The figure shows a schematic representation of the flowchart for the conventional slurry polymerization (SP) process used in a significant number of first-generation plants that are still in commercial production.
The catalyst used (Ti halide) and the cocatalyst (Al alkyl halide) are added to the reactor together with a liquid “carrier,” consisting of a mixture of organic solvents, in which the monomer is dissolved.
The reaction takes place in a stirred tank reactor, in which the slurry is thermally controlled by heat extraction via jackets, coils, and external recirculation through heat exchangers, as in the Hoechst process for PE described above. Typical temperatures and pressures are: 50 to 90 °C and 5 to 15 atm.
The catalyst introduces a degree of isotacticity into the resulting polymer. The isotactic form is insoluble in the carrier, while the atactic form dissolves in it.
In these conventional processes, deashing (catalyst recovery) must be performed, given the relatively low activity of the catalyst. To do this, after a decompression procedure in the “flash” evaporation vessel—in which most of the unreacted monomer is recovered—the slurry is treated with alcohol to deactivate the reaction and dissolve the catalyst in the form of alcoholates, which form a phase distinct from that of the slurry, allowing the alcoholates to be separated by extraction.
A wash with demineralized water completes the extraction process.
The next step involves separating the insoluble isotactic particles from the solvent + atactic PP solution by centrifugation. The crystalline fraction is dried with nitrogen and processed via extrusion. The liquid effluent from the centrifuge is fed into an evaporator where, through indirect heating, the solvent is removed. This, together with the effluent from the “deashing” stages, is sent to the solvent recovery plant, where it is rectified. Amorphous PP is extracted from the bottom of the evaporator; it can be used in grades that are relatively easy to process but have lower mechanical properties.
In other plants—or even in these same plants if certain post-treatment steps are omitted, and provided that the capacity of the devices designed to extract reaction heat allows it—it is possible to use high-activity (and high-stereospecificity) catalysts, for which recovery is not necessary (nor is the extraction of the virtually nonexistent amorphous fractions). The schematic flow diagram corresponding to these simplified processes (SSP) is shown in the figure.
The diagram shows that the catalyst removal step has been eliminated. Polymerization is followed by the following steps:
1) Monomer recovery via flash evaporation.
2) Centrifugation to separate the insoluble crystalline formations.
3) Drying of these formations.
4) Processing
This simplification of processes is important in terms of production costs. All process units dedicated to the recovery and recycling of the carrier that makes up the slurry are retained.
In this group of technologies, it is possible to observe the impact of the catalyst’s activity and stereospecificity on the process design. A plant using first-generation technology may, under certain circumstances, dispense with entire sections of the plant if the catalyst’s activity eliminates the need for catalyst recovery.