The purpose of the purification unit is to produce an ethylene gas
stream that has a purity of 99.9wt% of ethylene. The distillation column
that is going to be used to achieve this purity is usually a large one.
There are as many as 125 stages in a typical
column. The column is operated at a pressure of 700 to 2800kPa. Its
overhead condenser and bottom reboiler are operated at a temperature of
-40 to -90 degrees celcius, and -15 to -65 degrees celcius, respectively.
At such a high purity, the distillation column requires a high reflux
ratio. Sometimes acetylene is found in the gas stream. Instead of
removing the acetylene, hydrogen is added to hydrogenate it and form
ethylene.
| 
