Air separation method and apparatus

Abstract

A method and apparatus for separating air in which an oxygen-rich liquid stream is pumped and then heated within a heat exchanger to produce an oxygen product through indirect heat exchange with first and second boosted pressure air streams. The first boosted pressure air stream is cold compressed at an intermediate temperature of the heat exchanger, reintroduced into the heat exchanger at a warmer temperature and then fully cooled and liquefied. The second boosted pressure air stream, after having been partially cooled, is expanded to produce an exhaust stream that is in turn introduced into a lower pressure column producing the oxygen-rich liquid. The second boosted pressure air stream is partially cooled to a temperature no greater than the intermediate temperature at which the cold compression occurs so that both the first and second boosted pressure air streams are able to take part in the heating of the oxygen-rich stream.

I claim: 
     
         1 . A method of separating air comprising:
 separating compressed and purified air in a cryogenic rectification process such that an oxygen-rich liquid column bottoms is produced in a lower pressure column linked in a heat transfer relationship to a higher pressure column by a condenser reboiler, an oxygen-rich liquid stream is pumped to produce a pumped oxygen stream and at least part of the pumped oxygen stream is heated in a main heat exchange system to produce an oxygen product stream;   heating the at least part of the pumped oxygen stream within the main heat exchange system by further compressing part of the compressed and purified air stream to produce a first boosted pressure air stream and a second boosted pressure air stream, respectively, partially cooling the first boosted pressure air stream and the second boosted pressure air stream within the main heat exchange system, cold compressing the first boosted pressure air stream, after having been partially cooled, at an intermediate temperature to produce a cold compressed air stream, introducing the cold compressed air stream into the main heat exchange system at a warmer temperature than the intermediate temperature and fully cooling the cold compressed air stream to produce a liquid air stream;   expanding the second boosted pressure air stream in a turboexpander, after having been partially cooled, to produce an exhaust stream and introducing the exhaust stream into the lower pressure column to impart refrigeration into the cryogenic rectification process; and   expanding the liquid air stream and introducing the liquid air stream into at least one of the lower pressure column or higher pressure column;   the intermediate temperature being about equal to a vaporization or pseudo-vaporization temperature of the oxygen-rich liquid stream and the second boosted pressure air stream partially cooled to a temperature no greater than the intermediate temperature such that both the first and the second boosted pressure air stream assist in heating the oxygen-rich liquid stream at temperatures within the heat exchange system above the intermediate temperature.   
     
     
         2 . The method of  claim 1 , wherein the intermediate temperature is in a range of between 3.0 K below and 10.0 K above the vaporization or pseudo-vaporization temperature. 
     
     
         3 . The method of  claim 1 , wherein the liquid air stream is expanded within a liquid expander prior to introduction of the liquid air stream into at least one of the lower pressure column or the higher pressure column to impart additional refrigeration to the cryogenic rectification process. 
     
     
         4 . The method of  claim 1 , wherein:
 a first compressed air stream composed of a portion of the compressed and purified air is cooled in the main heat exchange system and introduced into the higher pressure column;   the first boosted pressure air stream is formed by compressing a second compressed air stream, composed of a further portion of the compressed and purified air, in a first booster compressor;   the second boosted pressure air stream is formed by compressing a third compressed air stream, composed of a yet further portion of the compressed and purified air, in a second booster compressor; and   the second booster compressor is coupled to and driven by the turboexpander.   
     
     
         5 . The method of  claim 1 , wherein:
 an argon and oxygen containing stream is removed from the lower pressure column and introduced into an argon column to separate the argon and the oxygen and thereby to produce an oxygen containing liquid as a column bottoms and an argon-rich vapor column overhead; and   an oxygen containing stream composed of the oxygen containing liquid is introduced into the lower pressure column to increase the oxygen recovery.   
     
     
         6 . The method of  claim 1 , wherein the cold compressor is externally driven by a motor. 
     
     
         7 . The method of  claim 6 , wherein the motor is a variable speed motor controlled by a variable speed drive and speed of the motor and therefore, the cold compressor is reduced during a turndown operation of the cryogenic rectification process when production of the oxygen product stream is reduced. 
     
