Polyethylene bulk/high-pressure process

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. 

As in any other process, so also in the high-pressure polymerization of ethylene, do capital costs, utilities, maintenance, manpower, and costs of raw materials contribute to the production costs of low-density polyethylene (LDPE). The cost structure is typical for the production of bulk chemicals but is strongly influenced by the requirements of a high-pressure process. 

The first application of supercritical fluids (SCFs) to the polymer industry occurred in the 1930s, with the development of a free-radical bulk polymerization of supercritical ethylene to produce low-density polyethylene (LDPE) [ 1,2]. LDPE is synthesized by a high-pressure process, at 180-300 °C and 1000-... 

The high-pressure process was developed from the high-pressure polyethylene process (LDPE). The polymerization is carried out in bulk. By means of this process mainly copolymers with a vinyl acetate content of up to 45% are produced. The important range lies between 5% and 30% vinyl acetate content, giving copolymers with thermoplastic properties. The maximum molecular weight achieved by the high-pressure process is comparatively low due to the high chain-transfer activity of the vinyl acetate in bulk polymerization. Therefore, the vinyl acetate content is limited. 

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. 

If the monomer and polymer are not mutually soluble, the bulk reaction mixture will be heterogeneous. The high pressure free radical process for the manufacture of low density polyethylene is an example of such reactions. This polyethylene is branched because of self-branching processes illustrated in reaction (6-89). Branches longer than methyls cannot fit into the polyethylene crystal lattice, and the solid polymer is therefore less crystalline and rigid than higher density (0.935-0.96 g cm ) species that are made by coordination polymerization (Section 9.5). 

Theonly important current application of tubular reactors in polymer syntheses is in the production of high pressure, low density polyethylene. In tubular processes, the newer reactors typically have inside diameters about 2.5 cm and lengths of the order of I km. Ethylene, a free-radical initiator, and a chain transfer agent are injected at the tube inlet and sometimes downstream as well. The high heat of polymerization causes nonisothermal conditions with the temperature increasing towards the tube center and away from the inlet. A typical axial temperature profile peaks some distance down the tube where the bulk of the initiator has been consumed. The reactors are operated at 200-300°C and 2000-3000 atm pressure. 

Multiple steady states are theoretically possible in many free radical polymerizations, but they are not usually observed in practice because the reaction is controlled at relatively low conversions (high [M]) where the viscosity of the medium presents less of a problem. This is particularly trueof bulk polymerizations such as those in the high-pressure polyethylene processes. 

Bulk polymerization. Bulk polymerization is the simplest and most direct method (from the standpoint of formulation and equipment) for converting monomer to polymer. It requires only monomer (and possibly monomer-soluble initiator or catalyst), and perhaps a chain transfer agent for molecular weight control, and as such gives the highest-purity polymer. However, extra care must be taken to control the process when the polymerization reaction is very exothermic and particularly when it is run on a large scale. Poly(methyl methacrylate), polystyrene, or low-density (high pressure) polyethylene, for example, can be produced from... 

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 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... 

Highly polar microdomains exist in reverse micelles of AOT and nonionic polyethylene oxide surfactants in ethane, even below 100 bar, both with and without cosolvents. Without cosolvents these domains are likely very small since values of Wo are small. The addition of the cosolvent octane provides a means to take up large amounts of water over a wide pressure range. The polarities in the interior of the micelles approach that of bulk water. The existence of polar microdomains in supercritical fluids at relatively low pressures presents an opportunity for new separation and reaction processes involving hydrophilic substances. 

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