Bulk polymerization reactors, thermal

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. 

Bulk Polymerization. Monomer and polymer (with traces of initiator) are the only constituents in bulk polymerizations. Obviously, the monomer must be soluble in the polymer for this type of process to effectively proceed. Bulk polymerization, also called mass or block polymerization, can occur in stirred-tank reactors, or can be unstirred, in which instance it is called quiescent bulk polymerization. The primary difficulty with bulk polymerizations is that as the polymerization proceeds and more polymer is formed, the viscosity increases, thermal conductivity decreases, and heat removal becomes difficult. 

As a sequel to the simple reactor model described above, two-zone cases for the bulk polymerization of styrene were also studied. Polymerizations in straight, empty tubes give rise to unfavorable temperature and velocity profiles which can lead to hydrodynamic or thermal instabilities. These instabilities may be avoided or postponed by manipulating the wall temperature. 

Crystal polystyrene is produced by thermally initiated (Section 6.5.4) bulk polymerization of styrene at temperature of I20°C or more. (The term crystal refers to the optical clarity of products made from this polymer, which is not crystalline.) The rate of polymerization would decrease with increasing conversion and decreasing monomer concentration if the reaction were carried out at constant temperature. For this reason, the polymerization is performed at progressively increasing temperatures as the reaction mixture moves through a series of reactors. The exothermic heat of polymerization is useful here in raising the reaction temperature to about 250°C as the process nears completion. 

Data on a thermal batch bulk polymerization at 150°C (run in a two-litre bench-scale reactor) are presented in Table III and compared with computer predictions. Here, the computer prediction of molecular weight is good (almost independent of conversion) while the predicted conversions are slightly low. Comparison of the model predictions with thermal polymerization data of Hui and Hamielec 6) and Husain and Hamielec (J7) also indicate that the... 

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. 

Superabsorbent polyacrylates are prepared by means of free-radical-initiated copolymerization of acrylic acid and its salts with a cross-linker (12,13). Two principal processes are used bulk, aqueous solution pol5unerization and suspension polymerization of aqueous monomer droplets in a hydrocarbon liquid continuous phase (14) (see Bulk and Solution Polymerizations Reactors Heterophase Polymerization). In either process, the monomers are dissolved in water at concentrations of 20-40 wt% and the polymerization is initiated by free radicals in the aqueous phase (15). The initiators, freeradical (qv) used include thermally decomposable initiators, reduction-oxidation systems, and photochemical initiators and combinations. Redox systems include persulfate/bisulfite, persulfate/thiosulfate, persulfate/ascorbate, and hydrogen peroxide/ascorbate. Thermal initiators include persulfates, 2,2 -azobis(2-amidinopropane)-dihydrochloride, and 2,2 -azobis(4-cyanopentanoic acid). Combinations of initiators are useful for polymerizations taking place over a temperature range. 

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. 

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. 

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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