EP2788700B1

EP2788700B1 - Air separation method and apparatus

Info

Publication number: EP2788700B1
Application number: EP12794600.2A
Authority: EP
Inventors: Henry Edward Howard; Mathew R. WATT; Thomas J. BERGMAN
Original assignee: Praxair Technology Inc
Current assignee: Praxair Technology Inc
Priority date: 2011-12-05
Filing date: 2012-11-14
Publication date: 2017-07-19
Anticipated expiration: 2032-11-14
Also published as: CA2857665A1; WO2013085679A3; CN103988037B; KR20140103109A; EP2788700A2; BR112014012911A2; MX2014006737A; US20130139547A1; ES2644280T3; RU2014127295A; WO2013085679A2; CN103988037A

Description

Field of the Invention

The present invention relates to a method and apparatus for separating air in which oxygen and nitrogen products are produced as co-products by rectifying compressed and purified air in a main distillation column to produce the nitrogen product and stripping crude liquid oxygen formed in the main distillation column within an auxiliary distillation column to produce the oxygen product.

Background of the Invention

Nitrogen is typically obtained at high purity by separating nitrogen from air within a cryogenic air separation plant often employing a single distillation column. In such a plant, air is compressed and then purified of higher boiling contaminants to produce a compressed and purified air stream. The compressed and purified air stream is then cooled to a temperature suitable for its cryogenic rectification within a main heat exchanger and then introduced into a single distillation column operating at about 3 bara or higher. The air, within the distillation column, is rectified to produce a nitrogen-rich vapor column overhead and an oxygen-rich liquid column bottoms known as crude liquid oxygen or kettle liquid. The oxygen-rich liquid is depressurized in an expansion valve and then introduced into a heat exchanger to condense a stream of the nitrogen-rich vapor column overhead and thereby produce liquid nitrogen to reflux the distillation column. The oxygen-rich liquid that is partially vaporized can be used to generate refrigeration for the plant or alternatively, refrigeration can be provided to the plant through liquid nitrogen addition to the main column or a stream entering the main heat exchanger.
In general, oxygen is not recovered from such single column nitrogen plants. However, there is an interest in also recovering oxygen from such plants. For example, float glass production typically requires nitrogen and low purity oxygen in a flow ratio of approximately 2:1. The oxygen is employed in the glass furnace to boost recovery and is typically required at a low purity of between 90 and 95 percent and at a pressure of between 10 and 20 psig. While such oxygen can be provided by a separate vacuum pressure swing adsorption plant or through the vaporization of delivered on-site liquid, the added expense is not justifiable. It is to be noted that both oxygen and nitrogen can be produced by a typical double column air separation plant having high and low pressure columns operatively associated in a heat transfer relationship. However, such a plant is not economical where, for example, float glass is to be produced because of the requirement of product compression and higher initial capital cost. The incremental modification of a small single column nitrogen plant to deliver both nitrogen and oxygen products can economically meet the requirements of processes such as float glass production where the nitrogen and oxygen products are required at pressure and at modest flow rate, for example less than 100 kcfh of nitrogen.
In the prior art, there are examples of single column nitrogen plants that have been modified to co-produce both nitrogen and oxygen products. For example, in US 4,783,210 , a stream of the crude liquid oxygen column bottoms is partially vaporized in a heat exchanger used in condensing a stream of the nitrogen-rich vapor column overhead produced in a distillation column. The resulting nitrogen rich liquid is used to reflux the column. A stream of the resulting liquid phase produced by the partial vaporization of the crude liquid oxygen is then stripped in a secondary or auxiliary distillation column to produce a column bottoms, rich in oxygen, that can be taken as an oxygen product. The auxiliary distillation column is reboiled with another stream of the nitrogen-rich vapor which condenses in the reboiling and can be taken as a liquid nitrogen product and also, used to reflux the distillation column.
US 5,074,898 discloses a single column nitrogen generator with an auxiliary distillation column to produce an oxygen product. In this patent, a crude liquid oxygen stream generated in a main distillation column is stripped in an auxiliary distillation column. The auxiliary distillation column is reboiled with a stream of the nitrogen-rich vapor column overhead produced in the main distillation column. This condenses the nitrogen-rich vapor to produce reflux for the main distillation column. Residual liquid generated in the auxiliary distillation column can be taken as a liquid product along with part of the condensed nitrogen vapor.
In both of these patents, the column bottoms is being used to condense nitrogen, it necessarily must be at a lower pressure than the nitrogen to accomplish nitrogen condensation. As a result, the oxygen product is also at low pressure. Furthermore, since part of the column bottoms is being taken as a product, there will be less bottoms fluid to condense nitrogen. Consequently, nitrogen production is limited. As will be discussed, the present invention provides a method and apparatus for producing nitrogen and oxygen co-products, that among other advantages, may allow the oxygen product to be produced at elevated pressure. In another aspect of the present invention, the auxiliary column bottoms is subjected to a staged vaporization to enable a greater fraction of oxygen to be obtained at lower power relative those processes contemplated in the prior art.
Furthermore, in US 4 560 397 A there is disclosed a method of separating air to produce oxygen and nitrogen coproducts, 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: a nitrogen product stream from the nitrogen-rich vapor column overhead; and a waste stream from the auxiliary column overhead; and
passing the nitrogen product stream and the waste stream in indirect heat exchange with the compressed and purified stream.

