High-Pressure Polyethylene Polymerization

We begin with a description of the high-pressure polymerization process since it is an authentic example of how the principles of thermodynamics and kinetics can be combined with creative engineering to develop an economically viable high-pressure process. These principles can be generalized and extended to other high-pressure processes. After describing the polyethylene process, we move on to more recent work on polyethylene and ethylene copolymers, followed by a discussion of other recent SCF studies with a variety of other polymers and monomers. 

Ethylene is compressed to 2,700 bar and a free-radical initiator, e.g., trace amounts of oxygen or a peroxide, is injected into the feed stream to promote the free-radical polymerization. The polyethylene polymer that is formed remains dissolved in the supercritical ethylene phase at the operating temperature, which ranges from 140 to 250°C. The heat of reaction is removed by through-wall heat transfer when the tubular reactor is used and by regulating the rate of addition of initiator when the autoclave reactor is used. 

Downstream of the reactor section the polyethylene-ethylene solution is expanded to a lower pressure, typically to about 350 bar. Because the solvent power of supercritical ethylene is lowered during the pressure reduction, the polyethylene formed in the reactor nucleates, precipitates from solution, and is collected in the separator. Note that this separation step is exactly analogous to the separation of naphthalene in the model extraction/separation process described in chapter 6. The low-pressure ethylene from the polyethylene separator is recompressed and recycled to the reactor. Interestingly, high-pressure ethylene is both the reactant and the solvent for the product in this process. 

There are some other facets to the operation of this process that are worth noting since they illustrate the SCF phenomena that are in operation the 

In the preceding discussion of the operation of the tubular reactor polymerization scheme, we stated that the heat of reaction is removed by through-wall heat transfer. What exactly occurs in the vicinity of the wall If the characteristic of the high-pressure (300 bar) naphthalene isobar is interpreted as a schematic representation of the solubility of polyethylene in ethylene at a pressure of 2,700 bar, we can use it to predict that polyethylene will precipitate in the boundary layer near any relatively cold surfaces in the reactor or downstream lines. If the precipitation of polyethylene does occur on these internal surfaces and if it is not appropriately removed, the buildup of the polymer on the wall can result in decreased heat transfer from the hot gas-polymer solution, and the attendant decrease in heat transfer can lead to the runaway reaction that is occasionally encountered in high-pressure polyethylene plants. 

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High-density polyethylene (HOPE), 200-201 High-pressure polyethylene polymerization, 18S1-192... 

Polydimethylsiloxanes, 231-236 Polyethylene, 62 aerogel, 364 fractionation, 198-205 and low molecular weight hydrocarbon mixtures, 70 and solvent mixtures, 63 and xylene gel, 364 see also High-pressure polyethylene polymerization... 

Simplest of the techniques requiring only monomer and monomer-soluble initiator, and perhaps a chain-transfer agent for molecular weight control. Characterized, on the positive side, by high polymer yield per volume of reaction, easy polymer recovery. Difficulty of removing unreacted monomer and heat control are negative features. Examples of polymers produced by bulk polymerization include poly(methyl methacrylate), polystyrene, and low-density (high pressure) polyethylene. 

The polyethylene produced by radical polymerization is referred to as low-density polyethylene (LDPE) or high-pressure polyethylene to distinguish it from the polyethylene synthesized using coordination catalysts (Sec. 8-1 lb). The latter polyethylene is referred to as high-density polyethylene (HDPE) or low-pressure polyethylene. Low-density polyethylene is more highly branched (both short and long branches) than high-density polyethylene and is therefore lower in crystallinity (40-60% vs. 70-90%) and density (0.91-0.93 g cm 3 vs. 0.94-0.96 g cm-3). 

