Polymerisation heat removal

The preparation of a synthetic latex is shown to be a very complex process that is affected by the monomers selected, surfactants, initiators and the polymerisation process. The semi-continuous process is the one most frequently used as it provides control of the polymerisation heat removal, as well as control of the composition of the copolymers comprising several types of monomer units. Some aspects of copolymerisation in emulsion and particle growth in the case of the semi-continuous process are discussed. The copolymers usually comprise 4 to 5 comonomers, some of them with functional groups. The functional groups serve as loci for crosslinking, improve colloid stability, increase polarity, improve adhesion and cause alkali-solubility and/or alkali swellability. High value polymer latices with special particle morphology, composition and other... 

EPM and EPDM mbbers are produced in continuous processes. Most widely used are solution processes, in which the polymer produced is in the dissolved state in a hydrocarbon solvent (eg, hexane). These processes can be grouped into those in which the reactor is completely filled with the Hquid phase, and those in which the reactor contents consist pardy of gas and pardy of a Hquid phase. In the first case the heat of reaction, ca 2500 kJ (598 kcal)/kg EPDM, is removed by means of cooling systems, either external cooling of the reactor wall or deep-cooling of the reactor feed. In the second case the evaporation heat from unreacted monomers also removes most of the heat of reaction. In other processes using Hquid propylene as a dispersing agent, the polymer is present in the reactor as a suspension. In this case the heat of polymerisation is removed mainly by monomer evaporation. 

One of the drawbacks associated especially with slurry and solution CSTR processes is the necessity of removing the solvent or diluent in a post-production step. In a gas phase reactor the polymerisation takes place in a fluidised bed of polymer particles. Inert gas or gas mixture is used for fluidisation. The gas flow is circulated through the polymer bed and a heat-ex-changer in order to remove the polymerisation heat. Gaseous ethylene and comonomer are fed into the fluidisation gas line of the reactor, and a supported catalyst is added directly to the fluidised bed (Fig. 7). Polymerisation occurs at a pressure of about 20-25 bar and a temperature of about 75-110 °C. The polymer is recovered as a solid powder which is, however, usually pelletised. Due to the limited cooling capacity of the fluidising gas, reactor... 

The decrease of temperature gradients in the fast polymerisation processes exerts a substantial influence on the molecular characteristics of the polymers. Fleat removal by a heat-conducting wall determines, in many cases, the possibility of the practical implementation of the process. Changing the input method of the reactants into a reaction zone, under conditions of external heat removal, affects the MW characteristics of the products of fast polymerisation processes [30, 31, 95, 96]. 

In addition, as the cooling zone length does not usually exceed several centimetres in tubular devices for fast chemical processes (cationic polymerisation of isobutylene, neutralisation of acidic media, and so on), tubular devices with L/d <100 are recommended for such processes. It will provide more than a 1.5 times (up to 1.65 times) increase of external heat removal (heat emission coefficient) output by additional turbulisation from the input and output nipples. 

Equations 3.2-3.10 allow the opportunity to calculate the MW and MWD of a polymer at any given method of catalyst loading under adiabatic conditions of polymerisation, i.e., in the absence of heat removal. 

Taking into consideration the fact that fast polymerisation processes are characterised by inequality of chemical reaction time and transfer time ( chem < it is clear that an increase of facilitates the decrease of and both these processes are comparable in duration. The increase in linear flow rate V, i.e., the intensification of heat and mass exchange in the system, is equivalent to a slow dovm of the polymerisation reaction itself, compared with the transfer process. Therefore, the conventional approaches to external heat removal, which normally have such a restrictive effect on conventionally designed fast polymerisation processes implemented in stirred tank reactors, play an essential role at both high V and values when quasi-plug flow tubular turbulent reactors are used. In this case, control of the external temperature can be significantly enchanced due to zone-type catalyst loading. 

In this case, the conditions defining the adequacy of the zone model of a reactor, with external heat removal to the real tubular turbulent reactor, should be achieved with 1) the length of the heat removal zone should be more than that of the reaction zone, i.e., L, >> V/, and 2) the amount of heat removed, from the reaction zone through the wall, should be significantly less than that of the heat generated during polymerisation in that zone ... 

I is the portion of heat removed in the cooling zone from the total thermal energy accumulated by the system at the inlet of this zone (taking into account both the polymerisation reaction and cooling process in all previous zones). 

