Technical Field

(0001) The present inventions relates to a nitrogen producing cryogenic air separation unit, and more particularly, to nitrogen producing air separation units with an excess air circuit capable of producing high pressure gaseous nitrogen without the use of a nitrogen product compressor and that is also capable of producing high rates of liquid nitrogen without adding additional compression stages in the main air compressor and/or without a nitrogen recycle compressor.

Background

(0002) Prior art nitrogen producing air separation units attempting to produce high pressure gaseous nitrogen product without use of a nitrogen product compressor and also capable of producing high levels or rates of liquid nitrogen typically required raising the nitrogen column pressure by increasing the pressure of the incoming feed air, typically by adding additional stages of feed air compression to the main air compressor arrangement.

(0003) Because the nitrogen column in such nitrogen producing air separation units is operating at higher pressures, the waste nitrogen from the column is also at a higher pressure and capable of generating more refrigeration when the waste nitrogen gas is expanded in the waste expander. The additional refrigeration allows the air separation unit to be capable of greater rates of liquid nitrogen production. The main disadvantage of this prior art solution is it requires more power from the main air compressor arrangement and entails a more difficult separation process within the higher pressure nitrogen column requiring more separation stages in order to reduce the power/recovery penalty that arises from the higher column pressure. (0004) To further increase the liquid nitrogen production rates in such conventional nitrogen producing air separation plants, a motor-driven nitrogen recycle compressor and additional turbine/expander may also be added to create a supplemental refrigeration circuit or supplemental source of refrigeration. In some cases, the recycle compressor function can be included on the same machine in combined service with the main air compressor. For such cases, a customized compressor design is required, which is an appreciable capital cost addition. In addition, the nitrogen producing air separation plants that employ conventional supplemental refrigeration circuits incur additional capital costs that result from larger heat exchangers, distillation columns, and piping components due to the high flow recirculation.

(0005) What is needed therefore is an improved nitrogen producing air separation unit and cycle that is capable of producing high pressure gaseous nitrogen without use of a nitrogen product compressor and capable of producing high liquid nitrogen production rates (e.g. more than 30% of the nitrogen product as liquid nitrogen) while eliminating the need for adding additional compression stages to the combined service compressor or a separate recycle compressor and/or reducing the size and number of additional compression stages to reduce the associated capital costs.

Summary of the Invention

(0006) The present invention may be characterized as a method of providing supplemental refrigeration in a nitrogen producing air separation unit comprising the steps of: (a) compressing and purifying the incoming feed air stream to produce a compressed, purified air stream; (b) splitting the compressed, purified air stream into an excess air stream and a compressed, purified feed air stream; (c) compressing the excess air stream in one or more excess air compressors to a pressure greater than about 24 bar(a); (d) cooling the compressed, purified feed air stream in a heat exchanger to produce a fully cooled feed air stream that is directed to a distillation column system and also cooling a first portion of the further compressed excess air stream in the heat exchanger to produce a liquid air stream that is directed to the distillation column system; (e) expanding a second portion of the further compressed excess air stream in an excess air expander to produce an excess air exhaust stream; (f) combining the excess air exhaust stream with a waste stream from the distillation column system to produce a combined excess air and waste stream; (g) expanding the combined excess air and waste stream in an excess air and waste expander to produce a waste exhaust stream; and (h) warming the waste exhaust stream in the heat exchanger to provide supplemental refrigeration to cool the first portion of the further compressed excess air stream and cool the fully cooled feed air stream.

(0007) The nitrogen producing air separation unit is configured to receive an incoming feed air stream and produce a high pressure gaseous nitrogen product, preferably without a nitrogen product compressor and also produce a liquid nitrogen product. To produce such nitrogen products, the distillation column system of the air separation unit comprises at least one nitrogen column and at least one nitrogen condenser. Preferably, the distillation column system preferably includes one nitrogen column and one nitrogen condenser. Alternative embodiments include arrangements where the distillation column system comprises a dual nitrogen column arrangement with one or two nitrogen condensers. The waste stream from the distillation column system is preferably a warmed, boil-off vapor stream from one of the nitrogen condensers.

