Method and apparatus for separating air by cryogenic distillation

A method for separating air by cryogenic distillation in a separation unit comprising a medium-pressure column and a low-pressure column, connected thermally together, with the low-pressure column comprising a vessel reboiler and an intermediate reboiler, and a pure oxygen column, the method comprising the steps of:

• i) introducing a purified and cooled gaseous air at a first pressure from an exchange line to the medium-pressure column;
• ii) sending an oxygen-rich liquid and a nitrogen-rich liquid from the medium-pressure column to the low-pressure column;
• iii) withdrawing a nitrogen-rich gas from the low-pressure column;
• iv) withdrawing an oxygen-rich liquid containing at most 97 mol % oxygen from the vessel reboiler of the low-pressure column;
• v) sending a first flow of oxygen-rich liquid to a vaporizer and sending the gaseous oxygen formed to the exchange line;
• vi) sending a second flow of oxygen-rich liquid to the top of the pure oxygen column, the pure oxygen column having a pure oxygen reboiler, wherein the second flow of oxygen-rich liquid is purified in order to form a vessel liquid containing at least 98 mol % oxygen;
• vii) sending a boosted airflow at a second pressure, higher than the first pressure, to the vessel reboiler of the pure oxygen column;
• viii) withdrawing a nitrogen-rich gas from the top of the medium-pressure column and sending the nitrogen-rich gas to the intermediate reboiler of the low-pressure column, and sending the condensed gas to the top of the medium-pressure column;
• and
• ix) sending a nitrogen-rich gas or air to the vessel reboiler of the low-pressure column and sending the liquid that condenses therein to the medium-pressure column,
• x) withdrawing an oxygen-rich top gas from the pure oxygen column and introducing said oxygen-rich top gas to the low-pressure column or to the exchange line for warming,
• wherein the vessel liquid is withdrawn from the pure oxygen column as a product and boosted air at the second pressure is sent to the vaporizer in order to vaporize the first flow of oxygen-rich liquid,
• wherein the boosted air at the second pressure is divided into two portions, a first portion of boosted air at the second pressure is sent to the vessel reboiler of the pure oxygen column and a second portion of boosted air at the second pressure is sent to the vaporizer,
• wherein the first flow of oxygen-rich liquid is partially vaporized in the vaporizer, with the liquid formed constituting the second flow of oxygen-rich liquid,
• wherein the first flow of oxygen-rich liquid is less rich in oxygen than the second flow of oxygen-rich liquid,
• wherein the first flow of oxygen-rich liquid is pressurised upstream of the vaporizer.