Patent Application
20110192194
The present invention relates to a method and apparatus in which a mixture comprising nitrogen and oxygen is separated into nitrogen-rich and oxygen-rich fractions through cryogenic rectification and one or more pressurized product streams are produced by pumping and heating one or more liquid streams composed of one of the nitrogen-rich or oxygen-rich product fractions. More particularly, the present invention relates to such a method and apparatus utilizing a banked arrangement of heat exchangers in which a heat exchange stream is partially cooled within the higher pressure heat exchanger utilized in heating the one or more pumped liquid streams and then further cooled within the lower pressure heat exchanger to reduce warm end temperature difference within the higher pressure heat exchanger and therefore, the amount of refrigeration required to be imparted in connection with the cryogenic rectification.
Nitrogen and oxygen containing mixtures, most commonly air, are separated into nitrogen-rich and oxygen-rich fractions and other component fractions found in air, for example, argon, by cryogenic rectification. In cryogenic rectification, the mixture is compressed and then purified to remove higher boiling contaminants such as carbon dioxide, water vapor and hydrocarbons. The resulting compressed and purified stream is then cooled to a temperature suitable for distillation. The distillation produces nitrogen-rich and oxygen-rich fractions of the air and potentially other fractions that can be taken as both liquid and gaseous products. There are different distillation column arrangements that are used for such purposes. For example, the distillation columns can consist of a higher pressure column and a lower pressure column thermally linked by a heat exchanger to vaporize an oxygen-rich liquid column bottoms of the lower pressure column and to condense a nitrogen-rich vapor column overhead of the higher pressure column.
Where there exists a demand for large amounts of high pressure oxygen in a gaseous form, in for example, gasification, the oxygen is often produced by pumping a stream of oxygen-rich liquid to pressure and then heating the liquid through indirect heat exchange with a boosted pressure stream that is typically part of the air to be distilled. In the column arrangement discussed above, the stream of oxygen-rich liquid is composed of the oxygen-rich liquid column bottoms of the higher pressure column. The boosted pressure air will be liquefied as a result of the heat exchange and introduced into the higher pressure column, the lower pressure column or divided between the columns after having been reduced to a pressure suitable for introduction into such columns. In addition to oxygen, high pressure nitrogen can also be a desired product, particularly in gasification applications. Such high pressure nitrogen can be produced by pumping a stream of the condensed, nitrogen-rich vapor and heating the resulting liquid through heat exchange with the boosted pressure stream.
It is to be noted that the heating of the pumped streams can be carried out in a banked arrangement of heat exchangers having a higher pressure heat exchanger for the heating of the pumped liquid and a lower pressure heat exchanger, having a lower maximum allowable working pressure than the higher pressure heat exchanger for cooling at least part of the air. In this regard, although as discussed above, the boosted pressure stream is commonly composed of air, other fluids can be used. For example, in U.S. Pat. No. 4,345,925, a fluid such as argon is compressed and then liquefied as a result of the indirect heat exchange occurring in the higher pressure heat exchanger. The resulting liquid then serves in various heat exchange functions related to the distillation columns. In particular, the fluid is vaporized and superheated in the lower pressure heat exchanger and then recompressed prior to its introduction into the higher pressure heat exchanger. Thus, the fluid circulates in a heat exchange loop that involves the heating of the pressurized oxygen-rich liquid.
An example of a cryogenic rectification plant in which air serves as the boosted pressure fluid can be found in United States Patent Application, Publication Number 2008/0307828. In this application, air is compressed and purified. Part of the resulting compressed and purified air is cooled within the lower pressure heat exchanger and then introduced into the higher pressure column as a main air stream. The higher pressure column is thermally linked to a lower pressure column by a condenser reboiler. The condenser reboiler condenses nitrogen-rich vapor column overhead of the higher pressure column against vaporizing oxygen-rich liquid column bottoms formed in the lower pressure column. Part of an oxygen-rich liquid stream composed of the oxygen-rich liquid column bottoms of the low pressure column can be pumped to form a pumped liquid oxygen stream and part of the condensed nitrogen-rich vapor can also be pumped to produce a pumped liquid nitrogen stream. The pumped liquid oxygen stream and the pumped liquid nitrogen stream are then heated in a higher pressure heat exchanger through indirect heat exchange with boosted pressure stream that is formed by compressing another part of the compressed and purified air in a booster compressor. The boosted stream is liquefied, expanded and introduced into the higher and lower pressure columns. A thermal balancing stream composed of a waste nitrogen stream produced in the lower pressure column is introduced into both the lower pressure heat exchanger and the higher pressure heat exchanger to inhibit warm end losses of refrigeration by such heat exchangers and also to decrease the difference in temperatures between the boosted pressure stream and the main air stream at the cold end of such heat exchangers.
