Air separation method and apparatus

Abstract

A method and apparatus to produce oxygen and nitrogen co-products in which a compressed a compressed and purified air stream is cooled, fully or partially condensed and then rectified in a main distillation column to form a nitrogen-rich vapor column overhead and crude liquid oxygen. A crude liquid oxygen stream is depressurized and then stripped in an auxiliary distillation column with a stripping gas to produce an oxygen-rich liquid. The nitrogen-rich vapor column overhead from the main distillation column is used to form a nitrogen product and the crude liquid oxygen is partially vaporized to produce the stripping gas, a residual oxygen-rich liquid and liquid nitrogen reflux to the main distillation column. The oxygen product is formed from the residual oxygen-rich liquid by either providing the heat exchange duty in condensing the compressed and purified air stream or by condensing nitrogen-rich vapor used in refluxing the main distillation column.

What is claimed is: 
     
         1 . A method of separating air to produce oxygen and nitrogen co-products, said method comprising:
 cooling a compressed and purified stream comprising the air;   at least partially condensing the compressed and purified stream and rectifying the compressed and purified stream within a main distillation column to produce a nitrogen-rich vapor column overhead and crude liquid oxygen;   depressurizing a crude liquid oxygen stream composed of the crude liquid oxygen, stripping the crude liquid oxygen stream within an auxiliary distillation column with an ascending stripping gas to produce a column overhead and an oxygen-rich liquid and partially vaporizing the oxygen-rich liquid through indirect heat exchange with a nitrogen-rich vapor stream composed of the nitrogen-rich vapor column overhead, thereby producing a liquid nitrogen stream, the stripping gas and a residual oxygen-rich liquid;   refluxing the main distillation column with at least part of the liquid nitrogen stream;   the at least partial condensing of the compressed and purified stream conducted by indirectly exchanging heat from the compressed and purified stream to a liquid oxygen enriched stream produced from the residual oxygen-rich liquid;   forming: an oxygen product stream from at least part of the liquid oxygen enriched stream after having indirectly exchanged heat with the compressed and purified stream;   a nitrogen product stream from the nitrogen-rich vapor; and a waste stream from the column overhead of the auxiliary distillation column; and   passing the oxygen product stream, the nitrogen product stream and the waste stream in indirect heat exchange with the compressed and purified stream.   
     
     
         2 . The method of  claim 1 , wherein:
 the oxygen-rich liquid is collected within the auxiliary distillation column; and   the oxygen-rich liquid is partially vaporized by passing an oxygen-rich liquid stream and the at least part of the nitrogen-rich vapor through a once-through heat exchanger to form the stripping gas and the residual oxygen-rich liquid that collects as a column bottoms of the auxiliary distillation column.   
     
     
         3 . The method of  claim 1  or  claim 2 , wherein the compressed and purified stream is at least partially condensed in a condenser located in a main distillation column bottom region such that condensed air mixes with downcoming liquid produced by the rectification to thereby produce the crude liquid oxygen as a column bottoms in the main distillation column. 
     
     
         4 . The method of  claim 3 , wherein:
 a stream of the residual oxygen-rich liquid is collected in a separation vessel;   the liquid oxygen enriched stream is formed of a liquid phase produced within the separation vessel and is partially vaporized in the condenser to produce a two-phase stream;   liquid and vapor phases of the two phase stream are disengaged within the separation vessel to form a vapor phase and a liquid phase together with the stream of the residual oxygen-rich liquid collected in the separation vessel; and   the oxygen product stream is formed from a vapor phase stream composed of the vapor phase.   
     
     
         5 . The method of  claim 4 , wherein:
 the liquid nitrogen reflux stream is formed from part of the liquid nitrogen stream;   a further part of the liquid nitrogen stream and the waste stream indirectly exchange heat with the crude liquid oxygen stream such that the crude liquid oxygen stream is subcooled prior to being depressurized; and   the further part of the liquid nitrogen stream is also warmed in the indirect heat exchange with the compressed and purified stream.   
     
     
         6 . The method of  claim 1 , wherein the liquid oxygen enriched stream is pressurized such that the oxygen product stream is also pressurized. 
     
     
         7 . A method of separating air to produce oxygen and nitrogen co-products, said method comprising:
 cooling a compressed and purified stream comprising the air;   rectifying the compressed and purified stream within a main distillation column to produce a nitrogen-rich vapor column overhead and a crude liquid oxygen column bottoms;   depressurizing a crude liquid oxygen stream composed of the crude liquid oxygen column bottoms, stripping the crude liquid oxygen stream within an auxiliary distillation column with an ascending stripping gas to produce a column overhead and an oxygen-rich liquid;   collecting the oxygen-rich liquid and partially vaporizing the oxygen-rich liquid through indirect heat exchange with at least part of a nitrogen-rich vapor stream, composed of the nitrogen-rich vapor column overhead, in a once-through heat exchanger thereby producing a liquid nitrogen stream, the stripping gas and a residual oxygen-rich liquid;   indirectly exchanging heat between the residual oxygen-rich liquid and a gaseous stream having a nitrogen concentration no less than that of air, thereby condensing the gaseous stream to produce a liquid condensate stream and partially vaporizing the residual oxygen-rich liquid into a liquid fraction and a vapor fraction;   refluxing the main distillation column with at least part of the liquid nitrogen stream;   forming: an oxygen product stream from the vapor fraction; a nitrogen product stream from the nitrogen-rich vapor column overhead; and a waste stream from the column overhead produced in the auxiliary distillation column; and   passing the oxygen product stream, the nitrogen product stream and the waste stream in indirect heat exchange with the compressed and purified stream.   
     
