Cooling of Polymerization Reactors

In Section 11.2, general principles of reactor safety and heat balance of reactors are presented, with an emphasis on specific aspects of polymerizations. Section 11.3 is devoted to safety-related thermodynamics and reaction engineering aspects of polymerization reactions. In Section 11.4, cooling of polymerization reactors is reviewed. The chapter is concluded by a section describing safety aspects of industrial processes, together with technical risk-reducing solutions. 

But in most cases a simplified heat balance, which comprises only the most important terms on the right-hand side of Equation (10), is sufficient for safety purposes. Such an expression will be used in Section 11.2.4. The problems linked with the cooling of polymerization reactors are presented in detail in Section 11.4. 

This can be seen in the simulation of the PID control of polymerization reactor temperature by Houston and Schork [34]. Reactor temperature is often controlled by manipulating the temperature or flow rate of coolant in the reactor cooling jacket. This scheme is hindered by the slow dynamics of heat removal and the nonlinear nature of the heat evolution process (especially in... 

A schematic of a continuous bulk SAN polymerization process is shown in Figure 4 (90). The monomers are continuously fed into a screw reactor where copolymerization is carried out at 150°C to 73% conversion in 55 min. Heat of polymerization is removed through cooling of both the screw and the barrel walls. The polymeric melt is removed and fed to the devolatilizer to remove unreacted monomers under reduced pressure (4 kPa or 30 mm Hg) and high temperature (220°C). The final product is claimed to contain less than 0.7% volatiles. Two devolatilizers in series are found to yield a better quaUty product as well as better operational control (91,92). 

Low density polyethylene is made at high pressures in one of two types of continuous reactor. Autoclave reactors are large stirred pressure vessels, which rely on chilled incoming monomer to remove the heat of polymerization. Tubular reactors consist of long tubes with diameters of approximately 2.5 cm and lengths of up to 600 m. Tubular reactors have a very high surface-to-volume ratio, which permits external cooling to remove the heat of polymerization. 

Two vinyl chloride polymerization reactors were being operated by the same team of operators. Reactor 3 was in the cool down and dump phase of the process, and reactor 4 was nearly full of monomer and in the polymerization phase. The foreman and three employees set to work... 

Plasma polymerization is usually carried out in a low pressure glow discharge sustained by either a dc or an ac electric field. Examples of the reactors used for this purpose are shown in Fig. 1. The simplest configuration involves a pair of circular parallel plate electrodes mounted inside a glass bell jar. The lower electrode usually serves as the substrate holder and is sometimes heated or cooled. Monomer is introduced through a feed tube and unconsumed monomer and gaseous products are withdrawn through a port in the base plate. 

The diene polymerizations were generally allowed to proceed for 48 hours, which ensured quantitative conversion. In the case of butadiene, the reactor was cooled periodically to draw residual butadiene monomer into the solution from the void volume of the reactor. 

MPa was obtained. The temperature was further elevated to 85°C and maintained at this temperature for 2 hours. At the end of polymerization the ethylene feed was stopped and the reactor quickly cooled, vented, and polyethylene powder isolated from hexane. Polymerization scoping reactions studies are provided in Table 1. 

The type of feed that can be used varies widely. In nonselective polymerization in chamber-type plants a C3-C4 olefin content of about 20 to 25%, is practical. When the olefin content of the fresh feed rises above 25%, enough spent gas of low olefin content must be recycled in order to limit the olefin content of the total feed. Reactor-type plants can employ a higher olefin content in the feed, the exact value depending on the cooling efficiency of the reactor. 

During normal operation, it is essential to ensure sufficient cooling in order to control the temperature of the reactor, hence to control the reaction course. This typical question should be addressed during process development. To ensure the thermal control of the reaction, the power of the cooling system must be sufficient to remove the heat released in the reactor. Special attention must be devoted to possible changes in the viscosity of the reaction mass as for polymerizations, and to possible fouling at the reactor wall (see Chapter 9). An additional condition, which must be fulfilled, is that the reactor is operated in the dynamic stability region, as described in Chapter 5. 

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