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;   rectifying the compressed and purified stream within a main distillation column to produce a nitrogen-rich vapor column overhead and crude liquid oxygen;   producing an oxygen-rich liquid and an auxiliary column overhead containing not less than 5.0 percent oxygen on a volume basis within an auxiliary distillation column by, at least in part, depressurizing a crude liquid oxygen stream composed of the crude liquid oxygen, stripping the crude liquid oxygen stream within the auxiliary distillation column with an ascending stripping gas 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;   forming an oxygen-rich vapor fraction from the residual oxygen-rich liquid by indirectly exchanging heat between a stream of the residual oxygen-rich liquid with a gaseous stream having a nitrogen concentration no less than that of air so that the stream of the residual oxygen-rich liquid partially vaporizes;   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 auxiliary column overhead; 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 auxiliary column overhead is solely produced by the stripping of the crude liquid oxygen stream within an auxiliary distillation column. 
     
     
         3 . The method of  claim 1 , wherein:
 the main distillation column is refluxed with part of the liquid nitrogen stream;   the stripping of the crude liquid oxygen stream takes place within a stripping section of auxiliary distillation column;   stripping the crude liquid oxygen stream within the auxiliary distillation column produces a nitrogen and oxygen containing vapor stream; and   the nitrogen and oxygen containing vapor stream is rectified within the auxiliary distillation column within a rectification section of the auxiliary distillation column located above the stripping section by introducing the nitrogen and oxygen containing vapor stream into the rectification section and refluxing the auxiliary distillation column and therefore, the rectification section with a further part of the liquid nitrogen stream, thereby increasing recovery of oxygen within the residual oxygen-rich liquid.   
     
     
         4 . 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 composed of the oxygen-rich liquid and the nitrogen-rich vapor stream 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.   
     
     
         5 . The method of  claim 4 , wherein:
 the gaseous stream is the compressed and purified stream;   the compressed and purified stream is partially condensed in a condenser;   the oxygen-rich vapor fraction is formed by:
 collecting the stream of the residual oxygen-rich liquid in a separation vessel; 
 introducing a liquid phase stream, formed of a liquid phase produced within the separation vessel, into the condenser and partially vaporizing the liquid phase stream in the condenser through indirect heat exchange with the compressed and purified stream, thereby producing a two-phase stream from the liquid phase stream; 
 introducing the two phase stream into the separation vessel and disengaging liquid and vapor phases of the two phase stream within the separation vessel to form an oxygen-rich vapor fraction and the liquid phase together with the stream of the residual oxygen-rich liquid collected in the separation vessel; and 
   the oxygen product stream is formed by discharging a stream of the oxygen-rich vapor fraction from the separation vessel.   
     
     
         6 . The method of  claim 5 , wherein the condenser is 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. 
     
     
         7 . The method of  claim 6 , wherein:
 the main distillation column is refluxed with part of the liquid nitrogen stream; and   the waste stream indirectly exchanges heat with the crude liquid oxygen stream such that the crude liquid oxygen stream is subcooled prior to being depressurized.   
     
     
         8 . The method of  claim 4 , wherein:
 the gaseous stream is composed of the nitrogen-rich vapor column overhead;   the heat is indirectly exchanged between the stream of the residual oxygen-rich liquid and the gaseous stream by depressurizing a liquid oxygen enriched stream and passing the stream of the residual oxygen-rich liquid in indirect heat exchange with the gaseous stream within a thermo-siphon reboiler thereby producing the vapor fraction from the partial vaporization of the stream of the residual oxygen-rich liquid and a condensate stream through condensation of the gas 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 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. 
     
     
         10 . The method of  claim 1 , wherein the stream of the residual oxygen-rich liquid is pressurized such that the oxygen product stream is also pressurized. 
     