Summary of the Invention

The present invention is a method of separating air to produce oxygen and nitrogen co-products as it is defined in claim 1 and an apparatus for separating air to produce oxygen and nitrogen co-products as it is defined in claim 7.
An oxygen-rich vapor fraction is formed 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. An oxygen product stream is formed from the vapor fraction, a nitrogen product stream is formed from the nitrogen-rich vapor and a waste stream is formed from the auxiliary column overhead of the auxiliary distillation column. The oxygen product stream, the nitrogen product stream and the waste stream are passed in indirect heat exchange with the compressed and purified stream.
As can be appreciated from the above discussion, since the oxygen product stream is formed from a vapor fraction that is in turn produced from the residual oxygen-rich liquid, the production of the oxygen product is no longer directly coupled to the production of reflux nitrogen to the main distillation column since the residual oxygen-rich liquid is not used in condensing the nitrogen-rich vapor from the main distillation column. As a consequence, the removal of the residual oxygen-rich liquid does not decrease nitrogen reflux to the main distillation column and therefore, nitrogen production as in the prior art.
Additionally, since the residual oxygen-rich liquid is not used in condensing nitrogen, such liquid can be produced at a higher pressure.
As indicated above, the auxiliary column overhead can solely be produced by the stripping of the crude liquid oxygen stream within an auxiliary distillation column. Alternatively, the main distillation column is refluxed with part of the liquid nitrogen stream. In the latter case, the stripping of the crude liquid oxygen stream takes place within a stripping section of the auxiliary distillation column and the stripping produces a nitrogen and oxygen containing vapor stream. 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. The effect of this is to increase recovery of oxygen within the residual oxygen-rich liquid by in effect trapping the oxygen that would otherwise escape the auxiliary column within the waste stream.
In either of the cases mentioned above, the oxygen-rich liquid is collected within the auxiliary distillation column. The oxygen-rich liquid is partially vaporized by passing an oxygen-rich liquid stream 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.
The gaseous stream is the compressed and purified stream. The compressed and purified stream is partially condensed in a condenser and the stream of the residual oxygen-rich liquid is collected in a separation vessel. A liquid phase stream, formed of a liquid phase produced within the separation vessel, is introduced into the condenser and partially vaporized in the condenser through indirect heat exchange with the compressed and purified stream, thereby producing a two-phase stream from the liquid phase stream. The two phase stream is introduced into the separation vessel and liquid and vapor phases of the two phase stream are disengaged 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. The oxygen product stream is then formed by discharging a stream of the oxygen-rich vapor fraction from the separation vessel.
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. Further, the main distillation column can be 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.
As an alternative to forming the gaseous stream from the compressed and purified stream, the gaseous stream can be composed of the nitrogen-rich vapor column overhead. In such embodiment, 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. This produces 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. The condensate stream is introduced into the main distillation column as reflux along with the liquid nitrogen stream.
In the above described embodiment, the waste stream can pass 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.
In any embodiment of the present invention, the stream of the residual oxygen-rich liquid can be pressurized such that the oxygen product stream is also pressurized. Further, A liquid nitrogen refrigeration stream can be introduced into the main distillation column to impart refrigeration.
The present invention also provides an apparatus for separating air to produce the oxygen and nitrogen co-products. Such apparatus includes a main heat exchanger, a main distillation column and an auxiliary distillation column. The main heat exchanger is configured to cool a compressed and purified stream comprising the air and the main distillation column is configured to rectify the compressed and purified stream to produce a nitrogen-rich vapor column overhead and crude liquid oxygen. The auxiliary distillation column is 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 is 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.
A means is provided 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 is 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 is connected to the main distillation column and the auxiliary distillation column so that a nitrogen product stream formed from the nitrogen-rich vapor 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. A means is also provided 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. Further, a means is provided for forming an oxygen-rich vapor fraction from the stream of the residual oxygen-rich liquid after having been partially vaporized. The main heat exchanger is 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.
The auxiliary column can be solely provided with a stripping section where the stripping of the crude liquid oxygen stream takes place. As an alternative, the auxiliary column can be provided with a stripping section and a rectification section, located above the stripping section. In the latter case, 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. This rectification is provided to thereby increase 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. 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.
The auxiliary distillation column is provided with 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. The main distillation column is connected to the once-through heat exchanger such that the nitrogen-rich vapor stream is condensed within the once-through heat exchanger.
The gaseous stream is the compressed and purified air stream. In such case, the stream of the residual oxygen-rich liquid heat exchange means and the oxygen-rich vapor fraction forming means is a condenser and a separation vessel. The condenser is connected to the main heat exchanger such that the compressed and purified stream is partially condensed. 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.
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. Further, the once-through heat exchanger can be connected to the main distillation column such that the main distillation column is refluxed with 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 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.
As an alternative, the gaseous stream can be composed of the nitrogen-rich vapor. In such case, the stream of the residual oxygen-rich liquid heat exchange means and the oxygen-rich vapor fraction forming means are formed by a thermo-siphon reboiler having a shell. The shell is connected to the auxiliary column to receive the stream of the residual oxygen-rich liquid and 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 is 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. 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 all pass within the main heat exchanger, in indirect heat exchange with the compressed and purified stream.
In the embodiment discussed above, the 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.
In any embodiment of the present invention, the main distillation column can be provided with a top inlet for introduction of a liquid nitrogen refrigeration stream to impart refrigeration.

