Slurry polymerisation

The slurry process is the oldest and still widely used method for manufacturing polymers of ethylene, propylene and higher a-olefins. In this process, the monomer dissolves in the polymerisation medium (hydrocarbon diluent) and forms a solid polymer as a suspension containing ca 40 wt-% of the polymer the polymerisation occurs below the melting point of the polymer. In slurry polymerisation, the temperature ranges from 70 to 90 °C, with the ethylene pressure varying between 7 and 30 atm. The polymerisation time is 1-4 h and the polymer yield is 95-98 %. The polymer is obtained in the form of fine particles in the diluent and can be separated by filtration. Removal of the catalyst residues from the polymer can be achieved by the addition of alcohol (isopropanol, methanol), followed by recovery and extraction of the catalyst residues. The polymer is freed from diluent by centrifuging and then dried. In the case of polypropylene manufacture, the atactic fraction remains in the diluent [28,37]. 

In some processes the polymerisation is carried out in a series of cascade reactors to allow variation in the hydrogen concentration through the operating steps in order to control the molecular weight distribution in the polymers formed. 

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The technologies suitable for LLDPE manufacture include gas-phase fluidised-bed polymerisation, polymerisation in solution, polymerisation in a polymer melt under high ethylene pressure, and slurry polymerisation. Most catalysts are fine-tuned for each particular process. 

Figure 3.52 Flow scheme of polypropylene production using the slurry polymerisation process (with the removal of catalyst residues)...

Polymer is separated from the polymerisation slurry and slurried with acetic anhydride and sodium acetate catalyst. Acetylation of polymer end groups is carried out in a series of stirred tank reactors at temperatures up to 140°C. End-capped polymer is separated by filtration and washed at least twice, once with acetone and then with water. Polymer is made ready for extmsion compounding and other finishing steps by drying in a steam-tube drier. 

Factors affecting laboratory polymerisation of the monomer have been discussed" and these indicate that a Ziegler-Natta catalyst system of violet TiCl3 and diethyl aluminium chloride should be used to react the monomer in a hydrocarbon diluent at atmospheric pressure and at 30-60°C. One of the aims is to get a relatively coarse slurry from which may be washed foreign material such as catalyst residues, using for example methyl alcohol. For commercial materials these washed polymers are then dried and compounded with an antioxidant and if required other additives such as pigments. 

A polymer is produced by the emulsion polymerisation of acrylonitrile and methyl methacrylate in a stirred vessel. The monomers and an aqueous solution of catalyst are fed to the polymerisation reactor continuously. The product is withdrawn from the base of the vessel as a slurry. 

Slurry processes in which dissolved ethylene is polymerised to form solid polymer particles suspended in a hydrocarbon diluent... 

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... 

Ethylene polymerisation in the presence of Phillips catalysts is carried out in a slurry using a hydrocarbon medium at pressures up to 40 atm and temperatures from 80 to above 100 °C. The temperature of catalyst preparation in its... 

The technologies that have been developed for the production of polyolefins, olefin homopolymers and copolymers are slurry, solution and gas-phase polymerisation bulk polymerisation of propylene in the liquid monomer as a special case of the slurry process has also emerged. The fundamental differences in the various olefin polymerisation processes reflect the different approaches that have been devised to remove the substantial heat of polymerisation. In addition, processes can be operated in a batch or a continuous mode. In the batch process the reagents are loaded into a polymerisation vessel, the polymer forms and the vessel is emptied before a new charge of reagents is introduced. In the continuous process, the catalyst precursor, activator and other necessary... 

In a typical procedure, the beads are mixed with an imprinting mixture (containing somewhat more solvent than usual) and mixed for some time with application of mild vacuum and/or sonication to ensure penetration of the mixture into the pore structure. The slurry is then heated to initiate polymerisation. After completion the beads are extensively washed and treated as for ground particles. 

By far the greatest part of PVC production across the world is now made by the suspension process. Vinyl chloride monomer (derived from a reaction between ethylene (derived from oil) and chlorine (derived from common salt) is dispersed in deionised water with the help of small quantities of chemical dispersants and polymerisation initiators (typically peroxide compounds). At moderately raised temperature (50 C) and pressure (0.7 MPa) polymerisation proceeds and the polymer can be removed from the resulting slurry by de-watering and steam stripping the unconverted vinyl chloride monomer. 

As coagulants for separating out solids suspended in a liquid. The polymer is more finely dispersed than if it had been obtained by classic emulsion polymerisation. Consequently, it should be more active in the context of steel mill slurries, coal slurries, potash sludges, and so on. 

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