Polyethylene manufacturing processes

Stirred autoclave Low capital cost Low heat transfer area Low space time yield 

Loop Low capital cost High heat removal High space time yield  

Stirred gas phase PFR characteristic allows for fast grade transfer More complex reactor design, higher capital cost 

Circulating bed Better polymer properties control Different reacting zone allows production of product not possible with traditional reactors Complex reactor design, high capital cost Still unproven technology 

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Polypropylene made with modern catalysts, on the other hand, has an insignificant amount of catalyst residues because of their very high activity and practically no atactic content. For this reason modern processes do not require post-reactor purification. Some catalysts, such as the ones used in the Spheripol process (Section 2.5), will even produce large spherical polypropylene particles that do not necessarily require pelletization. As a consequence of these many advances, modern polypropylene (and polyethylene) manufacturing processes have very few units basically one or more reactors in series, compressors, recovery systems for diluent (for some processes) and unreacted monomer, and an extruder for making pellets. 

There are many parallels between polypropylene and polyethylene manufacturing processes. The reactor configurations are similar, but, due to the different requirements of the polymer, it will be seen that there are significant differences between the processes as well. While propylene homopolymer can be produced in reactors of various configurations, for impact copolymer production, gas phase is the reactor of choice because of the stickiness of the polymer and the solubility of the copolymer in the monomer and diluent. 

Each type of process will be discussed in this chapter with the objective of providing chronological details of important innovations as the polyethylene manufacturing process technology developed from 1939 to 2012, so that future scientists may gain a better understanding of the growth of the polyethylene industry over the past 80 years. 

These high reactor efficiencies reported for the high-pressure process have significantly improved the cost analysis for this process compared to the gas-phase fluidized-bed process, which is often considered the low-cost polyethylene manufacturing process. 

The evaluation of new catalysts in a commercial polyethylene manufacturing process is a complex procedure. When a commercial reactor is taken off-line in order to evaluate a new catalyst and product, the polyethylene business undergoes a loss in revenue from production. 

The results in Table 1.1 show that the only significant variation of peak height ratios with temperature occurs when component 1 is compared with components 2-7 and component 2 is compared with components 4-7. As the temperature increases to 200 C components 1 and 2 increase while components 3-7 decrease (Fig 1.1). Components 1 and 2 were eluted in the C2 - C4 hydrocarbon region and components 3-7 were eluted coincident with the major components of the polymerization solvent known to be used in this polyethylene manufacturing process. These observations suggest that between 125 C and 200 C there is some thermal degradation of the polymerization solvent to C2 - C4 hydrocarbons. 

A variety of technological processes is used for polyethylene manufacture. 

It is estimated that 27,000 t/yr of CSM have been commercially used in the United States. However, due to environmental problems in the manufacturing process, it has been necessary to develop a process that is much mote expensive. As a result many companies using CSM ate trying to replace the CSM with CPE or other elastomers. The result is a decline in the usage of this polymer. Chlorosulfonated polyethylene is sold under the trade name Hypalon (DuPont—Dow Company). 

Gas phase olefin polymerizations are becoming important as manufacturing processes for high density polyethylene (HOPE) and polypropylene (PP). An understanding of the kinetics of these gas-powder polymerization reactions using a highly active TiCi s catalyst is vital to the careful operation of these processes. Well-proven models for both the hexane slurry process and the bulk process have been published. This article describes an extension of these models to gas phase polymerization in semibatch and continuous backmix reactors. 

Downstream of the compressor is a series of fractionators (generally the tallest towers in an ethylene plant) which separate the methane and hydrogen, the ethylene, the ethane, and the propane and heavier. All are heavy metallurgy to handle the pressures and insulated to maintain the low temperatures. There s also an acetylene hydrogenator or converter in there. Trace (very small) amounts of acetylene in ethylene can really clobber some of the ethylene derivative processes, particularly polyethylene manufacture. So the stream is treated with hydrogen over a catalyst to convert the little acetylene present into ethylene. 

In 1989, the NDF Company opened a facility in Georgetown, South Carolina to produce low density polyethylene. Manufacturing of the polyethylene is done in two 50-ton reactors that are encased individually within their own 8-story-high process unit. The main raw materials for the manufacturing operations include ethylene, hexane, and hutene. The polymerization is completed in the presence of a catalyst. The hase chemicals for the catalyst are aluminum alkyl and isopentane. The reactor and catalyst preparation areas are on a distributed control system (DCS). A simplihed process flow diagram is attached. 

Chemical reactions enhanced by catalysts or enzymes are an integral part of the manufacturing processes for the majority of chemical products. The total market for catalysts and enzymes amounts to 11.5 billion (2005), of which catalysts account for about 80%. It consists of four main applications environment (e.g., automotive catalysts), 31% polymers (e.g., polyethylene and polypropylene), 24% petroleum processing (e.g., cracking and reforming), 23% and chemicals, 22%. Within the latter, particularly the catalysts and enzymes for chiral synthesis are noteworthy. Within catalysts, BINAPs [i.e., derivatives of 2,2 -bis(diphenylphosphino) -1, l -bis-l,l -binaphthyl) have made a great foray into chiral synthesis. Within enzymes, apart from bread-and-butter products, like lipases, nitrilases, acylases, lactamases, and esterases, there are products tailored for specific processes. These specialty enzymes improve the volumetric productivity 100-fold and more. Fine-chemical companies, which have an important captive use of enzymes, are offering them to third parties. Two examples are described here ... 

A variety of technological processes arc used for polyethylene manufacture. They include polymerization in supercritical ethylene at a high ethylene pressure and temperature above the PE melting point (110-140°C), polymerization in solution at 120-150°C or in slurry, and polymerization in the gas phase... 

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