The industrial production of propylene glycol primarily employs the propylene oxide hydration method, using propylene oxide and water as the main raw materials and reacting them under specific temperature and pressure conditions. During the reaction, the epoxide ring in the propylene oxide molecule opens, and the addition of water molecules yields 1,2-propylene glycol. This process features a straightforward reaction pathway and mature equipment and technology, making it widely adopted in industrial production. To ensure the complete conversion of propylene oxide, reaction temperature, pressure, and material ratios must be controlled based on raw material properties and equipment specifications.
Propylene oxide and water are fed into the reactor at a specific ratio and undergo hydration under suitable conditions. While the target product, 1,2-propylene glycol, is formed during the reaction, small amounts of by-products-such as dipropylene glycol and tripropylene glycol-may also be produced. Factors such as the amount of water added, reaction temperature, and reaction time influence the product composition. Increasing the water-to-propylene oxide ratio helps improve the conversion rate of propylene oxide but also increases the load on subsequent dehydration and separation processes; therefore, the production process requires a balance between reaction efficiency and energy consumption.
The resulting mixture typically undergoes separation steps such as dehydration, distillation, and rectification. First, the bulk of the water is removed from the reaction system; subsequently, the differences in boiling points among the components are utilized to separate propylene glycol from by-products like dipropylene glycol and tripropylene glycol. Products requiring higher purity undergo further rectification to reduce impurity levels and enhance product purity. Precise control of temperature, pressure, and reflux ratio during rectification improves separation efficiency while minimizing energy consumption.
Following separation and purification, the propylene glycol undergoes quality testing, focusing on parameters such as purity, water content, color, acidity, and density. Products meeting specifications are cooled, stored, and packaged, whereas non-compliant products require further processing based on specific circumstances. In addition to the propylene oxide hydration method, research is also being conducted on producing propylene glycol from bio-based raw materials; such processes offer potential regarding renewable feedstocks and green production. However, in terms of current industrial application, the propylene oxide hydration method remains the process with the most mature technological foundation.