     
         8 . The method of  claim 1  or  claim 2 , wherein:
 a first compressed air stream composed of a portion of the compressed and purified air is cooled in the main heat exchange system and introduced into the higher pressure column; 
 the first boosted pressure air stream is formed by compressing a second compressed air stream, composed of a further portion of the compressed and purified air, in a first booster compressor; 
 the second boosted pressure air stream is formed by compressing a third compressed air stream, composed of a yet further portion of the compressed and purified air, in a second booster compressor; 
 the second booster compressor is coupled to and driven by the turboexpander; 
 refrigeration is also imparted by expanding the liquid air stream within a liquid expander prior to introduction of the liquid air stream into the distillation column; and 
 the cold compressor is externally driven by a motor. 
 
     
     
         9 . The method of  claim 8 , wherein:
 an argon and oxygen containing stream is removed from the lower pressure column and introduced into an argon column to separate the argon and the oxygen and thereby to produce an oxygen containing liquid as a column bottoms and an argon-rich vapor column overhead; and   an oxygen containing stream composed of the oxygen containing liquid is introduced into the lower pressure column to increase the oxygen recovery.   
     
     
         10 . The method of  claim 8 , wherein the motor is a variable speed motor controlled by a variable speed drive and speed of the motor and therefore, the cold compressor is reduced during a turndown operation of the cryogenic rectification process when production of the oxygen product stream is reduced. 
     
     
         11 . The method of  claim 9 , wherein:
 a nitrogen-rich vapor stream composed of a column overhead produced in the higher pressure column is condensed within the condenser reboiler to produce a liquid nitrogen reflux stream;   at least part of the liquid nitrogen reflux stream is introduced into the higher pressure column as reflux; and   a nitrogen-rich liquid stream having a nitrogen concentration less than that of nitrogen-rich vapor is withdrawn from the higher pressure column, subcooled, valve expanded and then introduced into the lower pressure column as reflux;   
     
     
         12 . The method of  claim 11 , wherein:
 the main heat exchange system comprises a higher pressure heat exchanger of a banked heat exchanger arrangement also having a lower pressure heat exchanger;   the first compressed air stream is fully cooled in the lower pressure heat exchanger and introduced into the higher pressure column;   the first boosted pressure air stream is partially cooled in the higher pressure heat exchanger and discharged at the intermediate temperature;   the cold compressor, after having compressed the first boosted pressure air stream at the intermediate temperature, returns the cold compressed air stream to the higher pressure heat exchanger at the warmer temperature;   the second boosted pressure air stream is partially cooled within the higher pressure heat exchanger, introduced to the turboexpander connected to the higher pressure heat exchanger to form the exhaust stream and the exhaust stream is introduced into the lower pressure column;   the at least part of the pumped liquid oxygen stream is warmed in the higher pressure heat exchanger; and   first and second nitrogen-rich vapor streams, made up, at least in part, of lower-pressure nitrogen-rich vapor produced in the lower pressure column, are introduced into the lower pressure and higher pressure heat exchangers, respectively, and with flow rates selected to fully cool the first compressed air stream and to balance cold end temperatures of the lower and higher pressure heat exchangers.   
     
     
         13 . The method of  claim 12 , wherein:
 a crude liquid oxygen stream composed of a crude liquid oxygen column bottoms of the higher pressure column is withdrawn from the higher pressure column, subcooled, valve expanded, partially vaporized in an argon condenser of the argon column to produce liquid and vapor phase streams;   the liquid and vapor phase streams are introduced into the lower pressure column for further refinement of the crude liquid oxygen column bottoms; and   a waste nitrogen stream composed, at least in part, of the nitrogen rich vapor is divided into the first and second nitrogen-rich vapor stream and the first of the nitrogen-rich vapor streams is partially warmed in at least one subcooling heat exchanger used in the subcooling of the crude liquid oxygen stream and the nitrogen-rich liquid stream.   
     
     
         14 . The method of  claim 13 , wherein a compressed, main feed air stream, composed of the compressed and purified air, is divided into the first compressed air stream, the second compressed air stream and the third compressed air stream. 
     
