US20210348839A1

US20210348839A1 - System and method for cryogenic air separation using a booster loaded liquid turbine for expansion of a liquid air stream

Info

Publication number: US20210348839A1
Application number: US17/126,220
Authority: US
Inventors: Jeremiah J. Rauch; Edward Joseph Colman; Zhengrong Xu
Original assignee: Individual
Current assignee: Praxair Technology Inc
Priority date: 2020-05-05
Filing date: 2020-12-18
Publication date: 2021-11-11

Abstract

A system and method for cryogenic air separation arrangement having a booster loaded liquid turbine for expansion of a liquid air stream or other fluid having liquid-like densities is provided. The disclosed booster loaded liquid turbines are relatively small to provide an aerodynamic and speed match between the turbine and the coupled gas compressor. The coupled gas compressor is a supplemental booster compressor and may be a dedicated warm booster compressor or alternatively a cold booster compressor.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority to U.S. provisional patent application Ser. No. 63/020,045 filed May 5, 2020 the disclosure of which is incorporated by reference.

TECHNICAL FIELD

The present invention relates to a system and method of cryogenic air separation, and more particularly to cryogenic air separation that employs the use of a booster-loaded liquid turbine for the expansion of a liquid air stream and the associated recovery of work therefrom.

BACKGROUND

Liquid turbines or dense phase turbines are currently used in high pressure air separation units to extract work from the liquid air stream or supercritical air stream that has been condensed against boiling product oxygen or against some product nitrogen from the air separation unit. This air stream is typically compressed to elevated pressures in order to effectively boil the product(s) but is subsequently expanded prior to introduction to the distillation columns of the air separation units.
Such liquid turbines tend to be used in either nitrogen liquefaction applications (e.g. less than 400 tons per day of liquid nitrogen product) typically with an oil brake coupled to the liquid turbine for dissipating the extracted power or in large volume air separation units (e.g. greater than about 2000 tons per day of oxygen product) and typically with a fixed speed generator coupled to the liquid turbine for recovery of power. When used, such liquid turbines or dense phase turbines generally improve the performance of the air separation unit but involve additional capital costs for the liquid turbine, generator, and associated controls. Because of the additional capital costs associated with the installation and use of the liquid turbine, generator, and associated controls, liquid turbines are currently deemed beneficial only in larger cryogenic air separation units.
Booster loaded liquid turbines are generally avoided in cryogenic air separation plants because most of the streams available to boost, such as the boiler air stream or other feed air stream upstream of the heat exchanger are gas streams, and there is an inherent aerodynamic and speed mismatch in coupling a booster stage for these gas streams with a turbine handling a fluid of liquid-like density. This inherent aerodynamic and speed mismatch will tend to result in lower efficiencies, and in some cases perhaps even the inability to design a viable turbine-booster pairing.
What is needed is a liquid turbine arrangement that has reduced capital costs compared to conventional generator-loaded liquid turbine arrangements but still yields improved performance of the air separation unit in the form of additional refrigeration and/or power savings during operation of the air separation unit.

