Heat removal, polymerization reactions

Both the emulsion and suspension processes use water as a heat sink. Polymerization reactions are easier to control in both these processes than in bulk or solution systems because stirring is easier and removal of the exothermic heat of polymerization is facilitated. 

Emulsion polymerization refers to a unique process employed for some radical chain polymerizations. It involves polymerization of monomers in the form of emulsions. Like suspension polymerization, the emulsion process uses water as a heat sink. Polymerization reactions are thus easier to control in both these processes than in bulk systems because stirring is easier and removal of the exothermic heat of polymerization is facilitated. Emulsion polymerization, however, differs from suspension polymerization in the type and smaller size of particles in which polymerization occurs and in the kind of initiator employed. The process is also quite different from suspension polymerization in its mechanism and reaction characteristics. 

It is important to note that problems to remove the heat from polymerization reactions are not only relevant in the case of thermal runaways. Local hot spots in the polymerization reactor can also cause undefined molecular mass distributions, product color-ization, or decomposition reactions and result, in this way, in a lower product quality. 

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. 

To accelerate the polymerization process, some water-soluble salts of heavy metals (Fe, Co, Ni, Pb) are added to the reaction system (0.01-1% with respect to the monomer mass). These additions facilitate the reaction heat removal and allow the reaction to be carried out at lower temperatures. To reduce the coagulate formation and deposits of polymers on the reactor walls, the additions of water-soluble salts (borates, phosphates, and silicates of alkali metals) are introduced into the reaction mixture. The residual monomer content in the emulsion can be decreased by hydrogenizing the double bond in the presence of catalysts (Raney Ni, and salts of Ru, Co, Fe, Pd, Pt, Ir, Ro, and Co on alumina). The same purpose can be achieved by adding amidase to the emulsion. 

During polymerization, when Initiator Is Introduced continuously following a predetermined feed schedule, or when heat removal Is completely controllable so that temperature can be programmed with a predetermined temperature policy, we may regard functions [mo(t ], or T(t), as reaction parameters. A common special case of T(t) Is the Isothernral mode, T = constant. In the present analysis, however, we treat only uncontrolled, batch polymerizations In which [mo(t)] and T(t) are reaction variables, subject to variation In accordance with the conservation laws (balances). Thus, only their Initial (feed) values, Imo] andTo, are true parameters. 

The other entries in Table 13.2 show that heat removal is not a problem for most ring-opening and condensation polymerizations. Polycaprolactam (also called Nylon 6) is an addition polymer, but with rather similar bond energies for the monomer and the polymer. The reaction exotherm is small enough that large parts are made by essentially adiabatic reaction in a mold. An equilibrium between monomer and polymer does exist for polycaprolactam, but it occurs at commercially acceptable molecular weights. 

Polymerization High viscosity may cause problems with heat removal from the reaction zone, reaction often uncontrollable... 

Heat is applied to the reactor to further concentrate the reactants and to supply the energy to activate the polymerization reactions. At the outset, the reactor temperature and pressure rise rapidly. Sensor measurements indicate the existence of a temperature gradient having as much as a 40°C difference between material at the top and at the bottom of the reactor. Shortly after the pressure reaches its setpoint, the entire mixture boils and the temperature gradient disappears. The solution is postulated to be well mixed at this time. The cumulative amount of water removed is one indication of the extent of polymerization. 

There is a potential thermal runaway upon the combined occurrence of two or more of the above listed factors. For example, an accumulation of reactants in combination with insufficient heat removal leads to a runaway of the desired reaction. The resulting temperature increase (now uncontrolled) may lead to an explosive decomposition if other exothermic reactions, such as decompositions or polymerizations, occur within the range of the temperature increase. 

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. 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

Polymerization of a monomer in a solvent overcomes many of the disadvantages of the bulk process. The solvent acts as diluent and aids in the transfer of the heat of polymerization. The solvent also allows easier stirring, since the viscosity of the reaction mixture is decreased. Thermal control is much easier in solution polymerization compared to bulk polymerization. On the other hand, the presence of solvent may present new difficulties. Unless the solvent is chosen with appropriate consideration, chain transfer to solvent can become a problem. Further, the purity of the polymer may be affected if there are difficulties in removal of the solvent. Vinyl acetate, acrylonitrile, and esters of acrylic acid are polymerized in solution. 

Solution Polymerization. By adding a solvent to the monomer-polymer mixtnre, heat removal can be improved dramatically over bnlk reactions. The solvent mnst be removed after the polymerization is completed, however, which leads to a primary disadvantage of solution polymerization. Another problem associated with radical chain polymerizations carried out in solution is associated with chain transfer to the solvent. As we saw in Section 3.3.1.2, chain transfer can significantly affect the molecular weight of the final polymer. This is particnlarly trne in solntion polymerization, where there are many solvent molecules present. In fact, chain transfer to solvent often dominates over chain transfer to other types of molecnles, so that Eq. (3.79) reduces to... 

Viscosity of Polymer Solutions. Polymer solutions are very common. Glues, pastes, and paints are just a few examples of commercially available aqueous suspensions of organic macromolecules. Also, recall from Chapter 3 that certain types of polymerization reactions are carried out in solution to assist in heat removal. The resulting polymers are also in solution, and their behavior must be fully understand in order to properly transport them and effect solvent removal, if necessary. 

In the use of polystyrene, the polymerization reaction is exothermic to the extent of 17 Kcal/mol or 200 BTU/lb (heat of polymerization). The polystyrene produced has a broad molecular weight distribution and poor mechanical properties. The residual monomer in the ground polymers can be removed using efficient devolatilization equipment. Several reviews are worthwhile consulting [42-44],... 

All prebiotic polymerization reactions, which are dehydration reactions, are thermodynamically unfavorable. This free energy barrier can be overcome in two ways. The first is to drive the dehydration reaction by coupling it to the hydration of a high energy compound, and the second method is to remove the water by heating. In principle, visible or ultraviolet light could drive these reactions, but so far no one has demonstrated adequately such processes. 

In radical polymerization, particularly in the large-scale production of poly(methyl methacrylate), the monomer may boil hence the process should be carried out with continuous removal of reaction heat to eliminate overheating in the reactive volume. In this case, the choice of cooling method needs careful consideration.1 2... 

Two types of polymerization units are designed by Universal Oil Products Company. The U.O.P. Reactor-type unit contains the catalyst in tubes which are surrounded by water in a jacket for the purpose of removing the heat liberated by the exothermic polymerization reaction. The steam generated in the water jacket normally is used to preheat the feed. A feed-to-products heat exchanger furnishes the remaining heat requirements. Conventional depropanizer and debutanizer columns are used to fractionate the product. Figure 3 shows a flow diagram of a reactor type of polymerization unit. 

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