Method and apparatus for producing pressurized oxygen by low-temperature separation of air

Abstract

The invention relates to a method and apparatus for producing pressurized oxygen by low-temperature separation of air in a distillation column system having a low-pressure column and a high-pressure column. All of the feed air is compressed to a first air pressure, at least 4 bar above the operating pressure of the high-pressure column. A first partial stream of the feed air, compressed to the first air pressure, is further compressed in a secondary compressor to a second higher air pressure, before being (pseudo-)liquefied and is introduced into the distillation system. A second partial stream of the feed air, compressed to the first air pressure, is cooled and depressurized in an expander, which drives the secondary compressor. A return stream is branched off from the (pseudo-)liquefied first partial stream, depressurized, heated and then introduced to the first partial stream upstream from the secondary compressor.

1 . A method for producing pressurized oxygen by low-temperature separation of air in a distillation column system for nitrogen-oxygen separation, comprising a low-pressure column ( 17 ) and a high-pressure column ( 18 ), said method comprising:
 compressing all of the feed air ( 1 ) to a first air pressure ( 3 ;  403 ,  463 ), which is at least 4 bar over the operating pressure of the high-pressure column ( 18 ),   cooling at least a portion of the compressed feed air ( 6 ) in a main heat exchanger ( 13 ),   further compressing a first partial stream ( 7 ,  9 ;  107 ) of the feed air ( 6 ), already compressed to said first air pressure, in a secondary compressor ( 10 ;  110 ) to a second air pressure that is higher than said first air pressure,   liquefying or pseudo-liquefying the further-compressed first partial stream ( 12 ;  112 ) in said main heat exchanger  13  and then introduced at least a part ( 15 ) of the liquefied or pseudo-liquefied first partial stream into said distillation column system for nitrogen-oxygen separation,   cooling a second partial stream ( 8 ,  23 ;  123 ) of the feed air ( 6 ), compressed to the first air pressure, in said main heat exchanger ( 13 ) to an intermediate temperature and then actively depressurizing the cooled second partial stream in a first expander ( 24 ;  124 ), whereby said first expander ( 24 ;  124 ) drives said secondary compressor ( 10 ,  110 ),   introducing the expanded second partial stream ( 25 ) into said distillation column system for nitrogen-oxygen separation,   withdrawing an oxygen product stream  39  in liquid form from said distillation column system for nitrogen-oxygen separation, and bringining said oxygen product stream  39  in liquid form to an elevated pressure ( 40 ),   evaporating or pseudo-evaporating and heating under elevated pressure the oxygen product stream  39 , in pressurized in liquid form, in said main heat exchanger ( 13 ) and recovering said oxygen product stream  39  as a pressurized oxygen product ( 42 ),   branching off a return stream ( 57 ;  357 ) off from the (pseudo-)liquefied first partial stream ( 14 ),   depressuring said return stream ( 57 ;  357 ) in a choke valve ( 58 ;  358 ) and heating the depressurized return stream in said main heat exchanger ( 13 ), and   feeding the heated return stream ( 59 ;  159 ) to said first partial stream ( 7 ;  107 ) at a point upstream from the secondary compressor ( 10 ;  110 ).   
     
     
         2 . The method according to  claim 1 , wherein compression of the feed air ( 1 ) to the first air pressure is performed in a main air compressor ( 3 ). 
     
     
         3 . The method according to  claim 1 , wherein compression of the feed air ( 1 ) to the first air pressure is performed in a main air compressor ( 403 ) and in a secondary air compressor ( 463 ). 
     
     
         4 . The method according to  claim 1 , wherein the division of the feed air into said first partial stream  107  and said second partial stream  123  occurs within said main heat exchanger  13 . 
     
     
         5 . The method according to  claim 1 , wherein the secondary compressor  110  is operated as a cold compressor, with a starting temperature more than 20K below ambient temperature. 
     
     
         6 . The method according to  claim 5 , wherein the return stream  159  is combined with the first partial stream  107  only downstream from the cooling of the first partial stream  107 , and the division of the feed air into said first partial stream  107  and said second partial stream  123  occurs within said main heat exchanger  13 . 
     
     
         7 . The method according to  claim 4 , wherein said expander  124  is mechanically coupled to said secondary compressor  110 , and an oil brake  161  sits on the common shaft of said expander  124  and secondary compressor  110 . 
     
     
         8 . The method according to  claim 4 , wherein a portion of said second partial stream ( 123 ) of the feed air ( 6 ) is depressurized in a second expander  224  which is coupled to a generator and said first expander ( 24 ;  124 ) mechanically connected only to the said secondary compressor  110 . 
     
     
         9 . The method according to  claim 1 , wherein the return stream  357  is branched off from the first partial stream at a point before the cold end of the main heat exchanger. 
     
     
         10 . The method according to  claim 2 , wherein the feed air is subjected to a pre-cooling stage  4  and the purification stage  5  after all of the feed air is compressed to said first air pressure. 
     
     
         11 . The method according to  claim 3 , wherein the feed air is subjected to a pre-cooling stage  4  and the purification stage  5  prior to all of the feed air being compressed up to said first air pressure in the secondary air compressor ( 463 ). 
     
     
         12 . An apparatus for producing pressurized oxygen by low-temperature separation comprising:
 a distillation column system for nitrogen-oxygen separation, said system comprising a low-pressure column ( 17 ) and a high-pressure column ( 18 ),   means for compressing all of the feed air ( 1 ) to a first air pressure ( 3 ;  403 ,  463 ), which is at least 4 bar above the operating pressure of said high-pressure column ( 18 ),   a main heat exchanger ( 13 ) for cooling at least a portion of the compressed feed air ( 6 ),   a secondary compressor ( 10 ;  110 ) for further compressing a first partial stream ( 7 ,  9 ;  107 ) of the feed air ( 6 ), that is already compressed to the first air pressure, to a second air pressure, which is higher than the first air pressure,   means for liquefying or pseudo-liquefying further-compressed first partial stream ( 12 ;  112 ) in said main heat exchanger and means for introducing at least a portion of resultant (pseudo-)liquefied first partial stream into said distillation column system for nitrogen-oxygen separation,   means for cooling a second partial stream ( 8 ,  23 ;  123 ) of the feed air ( 6 ). that is already compressed to the first air pressure, to an intermediate temperature in said main heat exchanger ( 13 ),   an expander ( 24 ;  124 ) for active depressurization of cooled second partial stream, whereby said expander ( 24 ;  124 ) is mechanically coupled to said secondary compressor ( 10 ,  110 ),   means for introducing expanded second partial stream ( 25 ) into said distillation column system for nitrogen-oxygen separation,   means for withdrawing off an oxygen product stream  39  in liquid state from said distillation column system for nitrogen-oxygen separation,   means ( 40 ) for increasing the pressure of the oxygen product stream  39  in the liquid state to an elevated pressure,   means in said main heat exchanger ( 13 ) for evaporation or pseudo-evaporation and heating of pressurized liquid oxygen product stream  39  and means for recovering heated pressurized oxygen product stream ( 42 ) from said main heat exchanger ( 13 ),   means for branching off a return stream ( 57 ;  357 ) from the (pseudo-)liquefied first partial stream ( 14 ),   a choke valve ( 58 ;  358 ) for depressurizing the return stream ( 57 ;  357 ),   means for heating the depressurized return stream in said main heat exchanger ( 13 ), and   means for supplying heated depressurized return stream ( 59 ;  159 ) to the first partial stream ( 7 ;  107 ) upstream from the secondary compressor ( 10 ;  110 ).