(0008) The present invention may also be characterized as a nitrogen producing air separation unit configured to receive an incoming feed air stream and produce a high pressure gaseous nitrogen product and a liquid nitrogen product, and further configured to be capable of taking more than 30% of the nitrogen product as liquid nitrogen product. The nitrogen producing air separation unit comprises: (i) an excess air circuit that includes an excess air stream diverted from the compressed, purified feed air stream, preferably at a location downstream of the main air compression and purification system; (ii) one or more excess air compressors configured to further compress the excess air stream to a pressure greater than about 24 bar(a), wherein a first portion of the further compressed excess air stream is directed to a distillation column system having at least one nitrogen column; (iii) an excess air expander configured to expand a second portion of the further compressed excess air stream to produce an excess air exhaust stream; and (iv) a waste and excess air expander configured to receive a vapor stream from the nitrogen condenser and the excess air exhaust stream and expand the vapor stream from the nitrogen condenser and the excess air exhaust stream to produce a waste exhaust stream. The waste exhaust stream is configured to be warmed in the heat exchanger against the compressed, purified feed air stream and the excess air stream to provide the supplemental refrigeration necessary to support the high liquid product make.

(0009) In one embodiment, the one or more excess air compressors further comprise a motor driven booster compressor configured to receive and further compress the excess air stream and one or more booster compressors driven by the waste and excess air expander and excess air expander. In this embodiment, the one or more excess air compressors configured to further compress the excess air stream to a pressure between about 34 bar(a) and 55 bar(a).

(00010) In another embodiment, the one or more excess air compressors further comprise a first booster compressor operatively coupled to and driven by the waste and excess air expander and a second booster compressor arranged in series with the first booster compressor, the second booster compressor operatively coupled to and driven by the excess air expander. The two booster compressors arranged in series are configured to further compress the excess air stream to a pressure between about 24 bar(a) and 35 bar(a).

(00011) In yet another embodiment, the nitrogen producing air separation unit comprises a feed air booster compressor downstream of the main air compression and purification system and configured to further compress the incoming feed air, the feed air booster compressor operatively coupled to and driven by the waste and excess air expander. In this embodiment, the one or more excess air compressors further comprise two booster compressors arranged in series and configured to receive and further compress the excess air stream wherein one of the booster compressors is operatively coupled to and driven by the excess air expander. Brief Description of the Drawings

(00012) While the present invention concludes with claims distinctly pointing out the subject matter that Applicant regards as the invention, it may be better understood when taken in connection with the accompanying drawings in which: (00013) Fig. l is a schematic process flow diagram of a conventional single column nitrogen producing cryogenic air separation unit that employs a nitrogen recycle loop to provide supplemental refrigeration to allow higher liquid nitrogen production;

(00014) Fig. 2 is a schematic process flow diagram of an embodiment of a nitrogen producing cryogenic air separation unit in accordance with an embodiment of the present invention;

(00015) Fig. 3 is a schematic process flow diagram of another embodiment of the present nitrogen producing cryogenic air separation unit;

(00016) Fig. 4 is a schematic process flow diagram of yet another embodiment of the present nitrogen producing cryogenic air separation unit;

(00017) Fig. 5 is a schematic process flow diagram of still another embodiment of the present nitrogen producing cryogenic air separation unit;

(00018) Fig. 6 is a schematic process flow diagram of an embodiment of the present nitrogen producing cryogenic air separation unit where the excess air and waste expander is generator loaded;

(00019) Fig. 7 is a schematic process flow diagram of another embodiment of the present nitrogen producing cryogenic air separation unit where the excess air and waste expander is also generator loaded; and

(00020) Fig. 8 is a schematic process flow diagram of still another embodiment of the present nitrogen producing cryogenic air separation unit with a dual column arrangement and where the excess air and waste expander is also generator loaded. Detailed Description