Thus, depending on the polymerisation conditions, particularly on the number of reactors in the cascade and the amount (portion) of a catalyst fed to each reactor, it is possible to influence the MW characteristics of the polymer obtained, over a specific but sufficiently wide range. Upon quasi-plug flow mode formation in turbulent flows, in the case of fast polymerisation processes, external heat removal becomes efficient enough and, as a consequence, has a notable effect on the temperature field of a reaction, as well as on the MW and MWD width of the polymeric product obtained. The occurrence of heat exchange in such reactors gives real opportunities to improve... 

In batch polymerisation (399), the components of the emulsion are charged into a stirred reactor, which is then heated to begin polymerisation. No material is added or removed during the entire polymerisation. Since most polymerisations are highly exothermal, the rate of heat generation can easily exceed the heat removal capabilities, and a mnaway reaction is possible. The batch polymerisation method offers little or no control over the copolymer composition, but depends upon the comonomer reactivity ratios and the partitioning of the comonomers in the latex particles (288, 340). 

In larger industrial reactors (often 1,000 to 100,0(X) litres), the reactor volume to surface area is high, resulting in poor heat removal. Cooling jackets are necessary, and reactions are carried out such that the jacket or reactor temperature is constant. In some polymerisations, there is effectively no heat removal. Rather, the emulsion recipe is designed so that the adiabatic temperature rise (i.e., with no heat transfer) in the reactor during polymerisation does not cause the maximum safe temperature limit to be exceeded (which would typically be below the boiling point of the aqueous continuous phase). 

The desired reactor temperature (270,271) is a balance between the heating time, the propagation rate constant, the initiator dissociation constant, and the heat removal capabilities of the reactor. Polymerisations are usually... 

Solution polymerisation. A solvent is used to solubilise both the monomers and the forming polymer, obtaining a homogeneous system. Solubilisation allows an easy control of the polymerisation heat and also decreases viscosity, allowing bulk stirring. On the other hand, it decreases the concentrations of the reactive species, lowering polymerisation rate. Solvent should be carefully selected with inert properties respect to the polymerisation reaction. Moreover, at the end of the reaction, the solvent should be completely removed. 

The ultimate goal of most of the investigations on emulsion copolymerisation is to be able to control the process in such a way as to produce a copolymer product (latex or coagulate) with desired properties. For this purpose the semi-continuous (sometimes called semi-batch) emulsion copolymerisation process is widely used in industry. The main advantages of this process as compared with conventional emulsion batch processes include a convenient control of emulsion polymerisation rate in relation with heat removal and control of chemical composition of the copolymer and particle morphology. These are important features in the preparation of speciality or high performance polymer latexes. 

Solution Polymerization. In this process an inert solvent is added to the reaction mass. The solvent adds its heat capacity and reduces the viscosity, faciUtating convective heat transfer. The solvent can also be refluxed to remove heat. On the other hand, the solvent wastes reactor space and reduces both rate and molecular weight as compared to bulk polymerisation. Additional technology is needed to separate the polymer product and to recover and store the solvent. Both batch and continuous processes are used. 

Since impurities can affect both the polymerisation reaction and the properties of the finished product (particularly electrical insulation properties and resistance to heat aging) they must be rigorously removed. In particular, carbon monoxide, acetylene, oxygen and moisture must be at a very low level. A number of patents require that the carbon monoxide content be less than 0.02%. 

Suspension polymerisation of styrene is widely practised commercially. In this process the monomer is suspended in droplets 5 -Min. in diameter in a fluid, usually water. The heat transfer distances for the dissipation of the exotherm are thus reduced to values in the range s-fisin. Removal of heat from the low-viscosity fluid medium presents little problem. The reaction is initiated by monomer-soluble initiators such as benzoyl peroxide. 

Alkali-washed material, stabilised with 0.25% of pyrogallol, was distilled at 103°C/4 mbar until slight decomposition began. The heating mantle was then removed and the still-pot temperature had fallen below its maximum value of 135°C when the residue exploded violently [1], The presence of solid alkali [2] or 5% of phenolic inhibitor is recommended, together with low-temperature high-vacuum distillation, to avoid formation of acidic decomposition products, which catalyse rapid exothermic polymerisation. 

In the first zone, the solution is heating for starting polymerisation, but because of the exothermic reaction, cooling becomes necessary in the second and third zones. The polymer solution is then extruded as fine strands into a devolatilising vessel. This vessel is heated to a temperature of 225°C and then the solvent and unreacted monomer is removed. The molten material is fed into an extruder, then cooled and finally chopped. 

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