Brief Description of the Drawings

While the present invention concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the present invention will be better understood when taken in connection with the accompanying drawings in which:

Fig. 1 is a schematic illustration of an air separation plant for carrying out a method in accordance with the present invention;
Fig. 2 is a schematic illustration of an alternative embodiment of an air separation plant for carrying out a method which embodiment does not fall within the scope of the claims; and
Fig. 3 is a schematic illustration of a yet further alternative embodiment of an air separation plant for carrying out a method in accordance with the present invention.

Detailed Description

With reference to Figure 1, an air separation plant 1 is illustrated that is capable of co-producing oxygen and nitrogen products ("N₂" and "O₂").
A compressed and purified air stream 10 is cooled in a main heat exchanger 12 to a temperature suitable for its rectification at or near saturation. As can be appreciated, air separation plant 1 could be part of an installation requiring compressed air and as such, compressed and purified air stream 10 could be formed from part of the air produced in such installation. Alternatively, such stream could be generated by compressing the air, typically from between 70 psia to 90 psia, and then purifying the air of higher boiling contaminants that would solidify or concentrate at cryogenic temperatures; for instance, carbon dioxide, water vapor and hydrocarbons. Main heat exchanger 12 can be of conventional brazed aluminum plate fin construction. As would occur to those skilled in the art, main heat exchanger 12 could be formed from a plurality of units operating in parallel.
The compressed and purified air stream 10, after having been cooled, is introduced into a condenser 14 located in a bottom region 16 of a main distillation column 18. Distillation column 18 contains mass-transfer contacting elements such as trays, structured packing, random packing or a combination of such elements that are generally indicated by reference numeral 20. The incoming air is partially condensed within condenser 14 to produce a vapor fraction 22 that ascends into the mass-transfer contacting elements 20 to contact a descending liquid phase that produces a nitrogen-rich vapor column overhead 23. Nitrogen-rich vapor column overhead 23 will typically be essentially pure nitrogen. The partial condensation of the air produces a liquid fraction 24 that constitutes about 20 percent of the incoming air and collects along with the liquid that has descending within elements 20 within the bottom region 16 of main distillation column 18 as crude liquid oxygen 26.
It is to be noted, alternative means for condensing the incoming air would be to locate condenser 14 outside of the main distillation column 18. However, such an embodiment does not fall within the scope of the claims. In connection with such means, the liquid fraction 24 and the liquid having descended within distillation column 20 could be separately combined to form the crude liquid oxygen 26 or they could be subcooled and fed separately to column 34. In another such means, the compressed and purified air stream could be introduced directly into the main distillation column shell. The air would be allowed to mix with vapor within the column and the recirculated vapor 22 from the condenser 14. In such configuration, the condenser would have no condensing-side headering, namely, the gas would simply be induced to flow into the exchanger through condensation. As a further alternative, a portion of the cooled air stream 10 could be directed to the shell of column 18 and a portion piped directly to condenser 14. Such a configuration would allow a modulation of the product oxygen and nitrogen flows. However, this configuration would require additional valves and control means. A further means would be to employ a separate stream of air that could be further compressed. The separate air stream would be split at either the warm end, if further compressed, or if not so compressed, at the cold end. The stream would then be totally condensed within the condenser 14. The condensed air would then be introduced into an interstage location of the main distillation column 18 through suitable piping. The condensed air would not be collected in the sump of the column and instead, liquefied air would be fed several stages up from the bottom of the main distillation column 14.
A crude liquid oxygen stream 28 composed of the crude liquid oxygen 26 is then preferably subcooled within a subcooling heat exchanger 30 and depressurized within an expansion valve 32, positioned between the main distillation column 18 and the auxiliary column 34, to a pressure of preferably 15 to 25 psia. The crude liquid oxygen stream 28 is then stripped within an auxiliary distillation column 34 by contacting such stream with an ascending stripping gas stream 52 within mass transfer contacting elements 36 that could be trays, structured packing, random packing or a combination of such elements. The stripping produces an auxiliary column overhead 38 that typically contains between 80 percent to 90 percent by volume of nitrogen and a residual oxygen-rich liquid 56 that typically contains 85 percent to 98 percent by volume of oxygen.
The oxygen-rich liquid 40 can be collected within a collection tray 42 or by similar means in which downcoming liquid is collected and rising vapor is allowed to pass through and into the mass transfer contacting elements 36. A stream of the oxygen-rich liquid 44, composed of the oxygen-rich liquid 40, is partially vaporized through indirect heat exchange with a nitrogen-rich vapor stream 46 composed of the nitrogen-rich vapor 23 within a once-through heat exchanger 48. As a result of the indirect heat exchange, a liquid nitrogen stream 50 is produced. The stream of the oxygen-rich liquid 44 is partially vaporized to produce the stripping gas 52 and a residual gas liquid stream 54 that collects in the bottom of the auxiliary distillation column 34 as a residual oxygen-rich liquid 56. It is to be noted that in alternative embodiment, a two phase stream from the boiling side of the once-through heat exchanger could be extracted from the auxiliary distillation column 34 and directed to a separate phase separator to produce the residual oxygen rich liquid stream 68 and a vapor phase would then be piped back to the auxiliary distillation column 34 to form the stripping gas 52. Although not preferred, this would be equivalent to that shown in the Figures.
As an alternative means to the once-through heat exchanger 48 and the collection tray 42, a thermo-siphon reboiler situated in the base of the auxiliary column 34 could be employed to partially vaporize the oxygen-rich liquid 40 falling from the mass-transfer contacting elements 36 into the base of the auxiliary column 34. The core of such reboiler would sit within the residual oxygen-rich liquid and produce the stripping gas through vaporization of the oxygen-rich liquid 40. This would not, however, be preferred given that such a heat exchanger would not operate with the temperature differences of a once-through heat exchanger; and consequently, the incoming air would have to be compressed to a higher pressure.