(00021) As discussed in more detail below, the disclosed cryogenic air separation systems and methods provide certain cost and performance benefits over conventional nitrogen producing cryogenic air separation units depicted in Fig. 1. The various embodiments of the present nitrogen producing air separation unit all utilize a uniquely configured excess air circuit. This excess air circuit includes multiple expanders and one or more booster compressors that allows the nitrogen producing air separation unit to produce high pressure gaseous nitrogen without the use of a nitrogen product compressor. This disclosed excess air circuits also allows the nitrogen producing air separation unit to achieve very high rates of liquid nitrogen production without adding additional compression stages in the main air compressor and/or without a separate nitrogen recycle compressor.

(00022) Turning now to Fig. 2, there is shown a schematic illustration of the present nitrogen producing cryogenic air separation unit 10. In a broad sense, the depicted air separation unit includes a main feed air compression train or system, an excess air circuit, a main heat exchange system, a distillation column system, and a nitrogen liquefaction system.

(00023) In the main feed compression train shown in Fig. 2, the incoming feed air 22 is typically drawn through an air suction filter house and is compressed in a multi-stage, intercooled main air compressor arrangement 24 to a pressure that can be between about 6.5 bar(a) and about 11 bar(a). This main air compressor arrangement 24 may include integrally geared compressor stages or a direct drive compressor stages, arranged in series or in parallel. The compressed air stream 26 exiting the main air compressor arrangement 24 is cooled in aftercooler and then fed to a pre-purification unit 28 to remove impurities including high boiling contaminants. The pre-purification unit 28, as is well known in the art, typically contains two beds of alumina and/or molecular sieve operating preferably in accordance with a temperature swing adsorption cycle in which moisture and other impurities, such as carbon dioxide, water vapor and hydrocarbons, are adsorbed. One or more additional layers of catalysts and adsorbents may be included in the pre-purifi cation unit 28 to remove other impurities such as carbon monoxide, carbon dioxide and hydrogen to produce the compressed, purified air stream 29. Particulates may be removed from the feed air in a dust filter disposed upstream or downstream of the pre-purifi cation unit 28.

(00024) As shown in Fig. 2, the compressed, purified air stream 29 may be split into a plurality of air streams, including an excess air stream 31 and a compressed, purified feed air stream 33. Excess air stream 31 may be further compressed in one or more excess air compressors, including a motor driven compressor 37A and a pair of booster compressors 37B, 37C and subsequently cooled in aftercoolers 39A, 39B, 39C to form a boosted pressure excess air stream 36. The boosted pressure excess air stream 36 is then directed to the main heat exchange system which includes heat exchanger 52. A first portion 38 of the boosted pressure excess air stream is partially cooled and exits the heat exchanger at an intermediate location as a partially cooled excess air stream 38 exits heat exchanger 52. The partially cooled excess air stream 38 is then expanded in expander 35 to produce exhaust stream 44 that is then further cooled in the heat exchanger 52 to form the cooled excess air exhaust stream 45.

(00025) As discussed in more detail below, the cooled excess air exhaust stream 45 is combined with a warmed waste stream 46 to form a combined excess air and waste stream 48. The combined excess air and waste stream 48 is then directed to a waste and excess air expander 40 where it is expanded to form a waste exhaust stream 49 that is warmed in heat exchanger 52. In this manner, a portion of the refrigeration for the air separation unit 10 is thus provided by the expansion of the combined excess air and waste stream 48 in expander 40 thus allowing a higher liquid nitrogen production by the air separation unit 10. The warmed waste stream 41 exits the warm end of the heat exchanger 52 and may be used as a purge gas stream during regeneration of the adsorbents and other layers in the pre-purification unit 28.

