitates and deposits in distillation kettle or salt tank. Therefore, salt should be regularly pressed out of the salt tank, filtered, and then re-distilled, and the filtered salt should be washed and then boiled back into the soap process. Hot air or water can be used as condensation medium in condenser. Hot water and cold water are mostly used as condensation medium domestic now. Controlling the temperature of the condensate medium of 1# condenser and 2# condenser can make the concentration of glycerin in the condensate of 1# condenser and 2# condenser reach more than 98%. High-quality medicinal glycerin or explosive-grade glycerin was prepared by deodorization, decolorization or ion exchange of glycerin collected from 1# condenser and 2# condenser. Generally, the 3# condenser has low temperature, heavy odor and poor quality. It often returns to the treatment process of saponification waste water. In order to ensure the quality of glycerin, there are also concentrated or burned-out ones. The vacuum system of crude glycerol distillation can adopt mechanical vacuum pump, steam jet vacuum pump or water-steam series jet vacuum pump, etc.