Polyethylene production, initial

Over a period of many years polymeric materials have gradually replaced metals in many applications. Among the five leading thermoplastics low and high density polyethylene, polyvinyl chloride, polypropylene, and polystyrene polyethylene is the largest volume plastic in the world. Polyethylene was initially made in the United States in 1943. In 1997, the estimated combined worldwide production of both low and high-density polyethylene was 1.230 x 1010 kg (2.712 x 1010 lb) [10]. Low density polyethylene is produced at pressures of 1030 to 3450 bar (1020 to 3400 atm) whereas high density polyethylene is produced at pressures of 103 to 345 bar (102 to 340 atm) [11]. 

In Brazil, for similar reasons, the Dow Chemical Company and Mitsui Co Ltd are engaging in a 50/50 joint venture to cultivate sugarcane for the production of ethanol and subsequently ethylene. Initially a 350 000 tons ethanol production unit is planned, eventually leading to polyethylene production by 2015. This will take about 120 000 hectares of sugar cane aimed at providing a cheaper and less economically volatile ethanol feedstock (see http //www.biofuelsdigest.eom/bdigest/2012/03/22/dow-mitsui-to-finalize-brazilian-bio-based-polyethylene-jv/, accessed 8 July 2013). 

The Unipol process, initially developed for polyethylene production, was later extended to polypropylene manufacture. The process consists of a large fluidized-bed gas-phase reactor for homopolymer and random copolymer production, and a second smaller reactor for impact copolymer production. The second reactor is smaller than the first one because only 20% of the production comes from the second reactor. This reactor typically has a lower pressure rating as copolymerization is usually carried out at lower temperatures and pressures. Condensed mode operation is used in the homopolymer reactor but an inert diluent is not required because propylene is partially fed as a liquid. The copolymerization reactor is operated purely in the gas phase. The Unipol process has a unique and complex product discharge system that allows for very efficient recovery of unreacted monomer, but this does add complexity and capital cost to the process. 

Metallocene polyethylene (MPE) is a new PE product initiated and polymerized by metallocene calalyst. It has a narrow molecular weight distribution (d = 2) uniform molecular chain structure higher crystallinity, strength, toughness, and transparency and excellent performance. However, MPE melt has a high viscosity that makes processing difficult, which limits its application. 

Free-radical polymerization [79, 93] Low-density polyethylene (LDPE) accounts for almost two-fifths of the global polyethylene production capacity, reaching about 45 million tons per year in 1996. LDPE is exclusively produced by free-radical polymerization in a tubular or autoclave reactor, each of which accounts for about 50% of the total capacity. In the tubular reactor, a small amount of initiator is injected into a turbulent monomer flow for initiating the exothermic free-radical chemistry. Under extreme operating conditions (T 140-300°C, p 1000-3500 atm), these... 

Countries produciug commodity LLDPE and their capacities, as well as production volumes of some U.S. companies, are Hsted iu Table 5. Iu most cases, an accurate estimate of the total LLDPE production capacity is compHcated by the fact that a large number of plants are used, iu turn, for the manufacture of either HDPE or LLDPE iu the same reactors. VLDPE and LLDPE resius with a uniform branching distribution were initially produced in the United States by Exxon Chemical Company and Dow Chemical Company. However, since several other companies around the world have also aimounced their entry into this market, the worldwide capacity of uniformly branched LLDPE resins in 1995 is expected to reach a million tons. Special grades of LLDPE resins with broad MWD are produced by Phillips Petroleum Co. under the trade name Low Density Linear Polyethylenes or LDLPE. 

Polymerization processes are characterized by extremes. Industrial products are mixtures with molecular weights of lO" to 10. In a particular polymerization of styrene the viscosity increased by a fac tor of lO " as conversion went from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1,800 K (3,240 R). Heat transfer coefficients in stirred tanks with high viscosities can be as low as 25 W/(m °C) (16.2 Btu/[h fH °F]). Reaction times for butadiene-styrene rubbers are 8 to 12 h polyethylene molecules continue to grow lor 30 min whereas ethyl acrylate in 20% emulsion reacts in less than 1 min, so monomer must be added gradually to keep the temperature within hmits. Initiators of the chain reactions have concentration of 10" g mol/L so they are highly sensitive to poisons and impurities. 

A somewhat different approach to the production of thermoplastic polyolefin rubbers has been adopted by Allied Chemical with their ET polymers. With these materials butyl rubber is grafted on to polyethylene chains using a phenolic material such as brominated hydroxymethyl phenol. The initial grades of these polymers, which were introduced commercially towards the end of the 1970s, had polyethylene butyl rubber ratios of 50 50 and 75 25. Both low-density and high-density polyethylene-based varieties were produced. 

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