(00026) In the embodiment of Fig. 2, a motor driven compressor 37A is shown raising the pressure of the excess air stream 31 prior to its feed to the first booster compressor 37B. The motor driven compressor 37A preferably is only a single stage and its capital cost will be less than that of a separate recycle compressors shown in the conventional arrangement depicted in Fig. 1. The preferred turbine booster loading configuration is as shown, with the first booster compressor 37B loaded by the waste and excess air expander 40 and the second booster compressor 37C loaded by the excess air expander 35. The first booster compressor 37B may require gearing between the expander 40 and booster compressor 37B for an effective design. The speed of the second booster compressor 37C matches quite well with the speed of the excess air expander 35, so the machines may be directly coupled with no gearing and provide an effective design.

(00027) Turning now to the compressed, purified and cooled air streams to be directed to the distillation column system, a second portion of the boosted pressure excess air stream 32 is further cooled in heat exchanger 52 and exits the cold end of the heat exchanger 52 as a fully cooled liquid air stream 55. The fully cooled liquid air stream 55 is then introduced into distillation column system, preferably at a location proximate the bottom of the distillation column 65 in a single column arrangement as shown in Fig. 2 or in the bottom section of the higher pressure column in a dual column distillation arrangement (See. Fig. 8).

(00028) The compressed, purified feed air stream 33 is also fully cooled in heat exchanger 52 and exits the cold end of heat exchanger 52 as a fully cooled feed air stream 56 that is also introduced into distillation column system, preferably at a location several stages above the bottom of the distillation column 65 in a single column arrangement or several stages above the bottom of the higher pressure column in a dual column arrangement (See. Fig. 8).

(00029) Cooling of the second portion 32 of the boosted pressure excess air stream and the compressed, purified feed air stream 33 in the heat exchanger 52 to produce cooled air streams suitable for rectification in the distillation column system is preferably accomplished by way of indirect heat exchange with the warming streams which may include: a waste stream 59A from the distillation column system; a nitrogen product stream 57A from the distillation column system; the waste exhaust stream 49 from the excess air circuit; and a recycle stream 58 A from the nitrogen liquefaction system. The heat exchanger 52 is preferably a brazed aluminum plate-fin type heat exchanger. Such brazed aluminum heat exchangers are advantageous due to their compact design, high heat transfer rates and their ability to process multiple streams. They are manufactured as fully brazed and welded pressure vessels.

(00030) The illustrated distillation column system includes a single distillation column 65 and a main condenser-reboiler 75. The distillation column 65 typically operates in the range from between about 7.5 bar(a) to about 17 bar(a). Fully cooled feed air stream 56 and liquid air stream 55 are fed into the distillation column 65 for rectification resulting from mass transfer between an ascending vapor phase and a descending liquid phase that is initiated by a nitrogen based reflux stream. A plurality of mass transfer contacting elements, that can be trays or structured packing or other known elements in the art of cryogenic air separation are disposed within the distillation column 65. This separation process within the distillation column 65 produces a nitrogen-rich column overhead 66 and crude oxy gen-enriched bottoms liquid also known as kettle liquid which is taken as kettle stream 67. The kettle stream is preferably subcooled in subcooler 53 via indirect heat exchange against: a first part of the nitrogen-rich column overhead 66 taken as the gaseous nitrogen product stream 57B; the boil-off stream or waste stream 59B from the main condenser-reboiler 75; and the recycle stream 58B from the nitrogen liquefaction system. The subcooled kettle stream 68 is directed to the main condenser-reboiler 75 to condense a clean shelf nitrogen stream 69 taken as second part of the nitrogen-rich column overhead 66.

(00031) The condensation produces a liquid nitrogen stream 71 exiting the main condenser-reboiler 75 that is separated into a first portion, referred to as the reflux stream 73, that is released into the distillation column 65 to initiate the formation of descending liquid phase therein and a second portion, referred to as the liquefaction feed stream 72, that is fed to the nitrogen liquefaction system. (00032) The boil-off stream from the main condenser-reboiler 75 is a waste stream that is warmed in subcooler 53 and main heat exchanger 52. The warmed waste stream 46 is combined with the cooled excess air exhaust stream 45 to form the combined excess air and waste stream 48 and directed to the waste and excess air expander 40 where it is expanded with the resulting exhaust stream directed to main heat exchanger to provide the supplemental refrigeration necessary to allow higher liquid nitrogen production. Combining the warmed waste stream 46 with the cooled excess air exhaust stream 45 may occur outside the main heat exchanger, as illustrated or may occur within the brazed aluminum heat exchanger with the use of a large side header configured to receive both streams from their respective heat exchange passages.