     
         15 . An apparatus for separating air comprising:
 a lower pressure column thermally linked to a higher pressure column by a condenser reboiler and configured to produce an oxygen-rich liquid as an oxygen-rich liquid column bottoms of the lower pressure column through cryogenic rectification of compressed and purified air;   a pump connected to the lower pressure column to pump an oxygen-rich liquid stream composed of the oxygen-rich liquid column bottoms to produce a pumped liquid oxygen stream;   means for forming a first boosted pressure air stream and a second boosted pressure air stream from part of the compressed and purified air;   a main heat exchange system connected to the pump and configured to heat at least part of the pumped liquid oxygen stream and thereby form an oxygen-rich product through indirect heat exchange with the first boosted pressure air stream, the second boosted pressure air stream and a cold compressed stream;   the main heat exchange system in flow communication with the distillation column system so that the liquid air stream is introduced into at least one of the lower pressure column or higher pressure column and having a first intermediate outlet positioned to discharge the first boosted pressure air stream at an intermediate temperature about equal to a vaporization or pseudo-vaporization temperature of the oxygen-rich liquid stream, an inlet to introduce the cold compressed air stream into the main heat exchange system at a warmer temperature than the intermediate temperature and a second intermediate outlet positioned to discharge the second boosted pressure air stream at a temperature no greater than the intermediate temperature so that both the first and second boosted pressure air stream thereby assist in heating the oxygen-rich liquid stream at temperatures within the heat exchange system above the intermediate temperature;   a cold compressor connected between the first intermediate outlet and the inlet to compress the first boosted pressure air stream and thereby to form the cold compressed stream;   a turboexpander connected between the second intermediate outlet and the lower pressure column to expand the second boosted pressure air stream and thereby to form an exhaust stream that is introduced into the lower pressure column to impart refrigeration into the apparatus; and   means for expanding the liquid air stream.   
     
     
         16 . The apparatus of  claim 15 , wherein the first intermediate outlet is positioned so that the intermediate temperature is in a range of between 3.0 K below and 10.0 K above the vaporization or pseudo-vaporization temperature. 
     
     
         17 . The apparatus of  claim 15 , wherein the liquid air expansion means comprises a liquid expander is positioned between the main heat exchange system of the distillation column system to expand the liquid air stream prior to introduction of the liquid air stream into the distillation column system and thereby to generate additional refrigeration. 
     
     
         18 . The apparatus of  claim 15 , wherein:
 the main heat exchange system is also configured to fully cool a first compressed air stream composed of a portion of the air to be rectified;   the main heat exchange system is connected to the higher pressure column so that the first compressed air stream is introduced into the higher pressure column;   the means for forming the first boosted pressure air stream and the second boosted pressure air stream is a first booster compressor and a second booster compressor;   the first booster compressor is connected to the main heat exchange means to compress a second compressed air stream, composed of a further portion of the air to be rectified and thereby to form the first boosted pressure air stream;   the second booster compressor is connected to the main heat exchange means to compress a third compressed air stream, composed of a yet further portion of the air to be rectified and thereby to form the second boosted pressure air stream; and   the second booster compressor is coupled to and driven by the turboexpander.   
     
     
         19 . The apparatus of  claim 15  wherein:
 an argon column is connected to the lower pressure column to receive an argon and oxygen containing stream from the lower pressure column and thereby separate the argon and the oxygen and produce an oxygen containing liquid as a column bottoms and an argon-rich vapor column overhead; and 
 the argon column connected to the lower pressure column so that an oxygen containing stream composed of the oxygen containing liquid is introduced into the lower pressure column to increase the oxygen recovery. 
 
     
     
         20 . The apparatus of  claim 15  wherein the cold compressor is connected to a motor to independently drive the cold compressor. 
     
     
         21 . The method of  claim 20 , wherein:
 the motor is a variable speed motor;   a variable speed drive is connected to the motor to control speed of the motor and therefore, the cold compressor to enable the speed of the cold compressor to be reduced during a turndown operation of the apparatus when production of the oxygen product stream is reduced.   
     
     
         22 . The apparatus of  claim 15  or  claim 16 , wherein:
 the main heat exchange system is also configured to fully cool a first compressed air stream composed of a portion of the air to be rectified;
 the main heat exchange system is connected to the higher pressure column so that the first compressed air stream is introduced into the higher pressure column; 
 the means for forming the first boosted pressure air stream and the second boosted pressure air stream is a first booster compressor and a second booster compressor; 
 the first booster compressor is connected to the main heat exchange means to compress a second compressed air stream, composed of a further portion of the air to be rectified and thereby to form the first boosted pressure air stream; 
 the second booster compressor is connected to the main heat exchange means to compress a third compressed air stream, composed of a yet further portion of the air to be rectified and thereby to form the second boosted pressure air stream; and 
 the second booster compressor is coupled to and driven by the turboexpander; 
 the cold compressor is connected to a motor to independently drive the cold compressor; and 
 the liquid air expansion means comprises a liquid expander is positioned between the main heat exchange system of the distillation column system to expand the liquid air stream prior to introduction of the liquid air stream into at least one of the lower pressure column or higher pressure column and thereby generate additional refrigeration. 
 