SUMMARY OF THE INVENTION

The present invention may be characterized as an air separation unit having a booster loaded liquid turbine arrangement configured to produce one or more oxygen enriched streams and one or more nitrogen enriched streams, the air separation unit comprising: (i) a main air compression system configured to receive an incoming air stream and produce a compressed air stream at a pressure of between about 5 bar(a) and 15 bar(a); (ii) a pre-purification unit configured to pre-purify the compressed air stream to produce a compressed, pre-purified air stream; (iii) a primary booster compressor configured to receive a portion of the compressed, pre-purified air stream and produce a further compressed, pre-purified air stream at a pressure of between about 25 bar(a) and 90 bar(a); (iv) a supplemental booster compressor configured to further compress the further compressed, pre-purified air stream and produce a still further compressed, pre-purified air stream at a pressure of between about 30 bar(a) and 95 bar(a); (v) a main heat exchanger configured to cool the still further compressed, pre-purified air stream to produce a liquid air stream; (vi) a liquid turbine operatively coupled to the supplemental booster compressor and configured to expand the liquid air stream and produce an expanded air stream at a pressure between 7 bar(a) and 12 bar(a) and wherein the work of expansion is used to drive the supplemental booster compressor; and (vii) a distillation column system comprising a higher pressure column and a lower pressure column linked in a heat transfer relationship via a condenser reboiler, the distillation column system configured to receive all or a portion of the expanded air stream in the higher pressure column for distillation of the expanded air stream into the one or more oxygen enriched streams and the one or more nitrogen enriched streams.
The present invention may also be characterized as a method for cryogenic air separation comprising the steps of: (a) compressing an incoming air stream in a main air compression system configured to a pressure of between about 5 bar(a) and 15 bar(a); (b) pre-purifying the compressed air stream to produce a compressed, pre-purified air stream; (c) further compressing all or a portion of the compressed, pre-purified air stream in a primary booster compressor to produce a further compressed, pre-purified air stream at a pressure of between about 25 bar(a) and 90 bar(a); (d) still further compressing the further compressed, pre-purified air stream in a supplemental booster compressor to produce a still further compressed, pre-purified air stream at a pressure of between about 30 bar(a) and 95 bar(a); (e) cooling the still further compressed, pre-purified air stream in a main heat exchanger to produce a liquid air stream; (f) expanding the liquid air stream in a liquid turbine operatively coupled to the supplemental booster compressor to produce an expanded air stream at a pressure between 7 bar(a) and 12 bar(a) and wherein the work of expansion drives the supplemental booster compressor; and (g) separating the expanded air stream into one or more oxygen enriched streams and the one or more nitrogen enriched streams in a distillation column system comprising a higher pressure column and a lower pressure column linked in a heat transfer relationship via a condenser reboiler, the distillation column system configured to receive all or a portion of the expanded air stream in the higher pressure column.
In some embodiments, the supplemental booster compressor may be a cold booster compressor configured to receive the further compressed, pre-purified air stream, preferably at a pressure of between about 40 bar(a) and 50 bar(a) that has been partially cooled in the main heat exchanger and produce a still further compressed, pre-purified air stream at a pressure of preferably between about 50 bar(a) and 60 bar(a). Alternatively, the supplemental booster compressor may be a warm booster compressor configured to further compress the further compressed, pre-purified air stream, preferably at a pressure of between about 70 bar(a) and 90 bar(a) and before any cooling in the main heat exchanger. The warm supplemental booster compressor then produces a still further compressed, pre-purified air stream at a pressure of preferably between about 75 bar(a) and 95 bar(a).
The portion of the compressed, pre-purified air stream received by the primary booster compressor in the present air separation unit and associated methods is preferably a boiler air stream comprising between about 25% to 45% by volume of the incoming compressed, pre-purified air stream. In some embodiments, the expanded air stream is directed to the higher pressure column of the distillation column system, whereas in other embodiments a first portion of the expanded air stream is directed to the higher pressure column of the distillation column system and a second portion of the expanded air stream is further reduced in pressure with an expansion valve and directed to the lower pressure column of the distillation column system.

BRIEF DESCRIPTION OF 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 diagram of a cryogenic air separation unit with a conventional generator loaded liquid turbine commonly used in the prior art;

FIG. 2 is a schematic diagram of a cryogenic air separation unit having a warm booster loaded liquid turbine in accordance with an embodiment of the present system and method for cryogenic air separation;

FIG. 3 is an illustration of an example of a booster loaded liquid turbine used in the embodiment of FIG. 2;

FIG. 4 is a schematic diagram of a cryogenic air separation unit with a cold booster loaded liquid turbine in accordance with another embodiment of the present system and method for cryogenic air separation; and

FIG. 5 is a graph showing data from computer simulations showing the total net compression power versus the fraction of the incoming air stream directed to the boiler air stream and comparing the prior art generated loaded liquid turbine to embodiments of a booster loaded liquid turbine.

