CRYOGENIC AIR SEPARATION UNIT WITH FLEXIBLE LIQUID PRODUCT MAKE

Abstract

A cryogenic air separation unit that provides flexibility in the production of liquid products is disclosed. The present cryogenic air separation unit and associated operating methods involves the use of a dual nozzle arrangement for the main heat exchanger that allows a turbine air stream draw from the main heat exchanger at different temperatures to provide refrigeration to the cryogenic air separation unit which, in turn, enables different production modes for the various liquid products.

Description

TECHNICAL FIELD

The present invention relates to a cryogenic air separation unit, and more particularly, to a system and method for providing flexibility in the production of liquid products from the cryogenic air separation unit. Still more particularly, the present system and method involves a dual nozzle arrangement for a main heat exchanger of a cryogenic air separation unit that allows a turbine air stream draw from the main heat exchanger at one of two different temperatures to provide supplemental refrigeration required for different liquid product makes. For sake of clarity, a cryogenic air separation unit as used herein refers to a system and apparatus for separation of air into by its components, namely oxygen, nitrogen, and argon by means of the fractional distillation process.

BACKGROUND

It has long been known to separate air by cryogenic rectification, or more specifically a fractional distillation process. In such processes, the incoming feed air to be separated is pressurized, purified and then cooled to a temperature suitable for its rectification and then introduced into one or more distillation columns. Each of the distillation columns has various mass transfer elements such as trays or packing, for example, structured packing, which bring liquid and vapor phases of the gaseous mixtures within the distillation columns into contact with one another and effectuate mass transfer between the vapor and liquid phases. The incoming feed air stream is thereby distilled within the distillation column or columns to form component streams enriched in the components of the gaseous mixture, namely nitrogen, oxygen or argon. The nitrogen, oxygen and argon streams can be taken as liquid products and/or gaseous products and are typically used in the cooling of the incoming feed air, which takes place through indirect heat exchange within a main heat exchanger. For example, it is well known to mechanically pump a liquid product, such as an oxygen-rich liquid column bottoms stream to the main heat exchanger where it is vaporized against the liquefying compressed air stream.

Additional or supplemental refrigeration for the cryogenic air separation unit is often generated by expanding a stream made up of a portion of the compressed and purified feed air in a turboexpander and introducing the expanded stream into at least one of the distillation columns. The additional refrigeration provided by the expansion of the portion of the compressed and purified feed air in the turboexpander typically offsets any refrigeration losses in the warm end of the cryogenic air separation unit and/or enables production of liquid products from the cryogenic air separation unit.

Most cryogenic air separation units are typically designed, constructed and operated to meet the base load product slate requirements for one or more end-user customers and optionally the local or merchant liquid product market demand. The base load product slate requirements typically include a target volume of high pressure gaseous oxygen, as well as various co-products such as gaseous nitrogen, liquid oxygen, liquid nitrogen, and/or liquid argon. The air separation units are designed and operated based, in part, on the selected design conditions, including the target product slate, typical day ambient conditions as well as the available utility/power supply costs and conditions.

Over time, the product slate requirements for some air separation units change, and in particular, there are often changes in local demand for liquid products from merchant customers in the area surrounding the air separation units. Other changes to product slate requirements may arise in a shorter timeframe when, for example, the production of higher volumes of liquid products (i.e. higher liquid make) is more economically feasible due to lower utility/power costs.

To meet these varying liquid product demands, typically expressed as a percent of the incoming feed air, it is desirable to change the air separation unit operating characteristics in order to adjust the product slate requirements. It is well known that the liquid production rate requires supplemental refrigeration and that the additional or supplemental refrigeration provided by the expansion of the compressed and purified feed air in the turboexpander is dependent on the temperature and flow of the compressed air stream directed to the turboexpander, as well as the pressure ratio across the turboexpander. Thus, in order to change liquid production rates, it is conventional practice to adjust the flow of the compressed air stream to the turboexpander. See, for example, U.S. Pat. No. 5,412,953.

Another possibility in controlling liquid production from a cryogenic air separation unit is to vary the expansion ratio of the turboexpander by increasing or decreasing the pressure of the compressed air stream to the turboexpander. This prior art solution can result in a poor turbine efficiency and control problems in that as the pressure changes at a fixed temperature, the volume flow of the turbine air often changes significantly and gets far away from the best efficiency condition. At the extreme condition, the turbine air stream to be expanded may even be liquefied at the exhaust or outlet of the turboexpander. In such situations, the turboexpander would not only suffer from poor efficiency, but also may incur potential damage as a result of such unintended liquefaction. At the other extreme, as the pressure of the turbine air stream is decreased at a fixed inlet temperature, the temperature of the expanded stream increases. If the exhaust stream temperature is above the saturation temperature of the stream feed to the distillation column, liquids within the distillation column may vaporize resulting in high local vapor flows, loss of separation performance and potential distillation column flooding.

It has been known to control the turboexpander inlet temperature of an air separation unit in order to prevent liquefaction in the turboexpander exhaust. One such example of controlling turbine air stream temperature is disclosed in U.S. Pat. No. 3,355,901. In this prior art system the turbine air stream is comprised of a mixture of two streams, the first stream being a compressed and purified stream of air that is cooled in the main heat exchanger and the second stream being a compressed and purified stream of air that bypasses the heat exchanger. The first and second streams are then combined and introduced into the inlet of the turboexpander. By controlling the relative flows of the two streams, the temperature of the turbine air stream is controlled. More specifically, the disclosed control system senses the turboexpander exhaust temperature which is fed as an input into the control system to control a valve that in turn controls flow of one of the two streams, namely the first stream that is cooled within the main heat exchanger.

Another example of controlling the turbine air stream temperature is disclosed in U.S. Pat. Nos. 8,020,408 and 9,038,413. In these prior art systems the turbine air stream is also comprised of a mixture of two streams. The first stream is a compressed and purified stream of air that is partially cooled in the main heat exchanger to a first temperature and the second stream being is a compressed and purified stream of air that is partially cooled in the main heat exchanger to a second temperature. The first and second streams are then combined in a downstream static mixer before being introduced into the inlet of the turboexpander. The flow rates of the two streams from the dual nozzles of the main heat exchanger are adjusted to control the inlet temperature to the turboexpander supplying refrigeration and to minimize potential deviation of the turboexpander exhaust from a saturated vapor state.

