Ethylene polymerization, heat removal

Similar to other ethylene polymerization processes, removal of the heat of polymerization from the reactor is a critical feature of the process. In this regard, the tubular high-pressure process has a more efficient heat removal process than the stirred autoclave reactor. 

Polymerization of methyl methacrylate to Plexiglas is done in the bulk process. High pressure polymerization of ethylene is done this way also. But other addition polymerizations frequently become too exothermic and without adequate heat removal system, the reaction tends to run away from optimum conditions. 

The heat of polymerization of ethylene is high (93.6 kJ/mol). Heat removal is thus a key issue in commercial polymerization processes. Polyolefins are produced primarily by suspension (slurry), gas-phase, or solution processes (20). Solution processes have been developed by various companies using hydrocarbons, such as heptane or cyclohexane, or hydrocarbon mixtures as solvents. The reaction temperature is in the range of 200-300°C. An advantage of these processes is that they readily accommodate a wide range of comonomer types and product densities. Like the high-pressure process, which is also a solution process, they are unable to accommodate highly viscous products. 

DP 2 For heats of polymerization assume ca. 100 kJ per mol of ethylene or propylene (disregard finer details). Consult e.g. Section 7.2 and references given at its end for heat removal aspects. 

Conditions used in PE processes vary widely. Because the heat of polymerization for ethylene is quite high (variably reported to be between 22 and 26 kcal/mole), efficient heat removal is crucial for polyethylene processes. Selection of process must also accommodate catalyst features, such as its kinetic profile. Table 7.1... 

Free radical polymerization of neat monomer in the absence of solvent and with only initiator present is called bulk or mass polymerization. Monomer in the liquid or vapor state is well mixed with initiator in a heated or cooled reactor as appropriate. The advantages of this method are that it is simple, and because of the few interacting components present, there is less possibility for contamination. However, vinyl-type polymerizations are highly exothermic so that control of the temperature of bulk polymerization may be difficult. Also, in the absence of a solvent viscosities may become very high toward the end of a polymerization, which could make stirring difficult, and add to the difficulty of heat removal from the system. The advantages of this system, however, are sufficiently attractive for this to be used commercially for the free radical polymerization of styrene, methyl methacrylate, vinyl chloride, and also for some of the polymerization processes of ethylene [7]. 

During the late 1970s, Union Carbide developed a low-pressure polymerization process (Unipol process) capable of producing polyethylene in the gas phase that required no solvents. The process employed a chromium based catalyst. In this process (Figure 4.1) ethylene gas and solid catalysts are fed continuously to a fluidized bed reactor. The fluidized material is polyethylene powder which is produced as a result of polymerization of the ethylene on the catalyst. The ethylene, which is recycled, supplies monomer for the reaction, fluidizes the solid, and serves as a heat-removal medium. The reaction is exothermic and is normally run at temperatures 25-50°C below the softening temperatures of the polyethylene powder in the bed. This operation requires very good heat transfer to avoid hot spots and means that the gas distribution and fluidization must be uniform. 

Addition polymerization is exothermic, and one of the major constraints to high production rates is the problems associated with heat removal. In processes using ethylene, the pressure of the gas determines solubility in the liquid phases (i.e., water and vinyl acetate monomer droplets) and in the polymer particles. This concentration of ethylene at the point of polymerization determines the ethylene content of the final polymer. Use of high pressures in such systems eliminates refluxing of the vinyl acetate, losing a very eflective heat removal mechanism available to simple batch-process PVA production. Refluxing,... 

A very efficient alternative for heat removal is to use overhead condensers. This modification uses the latent heat of evaporation of the monomer to remove the heat of polymerization. Monomer is evaporated in the reactor, condensed in the overhead condenser, and the cooled liquid monomer is returned to the reactor. This design works well for propylene polymerization, but it is not a good option for ethylene because of its much lower boiling point. Overhead condensers are used in the El Paso bulk polypropylene process [72]. 

Later bulk polymerization processes were developed where liquid propylene was either used as the only diluent in a loop reactor or permitted to boil out to remove the heat of reaction. The second was done in stirred vessels with vapor space at the top. More recently, gas-phase polymerizations of propylene were introduced. The technology is similar to the gas-phase technology in ethylene polymerizations [15] described in Sect. 6.1. 

Polymerization of ethylene is quite exothermic (3.4 x 10 J/kg) and since the heat capacity of gas is much lower than that of liquid, removal of the heat of polymerization can be problematic compared to solution and slurry processes. This was usually accomplished by lowering the activity of gas-phase catalysts by say 50-75% to reduce the rate of local heat generated. To compensate, the residence time was then extended to several hours. As a result of these differences, gas-phase processes tend to have a much larger polymer inventory in the reactor. The gas-phase approach is also more rigid in its catalyst requirements. The kinetic profile of a catalyst for a gas-phase process should preferably have a steady activity lasting 2-3 h. The particle size for consistent fluidization is also sometimes important, and smaller particles are preferred for heat removal. 

Heat removal and viscosity increases during polymerization are facilitated by using a diluent solvent. However recovery and repurification of the solvent, together with flammability hazards, have limited this technique. Heat removal can be conveniently carried out using the latent heat of vaporization, e.g. the cationic polymerization of isobutene to make polyisobutylene (butyl rubber) is maintained at — 100°C by the refluxing ethylene solvent. 

Eor all these reasons heat removal and reliable temperature control are key factors in all technical ethylene polymerization processes to ensure an economical and safe process. The different processes may chose different ways to limit or remove the reaction heat (e.g., by limited conversion per reactor pass, cooling of unreacted monomer, large surface area for heat exchange) in all concepts heat management is a key aspect of the reactor design. 

The driving force for developing ethylene polymerization in a loop reactor was problems encountered in the autoclave process with fouling and related problems with heat removal. In the loop process two important features support effective heat removal the high surface-to-volume ratio offered by the pipe and the turbulent flow... 

Some technological problems involved in building large LDPE production units are process operation, size of compressors, reactor structure, high-pressure valves, and safety problems [7]. Due to high exothermicity of the polymerization reaction, the removal of reaction heat is a critical design problem. Factors that affect the heat removal include reactor surface/volume ratio, reaction mixture and feed ethylene temperature difference, thickness of the polyethylene layer at the inner wall of the reactor, reaction mixture flow rate, and reactor material heat conductance. It should be noted that the thickness of the laminar layer at the reactor wall is affected by the reaction mixture flow rate. 

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