     
         8 . The method of  claim 7 , wherein:
 the gaseous stream is composed of the nitrogen-rich vapor column overhead;   the heat is indirectly exchanged between the residual oxygen-rich liquid and the gaseous stream by depressurizing a liquid oxygen enriched stream, composed of the residual oxygen-rich liquid, and the liquid oxygen enriched stream is passed in indirect heat exchange with the gaseous stream; and   the condensate stream is introduced into the main distillation column as reflux along with the liquid nitrogen stream.   
     
     
         9 . The method of  claim 8 , wherein the heat is indirectly exchanged from the liquid oxygen enriched stream and the second nitrogen-rich vapor stream within a thermo-siphon reboiler and the liquid fraction and the vapor fraction collect in a shell of the thermo-siphon reboiler. 
     
     
         10 . The method of  claim 9 , wherein the waste stream passes in indirect heat exchange with the crude liquid oxygen stream prior to the depressurization of the crude liquid oxygen stream so that the crude liquid oxygen stream is subcooled. 
     
     
         11 . The method of  claim 1  or  claim 7 , wherein a liquid nitrogen refrigeration stream is introduced into the main distillation column to impart refrigeration. 
     
     
         12 . An apparatus for separating air to produce oxygen and nitrogen co-products, said apparatus comprising:
 a main heat exchanger configured to cool a compressed and purified stream comprising the air;   means for at least partially condensing the compressed and purified stream;   a main distillation column configured to rectify the compressed and purified stream to produce a nitrogen-rich vapor column overhead;   the compressed and purified stream at least partial condensing means is connected to the main heat exchanger and operatively associated with the main distillation column such that the compressed and purified stream is rectified after having been cooled and at least partially condensed and crude liquid oxygen is produced from the at least partial condensation of the compressed and purified stream and the rectification of the compressed and purified stream;   an expansion valve positioned between the main distillation column and an auxiliary distillation column such that a crude liquid oxygen stream composed of the crude liquid oxygen is depressurized prior to introduction into the auxiliary distillation column, the auxiliary distillation column configured such that the crude liquid oxygen stream is stripped with an ascending stripping gas within the auxiliary distillation column to produce a column overhead and an oxygen-rich liquid;   means for partially vaporizing the oxygen-rich liquid through indirect heat exchange with a nitrogen-rich vapor stream composed of the nitrogen-rich vapor column overhead, thereby producing a liquid nitrogen stream, the stripping gas and a residual oxygen-rich liquid;   the oxygen-rich liquid partial vaporization means connected to the main distillation column such that the main distillation column is refluxed with at least part of the liquid nitrogen stream;   the compressed and purified at least partial condensing means connected to the auxiliary distillation column such that the at least partial condensing of the compressed and purified stream is conducted by indirectly exchanging heat from the compressed and purified stream to a liquid oxygen enriched stream, composed of the residual oxygen-rich liquid;   means for forming an oxygen product stream from the liquid oxygen enriched stream after having indirectly exchanged heat with the compressed and purified stream; and   the main heat exchanger connected to the oxygen product stream forming means, the auxiliary distillation column and at least one of the oxygen-rich liquid partial vaporization means and the main distillation column such that the oxygen product stream, a nitrogen product stream, formed from the nitrogen-rich vapor column overhead, and a waste stream, composed of the column overhead of the auxiliary distillation column, pass within the main heat exchanger, in indirect heat exchange with the compressed and purified stream.   
     
     
         13 . The apparatus of  claim 12 , wherein:
 the auxiliary distillation column has means for collecting the oxygen-rich liquid;   the oxygen-rich liquid partial vaporization means is a once-through heat exchanger connected to an auxiliary distillation column and the oxygen-rich liquid collecting means such that the oxygen-rich liquid is partially vaporized within the once-through heat exchanger through passage of an oxygen-rich liquid stream, composed of the oxygen-rich liquid and the residual oxygen-rich liquid collects as a column bottoms of the auxiliary distillation column; and   the main distillation column connected to the once-through heat exchanger such that the nitrogen-rich vapor stream is condensed within the once-through heat exchanger.   
     
     
         14 . The apparatus of  claim 13 , wherein the condenser is located in a bottom region of the main distillation column such that condensed air mixes with downcoming liquid produced by the rectifying of the compressed and purified stream to produce the crude liquid oxygen as a column bottoms in the main distillation column. 
     
