The moisture content of oligomeric polyols should be less than 0.05% no matter how to produce CPU products. If the value is higher than this value, the dehydration operation should be carried out. The most commonly used methods are intermittent water removal, continuous spray dehydration and continuous membrane dehydration.

Continuous dehydration method is used for dehydration of large quantities of polyols.

The interval dehydration method is best carried out in stainless steel kettle. Frame (anchor) agitator and end face mechanical seal are used. Steam or electric heating is used. The dehydration temperature of polyether is 100-110 ℃, and polyester is 110-140 ℃. It usually takes 1-4 h under high vacuum degree. The reaction time varies with the size of reaction kettle.

Spray dehydration is a method of spray dehydration. The oligomer polyols are added to a preheated storage tank, heated up to 90 degrees centigrade, and sent to the spray drying chamber nozzle at a certain speed through the metering pump, and the dry hot air is passed into the drying chamber at a certain speed. The liquid polyols spray from the nozzle in the form of mist and dry by contacting with hot dry air. The dehydrated polyols are concentrated at the bottom of the drying chamber and then sent to the synthetic prepolymer. This method can be used for continuous drying, and has the characteristics of short time and good dehydration effect. For example, Bayer company in Germany used this method to dry polyester. When the conveying speed of metering pump is 2.3kg/min, the temperature of hot and dry air is 155 ℃ and the rate is 2.8m3/min, the moisture content of dried polyester can be reduced to below 0.05%.

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The membrane dehydration was carried out in a vacuum film dryer. The oligomeric polyols are sent into the film evaporation tower, which is scraped into thin film by the rotating scraper, and flows down slowly from the upper part of the tower. The pressure in the tower reaches 106pa. The jacket of the tower is heated with 0.39-0.59mpa steam, and the temperature in the tower is about 140 ℃. The water vapor is extracted from the top of the tower, and the dry polyols flow to the bottom of the tower to synthesize the prepolymer. This method is characterized by short heating time (several minutes), large evaporation area and good dehydration effect.