In any cryogenic rectification plant, refrigeration must be imparted for such reasons as warm end losses in the heat exchangers, heat leakage into the plant and to produce liquids. In the higher pressure heat exchanger, the magnitude of the warm end temperature difference between the boosted stream being cooled versus the vaporized liquid stream or streams being heated represents a loss of such refrigeration. In order to overcome such loss of refrigeration, more refrigeration must be introduced into the plant. In the published patent application, discussed above, this refrigeration can be produced by introducing yet another part of the compressed and purified air into a booster compressor, partially cooling such air in either the higher or the lower pressure heat exchanger and then, introducing the partially cooled stream into a turboexpander to generate the refrigeration in an exhaust stream that can be introduced into the higher pressure column or the lower pressure column. The greater the degree of refrigeration that is required for a plant, the greater the energy that will be expanded in the associated booster compressor. Since, the total energy expenditure is an important consideration in the cost of production in a cryogenic rectification plant, it is desired to minimize energy requirements for the plant. As will be discussed, among other advantages, the present invention provides a method and a cryogenic rectification plant for conducting such a method in which warm end temperature differences produced in the higher pressure heat exchanger are reduced to decrease the refrigeration requirements and therefore, the costs involved in operating the plant.
The present invention, in one aspect, provides a method of separating a mixture comprising nitrogen and oxygen. In accordance with this aspect of the present invention, a cryogenic rectification process is conducted that comprises compressing, purifying, cooling and distilling the mixture into oxygen into nitrogen-rich and oxygen-rich fractions, imparting refrigeration into the cryogenic rectification process and producing at least one pressurized product stream by pumping and heating at least part of one of the nitrogen-rich and oxygen-rich fractions in a liquid state.
The cryogenic rectification process is conducted so as to produce a boosted pressure stream and a heat exchange stream and the cryogenic rectification process utilizes a banked heat exchanger arrangement having a lower pressure heat exchanger for the cooling of at least part of the mixture and a higher pressure heat exchanger for heating the at least part of the one of the nitrogen-rich and oxygen-rich fractions. The boosted pressure stream and the heat exchange stream are introduced into the higher pressure heat exchanger in indirect heat exchange with the at least part of the one of the nitrogen-rich and oxygen-rich fractions. The heat exchange stream is partially cooled in the higher pressure heat exchanger, thereby decreasing a warm end temperature difference within the higher pressure heat exchanger and therefore, the refrigeration required to be imparted to the cryogenic rectification process. The boosted pressure stream is cooled within the higher pressure heat exchanger and the heat exchange stream is further cooled in the lower pressure heat exchanger.
The reduction of the warm end temperature difference within the higher pressure heat exchanger reduces refrigeration losses to in turn decrease the amount of refrigeration that is required for the cryogenic rectification process. The decrease in refrigeration requirement translates into a reduction in the power consumption of the plant due to lower compression requirements for generating the refrigeration in the first instance. As used herein and in the claims, the term “partially cooled” means cooled to a temperature intermediate the temperatures that can be achieved at the warm and cold ends of a heat exchanger. The term, “fully cooled” as used herein and in the claims means cooled to a temperature of the cold end of a heat exchanger and “fully warmed” means warmed to a temperature of the warm end of the heat exchanger.