     
         15 . The apparatus of  claim 12 , wherein:
 the at least one oxygen product stream forming means is a separation vessel connected to the auxiliary distillation column such that a stream of the residual oxygen-rich liquid is collected in a separation vessel;   the separation vessel is connected to the condenser so that the liquid oxygen enriched stream is formed of a liquid phase produced within the separation vessel and is partially vaporized in the condenser to produce a two-phase stream that is introduced into the separation vessel, liquid and vapor phases of the two phase stream are disengaged within the separation vessel to form a vapor phase and a liquid phase together with the stream of the residual oxygen-rich liquid collected in the separation vessel; and   the main heat exchanger is connected to the separation vessel so that the oxygen product stream is formed from a vapor phase stream composed of the vapor phase.   
     
     
         16 . The apparatus of  claim 15 , wherein:
 the once-through heat exchanger is connected to the main distillation column such that the main distillation column is refluxed with a part of the liquid nitrogen stream;   a subcooling heat exchanger is connected to the once-through heat exchanger, the auxiliary distillation column and the expansion valve such that a further part of the liquid nitrogen stream and the waste stream indirectly exchange heat with the crude liquid oxygen stream within the subcooling heat exchanger and the crude liquid oxygen stream is subcooled prior to passage through the expansion valve; and   the further part of the liquid nitrogen stream also passes through the main heat exchanger, in indirect heat exchange with the compressed and purified stream.   
     
     
         17 . An apparatus for separating air to produce oxygen and nitrogen co-products, said method comprising:
 a main heat exchanger configured to cool a compressed and purified stream comprising the air;   a main distillation column connected to the main heat exchanger and configured to rectify the compressed and purified stream to produce a nitrogen-rich vapor column overhead and a crude liquid oxygen column bottoms;   an auxiliary distillation column in flow communication with the main distillation column and configured to strip a crude liquid oxygen stream composed of the crude liquid oxygen column bottoms with an ascending stripping gas to produce a column overhead and an oxygen-rich liquid, the auxiliary distillation column having means for collecting the oxygen-rich liquid;   an expansion valve positioned between the auxiliary distillation column and the main distillation column to depressurize the crude liquid oxygen stream;   a once-through heat exchanger connected to the collection means and the auxiliary distillation column such that the oxygen-rich liquid is partially vaporized through indirect heat exchange with at least part of a first nitrogen-rich vapor stream, composed of the nitrogen-rich vapor column overhead, thereby producing a liquid nitrogen stream, the stripping gas and a residual oxygen-rich liquid;   means for indirectly exchanging heat between the residual oxygen-rich liquid and a gaseous stream having a nitrogen concentration no less than that of air, thereby condensing the gaseous stream to produce a liquid condensate stream and partially vaporizing the residual oxygen-rich liquid into a liquid fraction and a vapor fraction;   the once-through heat exchanger and the indirect heat exchange means in flow communication with the main distillation column such that the main distillation column is refluxed with at least part of the liquid nitrogen stream; and   the main heat exchanger also connected to the indirect heat exchange means, the main distillation column and the auxiliary distillation column such that an oxygen product stream, formed from the vapor fraction, a nitrogen product stream, formed from the nitrogen-rich vapor column overhead, a waste stream, formed from the column overhead produced in the auxiliary distillation column pass within the main heat exchanger, in indirect heat exchange with the compressed and purified stream.   
     
     
         18 . The apparatus of  claim 17 , wherein:
 the indirect heat exchange means is connected to the main distillation column so that the gaseous stream is composed of the nitrogen-rich vapor column overhead and the condensate stream is introduced into the main distillation column as reflux along with the liquid nitrogen stream;   the once-through heat exchanger is connected to the auxiliary distillation column so that the residual oxygen-rich liquid collects within the auxiliary distillation column;   the indirect heat exchange means is connected to the auxiliary column so that a liquid oxygen-enriched stream, composed of the residual oxygen-rich liquid, passes in indirect heat exchange with the gaseous stream; and   another expansion valve is positioned between the auxiliary column and the indirect heat exchange means for depressurizing the liquid oxygen-enriched stream.   
     
     
         19 . The apparatus of  claim 18 , wherein the indirect heat exchange means is a thermo-siphon reboiler having a shell within which the liquid fraction and the vapor fraction collect. 
     
     
         20 . The apparatus of  claim 19 , wherein a subcooling heat exchanger is positioned between the auxiliary distillation column, the main distillation column and the main heat exchanger such that the waste stream passes in indirect heat exchange with the crude liquid oxygen stream prior to the depressurization of the crude liquid oxygen stream and prior to the warming the waste stream in the main heat exchanger. 
     
     
         21 . The apparatus of  claim 12  or  claim 17 , wherein the main distillation column has a top inlet for introduction of a liquid nitrogen refrigeration stream to impart refrigeration.