The prior art dual nozzle arrangements disclosed in U.S. Pat. Nos. 9,038,413 and 8,020,408 have several disadvantages, specifically a likely higher pressure drop resulting from the split paths and multiple open or partially open valve arrangements required by the disclosed system and potential entropy loss that would likely occur when the colder second stream is mixed with warmer first stream.

What is needed therefore, is an improved dual nozzle arrangement of a main heat exchanger of an air separation unit that allows for changes or adjustments in liquid production rates from the air separation unit by means of adjusting the inlet temperature of the compressed turbine air stream to the turboexpander in either a very short timeframe rapid changes to liquid product make due to utility/power costs) or in longer timeframes (i.e. longer term changes in product slate requirements from local merchant customers). Preferably, the improved dual nozzle arrangement of the main heat exchanger of an air separation unit allows withdrawal of a turbine air stream from the main heat exchanger at one of two different temperatures to provide the supplemental refrigeration required for different liquid product makes.

SUMMARY OF THE INVENTION

The present invention may be characterized as an air separation unit comprising: (i) a main compressor arrangement configured to compress the feed air stream; (ii) an adsorption based pre-purifier configured to produce a compressed and purified feed air stream; (iii) a feed air stream circuit configured to divide the compressed and purified feed air stream into at least two streams including a boiler air stream and a turbine air stream; wherein the boiler air stream is further compressed and cooled in a main heat exchanger to a cold-end temperature; (iv) the turbine air stream is optionally compressed and also partially^- cooled in the main heat exchanger, wherein the partially cooled turbine air stream is discharged from the main heat exchanger as a first subsidiary stream at a first temperature wanner than the cold-end temperature or as a second subsidiary stream at a second temperature colder than the first temperature but warmer than the cold-end temperature. The partially cooled first subsidiary stream or the partially cooled second subsidiary stream is then expanded to produce an exhaust stream which is introduced into the distillation column system of the air separation unit to produce products, including one or more liquid products.

The air separation unit is further configured to operate in a low liquid make mode with the turboexpander configured to expand only the first subsidiary stream or to operate in a high liquid make mode with the turboexpander configured to expand only the second subsidiary stream. The low liquid make mode produces the liquid products in the amount of about 2.5 percent or less of the total feed air stream whereas the high liquid make mode produces the liquid products in the amount of about 4.5 percent or more of the total feed air stream. Some embodiments of the air separation unit can also operate in a medium liquid make variable mode with the turboexpander configured to expand a mixture of the first subsidiary stream and the second subsidiary stream. Such medium liquid make variable mode producing the liquid products in the amount of between about 2 percent and about 5 percent of the total feed air stream.

The turbine air stream preferably comprises between about 60 percent and 75 percent of the compressed and purified feed air stream. In some embodiments of the present air separation unit, a portion of the turbine air stream is fully cooled within the main heat exchanger to the cold-end temperature and at a medium pressure greater than the pressure of the compressed and purified feed air stream and less than the pressure of the compressed boiler air stream. This third subsidiary medium pressure stream preferably bypasses the turboexpander and is passed directly to the distillation columns.

The different operating modes of the cryogenic air separation unit are preferably enabled by controlling a first valve disposed between the main heat exchanger and the turboexpander that is configured to adjust the flow of the first subsidiary stream a second valve disposed between the main heat exchanger and the turboexpander and configured to adjust the flow of the second subsidiary stream. Adjusting the flow of the first subsidiary stream and second subsidiary stream exiting the main heat exchanger adjusts the inlet temperature of the stream sent to the turboexpander and the corresponding outlet temperature of the exhaust stream. The first valve may be an on-off valve configured to be in an open position allowing flow of the first subsidiary stream to exit the main heat exchanger or a closed position preventing flow of the first subsidiary stream from exiting the main heat exchanger. Likewise, the second valve may also be an on-off valve configured to be in an open position allowing flow of the second subsidiary stream to exit the main heat exchanger or a closed position preventing flow of the second subsidiary stream from exiting the main heat exchanger.

In yet other embodiments of the present cryogenic air separation unit, there is included a bypass circuit configured to direct all or a portion of the turbine air stream to bypass the one or more turbine air stream compressors. Where the bypass circuit arrangement is used the air separation unit is configured to operate in one or more medium liquid make modes with all or a portion of the turbine air stream bypassing at least one of the one or more compressors prior to entering the main heat exchanger. The bypassed turbine air stream is then partially cooled in the main heat exchanger and discharged therefrom as the first subsidiary stream at a first temperature warmer than the cold-end temperature or as the second subsidiary stream at a second temperature colder than the first temperature but warmer than the cold-end temperature. The partially cooled first subsidiary stream or the partially cooled second subsidiary stream is then expanded to produce an exhaust stream which is introduced into the distillation column system of the air separation unit to produce products, including one or more liquid products. In such medium liquid make modes, liquid products can be produced at a level of between about 2 percent and about 5 percent of the total feed air stream.

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 invention will be better understood when taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic representation of a cryogenic air separation unit in accordance with the present invention;

FIG. 2 is a schematic representation of yet another embodiment of a cryogenic air separation unit in accordance with the present invention;

FIG. 3 is a schematic representation of a cryogenic air separation unit in accordance with the present invention;

FIG. 4 is a schematic representation of a cryogenic air separation unit in accordance with the present invention; and

FIG. 5 is a perspective view of a heat exchanger distributor suitable for use with embodiments of the heat exchanger used in the present invention.

DETAILED DESCRIPTION

The improved dual nozzle arrangement of a main heat exchanger in an air separation unit provided herein allows for rapid changes or adjustments in liquid production rates in a very short timeframe resulting from external operating conditions such as changes in utility/power costs. In addition, the design of the main heat exchanger with dual nozzles allows the performance of the main heat exchanger to be altered in longer timeframes, when for example changes in base load product slate requirements occur. By designing a main heat exchanger that is capable of satisfying different product slate requirements, and particularly different liquid make modes, the flexibility of the air separation unit is improved.