The cryogenic rectification process can generate a nitrogen-rich vapor stream that is divided into two nitrogen-rich vapor streams that are fully warmed within the higher pressure heat exchanger and the lower pressure heat exchanger so as to balance cold end temperatures of the higher pressure heat exchanger and the lower pressure heat exchanger. The mixture can be air and in such case, a feed stream composed of the air, after having been compressed and purified, is divided into a first subsidiary compressed air stream; a second subsidiary compressed air stream and a third subsidiary compressed air stream. The first subsidiary compressed air stream is fully cooled within the lower pressure heat exchanger, at least part of the second subsidiary compressed air stream is further compressed to form the boosted pressure stream that forms a liquid air stream after having been fully cooled within the higher pressure heat exchanger and the heat exchange stream is the third subsidiary compressed air stream. A first part of the second subsidiary compressed air stream can be further compressed to produce the boosted pressure air stream and a second part of the third subsidiary compressed air stream can be further compressed to a pressure below that of the boosted pressure air stream and is thereafter, yet further compressed, partially cooled within the higher pressure heat exchanger and introduced into a turboexpander to produce an exhaust stream. The exhaust stream is introduced into the distillation column to impart at least part of the refrigeration into the cryogenic rectification process.
In a specific embodiment of the present invention, the mixture is distilled in a higher pressure column operatively associated in a heat transfer relationship with a lower pressure column by a condenser-reboiler configured to condense a higher pressure nitrogen-rich column overhead stream, removed from the higher pressure column, by reboiling an oxygen-rich liquid column bottoms of the lower pressure column. The first subsidiary compressed air stream and the third subsidiary compressed air stream, after having been fully cooled, are introduced into the higher pressure column. The liquid air stream is expanded and introduced into at least one of the higher pressure column and the lower pressure column. A crude liquid oxygen stream composed of a liquid column bottoms of the higher pressure column is subcooled, reduced in pressure to that of the lower pressure column and introduced into the lower pressure column for further refinement. First and second parts of a high pressure nitrogen-rich liquid stream, formed from condensing the higher pressure nitrogen-rich column overhead stream, is used to reflux the higher pressure column and the lower pressure column, respectively. The second of the parts of the higher pressure nitrogen-rich liquid stream is subcooled, reduced in pressure to that of the lower pressure column prior to being introduced as the reflux into the lower pressure column and the crude liquid oxygen stream and the second of the parts of the higher pressure nitrogen-rich liquid stream are subcooled through indirect heat exchange with a lower pressure nitrogen-rich column overhead stream withdrawn from the lower pressure column. The at least one liquid stream is one of an oxygen-enriched stream, composed of the oxygen-rich liquid column bottoms of the lower pressure column and a third part of the high pressure nitrogen-rich liquid stream.
The lower pressure nitrogen-rich column overhead stream can be divided into the two nitrogen-rich vapor streams that are utilized to balance the cold end temperatures of the higher pressure heat exchanger and the lower pressure heat exchanger. The exhaust stream is introduced into the higher pressure column and the liquid air stream is expanded in a liquid expander.
In another aspect, the present invention provides an apparatus for separating a mixture comprising nitrogen and oxygen. In accordance with this aspect of the present invention, the apparatus comprises a cryogenic rectification plant configured to compress, purify, cool and distill the mixture into nitrogen-rich and oxygen-rich fractions. The cryogenic rectification plant has at least one pump for pumping at least part of a liquid stream composed of one of the nitrogen-rich and oxygen-rich fractions in the liquid state and a banked heat exchanger arrangement having lower pressure heat exchanger configured to cool at least part of the mixture and a higher pressure heat exchanger in flow communication with the at least one pump for heating the at least part of the liquid stream and thereby producing a pressurized product stream. Additionally, a means is provided for producing a boosted pressure stream, a means is provided for producing a heat exchange stream and a means is provided for imparting refrigeration into the cryogenic rectification plant. The higher pressure heat exchanger is connected to the boosted pressure stream producing means and the heat exchange stream producing means and is configured to partially cool the heat exchange stream by indirectly exchanging heat from the heat exchange stream to the at least part of the liquid stream, thereby decreasing a warm end temperature difference within the higher pressure heat exchanger and therefore, the refrigeration required to be imparted to the cryogenic rectification plant and to cool the boosted pressure stream by indirectly exchanging heat from the boosted pressure stream to the at least part of the liquid stream. The lower pressure heat exchanger is connected to the higher pressure heat exchanger and is configured to further cool the heat exchange stream after having been partially cooled within the higher pressure heat exchanger.
