Air separation

A method of separating a mixture, comprising nitrogen and oxygen, comprising the steps of:

• a) introducing a stream of the mixture into a higher pressure rectification column and separating it into oxygen-enriched liquid and nitrogen vapour;
• b) condensing at least part of nitrogen vapour and employing a first stream of the condensate as reflux in the higher pressure rectification column and a second stream of the condensate as reflux in a lower pressure rectification column;
• c) introducing a stream of the oxygen-enriched liquid into an intermediate vessel below liquid-vapour mass exchange devices therein at a pressure intermediate the pressure at the top of the higher pressure rectification column and the pressure at the bottom of the lower pressure rectification column, and separating the oxygen-enriched liquid by rectification therein into an oxygen-depleted vapour and liquid further enriched in oxygen;
• d) rebelling a part of the further-enriched liquid and thereby forming more oxygen depleted vapour;
• e) condensing at least part of the oxygen-depleted vapour by indirect heat exchange with liquid from an intermediate mass exchange level of the lower pressure rectification column, and introducing at least some of the further-enriched liquid into the lower pressure rectification column;
• f) introducing at least part of the said condensed vapour of step (e) into the lower pressure rectification column, or taking at least part of the said condensed vapour as product, or both;
• g) separating an oxygen product from fluid introduced into the lower pressure rectification column;
• h) withdrawing a gaseous nitrogen product from the higher pressure rectification column;
• and
• i) reboiling liquid oxygen separated in the lower pressure rectification column by heat exchange with the condensing nitrogen vapour of step (b);
• wherein no liquid nitrogen reflux for the higher and lower pressure rectification columns is formed by indirectly heat exchanging liquid from an intermediate mass exchange region of the lower pressure rectification column with nitrogen vapour from the higher pressure rectification column.

A method of separating a mixture, comprising nitrogen and oxygen, comprising the steps of:

• a) introducing a stream of the mixture into a higher pressure rectification column and separating it into oxygen-enriched liquid and nitrogen vapour;
• b) condensing at least part of nitrogen vapour and employing a first stream of the condensate as reflux in the higher pressure rectification column and a second stream of the condensate as reflux in a lower pressure rectification column;
• c) passing a stream of the oxygen-enriched liquid through a pressure-reducing valve to form a further mixture comprising liquid further enriched in oxygen and vapour depleted of oxygen and introducing the further mixture into an intermediate vessel at a pressure intermediate the pressure at the top of the higher pressure rectification column and the pressure at the bottom of the lower pressure rectification column so as to separate therein the vapour phase from the liquid phase;
• d) reboiling a part of the further enriched liquid and thereby forming more oxygen depleted vapour;
• e) reducing the pressure of a stream of the further-enriched liquid and employing it to condense at least some of the oxygen-depleted vapour so as to form condensed vapour and an at least partially vaporised, further enriched liquid, and introducing at least part of the partially vaporised, further enriched liquid into the lower pressure rectification column;
• introducing at least part of the said condensed vapour of step (e) into the lower pressure rectification column, or taking at least part of the said condensed vapour as product, or both;
• g) separating an oxygen product from fluid introduced into the lower pressure rectification column;
• h) reboiling liquid oxygen separated in the lower pressure rectification column by heat exchange with the condensing nitrogen vapour of step (b);
• and
• i) withdrawing a gaseous nitrogen product from the higher pressure column.

A method of separating a mixture, comprising nitrogen and oxygen, comprising the steps of:

• a) introducing a stream of the mixture into a higher pressure rectification column and separating it into oxygen-enriched liquid and nitrogen vapour;
• b) condensing at least part of nitrogen vapour and employing a first stream of the condensate as reflux in the higher pressure rectification column and a second stream of the condensate as reflux in a lower pressure rectification column;
• c) passing a stream of the oxygen-enriched liquid through a pressure-reducing valve to form a further mixture comprising liquid further enriched in oxygen and vapour depleted of oxygen and introducing the further mixture into an intermediate vessel at a pressure intermediate the pressure at the top of the higher pressure rectification column and the pressure at the bottom of the lower pressure rectification column so as to separate therein the vapour phase from the liquid phase;
• d) reboiling a part of the further-enriched liquid and thereby forming more oxygen depleted vapour;
• e) condensing at least part of the oxygen-depleted vapour by indirect heat exchange with liquid from an intermediate mass exchange level of the lower pressure rectification column, and introducing at least some of the further-enriched liquid into the lower pressure rectification column;
• f) introducing at least part of the said condensed vapour of step (e) into the lower pressure rectification column, or taking at least part of the said condensed vapour as product, or both;
• g) separating an oxygen product from fluid introduced into the lower pressure rectification column;
• and
• h) withdrawing a gaseous nitrogen product from the higher pressure column;
• and
• i) reboiling liquid oxygen separated in the lower pressure rectification column by heat exchange with the condensing nitrogen vapour of step (b);
• wherein no liquid nitrogen reflux for the higher and lower pressure rectification columns is formed by indirectly heat exchanging liquid from an intermediate mass exchange region of the lower pressure rectification column with nitrogen vapour from the higher pressure rectification column.