DETAILED DESCRIPTION

With reference to FIG. 1, a prior art cryogenic air separation unit 10 employing a generator loaded liquid turbine 50 is illustrated. FIG. 2 and FIG. 4, on the other hand, illustrate embodiments of a cryogenic air separation unit 110, 210 employing a booster-loaded loaded liquid turbine 150, 250.
As many of the components and streams in the prior art cryogenic air separation unit having a generator loaded liquid turbine 50 are the same or similar to the present cryogenic air separation unit 110, 210 employing the booster-loaded loaded liquid turbines 150, 250, much of the following description is applicable to each of FIG. 1, FIG. 2, and FIG. 4 and the reference numerals identifying the similar components are the same in each of the drawings. The differences between the prior art cryogenic air separation unit of FIG. 1 having a generator loaded liquid turbine and the those of FIGS. 2 and 4 having booster-loaded loaded liquid turbines, will be highlighted, as appropriate.
Cryogenic air separation units 10. 110, 210 are each configured to receive an incoming feed air stream 12 and produce a plurality of product streams and/or waste streams that optionally include a high pressure gaseous oxygen stream 102, a low pressure gaseous oxygen stream 104, a liquid oxygen stream 95, a liquid nitrogen product stream 90, a plurality of gaseous nitrogen enriched streams 101, 106, 108, and a crude argon stream 63. Production of the streams is preferably achieved via use of a triple column fractional distillation process. In a broad sense, the depicted air separation units 10, 110, and 210 all include a main feed air compression train, a turbine air circuit, a boiler air circuit; a main or primary heat exchanger 60; and a distillation column system 70.
The incoming feed air streams 12 is typically drawn through an air suction filter house (not shown) and compressed in a multi-stage, intercooled main air compressor arrangement 20 to a pressure that can be between about 5 bar(a) and about 15 bar(a). This main air compressor arrangement 20 may include integrally geared compressor stages or a direct drive compressor stages, arranged in series or in parallel. The compressed air stream 14 exiting the respective main air compressor arrangement 20 is fed to an aftercooler with or without integral demister to remove the free moisture in the incoming feed air stream. The cool, compressed air feed 14 is then purified in a pre-purification unit 25 to remove high boiling contaminants from the cool, compressed air feed. The pre-purification unit 25, as is well known in the art, typically contains two beds or more 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 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 subsequently removed from the compressed, pre-purified feed air in a dust filter disposed downstream of the pre-purification unit 25 to produce the compressed, purified air streams 28.
The compressed and purified air stream 28 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 an argon column arrangement, which may include an argon column 76. Prior to such distillation however, the compressed and pre-purified air streams 28 is typically split into a plurality of feed air streams, which includes at least a boiler air stream 42 and a turbine air stream 32.
The boiler air stream 42 comprises generally between about 25% to 45% of the compressed and purified feed air stream 28. The boiler air streams 42 may be further compressed in a booster compressor arrangement 44 to a targeted pressure between about 25 bar(a) and about 90 bar(a) and subsequently cooled in aftercooler (not shown) to form a boosted pressure air stream 45. The target pressure of the boosted pressure air streams 45 is generally dictated by the product requirements for the high pressure gaseous oxygen product stream 102. The boosted pressure air stream 45 is then further cooled to temperatures required for rectification in the main heat exchanger 60 where it is used to boil liquid oxygen streams 97 via indirect heat exchange to produce the high pressure gaseous oxygen product stream 102 and/or a low pressure gaseous oxygen product stream 104. The cooled and further compressed boiler air streams 46 exiting main heat exchanger 60 is often a liquid air stream or other dense phase fluid which is expanded in liquid turbine 50 (See FIG. 1) operatively coupled to a generator system 52. In the embodiment shown in FIG. 2 the liquid air stream or other dense phase fluid which is expanded in liquid turbine 150 is coupled to a warm booster compressor 121 while in the embodiment shown in FIG. 4 the liquid air stream or other dense phase fluid which is expanded in liquid turbine 150 is coupled to a cold booster compressor 221.