     
         11 . The method of  claim 1 , 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;   a main distillation column configured to rectify the compressed and purified stream to produce a nitrogen-rich vapor column overhead and crude liquid oxygen;   an auxiliary distillation column connected to the main distillation column and configured such that a crude liquid oxygen stream, composed of the crude liquid oxygen, is stripped with an ascending stripping gas within the auxiliary distillation column and an oxygen-rich liquid and an auxiliary column overhead containing not less than 5.0 percent oxygen by volume are produced, at least in part, as a result of the stripping of the crude liquid oxygen stream;   an expansion valve positioned between the main distillation column and an auxiliary distillation column such that the crude liquid oxygen stream is depressurized prior to introduction into the auxiliary distillation column;   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 main heat exchanger connected to the main distillation column and the auxiliary distillation column so that a nitrogen product stream composed of the nitrogen-rich vapor column overhead and a waste stream formed from the auxiliary column overhead of the auxiliary distillation column indirectly exchange heat with the compressed and purified air stream;   means for indirectly exchanging heat between a stream of the residual oxygen-rich liquid with a gaseous stream having a nitrogen concentration no less than that of air so that the stream of the residual oxygen-rich liquid partially vaporizes and means for forming an oxygen-rich vapor fraction from the stream of the residual oxygen-rich liquid after having been partially vaporized; and   the main heat exchanger connected to the oxygen-rich vapor fraction forming means, the main distillation column and the auxiliary distillation column such that an oxygen product stream, composed of the oxygen-rich vapor fraction, a nitrogen product stream, composed of the nitrogen-rich vapor column overhead, and a waste stream, composed of the auxiliary 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 column is solely provided with a stripping section where the stripping of the crude liquid oxygen stream takes place. 
     
     
         14 . The apparatus of  claim 12 , wherein:
 the auxiliary column has a stripping section and a rectification section, located above the stripping section;   the stripping of the crude liquid oxygen stream takes place within a stripping section of auxiliary distillation column and a nitrogen and oxygen containing vapor stream is produced in the stripping section that enters the rectification section for rectification of the nitrogen and oxygen containing vapor stream, thereby increasing recovery of oxygen within the residual oxygen-rich liquid;   the oxygen-rich liquid partial vaporization means is connected to the main distillation column such that the main distillation column is refluxed with part of the liquid nitrogen stream and is also connected to the auxiliary distillation column such that the auxiliary distillation column and therefore, the rectification section is refluxed with a further part of the liquid nitrogen stream; and   another expansion valve is positioned between the oxygen-rich liquid partial vaporization means and the auxiliary distillation column so that pressure of the further part of the liquid nitrogen stream is reduced to that of the auxiliary distillation column.   
     
     
         15 . 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.   
     
     
         16 . The apparatus of  claim 15 , wherein:
 the gaseous stream is the compressed and purified air stream;   the residual oxygen-rich liquid heat exchange means and the oxygen-rich vapor fraction forming means is a condenser connected to the main heat exchanger such that the compressed and purified stream is partially condensed and a separation vessel;   the separation vessel is connected to the auxiliary distillation column such that the stream of the residual oxygen-rich liquid collects in the separation vessel;   the separation vessel is connected to the condenser so that a liquid phase stream, composed of a liquid phase produced within the separation vessel 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 the oxygen-rich vapor fraction and the liquid phase; and   the main heat exchanger is connected to the separation vessel so that the oxygen product stream is formed from the oxygen-rich vapor fraction.   
     
     
         17 . The apparatus of  claim 16 , 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; 
     
     
         18 . The apparatus of  claim 17 , wherein:
 the once-through heat exchanger is connected to the main distillation column such that the main distillation column is refluxed with part of the liquid nitrogen stream; and   a subcooling heat exchanger is connected to the once-through heat exchanger, the auxiliary distillation column and the expansion valve such that the waste stream indirectly exchanges 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.   
     
     
         19 . The apparatus of  claim 15 , wherein:
 the gaseous stream is composed of the nitrogen-rich vapor;   the stream of the residual oxygen-rich liquid heat exchange means and the oxygen-rich vapor fraction forming means are a thermo-siphon reboiler having a shell;   the shell connected to the auxiliary column to receive the stream of the residual oxygen-rich liquid;   another expansion valve is positioned between the shell and the auxiliary column so that the stream of the residual oxygen-rich liquid is depressurized;   the thermo-siphon reboiler connected to the main distillation column to receive the gaseous stream and thereby condense the gaseous stream through indirect heat exchange with the stream of the residual oxygen-rich liquid and thereby form the oxygen-rich vapor fraction within the shell and discharge a condensate stream to the main distillation column as reflux along with the liquid nitrogen stream; and   the main heat exchanger is connected to the shell, 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 auxiliary column overhead produced in the auxiliary distillation column pass within the main heat exchanger, in indirect heat exchange with the compressed and purified stream.   
     
     
         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  wherein the main distillation column has a top inlet for introduction of a liquid nitrogen refrigeration stream to impart refrigeration.