Method and device for separating air by cryogenic distillation

A process for separating air by cryogenic distillation in a set of columns comprising a first column operating at a first pressure, a second column operating at a second pressure lower than the first pressure, and a third column operating at a third pressure lower than the second pressure, the processing comprising the steps of:

• i) sending compressed, purified and cooled air to the first column where it is separated in order to form an oxygen-enriched liquid and a nitrogen-enriched gas;
• ii) condensing a portion of the nitrogen-enriched gas from the first column in a bottom vaporizer-condenser of the second column;
• iii) sending an oxygen-enriched liquid from the first column to the second column;
• iv) sending a second oxygen-enriched liquid from the bottom of the second column to the third column;
• v) sending a nitrogen-enriched gas from the top of the second column to a first vaporizer-condenser of the third column wherein the nitrogen-enriched gas is condensed and then sent to the second column;
• vi) withdrawing a nitrogen-rich gas from a top portion of the third column;
• vii) withdrawing a liquid containing at least 85% oxygen from the bottom of the third column, and then pressurizing and vaporizing the liquid to form a gaseous product containing at least 85% oxygen,
• wherein the first vaporizer-condenser of the third column is a vaporizer-condenser positioned at an intermediate point of the third column, the third column has a second vaporizer-condenser which is a bottom vaporizer-condenser, a nitrogen-enriched gas from the second column is compressed in a compressor having an inlet temperature below ambient temperature and is sent to the second vaporizer-condenser in order to be condensed therein, the vaporizer-condenser of the second column is a bottom vaporizer-condenser which is the only vaporizer-condenser present in the second column, the overhead gas from the first column is sent to the vaporizer-condenser of the second column without having been compressed.

A unit for separating air by cryogenic distillation comprising:

• a set of columns comprising a first column operating at a first pressure, a second column operating at a second pressure lower than the first pressure, and a third column operating at a third pressure lower than the second pressure, the second column having a bottom vaporizer-condenser, the third column having a first vaporizer-condenser and a second vaporizer-condenser, wherein the first vaporizer-condenser of the third column is a vaporizer-condenser positioned at an intermediate point of the third column, wherein the second vaporizer-condenser is a vaporizer-condenser positioned at a bottom location of the third column;
• a line configured to send compressed, purified and cooled air to the first column where the compressed, purified and cooled air is separated to form an oxygen-enriched liquid and a nitrogen-enriched gas;
• a line configured to send a portion of the nitrogen-enriched gas from the first column to be condensed in the bottom vaporizer-condenser of the second column;
• line configured to send oxygen-enriched liquid from the first column to the second column;
• a line configured to send an oxygen-enriched liquid from the bottom of the second column to the third column;
• a line configured to send a nitrogen-enriched gas from the top of the second column to the first vaporizer-condenser of the third column where it is condensed;
• a line configured to send the condensed liquid from the first vaporizer-condenser to the second column;
• a line configured to withdraw a nitrogen-rich gas from the top of the third column;
• a line configured to withdraw a liquid containing at least 85% oxygen from the bottom of the third column;
• means for pressurizing the liquid and a heat exchanger for vaporizing the pressurized liquid in order to form a gaseous product containing at least 85% oxygen;
• a compressor configured to receive and compress a nitrogen-enriched gas from the second column, wherein the compressor is in fluid communication with an intermediate portion of the heat exchanger, such that the compressor has an inlet temperature below ambient temperature,
• wherein the second vaporizer-condenser is in fluid communication with the compressor such that the second vaporizer-condenser is configured to receive and condense a compressed nitrogen-enriched gas from the compressor after the compressed nitrogen-enriched gas cools in the heat exchanger in order to form a condensed nitrogen-enriched stream, wherein the vaporizer-condenser of the second column is a bottom vaporizer-condenser which is the only vaporizer-condenser present in the second column;
• means for sending the overhead gas from the first column to the vaporizer-condenser of the second column without having been compressed;
• means for sending the pressurized rich liquid from the top of the second column to the top of the first column;
• and
• a line configured to withdraw a nitrogen enriched gas from the top of the first column and means for reheating it in order to form a product of the process.