Styrene polymer reactors

Classification of Processes and Reactors. Most styrene polymers are produced by batch suspension or continuous mass processes. Some are produced by batch mass processes. Mass in this sense includes bulk polymerization of the polymer... 

Fig. 1. Process flow sheet for the continuous conversion of latex in a counterrotating, tangential twin-screw extruder as it might be arranged for the production of acrylonitrile-butadiene-styrene polymer (Nichols and Kheradi, 1982). Polystyrene (or styrene-acrylonitrile) melt is fed upstream of the reactor zone where the coagulation reaction takes place. Washing (countercurrent liquid-liquid extraction) and solids separation are conducted in zones immediately downstream of the reactor zone. The remainii zones are reserved for devolatilization and pumping.

Zhang, X. Yoon, S.C. Lim, J.G. Lee, Y.S. Supported catalyst for producing syndiotactic styrenic polymer with high productivity and significantly reduced reactor fouling, U.S. Patent 6,828,270, December 7, 2004. 

There are different tubular and column plug flow reactors as well as screw reactors [1]. Plug flow reactors are used for various gas-phase reactions occuring within industrial-scale production, particularly for the reactions of nitrogen oxide oxidation, ethylene chloration, and high-pressure ethylene polymerisation. They are also used for some liquid-phase and gas-liquid reactions, e.g., styrene polymer production in a column, plastic and rubber production, synthesis of ammonia and methanol, and sulfation of olefins [2]. 

Significant effort has recently been put in for the elimination of polymer wastes from electric and electronic equipment (WEEE) by pyrolysis. WEEE includes mainly epoxy resins and styrene polymers. They often contain brominated aromatics, which are highly contaminant. However, their elimination by simple thermal treatments is no longer possible as one of the most important drawbacks in dealing with thermal treatment of WEEE is the likely production of supertoxic halogenated dibenzodiox-ins and dibenzofurans. A pyrolysis method at low temperature range was developed, which limited the formation of such toxic by-products and reduced pyrolysis costs, even at relatively long residence times in the reactor. 

Perhaps the largest-scale application of vacuum distillation is the separation of ethyl benzene from styrene monomer. Such a column typically operates at a top pressure of 50 mm Hg absolute, or higher, so that the distillate may be condensed in an air cooler. The distillate primarily consists of ethyl benzene but also contains any water, benzene, and toluene present in the feed. This distillate also contains up to 5% styrene. The bottoms product is styrene monomer, purified to a specification of 400 to 1,000 ppm ethyl benzene content. The bottoms product also contains some tars formed in the reactors and polymer that is present in the feed or formed in the distillation column. The bottoms product specification is fixed by market requirements however, the styrene present in the distillate merely recycles through the dehydrogenation reactor. 

In an adiabatic tubular reactor, styrene is converted partially to polymer at high pressure and the mixture of monomer and polymer is sprayed into a vacuum chamber with evaporation of monomer and recovery of polymer. If the heat of vaporization is 355 J/g and the monomer enters the tube at 50°C, what fraction can be converted to polymer per pass and still allow polymer recovery at 50°C ... 

The mbber latex is usually produced in batch reactors. The mbber can be polybutadiene [9003-17-2] or a copolymer of 1,3-butadiene [106-99-0] and either acrylonitrile [107-13-1] or styrene [100-42-5]. The latex normally has a polymer content of approximately 30 to 50% most of the remainder is water. 

Solution Polymerization These processes may retain the polymer in solution or precipitate it. Polyethylene is made in a tubular flow reactor at supercritical conditions so the polymer stays in solution. In the Phillips process, however, after about 22 percent conversion when the desirable properties have been attained, the polymer is recovered and the monomer is flashed off and recyled (Fig. 23-23 ). In another process, a solution of ethylene in a saturated hydrocarbon is passed over a chromia-alumina catalyst, then the solvent is separated and recyled. Another example of precipitation polymerization is the copolymerization of styrene and acrylonitrile in methanol. Also, an aqueous solution of acrylonitrile makes a precipitate of polyacrylonitrile on heating to 80°C (176°F). 

Methods have been developed for improving batch process productivity in the manufacture of styrene-butadiene latex by the continuous addition of reactants so the reaction occurs as the reactor is being filled. These are not continuous processes even though the reactants are added continuously during most of a batch cycle. The net result is that reactants can be added almost as fast as heat can be removed. There is relatively little hazardous material in the reactor at any time because the reactants, which are flammable or combustible, are converted to non-hazardous and non-volatile polymer very quickly. 

Low Conversion Reactors. The major problem in temperature control in low conversion reactors is the orders cf magnitude increase in viscosity as the conversion increases. Fig.8 shows the viscosity of a polystyrene solution as the function of percent PS. The data are for polystyrene with a Staudinger molecular weight of 60,000 at 100 C and 150 C in a cumene solution, a satisfactory analog for styrene monomer solutions. As the polymer concentration increases from 0 to 60%, viscosity increases from about 1 cp to 10 cp. 

In this work, the characteristic "living" polymer phenomenon was utilized by preparing a seed polymer in a batch reactor. The seed polymer and styrene were then fed to a constant flow stirred tank reactor. This procedure allowed use of the lumped parameter rate expression given by Equations (5) through (8) to describe the polymerization reaction, and eliminated complications involved in describing simultaneous initiation and propagation reactions. 

Three polymer seeds were prepared in a batch reactor. The reactor with styrene and benzene was cooled to 0 C in an ice bath, initiator was injected into the reactor and reaction began with a gradual increase in temperature. Table II presents the initial conditions used in preparing the seed polymer and the molecular weights of the seed polymer. The molecular weight distribution of the pol3nner seeds are shown in Figure 5. 

This monomer is ethylene when R is hydrogen, propylene when R is a methyl group, styrene when R is a benzene ring, and vinyl chloride when R is chlorine. The polymers formed from these four monomers account for the majority of all commercial plastics. The polymers come in great variety and are made by many different processes. All of the polymerizations share a characteristic that is extremely important from the viewpoint of reactor design. They are so energetic that control of the reaction exotherm is a key factor in all designs. 

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