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PURIFICATION (how cold could you go)

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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