Turning now to FIGS. 1-4, there is shown simplified illustrations of a cryogenic air separation unit 10. In a broad sense, the cryogenic air separation unit 10 includes a main feed air compression train 20, a turbine air compression circuit 30, a boiler air compression circuit 40, a heat exchanger arrangement 50, a turbine based refrigeration circuit 60 and a distillation column system 70. As used herein, the main feed air compression train 20, the turbine air compression circuit 30, and the booster air compression circuit 40, collectively comprise a ‘warm-end’ air compression circuit. Similarly, the heat exchanger arrangement 50, portions of the turbine based refrigeration circuit 60 and portions of the distillation column system 70 are referred to as the ‘cold-end’ equipment that are typically housed in one or more insulated cold boxes.

WARM END AIR COMPRESSION CIRCUIT

In the main feed compression train 20 shown in FIGS. 1-4, the incoming feed air 22 is drawn through an air suction filter house and compressed in a multi-stage, intercooled main air compressor arrangement 24 to a pressure that can be between about 5 bar(a) and about 15 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 exiting the main air compressor arrangement 24 is typically fed to an aftercooler with integral demister to remove the free moisture in the stream. The heat of compression from the final stages of compression for the main air compressor arrangement 24 is typically removed in one or more aftercooler(s) by cooling the compressed feed air with cooling tower water. The condensate from this aftercooler as well as some of the intercoolers in the main air compression arrangement is preferably piped to a condensate tank and used to supply water to other portions of the air separation unit.

The cool, dry compressed air feed 26 is then purified in a pre-purification unit 28 to remove high boiling contaminants from the cool, dry compressed air feed 26. The pre-purification unit 28, as is well known in the art, typically contains two beds of alumina and/or molecular sieve operating in accordance with a temperature and/or pressure swing adsorption cycle in which moisture and other impurities, such as carbon dioxide, water vapor and hydrocarbons, are adsorbed. While one of the beds is used for pre-purification of the cool, dry compressed air feed 26 while the other bed is regenerated, preferably with a portion of the waste nitrogen from the air separation unit. The two beds switch service periodically. Particulates are removed from the compressed, pre-purified feed air in a dust filter disposed downstream of the pre-purification unit 28 to produce the compressed, purified feed air stream 29.

As described in more detail below, the compressed, purified feed air stream 29 is separated into oxygen-rich, nitrogen-rich, and argon-rich fractions in a plurality of distillation columns including a higher pressure column 72, a lower pressure column 74, and optionally, an argon column arrangement, which may include the illustrated superstaged argon column 76, an argon condenser 78 which are coupled to the lower pressure column 74 and arranged or configured with to produce a crude argon stream 170.

Prior to such distillation however, the compressed, pre-purified feed air stream 29 is split into a plurality of feed air streams, including a boiler air stream 42 and a turbine air stream 32 described in more detail below. The boiler air stream 42 and turbine air stream 32 are further compressed and cooled to temperatures required for fractional distillation in the distillation columns. Cooling the boiler air stream 42 to cryogenic temperatures of between about 96 Kelvin and 100 Kelvin is preferably accomplished by way of indirect heat exchange in a primary or main heat exchanger 52 with the warming streams which include the one or more oxygen, nitrogen and/or argon streams from the distillation column system 70. Refrigeration for the cryogenic air separation unit 10 is also typically generated by the turbine air stream 32 and associated cold and/or warm turbine arrangements disposed within the turbine based refrigeration circuits 60 and/or any optional closed loop warm refrigeration circuits.

In the illustrated embodiments, the compressed and purified feed air stream is divided into a first stream 42, and a second stream 32. First stream 42, often referred to as the boiler air stream, is generally about 25% to 40% of the compressed and purified feed air stream and is yet further compressed within a boiler air stream compressor arrangement 40, which preferably comprises yet another single or multi-stage intercooled compressor 41 and aftercooler 43. As with the main air compressor arrangement 20, this boiler air stream compressor arrangement 40 may include an integrally geared compressor or a direct drive compressor. This boiler air stream compressor arrangement 40 further compresses the first boiler air stream 42 to a targeted pressure between about 25 bar(a) and about 70 bar(a) to produce a further compressed boiler air stream 49. The further compressed boiler air stream 49 is directed or introduced into aftercooler 43 to produce the compressed boiler air stream 45. Compressed boiler air stream 45 is then directed to main heat exchanger 52 where it is used to boil a liquid oxygen stream 188 via indirect heat exchange to produce a high pressure gaseous oxygen product stream 190. The compressed and cooled boiler air stream 45 is may be subsequently divided into air streams 46A and 46B which are then partially expanded in expansion valve(s) 47 and 48 and for introduction into the lower pressure column 74 and higher pressure column 72 respectively. The target pressure of the compressed boiler air stream 45 is generally dictated by the product requirements for the high pressure gaseous oxygen product stream 190. The temperature of the cooled and compressed boiler air stream 46 exiting the main heat exchanger 52 is preferably between about 96 Kelvin and 100 Kelvin which represents a cold-end temperature of the main heat exchanger. In many applications, the boiler air compressor 41, turbine air compressor 34, turbine booster compressor 36 and turbine 62 could be configured in one integrally geared machine.

As illustrated, second stream 32, often referred to as the turbine air stream 32, is generally about 60% to 75% of the compressed and purified feed air stream 29 and is optionally further compressed in one or more turbine air compressors 34 and 36, cooled in aftercoolers 37 and directed as stream 35 to the main heat exchanger 52 where it is partially cooled prior to being directed to the turbine based refrigeration circuit 60, as described below. The target pressure of the further compressed turbine air stream 35 is preferably between about 20 bar(a) and about 60 bar(a).

In some embodiments of the present system (See FIGS. 3 and 4), liquid production in the cryogenic air separation unit 1, including a pressurized liquid oxygen product stream and a liquid nitrogen product stream, may be further varied by varying the pressure in the turbine air stream to the turboexpander. This variation in pressure can be effectuated by a turbine air stream bypass circuit 39 which includes a bypass line having a bypass valve 38 that can be set in an open or closed position.

The bypass circuit is configured to direct all or a portion of the turbine air stream to bypass at least one of the one or more turbine air compressors 36. In the illustrated embodiments, the bypassed turbine air stream 31 is diverted via bypass valve 38 directly to the main heat exchanger 52 or other heat exchanger used in the turbine based refrigeration circuit 60. Preferably, the target pressure of the bypassed turbine air stream 31 is between about 10 bar(a) and about 30 bar(a). Other means for varying the pressure in the turbine air stream include use of variable inlet vanes on one or more of the turbine air compressors to vary the pressure of turbine air stream sent to the turboexpander. Also, in some embodiments that utilize the bypass circuit, it may be advantageous to provide a source of make-up nitrogen that is directed to the turbine air stream compressors in lieu of the turbine air stream so as to not damage the turbine air compressor. In such arrangements, the compressed make-up nitrogen may be vented or directed to another location of the cryogenic air separation unit.