(00033) In the illustrated embodiment, the nitrogen liquefaction system is depicted as a subcooler 80 that is configured to subcool the liquefaction feed stream 73 to produce a subcooled liquid nitrogen stream 82. A liquid nitrogen product stream 84 is taken as a first portion of the subcooled liquid nitrogen stream while the remaining portion 86 of the subcooled liquid nitrogen stream is used as the cooling medium in subcooler 80 after being let down in pressure. The warmed nitrogen stream exiting subcooler 80 is recycled to the main heat exchanger 52 as recycle stream 58A. The recycle stream exiting the main heat exchanger 52 is a nitrogen vapor stream that may be recycled to the main air compression train or system.

(00034) The present system and method differs from the conventional high liquid make nitrogen producing air separation units (See Fig. 1) in that a diverted excess air stream 31 is used to generate supplemental refrigeration in order to eliminate the recycle compressors of the conventional high liquid make nitrogen producing air separation units (See Fig. 1). In order to most effectively utilize the excess air stream 31 it is necessary that the excess air stream be expanded over a high pressure ratio. This is accomplished by first raising the pressure of the excess air stream using one or more upstream motor-driven and/or booster compressors and also by using at least two turbines/expanders. The boosting is preferably done with some or all of the booster compressors absorbing the refrigeration energy of each turbine/expander. Also, the refrigeration needs of the present system allow for a relatively warm inlet temperature to the waste and excess air expander 40 without creating an excessively pinched temperature profile in the main heat exchanger 52. Comparatively, the power consumption for the arrangement depicted in Fig. 2 is similar to the power consumption of the conventional high liquid production air separation unit shown in Fig. 1 at comparable feed air and nitrogen product flows.

(00035) Using a relatively warm inlet temperature allows the present system to take advantage of the higher energy a warmer excess air expander provides so that the pressure rise in the booster is higher. In order to further expand the excess air after the excess air expander 35 the flow is further cooled in the main heat exchanger 52 and combined with the warming waste stream for expansion in the waste and excess air expander. Combining these streams and feeding the combined stream to the waste and excess air expander 40 is an important and key feature of the present system and method. By using the combined stream and the waste and excess air expander 40 for the second stage of expansion, there is no need for a third expander.

(00036) Turning to Fig. 3, there is shown a schematic diagram of an alternate embodiment of the present system and method. Many of the features, components and streams associated with the nitrogen producing air separation unit 11 shown in Fig. 3 are similar or identical to those described above with reference to the embodiment of Fig. 2 and for sake of brevity will not be repeated here. The key differences between the nitrogen producing air separation unit 11 illustrated in Fig. 3 compared to the air separation unit 10 in the arrangement shown in Fig. 2 is the absence of the upstream, motor-driven booster compressor 37A and aftercooler 39A in the excess air circuit. (00037) In the embodiment of Fig. 3, since the small motor driven compressor 37A is eliminated, the excess air flow must be increased to produce the same liquid product rate. In this embodiment, there are capital cost savings realized by eliminating the small motor driven compressor 37A and aftercooler 39A which is partially offset by a slightly higher overall power cost required to compress the higher excess air flow. Note that for a lower liquid make production rate, the power savings incentive for the motor driven compressor of Fig. 2 is reduced as the power penalty associated with the embodiment of Fig. 3 becomes smaller as the liquid production rate decreases.

(00038) Also, without the motor driven compressor 37A, the pressure of the excess air stream 36 directed to the heat exchanger 52 is lower compared to the pressure of excess air stream 36 in Fig. 2. This means that the liquid air condensing pressure will also be lower, probably subcritical. As a result, each expander 35, 40 in the embodiment of Fig. 3 must be designed to operate at a somewhat lower temperature than corresponding expander in the embodiment of Fig. 2. Depending on the liquid make flow requirements and the product nitrogen pressures, the waste and excess air expander 40 in any of the disclosed configurations may operate at a sufficiently low temperatures that the waste exhaust stream 49 may be reconfigured to feed the cold end of the subcooler 53.