The cryogenic rectification plant can also be configured to generate two nitrogen-rich vapor streams and the higher pressure heat exchanger and the lower pressure heat exchanger are also configured to receive and to fully warm the two nitrogen-rich vapor streams so that cold end temperatures of the higher pressure heat exchanger and the lower pressure heat exchanger are balanced. The mixture can be air and in such case, the cryogenic rectification plant has a main air compressor and a pre-purification unit connected to the main air compressor to purify the air after having been compressed. The boosted pressure producing means comprises a booster compressor connected to the pre-purification unit and the lower pressure heat exchanger is also connected to the pre-purification unit so that a feed stream composed of the mixture after having been compressed in the main air compressor and purified in the pre-purification unit is divided into a first subsidiary compressed air stream that is fully cooled in the lower pressure heat exchanger and a second subsidiary compressed air stream that at least in part is further compressed in the booster compressor to form the boosted pressure stream and that forms a liquid air stream after having been fully cooled within the higher pressure heat exchanger. The heat exchange stream producing means comprises the higher pressure heat exchanger also connected to the pre-purification unit so that the feed stream after having been compressed and purified is further divided into a third subsidiary compressed air stream that forms the heat exchange stream. The booster compressor can be a multi-stage machine. A first part of the second subsidiary compressed air stream is discharged from a final stage of the booster compressor and forms the boosted pressure air stream. The refrigeration imparting means, at least in part, comprises a further booster compressor connected to an intermediate stage of the booster compressor to further compress a second part of the second subsidiary compressed air stream. The higher pressure heat exchanger connected to the further booster compressor so that the second part of the third subsidiary compressed air stream, after having been further compressed, is partially cooled within the higher pressure heat exchanger and a turboexpander is connected to the higher pressure heat exchanger to expand the first part of the second subsidiary compressed air stream and thereby to produce an exhaust stream. The turboexpander is connected to the distillation column so that the exhaust stream is introduced into the distillation column.
The cryogenic rectification plant can have a higher pressure column and a lower pressure column to distill the mixture. The higher pressure column is operatively associated in a heat transfer relationship with the lower pressure column by a condenser-reboiler. At least part of a higher pressure nitrogen-rich column overhead stream, discharged from the higher pressure column, is condensed by reboiling an oxygen-rich liquid column bottoms of the lower pressure column. The lower pressure heat exchanger is connected to the higher pressure column so that the first subsidiary compressed air stream and the second subsidiary compressed air stream are introduced into the higher pressure column. The higher pressure heat exchanger is in flow communication with at least one of the higher pressure column and the lower pressure column so that the liquid air stream is introduced into at least one of the higher pressure column and the lower pressure column. An expansion device is positioned between the higher pressure heat exchanger and the at least one of the higher pressure column and the lower pressure column to expand the liquid air stream. The higher pressure column is connected to the lower pressure column so that a crude liquid oxygen stream composed of liquid column bottoms of the higher pressure column is introduced into the lower pressure column so as to be further refined and first and second parts of a high pressure nitrogen-rich liquid stream, formed from condensing the higher pressure nitrogen-rich overhead stream, are introduced into the higher pressure column and the lower pressure column, respectively, as reflux. A subcooler, positioned between the lower pressure column and the lower pressure heat exchanger or incorporated into the lower pressure heat exchanger, is configured to subcool the crude liquid oxygen stream and the second of the parts of the higher pressure nitrogen-rich liquid stream. Expansion valves are located between the subcooler and the lower pressure column to expand the crude liquid oxygen stream and the second of the parts of the higher pressure nitrogen-rich liquid stream prior to their introduction into the lower pressure column. The subcooler is connected to the lower pressure column so that a lower pressure nitrogen-rich column overhead stream discharged from the lower pressure column passes in indirect heat exchange with the crude liquid oxygen stream and the second of the parts of the higher pressure nitrogen-rich liquid stream and the at least one liquid stream is an oxygen-enriched stream, composed of the oxygen-rich liquid column bottoms of the lower pressure column or a third part of the higher pressure nitrogen-rich liquid stream.
