Feed compression method and apparatus for air separation process

Abstract

A method and apparatus for separating air to produce a gaseous oxygen product in which the air is separated in an air separation plant to conduct a cryogenic rectification process that utilizes higher and lower pressure compressed air streams. The higher and lower pressure compressed air streams are generated in two multistage compressors linked together so that the lower pressure compressed air stream is produced from intermediate stages and the higher pressure compressed air stream is produced from higher pressure compression stages. During turn-down operational conditions, one of the two multistage compressors can be shut down to decrease the flow of air and therefore, the production of the oxygen product.

1 . A method of separating air to produce a gaseous oxygen product comprising:
 separating the air in a cryogenic rectification process that is configured to produce an oxygen-rich stream by cryogenically rectifying the air within a distillation column system and warming the oxygen-rich stream, thereby to produce the gaseous oxygen product and that utilizes a first compressed air stream and a second compressed air stream having a lower pressure than the first compressed air stream;   producing the first compressed air stream and the second compressed air stream to feed the cryogenic rectification process by compressing the air in two multistage compressors linked together such that during a normal mode of operation, a higher pressure air stream is produced from higher pressure stages of the two multistage compressors and a lower pressure air stream is produced from lower pressure intermediate stages of the two multistage compressors, removing heat of compression from the higher pressure air stream and the lower pressure air stream and purifying the lower pressure air stream and the higher pressure air stream to produce a higher pressure compressed and purified air stream and a lower pressure compressed and purified air stream and forming the higher pressure air stream from at least part of the higher pressure compressed and purified air stream and the lower pressure air stream from at least part of the lower pressure compressed and purified air stream; and   operating the cryogenic rectification process in a turn-down mode of operation by turning off one of the two multistage compressors and producing the higher pressure air stream and the lower pressure air stream from higher and lower pressure intermediate stages, respectively, of the other of the two multistage compressors, thereby decreasing air flow to the cryogenic rectification process and therefore, production of the oxygen-rich stream and the oxygen product.   
     
     
         2 . The method of  claim 1 , wherein:
 the first compressed air stream is formed from the lower pressure compressed and purified air stream and the second compressed air stream is formed from a first part of the higher pressure compressed and purified air stream;   a second part of the higher pressure compressed and purified air stream is further separately compressed in a booster compressor to produce a boosted pressure air stream;   the boosted pressure air stream partially cooled and then expanded to produce an exhaust stream; and   the exhaust stream is introduced into the distillation column system to impart refrigeration into the cryogenic rectification process.   
     
     
         3 . The method of  claim 2 , wherein:
 the air contained in the first compressed air stream is rectified in a first separation zone of a distillation column system to produce a kettle liquid and a nitrogen-rich vapor;   a kettle liquid stream composed of the kettle liquid is introduced into a second separation zone of the distillation column system for further refinement, the second separation zone operating at a lower operational pressure than the first separation zone;   down coming liquid entering a bottom region of the second separation zone is partly vaporized through indirect heat transfer with a nitrogen-rich vapor stream composed of the nitrogen-rich vapor produced in the first separation zone thereby condensing the nitrogen-rich vapor stream and forming reflux for the first separation zone and the second separation zone and a crude oxygen liquid from residual liquid not vaporized through the indirect heat exchange;   a crude oxygen liquid stream composed of the crude oxygen liquid is stripped in a third separation zone of the distillation column system operating at the lower operational pressure such that a nitrogen containing vapor and an oxygen-rich liquid are produced, the oxygen-rich liquid having a lower nitrogen content than the crude oxygen liquid;   a nitrogen containing vapor stream composed of the nitrogen containing vapor is introduced into the second separation zone;   the third separation zone is reboiled with the second compressed air stream to form a liquid air stream;   intermediate reflux streams, composed at least in part from the liquid air stream, are introduced into the first separation zone and the second separation zone; and   the oxygen-rich stream is withdrawn from the third separation zone and is composed of the oxygen-rich liquid.   
     