The liquid nitrogen stream 50, in its entirely, or a part thereof, as illustrated, can be introduced into the main distillation column 18 as a reflux stream 58. The remainder, as a further liquid nitrogen stream 60 (depressurized through valve 63) and a waste stream 62, composed of the auxiliary column overhead 38, can be first introduced into subcooling heat exchanger 30 in indirect heat exchange with the crude liquid oxygen stream 28 for purposes of the subcooling thereof and then passed in indirect heat exchange with the incoming compressed and purified air stream 10 to help cool the same within the main heat exchanger 12. Alternatively, stream 60 and stream 62 may be combined and warmed. After having been warmed, the further liquid nitrogen stream 60 would be discharged from main heat exchanger 12 as a gaseous nitrogen co-product ("N₂") and the wastestream 62 could be used in the regeneration of adsorbents employed in a pre-purification unit used to pre-purify the incoming air and used in connection with air separation plant 1. Furthermore, it is also possible to recycle all or part of such stream by recompressing it and combining it with the incoming air. It should be noted that utilization of stream 60 is optional, the vaporization of liquid nitrogen stream 60 serves to provide additional subcooling too stream 28 and hence enables greater oxygen production.
It should be noted that liquid nitrogen reflux stream 60 could be extracted from an interstage location of column 20 prior to passage through valve 63.
It is also to be noted that the subcooling heat exchanger 30 or 30' to be discussed could be integrated with the main heat exchanger 12. It is equally possible that an embodiment of the present invention could be constructed without the use of such subcooling heat exchangers 30 or 30'. In any case, in the illustrated embodiment, the further liquid nitrogen stream 60 can be depressurized in an expansion valve 63 before passing through subcooling heat exchanger 30 and main heat exchanger 12.
The nitrogen co-product is obtained by warming a nitrogen product stream 64, composed of the nitrogen-rich vapor column overhead 23, within the main heat exchanger 12, through indirect heat exchange with the incoming compressed and purified air stream 10 to also help cool the same. As illustrated, nitrogen product stream 64 and nitrogen-rich vapor stream 46 are produced by dividing a nitrogen-rich vapor stream 66 removed from the top of main distillation column 18 into the two foregoing streams. However, the nitrogen product stream 64 and the nitrogen-rich vapor stream 46 could be separately removed from the main distillation column 18. It is to be noted that since all of the liquid nitrogen stream 50 could be introduced into the main distillation column 18 as reflux, the nitrogen co-product could be formed solely from the warmed nitrogen product stream 64.
The oxygen co-product is produced by removing a stream of the residual oxygen-enriched liquid 68, composed of the residual oxygen-rich liquid 56, and introducing such stream into a separation vessel 70. A liquid phase stream 72, composed of the liquid phase 74, is partially vaporized in the condenser 14 to produce a two-phase stream 76 through indirect heat exchange with the incoming compressed and purified air stream 10, thereby partially condensing the compressed and purified air stream 10. Liquid and vapor fractions of the two phase stream 76 are disengaged within the separation vessel to form a vapor fraction and a liquid phase 74 produced by the disengagement with the stream and addition of the residual oxygen-rich liquid 68. A control valve (not shown) may be employed to modulate the operating pressure of vessel 70. The oxygen product stream is formed from a vapor phase stream 80 composed of the vapor fraction 78. A drain stream 82 that is passed through a valve 84 can optionally be collected as a liquid product (or directed as a contaminant drain vaporizer). Alternatively a portion of stream 68 may be extracted as liquid product and sent to suitable storage.
It is to be noted that the stream of the residual oxygen-rich liquid 68 can be pressurized as a result of the auxiliary distillation column 34 being situated at a height above the main distillation column 18. This results in a pressurized oxygen product. A yet further alterative, is to pressurize the stream of the residual oxygen-rich liquid by means of a mechanical pump or the use of the pump in connection with the liquid head produced by situating the auxiliary distillation column 34 above the main distillation column 18.
As illustrated, condenser 14 acts as a thermo-siphon. As an alternative means for forming the oxygen product, it is possible that separation vessel 70 could be constructed to house the condenser 14. In such case the two-phase stream would exit the condenser into the interior of the separation vessel 70.
However, it is believed that the illustrated embodiment is more cost effective to such an arrangement. Additionally, if more than roughly 20 percent of the oxygen product were to be taken as a liquid product by way of drain stream 82, then it would be possible to reconfigure the condenser 14 as a once-through heat exchanger with a potential further savings in power consumption. In such an arrangement, the stream of the residual oxygen-rich liquid 68 would be piped directly to the condenser 14. The partially vaporized product would then be separated in a phase separation vessel and there would not be a return stream, such as oxygen-enriched liquid stream being introduced into the condenser 14.
With reference to Figure 2, an alternative embodiment is illustrated as an air separation plant 2. It is to be noted that where the elements illustrated in Figure 2 would have the same description as in the discussion above with respect to Figure 1, such elements shown in Figure 2 will employ the same reference numbers as those in Figure 1; and for the sake of brevity, will not be further discussed. Air separation plant 2 is specifically designed to allow a greater fraction of the nitrogen contained in air to be recovered as product. This is accomplished through an increase in liquid nitrogen reflux to the main distillation column 18.
In air separation plant 2, the compressed and purified air stream 10 after having been cooled within main heat exchanger 12 is introduced into main distillation column 20 and rectified to produce a nitrogen-rich vapor column overhead 23 and a crude liquid oxygen column bottoms 26'. A crude liquid oxygen stream 28' can optionally be subcooled in a subcooling unit 30' and stripped within auxiliary distillation column 34 after having been depressurized within expansion valve 32. A first nitrogen-rich vapor stream 46 is condensed in the once-through heat exchanger 48 to produce a first liquid nitrogen stream 50 and residual oxygen-rich liquid 56 in the manner described with respect to air separation plant 1. The resulting stream of the residual oxygen-rich liquid 68 produced in auxiliary distillation column 34 is introduced into a thermo-siphon reboiler 86 after having been depressurized in an expansion valve 90. The stream of the residual oxygen-rich liquid 68 is partially vaporized through indirect heat exchange with a gaseous stream 92 that can be composed of the nitrogen-rich vapor column overhead 23 of main distillation column 18. Gaseous stream 92 passes into a core 94 located within a shell 96 of the thermo-siphon reboiler 86 to accomplish the heat exchange. The gaseous stream 92 is condensed to produce a condensate stream 98 that is introduced into the main distillation column 18 as part of the reflux for such column. All of the liquid nitrogen stream 50, produced in the once-through heat exchanger 48, is introduced into the main distillation column 18 as column reflux. This staged condensation of the nitrogen-rich vapor column overhead will increase the liquid nitrogen reflux and therefore, the increased ability of the air separation plant 2 to produce the nitrogen product.