(00039) The embodiment depicted in Fig. 4 shows a further arrangement of the nitrogen producing air separation unit 12 where both turbine-booster arrangements (37D, 40) and (37C, 35) operate at near ideal design parameters so that they can achieve good efficiency with no gearing. Again, as many of the features, components and streams associated with the nitrogen producing air separation unit 12 shown in Fig.

4 are similar or identical to those described above with reference to the embodiment of Figs. 2 and 3, the associated descriptions will not be repeated here. Similarly to the embodiments of Figs. 2 and 3, the column pressure in the arrangement depicted in Fig.

5 has been set to deliver the gaseous nitrogen product 57 to the customer without further compression (i.e. without the need for a nitrogen product compressor).

(00040) The first key difference is the booster compressor 37D powered by the waste and excess air expander 40 further compresses the entire compressed, purified air stream 29, rather than just the excess air stream. Now booster compressor 37B is designed or configured to handle much more flow and creates a lower pressure ratio. As a result, the optimal speed of booster compressor 37B is much lower than the speed of the booster compressors coupled to the waste and excess air expander depicted in Figs. 2 and 3. More importantly, the speed of booster compressor 37B is comparable or generally matches the optimal speed of waste and excess air expander 40. The concomitant benefits of eliminating the gearing are less mechanical losses and some capital cost reduction.

(00041) With the lower pressure excess air stream 31 exiting the first booster compressor 37D, the motor driven compressor 37A is needed to raise the pressure of the excess air stream unless the design liquid rate is significantly lower than about 60% of the total nitrogen product, and/or the product pressure is higher than about 120 psig. For a liquid rate make of about 60%, the motor driven compressor 37A depicted in Fig. 5 will preferably be one or two stages.

(00042) The operational liquid turndown range of the arrangement depicted in Fig. 4 will be more limited than that of the other configurations, especially when no product gas nitrogen compression is used. This is because the discharge pressure of the main air compressor 24 must increase as the liquid production rate is decreased in order to provide the air stream 33 from the first booster to the distillation column 65 at sufficient pressure. To turn down the liquid production rate or liquid make rate, the excess air stream 31 flow is decreased. This will reduce the flow of the compressed, air stream 26 from main air compressor 24. But the lower excess air stream flow will also tend to reduce the discharge pressure of the motor driven compressor 37A. Also, with the lower excess air stream flow, the power required to drive the booster compressors is less so the booster compressors will generate less pressure rise even though they are compressing less flow. This is unlike the other illustrated embodiments, where the discharge pressure from main air compressor 24 will decrease slightly as the liquid production rate is decreased, which benefits turndown operation. The use of a variable speed drive such as a direct drive motor for the motor driven compressor can resolve this issue for this configuration if a wide liquid turndown range is important.

(00043) Fig. 5 depicts yet another embodiment of the present system and method that incorporates a nitrogen-based refrigeration cycle that includes a nitrogen recycle compressor and a recycle nitrogen expander. Again, as many of the features, components and streams associated with the nitrogen producing air separation unit 15 shown in Fig. 5 are similar or identical to the embodiment of Figs. 2-4, the associated descriptions will not be repeated here. Also, similarly to the embodiments of Figs 2-4, the gaseous nitrogen product 57 is delivered to the customer without the need for a nitrogen product compressor.

(00044) In the embodiment of Fig. 5, the excess air stream 31 is a diverted portion of the compressed, purified air stream 29 and is combined with a warmed waste stream 46. The warmed waste stream 46 is raised in pressure to the same pressure as the excess air stream 31 by boosting it in booster compressor 37B after it has been fully warmed in the main heat exchanger 52 using the work energy from the waste and excess air expander 40. Since this booster compressor 37B must only raise the pressure of the warmed waste stream 46 to match the pressure of the excess air stream 31, only a fixed amount of excess air is passed through the waste and excess air expander 40 to power the booster compressor 37B. A gear may be needed between the waste and excess air expander 40 and the booster compressor 37B to enable good aerodynamic efficiencies of the machines.

