VCM supply, storage and unloading

For both emulsion and suspension processes, the VCM gas is polymerised in an aqueous medium. 

Polymerisation reactions are exothermic, thus the reactors must be equipped with cooling facilities. The pressure in the reactor is usually in the range of 0.4 - 1.2 MPa and the reaction temperature is in the range of 35 - 70 °C. At the end of the reaction, 85 - 95 % of the VCM is converted into PVC. 

The unconverted VCM is vented-off to a gas-holder or straight to a VCM recovery unit, before stripping operations. The objective is to reduce the pressure close to atmospheric. Because of the surfactants or emulsifiers present in PVC water mixture, the reactor contents are liable to foam during this venting process, especially in the emulsion process. This removal of unconverted monomer can be performed either in the polymerisation vessel itself, or in a blowdown tank. Some t5q)es of vessel will be equipped to catch any carry over, should it occur. 

In the suspension PVC (S-PVC) process, a suspension of PVC particles having a mean particle size of between 50 and 200 pm is produced. Besides particle size, the essential differences between S-PVC grades result from the average length of polymer chains and from the porosity of the particles. Suspension PVC is always produced batch wise in a stirred vessel. 

The monomer is dispersed in demineralised water by the combination of mechanical stirring and surfactants. Partially hydrolysed polyvinyl acetates are the most widely used type of suspension agent. The polymerisation takes place inside the VCM droplets under the influence of VCM soluble initiators such as peresters, percarbonates or peroxides. A phase of solid PVC primary particles builds up. The PVC particles present at the end of the polymerisation process, result from the complex aggregation of such primary particles, giving S-PVC its characteristic cauliflower aspect under an optical microscope. 

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