REMOVAL OF ORGANIC COMPOUNDS FROM A WATER-SOLUBLE POLYMER

 

Polymers which are water-soluble at all compositions at room temperature, because of hydrogen-bonding, can, depending on composition, separate into two phases as the temperature is raised. As the temperature is raised further, however, the mixtures become one phase again at all compositions. This situation is shown schematically in the diagram above. For example, at 200°C mixtures of a polymer and water with a polymer weight fraction of less than ~0.6 split into 2 phases one of nearly pure water and one containing 60% w/w of the polymer.

 

Impurities in the polymer, which have some water solubility at 200°C can be washed out with water at that temperature. The figure below shows schematically how this is done. A solution of polymer in water is put into the cell and heated to 200°C. Two phases are formed with the polymer-rich phase at the bottom. The right-hand valves are closed and water at 200°C pumped into the bottom of the cell and wastewater containing the impurities and a small amount of polymer passes out of the top. When the impurities have been removed to the specified level, the left-hand valves are closed and the right-hand valves opened. Nitrogen is then passed into the top of the cell and the product is pushed out of the bottom. The process is carried out at a pressure sufficient to maintain water as a liquid and heat transferred to or removed from the various streams as required, as shown in other applications. Finally water is evaporated from the product to give the purified polymer. This process can be made continuous.

 

 

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