Bulk polymerization reactors, thermal control

The thermal control of bulk polymerization reactors is challenging because the high heat of reaction of the polymerization is accompanied by the low heat capacity and heat conductivity of the polymers and the high viscosity of the reaction mixture [ 1 ]. A better thermal control can be achieved by carrying out the polymerization in a phase dispersed in a basically inert continuous medium. This reduces the overall viscosity of the reaction medium allowing an efficient heat transfer. In addition, it lowers the rate of heat generation per unit volume. 

However, a commercially feasible process for bulk polymerization in a continuous stirred tank reactor has been developed by Montedison Fibre [103,104]. The heat of reaction is controlled by operating at relatively low-conversion levels and supplementing the normal jacket cooling with reflux condensation of unreacted monomer. Operational problems with thermal stability are controlled by using a free radical redox initiator with an extremely high decomposition rate constant. Since the initiator decomposes almost completely in the reactor. 

Emulsion polymerization is mostly carried out in stirred tank reactors operated semicontinuously. The reason for using semicontinuous operation is that under similar reaction conditions, the polymerization rate is higher than in bulk (see Section ), which makes the thermal control of the reactor difficult even with the relatively low overall viscosity of the reaction medium. Therefore, heat generation rate is controlled by feeding the monomers slowly. In addition, the semicontinuous operation allows better control of polymer characteristics. Continuous stirred tank reactors are used for the production of some high-tonnage emulsion polymers such as styrene-butadiene rubber. 

Bead Polymerization Bulk reaction proceeds in independent droplets of 10 to 1,000 pm diameter suspended in water or other medium and insulated from each other by some colloid. A typical suspending agent is polyvinyl alcohol dissolved in water. The polymerization can be done to high conversion. Temperature control is easy because of the moderating thermal effect of the water and its low viscosity. The suspensions sometimes are unstable and agitation may be critical. Only batch reactors appear to be in industrial use polyvinyl acetate in methanol, copolymers of acrylates and methacrylates, polyacrylonitrile in aqueous ZnCb solution, and others. Bead polymerization of styrene takes 8 to 12 h. 

The autoacceleration effect (Trommsdorf effect) is less pronounced in solution polymerization than in bulk or suspension polymerization due to lower viscosity of the polymerizing solution. To prevent a thermal runaway reaction, the reactants are often added gradually to the reactor. The polymer molecular weight is controlled through the use of a chain transfer agent and by initiator concentration and type. Monomer concentration, solvent type, and reaction temperature also affect the molecular weight. 

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