Batchwise suspension polymerization

Today 80% of PVC is manufactured by the technique of batchwise suspension polymerization (S-PVC), the remaining part being shared between emulsion and bulk polymerization. A distinctive characteristic of the S-PVC processes is the large size of the reactors, 50 to 200m3, and operation at 10 to 12 bar pressure. The reaction time has been reduced from 18 h in the 1960s to only 3.5-5 h today by making use of fast initiation systems and recipe optimization. 

The original wartime process was mn batchwise in reactors similar to those used for suspension polymerization. Since then, in many plants, the reactors have been hooked together as a series of continuous stirred tanks. 

Suspension polymerization is mostly carried out batchwise, also on large scales. In a stirred reactor, filled with a certain amount of water, liquid monomer and a surfactant are added, and the monomer is dispersed. TTie catalyst or initiator, often an organic peroxide, is fed with the monomer. The mixture is heated till the reaction starts, then the temperature is allowed to rise to the desired level, and kept constant by cooling. 

Emulsion polymerization is usually carried out batchwise, for the same reasons as suspension polymerization, namely to ensure a very high degree of conversion, and to limit the molecular mass distribution. As a rule stirr reactors are used. The requirements for agitation are less severe than with suspension polymerization, since much more emulsifier is used, so that the dispersions are much better stabilized. Since reaction rates are approximately constant during much of the process, the heat fluxes through reactor walls are moderate. 

Most vinyl chloride is polymerized by a batchwise suspension process. Jacketed, stirred reactors like the one in Figure 5.13 vary in size from 2,000 to 10,000 gal (8 0 m ). 

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