(00045) The combined excess air and waste stream 48 is then directed to the waste and excess air expander 40 where it is expanded to form a waste exhaust stream 49 that is warmed in heat exchanger 52. In this manner, a portion of the refrigeration for the air separation unit 10 is thus provided by the expansion of the combined excess air and waste stream 48 in expander 40. The warmed waste stream 41 exits the warm end of the heat exchanger 52 and may be used as a purge gas stream during regeneration of the adsorbents and other layers in the pre-purification unit 28.

(00046) Because the excess air stream 31 is only provided at a pressure a similar to the discharge pressure of the main air compressor 24 and expanded once across a single turbine/expander, the supplemental refrigeration provided by the excess air circuit is limited. The remainder of the supplemental refrigeration required for high liquid nitrogen production is provided by a nitrogen-based refrigeration circuit. The nitrogen-based refrigeration circuit includes a nitrogen recycle compressor 97A, a booster compressor 97 C and a recycle nitrogen expander 95. A portion of product nitrogen 57 may be provided to the nitrogen-based refrigeration circuit together with a nitrogen recycle exhaust stream 98 and is compressed in recycle compressor 97A and further compressed in booster compressor 97C, with aftercoolers 99A and 99C used to remove some of the heat of compression. The compressed, cooled nitrogen recycle stream 96 is then directed to the heat exchanger 52 where the stream is split. A first portion of the compressed, cooled nitrogen recycle stream 91 is expanded in the nitrogen recycle expander 95 with the resulting nitrogen recycle exhaust stream 98 being warmed in heat exchanger 52 and recycled to the nitrogen recycle compressor 97A. A second portion of the compressed, cooled nitrogen recycle stream 92 is fully cooled in the heat exchanger 52 to produce a liquid nitrogen feed stream 92 that is directed to the distillation column 65. In this manner, the nitrogen recovery is enhanced, in part, because the stream of liquid nitrogen 92 is supplied to a location proximate the top of the distillation column 65 in the illustrated single column arrangement rather than the liquid air stream 55 in the previously disclosed embodiments. In a dual column arrangement the stream of liquid nitrogen 92 may be supplied to a location proximate the top of the higher pressure column.

(00047) Advantages of the air separation unit arrangement depicted in Fig. 5 include the capability of a wider and more efficient turndown operation without sacrificing nitrogen recovery. Also, the net power consumption of the embodiment depicted in Fig. 5 is comparable to the power consumption of the prior art arrangement depicted in Fig. 1. Nitrogen recovery is enhanced, in part, because the stream of liquid nitrogen 92 is supplied to the distillation column 65 rather than liquid air stream 55 in the previously disclosed embodiments in Figs. 2-4.

(00048) Turning now to Figs. 6 and 7, there are shown schematic diagrams of yet further embodiments of the present system and method. In these embodiments, the waste and excess air expander is generated loaded rather than boosted loaded. Such arrangements are suitable for applications where gearing between the booster compressor and waste and excess air expander is to be avoided. (00049) Specifically, Fig. 6 is an alternative arrangement of the nitrogen producing air separation unit 16 to that shown and described above with reference to Fig. 2 with a generator 140 loaded excess air and waste expander 40. Without the booster compressor 37B coupled to the waste and excess air expander 40, the liquid making capability of the air separation cycle may be reduced. However, to mitigate or ameliorated this issue, the discharge pressure from the motor driven compressor 37A is increased and/or the flow of the excess air stream 31 is increased. A higher discharge pressure from the motor driven compressor 37A may require an additional compression stage. Alternatively, one can keep the existing motor driven compressor 37A and the flow of the excess air stream 31 similar to the embodiment of Fig. 2 but increase the discharge pressure from the main air compressor 24.

