Patent GB-1050475-A (GB)
IPC: C01B3/36
Abstract: A process for the production of ammonia synthesis gas with a high free oxygen content wherein hydrogen is prepared by cracking hydrocarbons with a gas having a high free oxygen content and is then brought into contact with liquid nitrogen obtained by the liquefaction and rectification of air at a low temperature in at least two rectification columns arranged in series and operating under decreasing pressures and in mutual heat exchange in such a manner that a first portion of the gaseous nitrogen separated at the head of the first rectification column operating under the higher pressure is condensed by heat exchange with a liquid having a high free oxygen content, e.g. above 95% or from 60-70% separated in the second rectification column operating under the lower pressure, is improved by condensing at least a part of a second portion of the gaseous nitrogen separated at the head of the first rectification column by heat exchange with liquid nitrogen previously brought into contact with hydrogen and thereafter rectifying the condensed nitrogen, at least a portion of said liquid nitrogen which has been brought into contact with the hydrogen being obtained by the liquefaction of pure gaseous nitrogen in heat exchange with at least part of the liquid having a high free oxygen content separated at the bottom portion of the second rectification column.  The hydrogen may be purified by washing with liquid nitrogen, the residual liquid from this washing operation being used to condense at least a part of the second part of the nitrogen separated at the head of the first rectification column.  Purified hydrogen mixed with liquid nitrogen may be contacted in heat exchange with at least a part of the second protion of nitrogen separated at the head of the first rectification column.  The pressure of the nitrogen in the first rectification column may be near the pressure of the hydrogen, and the high free oxygen liquid which condenses the nitrogen at the head of the rectifier is compressed before entering the rectifier. The pressure of the nitrogen in the first rectification column prior to liquefaction may be lower than that <PICT:1050475/C1/1> of the hydrogen, and after liquefaction, may be brought to a pressure close to that of hydrogen. Impure hydrogen introduced at line 107 and partially purified in condensers 102, 100 and 98 by heat exchange with product leaving by line 103 is washed in column 88 by a pure nitrogen spray from expansion valve 87, and pure hydrogen leaving by line 97 meets liquid nitrogen from expansion valves 89 and 91 to form the product.  Air from turbo compressor 1 is introduced into high pressure rectification column 21 via line 8 and into low pressure rectification column 22 via line 20.  The liquid having a high free oxygen content (at least 95% oxygen) leaves the bottom of low pressure rectification column 22 via 45, and a part enters a combined condenser and vaporizer unit 23 at the top of high pressure rectification column 21 via line 47 to condense and ensure the reflux of nitrogen in the column, and is returned via line 48 to column 22. Part of the nitrogen withdrawn from column 21 via line 30 is liquefied in heat exchanger 32 by a product mixture of pure hydrogen and liquid nitrogen arriving through line 105, the liquid nitrogen in the mixture having been cooled by contact with the high free oxygen mixture from the bottom of column 22 in exchangers 19, 53 and 51, the liquefied nitrogen reaching the hydrogen to form the mixture via lines 83 and 85 (possibly via by-pass lines 79, 80 and 82) heat exchange 86, and expansion valves 89 and 91.  The other part of the nitrogen withdrawn via line 30 is liquefied in exchanger 35 by heat exchange with the residual liquid from washing column 88 introduced via line 94, and the liquefied nitrogen from exchangers 32 and 35 are conveyed via lines 33 and 36 respectively, and after combination via lines 33 and 39 to the top of column 22. Pure gaseous nitrogen leaving the top of column 22 passes via lines 59, 60 and 62 (some may be discharged at 63) 65, compressor 66 to bring it to slightly above the pressure of washing column 88, lines 67, 70 and 74, 71 to heat exchangers 19, 53 and 51.  Liquid containing 40% oxygen withdrawn from the bottom of column 21 via line 24 is cooled in heat exchanger 26, filtered at 27A and 27B and enters via expansion valve 29 into column 22. A liquid with a relatively high nitrogen content withdrawn via line 41 is similarly cooled and enters via valve 44. Impure nitrogen withdrawn from column 22 via line 56, is conveyed, after cooling the column 21 extracts in exchanger 26, via line 57 and discharge 58.