The higher pressure heat exchanger and the lower pressure heat exchanger can be connected to the subcooler so that the lower pressure nitrogen-rich column overhead stream divides into the two nitrogen-rich vapor streams that are utilized to balance the cold end temperatures of the higher pressure heat exchanger and the lower pressure heat exchanger. Further the turboexpander is connected to the higher pressure column so that the exhaust stream is introduced into the higher pressure column and the expansion device is a liquid expander.
While the specification concludes with claims distinctly pointing out the subject matter that Applicant regards as his invention it is believed that the invention will be better understood when taken in connection with the accompanying sole FIGURE that illustrates an apparatus for carrying out a method in accordance with the present invention.
With reference to the sole FIGURE, a cryogenic rectification plant 1 in accordance with the present invention is illustrated that is designed to cryogenically rectify air or another mixture that contains nitrogen and oxygen into nitrogen and oxygen fractions as will be discussed below. For example, the feed to a cryogenic rectification plant of the present invention could be derived from another air separation plant and as such, the feed might be richer in oxygen in a concentration that is higher than air. Furthermore, although the present invention is illustrated in connection with a double column system having a higher pressure column operatively associated with a lower pressure column in a heat transfer relationship by virtue of a condenser reboiler to condense a nitrogen-rich vapor column overhead in the higher pressure column against vaporizing an oxygen-rich liquid column bottoms of the lower pressure column, the invention is not limited to such a column arrangement. In this regard, the present invention has application to any cryogenic rectification plant employing a banked arrangement of heat exchangers in which a liquid stream enriched in a separated component, typically, nitrogen and oxygen, is pumped and then heated in a higher pressure heat exchanger to form a pressurized product either as a high pressure vapor or as a supercritical fluid.
A feed air stream 10 is compressed in a main compressor 12. After removal of the heat of compression by a first after-cooler 14, feed air stream 10 is purified within a pre-purification unit 16 to produce a compressed and purified air stream 17. Here it is appropriate to point out that although the after-cooler 14 is shown as a separate unit, such compressors as main compressor 12 could be multiple stage machines with intercoolers and an after-cooler installed by the manufacturer as part of the compressor. As such, the after-cooler might not be a separate unit as illustrated and instead, could be part of the compressor itself. The foregoing comments would be equally applicable to any of the compressor and after-cooler arrangements discussed hereinafter. Pre-purification unit 16, as well known to those skilled in the art can contain beds of adsorbent, for example alumina or carbon molecular sieve-type adsorbent to adsorb the higher boiling impurities contained within the air and therefore feed air stream 10. For example such higher boiling impurities as well known would include water vapor and carbon dioxide that will freeze and accumulate at the low rectification temperatures contemplated by apparatus 1. In addition, hydrocarbons can also be adsorbed that could collect within oxygen-rich liquids and thereby present a safety hazard.
A first subsidiary compressed air stream 18 is produced from a first part of the compressed and purified air stream 17. A booster compressor 20 is in flow communication with purification unit 16 to compress a second subsidiary compressed air stream 22 formed from a second part of the compressed and purified air stream 17 and a second after-cooler 23 is provided that is connected to booster compressor 20 to remove the heat of compression from the second subsidiary compressed air stream 22 after having been further compressed. This forms a boosted pressure stream 24 having a higher pressure than the first subsidiary compressed air stream 18. It is to be noted that main air compressor 10 and booster compressor 20 are shown as single units. However, as is known in the art, two or more compressors can be installed in parallel to form either the main air compressor 10 or the booster compressor 20. The two compressors can be of equal size or unequal size. For example, the capacity can be split 70/30 or 60/40 in order to better match customer demand. Typically, second subsidiary compressed air stream 22 will have a flow that ranges from between about 25 percent and about 40 percent of the flow of the compressed air stream 17.