Ultimately, the expanded streams 47 exiting the liquid turbines 50, 150, 250 are divided into two separate portions and introduced into the higher pressure column 72 and the lower pressure column 74 of the distillation column system 70. In the illustrated embodiment, the portion of the expanded stream 47 directed to the lower pressure column 74 may be further reduced in pressure via expansion valve 48 yielding a low pressure air stream 49 for introduction into the lower pressure column 74.
The turbine air stream 32 is generally about 55% to 75% of the compressed and purified feed air stream 28 and is optionally further compressed in one or more turbine air compressors 33, cooled in an aftercooler and then directed as stream 34 to the main heat exchanger 60 where it is partially cooled prior to being directed to a turbine based refrigeration circuit that includes a turboexpander 36 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). The partially cooled feed air stream 35 is expanded in turboexpander 36 to an exhaust stream 38 that is directed to the higher pressure column 72 or lower pressure column 74 of the distillation column system 70. In the illustrated embodiment, the exhaust stream 38 is shown directed to the higher pressure column 72.
In some embodiments of the present system, liquid production in the cryogenic air separation units, 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 sent to the turboexpander. This variation in pressure can be effectuated by a turbine air stream bypass circuit (not shown) which includes a bypass line having a bypass valve that can be set in an open or closed position. In such embodiments, 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. If a bypass circuit is employed, the target pressure of the bypassed turbine air stream is preferably between about 10 bar(a) and about 30 bar(a). 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.
The main or primary heat exchanger 60 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 unit units, a heat exchanger comprising a single core may be sufficient. For larger air separation unit units handling higher air flows, the main heat exchanger may be constructed from several heat exchange cores which may be divided into high pressure cores and low pressure cores.
The components of the feed air stream, namely oxygen, nitrogen, and argon are separated within the distillation column system 70 that preferably includes a plurality of distillation columns in which vapor and liquid are counter-currently contacted in order to produce a gas/liquid mass-transfer based separation of the respective feed streams. Such columns will preferably employ structured packing or trays as the mass transfer contacting elements. The illustrated distillation column system 70 includes: a higher pressure column 72 typically configured to operate in the range from between about 20 bar(a) to about 60 bar(a); a lower pressure column 74 typically configured to operate in the range from between about 1.1 bar(a) to about 1.5 bar(a); a main condenser-reboiler 75; and an argon column arrangement. The illustrated argon column arrangement preferably is configured as an argon superstaged or ultra superstaged column 76 with an argon condenser 77 disposed in a separate shell 78.
The higher pressure column 72 and the lower pressure column 74 are preferably inked in a heat transfer relationship such that a portion of the nitrogen-rich vapor column overhead, extracted from proximate the top of higher pressure columns 72 as a stream 88 is condensed within the condenser-reboiler 75 typically located in the base of lower pressure column 74 against boiling an oxygen-rich liquid column bottoms 92. The boiling of oxygen-rich liquid column bottoms 92 initiates the formation of an ascending vapor phase within lower pressure columns 74. The condensation produces a liquid nitrogen rich stream 86 that may be divided into a reflux stream 87 that refluxes one or more of the distillation columns to initiate the formation of the descending liquid phase and a shelf nitrogen stream 89 that may be subcooled in heat exchanger 55 and taken as a liquid nitrogen product stream 90. Also, another portion of the nitrogen-rich vapor column overhead from the higher pressure column 72 may be taken and warmed in the main heat exchanger 60 to produce a high pressure gaseous nitrogen stream 101.
