Separation vinyl chloride monomer process

Telescope the Process by Combining Stages. This has been done successfully in the conversion of propylene to acrylonitrile by direct ammoxidation rather than oxidation to acrolein followed by reaction with ammonia in a separate stage, as was described in the earlier patent literature. The oxychlorination of ethylene and HC1 directly to vinyl chloride monomer is another good example of the telescoping of stages to yield an economic process. 

The flow sheet for a balanced chlorination-oxychlorination of ethylene to vinyl chloride monomer is shown in Figure 2. Currently this process, with its variations involving fixed and fluid beds and different methods of heating and separation, dominates the commercial production of vinyl chloride with 93% of VCM being made by this route. 

Bulk polymerization is the simplest process for producing PVC, since there are no difficulties on the separation and recovery of the residual monomer, or on the finishing of the produced polymer. The most well-known bulk vinyl chloride monomer (VCM) polymerization process these days is the Rhone-Poulenc two-stage process. According to this process, only VCM and oil-soluble initiators are introduced into the reactor, since there is no need for water and suspending agents. Due to the absence of water, productivity is very high, with respect to the other VCM polymerization processes (i.e., suspension, emulsion, solution). 

Although vinyl chloride has been polymerized with organometallics [2], commercial poly(vinyl chloride) (PVC) is made by a free radical polymerization. The dominant process is the suspension process. Vinyl chloride (most commonly referred to as vinyl chloride monomer or VCM) is suspended as droplets in water and an initiator that is soluble in the VCM is added. VCM has a boiling point of -13 °C. It is reacted under pressure as a liquid. The resultant PVC powder is separated from the water and dried. PVC can also be prepared by a mass or bulk polymerization where the initiator is added to the liquid VCM. A third polymerization technique, an emulsion polymerization, employs liquid VCM, water, and an emulsifier. A water soluble initiator is added to the VCM/water emulsion. 

Gaseous monomers may also be trapped within the processing equipment and accidents have occurred as a consequence of the resulting pressure buildup. In the case of the polyacetals and poly(vinyl chloride) it is reported that at elevated temperatures these materials form a more or less explosive combination so that it is important to separate these materials rigorously at the processing stage. 

Figure 12-4. The European Vinyls Corp. process for producing polyvinyl chloride using suspension polymerization (1) reactor, (2) blow-down vessels (to separate unreacted monomer), (3) stripping column, (4) reacted monomer recovery, (5) slurry centrifuge, (6) slurry drier.

Suspension polymerization. In this process, monomers and initiator are suspended as droplets in water or a similar medium. The droplets are maintained in suspension by agitation (active mixing). Sometimes a water-soluble polymer like methylcellulose or a finely divided clay is added to help stabilize or maintain the droplets. After formation, the polymer, is separated and dried. This route is used commercially for vinyl-type polymers such as polyvinyl chloride and polystyrene. 

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