A higher pressure heat exchanger 26 is connected to second after-coolers 23 and 101 and a lower pressure heat exchanger 28 is in flow communication with purification unit 16 to receive the first subsidiary compressed air stream 18. Both the higher pressure heat exchanger 26 and the lower pressure heat exchanger 28 are preferably of brazed aluminum construction and consist of layers of parting sheets separated by side bars to produce flow passages for the streams to be heated and cooled. Each of the flow passages are provided with fins as well known in the art to enhance the surface area for heat transfer within said heat exchangers. The higher pressure heat exchanger 26 is so named due to the fact that it has a higher maximum allowable working pressure as compared with lower pressure heat exchanger 28. The higher pressure heat exchanger 26 is configured to fully cool the boosted pressure stream 24 to produce a liquid air stream 30 and the lower pressure heat exchanger 28 is configured to fully cool the first subsidiary compressed air stream 18 to produce a main feed air stream 32. As can be appreciated, other types of heat exchangers could be used, for example, higher pressure heat exchanger 26 could be spiral wound, printed circuit or of stainless steel plate-fin construction. Moreover, although each of the higher pressure heat exchanger 26 and the lower pressure heat exchanger 28 are illustrated as single units, in practice, each could consist of several heat exchangers linked together in parallel.
The lower pressure heat exchanger 28 will have a larger cross-sectional area for flow and a large total volume than the higher pressure heat exchanger 26. Typically the average density of the higher pressure heat exchanger 26 will be greater than the lower pressure heat exchanger 28 where density is the empty weight divided by volume. A typical density is about 1000 kg/m3. A typical working pressure of the higher pressure heat exchanger 26 is about 1200 psig and greater.
An air separation unit 34 is provided that has a higher pressure column 36 operatively associated with a lower pressure column 38 in a heat transfer relationship by means of a condenser-reboiler 50. Optionally, air separation unit 34 can also include an argon column that is connected to lower pressure column 38 for producing an argon product. It is understood that each of the higher pressure column 36 and the lower pressure column 38 contain liquid-vapor mass transfer elements such as sieve trays or packing, either random or structured. Such elements as well known in the art enhance liquid-vapor contact of liquid and vapor phases of the mixture to be separated in each of such columns for rectification purposes. The rectification of the air within such distillation columns produces nitrogen-rich and oxygen-rich fractions of the air as nitrogen-rich column overhead of the higher pressure column 36 and a nitrogen-rich column overhead of the lower pressure column 38, at of course a lower pressure than the nitrogen-rich vapor produced in the higher pressure column 36 and an oxygen-rich liquid as a liquid column bottoms of the lower pressure column 38. As will be discussed, streams of these fractions can be directly taken as products or condensed and/or pressurized and warm to form products of the cryogenic rectification plant 1.
The liquid air stream 30 is expanded to a pressure suitable for its introduction into higher pressure column 36 by way of a liquid turboexpander 40. Energy from liquid turboexpander 40 can be recovered and thus the liquid turboexpander can generate part of the refrigeration requirement for the cryogenic rectification plant 1. Alternatively, an expansion valve can be used (or a combination of the two). After having been expanded, liquid air stream 30 is divided into a first subsidiary expanded stream 42 and a second subsidiary expanded stream 44. Second subsidiary expanded stream 44 is expanded by an expansion valve 46 to pressure suitable for its introduction into lower pressure column 38 as a further expanded stream 47. Thus, both first and second subsidiary expanded streams 42 and 44 are introduced into intermediate locations of higher and lower pressure columns 36 and 38, respectively at points thereof that would match the composition of the mixture being separated in the columns. It is understood, however, that embodiments of the present invention are possible in which the liquid air stream 30 is introduced into either the higher pressure column 36 or the lower pressure column 38.
The rectification of the air within higher pressure column 36 produces a crude liquid oxygen column bottoms and a nitrogen-rich vapor column overhead. Part of a nitrogen-rich vapor column overhead stream 48 is condensed in condenser-reboiler 50 against vaporizing an oxygen-rich column bottoms that is produced by the rectification occurring in the lower pressure column. In this regard, such rectification also produces, within lower pressure column 38, a nitrogen-rich vapor column overhead. The resultant condensation produces a nitrogen-rich liquid stream 52. First part 54 of nitrogen-rich liquid stream 52 is returned to higher pressure column 36 as reflux. A second part 56 is subcooled within a subcooling unit 29 that as illustrated is an integral part of lower pressure heat exchanger 28 by provision of suitable passages therein. However, as would be known to those skilled in the art, subcooling unit 29 could in fact be a separate heat exchanger or separate heat exchangers operating in parallel. The resulting subcooled liquid nitrogen stream 58 is then further optionally subdivided into parts 60 and 62. Part 60 of the subcooled liquid nitrogen stream 58 is then expanded within an expansion valve 64 to a pressure suitable for its introduction to lower pressure column 38 and then introduced into lower pressure column 38 as reflux. Part 62 of subcooled liquid nitrogen stream 58 can be taken as an optional liquid product. The distillation of the air produces a crude liquid oxygen column bottoms, also known in the art as kettle liquid, in the higher pressure column 36. A crude liquid oxygen stream 58 composed of the crude liquid oxygen column bottoms is also subcooled within the subcooling unit 29 incorporated into lower pressure heat exchanger 28 and then expanded in an expansion valve 68 to be introduced into lower pressure column 38 for further refinement.