COLD END SYSTEMS AND EQUIPMENT

The main heat exchanger 52 is preferably a brazed aluminum plate-fin type heat exchanger. Such 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. For small air separation units, a heat exchanger comprising a single core may be sufficient whereas for larger air separation units handling higher flows, the main heat exchanger may be constructed from several cores connected in parallel or series.

Turbine based refrigeration circuits 60 are often referred to as either a lower column turbine (LCT) arrangement or an upper column turbine (UCT) arrangement which are used to provide refrigeration to a two-column or three column fractional distillation system 70. In the LCT arrangement shown in FIGS. 1 and 2, the compressed turbine air stream 35 is preferably at a pressure in the range from between about 20 bar(a) to about 60 bar(a). The compressed turbine air stream 35 is directed or introduced into main heat exchanger 52 where it is partially cooled to a temperature in a range of between about 130 Kelvin and about 220 Kelvin to form a partially cooled, compressed turbine air stream 61 that is subsequently introduced into a turboexpander 62 to produce an exhaust stream 64 that is introduced into the higher pressure column 72 of distillation column system 70. In some embodiments, turboexpander 62 may be coupled with one or more turbine air compressors 34 and/or 36 used to further compress the turbine air stream, either directly or by appropriate gearing.

In the illustrated embodiments of the present cryogenic air separation unit 10, the heat exchanger associated with the turbine based refrigeration circuit 60 includes a dual nozzle arrangement. In the dual nozzle arrangement, the heat exchanger 52 is configured to receive the turbine air stream 35 and discharge a first subsidiary stream 66 at a first temperature wanner than the cold-end temperature and/or discharge a second subsidiary stream 68 at a second temperature colder than the first temperature but warmer than the cold-end temperature. The first subsidiary stream 66 of the compressed turbine air stream that exits the heat exchanger 52 at a first location is partially cooled only to temperatures in a range of between about 140 Kelvin and about 220 Kelvin whereas the second subsidiary stream of the compressed turbine air stream that exits the heat exchanger at a second location is partially cooled to a colder temperature than the first subsidiary stream, preferably to temperatures between about 130 Kelvin and about 140 Kelvin.

The dual nozzle arrangement further includes a first valve 67 disposed between the heat exchanger 52 and the turboexpander 62 and configured to adjust the flow of the first subsidiary stream 66 which in turn controls the inlet temperature to the turboexpander 62 and the corresponding outlet temperature. The dual nozzle arrangement also includes a second valve 69 that is disposed between the heat exchanger 52 and the turboexpander 62 and configured to adjust the flow of the second subsidiary stream 68 which thus provides a different inlet temperature to the turboexpander 62 and different corresponding outlet temperature. In the embodiments of FIGS. 2 and 4, the dual nozzle arrangement includes a third valve 63 disposed between the heat exchanger 52 and the turboexpander 62 and configured to adjust the mixing of the first and second subsidiary streams and therefore produce a combined stream 61 that is directed to the turboexpander 62 that has a variable inlet temperature between that of the first subsidiary stream and the second subsidiary stream.

The first valve 67 and second valve 69 may be simple ‘on-off’ valves configured in either an open position allowing flow of the subsidiary stream to exit the heat exchanger at that particular nozzle and associated temperature or a closed position preventing flow of the subsidiary stream from exiting that particular nozzle. Alternatively is other embodiments, the first and second valves may be flow control valves having an actuator and a controller that is configured to modulate the flow over a range of different flow rates. The turboexpander 62 is configured to expand the first subsidiary stream 66 or the second subsidiary stream 68 or a combined stream 61 which is introduced into the at least one distillation column, preferably the higher pressure column 72, of the cryogenic air separation unit 10.

While the turbine based refrigeration circuit 60 illustrated in FIGS. 1 and 2 is preferably shown as a lower column turbine (LCT) circuit arrangement where the expanded exhaust stream is fed to the higher pressure column 72 of the distillation column system 70, it is contemplated that the turbine based refrigeration circuit alternatively may be an upper column turbine (UCT) circuit where the turbine exhaust stream is directed to the lower pressure column. Still further, the turbine based refrigeration circuit may be a combination of an LCT circuit and UCT circuit and/or even other variants such as partial lower column turbine (PLCT).

Although not shown, in the alternate embodiments that employ a UCT arrangement the turbine air stream is also partially cooled in the dual nozzle heat exchanger. The first subsidiary stream exits the heat exchanger at a first temperature warmer than the cold-end temperature of the heat exchanger. The second subsidiary stream exits the heat exchanger at a second temperature colder than the first temperature but also warmer than the cold-end temperature. Similar to the LCT based embodiments, the dual nozzle arrangement includes a first valve configured to adjust the flow of the first subsidiary stream and a second valve configured to adjust the flow of the second subsidiary stream. The dual nozzle arrangement optionally includes a third flow control valve disposed in operative association with the first valve and second valve to control the mixing of the first subsidiary stream and the second subsidiary stream. The resulting partially cooled stream of either the first subsidiary stream or the second subsidiary stream or a mixture of both the first and second subsidiary streams is directed to the turboexpander. The exhaust stream from the turboexpander is then directed to the lower pressure column in the two-column or three column distillation column system. The cooling or supplemental refrigeration created by the expansion of the exhaust stream is thus imparted directly to the lower pressure column thereby alleviating some of the cooling duty of the main heat exchanger.

In all contemplated embodiments, the aforementioned components of the feed air streams, namely oxygen, nitrogen, and argon are separated within the distillation column system 70 that includes a higher pressure column 72 and a lower pressure column 74 using a well-known process of fractional distillation. It is understood that if argon were a necessary product, an argon column 76 could be incorporated into the distillation column system 70. In the fractional distillation of air, the higher pressure column 72 typically operates at a pressure in the range from between about 20 bar(a) and about 60 bar(a) whereas the lower pressure column 74 typically operates at pressures between about 1.1 bar(a) and about 1.5 bar(a).

