Production of Polyester Polyols - Two Other Methods

Carrier Gas Melting Method

In this method, inert gases (such as nitrogen or carbon dioxide) are bubbled through a mixture of carboxylic acids and diols. During the high-temperature condensation reaction, water vapor generated is carried out of the reactor by the gas flow. However, a small amount of diol is also volatilized and carried away with the gas. Therefore, to account for diol loss, an extra amount of volatile diol is added during the raw material preparation. This method is suitable for large-scale production facilities.

Azeotropic Distillation Method

This method utilizes an azeotropic solvent that, along with water generated during the reaction, is removed from the reaction system and returned to the reactor through a recycling process. Inert solvents such as toluene or xylene are commonly used as azeotropes. For example, toluene forms an azeotropic vapor with water, and upon condensation through a condenser, the water and toluene separate into layers. The heavier water settles at the bottom of the water separator and is discharged in batches, while the upper toluene solvent overflows back into the reactor. This process is conducted at atmospheric pressure and lower temperatures (145–190°C). At the end of the process, the azeotropic solvent is distilled off, and any remaining solvent in the polyester polyol is removed by vacuum distillation. Despite the mild reaction conditions, this method is less commonly used in industrial applications compared to the other two methods.

Process Control and Monitoring

For all three methods, strict monitoring of acid value, hydroxyl value, and viscosity is required to ensure the production of qualified polyester polyols. Polyester polyols are generally produced using batch methods, but continuous processes suitable for large-scale production have also emerged.