Process Description
The feed stream consists of ambient air at 300 K and 101.32 kPa. It is first passed through an air filter to remove impurities such as dust, pollen and mould and then passed to a multi-stage compressor with inter-cooling where it is compressed to 911.85 kPa.
The remaining carbon dioxide and water is then removed from the stream to prevent formation and deposition of dry ice and ice respectively in the system at cryogenic temperatures. The stream is passed through a molecular sieve adsorber which contains a zeolite and alumina to adsorb the carbon dioxide and water respectively. It is assumed that all of the impurities are removed by the adsorber. The air stream is then cooled further to 107 K in the main heat exchanger.
The first distillation column operates at high pressure and the products include an oxygen-rich stream at the bottom and a nitrogen-rich stream at the top. The bottom product is passed through a reboiler and then throttled to 101.32 kPa, then fed into the second distillation column, which operates at a lower pressure, for further separation. The top product is passed through a condenser and then throttled to 101.32 kPa and also fed into the second distillation column. The bottom product of the second distillation column contains 99.5% pure oxygen while the top product contains 99% pure nitrogen. The nitrogen is used to purge the system while the oxygen is fed to the ethylene oxide (EO) reactor.
Heat integration was incorporated into the design to save on energy and therefore minimise on operating costs. The pure nitrogen product stream was used to cool down the air feed stream to cryogenic temperatures (107 K). In addition, heat transfer between the top product of the first distillation column and the bottom product of the second distillation column provided the heat required by the condenser-reboiler