The higher pressure column 72 and the lower pressure column 74 are preferably linked in a heat transfer relationship such that a nitrogen-rich vapor column overhead, extracted from the top of higher pressure column 72 as a stream 73, is condensed within a condenser-reboiler 75 located in the base of lower pressure column 74 against boiling an oxygen-rich liquid column bottoms 77. The boiling of oxygen-rich liquid column bottoms 77 initiates the formation of an ascending vapor phase within lower pressure column 74. The condensate from the condenser-reboiler 75 is a liquid nitrogen containing stream 81 that is that divided into streams 83 and 84 and directed as reflux streams to the higher pressure column 72 and the lower pressure column 74, respectively to initiate the formation of descending liquid phases in such columns.

In some of the illustrated embodiments, exhaust stream 64 is introduced into the higher pressure column 72 along with the cooled, compressed boiler air stream 46A and preferably a third subsidiary stream 44. In such embodiments, the third subsidiary stream 44 is a medium pressure air stream at a pressure of between about 20 bar(a) and about 60 bar(a) that is discharged from the main heat exchanger 52 at the cold-end temperature and bypasses the turboexpander 62. This stream 44 would save power by liquefying air at a lower pressure than the boiler air 46, and prevent any dead pass in the heat exchanger 50 during different operation modes with a different nozzle.

Within the higher pressure column 72, there is a mass transfer occurring between an ascending vapor phase with a descending liquid phase that is initiated by reflux stream 83 to produce a crude liquid oxygen column bottoms 86, also known as kettle liquid and the nitrogen-rich column overhead 87. A plurality of mass transfer contacting elements, illustrated as trays 71 are used to facilitate the mass transfer between the ascending vapor and descending liquid in the higher pressure column 72. A stream 88 of the crude liquid oxygen column bottoms 86 or kettle liquid is withdrawn from the higher pressure column 72, subcooled in subcooling units 99A and 99B and expanded in an expansion valve 96 to the pressure at or near that of the lower pressure column 74 and is then introduced into the lower pressure column 74 for further distillation.

Lower pressure column 74 is also provided with a plurality of mass transfer contacting elements, illustrated as structured packing elements 79. As stated previously, the distillation process produces an oxygen-rich liquid column bottoms 77 extracted as an oxygen-rich liquid stream 90 and a nitrogen-rich vapor column overhead 91 that is extracted as a nitrogen product stream 95. The oxygen-rich liquid stream 90 is preferably pumped via pump 180 to a higher pressure and then taken as a high pressure liquid oxygen product stream 185 and/or directed to the main heat exchanger where it is vaporized to produce a high pressure oxygen vapor product stream 190.

Additionally, a waste nitrogen stream 93 is also extracted from the lower pressure column 74 to control the purity of nitrogen product stream 86. Both the nitrogen product stream 95 and nitrogen waste stream 93 are passed through subcooling units 99A and 99B designed to subcool the kettle liquid stream 88 to be used as reflux to the lower pressure column 74. A portion of the subcooled kettle liquid stream may optionally be taken as a liquid product stream 98 and the remaining portion (shown as stream 94) may be introduced into lower pressure column 74 after passing through expansion valve 96. After partial warming by passage through subcooling units 99A and 99B, the nitrogen product stream 95 and nitrogen waste stream 93 are fully warmed within main heat exchanger 52 to produce a warmed nitrogen product stream 195 and a warmed nitrogen waste stream 193. Although not shown, the nitrogen waste streams may be used to regenerate the adsorbents within the pre-purification unit 28.

HEAT EXCHANGER DISTRIBUTOR

Conventional techniques to withdraw only a portion of a stream passing through passage of a brazed aluminum heat exchanger often involve a complete withdrawal of the stream from the passage, and diverting a portion of the withdrawn stream for its desired use and subsequent reintroduction of the remaining portion of the stream back to the brazed aluminum heat exchanger. Such conventional partial extractions suffer from two disadvantages, namely a relatively high pressure drop attributable to the flow circuit outside the heat exchanger passage and an increased passage length of the heat exchanger required or attributable to the inactive length of the heat exchanger passage between the extraction point and the reintroduction point. Another problem often encountered with existing schemes for partial withdrawal of a stream from a heat exchanger is that mal-distribution or inefficient use of heat transfer area could result, particularly when using the heat exchanger in off-design conditions.

To overcome these disadvantages, the embodiments of the main heat exchanger 52 in the present cryogenic air separation unit 10 preferably include a distributor 200 as shown in FIG. 5 that is configured to allow partial or complete withdrawal of a stream at a desired intermediate location along the length of the main heat exchanger 51. Advantageously, the illustrated distributor 200 avoids the full withdrawal of the stream and subsequent reintroduction of a portion of the stream back to the heat exchanger when only a portion of the stream needs to be extracted. The distributor design has little to no impact on flow distribution and almost negligible impact on pressure drop compared to conventional schemes.

The illustrated distributor 200 is a machined distributor manufactured by using a solid aluminum slab 202 as shown in FIG. 5. The thickness of the aluminum slab 202 is preferably selected to match the height of the brazed aluminum heat exchanger passage. The machined aluminum slab 202 includes a plurality of vertically oriented channels 205 on one side of the aluminum slab 202 to permit the continuing stream to pass through the distributor 200. On the other side of the aluminum slab 202 is a plurality of slots 210. Each of the slots 210 has a first opening 212 disposed at a top surface 214 of the aluminum slab 202 adjacent to the vertically oriented channels 205 and a second opening 216 disposed at an end edge 218 of the aluminum slab 202. Each of the slots 210 further include a first angled path section 220 extending from the first opening 212 and a second horizontally oriented path section 222 extending from the first angled path section 220 to the second opening 216. The plurality of slots 210 are configured to receive a portion of the stream at the first opening 212 and divert said portion via the first angled path section 220 and the second horizontally oriented path section 222 to the second opening 216. The portion of the stream then exits the aluminum slab 202 via the second opening 216 and exits the brazed aluminum heat exchanger 52 via a nozzle in an orientation generally orthogonal to the flow path orientation of the heat exchanger passage.

A similar distributor can also be used to inject a stream into the brazed aluminum heat exchanger via the plurality of slots by receiving a stream at the second opening and passing the received stream via the second horizontally oriented path section to the angled or inclined first path section and through the first opening where it joins the continuing stream within the heat exchanger.