     
         4 . The method of  claim 3 , wherein:
 a third part of the higher pressure compressed and purified air stream is divided into two further subsidiary streams that during the normal mode of operation are separately compressed within two additional booster compressors and recombined to produce a further compressed third part of the compressed and purified air stream and during the turn-down mode of operation the third part of the higher pressure compressed and purified air stream is further compressed within one of the two additional booster compressors to form the further compressed third part of the compressed and purified air stream with the other of the two additional booster compressors turned off;   the further compressed third part of the compressed and purified air stream is fully cooled;   the oxygen-rich stream is vaporized and warmed in part through indirect heat exchange with the further compressed third part of the compressed and purified air stream, thereby producing further liquid air; and   the intermediate reflux is introduced in the first separation zone and the second separation zone by subcooling a further liquid air stream composed of the further liquid air and a liquid air stream, composed of the liquid air produced in reboiling the third separation zone, through indirect heat exchange with a waste nitrogen stream produced in the second separation zone prior to the waste nitrogen stream being fully warmed, part of the further liquid air stream is valve expanded and combined with the liquid air stream to produce a first intermediate reflux stream which is further valve expanded and introduced into the second separation zone and introducing a second intermediate reflux stream that is introduced into the first separation zone;   the kettle liquid stream is introduced into the second separation zone by subcooling the kettle liquid stream through indirect heat exchange with the waste nitrogen stream prior to being fully warmed, valve expanding the kettle liquid stream and introducing the kettle liquid stream into the second separation zone; and   a first reflux stream composed of part of the condensed nitrogen-rich vapor refluxes the first separation zone and a second reflux stream composed of another part of the condensed nitrogen-rich vapor is subcooled through indirect heat exchange with the waste nitrogen stream, valve expanded and refluxes the second separation zone.   
     
     
         5 . The method of  claim 4 , wherein the oxygen-rich stream is pumped prior to the oxygen-rich stream being vaporized and warmed through the indirect heat exchange with the further compressed third part of the compressed and purified air stream. 
     
     
         6 . The method of  claim 3  or  claim 4  or  claim 5 , wherein:
 the distillation column system has a higher pressure distillation column that houses the first separation zone, a lower pressure distillation column that houses the second separation zone and a side stripping column that houses the third separation zone; and 
 the kettle liquid stream is introduced into the second separation zone in the rectification column. 
 
     
     
         7 . An apparatus for producing a gaseous oxygen product comprising:
 an air separation plant configured to produce an oxygen-rich stream and utilizing a first compressed air stream and a second compressed air stream, the air separation plant having a main heat exchanger to cool the first compressed air stream and the second compressed air stream, a distillation column system connected to the main heat exchanger to rectify the air contained in the first compressed air stream and the second compressed air stream, thereby to produce the oxygen-rich stream and to return the oxygen-rich stream to the main heat exchanger such that the oxygen-rich stream is fully warmed to produce the gaseous oxygen product, a compression system connected to a purification system to produce the first compressed air stream and the second compressed air stream;   the compression system having two multistage compressors linked together such that during a normal mode of operation, a higher pressure air stream is produced from higher pressure stages of the two multistage compressors and a lower pressure air stream is produced from lower pressure intermediate stages of the two multistage compressors and after-coolers connected to the higher pressure stages and the lower pressure intermediate stages for removing heat of compression from the higher pressure air stream and the lower pressure air stream   the purification system configured to purify the lower pressure air stream and the higher pressure air stream to produce a higher pressure compressed and purified air stream and a lower pressure compressed and purified air stream;   the purification system connected to the main heat exchanger such that the first compressed air stream is formed from at least part of the higher pressure compressed and purified air stream and the second compressed air stream is formed from at least part of the lower pressure compressed and purified air stream; and   a control system for controlling the compression system such that the air separation plant is able to be selectively operated in the normal mode of operation and in a turn-down mode of operation in which one of the two multistage compressors is turned off and producing the higher pressure air stream and the lower pressure air stream from higher and lower pressure intermediate stages, respectively, of the other of the two multistage compressors, thereby decreasing air flow to the cryogenic rectification process and therefore, production of the oxygen-rich stream and the oxygen product.   
     
     
         8 . The apparatus of  claim 7 , wherein:
 the purification system is connected to the main heat exchanger such that the first compressed air stream is formed from the lower pressure compressed and purified air stream and the second compressed air stream is formed from a first part of the higher pressure compressed and purified air stream;   a booster compressor is connected to the purification system such that a second part of the higher pressure compressed and purified air stream is further compressed in the booster compressor to produce a boosted pressure air stream;   the booster compressor is connected to the main heat exchanger and the main heat exchanger is configured such that the boosted pressure air stream is partially cooled within the main heat exchanger;   a turbo-expander is positioned between the main heat exchanger and the distillation column system such that the boosted pressure air stream, after having been partially cooled, is expanded to produce an exhaust stream and the exhaust stream is introduced into the distillation column system to impart refrigeration into the cryogenic rectification process.   
     
