High-pressure polyethylene process

I 4 Loiv-Density Polyethylene High-Pressure Process... 

Branching can to some extent reduce the ability to crystallise. The frequent, but irregular, presence of side groups will interfere with the ability to pack. Branched polyethylenes, such as are made by high-pressure processes, are less crystalline and of lower density than less branched structures prepared using metal oxide catalysts. In extreme cases crystallisation could be almost completely inhibited. (Crystallisation in high-pressure polyethylenes is restricted more by the frequent short branches rather than by the occasional long branch.)... 

A sophisticated model of the high-pressure process for polyethylene is described in... 

In the high-pressure process for the production of polyethylene, ethylene is compressed in a two-step process. In the primary step, the gas is compressed in a two-stage compressor to 25 to 30 MPa. This is followed by compression in a hypercompressor to 150 to 320 MPa. 

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. 

Many industrially important chemical processes are high pressure processes. Examples are the production of ammonia and the production of low density polyethylene. Basically, the pressure affects both the equilibrium yield of a chemical reaction and the reaction rate. Here, only the influence on the equilibrium yield is discussed. 

The first section of this chapter describes the most important high pressure process run under homogeneous conditions to manufacture Low Density PolyEthylene (LDPE). The radical polymerization of ethylene to LDPE is carried out in tubular reactors or in stirred autoclaves. Tubular reactors exhibit higher capacities than stirred autoclaves. The latter are preferred to produce ethylene copolymers having a higher comonomer content. 

R.G. Klimesch, The future of the high pressure process, Lecture at the conference The 1990 s Polyethylene of the Plastic and Rubber Institute, London 1992. 

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. 

In many cases, the linear low-density polyethylene (LLDPE) produced in low-pressure processes competes for the same market as LDPE. For this reason, in Figure 8.2-7 capital- and operation costs of the high-pressure polymerization are compared with those of a low-pressure solution process having the same capacity. Also, the production costs of the low-pressure process are dominated by the costs of the monomer, but some differences can be noted which are typical for the economics of low- and high-pressure processes. 

The mechanism of the polymerization of this monomer has been studied in far greater detail than any other. It is clear from the outset that a much more complex mechanism is involved than is the case for olefins. A large proportion of the initiator is used to form polymer whose molecular weight is only a few hundreds and the overall molecular weight distribution is so broad as to be rivalled only by those found in polyethylene produced by the high pressure process (19, 39). The initiator disappears almost instantaneously on mixing the reactants (19, 38). Under these conditions, an almost monodisperse polymer would be expected if chain transfer or termination processes are absent. 

Three processes are used commercially to make linear polyethylene-solution, slurry, and gas phase. All are called low-pressure processes (< 50 atm) to distinguish them from the free radical or high-pressure process that makes highly branched polyethylene. In the solution mode a hydrocarbon solvent at 125-170°C dissolves the polymer as it forms. The reaction usually slows as the solution becomes viscous because it becomes difficult to stir ethylene into the liquid phase. In contrast, The slurry process uses a poor solvent and low temperature (60-110°C) to prevent dissolving or even swelling of the polymer. Each catalyst particle creates a polymer particle several thousand times larger than itself. There is no viscosity limitation in the slurry method the diluent serves to transfer heat and to keep the catalyst in contact with ethylene and other reactants. Finally, the gas-phase process is much like the slurry method in that polymer particles are formed at similar temperatures. A bed of catalyst/polymer is fluidized by circulating ethylene, which also serves as a coolant. 

The reaction is exothermic and may form polymer from a molecular weight of 1000 to well over 1 million. The high-pressure process, which normally produces types I and II, uses oxygen, peroxide, or other strong oxidizers as catalyst. Pressure of reaction ranges from 15.000 to 50.000 psi ( 1,020-3,400 atmospheres). The polymer formed in this process is highly branched, with side branches occurring every 15-40 carbon atoms on the. chain backbone. Ciystallinity of this polyethylene is approximately 40-60%, Amorphous content of the polymer increases as the density rs reduced,... 

Polyethylenes produced commercially via high pressure, free radical processes have densities around 0.92 g/cc and are referred to simply as "low density" polyethylenes. It has been well established from infrared measurements that these low density polyethylenes possess appreciable quantities of ethyl and butyl branches (1-3) but it was not until C-13 NMR became available that an absolute identification, both qualitatively and quantitatively, of the short branches became possible (4-8). Long chain branching is also present in high pressure process low density polyethylenes and carbon-13 NMR was useful here also in establishing the identity and relative amounts of long versus short chain branches (9-11). 

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