Polyethylene solution/bulk process

Solution polymerizafion. Highly exothermic reactions can be handled by this process. The reaction is carried out in an excess of solvent that absorbs and disperses the heat of reaction. The excess solvent also prevents the formation of slush or sludge, which sometimes happens in the bulk process when the polymer volume overtakes the monomer. The solution process is particularly useful when the polymer is to be used in the solvent, say like a coating. Some of the snags with this process its difficult to remove residual traces of solvent, if that s necessary the same is true of catalyst if any is used. This process is used in one version of a low-pressure process for high-density polyethylene and for polypropylene. 

Producers use four routes to make polyechylencj the bulk- or high-pressure process, the solution-phase process, the slurry-phase process, and the gas-phase process. Organizing your thinking around processes and products is not all that straightforward. Some of the processes can be used to produce ail the polyethylene forms, some only a few or one. That calls for a few words first by product and then more by process. 

Blow-fill technology is an aseptic process whereby the container is formed from thermoplastic granules, filled with sterile solution and sealed, all within one automatic operation. The bulk solution should have a low bioburden and is delivered to the machine through a filling system that has been previously sanitized and steam sterilized in situ. Concern has been expressed that the machine itself may generate particles. The plastic granules are composed usually of polyethylene, polypropylene or one of their copolymers and are heat extruded at 200°C into a tube. The two halves of a mould close around this tube and seal the base. The required quantity of sterile fluid is filled into the container, which is then sealed. Products packed in this way include intravenous solutions, and small volume parenteral, ophthalmic and nebulizer solutions. The... 

In slurry, bulk-monomer, or gas-phase processes, the polymer is usually of higher density or crystallinity (e.g.. high-density polyethylene, linear-low-density polyethylene, isotactic polypropylene) and is thus insoluble in the reactor diluent or fluidizing gas stream. The continuous operation of these processes suggests the use of morphologically uniform catalyst particles which can be fed into the reactor smoothly without clumping, which in turn implies fixing the solution-soluble catalyst on an insoluble carrier. 

Solution polymerization is used for the manufacture of polyethylene, polypropylene, and polystyrene, but by far the most widely used process for polystyrene and PVC is suspension polymerization. In the latter process (also known as bead, pearl, or granular polymerization because of the form in which the final products may be obtained), the monomer is dispersed as droplets (0.01-0.05 cm in diameter) in water by mechanical agitation. Various types of stabilizers, which include water-soluble organic polymers, electrolytes, and water-insoluble inorganic compounds, are added to prevent agglomeration of the monomer droplets. Each monomer droplet in the suspension constitutes a small bulk polymerization system and is transformed finally into a solid bead. Heat of polymerization is quickly dissipated by continuously stirring the suspension medium, which makes temperature control relatively easy. 

Several polymerization processes are carri out in single liquid phase systems. The most widespread process of this type is the high pressure polymerization of ethylene (for "low-density" polyethylene). Other well Imown examples are the newest high temperature versions of processes for the polymerization of ethylene with Ziegler-type catalysts (for "high density" polyethylene), the preparation of some synthetic rubbers (polybutadiene, polyisoprene, ethylene-propylene co- and ter-polymers), the "bulk" polymerization of styrene, etc. All these processes have in common that the reaction product is a polymer melt or solution, that is relatively viscous. Since at the same time the intrinsic reaction rates are usually quite high, the conversion rates are often limited by diffusion. These processes are usually carried out in stirred reactors, for which the effects of micro-mixing have to be taken into account. 

The polymerization of olefins with coordination catalysts is performed in a large variety of polymerization processes and reactor configurations that can be classified broadly into solution, gas-phase, or slurry processes. In solution processes, both the catalyst and the polymer are soluble in the reaction medium. These processes are used to produce most of the commercial EPDM rubbers and some polyethylene resins. Solution processes are performed in autoclave, tubular, and loop reactors. In slurry and gas-phase processes, the polymer is formed around heterogeneous catalyst particles in the way described by the multigrain model. Slurry processes can be subdivided into slurry-diluent and slurry-bulk. In slurry-diluent processes, an inert diluent is used to suspend the polymer particles while gaseous (ethylene and propylene) and liquid (higher a-olefins) monomers are fed into the reactor. On the other hand, only liquid monomer is used in the slurry-bulk pro-... 

The initiator f and chain transfer agent g can be metered into the ethylene stream as it enters the reactor or at various points within it. From the reactor the product stream h containing a mixture of unreacted ethylene, oils, waxes, and polyethylene proceeds to a two stage separation process. The product stream is initially let down into a high pressure separator 4 wherein the polyethylene precipitates and is drained off with some ethylene i to a low pressure separator 5. The low molecular weight oils and waxes remain in solution in the bulk of the ethylene, and this stream j is let down into a separate low pressure separator 6. Here the ethylene is stripped from the oils and waxes, which are discharged in waste stream k. The ethylene for recycle I proceeds to a cooler 7, from which it is piped to... 

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