Separation of air

A plant for separating air by rectification comprising at least two compression stages in series for compressing a flow of air, a main heat exchanger having at least one first passage for the cooling at a first pressure of a first stream of the compressed air to a temperature suitable for its rectification, the at least one first passage communicating with a first chosen one of the compression stages, whereby the first pressure is essentially the outlet pressure of the first chosen compression stage, and at least one second passage for the cooling at a second pressure of a second stream of the compressed air to a temperature above that suitable for its rectification, the at least one second passage communicating with a second chosen one of the compression stages whereby the second pressure is essentially the outlet pressure of the second chosen compression stage, a double rectification column including a higher pressure column, a lower pressure column, and a condenser-reboiler placing the higher pressure column in indirect heat exchange relationship with the lower pressure column, the higher pressure column being arranged to operate under a pressure at its bottom not greater than the first pressure, and the lower pressure column being arranged to operate so as to produce therein an bottom liquid oxygen fraction;

• an inlet to the higher pressure column communicating with the at least one first passage;
• an expansion turbine for expanding with the performance of external work the second stream of the compressed air, the expansion turbine being arranged to operate at an inlet pressure not greater than the second pressure;
• an inlet to the lower pressure column communicating with the expansion turbine, and an outlet from a bottom region of the lower pressure column for an oxygen product, wherein the first chosen compression stage is downstream of the second chosen compression stage, whereby the second pressure is less than the first pressure, there is no expansion means intermediate the at least one first passage and the higher pressure column, and the expansion turbine is the sole expansion turbine in the plant.

A plant for separating air by rectification comprising at least two compression stages in series for compressing a flow of air, a main heat exchanger having at least one first passage for the cooling at a first pressure of a first stream of the compressed air to a temperature suitable for its rectification, the at least one first passage communicating with a first chosen one of the compression stages, whereby the first pressure is essentially the outlet pressure of the first chosen compression stage, and at least one second passage for the cooling at a second pressure of a second stream of the compressed air to a temperature above that suitable for its rectification, the at least one second passage communicating with a second chosen one of the compression stages whereby the second pressure is essentially the outlet pressure of the second chosen compression stage, a double rectification column including a higher pressure column, a lower pressure column, and a condenser-reboiler placing the higher pressure column in indirect heat exchange relationship with the lower pressure column, the higher pressure column being arranged to operate under a pressure at its bottom not greater than the first pressure, and the lower pressure column being arranged to operate so as to produce therein an bottom liquid oxygen fraction;

• an inlet to the higher pressure column communicating with the at least one first passage;
• an expansion turbine for expanding with the performance of external work the second stream of the compressed air, the expansion turbine being arranged to operate at an inlet pressure not greater than the second pressure;
• an inlet to the lower pressure column communicating with the expansion turbine, and an outlet from the lower pressure column for withdrawing an oxygen product in vapor state, wherein the first chosen compression stage is downstream of the second chosen compression stage, whereby the second pressure is less than the first pressure, there is no expansion means intermediate the at least one first passage and the higher pressure column, and the expansion turbine is the sole expansion turbine in the plant.