Polyethylene Process Development

The subsequent development of the process for high-density polyethylene was the result of careful systematic observation by the experiment team. Further work on the oligomerization of ethylene to the commereially-valuable Ce and Cs a-olefins led to the discovery that the displacement reaction was catalyzed by traces of nickel in the reactor. The displacement reaction became faster than the growth reaction as the concentration of nickel was increased, and at 120°C, only the dimer was produced. In subsequent investigations of other metals as co-catalysts in conjunction with triethylaluminium, it was shown that zirconium acetylacetonate inhibited the displacement reaction completely, and that polyethylene was the only product. Further work led to the development of the well-known titanium chloride/triethyl alununum catalyst, which operated at low temperatures and atmospheric pressure. The reaction became known as the Mul-heim Atmospheric Polyethylene Process.  

At about the same time that news of the Ziegler discovery was released, the Phillips Petroleum Company in the United States atmounced that it had developed a medium-pressure, catalytic process (500 psig) to produce a high-density, crystalline polyethylene. The process was discovered when traces of ethylene in a flue gas had polymerized over conventional cracking catalysts. The Phillips catalyst contained chromic oxide supported on silica. The Standard Oil Company of Indiana (later Amoco) also introduced a medium pressure process using a catalyst comprising molybdenum oxide supported on carbon or alumina, but it did not enjoy the success of the Ziegler or Phillips processes and was only operated in three full-scale plants.  

The Phillips process, used a different type of catalyst from Ziegler, was available for license almost immediately and became a commercial success by producing a linear, highly crystalline prodnct with higher density than the high-pressure polymer discovered by ICI. The new polymer became known as high-density polyethylene (HDPE), whereas the original ICI polyethylene was thereafter known as low density polyethylene (LDPE). Phillips did not develop a catalyst for the production of polypropylene. 

Ziegler, working in the academic environment of the Max Planck Institut, did not develop his own process for the polymerization of olefins, but did offer the rights to his catalysts for license. Licensees were required to develop their own versions of a polymerization process that was based on Ziegler s discovery. It is not surprising that several different versions of the Ziegler processes evolved to produce HDPE, each based on somewhat different catalyst formulations and operating procednres. 

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Pulp-like olefin fibers are produced by a high pressure spurting process developed by Hercules Inc. and Solvay, Inc. Polypropylene or polyethylene is dissolved in volatile solvents at high temperature and pressure. After the solution is released, the solvent is volatilised, and the polymer expands into a highly fluffed, pulp-like product. Additives are included to modify the surface characteristics of the pulp. Uses include felted fabrics, substitution in whole or in part for wood pulp in papermaking, and replacement of asbestos in reinforcing appHcations (56). 

The focus of commercial research as of the mid-1990s is on catalysts that give desired and tailored polymer properties for improved processing. Development of metallocene catalyst systems is an example. Exxon, Dow, and Union Carbide are carrying out extensive research on this catalyst system for the production of polyethylene and polypropylene. 

The third process for cross-linking is the Sioplas process developed by Dow. The first stage of this involves the grafting of an easily hydrolysable trialkoxyvinylsilane onto the polyethylene chain, the site activation having been achieved with the aid of a small amount of peroxide. The compound is then extruded onto the wire, which is collected on a drum. The drum is then exposed to hot water, or, more commonly, low-pressure steam. The water hydrolyses the alkoxy groups, which then condense to form a siloxane cross-link. ... 

Sclair A process for polymerizing ethylene. Depending on the co-monomer used, the product can be linear low-density polyethylene (LLDPE) or high-density polyethylene (HDPE). Developed by DuPont in 1960 and widely licensed. Engineered by Uhde under the name Sclairtech. Nova Chemicals (Alberta) acquired the technology in 1994. 

UNIPOL [Union Carbide Polymerization] A process for polymerizing ethylene to polyethylene, and propylene to polypropylene. It is a low-pressure, gas-phase, fluidized-bed process, in contrast to the Ziegler-Natta process, which is conducted in the liquid phase. The catalyst powder is continuously added to the bed and the granular product is continuously withdrawn. A co-monomer such as 1-butene is normally used. The polyethylene process was developed by F. J. Karol and his colleagues at Union Carbide Corporation the polypropylene process was developed jointly with the Shell Chemical Company. The development of the ethylene process started in the mid 1960s, the propylene process was first commercialized in 1983. It is currently used under license by 75 producers in 26 countries, in a total of 96 reactors with a combined capacity of over 12 million tonnes/y. It is now available through Univation, the joint licensing subsidiary of Union Carbide and Exxon Chemical. A supported metallocene catalyst is used today. 

