Polyethylene polymerization Processes
The heat of polymerization of ethylene is 93.6 kJ/mol (334 kJ/gm). The temperature rise in the gas phase is 16°C for 1 % conversion to polymer. Heat removal is thus key factor in commercial polymerization process. Two types of processes used in commercial production.
High-pressure process: The chance observation in 1933 by an ICI research team that traces of waxy polymer were formed when ethylene and benzaldehyde were subjected to a temperature of 170°C and pressure of 190 MPa, led to the invention of polyethylene high-pressure process. High-pressure (around 2000 bar) process is used to manufacture LDPE
Low-pressure process: During 1950 three research groups working independently discovered three different catalysts, which allowed the production of polyethylene at low pressure. Low-pressure process is used to produce HDPE Of the three discoveries at Standard Oil (Indiana), Phillips Petroleum, and Karl Ziegler at Max-Plank Institute (Germany) the later two have been extensively commercialized.
In 1978 Union Carbide announced their Unipol process to produce LLDPE It is a low-pressure process in which butene or hexene is copolyemrized with ethylene to form branched structure Essentially this process enabled to produce branched polyethylene similar to LDPE by low-pressure economical process LLDPE is now replacing LDPE market The high-pressure process is being replaced by catalytic low-pressure process.
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Polymerization Chemistry:
High-pressure process involves free radical addition polymerization.
Low-pressure process involves coordination catalysis.
Free Radical Catalysis:
Free radical catalysis is used in high-pressure process. Employing pressures of 200 MPa and temperatures above 160°C enables the polyethylene produced to dissolve in the unreacted ethylene. 20% conversion of the monomer is achieved in 40 s Chain transfer agents such as hydrogen, propane, propene, acetone, methyl ethyl ketone are used to control molecular weight. Oxygen was used as initiator in the early commercial processes. With the developments of high-pressure pumps, and new initiators, modern plants are able to maintain precise control of temperature profiles by injection of solution of liquid catalysts (initiators).
Typical initiators used in high-pressure process are required to have half-life of about 1s. Following is a list of some commercial liquid initiator used in HP process .
1. Di-(2-ethyl) peroxycarbonate
2. ten-Butyl 2,2-dimethyiperoxy-propanoate
3. Di-(3,5,5-trimethyl-hexanoyl)-peroxide
4. ten-Butyl 3,5,5-trimethyl-peroxyhexanoate
5. Di-tert-butyl peroxide
Beside the polymerization reaction, the decomposition of ethylene into carbon and a mixture of methane and hydrogen is also highly exothermic. High-pressure plants are designed with relief valves to
protect the equipment from overpressurization due to decomposition. This decomposition reaction is called as runaway polymerization reaction
Polymerization reaction:
Runaway Polymerization reaction:
Under the turbulent flow conditions in commercial plants the propagation of runaway reaction can be rapid leading to explosion.
Coordination Catalysis:
Coordination catalysts containing transition metals are used in lowpressure process. Most of the catalysts are oxygen and water sensitive They react easily with impurities especially with polar compounds. They are usually handled under inert gas and with high purity reactants Catalytic polymerization presents numerous
advantages.
1- Polymers produced by catalytic polymerization cannot be produced by other methods For example HOPE and LLOPE cannot be produced by free radical polymerization.
2- Although the price of the catalyst can be very high, the polymerization rates can be extremely high (~ $400 per kg), making the process industrially attracting. Rates are usually measured in kg of polymer produced per hour and per kg (or mole) of catalyst. For example in recent Ziegler-Natta polymerization processes catalyst efficiency of 106 gm of polyethylene per gram of catalyst per hour has been reported.
The amount of toxic metal residue of catalyst is low enough in polymer that it need not be extracted.
3. Catalytic polymerization many times involves use of less Severe process conditions, making plant operation safe and economical. For example free radical high-pressure involves pressure of 2000 barg, whereas catalytic needs pressure of 1-30 barg.
Three main types of coordination catalyst:
1. Ziegler Catalysts or Ziegler-Natta catalysis
2. Philips Catalysts
3. Single-Site or Metallocene catalysts
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