Encrustation Prevention in PVC Reactors

Poly(vinyl chloride) is one of the world's most important bulk thermoplastics. Approximately 15 million tons are produced annually. About 80% of this is made by the suspension process. Suspension polymerization of vinyl chloride is almost invariably accompanied by the formation of scale on the internal surfaces of the polymerization reactor, including moving surfaces such as those of the impeller. The formation of scale, or deposits of PVC, on the internal surfaces of reactors has always been a problem in the manufacture of PVC because of reduced heat transfer, large alteration of the agitation regime present in the reactor, and broken-off scale fragments that block transfer systems and contaminate the product. Therefore scale must be cleaned periodically. Until the early 1970s these deposits were removed manually; PVC workers were exposed to high levels of VCM and considerable production time was lost. The introduction of large reactors in 1975 made it practical to mechanize the cleaning. High-pressure water sprays are employed for this purpose. However, such devices remove only the slightly adherent sandy deposits. For strongly attached deposits, a thorough manual cleaning is necessary after a series of polymerization batches. Manual cleaning requires extensive safety measures to protect the workers from VCM vapor inside the reactor. In addition, such cleaning is costly and undesirable. In more recent years, methods have been developed to suppress or inhibit the formation of scale on the reactor walls. Proposed solutions in this field are sparese. The patient literature claims many chemical treatments to resuce reactor fouling. Many PVC producers have developed effective buildup-suppressant treatments, which significantly reduce the need for reactior cleaning. They vary from treatments to render the reactor wall hydrophilic and reduce the adsorption of VCM (e.g., application of a sulfonic acid polymer to the reactor surface [1]) to systems containing polymerization inhibitors and radical traps (e.g., organic dyes and phenolic resins). The most successful of these probably work by a combination of both mechanisms. In this paper the soultions are reviewed brifly and emphasis is given to suspension polymerization where the initiator is monomer soluble.