Process and apparatus for separating air by cryogenic distillation

A process for separating air by cryogenic distillation in an apparatus comprising:

• i) a first set of distillation columns comprising at least a first column operating at a first pressure referred to as high pressure and a second column operating at a second pressure lower than the first pressure, the top of the first column being thermally connected to the bottom of the second column by means of a first reboiler-condenser, a first purification unit and a first heat exchange line;
• and
• ii) a second set of distillation columns comprising at least a third column operating at a third pressure lower than the first pressure and a fourth column operating at a fourth pressure lower than the first, the second and the third pressures, the top of the third column being thermally connected to the bottom of the fourth column by means of a second reboiler-condenser, a second purification unit and a second heat exchange line,
• the method comprising the steps of:
• sending a first compressed air to the first purification unit to produce a first purified air;
• sending the first purified air from the first purification unit to the first exchange line to produce a first cooled air;
• sending the first cooled air from the first exchange line to the first column under conditions effective for the rectification of air;
• sending a second compressed air to the second purification unit to produce a second purified air;
• sending the second purified air from the second purification unit to the second exchange line to produce a second cooled air;
• sending the second cooled air from the second exchange line to the third column under conditions effective for the rectification of air, wherein the third column is operating at the third pressure;
• sending a head fluid from the third column to the second column or mixing the head fluid from the third column with a head gas from the second column;
• and
• sending an intermediate fluid from the second column to the third column.

An air separation apparatus comprising a first air separation unit comprising:

• i) a first set of distillation columns comprising at least a first column capable of operating at a first pressure referred to as high pressure and a second column capable of operating at a second pressure lower than the first pressure, the top of the first column being thermally connected to the bottom of the second column by means of a reboiler-condenser, a first purification unit and a first heat exchange line, means for sending compressed air to the first purification unit, means for sending purified air from the first purification unit to the first exchange line and means for sending cooled air from the first exchange line to the first column;
• ii) a second set of distillation columns comprising at least a third column capable of operating at a third pressure lower than the first pressure and a fourth column capable of operating at a fourth pressure lower than the first, the second and the third pressures, the top of the third column being thermally connected to the bottom of the fourth column by means of a reboiler-condenser, a second purification unit and a second heat exchange line, means for sending compressed air at the third pressure to the second purification unit, means for sending purified air from the second purification unit to the second exchange line and means for sending cooled air from the second exchange line to the third column;
• means for sending a head fluid from the third column to the second column or to a head gas pipe of the second column;
• and
• means for sending an intermediate fluid from the second column to the third column.