A nitrogen-rich vapor stream 70 can be removed from the top of lower pressure column 38 that consists of the lower pressure nitrogen-rich vapor column overhead produced as a result of the distillation occurring within the lower pressure column 38. Although not illustrated, as known in the art, a waste nitrogen stream could also be removed below the top of low pressure column 38 in order to maintain the purity of nitrogen-rich vapor stream 70 if the same were required in a resulting product. Since this has not been done in the illustrated embodiment, nitrogen-rich vapor stream 70 is not of typical product purity in that it is contaminated with higher amounts of oxygen than a nitrogen product stream. The nitrogen-rich vapor stream 70 is then subdivided into two subsidiary nitrogen-rich vapor streams 71 and 72. Subsidiary nitrogen-rich vapor stream 71 is partially warmed in the subcooling unit portion of the lower pressure heat exchanger 28 in order to subcool second part 56 of the nitrogen-rich liquid stream 56 and the crude liquid oxygen stream 58. The subsidiary nitrogen-rich vapor stream 71 is then fully warmed within lower pressure heat exchanger 28 to form a waste nitrogen stream 73. As illustrated, the waste nitrogen stream 73 can be used to regenerate adsorbent beds within the pre-purification unit 16 in a manner known in the art. The subsidiary nitrogen-rich vapor stream 72 is fully warmed in the higher pressure heat exchanger to form waste nitrogen stream 74. The flow rates of these streams are selected to balance the cold end temperature difference of the higher pressure heat exchanger 26 and the lower pressure heat exchanger 28. In this regard, if the temperature of liquid air stream 30 were too high, the liquid produced by the liquid turboexpander 40 after expansion to column pressure will produce too much vapor and as result, the desired distillation will not occur within the distillation columns.
An oxygen-rich liquid stream 75, composed of the oxygen-rich liquid column bottoms of lower pressure column 38, can be removed from the lower pressure column 38. A first part 76 of the oxygen-rich liquid stream 75 can be pressurized by a pump 78 to produce a pumped liquid oxygen stream 80. A second part 82 of the oxygen-rich liquid stream 75 can optionally be taken as a product. Pumped liquid oxygen stream 80, the two nitrogen-rich vapor streams 71 and 72, and as will be discussed, second subsidiary nitrogen vapor stream 84 and pumped liquid nitrogen stream 92 constitute return streams of the air separation unit 34 that are used to cool the incoming air within higher pressure heat exchanger 26 and lower pressure heat exchanger 28. As illustrated, optionally, nitrogen-rich vapor stream 48 can be divided into first and second subsidiary nitrogen vapor streams 82 and 84. First subsidiary nitrogen vapor stream 82 is introduced into condenser reboiler 50 and second subsidiary nitrogen vapor stream 84 is fully warmed within the lower pressure heat exchanger 28 and forms a nitrogen product stream 86. A third portion 88 of the nitrogen-rich liquid stream 52 can optionally be pumped in a pump 90 to produce a pumped liquid nitrogen stream 92 that is fully warmed within the higher pressure heat exchanger 26 to produce a pressurized nitrogen product stream 94. It is to be noted that if pressurized nitrogen products were desired at different pressures, pumped liquid nitrogen stream 92 could be subdivided and pumped to the different pressures. Pumped liquid oxygen stream 80 is similarly fully warmed within higher pressure heat exchanger 26 to produce a pressurized oxygen product stream 96.
