Once ethylene is formed, it needs to be purified. The gas
stream that exits the reactor is a mixture of ethylene and a number of
other gases. These gases are considered to be impurities in the gas
stream. Practically all of these gases (except for ethylene need to be
removed (extracted from the gas stream.) This step is necessary since the
next step in the process requireds the ethylene gas stream (feed) to be
extremely pure. The purification unit is nothing more than a separation
(distillation) unit (discussed earlier). The unit is operated at a very
low temperature and high pressure. The operating temperatures and
pressures ranges from -15 to -90 degrees celcius, and 700 to 2800 kPa,
respectively. These temperatures and pressures are needed to turn the
unwanted gases into liquids leaving ethylene by it self in the gas phase
at a very pure form. Once the purification process is done, the pure
ethylene gas stream is fed into the polymerization unit.
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