Technology

Process Summary

The autoclave reactor polymerization of ethylene is a non-catalytic process that operates with organic peroxides as radical initiators at an operating pressure from 1,300 to 2,000 bar and a reactor temperature of up to 310°C. The Ethylene conversion yield is up to 21% with a reactor capacity of up to 150,000 ton per year^[1].

Process Stages

The different process stages are described according to Borealis Povoo LDPE plant^[2].

1. Ethylene Pressure Increase
   
   The gas fed into the process comes from cracker as 15 bar gas. Ethylene pressure is increased in three steps to reach desired reaction pressure. Every time the pressure is increased, the temperature increases thus, cooling is needed after every increase. The first step is to feed the source ethylene into booster compressor. There the pressure is compressed from 15 to 40 bars and cooled down afterwards. The 40 bar gas is then fed to the primary compressor and pressure is increased to 250 bars. The secondary compressor is the most critical one since it converts the pressure from 250 to 2000 bars. After the secondary compressor and the cooling, the gas is then fed into the reactor along with reaction initiators.
    
2. Ethylene Circulation
   
   During the reaction, about 20% of the gas fed into the reactor polymerizes and the rest is separated. After the reaction the gas that did not polymerize is fed back into the process. Immediately after the reaction the product and any non-polymer compounds are separated in two stages. The pressure is also lowered in these stages. The first stage is high pressure separator. From the HPS, the gas mixture is cooled down and the gas is separated from the blend and fed back into the secondary compressor. This cycle is called recycle gas. At the second stage, the low pressure separator, the remaining gas is separated along with remains of initiators from the polymer. From the LPS the gas cycles into the so called Flash gas circulation. In the Flash gas circulation the gas it cooled down, cleaned from solvents, compressor oils and wax. These contaminants are separated into a slop system and then the gas is pressurized back to 40 bars and fed back to the primary compressor. Some amount if gas is recycled back to cracker in order to keep process gas clean from any inert components.
    
3. Extrusion and post-reaction
   
   Once the polymer mass is separated from the gas in LPS, it is then fed into an extruder. This extruder then pushes the mass trough a die and after the die the polymer is immediately cooled down and cut into granulates. The cooling happens by circulating water in the cutting chamber and with this water, granulates are moved to drying. While drying, granulates are weighted and this way the production rate is measured. After drying, granulates are conveyed with air to large silos for storing. Once in the silos, granulates still exert some ethylene gas, thus ventilation is needed for 12 hours to remove any risk of the gas forming a flammable and explosive atmosphere in the silo.

References

1. Csernyik István, 2010, LDPE Technology, MOL Hungary.
2. Daniel Lignell, 1^st Jan 2015, Possibilities of autoclave LDPE process, oai:www.theseus.fi:10024/96353.