As indicated above, a portion of the expanded air stream 47 exiting the liquid turbine is introduced into the higher pressure column 72 for distillation by contacting an ascending vapor phase of such mixture within a plurality of mass transfer contacting elements which may comprise trays or structured packing, with a descending liquid phase that is initiated by the reflux streams. This distillation process produces crude liquid oxygen column bottoms 82 also known as kettle liquid, and a high pressure nitrogen-rich column overhead. Similarly, a portion of the expanded air stream 47 exiting the liquid turbine is further reduced in pressure by the expansion valve 148 and introduced into the lower pressure column 74 for distillation. The separation occurring within lower pressure column 74 produces a nitrogen-rich vapor column overhead that is extracted as a low pressure nitrogen stream 58 and a low pressure nitrogen waste stream 56. The separation further yields an oxygen-rich liquid column bottoms 92, a portion of which may be extracted from the lower pressure column 74 as an oxygen-rich liquid stream 94.
As shown in the drawings, a portion of the oxygen-rich liquid stream 94 may be pumped via pump 96 to yield a pumped liquid oxygen stream 97 which is directed to the associated main heat exchanger 60. A first portion of the pumped liquid oxygen stream 97 is warmed to produce a high pressure gaseous oxygen product stream 102, while a second portion is reduced in pressure via valve 98 and the resulting lower pressure oxygen stream 99 is then warmed in main heat exchanger 60 to produce a low pressure oxygen product stream 104. Another portion of the oxygen-rich liquid stream 94 taken from the lower pressure column 74 may be taken as a liquid oxygen product stream 95. Additionally, nitrogen waste streams 106 may also be extracted from lower pressure column 74 to control the purity of the low pressure nitrogen product streams 108.
The low pressure nitrogen product stream 58 and the nitrogen waste stream 56 are preferably passed through one or more subcooling units 55 designed to subcool: (i) the liquid nitrogen product stream 89; and (ii) the respective kettle streams 186,286 used to reflux the argon column 76.
The argon column arrangement is configured to receive an oxygen-rich stream 62 typically having a concentration of about 8% to 15% by volume argon from an intermediate location of the lower pressure column 74 and separate argon and oxygen using the oxygen-rich stream 62 as an ascending vapor stream against a downflowing argon-rich reflux stream 67 received from an argon condenser 77. The mass transfer contacting elements within the argon column arrangement could be trays or structured packing.
The resulting argon-rich vapor overhead stream 65 from the argon column 76 is then preferably directed to the argon condenser 77 where the argon-rich vapor overhead stream is condensed into a crude liquid argon stream 66. A first portion of the crude liquid argon stream 66 is used as the argon-rich reflux stream 67 and the remaining portion of the crude liquid argon 66 taken as a crude argon stream 63. The oxygen rich column bottoms stream 64 from the argon column 76 is preferably returned to the lower pressure column. The cooling duty of the argon condenser 77 is provided via a subcooled stream 85 of the oxygen-enriched stream 84 taken from the kettle liquid 82 of the higher pressure column 72. The vaporized portion 68 of the condensing media as well as a portion of the excess liquid in the bottom of the shell 78 may be returned to the lower pressure column 74 as stream 69. The argon condenser 77 may be configured as a conventional argon condenser or as a down-flowing once-through argon condenser and is preferably disposed in a separate shell 78 or alternatively disposed within the lower pressure column 74.