The dimensions of the vertical oriented channels 205 on the one side of the aluminum slab 202 and the slots 210 on the other side of the aluminum slab 202 are selected based on the expected flow rates of the streams to achieve the desired splits of flows between the withdrawn (or injected) portion of the stream and the remaining carry-on portion of the stream. Unlike some of the conventional heat exchanger distributors, the illustrated distributor design maintains flow distribution within the heat exchanger passages and full utilization of heat transfer area for off-design conditions.

OPERATING MODES FOR LIQUID PRODUCT MAKE

Referring again back to FIG. 1, some embodiments of the present cryogenic air separation unit are configured for operating in either a low liquid make first mode with the turboexpander configured to expand only the first subsidiary stream or a high liquid make second mode with the turboexpander configured to expand only the second subsidiary stream. The low liquid make first mode is targeted to produce liquid products in the amount of about 2.5 percent or less of the total feed air stream. Conversely, the high liquid make second mode is targeted to produce liquid products in the amount of about 4.5 percent or more of the total feed air stream.

The high liquid make second mode may be desired during the later years of the cryogenic air separation unit when the merchant liquid market has developed sufficiently while the low liquid make first mode may be preferred during the initial few years (i.e. liquid ramp years) after commissioning of the cryogenic air separation unit when the merchant liquid market is not yet matured. Of course, there could be scenarios where it is just the opposite is true and the high liquid make second mode is desired during early years or early phases of a project while the low liquid make first mode may be desired several years after start-up.

Alternatively, more rapid changes in product slate or liquid product requirements may occur as a result of customer product demand issues either for more liquid products or more pressurized gaseous products occurs. In such situations, the present cryogenic air separation unit is capable of switching between the high liquid make first mode and the low liquid make second mode. Still further, an operator can also switch between the low liquid make second mode and the high liquid make first mode based on other operating characteristics such as cost of power, as the power consumption is greater in the high liquid make first mode compared to the low liquid make second mode.

When integrated with a cryogenic air separation unit that includes turbine air stream bypass configuration, as shown in FIG. 3 the cryogenic air separation unit may operate in yet additional modes, including a third mode where the cryogenic air separation unit is in a low liquid make configuration with all or a portion of the turbine air stream bypassing at least one of the one or more turbine air stream compressors prior to entering the main heat exchanger and the turboexpander configured to expand only the first subsidiary stream. Alternatively, the cryogenic air separation unit may operate in a high liquid make fourth mode with none of the turbine air stream bypassing any of the one or more turbine air stream compressors prior to entering the main heat exchanger in which case, the turboexpander is configured to expand only the second subsidiary stream, which is equivalent to the first operating mode in the embodiment of FIG. 1. The other two modes are best characterized as medium liquid make modes, including a fifth mode with none of the turbine air stream bypasses any of the one or more turbine air stream compressors prior to entering the main heat exchanger and the turboexpander configured to expand only the first subsidiary stream (which is similar to the second high liquid mode with reference to FIG. 1) and a sixth mode where all or a portion of the turbine air stream bypasses at least one of the one or more turbine air stream compressors prior to entering the main heat exchanger and the turboexpander configured to expand only the second subsidiary stream.

Some other embodiments of the described cryogenic air separation units are configured to operate to include a potential medium liquid make variable mode where the turboexpander is configured to expand a mixture of the first subsidiary stream and the second subsidiary stream. In such embodiments, such as those illustrated in FIGS. 2 and 4, a third valve is disposed between the main heat exchanger and the turboexpander and configured to control the mixing of the first subsidiary stream and the second subsidiary stream. By mixing the first subsidiary stream and the second subsidiary stream, the inlet temperature of the combined stream sent to the turboexpander may be varied or controlled to a temperature somewhere between the first temperature and the second temperature. The medium liquid make variable mode is preferably targeted to produce liquid products in the amount of between about 2 percent and about 5 percent of the total feed air stream.

EXAMPLES

A number of computer simulations were run using cryogenic air separation unit operating models to characterize: (i) relative power consumption; (ii) liquid product make; and (iii); lower column turbine (LCT) efficiency when operating the present cryogenic air separation units as described herein in the above-described operating modes. Details of the various operating modes are evidenced by the listed temperatures, pressures and flows of selected streams depicted in the Figs.

As seen in Table 1, Low Liquid Mode represents the low liquid make first mode of the embodiments shown in FIG. 1 wherein High Liquid Mode represents the high liquid make second mode of the embodiments shown in FIG. 1. The respective temperatures, pressures and flows of selected streams are depicted as well as the liquid product makes, relative power consumption; and; the lower column turbine (LCT) efficiencies.

	TABLE 1
	Low Liquid Mode	High Liquid Mode
	Flow	T			T
Stream	(kcfh)	(K)	P (bar)	Flow	(K)	P (bar)
Stream 29	7631	284.3	10.15	7646	284.3	12.0
Stream 46	2364	96.96	53.04	2225	97.46	53
Stream 35	5245	284.3	10.15	5386	301.7	24.8
Stream 44	127.6	96.96	9.7	148.6	97.46	24.5
Stream 66	—	—	—	5237	160.1	24.6
Stream 68	5245	131.3	9.7
Stream 61	—	—	—
Stream 64	5117	113	5.55	5237	109.9	5.7
Liquid	1.7%	6.9%
Make
Power	25901 kW	30121 kW
Con-
sumption
LCT	90%	85%
Turbine
Efficiency

As seen in Table 2, Low Liquid Mode represents the low liquid make first mode of the embodiments shown in both FIG. 1 and FIG. 3 wherein High Liquid Mode represents the high liquid make second mode of the embodiments shown in FIG. 3. The Medium Liquid Modes represent changes to the flows split between the boiler air stream and the turbine air stream (with and without the turbine air stream bypass. The respective temperatures, pressures and flows of selected streams are depicted as well as the liquid product makes, relative power consumption; and; the lower column turbine (LCT) efficiencies.

