Reactors suspension polymerization

Tubular reactor Suspension polymerization to prepare large porous polymer beads in a tubular reactor less polydisperse particle size distribution than in batch stirred-tank reactor 176... 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

Hydroxyhydroquinone and pyrogaHol can be used for lining reactors for vinyl chloride suspension polymerization to prevent formation of polymer deposits on the reactor walls (98). Hydroxyhydroquinone and certain of its derivatives are useful as auxiUary developers for silver haUde emulsions in photographic material their action is based on the dye diffusion-transfer process. The transferred picture has good contrast and stain-free highlights (99). 5-Acylhydroxyhydroquinones are useful as stabilizer components for poly(alkylene oxide)s (100). 

Suspension Polymerization. In this process the organic reaction mass is dispersed in the form of droplets 0.01—1 mm in diameter in a continuous aqueous phase. Each droplet is a tiny bulk reactor. Heat is readily transferred from the droplets to the water, which has a large heat capacity and a low viscosity, faciUtating heat removal through a cooling jacket. 

The original wartime process was mn batchwise in reactors similar to those used for suspension polymerization. Since then, in many plants, the reactors have been hooked together as a series of continuous stirred tanks. 

Processes that are essentially modifications of laboratory methods and that allow operation on a larger scale are used for commercial preparation of vinyhdene chloride polymers. The intended use dictates the polymer characteristics and, to some extent, the method of manufacture. Emulsion polymerization and suspension polymerization are the preferred industrial processes. Either process is carried out in a closed, stirred reactor, which should be glass-lined and jacketed for heating and cooling. The reactor must be purged of oxygen, and the water and monomer must be free of metallic impurities to prevent an adverse effect on the thermal stabiUty of the polymer. 

Polymerization Kinetics of Mass and Suspension PVC. The polymerization kinetics of mass and suspension PVC are considered together because a droplet of monomer in suspension polymerization can be considered to be a mass polymerization in a very tiny reactor. During polymerization, the polymer precipitates from the monomer when the chain size reaches 10—20 monomer units. The precipitated polymer remains swollen with monomer, but has a reduced radical termination rate. This leads to a higher concentration of radicals in the polymer gel and an increased polymerization rate at higher polymerization conversion. 

Beaded polymeric supports are produced by a two-phase suspension polymerization in which microdrops of a monomer solution are directly converted to the corresponding microbeads. The size of a microdroplet is usually determined by a number of interrelated manufacturing parameters, which include the reactor design, the rate of stirring, the ratio of the monomer phase to water, the viscosity of both phases, and the type and concentration of the droplet stabilizer. 

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.

In the suspension polymerization process, the autoclave reactor is filled with water. PVA, polyvinyl alcohol is the dispersing agent that helps stabilize the suspension. Lauroyl peroxide is the free radical catalyst that starts it all off. The reaction temperature is around 130°F, and the process takes 10—12 hours per batch, with 95% conversion. 

Fig. 1.3 S mall scale oscillatory baffled reactor (OBR) for gram scale suspension polymerization.

The final polymerizations—i.e., the prepolymerization in a w/o emulsion followed by suspension polymerization were carried out in a 40-liter stainless steel reactor. The stirrer speed was varied between 200 and 420 rpm. Prepolymerizations were performed at 60°-100°C, the ensuing suspension polymerizations at up to 140 °C. 

The suspension process is practiced by only a few companies because it offers a higher degree of production control and product engineering during polymerization step. This process suspends the water-based reactant in a hydrocarbon-based solvent. The net result is that the suspension polymerization creates the primary polymer particle in the reactor rather than mechanically in postreactions stages. Performance enhancements can also be during or just after the reaction stage. 

Today 80% of PVC is manufactured by the technique of batchwise suspension polymerization (S-PVC), the remaining part being shared between emulsion and bulk polymerization. A distinctive characteristic of the S-PVC processes is the large size of the reactors, 50 to 200m3, and operation at 10 to 12 bar pressure. The reaction time has been reduced from 18 h in the 1960s to only 3.5-5 h today by making use of fast initiation systems and recipe optimization. 

Kiparissides, C., Daskalakis, G., Achilias, D.D., Sidiropoulou, Dynamic simulation of industrial poly(vinyl chloride) batch suspension polymerization reactors, Ind. Eng. Chem. Res., 1997, 36,1253-1267... 

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