Method and apparatus for separating air by cryogenic distillation

A method for separating air by cryogenic distillation using a column system comprising of a higher pressure column operating at a first pressure and a lower pressure column operating at a second pressure lower than the first pressure, the top of the higher pressure column being thermally coupled to the bottom of the lower pressure column, in which:

• i. compressing a first air flow constituting between 75% and 98% of the air sent to the column system to a third pressure above the first pressure, and then cooling and sending the first air flow at the third pressure to a first adsorption unit in order to be purified of water and of carbon dioxide and the purified first flow before sending at least a first portion of first air flow to the higher pressure column and optionally to the lower pressure column;
• ii. compressing a second air flow constituting between 2% and 25% of the air sent to the column system to a fourth pressure between 1.2 and 2 bar abs and above the second pressure but lower than the third pressure, preferably cooled by direct contact in an air cooling tower, and then sending at the fourth pressure to a second adsorption unit in order to be purified of water and of carbon dioxide and the purified second flow before being sent to the lower pressure column;
• iii. separating air in the higher pressure column to form an oxygen-enriched liquid and a nitrogen-enriched gas;
• iv. sending the oxygen-enriched liquid and the nitrogen-enriched liquid from the higher pressure column to the lower pressure column;
• v. withdrawing a liquid with a purity of greater than 99% of oxygen from the column system, pressurizing the liquid in a pump and then vaporizing said liquid by heat exchange with at least one portion of the first air flow;
• vi. sending an argon-enriched gas from the lower pressure column to a third column and withdrawing an argon-rich fluid at the top of the third column;
• and
• vii. sending air the lower pressure column constitutes between 10% and 25% of the total air sent to the column system;
• wherein the argon-rich fluid contains between 20% and 80% of the argon contained in the first and second air flows.