the apparatus which is illustrated in the drawings is intended to produce on the one hand , under 55 bars , gaseous nitrogen of high purity ( typically containing less than 10 ppm oxygen ), at a flow at least equal to 20 % of the flow of air being treated , and on the other hand , under 65 bars , oxygen at a purity of 95 to 99 . 5 %, with a high yield of extraction . these two gases under elevated pressure will be used on a same site : hydrogen will be produced by reacting oxygen with heavy hydrocarbons , and this hydrogen will be reacted with nitrogen to give ammonia . the apparatus essentially comprises an air compressor 1 , a device 2 for purifying air by adsorption , a heat exchange line 3 of the counter - current type , a double distillation column 4 , an expansion turbine 5 , a pump for liquid oxygen 6 , a nitrogen compressor 7 , an oxygen compressor 8 and a source of power 9 consisting for example of a steam turbine . double column 4 comprises a mean pressure column 10 surmounted by a low pressure column 11 in which the upper end portion defines a minaret 11a for the production of pure nitrogen under low pressure . a condensor - vaporizer 12 sets up heat exchange relationship between the head vapor ( substantially pure nitrogen ) of column 10 and the vat liquid ( oxygen of given purity ) of column 11 . in known manner , a duct provided with an expansion valve 13 enables to cause &# 34 ; rich liquid &# 34 ; ( oxygen enriched air ) to rise from the bottom of column 10 to an intermediate point of column 11 ; a duct provided with an expansion valve 14 enables to cause &# 34 ; lower poor liquid &# 34 ; ( impure nitrogen ) to rise from an intermediate point of column 10 to the base of minaret 11a ; and a duct provided with an expansion valve 15 enables to cause &# 34 ; upper poor liquid &# 34 ; ( substantially pure nitrogen ) to rise from the top column 10 to the top of the minaret . the nitrogen compressor 7 consists of a single three stage compressor . the first two stages each comprise two compression wheels and have average compression ratios per wheel of 2 and 1 . 73 respectively , while the third stage comprises a single compression wheel having a compression ratio of 1 . 83 . the global compression ratio of the compressor is therefore 22 . each wheel has a refrigerating agent at its outlet . the oxygen compressor 8 also consists of a single compressor . this compressor has six wheels having an average compression ratio of 1 . 37 per wheel . the global compression ratio is therefore 6 . 5 . the shaft 16 of compressor 7 is connected to shaft 17 of compressor 1 by means of a coupling 18 , and the unit is operated by the common source of energy 9 . possibly , shaft 16 may drive the different stages of compressor 7 by means of speed multipliers which are suitable for each stage . compressor 8 is driven by a separate source of energy 19 . the low pressure is selected so that once multiplied by the compression ratio of compressor 7 , it provides the desired elevated pressure for the production of nitrogen . thus , by neglecting the losses of charge , for an elevated pressure of nitrogen of 55 bars , the chosen low pressure is 55 / 22 = 2 . 5 bars . for a temperature spread of 2 ° c . in the vaporizer - condensor 12 , this corresponds to a mean pressure of the order of 11 bars . thus , the air which is introduced is compressed at 11 bars in compressor 1 , it is purified at 2 , cooled from the hot end to the cold end of the heat exchange line 3 , and introduced in the vicinity of its dew point at the bottom of column 10 . the pure low pressure nitrogen which exits in gas form from the top of minaret 11a and which is heated at room temperature from the cold end to the hot end of the heat exchange line , is introduced at the suction side of the first stage of compressor 7 , except possibly for a flow of nitrogen produced at low pressure via duct 20 . the high pressure nitrogen is produced by compression in the third stage of the compressor and is removed via duct 21 . compressor 7 is also used as a compressor for cycle nitrogen . for this purpose , mean pressure nitrogen is withdrawn at the top of column 10 , via duct 22 , is heated at room temperature in the heat exchange line and is introduced via duct 23 at the suction side of the second stage of compressor 7 . high pressure cycle nitrogen exits after being compressed in this second stage via duct 24 , it is cooled , liquefied , and sub - cooled in the heat exchange line , expanded at mean pressure in an expansion valve 25 and is introduced at the top of column 10 . through flow control means not illustrated , the flow of nitrogen which circulates in duct 24 is higher by a given quantity than the flow of nitrogen which circulates in duct 23 . the difference constitutes an additional flow of liquid nitrogen , so called rectification support , which is introduced under reflux at the top of column 10 . this flow is withdrawn from the flow of low pressure nitrogen which is sucked by the first stage of compressor 7 . impure nitrogen , constituting the residual gas of the apparatus , is withdrawn at the base of minaret 11a , it is heated at an intermediate temperature in the heat exchange line , it exits from the latter , is expanded at atmospheric pressure in a turbine 5 which ensures cold conditions of the apparatus , then is introduced into the exchange line , it is heated at room temperature , and finally is used to regenerate the bottles of adsorbent material of the apparatus 2 and withdrawn from the apparatus via duct 26 . the desired flow of liquid oxygen is withdrawn at the bottom of column 11 , it is pressurized by pump 6 to an intermediate pressure of oxygen , it is vaporized and heated at room temperature in exchange line 3 , and is then compressed at a production pressure by means of compressor 8 . in order to have a maximum limit of the thermodynamic irreversibilities in the exchange line , steps are taken so that the vaporization of liquid oxygen under the intermediate pressure of oxygen be carried out by condensation of nitrogen under the elevated cycle pressure , with a sub - critical value for this elevated pressure , for example 30 bars . this value corresponds to a vaporization of liquid oxygen under about 11 bars , which is therefore the pressure provided by pump 6 . the above considerations give pressures for the various stages of the compressor 7 : 2 . 5 bars at the inlet of the first stage , 11 bars at the inlet of second stage , 30 bars at the inlet of the third stage and 55 bars at the outlet of this third stage . it can be shown that the process described above results in an increase of investment costs , as compared to the known process where the low pressure is selected to be slightly higher than 1 bar , as soon as the flow of nitrogen produced is at least equal to 20 % of the flow of air being treated . when the production of nitrogen is higher than about 30 % of the air flow , it additionally represents a gain of energy . on the other hand , the fact of purifying the air which is introduced under 11 bars is very advantageous on a cost point of view .