Process and device for low temperature air fractionation

Abstract

The process and the device serve for low temperature air fractionation in a distillation column system for obtaining nitrogen which has a single column ( 12 ). Feed air ( 8 ) is cooled in a main heat exchanger ( 9 ) and introduced ( 11, 43 ) into the single column ( 12 ). The single column ( 12 ) has a top condenser ( 13 ) in which vapour from the upper region of the single column is at least in part condensed. A nitrogen product stream ( 15, 16, 17 ) is withdrawn from the upper region of the single column ( 12 ). A first residual fraction ( 14, 19 ) is withdrawn in the liquid state from the single column ( 12 ), at least in part vaporized in the top condenser ( 13 ) and subsequently taken off from the top condenser as vaporized first residual fraction ( 19 ). A first part ( 20 ) of the vaporized first residual fraction ( 19 ) is expanded in a work-producing manner in an expansion machine ( 21 ). A second residual fraction ( 18, 29 ) is withdrawn from the lower or intermediate region of the single column ( 12 ), recompressed ( 30 ) and subsequently passed ( 32 ) back at least to a first part of the single column ( 12 ). A second part of the vaporized first residual fraction ( 19 ) is not passed into the expansion machine ( 21 ) but is taken off as gaseous impure oxygen product ( 60 ) at about the inlet pressure of the expansion machine ( 21 ).

1 . A process for the low temperature air fractionation in a distillation column system for obtaining nitrogen which has a single column ( 12 ) in which
 feed air ( 8 ) is cooled in a main heat exchanger ( 9 ) and introduced ( 11 ,  43 ) into the single column ( 12 ),   the single column ( 12 ) has a top condenser ( 13 ) in which vapor from the upper region of the single column is at least in part condensed,   a nitrogen product stream ( 15 ,  16 ,  17 ) is withdrawn from the upper region of the single column ( 12 ),   a first residual fraction ( 14 ,  19 ) is withdrawn in the liquid state from the single column ( 12 ), at least in part vaporized in the top condenser ( 13 ) and subsequently taken off from the top condenser as vaporized first residual fraction ( 19 ),   at least a first part ( 20 ) of the vaporized first residual fraction ( 19 ) is expanded in a work-producing manner in an expansion machine ( 21 ) and   a second residual fraction ( 18 ,  29 ) is withdrawn from the lower or intermediate region of the single column ( 12 ), recompressed ( 30 ) and subsequently passed ( 32 ) back at least to a first part of the single column ( 12 ),   
     characterized in that
 a second part of the vaporized first residual fraction ( 19 ) is not passed into the expansion machine ( 21 ) but is taken off as gaseous impure oxygen product ( 60 ) at about the inlet pressure of the expansion machine ( 21 ) and/or 
 a second part of the second residual fraction is taken off downstream of the recompression ( 30 ) as gaseous impure oxygen product ( 160 ). 
 
   
   
       2 . A process according to  claim 1 , characterized in that the first and second parts of the first residual fraction are introduced into the main heat exchanger ( 9 ), wherein the first part ( 20 ) is withdrawn from the main heat exchanger ( 9 ) at an intermediate temperature and passed to the expansion machine ( 21 ) and the second part is warmed to about ambient temperature and is taken off as gaseous impure oxygen product ( 60 ). 
   
   
       3 . A process according to  claim 2 , characterized in that the first and second parts of the first residual fraction are introduced ( 19 ) together into the main heat exchanger ( 9 ). 
   
   
       4 . A process according to  claim 1 , characterized in that the second residual fraction ( 18 ,  29 ) is recompressed by means of a cold compressor ( 30 ). 
   
   
       5 . A process according to  claim 1 , characterized in that the mechanical energy generated in the work-producing expansion ( 21 ) is used at least in part for recompressing ( 30 ) the second residual fraction. 
   
   
       6 . A process according to  claim 1 , characterized in that
 an oxygen-containing stream ( 36 ) is withdrawn from the single column ( 12 ) at an intermediate point and passed ( 39 ) to a pure oxygen column ( 38 ) and   a pure oxygen product stream ( 41 ) is withdrawn in the liquid state from the lower region of the pure oxygen column ( 38 ),   the pure oxygen product stream ( 41 ,  56 ), optionally after pressure elevation ( 55 ) in the liquid state, is vaporized and warmed against feed air ( 8 ) in the main heat exchanger ( 9 )   and finally is obtained as gaseous product ( 57 ).   
   
   
       7 . A process according to  claim 1 , characterized in that the first residual fraction ( 14 ) is taken off the bottom of the single column ( 12 ). 
   
   
       8 . A process according to  claim 1 , characterized in that the second residual fraction ( 18 ) is taken off from an intermediate point of the single column ( 12 ), which intermediate point is arranged above the bottom, in particular above the point at which the first residual fraction ( 14 ) is withdrawn. 
   
   
       9 . A process according to  claim 1 , characterized in that the exit pressure of the expansion machine ( 21 ) is less than 0.5 bar above atmospheric pressure. 
   
   
       10 . A process according to  claim 1 , characterized in that the second residual fraction ( 18 ,  29 ) is recompressed ( 30 ) to a pressure which is less than 0.5 bar above the operating pressure of the single column ( 12 ). 
   
   
       11 . Apparatus for low temperature air fractionation:
 having a distillation column system ( 12 ) for obtaining nitrogen which has a single column ( 12 ),   having a main heat exchanger ( 9 ) for cooling feed air ( 8 )   having means for introducing cooled feed air into the single column ( 12 ),   having a nitrogen product line ( 15 ,  16 ,  17 ) which is connected to the single column ( 12 ),   having a top condenser ( 13 ) for condensing vapor from the upper region of the single column,   having means for withdrawing a first residual fraction ( 14 ,  19 ) in the liquid state from the single column ( 12 ) and for introducing the first residual fraction into the top condenser,   having means for taking off the at least in part vaporized first residual fraction ( 19 ) from the top condenser ( 13 ),   having a turbine line ( 20 ) for introducing at least a first part of the vaporized first residual fraction ( 19 ) into an expansion machine ( 21 ),   having means for withdrawing a second residual fraction ( 18 ,  29 ) from the lower or intermediate region of the single column ( 12 ),   having means for recompressing ( 30 ) the second residual fraction and   having means for recycling at least a first part of the recompressed second residual fraction to the single column ( 12 ),   
     characterized by an impure oxygen product line ( 60 ,  160 )
 for taking off a second part of the vaporized first residual fraction ( 19 ) as gaseous impure oxygen product at about the inlet pressure of the expansion machine ( 21 ) and/or 
 for taking off a second part of the second residual fraction as gaseous impure oxygen product downstream of the means for recompression ( 30 ).