 
     
     
         23 . The apparatus of  claim 22 , wherein:
 an argon column is connected to the lower pressure column to receive an argon and oxygen containing stream from the lower pressure column and thereby separate the argon and the oxygen and produce an oxygen containing liquid as a column bottoms and an argon-rich vapor column overhead; and   the argon column connected to the lower pressure column so that an oxygen containing stream composed of the oxygen containing liquid is introduced into the lower pressure column to increase the oxygen recovery.   
     
     
         24 . The method of  claim 22 , wherein:
 the motor is a variable speed motor;   a variable speed drive is connected to the motor to control speed of the motor and therefore, the cold compressor to enable the speed of the cold compressor to be reduced during a turndown operation of the apparatus when production of the oxygen product stream is reduced.   
     
     
         25 . The apparatus of  claim 23 , wherein:
 the condenser reboiler is connected to the higher pressure column so that a nitrogen-rich vapor stream composed of a column overhead produced in the higher pressure column is condensed within the condenser reboiler to produce a liquid nitrogen reflux stream and at least part of the liquid nitrogen reflux stream is introduced into the higher pressure column as reflux;   the higher pressure column is connected to the lower pressure column so that a nitrogen-rich liquid stream having a nitrogen concentration less than that of nitrogen-rich vapor is withdrawn from the higher pressure column and then introduced into the lower pressure column as reflux;   a subcooling heat exchanger is positioned between the higher pressure column and the lower pressure column and is configured so that the nitrogen-rich liquid stream is subcooled prior to introduction into the lower pressure column; and   an expansion valve is positioned between the subcooling heat exchanger and the lower pressure column so that the nitrogen-rich liquid stream is reduced in pressure to that of the lower pressure column prior to the introduction of the nitrogen-rich liquid stream into the lower pressure column.   
     
     
         26 . The apparatus of  claim 25 , wherein:
 the heat exchange system is a banked heat exchanger arrangement having a higher pressure heat exchanger and a lower pressure heat exchanger;   the lower pressure heat exchanger is connected to the higher pressure column to fully cool the first compressed air stream and to introduce the third compressed air stream, after having been fully cooled, into the higher pressure column;   the higher pressure heat exchanger is connected to the first booster compressor and the second booster compressor and the pump and has the first intermediate outlet, the second intermediate outlet and the intermediate inlet;   the higher and lower pressure heat exchanger are in flow communication with the lower pressure column to receive first and second nitrogen-rich vapor streams, made up, at least in part, of lower pressure nitrogen-rich vapor produced in the lower pressure column and to fully warm the first and second nitrogen-rich vapor streams; and   means for controlling flow rates of the first and second nitrogen-rich vapor streams such that cold end temperatures of the lower and higher pressure heat exchangers are balanced.   
     
     
         27 . The apparatus of  claim 26 , wherein:
 an argon condenser is connected to the argon column to condense argon reflux for the argon column, the argon condenser connected to the higher pressure column and also configured so that a crude liquid oxygen stream composed of a crude liquid oxygen column bottoms of the higher pressure column is partially vaporized in the argon condenser against condensing the argon reflux to the argon column;   the subcooling heat exchanger is also connected to the argon condenser and is also configured such that the crude liquid oxygen stream is subcooled prior to being partially vaporized in the argon condenser;   an additional expansion valve is positioned between the subcooling heat exchanger and the argon condenser to expand the crude liquid oxygen stream;   the subcooling heat exchanger is connected to the lower pressure column so that a waste nitrogen stream composed, at least in part, of the lower pressure nitrogen rich vapor produced as column overhead of the lower pressure column is divided into the first and second nitrogen-rich vapor streams and the first of the nitrogen-rich vapor streams is partially warmed in the subcooling heat exchanger; and   the argon condenser is also connected to the lower pressure column so that liquid and vapor phase streams, composed of liquid and vapor phases produced through the partial vaporization of the crude liquid oxygen stream, are introduced into the lower pressure column for further refinement of the crude liquid oxygen column bottoms.   
     
     
         28 . The method of  claim 27 , wherein the lower pressure heat exchanger, the first booster compressor and the second booster compressor are connected so that a compressed main feed air stream composed of the compressed and purified air is divided into the first compressed air stream, the second compressed air stream and the third compressed air stream.