	TABLE 2
	Mode
		Medium	Medium
		Liquid Mode	Liquid Mode
	Low Liquid	(No Turbine	(With Turbine	High Liquid
	Mode	Bypass)	Bypass)	Mode
	Flow	T	P	Flow	T	P	Flow	T	P	Flow	T	P
Stream	(kcfh)	(K)	(bar)	(kcfh)	(K)	(bar)	(kcfh)	(K)	(bar)	(kcfh)	(K)	(bar)
Stream 29	7631	284.3	10.2	7631	284.3	10.2	7631	284.3	12.0	7644	284.3	10.9
Stream 46	2364	96.9	53.0	2003	96.9	51.7	2388	97.6	53.0	2148	98.1	53.0
Stream 35	5245	284.3	10.2	5606	301.7	15.3	5221	284.3	12.0	5461	301.7	21.8
Stream 44	128	96.9	9.7	128	96.9	15.3	213	96.4	11.7	302	98.1	51.5
Stream 66	—	—	—	5479	135.0	15.3	5008	142.0	11.8	5461	155.6	21.6
Stream 68	5245	131.3	9.7	—	—	—	—	—	—	—	—	—
Stream 61	—	—	—	—	—	—	—	—	—
Stream 64	5117	113	5.6	5479	102.7	5.6	5008	116.3	5.6	5159	113	5.9
Liquid	1.7%	3.3%	3.0%	5.6%
Make
Power	25901 kW	27496 kW	26645 kW	29566 kW
LCT	90%	88%	90%	85%
Efficiency

Although the present invention has been discussed with reference to one or more preferred embodiments, as would occur to those skilled in the art that numerous changes and omissions can be made without departing from the spirit and scope of the present inventions as set forth in the appended claims.

Claims

1. An air separation unit having one or more distillation columns for the fractional distillation of a feed air stream and configured to produce one or more liquid products, the air separation unit comprising:

a main compressor arrangement configured to compress the feed air stream;

an adsorption based pre-purifier configured for removing water vapor, carbon dioxide, nitrous oxide, hydrocarbons or other contaminants from the compressed feed air stream to produce a compressed and purified feed air stream;

a feed air stream circuit configured to divide the compressed and purified feed air stream into at least two streams including a boiler air stream and a turbine air stream;

one or more boiler air stream compressors configured to compress the boiler air stream;

a main heat exchanger configured to receive the compressed boiler air stream and cool the compressed boiler air stream to a cold-end temperature via indirect heat exchange with an oxygen stream from at least one distillation column of the air separation unit;

one or more turbine air stream compressors configured to compress all or a portion of the turbine air stream;

the main heat exchanger further configured to receive the turbine air stream and discharge a first subsidiary stream at a first temperature warmer than the cold-end temperature and a second subsidiary stream at a second temperature colder than the first temperature but warmer than the cold-end temperature;

a turboexpander configured to expand the first subsidiary stream or the second subsidiary stream to produce an exhaust stream having an outlet temperature and wherein the exhaust stream is introduced into the at least one distillation column of the air separation unit;

a first valve disposed between the main heat exchanger and the turboexpander and configured to adjust the flow of the first subsidiary stream and therefore, an inlet temperature to the turboexpander and the outlet temperature;

a second valve disposed between the main heat exchanger and the turboexpander and configured to adjust the flow of the second subsidiary stream and therefore, the inlet temperature to the turboexpander and the outlet temperature; and

the one or more distillation columns configured for fractional distillation of the streams into a plurality of products including the one or more liquid products;

wherein the air separation unit is further configured to operate in a low liquid make mode with the turboexpander configured to expand only the first subsidiary stream:

wherein the air separation unit is configured to operate in a high liquid make mode with the turboexpander configured to expand only the second subsidiary stream.

2. The air separation unit of claim 1, wherein the low liquid make mode produces the liquid products in the amount of about 2.5 percent or less of the total feed air stream.

3. The air separation unit of claim 1, wherein the high liquid make mode produces the liquid products in the amount of about 4.5 percent or more of the total feed air stream.

4. The air separation unit of claim 1, wherein the air separation unit is still further configured to operate in a medium liquid make variable mode with the turboexpander configured to expand a mixture of the first subsidiary stream and the second subsidiary stream.

5. The air separation unit of claim 4, further comprising a third valve disposed between the main heat exchanger and the turboexpander and configured to adjust the mixing of the first subsidiary stream and the second subsidiary stream and therefore, the inlet temperature to the turboexpander and the outlet temperature.

6. The air separation unit of claim 4, wherein the medium liquid make variable mode produces the liquid products in the amount of between about 2 percent and about 8 percent of the total feed air stream.

7. The air separation unit of claim 1, wherein the first temperature is between about 140 Kelvin and about 220 Kelvin.

8. The air separation unit of claim 1, wherein the second temperature is between about 130 Kelvin and about 140 Kelvin.

9. The air separation unit of claim 1, wherein the one or more boiler air stream compressors are configured to compress the boiler air stream to a pressure between about 25 bar(a) and about 70 bar(a).

10. The air separation unit of claim 1, wherein the turbine air stream compressors are configured to compress the turbine air stream to a pressure between about 20 bar(a) and about 60 bar(a).

11. The air separation unit of claim 1, wherein the main heat exchanger is further configured to discharge a third subsidiary stream at the cold-end temperature and at a medium pressure greater than the pressure of the compressed and purified feed air stream and less than the pressure of the compressed boiler air stream, wherein the third subsidiary stream bypasses the turboexpander and is directed to the at least one distillation column of the air separation unit.

12. The air separation unit of claim 1, wherein the feed air stream circuit is further configured to divide the compressed and purified feed air stream such that the turbine air stream is between about 60 percent and 75 percent of the compressed and purified feed air stream.

13. The air separation unit of claim 1, further comprising a first distributor disposed within the main heat exchanger and wherein the first valve is an on-off valve in fluid communication with the first distributor and configured to be in an open position allowing flow of the first subsidiary stream to exit the main heat exchanger or a closed position preventing flow of the first subsidiary stream from exiting the main heat exchanger.

14. The air separation unit of claim 13, further comprising a second distributor disposed within the main heat exchanger and wherein the second valve is an on-off valve in fluid communication with the second distributor and configured to be in an open position allowing flow of the second subsidiary stream to exit the main heat exchanger when the first valve is in the closed position or in a closed position preventing flow of the first subsidiary stream from exiting the main heat exchanger when the first valve is in the open position.

15. The air separation unit of claim 1, wherein the one or more turbine air stream compressors further comprise at least one turbine loaded booster compressor.

