Glycerin boiling point is 290 ℃ under atmospheric pressure,begin to decompose and polymerize when the temperature is 204 ℃,so crude glycerol distillation needs to be done under pressure.Under the pressure of 20mmHg, the boiling point of glycerol is 181.34 ℃, while under 10mmHg the pressure is 166.11 ℃,in order to further reduce the evaporation temperature during glycerol distillation, the steam distillation under pressure can be used in industry.
The distillation process of crude glycerol is transfer glycerol to glycerol vapour with high temperature,it is distilled with water and separated from other components that cannot be steamed at this temperature.Then condense the glycerine vapor and water vapor sectionally, separate the glycerine from the water.Glycerol products with a content of 99% or more can be obtained under good control.
According to the principle of distillation, each component of mixed liquid will produce steam when heated,The vapor pressure on the mixed liquid level meter is equal to the partial vapor pressure of each component at this temperature.If the vapor pressure is equal to the external pressure, the mixture begins to boil.If water vapor passes through the mixture,the vapor pressure on the liquid surface consists of the water steam pressure and the vapor pressure of the distilled liquid.At this point, the mixture will boil and evaporate at a lower temperature.During the distillation of glycerin, when outside pressure is 30mmHg, glycerol boiling gasification at the temperature of 190.87 ℃.The water vapor pressure on the crude glycerine is 15mmHg when connect with water steam,the vapor pressure of glycerol is 30- 15= 15mmHg,glycerin becomes boiling evaporation at the temperature of 174. 86 ° C .The reduction of distillation temperature reduces the decomposition and polymerization of glycerol,increase the distillation yield and quality of the glycerol.So the lower distillation temperature the better, usually distillation temperature is under 170 ℃.
When the saponified waste liquid crude glycerol is distilled, the salts in the crude glycerol are gradually separated out and deposited in the distillation kettle or salt box.Therefore, the salt should be pressed out of the salt box regularly,after filtration, the filtrate is redistilled and the salt is washed back to the boiling process.The condenser can use hot air (1980s) or water as the condensing medium, domestic now use hot water and cold water as the medium.By controlling the temperature of the condensing medium of first cooling and second cooling, the glycerine concentration of first cooling condensate can reach above 98%.High quality pharmaceutical or explosive grade glycerol is obtained by decolorization, decolorization or step – by – step ion exchange.