     
         9 . The apparatus of  claim 8 , wherein:
 the distillation column system has a first separation zone, a second separation zone and a third separation zone, the first separation zone having a higher operational pressure than the second separation zone;   the first separation zone connected to the main heat exchanger so as to receive the first compressed air stream for rectification;   the second separation zone connected to the first separation zone such that a kettle liquid stream composed of kettle liquid produced in the first separation zone is introduced into the second separation zone for further refinement;   a condenser-reboiler is connected to the second separation zone and the first separation zone such that a nitrogen-rich vapor stream produced in the first separation zone indirectly exchanges heat to all down-coming liquid flowing towards a bottom region of the second separation zone, thereby condensing the nitrogen-rich vapor stream and partly vaporizing the down-coming liquid to produce boil-up in the second separation zone and a crude oxygen liquid collected in the bottom region of the second separation zone from down-coming liquid that is not vaporized;   the first separation zone and the second separation zone are in flow communication with the condenser reboiler such that reflux streams produced as a result of the condensing of the nitrogen-rich vapor stream are introduced into the first separation zone and the second separation zone;   the third separation zone is connected to a bottom region of the second separation zone to receive a crude oxygen liquid stream, composed of the crude oxygen liquid and to return a nitrogen containing vapor stream back to the bottom region of the second separation zone, the third separation zone configured to strip the crude oxygen liquid, thereby to produce the nitrogen containing vapor stream and an oxygen-rich stream having a lower nitrogen content than the crude oxygen liquid;   a reboiler is located in the third separation zone and in flow communication with the main heat exchanger receives the second compressed air stream, thereby to reboil the third separation zone and to produce liquid air;   the first separation zone and the third separation zone are in flow communication with the reboiler such that at least one intermediate reflux stream composed, at least in part, of the liquid air is introduced into at least one of the first separation zone and the second separation zone as intermediate reflux; and   the main heat exchanger is in flow communication with the third separation zone such that the oxygen-rich stream warms to form the oxygen product.   
     
     
         10 . The method of  claim 9 , wherein:
 two additional booster compressors are positioned between the purification system and the main heat exchanger such that during the normal mode of operation a third part of the higher pressure compressed and purified air stream is divided into two further subsidiary streams that are separately compressed and combined to form a further compressed third part of the higher pressure compressed and purified air stream;   the control system also controls the two additional booster compressors such that during the normal mode of operation both of the two additional booster compressors are in operation and in the turn-down mode of operation, the third part of the higher pressure compressed and purified air stream compressed within one of the two additional booster compressors to form the further compressed third part of the higher pressure compressed and purified air stream and the other of the two additional booster compressors is turned off;   the main heat exchanger is configured such that the further compressed third part of the higher pressure compressed and purified air stream fully cools within the main heat exchanger;   a vaporizer is connected between the main heat exchanger and the third separation zone such that the oxygen-rich stream is removed from the third separation zone as a liquid and vaporized and warmed in part through indirect heat exchange with the further compressed third part of the compressed and purified air stream, thereby producing further liquid air;   at least one subcooling unit is connected between the reboiler, the second separation zone and the vaporizer such that a further liquid air stream composed of the further liquid air and a liquid air stream, composed of the liquid air, are subcooled through indirect heat exchange with a waste nitrogen stream produced in the second separation zone, the at least one subcooling unit also connected to the main heat exchanger such that the waste nitrogen stream fully warms within the main heat exchanger;   the second separation zone is connected to the at least one subcooling unit such that a part of the further liquid air stream combines with the liquid air stream to produce the intermediate reflux stream;   the first separation zone and the second separation zone are in flow communication with the at least one subcooling unit such that a first intermediate reflux stream composed of part of the intermediate reflux stream is introduced into the second separation zone and a second intermediate reflux stream composed of a further part of the intermediate reflux stream is introduced into the first separation zone;   the first separation zone is connected to the condenser reboiler such that a first of the reflux streams is introduced into the first separation zone;   the at least one subcooling unit is also connected to the higher pressure distillation column such that the kettle liquid stream subcools within the at least one subcooling unit through indirect heat exchange with the waste nitrogen stream, a second of the reflux streams subcools within the at least one subcooling unit through indirect heat exchange with the waste nitrogen stream;   the second separation zone is connected to the at least one subcooling unit such that the second separation zone is refluxed with the second of the reflux streams; and   expansion valves are positioned between the at least one subcooling unit and the second separation zone such that the part of the further liquid air stream is expanded prior to combining with the liquid air stream and the first intermediate reflux stream is expanded prior to entering the second separation zone, the second of the reflux streams is expanded prior to entering the first separation zone and the kettle liquid stream is expanded before introduction into the second separation zone.   
     
     
         11 . The method of  claim 9  or  claim 10 , wherein:
 the distillation column system has a higher pressure distillation column that houses the first separation zone, a lower pressure distillation column that houses the second separation zone and a side stripping column that houses the third separation zone; and 
 the bottom region of the second separation zone is a sump in the lower pressure distillation column that contains the condenser reboiler.