16. An air separation unit having one or more distillation columns for the fractional distillation of a feed air stream and configured to produce one or more liquid products, the air separation unit comprising:

a main compressor arrangement configured to compress the feed air stream;

an adsorption based pre-purifier configured for removing water vapor, carbon dioxide, nitrous oxide, hydrocarbons or other contaminants from the compressed feed air stream to produce a compressed and purified feed air stream;

a feed air stream circuit configured to divide the compressed and purified feed air stream into at least two streams including a boiler air stream and a turbine air stream;

one or more boiler air stream compressors configured to compress the boiler air stream;

a main heat exchanger configured to receive the compressed boiler air stream and cool the compressed boiler air stream to a cold-end temperature via indirect heat exchange with an oxygen stream from at least one distillation column of the air separation unit;

one or more turbine air stream compressors configured to compress the turbine air stream;

a bypass circuit configured to direct all or a portion of the turbine air stream to bypass at least one of the one or more of compressors;

the main heat exchanger further configured to receive the turbine air stream and discharge a first subsidiary stream at a first temperature warmer than the cold-end temperature and a second subsidiary stream at a second temperature colder than the first temperature but warmer than the cold-end temperature;

a turboexpander configured to expand the first subsidiary stream or the second subsidiary stream to produce an exhaust stream having an outlet temperature and wherein the exhaust stream is introduced into the at least one distillation column of the air separation unit;

a first valve disposed between the main heat exchanger and the turboexpander and configured to adjust the flow of the first subsidiary stream and therefore, an inlet temperature to the turboexpander and the outlet temperature;

a second valve disposed between the main heat exchanger and the turboexpander and configured to adjust the flow of the second subsidiary stream and therefore, the inlet temperature to the turboexpander and the outlet temperature; and

the one or more distillation columns configured for fractional distillation of the streams into a plurality of products including the one or more liquid products;

wherein the air separation unit is configured to operate in a low liquid make mode with all or a portion of the turbine air stream bypassing at least one of the one or more compressors prior to entering the main heat exchanger and the turboexpander configured to expand only the first subsidiary stream;

wherein the air separation unit is configured to operate in a high liquid make mode with none of the turbine air stream bypassing any of the one or more compressors prior to entering the main heat exchanger and the turboexpander configured to expand only the second subsidiary stream;

wherein the air separation unit is configured to operate in a medium liquid make mode with none of the turbine air stream bypassing any of the one or more compressors prior to entering the main heat exchanger and the turboexpander configured to expand only the first subsidiary stream; and

wherein the air separation unit is configured to operate in an alternate medium liquid make mode with all or a portion of the turbine air stream bypassing at least one of the one or more compressors prior to entering the main heat exchanger and the turboexpander configured to expand only the second subsidiary stream.

17. The air separation unit of claim 16, wherein the high liquid make mode produces the liquid products in the amount of about 4.5 percent or more of the total teed air stream.

18. The air separation unit of claim 16, wherein the low liquid make mode produces the liquid products in the amount of about 2.5 percent or less of the total feed air stream.

19. The air separation unit of claim 16, wherein the medium liquid make mode produces the liquid products in the amount of between about 2 percent and about 5 percent of the total feed air stream.

20. The air separation unit of claim 16, wherein the alternate medium liquid make mode produces the liquid products in the amount of between about 2 percent and about 5 percent of the total feed air stream.

21. The air separation unit of claim 16, further comprising a third valve disposed between the main heat exchanger and the turboexpander and configured to adjust the mixing of the first subsidiary stream and the second subsidiary stream and therefore, the inlet temperature to the turboexpander and the outlet temperature.

22. The air separation unit of claim 20, wherein the air separation unit is still further configured to operate in a liquid make variable mode with none of the turbine air stream bypassing any of the one or more compressors prior to entering the main heat exchanger and the turboexpander configured to expand a mixture of the first subsidiary stream and the second subsidiary stream.

23. The air separation unit of claim 22, wherein the liquid make variable mode produces the liquid products in the amount of between about 2 percent and about 8 percent of the total feed air stream.

24. The air separation unit of claim 21, wherein the air separation unit is still further configured to operate in a liquid make variable mode with all or a portion of the turbine air stream bypassing at least one of the one or more compressors prior to entering the main heat exchanger and the turboexpander configured to expand a mixture of the first subsidiary stream and the second subsidiary stream.

25. The air separation unit of claim 24, wherein the liquid make variable mode produces the liquid products in the amount of between about 2 percent and about 5 percent of the total feed air stream.

26. The air separation unit of claim 16, wherein the first temperature is between about 140 Kelvin and about 220 Kelvin.

27. The air separation unit of claim 16, wherein the second temperature is between about 130 Kelvin and about 140 Kelvin.

28. The air separation unit of claim 16, wherein the one or more boiler air stream compressors are configured to compress the boiler air stream to a pressure between about 25 bar(a) and about 70 bar(a).

29. The air separation unit of claim 16, wherein the turbine loaded booster compressor is configured to compress the turbine air stream to a pressure between about 20 bar(a) and about 60 bar(a).

30. The air separation unit of claim 16, wherein the main heat exchanger is further configured to discharge a third subsidiary stream at the cold-end temperature and at a medium pressure greater than the pressure of the compressed and purified feed air stream and less than the pressure of the compressed boiler air stream, wherein the third subsidiary stream bypasses the turboexpander and is directed to the at least one distillation column of the air separation unit.

31. The air separation unit of claim 16 wherein the feed air stream circuit is further configured to divide the compressed and purified feed air stream such that the turbine air stream is between about 60 percent and 75 percent of the compressed and purified teed air stream.

32. The air separation unit of claim 16, further comprising a first distributor disposed within the main heat exchanger and wherein the first valve is an on-off valve in fluid communication with the first distributor and configured to be in an open position allowing flow of the first subsidiary stream to exit the main heat exchanger or a closed position preventing flow of the first subsidiary stream from exiting the main heat exchanger.

33. The air separation unit of claim 32, further comprising a second distributor disposed within the main heat exchanger and wherein the second valve is an on-off valve in fluid communication with the second distributor and configured to be in an open position allowing flow of the second subsidiary stream to exit the main heat exchanger when the first valve is in the closed position or in a closed position preventing flow of the first subsidiary stream from exiting the main heat exchanger when the first valve is in the open position.

34. The air separation unit of claim 16, wherein the one or more turbine air stream compressor further comprise at least one turbine loaded booster compressor.