Vinyl chloride monomer recovery

TABLE 1 Vinyl Chloride Monomer Recovery Costs... 

Figure 3.16. Typical incinerator-heat recovery-scrubber system for vinyl-chloride-monomer process waste...

FIGURE 21 Membrane-based vinyl chloride monomer (VCM) recovery system in a PVC polymerization plant. (Membrane Technology and Research, Inc.) [From Baker, R. W., etal. (2000, December). Chem. Eng. Prog. pp. 51-57.]... 

Lahiere R. J., Heliums M. W, Wijmans J. G., Kaschemekat J. 1993. Membrane vapor separation Recovery of vinyl chloride monomer from PVC reactor vents. Industrial and Engineering Chemistry Research 32 2236-2241. 

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). 

R. J. Lahiere, M. W. Heliums, J. G. Wijmans, J. Kaschemekat, Membrane vapor separation recovery of vinyl chloride monomer from pvc reactor vents. Ind. Eng. Chem. Res., 32, 2236-2241 (1993). 

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.

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

Then, in the 1970s, a number of poly(vinyl chloride) producers were completely surprised when it was found that long-term (20-year) exposure to vinyl chloride could cause rare forms of tumors.22 After the discovery that vinyl chloride was a carcinogen, venting was not permissible. Containment and recovery of the monomer was mandatory. As a result, some older processes and manufacturing facilities could not be economically modified to incorporate containment, and as a result such operations were discontinued. This case is but one example of the impact that necessary and regulated environmental controls can have on manufacturing processes and operations. 

Intermolecular reactions between carbon-centred radicals on adjacent chains will produce crosslinking, which will compete with the chain-scission and intramolecular reactions. The effect on the molar-mass distribution will depend on the competition between these reactions. For example the poly(acrylates) will preferentially crosslink rather than degrade and the recovery of monomer from these polymers is less than 1%, compared with close to 100% for poly(methacrylates). Poly(vinyl chloride) (PVC) is also a polymer that undergoes crosslinking in preference to scission, but the major observation of importance is the rapid elimination of hydrogen chloride by an auto-catalysed reaction as shown in Scheme 1.53. This is discussed in more detail later. 

The polymerization of vinyl monomers in liquid and supercritical CO2 has been studied extensively. Patents were issued in 1968 to the Sumitomo Chemical Company [81] and in 1970 to Fukui et al. [82] for the preparation of homopolymers of polystyrene, poly(vinyl chloride), poly(acrylonitrile) (PAN), poly-(acrylic acid) (PAA), and poly(vinyl acetate) (PVAc), as well as the random copolymers PS-co-PMMA and PVC-co-PVAc. Additionally, a patent was issued in 1995 to Bayer AG [83] for the preparation of styrene/acrylonitrile copolymers in SCCO2. In 1986, the BASF Corporation was issued a Canadian patent for the precipitation polymerization of 2-hydroxyethylacrylate and various N-vinylcarboxamides in compressed carbon dioxide [84]. In 1988, Terry et al. attempted to homopolymerize ethylene, 1-octene, and 1-decene in SCCO2 for the purpose of increasing the viscosity of CO2 for enhanced oil recovery [85]. These reactions utilized free-radical initiation with benzoyl peroxide and r-butylperoctoate at 71 °C and 100-130 bar for 24-48 h. Although the resulting polymers were not well characterized, they were found to be relatively... 

The irradiation of mixed lattices for subsequent combination of the mptured chains is another approach it has been carried out with natural rubber and poly(vinyl chloride) lattices to prepare graft (and block) copolymers in fairly high yields without the problem of monomer recovery. The same method has been used to graft polychloroprene onto synthetic polyisoprene dispersions and onto polybutadiene lattices of various compositions. 

Polymers. Foam is often a particular problem in the production of polymers. There are numerous situations where foam can reduce the production capacities of vats and vessels and cause problems in pumps, meters, and other equipment, particularly distillation and evaporation equipment. Foam is frequently a problem when stripping off a monomer from a polymer. Examples are in the production of styrene-butadiene [9003-55-8] and acrylonitrile-butadiene [9003-18-3] rubber latices. These latices are stabilized by surfactants that greatly contribute to foaming difficulties. Another problem foam area is in the stripping of imreacted monomer from poly(vinyl chloride) suspensions. In this process, vinyl chloride [75-01-4], a gas at room temperature, is liquefied by pressure, emulsified in water with surfactants and catalysts, and heated to bring about polymerization. The recovery of unpolymerized monomer by distillation from this mixture produces a severe foaming problem. 

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