As can be appreciated, other means in place of thermo-siphon reboiler 86 could be employed including a once-through heat exchanger if the drain stream 106 were of sufficient flow. Furthermore, although not illustrated, in place of the nitrogen-rich vapor, a stream having a nitrogen concentration no less than air could be introduced into the core 94 of the thermo-siphon reboiler 86. For example, it is possible to compress an air stream and liquefy such stream in the main heat exchanger 12. Thereafter, it could be introduced into an intermediate location of the main distillation column 18. A gaseous stream having about the same composition as such liquid stream could then be passed into the core 94 of the thermo-siphon reboiler 86 and condensed. The resulting condensate stream would be introduced into a location of the main distillation column at which the composition of the downcoming column liquid was the same or nearly the same as such condensate or introduced into the auxiliary distillation column 34 for stripping and production of the oxygen co-product.
The partial vaporization of the stream of the residual oxygen-rich liquid 68 creates vapor and liquid fractions that collect in the shell as a vapor fraction 100 and a liquid phase 102. A vapor phase stream 104, composed of the vapor fraction 100, is passed in indirect heat exchange with the incoming compressed and purified air stream 10, within main heat exchanger 12, to warm the vapor phase stream 100, to produce the oxygen product ("O₂") and to help cool the compressed and purified air stream 10. A drain stream 106 composed of the liquid phase 102 can be depressurized in a valve 108 and collected as a liquid oxygen product. In such embodiment, optionally, the waste stream 62 can be passed in indirect heat exchange with the crude liquid oxygen stream 28' within the subcooling heat exchanger 30'. The waste stream 62 is also, subsequently passed in indirect heat exchange with the compressed and purified air stream 10, within main heat exchanger 12, to warm the waste stream 62 and help cool the compressed and purified air stream 10. The waste stream 62 could be recycled back into the incoming air. This may be accomplished after warming in exchanger 12 by direct mixing with feed air or after further compression. As in air separation plant 1, the nitrogen product stream 64, composed of the nitrogen-rich vapor column overhead 23, is warmed within the main heat exchanger 12 through indirect heat exchange with the incoming compressed and purified air stream 10 to also help cool the same.
With reference to both Figure 1 and Figure 2, refrigeration is imparted to either air separation plant 1 or air separation plant 2 by means of a liquid nitrogen stream 110 that is passed through a flow control valve 112 and then introduced into a top inlet 114 located in the top of main distillation column 18. Such stream can be obtained from an on-site storage tank and may be of high or low pressure. The addition of such stream compensates for ambient heat leak as well as warm end losses incurred with the operation of main heat exchanger 12. It should be noted that liquid stream 110 could be introduced directly into one of the vapor streams entering heat exchangers 30' or 12. As could be appreciated, a portion of the incoming air could be expanded prior to introduction into the main distillation column 18. In this regard, a portion of the compressed and purified air stream after partial cooling within main heat exchanger 12 could be expanded and then mixed with waste gas stream 62. Another possibility would be to back-pressure the distillation column system of the main distillation column 18 and the auxiliary distillation column 34 and waste stream 62 could be partially warmed within main heat exchanger 12, expanded in a turboexpander and the further warmed within main heat exchanger 12. The use of a turboexpander may afford the possibility of taking a liquid product from the column system. A portion of the liquid nitrogen obtained from condensers 48 or 96 may be sent to suitable storage. A yet further alternative is that a liquid oxygen stream could be introduced or combined with the stream of the residual oxygen-rich liquid 68 and then introduced into the shell 96 of the thermo-siphon reboiler 86.
With reference to Figure 3, an air separation plant 1' is illustrated that constitutes an alternative embodiment to air separation plant 1 shown in Figure 1. In air separation plant 1, an auxiliary column 34' is shown that is provided with a stripping section 36a that can be provided with mass transfer contacting elements 36 as discussed above with respect to Figure 1. The crude liquid oxygen stream 28 is stripped within such stripping section 36a as described above and rather than directly producing an auxiliary column overhead, produces a nitrogen and oxygen containing vapor. Auxiliary column 34a is also provided with a rectification section 36b situated above the stripping section 36a to rectify such nitrogen and oxygen containing vapor and thereby recover some of the oxygen that would otherwise have been vented from the plant in the waste stream 62. This rectification section is refluxed by liquid nitrogen stream 60' that is introduced into the top of auxiliary column 34' after having been depressurized by a valve 63'. As can be appreciated, the same modification could be made to the plant shown in Figure 2. In such case, the auxiliary column would be modified in a like manner to that shown in Figure 3. In case of the Figure 2 embodiment, part of the condensate stream 98 would be used to reflux the auxiliary column 34'.
As can be appreciated, all that is contemplated is recovering some of the oxygen that would otherwise be lost in the waste to increase oxygen recovery. However, a balance must be struck between increased oxygen recovery and the fact that liquid nitrogen is being taken from the liquid nitrogen that would otherwise have been used in refluxing the main distillation column and therefore, the production of nitrogen. Any greater rectification would be accompanied by an increase in the flow of liquid nitrogen stream 60' and therefore, a decrease in the available product nitrogen 64 since the reflux rate of the main column must be maintained in order to maintain product nitrogen purity. A more concrete manner in stating such a limitation is that the performance of the rectification section 36b, when taken in connection with that of the stripping section 36a should be that the auxiliary column overhead 38 within the auxiliary column 36 should contain no less than 5.0 percent oxygen on a volume basis. In order to accomplish this, typically, a ratio of the flow rate of the liquid nitrogen stream 60' to the total available nitrogen flow from column 18 (the sum of streams 60 and 64) should be between 0.1 and 0.4 and a liquid to vapor ratio in the rectification section 36b of between 0.23 or less.

