Polyethylene 3-process

High pressures are required for many commercial chemical processes. For example, the synthesis of ammonia is carried out at reactor pressures of up to 1000 bar, and high-density polyethylene processes operate up to 1500 bar. 

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 The catalyst powder is continuously added to the bed and the granular product is continuously withdrawn.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. 

Sometimes a lower density polyethylene is made with both this type of catalysis or Ziegler-Natta. Branching is controlled by the addition of small amounts of 1-alkenes added to the ethylene. 1-Hexene would give a C4 branch, 1-octene a Ce branch, etc. If enough 1-alkene is used the polymer is called linear low-density polyethylene (LLDPE). It is made by a high-density polyethylene process but branching gives a lower density. 

Fig. 3-18 Flow diagram of high-pressure polyethylene process. After Doak [1986] (by permission of Wiley-Interscience, New York).

The major hazard that can occur in the high-pressure polyethylene process is a runaway of the reactor and decomposition of ethylene as well as fires, explosion, and disintegration of high-pressure parts. Although the last incidents are well understood, the reasons for runaway and ethylene decomposition have been evaluated only recently. Experience over twenty years has shown that decomposition mostly takes place in the reactor and in the high-pressure separator, but decompositions have also been reported from ethylene-feed and product lines. 

Most high density polyethylene processing technologies require the melting of HDPE. Typical HDPE melt viscosities are between 1,000 and 100,000 Pa s( 10,000 10 P) the melt viscosity of HDPE strongly... 

The Air Products technology will be able to recover almost 100% of the hydrocarbon off-gas from the slurry phase polyethylene process and more than 97% of the high-purity nitrogen that is used in degassing. Both of these gas streams will be recycled and re-used to reduce the VOC and NOx environmental emissions that are typically associated with flaring or burning plant off-gasses55. 

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. 

After the completion of MnOj separation, the water sample is transferred to the proper size polyethylene processing container. Adjust to pH 1-4 with nitric acid, add AMP as a slurry in water to extract the caesium (use 0.2 g AMP/1 of sample), stir the sample thoroughly and let the AMP settle, filter or decant the supemate (discard the supemate or save for Sr analysis, if required), separate the AMP by centrifugation and purify the Cs for beta or gamma counting. 

In this case, the aluminum alkyl is functioning as a cocatalyst, sometimes also called an "activator." Titanium alkyls, believed to be active centers for polymerization, are created through transfer of an alkyl from aluminum to titanium, known as "alkylation." Molar ratios of cocatalyst to transition metal (Al/Ti) are typically 30 for commercial polyethylene processes using Ziegler-Natta catalysts (lower ratios are used for polypropylene). The vast majority of aluminum alkyls sold into the polyethylene industry today is for use as cocatalysts. With TEAL, the most widely used cocatalyst, alkylation proceeds as in eq 4.8 ... 

Zinc alkyls have been known to chemical science since the mid-nineteenth century and were among the first organometallic compounds produced and characterized. Sir Edward Frankland, an English chemist and pioneer in organometallic chemistry, synthesized diethylzinc (DEZ) from zinc metal and ethyl iodide (3). Remarkably, more than century and a half after its discovery, diethylzinc remains today an important industrial metal alkyl. Though quantities are substantially smaller than those of aluminum alkyls, diethylzinc has several niche applications in polyethylene processes. 

For most industrial polyethylene processes (slurry and gas phase), thermal stability of the cocatalyst is not a factor since most operate in the temperature range 80-110 °C. However, solution processes operate at high enough temperatures where thermal decomposition of the cocatalyst could become a factor. Fortunately, residence times are typically short in solution processes. 

Conditions used in PE processes vary widely. Because the heat of polymerization for ethylene is quite high (variably reported to be between 22 and 26 kcal/mole), efficient heat removal is crucial for polyethylene processes. Selection of process must also accommodate catalyst features, such as its kinetic profile. Table 7.1... 

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