Coordination copolymerization of ethylene with small amounts of an a-olefin such as 1-butene, 1-hexene, or 1-octene results in the equivalent of the branched, low-density polyethylene produced by radical polymerization. The polyethylene, referred to as linear low-density polyethylene (LLDPE), has controlled amounts of ethyl, n-butyl, and n-hexyl branches, respectively. Copolymerization with propene, 4-methyl-1-pentene, and cycloalk-enes is also practiced. There was little effort to commercialize linear low-density polyethylene (LLDPE) until 1978, when gas-phase technology made the economics of the process very competitive with the high-pressure radical polymerization process [James, 1986]. The expansion of this technology was rapid. The utility of the LLDPE process Emits the need to build new high-pressure plants. New capacity for LDPE has usually involved new plants for the low-pressure gas-phase process, which allows the production of HDPE and LLDPE as well as polypropene. The production of LLDPE in the United States in 2001 was about 8 billion pounds, the same as the production of LDPE. Overall, HDPE and LLDPE, produced by coordination polymerization, comprise two-thirds of all polyethylenes. 

A number of processes have been developed to obtain products of different physical properties. The nature of the product is affected by the addition of diluents or other additives before carrying out the polymerization. Autoclaves or stirred-tank reactors, and tubular reactors, or their combinations have been developed for the industrial production of high-pressure polyethylene.206,440 Pressures up to 3500 atm and temperatures near 300°C are typically applied. 

Application of high-pressure vibrational spectroscopy in order to study and to monitor technically relevant fluid phase processes under extreme conditions is exemplified by high-pressure ethene polymerization. Several vibrational bands in the IR and the NIR may be used to detect concentrations directly in the ethene/polyethylene system (Buback, 1984). Some of these are plotted in Fig. 6.7-20. The conversion of unsaturated (ethylenic)... 

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. 

The original process for high pressure polyethylene was based on use of a high pressure autoclave and used air to introduce free radicals sufficient to initiate polymerization of ethylene. Principal features of the autoclave process are summarized in Table 7.2. 

Impurities can also act as chain transfer agents. In some instances, as in the production of low density polyethylene via high pressure radical polymerization, impurities and/or the so-called inerts (methane, ethane, and propane), which come as impurities in the ethylene, are used as effective chain transfer agents to lower the MW of the polymer. 

Polyethylene (also known as polythene) was synthesized by accident in 1932 when scientists at Imperial Chemical Industries (ICI) investigated the reaction between ethylene and various compounds at high pressure. Polyethylene is generally commercially polymerized from monomer ethylene gas under high pressure (1000—3000 atmospheres) and at temperatures of 80-300°C (Brydson, 1999). A free radical initiator such as benzoyl peroxide, azodi-isobutyronitrile or oxygen is added. The reaction is exothermic and must be cooled to control the rate of polymerization and molecular weight. 

Ethylene-vinyl acetate copolymers can be thought of as modified high pressure polyethylenes. Because of the free-radical polymerization process they have structural characteristics such as short-chain and long-chain branching in addition to the effects due to the incorporation of the vinyl acetate comonomer. Ethylene and vinyl acetate have a reactivity ratio which is close to 1 and as a result EVA copolymers contain vinyl acetate which is homogeneously distributed among the polymer chains. The major effect of the VA on polymer properties is to reduce... 

ALKATHENE, the high-pressure polyethylene (HPPE) or low-density PE (LDPE p = 915-940 kg m ) polymerization in an autoclave with benzoyl peroxide and... 

During the mid-1930s, free radical ethylene pol)mierization was developed by ICI. At temperatures above 150°C and pressures exceeding 1000 atm, radical initiators produce radicals which initiate free radical ethylene polymerization. Due to inter- and intramolecular chain transfer reaction via H atom transfer with the free radical at the polymer chain end, high pressure polyethylene contains short- and long-chain branches which reduce polyethylene (PE) crystallinity and consequently also PE density. Therefore this class of polyolefins became known as low density polyethylene (LDPE). The history of polyolefins was compiled by Seymour and Cheng [1]. 

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