As well known in the art, any cryogenic rectification plant must be refrigerated for such reasons as overcoming warm end heat exchange losses, heat leakage into the cold box containing the distillation columns and to allow for the production of liquid products. In cryogenic rectification plant 1, a part 98 of the second subsidiary compressed air stream 22 is extracted from an intermediate stage of booster compressor 20 and then, is further compressed in a booster compressor 100. Part 98 of the second subsidiary compressed air stream 22 will typically be between about 5 percent and about 20 percent of the flow of the compressed and purified air being discharged from pre-purification unit 16. After removal of the heat of compression in an after-cooler 101, such stream is partially cooled within the higher pressure heat exchanger 26 to produce a partially cooled stream 103 that is introduced into a turboexpander 104 to produce an exhaust stream 105 that is introduced into the higher pressure column 36 to impart the required refrigeration into the cryogenic rectification plant 1. As would be known in the art, this is but one option for imparting refrigeration into a cryogenic rectification plant. For example, depending upon the product make desired, the exhaust stream could be introduced into the lower pressure column 38 or a waste nitrogen stream could be expanded.
As indicated above, the boosted pressure stream 24 is fully cooled within the higher pressure heat exchanger 26. This being said, embodiments are possible in which boosted pressure stream 24 is removed prior to the cold end of the higher pressure heat exchanger 26 and as such, has a warmer temperature. In any event, the purpose of the boosted pressure stream 24 is to provide the major part of the heat transfer duty in heating the pumped liquid oxygen stream 80 and the pumped liquid nitrogen stream 92 in producing the pressurized nitrogen product stream 94 and the pressurized oxygen stream 96. In this regard, both of the pumped liquid oxygen and nitrogen streams 80 and 92 could be pressurized to a supercritical pressure and upon heating to supercritical temperatures, the resulting pressurized product nitrogen stream 94 and the pressurized production oxygen stream 96 would be supplied as supercritical fluids. However, the present invention also contemplates that such fluids would be pressurized to subcritical pressures and as such, would be vaporized upon heating to be supplied as high pressure vapor streams. It is to be noted that in place of the boosted pressure stream 24 being derived from air, other fluids could be used such as argon as shown in U.S. Pat. No. 4,345,925, discussed above, in which the boosted pressure stream circulates in a closed heat exchange loop.
In all cases, however, there is a refrigeration loss with respect to such boosted pressure streams having a magnitude that increases with the degree of warm end temperature difference. The warm end temperature difference in case of the higher pressure heat exchanger 26 is the difference, as measured at the warm end thereof, between the average temperature of the streams being cooled, namely, boosted pressure stream 24 and the part 98 of the second subsidiary compressed air stream 22 and the average temperature of the streams being warmed, namely, pumped liquid oxygen and nitrogen streams 94 and 96 and the subsidiary nitrogen-rich vapor stream 72. The greater the degree of such warm end temperature difference, the greater the amount of refrigeration that will be required. Practically, in the case of cryogenic rectification plant 1, the greater refrigeration requirement would be supplied by booster compressor 100 and therefore, a higher power consumption by such compressor. In order to decrease the warm end temperature difference, the degree of refrigeration required and therefore, the power consumption, a third subsidiary compressed air stream 106 is produced from part of the compressed and purified air stream 17. Such stream serves as a heat exchange stream that is partially cooled within the higher pressure heat exchanger 26 and then further cooled within the lower pressure heat exchanger 28 to form a cooled air stream 108 that can be combined with first subsidiary compressed air stream 18 within the lower pressure heat exchanger 28. The combination of the two streams may not be necessarily carried out within the heat exchanger. They can proceed separately to the lower column. The resultant combined stream 110 can be further combined with the exhaust stream 105 and introduced into the higher pressure column as a stream 112.
As would be known, there are other possibilities for forming such a heat exchange stream. For example, a part of the waste nitrogen stream 73 could be compressed and then partially cooled within higher pressure heat exchanger 26 in place of the illustrated heat exchange stream 106. Such stream could then be further cooled within the lower pressure heat exchanger 28 and then introduced into an intermediate location of the higher pressure column 36.
The following Table is a stream summary derived from a calculated example of the operation of the cryogenic rectification plant 1.
As would occur to those skilled in the art, although the present invention has been discussed with respect to a preferred embodiment, numerous changes, addition and omissions to such embodiment could be made in accordance with the spirit and scope of the present invention as set forth in the appended claims.
