Polymerization

In emulsion polymerization, the water-insoluble monomer is predispersed in an aqueous phase. In contrast to bead polymerization, the dispersion is not stabilized physically by turbulence, but chemically using emulsifiers. Polymerization does not take place in the dispersed monomer droplets, whose diameter is 10–1000 μm, but in the much smaller latex particles with diameters of about 0.3–0.8 μm. These contain both polymer and monomer molecules and are surrounded by emulsifier molecules that stabilize them against the aqueous phase. Owing to the small particle size, the heat of reaction can easily be dissipated into the aqueous phase. Heat transfer between the vessel wall and the aqueous phase is very good on account of the low viscosity of the emulsion and the high thermal conductivity of water.

In homogeneous solvent polymerization, the viscosity is lowered by adding a chemically inert solvent. Both the monomer and the polymer are present in solution during the entire process. In many cases, heat removal is improved by simultaneous evaporative cooling induced by boiling off the solvent.

Equipment to produce high-impact polystyrene (HIPS) generally consists of a cascade of three to five reactors, divided into pre-polymerization and post-polymerization stages. In the pre-polymerization stage, the target morphology and particle size are already essentially predefined. The reaction is generally carried out at 100–150 °C with yields of up to 15–30 %. During post-polymerization, the polymerization reaction is continued to give higher yields with correspondingly higher viscosities. Post-polymerization is usually carried out at temperatures of 140–190 °C.

In the reactor cascade, the heat of polymerization is removed by evaporating the styrene monomer. The gaseous monomer is then condensed and fed back into the reactor. This type of heat removal requires high homogeneity and good surface entrainment. For this reason, these reactors are often equipped with the Ekato Paravisc. Temperature homogeneity is the key variable influencing the molecular weight distribution and thus the attainable product quality.

Isobutylene-isoprene rubber (IIR) is a copolymer of isobutylene and isoprene. This material is used for high-performance, long-distance car tyres. To achieve a high molecular weight, the strongly exothermic reaction must be carefully controlled at very low temperatures as low as –90 °C to –100 °C. The most commonly used process for synthesising IIR is low-temperature cationic polymerization. This type of polymerization involves generating a suspension of very fine rubber particles in methyl chloride. Because the reaction is very exothermic, the reactor is designed as a draft tube with very high axial flow rates. The cylindrical chamber of the reactor is equipped with tube bundle heat exchangers. In addition, the extremely fast reaction necessitates fast homogenization of the material feeds. The impeller is introduced from below, which requires the corresponding submerged seal with flushing device.

Polyesters are synthesised by condensation polymerization (or polycondensation) of polyfunctional carboxylic acids with polyfunctional alcohols. Polycondensation, in which water is always a product, is endothermic, in contrast to other classical polymerization reactions. Shifting the chemical equilibrium of the reversible reaction to the side of the polyester requires that the water produced by the condensation reaction is continuously removed from the reaction mixture. At high viscosities, water can only be removed by evaporation from the surface of the reaction mixture. This means that the contents of the reactor must be efficiently circulated using axial pumping impellers with a small wall clearance to achieve high degrees of polymerization. Whereas a purely radial pumping agitator produces high polymerization degrees only close to the surface, this can be achieved throughout the reaction vessel if there is also axial exchange. The axial flow can be improved significantly by using a draft tube. Thermoplastic polyesters, especially polyethylene terephthalate, are economically important materials that are used to manufacture fibres and bottles. One group of polycondensates, the polycarbonates, are gaining importance as high-performance plastics.

Precipitation polymerization of HDPE is carried out at low pressures in an autoclave. Unimodal HDPE is produced in parallel reactors, whereas bimodal HDPE is produced in series reactors. Current reactors have a volume of up to 300 m3 and capacities of up to 500 kt/a. The catalyst is prepared batchwise in a vessel, diluted in another vessel and then added to the reactor. The continuously operating reactor is also fed with monomer, hydrogen and hexane. The exothermic reaction takes place at a pressure of 5–10 bar and a temperature of 75–85 °C. Heat is removed with an external heat exchanger. The molecular weight, molecular weight distribution and density of the product are controlled by adjusting the type and concentration of the catalyst and co-monomer as well as the quantity of hydrogen. The processing chain ends with the post-reactor in which the monomer reaches a conversion rate of 99%. The resulting suspension is fed to a receiver and then centrifuged, dried in a fluidised bed with hot nitrogen and finally sieved. Stabilisers and additives are mixed in before it is extruded. Due to the the tall shape of these polymerization reactors the main mixing task is to achieve very short blend times. In addition, high wall velocities must prevent scaling on the vessel walls. EKATO developed the ISOJET VDT concept to carry out these tasks efficiently. The flow mechanics induced by the multiple Isojet stages stacked on top of each other act as a virtual draft tube (VDT) that accelerates the downward axial flow. This allows extremely short blend times, even in very thin and tall vessels. The special design of this mixing system quickly equalises any concentration or temperature gradients, thus leading to high product qualities. The flow pattern close to the walls shows correspondingly high upward flow velocities that prevent incrustations and deposits.

The CFD simulation of the flow velocities for the Viscoprop clearly shows that the flow currents near the agitator shaft and close to the impellers are fast and directed downwards in an axial direction. This is achieved by the optimized shape of the Viscoprop impellers and tailoring of the mixing system, including baffles, to the reaction vessel. Close to the reactor wall there is a corresponding flow profile directed upwards that provides high wall speeds which reduces deposits forming at the wall. Another benefit of a axial pumping, multi-stage set up is a more narrow particle size distribution.

The synthesis of ABS is generally carried out in two steps. In the first step, the butadiene monomer undergoes emulsion polymerization to produce a polybutadiene dispersion (PBL). This is then reacted in the second step with the styrene-acrylonitrile (SAN) copolymer in an emulsion to achieve the target rubber concentration. Before continuing processing with styrene-acrylonitrile, it is important to adjust the particle size of the PBL dispersion to the desired value. Larger particles produce a greater impact toughness in the final product, but a lower surface gloss. The optimum size range of the PBL particles is approx. 0.3–0.5 µm. Loading of the latex particles in the shear field of the agitator increases with increasing particle size. If the emulsifier envelope of two neighbouring particles is destroyed by high local shear introduced by an unsuitable agitator system, they will coagulate to even larger latex particles. This results in substantial changes to the mechanical properties of the end product. It also leads to the formation of thicker wall deposits that inhibit the dissipation of heat generated by the reaction. As a consequence, frequent cleaning cycles with considerable loss of productivity must be accepted. The Ekato Isojet B, a very low shear impeller, is ideal for PBL reactors, particularly as a multi-stage version. At the same time, using baffles as additional heat exchangers provides very efficient cooling.