Flow diagram of an HDPE STR

T5q)ical reaction conditions are 90- 110°C and 3-4.5 MPa. The diluent used is normally isobutane, which facilitates the separation by vaporisation in a low pressure flash tank and permits higher operating temperatures than longer chained organic solvents. After treatment in purification beds to remove catalyst poisons, the ethylene monomer, comonomer and recycled diluent are fed to the loop reactor. The catalyst is flushed out of the feed container into the loop reactor by the diluent. The polymer concentration in the reactor slurry may range from 30 to 50 wt-%. The slurry from the reactor is passed through a sedimentation zone, where the polymer concentration increases up to 55 - 65 wt-%, and the slurry is then fed to the flash tank, where the hydrocarbons evaporate at a low pressure of about 0.15 MPa. 

The evaporated gases from the flash tank are filtered, compressed and fed to a distillation column, where the unreacted ethylene is separated from the diluent and comonomer. Due to the high conversion of ethylene in the loop reactor (96 - 98 %) the amount of separated ethylene rich stream is quite small and can be partly recycled back into the loop reactor, but a small purge stream is needed to control the accumulation of light inert material. The diluent and the comonomer from the distillation are recycled back into the reactor after having poisons such as water in an alumina or molecular sieve bed removed. 

From the purge bin, the powder, with a low residual hydrocarbon level, is fed to the extruder feed bins, from where the powder is fed to the extruder together with necessary stabilisers and additives. The powder is melted and homogenised and cut to pellets in an imderwater pelletiser. The pelletiser water is recycled and a small side-stream has to be discharged. This waste water contains small amounts of hydrocarbons due to the direct contact with the pellets. 

In addition to the general type of slurry loop process described above, slightly different loop reactor processes also exist. A short summary of two of these processes is shown below. 

Slurry process with horizontal loop reactors 

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