Claims

A method of separating air to produce oxygen and nitrogen co-products, said method comprising:
cooling a compressed and purified stream (10) comprising the air;

partially condensing the compressed and purified stream is in a condenser (14) located in a bottom region (16) of a main distillation column (18);

rectifying the vapor fraction (22) of the partially condensed compressed and purified stream within the main distillation column (18) to produce a nitrogen-rich vapor column overhead (23) and wherein the condensed air mixes with downcoming liquid produced by the rectification in the main distillation column to thereby produce crude liquid oxygen column bottoms (26) in the main distillation column (18);

producing an oxygen-rich liquid (40) and an auxiliary column overhead (38) containing not less than 5.0 percent oxygen on a volume basis within an auxiliary distillation column (34) by, at least in part, depressurizing a crude liquid oxygen stream (28) composed of the crude liquid oxygen column bottoms (26), stripping the crude liquid oxygen stream (28) within the auxiliary distillation column (34) with an ascending stripping gas (52) and partially vaporizing the oxygen-rich liquid (44) through indirect heat exchange with a nitrogen-rich vapor stream (46) composed of the nitrogen-rich vapor column overhead (23), thereby producing a liquid nitrogen stream (50), the stripping gas (52) and a residual oxygen-rich liquid (56);

collecting the oxygen-rich liquid (40) within the auxiliary distillation column (34);

partially vaporizing the oxygen-rich liquid (40) by passing an oxygen-rich liquid stream composed of the oxygen-rich liquid (40) and the nitrogen-rich vapor stream (46) through a once-through heat exchanger (48) to form the stripping gas (52) and the residual oxygen-rich liquid (56) that collects as a column bottoms of the auxiliary distillation column (34).

