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. Heat is removed with an external heat exchanger. The exothermic reaction takes place at a pressure of 5–10 bar and a temperature of 75–85 ☌. The continuously operating reactor is also fed with monomer, hydrogen and hexane. The catalyst is prepared batchwise in a vessel, diluted in another vessel and then added to the reactor. Current reactors have a volume of up to 300 m3 and capacities of up to 500 kt/a.
#Emulsion and suspension polymerization series
Unimodal HDPE is produced in parallel reactors, whereas bimodal HDPE is produced in series reactors. Precipitation polymerization of HDPE is carried out at low pressures in an autoclave. One group of polycondensates, the polycarbonates, are gaining importance as high-performance plastics. Thermoplastic polyesters, especially polyethylene terephthalate, are economically important materials that are used to manufacture fibres and bottles. The axial flow can be improved significantly by using a draft tube. 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. 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. At high viscosities, water can only be removed by evaporation from the surface of the reaction mixture. 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. Polycondensation, in which water is always a product, is endothermic, in contrast to other classical polymerization reactions. Polyesters are synthesised by condensation polymerization (or polycondensation) of polyfunctional carboxylic acids with polyfunctional alcohols.
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