Nitrogen generation

Abstract

Air is separated in a single fractionation column ( 116 ) to produce a top nitrogen gaseous fraction ( 142 ) and a bottom liquid fraction ( 120 ) containing less than (80) mole percent of oxygen. A liquid nitrogen product ( 136 ) is also produced. The necessary refrigeration is created by expansion with the performance of external work of firstly a stream of compressed air in an expansion turbine ( 114 ) and of secondly a stream of vaporised bottom fraction in an expansion turbine ( 128 ). At least part of the feed to the column ( 116 ) comes from the turbine ( 114 ). The outlet pressure of the turbine ( 114 ) is essentially the gaseous nitrogen product pressure. A double fractionation column may be used instead of the single fractionation column ( 116 ).

1 . A method of generating nitrogen, wherein compressed air is separated at elevated pressure by fractionation to produce a gaseous nitrogen product fraction and an oxygen-enriched liquid product fraction containing less than 80 mole percent of oxygen, a gaseous nitrogen product is taken at elevated pressure from the top gaseous nitrogen product fraction, a stream of the product oxygen-enriched fraction is vaporised by indirect heat exchange with condensing nitrogen separated in the fractionation, a liquid nitrogen product is generated with the gaseous nitrogen product by condensing nitrogen separated in the fractionation, and refrigeration for the method is created (i) by expanding a stream of compressed air in a first expansion turbine with the performance of external work and (ii) by expanding at least part of the vaporised stream of the product oxygen-enriched fraction in a second expansion turbine with the performance of external work, characterised in that the said stream of compressed air enters the first expansion turbine at a pressure substantially in excess of the pressure at which the gaseous nitrogen product is taken but leaves the first expansion turbine at essentially the gaseous nitrogen product pressure, and in that at least part of the air separated in the said fractionation column is supplied from the first expansion turbine:  
     
     
         2 . The method according to  claim 1 , in which the fractionation is performed in a single fractionation column.  
     
     
         3 . The method according to  claim 2 , in which a stream of a liquid mixture of nitrogen and oxygen is withdrawn from the bottom or an intermediate mass exchange region of the single column, is reduced in pressure, is heat exchanged with condensing nitrogen separated in the single column, and is vaporised thereby, and a stream of the resulting vapour is compressed in a recycle compressor and is returned to a bottom mass exchange region of the single column.  
     
     
         4 . The method according to  claim 1  in which the fractionation is performed in a double fractionation column comprising a higher pressure column and a lower pressure column.  
     
     
         5 . The method according to  claim 4  in which all the gaseous nitrogen product is taken from the higher pressure column.  
     
     
         6 . The method according to  claim 4 , in which liquid nitrogen separated in the lower pressure column is pumped to the higher pressure column.  
     
     
         7 . The method according to  claim 1 , in which the proportion of nitrogen product produced in liquid state is varied by varying the inlet pressure of the first expansion turbine.  
     
     
         8 . The method according to  claim 1 , in which at least 85% by volume of the gaseous air to be separated by fractionation flows through the first expansion turbine.  
     
     
         9 . A nitrogen generator for performing the method claimed in  claim 1 , comprising a fractionation column operable to produce an elevated pressure gaseous nitrogen product and an oxygen-enriched air product containing less than 80 mole % of oxygen, the nitrogen generator also being able to co-generate a liquid nitrogen product, a first expansion turbine for expanding with the performance of external work a first stream of compressed air, a nitrogen condenser for condensing nitrogen separated in the fractionation column by indirect heat exchange with a vaporising stream of the product oxygen-enriched liquid air, and a second expansion turbine for expanding a stream of the vaporised product oxygen-enriched air with the performance of external work, characterised in that the outlet of the first expansion turbine communicates with the fractionation column.  
     
     
         10 . The nitrogen generator according to  claim 9 , in which the fractionation column is a single fractionation column  
     
     
         11 . The nitrogen generator according to  claim 10 , additionally comprising a further condenser for condensing nitrogen, the further condenser having passages therethrough for a stream of liquid comprising a mixture of oxygen and nitrogen taken from the fractionation column, the said passages communicating at their outlet end with a recycled compressor for returning the vaporised mixture to the bottom of the fractionation column.  
     
     
         12 . The nitrogen generator according to  claim 9 , wherein the fractionation column is a double fractionation column comprising a higher pressure column and a lower pressure column.  
     
     
         13 . The nitrogen generator according to  claim 12 , additionally including a pump for sending condensed nitrogen separated in the lower pressure column to the higher pressure column.