MONOMER RECOVERY

In principle, recovery of monomers and repolymerization shonld result in a product that is indistinguishable from the original polymer. PS and poly(methyl methacrylate), when heated with a free-radical source under vacuum, give high yields of the respective monomers (see Section 11.3). Poly(dimethyl siloxane) heated with KOH in a vacuum also produces a cyclic monomer. All of these processes are used for in-plant scrap treatment, but economics do not yet favor applying them to postconsumer plastic waste. 

Somewhat similar is the devulcanization of rubber, in which a sulfur cross-linked network is converted to a system of more-or-less thermoplastic components. Rubber reclaiming has fallen on hard times for various reasons, including the difficulty of making an environmentally attractive process. 

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Although bulk polymerization of acrylonitrile seems adaptable, it is rarely used commercially because the autocatalytic nature of the reaction makes it difficult to control. This, combined with the fact that the rate of heat generated per unit volume is very high, makes large-scale commercial operations difficult to engineer. Lastiy, the viscosity of the medium becomes very high at conversion levels above 40 to 50%. Therefore commercial operation at low conversion requires an extensive monomer recovery operation. 

The monomer recovery process may vary ia commercial practice. A less desirable sequence is to filter or centrifuge the slurry to recover the polymer and then pass the filtrate through a conventional distillation tower to recover the unreacted monomer. The need for monomer recovery may be minimized by usiag two-stage filtration with filtrate recycle after the first stage. Nonvolatile monomers, such as sodium styrene sulfonate, can be partially recovered ia this manner. This often makes process control more difficult because some reaction by-products can affect the rate of polymerization and often the composition may vary. When recycle is used it is often done to control discharges iato the environment rather than to reduce monomer losses. 

The problems of monomer recovery, reaction medium viscosity, and control of reaction heat are effectively dealt with by the process design of Montedison Fibre (53). This process produces polymer of exceptionally high density, so although the polymer is stiU swollen with monomer, the medium viscosity remains low because the amount of monomer absorbed in the porous areas of the polymer particles is greatly reduced. The process is carried out in a CSTR with a residence time, such that the product k jd x. Q is greater than or equal to 1. is the initiator decomposition rate constant. This condition controls the autocatalytic nature of the reaction because the catalyst and residence time combination assures that the catalyst is almost totally expended in the reactor. 

Chemistry of SBR. There are three steps in the manufacturing of SBR polymerization, monomer recovery and finishing. The polymerization step determines the basic characteristics of SBR, whereas the product form (latex or dry rubber, oil extended or not) depends on the finishing step. 

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.

ICI Acrylics believes that greater cooperation between companies and a revised approach to life cycle analysis are the keys to the industry s future environmental sustainability. The company has invested over 2m pounds sterling in an on-going monomer recovery project, which encompasses a joint research programme with Mitsubishi Rayon. The project focuses on increasing the efficiency of acrylic depolymerisation and overcoming technical issues such as its use in recycling flame retardant acrylics. ICI ACRYLICS... 

Polymers and resins Water purification, including removal of phenol, chlorophenols, ketones, alcohols, aromatics, aniline, indene, polynuclear aromatics, nitro- and chlor-aromatics, PCB, pesticides, antibiotics, detergents, emulsifiers, wetting agents, kraftmill effluents, dyestuffs recovery and purification of steroids, amino acids and polypeptides separation of fatty adds from water and toluene separation of aromatics from ahphatics separation of hydroquinone from monomers recovery of proteins and enzymes removal of colours from symps ... 

Monomer recovery Decant water layer Dissolved and separable organics. Source of high BOD and COD discharges... 

TABLE 1 Vinyl Chloride Monomer Recovery Costs... 

During 1973, at a chloroprene polymerization plant in the United States, airborne concentrations of chloroprene were found to range from 14 to 1420 ppm [50-5140 mg/m ] in the make-up area, from 130 to 6760 ppm [470-24 470 mg/m in the reactor area, from 6 to 440 ppm [22-1660 mg/m ] in the monomer recovery area and from 113 to 252 ppm [409-912 mg/ni l in the latex area (Infante et al., 1977). Concentrations in the air inside a Russian polychloroprene rubber plant were 14.5-53.4 mg/m (Mnatsakayan et al., 1972). In a Russian chloroprene latex manufacturing facility, chloroprene concentrations varied from 1 to 8 mg/m (Volkova et al., 1976). 

For the purposes of the F EI Guide a process unit is defined as any primary item of process equipment. For example, in the process area of a styrene/butadiene latex plant, process units could include monomer mix feed preparation, reactors, stripppers, monomer recovery, aqueous room, and styrene scrubber. A warehouse also may be treated as a process unit. In particular, materials stored within a fire-walled area, or within the total storage area where fire walls are not provided, would constitute such a process unit. 

R.W. Baker and M.L. Jacobs, Improve Monomer Recovery from Polyolefin Resin Degassing, Hydrocarbon Process. 75, 49 (1996). 

Baker, R.W. and Jacobs, M. (1996) Improved monomer recovery from polyolefin resin degassing. Hydrocarbon Processing, 75(3), 49. 

Rate and Distribution. The distinction between polymerization rate and distribution is particularly important with polymers. Rate considerations answer questions of quantity. What is the conversion per pass What reactor size is required Will monomer recovery be needed ... 

J. Aguado, D. P. Serrano, M. D. Romero, and J. M. Escola, Catalytic conversion of polyethylene into fuels over mesoporous MCM-41, Chem. Commun. 725 (1996). W. Kaminsky, and J. Eranck, Monomer recovery by pyrolysis of poly(methylme-thacrilate), J. Anal. Appl. Pyrolysis, 19, 311 (1991). 

W. Kaminsky and C. Eger, Pyrolysis of filled PMMA for monomer recovery, J. Anal. App. Pyrolysis, 58-59, 781-787 (2001). 

W. Kaminsky and J. Franck, Monomer recovery by pyrolysis of poly(methylmeth-acrylate) (PMMA), Journal of Analytical and Applied Pyrolysis, 19,311-318 (1991). 

Monomer Recovery of Plastic Waste in a Fluidized Bed Process... 

The monomer recovery is the highest by feeding pure PMMA pellets (98.4 wt%) in the laboratory plant. But highly filled PMMA yielded monomer concentrations of 83-90 wt%. A short residence time is preferred. The good results of the laboratory-scale experiments were confirmed by the experiments in the technical-scale plant yielding 97% MMA for pure PMMA pellets and 83.5% for silica-filled PMMA. 

They also rednce monomer recovery as well as recovery of other comonomers during pyrolysis becanse of the distnrbance of the step-by-step radical degradation process [29]. 

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