(00050) Similarly, Fig. 7 illustrates an alternative arrangement of the nitrogen producing air separation unit 17 to that shown and described above with reference a generator loaded waste expansion version of the Fig. 3. Since this embodiment does not have a separate, motor driven compressor, this arrangement of the will be most applicable to lower liquid product rate applications. However, liquid production can be increased in this configuration, again by increasing the flow of the excess air stream 31 flow or by increasing the discharge pressure of the main air compressor 24, if that is feasible or possible.

(00051) It should also be pointed out that for lower liquid rate applications most suitable for the embodiments of Figs 6 and 7, with relatively low flows of the excess air stream 31, it may be preferable to operate the excess air expander 35 at or near ambient temperatures. In other words, the boosted excess air stream 36 would be passed directly to the excess air expander 35 from the aftercooler 39C without any further cooling in the heat exchanger 52. With lower excess air flow rates this can be done without the occurrence of an excessively large warm end temperature difference in the heat exchanger 52. Hence, the penalty for lost refrigeration at the warm end of the heat exchanger 52 is not large. In this way, a further benefit for operating the excess air expander 35 at an even warmer temperature, with the subsequent increase in refrigeration production is exploited.

(00052) As suggested above, the present system and method are equally applicable to single column nitrogen producing air separation units and dual column nitrogen producing air separation units. For example, the present nitrogen producing air separation unit may be configured with a single nitrogen column and single nitrogen condenser as generally shown and described with reference to Figs, 2-7. Alternatively, the present nitrogen producing air separation unit may be configured to include two nitrogen columns and the at least one nitrogen condenser. Still further, as shown in Fig. 8, the present nitrogen producing air separation unit 18 may comprises two or more nitrogen columns, including a lower pressure column 170 and a higher pressure column 165 linked in a heat transfer relationship with a main condenser-reboiler 275 together with additional nitrogen condenser 175, and streams 167, 168, together with optional pump 177 to supply a reverse reflux nitrogen stream 178 from the top nitrogen condenser 175 to the higher pressure column 165 so that the nitrogen comes out at a single pressure. This would be the configuration needed to avoid any product compression. The higher pressure column 165 of the two column distillation column arrangement typically operates in the pressure range of between about 8.5 bar(a) to about 17 bar(a).

(00053) While the present nitrogen producing air separation unit capable of producing high pressure gaseous nitrogen without the use of a nitrogen product compressor and also capable of producing high rates of liquid nitrogen without adding additional compression stages in the feed air compressor has been described with reference to several preferred embodiments, it is understood that numerous additions, changes and omissions can be made without departing from the spirit and scope of the present inventions as set forth in the appended claims.

(00054) For example, a variation of the embodiments of Fig. 2 or Fig. 4 may use an integrally geared ‘bridge’ machine that operatively couples all the turbine/expander and compressor stages in the excess air circuit. The integrally geared ‘bridge’ machine preferably drives compressor 37 A, and the two booster compressors 37B and 37C from the collective work or energy supplied by the motor, the excess air expander 35, and waste and excess expander 40. The integrally geared ‘bridge’ machine typically includes a large diameter bull gear with several meshing pinions upon the ends of which the various compression impellers are mounted forming the plurality of compression stages. The pinions may also have differing diameters to best match the speed requirements of the coupled compression impellers and expanders. Since the excess air expander 35 and booster compressor 37C operate optimally at about the same speeds, they will preferably be driven from the same pinion.

(00055) Another contemplated variation would be the design of the various heat exchangers. The liquid nitrogen subcooler 80 as well as the subcooler 53 and the heat exchanger 52 are separate heat exchangers as generally shown and described with reference to Figs, 2-7. In practice, these components may be separate heat exchanger cores or may be integrated so that the liquid nitrogen subcooler 80 is combined with the subcooler 53. Alternatively, subcooler 53 may be integrated with heat exchanger 52 or both subcoolers 53 and 80 may be combined with the heat exchanger 52.