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. 

Higher extraction volumes in high-pressure processes represent higher costs and attention must be paid to limiting them as far as possible. The main impact comes from the bulk density of the feed material (see Table 6.6-1). 

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-... 

PE-LLD and even PE-VLD can further be synthesized with metallocenes and methylalumoxanes in the bulk ethylene (high-pressure) process. The polymerization is performed in a stirred tank reactor at temperatures above 120°C and pressures of at least 50 MPa [65, 66]. The copolymer continuously leaves the reactor with excess ethylene, then the ethylene is vented and recycled into the polymerization reactor. The polymer melt is transferred into pellets. In this case the comonomers are propylene, 1-butene, and 1-hexene. 

Industrially, ethylene is polymerized by the high-, medium-, or low-pressure process in bulk, solution, or in the gas phase (Table 25-1). The high-pressure-process polymerizes by a free radical mechanism addition of about 0.05% oxygen to ethylene presumably produces CH2=CH(OOH), which decomposes to provide the start free radicals. Correspondingly, hydroxyl groups have been found in high-pressure poly (ethylene). Inter-molecular transfer by polymer or initiator free radicals produce main-chain free radicals that initiate the polymerization of ethylene ... 

As a consequence, heat transfer to gases in high-pressure processes is enhanced when the process conditions become pseudocritical. A successful method is to calculate the improved heat transfer when designing a heat transfer apparatus for options of heating and cooling as w is the relation of the thermophysical data at wall and bulk temperature. Polyakov [36] and Krasnoshchekov [37] developed the following equations ... 

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. 

A schematic of a continuous bulk SAN polymerization process is shown in Figure 4 (90). The monomers are continuously fed into a screw reactor where copolymerization is carried out at 150°C to 73% conversion in 55 min. Heat of polymerization is removed through cooling of both the screw and the barrel walls. The polymeric melt is removed and fed to the devolatilizer to remove unreacted monomers under reduced pressure (4 kPa or 30 mm Hg) and high temperature (220°C). The final product is claimed to contain less than 0.7% volatiles. Two devolatilizers in series are found to yield a better quaUty product as well as better operational control (91,92). 

The carbon monoxide product is removed from the top of the column and warmed against recycled high pressure product. The warm low pressure stream is compressed, and the bulk of it is recycled to the system for process use as a reboder medium and as the reflux to the carbon monoxide column the balance is removed as product. The main impurity in the stream is nitrogen from the feed gas. Carbon monoxide purities of 99.8% are commonly obtained from nitrogen-free feedstocks. 

Traditionally, inerts have been obtained from sources such as high-pressure gas cylinders or tube trailers or through evaporation of cryogenic liquids from bulk tanks. Other sources of inerts include (NFPA 69, Standard on Explosion Prevention Systems, National Fire Protection Association, 2002 FM Global, Loss Prevention Data Sheet 7-59, Inerting and Purging of Tanks, Process Vessels, and Equipment, 2000)... 

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