refluxing the main distillation column (18) with at least part (58) of the liquid nitrogen stream (50);

forming an oxygen-rich vapor fraction (78) from the residual oxygen-rich liquid (56) by indirectly exchanging heat between a stream (68) of the residual oxygen-rich liquid (56) with a gaseous stream (76) having a nitrogen concentration no less than that of air so that the stream (68) of the residual oxygen-rich liquid (56) partially vaporizes, wherein the gaseous stream (76) is the compressed and purified stream (10);

forming: an oxygen product stream (80) from the vapor fraction (78); a nitrogen product stream (64) from the nitrogen-rich vapor column overhead (23); and a waste stream (62) from the auxiliary column overhead (38); and

passing the oxygen product stream (80), the nitrogen product stream (64) and the waste stream (62) in indirect heat exchange with the compressed and purified stream (10);

wherein ;the oxygen-rich vapor fraction (78) is formed by:
collecting the stream (68) of the residual oxygen-rich liquid (56) in a separation vessel (70);

introducing a liquid phase stream (72), formed of a liquid phase produced within the separation vessel (70), into the condenser (14) and partially vaporizing the liquid phase stream (72) in the condenser (14) through indirect heat exchange with the compressed and purified stream (10), thereby producing a two-phase stream (76) from the liquid phase stream (72);

introducing the two phase stream (76) into the separation vessel (70) and disengaging liquid and vapor phases of the two phase stream (76) within the separation vessel (70) to form an oxygen-rich vapor fraction (78) and the liquid phase (74) together with the stream (68) of the residual oxygen-rich liquid collected in the separation vessel (70); and

the oxygen product stream (80) is formed by discharging a stream of the oxygen-rich vapor fraction (78) from the separation vessel (70).
The method of claim 1, wherein the auxiliary column overhead (38) is solely produced by the stripping of the crude liquid oxygen stream (28) within an auxiliary distillation column (34).
The method of claim 1, wherein:
the main distillation column (18) is refluxed with part (58) of the liquid nitrogen stream (50);

the stripping of the crude liquid oxygen stream (28) takes place within a stripping section (36a) of auxiliary distillation column (34');

stripping the crude liquid oxygen stream (28) within the auxiliary distillation column (34') 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 (36b) of the auxiliary distillation column (34') located above the stripping section (36a) by introducing the nitrogen and oxygen containing vapor stream into the rectification section (36b) and refluxing the auxiliary distillation column (34') and therefore, the rectification section with a further part of the liquid nitrogen stream (50), thereby increasing recovery of oxygen within the residual oxygen-rich liquid (56).
The method of claim 1, wherein:
the main distillation column (18) is refluxed with part (58) of the liquid nitrogen stream (50); and

the waste stream (62) indirectly exchanges heat with the crude liquid oxygen stream (28) such that the crude liquid oxygen stream (28) is subcooled prior to being depressurized.
The method of claim 1, wherein the stream (68) of the residual oxygen-rich liquid (56) is pressurized such that the oxygen product stream (80) is also pressurized.
The method of claim 1, wherein a liquid nitrogen refrigeration stream (110) is introduced into the main distillation column (18) to impart refrigeration.
An apparatus for separating air to produce oxygen and nitrogen co-products, said apparatus comprising:
a main heat exchanger (12) configured to cool a compressed and purified stream (10) comprising the air;

a condenser (14) located in a bottom region (16) of a main distillation column (18) configured for partially condensing the compressed and purified stream (10);

the main distillation column (18) configured to rectify the vapor fraction (22) of the partially condensed compressed and purified stream to produce a nitrogen-rich vapor column overhead (23) and wherein the condensed air mixes with downcoming liquid produced by the rectification in the main distillation column (18) to thereby produce_crude liquid oxygen column bottoms (26);

an auxiliary distillation column (34) connected to the main distillation column (18) and configured such that a crude liquid oxygen stream (28), composed of the crude liquid oxygen column bottoms (26), is stripped with an ascending stripping gas (52) within the auxiliary distillation column (34) and an oxygen-rich liquid (40) and an auxiliary column overhead (38) 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 (28), the auxiliary distillation column (34) having means for collecting the oxygen-rich liquid;

an expansion valve (32) positioned between the main distillation column (18) and the auxiliary distillation column (34) such that the crude liquid oxygen stream (28) is depressurized prior to introduction into the auxiliary distillation column;