The process developed at CPRR is said to be similar in its layout to those used in private industry. Most plastic reclamation systems are designed to work with rigid containers, such as PET beverage bottles, and HDPE milk or household product containers, because they are currently the easiest postconsumer items to collect and sort. PET beverage bottles are actually not one, but several materials a PET body (clear or green), a pigmented high-density polyethylene (HDPE) base cup, aluminum cap, label, and adhesives. To separate these components, either a dry or wet separation method based on one or more of the different physical properties of the materials can be used. 

Envirocare of Utah, Inc. (Envirocare) has commercialized the polyethylene encapsulation process developed by Brookhaven National Laboratory (BNL) as an ex situ stabilization technology for hazardous and mixed wastes (wastes with both hazardous and radioactive components). 

As mentioned in section 7.1, technologies have been developed in recent years wherein combinations of processes are used to produce polyethylene. A case in point is the Borstar process developed by Borealis and started up in 1995. Borstar is capable of producing the entire range of polyethylenes from LLDPE to HOPE (14). 

Numerous industrial applications of applied thermodynamics have been reported in the literature for engineering analysis of wide varieties of chemical systems and processes. For example, Chen and Mathias reported examples of physical property modeling for the high-density polyethylene process and for sulfuric acid plants. Here, we present two recent examples that are illustrative of numerous applications of applied thermodynamics models in the industry for various process and product development studies. 

Several commercial processes are used to produce high-density polyethylene. All employ more moderate pressures and most also use lower temperatures than the low-density polyethylene processes. The Ziegler-developed process uses the mildest conditions, 200-400 kPa (2 atm) and 50-75°C, to polymerize a solution of ethylene in a hydrocarbon solvent using a titanium tetrachloride/aluminum alkyl-based coordination catalyst. After quenching the polymerized mixture with a simple alcohol, the catalyst residues may be removed by extraction with dilute hydrochloric acid or may be rendered inert by a proprietary additive. The product is almost insoluble in the hydrocarbon solvent, so is recovered by centrifuging and drying. The final product is extruded into uniform pellets and cooled for shipping to fabricators. 

In the solution process, a hydrocarbon solvent at a temperature of 125-250 °C dissolves the polymer as it forms [2,13,19,710-716]. This was the earliest commercial process developed for linear polyethylene. To prevent the solution from becoming too viscous, the polymer concentration is usually low in comparison with those of other processes, and the polymer MW must usually be kept low. In the recovery of the polymer, the solvent must be vaporized, leaving a polymer melt or sometimes a pulp, which is then pelletized in a further solvent removal step. 

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. 

In the following sections of this chapter, the catalytic conversion of individual plastics (polyethylene, polypropylene and polystyrene) is first reviewed, followed by a description of the processes developed for the catalytic cracking of plastic and rubber mixtures. Finally, methods based on a combination of thermal and catalytic treatments are considered. However, taking into account that the key factor in the catalytic conversion of plastic wastes is the catalyst itself, we will first describe the main properties of the most widely used catalytic systems for the degradation of polymers. 

Ziegler s timing was also fortunate in bringing out a new product at a time when the chemical industry was in an aggressive, expansionist mood. Had he been a few years earlier, that wave would not yet have crested had he been even a year or two later, he might have lost out to the other linear, low-pressure polyethylene processes that were developed independently and contemporaneously. 

Zletz was attempting to use the supported cobalt catalyst for alkylation with ethylene and found, to his surprise, that considerable solid polymer was formed. Like Hogan, he was not a trained polymer chemist, but had enough curiosity, initiative, and freedom to pursue the interesting bypath. Other metals and other supports were tested by Zletz and his coworkers and led eventually to improved catalysts such as molybdenum on alumina that formed the basis for development of a practical low-pressure polyethylene process. 

Linear polyethylenes are produced in solution, slurry, and increasingly, gas-phase low-pressure processes. The Phillips process developed during the mid 1950s used supported chromium trioxide catalysts in a continuous slurry process (or particle-form process) carried out in loop reactors. Earlier, Standard Oil of Indiana patented a process using a supported molybdenum oxide catalyst. The polyethylenes made by both these processes are HDPE with densities of 0.950-0.965 g/cm and they are linear with very few side-chain branches and have a high degree of crystallinity. 

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