means (48) for partially vaporizing the oxygen-rich liquid (44) through indirect heat exchange with a nitrogen-rich vapor stream composed of the nitrogen-rich vapor column overhead (23), thereby producing a liquid nitrogen stream (50), the stripping gas (52) and a residual oxygen-rich liquid (56);

wherein the oxygen-rich liquid partial vaporization means (48) is a once-through heat exchanger connected to an auxiliary distillation column (34) and the oxygen-rich liquid collecting means such that the oxygen-rich liquid (40) is partially vaporized within the once-through heat exchanger (48) through passage of an oxygen-rich liquid stream, composed of the oxygen-rich liquid (40) and the residual oxygen-rich liquid (56) collects as a column bottoms of the auxiliary distillation column (34); and

the main distillation column (18) connected to the once-through heat exchanger (48) such that the nitrogen-rich vapor stream (46) is condensed within the once-through heat exchanger (48) and such that the main distillation column is refluxed with at least part (58) of the liquid nitrogen stream (50);

the main heat exchanger (12) connected to the main distillation column (18) and the auxiliary distillation column (34) so that a nitrogen product stream (64) composed of the nitrogen-rich vapor column overhead (23) and a waste stream (62) formed from the auxiliary column overhead (38) of the auxiliary distillation column (34) indirectly exchange heat with the compressed and purified air stream (10);

means (70) for indirectly exchanging heat between a stream (68) of the residual oxygen-rich liquid (56) with a gaseous stream (76, 92) having a nitrogen concentration no less than that of air so that the stream (68) of the residual oxygen-rich liquid (56) partially vaporizes and means for forming an oxygen-rich vapor fraction (78) from the stream of the residual oxygen-rich liquid after having been partially vaporized, wherein the gaseous stream (76) is the compressed and purified air stream (10); wherein:
the residual oxygen-rich liquid heat exchange means is the condenser (14) and the oxygen-rich vapor fraction forming means is a separation vessel (70);

the separation vessel (70) is connected to the auxiliary distillation column (34) such that the stream (68) of the residual oxygen-rich liquid (56) collects in the separation vessel (70);

the separation vessel (70) is connected to the condenser (14) so that a liquid phase stream (72), composed of a liquid phase produced within the separation vessel is partially vaporized in the condenser (14) to produce a two-phase stream (76) that is introduced into the separation vessel (70), liquid and vapor phases of the two phase stream are disengaged within the separation vessel to form the oxygen-rich vapor fraction (78) and the liquid phase (74);

the main heat exchanger (12) is connected to the separation vessel (70) so that the oxygen product stream (80) is formed from the oxygen-rich vapor fraction (78); and

the main heat exchanger (12) is connected to the oxygen-rich vapor fraction forming means (70), the main distillation column (18) and the auxiliary distillation column (34) such that an oxygen product stream (80), composed of the oxygen-rich vapor fraction (78), a nitrogen product stream 64), composed of the nitrogen-rich vapor column overhead (23), and a waste stream (62), composed of the auxiliary column overhead (38) of the auxiliary distillation column (34), pass within the main heat exchanger (12), in indirect heat exchange with the compressed and purified stream (10).
The apparatus of claim 7, wherein the auxiliary column (34') is solely provided with a stripping section (36a) where the stripping of the crude liquid oxygen stream (28) takes place.
The apparatus of claim 7, wherein:
the auxiliary column (34') has a stripping section (36a) and a rectification section (36b), located above the stripping section (36a);

the stripping of the crude liquid oxygen stream (28) takes place within a stripping section (36a) of auxiliary distillation column (34') and a nitrogen and oxygen containing vapor stream is produced in the stripping section (36a) that enters the rectification section (36b) for rectification of the nitrogen and oxygen containing vapor stream, thereby increasing recovery of oxygen within the residual oxygen-rich liquid (28);

the oxygen-rich liquid partial vaporization means (48) is connected to the main distillation column (18) such that the main distillation column is refluxed with part (58) of the liquid nitrogen stream (50) and is also connected to the auxiliary distillation column (34') such that the auxiliary distillation column and therefore, the rectification section (36b) is refluxed with a further part (60') of the liquid nitrogen stream (50); and

another expansion valve (63') is positioned between the oxygen-rich liquid partial vaporization means (48) and the auxiliary distillation column (34') so that pressure of the further part (60') of the liquid nitrogen stream (50) is reduced to that of the auxiliary distillation column (34').
The apparatus of claim 7, wherein:
the once-through heat exchanger (48) is connected to the main distillation column (18) such that the main distillation column is refluxed with part (58) of the liquid nitrogen stream (50); and

a subcooling heat exchanger (30) is connected to the once-through heat exchanger (48), the auxiliary distillation column (34) and the expansion valve (32) such that the waste stream (62) indirectly exchanges heat with the crude liquid oxygen stream (28) within the subcooling heat exchanger (30) and the crude liquid oxygen stream (28) is subcooled prior to passage through the expansion valve (32).
The apparatus of claim 7 wherein the main distillation column (18) has a top inlet (114) for introduction of a liquid nitrogen refrigeration stream (110) to impart refrigeration.