Booster Loaded Liquid Turbine Coupled to Warm Booster Compressor

Turning now to FIG. 2, the differences between the illustrated warm booster loaded liquid turbine and the prior art generator-loaded liquid turbine of FIG. 1 include: a booster loaded liquid turbine 150; a warm booster bypass circuit; and a liquid turbine bypass circuit. Details of the warm booster loaded liquid turbine (i.e. liquid turbine 150 that is operatively coupled to a discrete warm booster compressor 121) is discussed below and with reference to the discussion of FIG. 3.
The warm booster bypass circuit comprises an air circuit that includes a diversion valve 123 configured to allow the further compressed boiler air stream 122 to bypass the warm booster compressor 121 and send the further compressed boiler air stream 122 directly to the main heat exchanger 60 during air separation unit start-up operation and other operating conditions where the liquid turbine is off-line. In situations where the diversion valve 123 is closed, the further compressed boiler air stream 122 will be still further compressed in warm booster compressor circuit that includes the warm booster compressor 121 and a recirculation valve 124 as well as the control valve 126.
When diversion valve 123 is closed, the further compressed boiler air stream 122 is directed to the warm booster compressor 121. After additional compression in the warm booster compressor 121, the still further compressed boiler air stream 128 is recirculated back upstream of the warm booster compressor 121 via an open recirculation valve 124 (with control valve 126 in ‘closed’ position) or the still further compressed boiler air stream 128 is directed to the main heat exchanger 60 via the open control valve 126 (with the recirculation valve 124 in ‘closed’ position). When the control valve 126 is ‘open’ then recirculation valve 124 and diversion valve 123 are ‘closed’. Conversely, when the diversion valve 123 is ‘open’ then recirculation valve 124 and control valve 126 are ‘closed’.
The liquid turbine bypass circuit includes bypass valve 153 that directs cooled compressed, boiler air stream 146 to the higher pressure column 72 while bypassing the liquid turbine circuit. The liquid turbine circuit preferably comprises valve 151, liquid turbine 150, and valve 152 which directs the resulting expanded air stream 154 to the higher pressure column 72. The liquid turbine 150 is generally bypassed concurrently with bypassing the warm booster compressor 121. When bypassing the liquid turbine 150, bypass valve 153 is set in the ‘open’ position while valves 151 and 152 are set in the ‘closed’ position.
By using the recovered power from the liquid turbine 150 to drive a dedicated warm booster compressor 121 disposed downstream of the primary boiler air compressor 144, the required discharge pressure of the primary boiler air compressor 144 would be lower thereby reducing the operating costs associated with the primary boiler air compressor 144. Alternatively, one can maintain the discharge pressure of the primary boiler air compressor 144 at design conditions and use the dedicated warm booster compressor 121 driven by the liquid turbine expansion to provide a net higher pressure of the boiler air stream 145 directed to the main heat exchanger 60 which in turn allows a reduction in the air flow split to the boiler air circuit. Such net higher pressure of the boiler air stream 145 translates to improved performance of the air separation unit 110, 210 in in the form of additional refrigeration capacity and/or power savings during operation of the air separation unit 110, 210 as more of the incoming air flow 12 can be directed to the turbine air circuit and less of the incoming air flow 12 to the boiler air circuit. In addition, there would be a capital cost savings by avoiding the capital costs associated with the generator and associated controls, partially offset by the added costs of the booster loaded liquid turbine arrangement.
Turning now to FIG. 3, there is shown an embodiment of the booster loaded liquid turbine 500 that employs an advanced aerodynamic design similar to other booster loaded gas turbine chassis. As seen therein, one embodiment of the booster loaded liquid turbine 500 includes a rotor assembly 510, a plurality of active magnetic bearings 520, 530 and a plurality of seals including a seal 540 disposed between the rotor assembly 510 and the liquid turbine 150 as well as a seal 550 disposed between the rotor assembly and the warm compressor 121.
However, the size of the liquid or dense phase turbine 150 and the coupled compressor 121 are relatively small such that there is an aerodynamic and speed match between the turbine and the coupled gas compressor. Specifically, the pressure ratio across the dedicated warm booster compressor is preferably only about 1.1 or less, as shown in the Example of FIG. 3 with the incoming purified, compressed air being at 1150 psia and the further compressed air stream exiting the warm booster compressor 121 being about 1250 psia. Also, due primarily to the smaller size and lower amount of recovered power, compared to conventional booster loaded gas turbines, use of an aftercooler to cool the further compressed stream is optional and is preferably avoided to further reduce capital and operating costs. On the turbine side the incoming liquid or dense phase fluid is at 1250 psia and expanded to an exhaust stream of roughly 100 psia. The recovered work of the expansion of the fluid having liquid-like densities in turbine 150 is transferred to the warm booster compressor 121 via rotor assembly 510.

Booster Loaded Liquid Turbine Coupled to Cold Booster Compressor

FIG. 4 shows a schematic diagram of a cryogenic air separation unit having a booster loaded liquid turbine 250 coupled to a cold-compressor 221. In this embodiment, the booster loaded liquid turbine is used for cold compression of all or a portion of the boiler air stream that was introduced into the main heat exchanger. In many regards the air separation unit of FIG. 4 is similar to that of the air separation unit of FIG. 2 except that the dedicated booster compressor is a cold compressor in lieu of the warm compressor shown in FIG. 2.
The differences between the illustrated cold booster loaded liquid turbine in FIG. 4 and the prior art generator-loaded liquid turbine of FIG. 1 include: a liquid turbine 250 that is operatively coupled to a cold booster compressor 221; a cold booster bypass circuit; and a liquid turbine bypass circuit.
In the embodiment of FIG. 4, the boiler air stream 142 in the boiler air circuit is compressed to a certain design pressure by the primary boiler air compressor 144 and then directed to the main heat exchanger 60. The cold booster bypass circuit comprises an air circuit that includes a bypass valve 223 configured to allow the further compressed boiler air stream 145 to bypass the cold booster compressor 221 and proceed through the main heat exchanger 60 directly to the higher pressure column 72 during air separation unit operating conditions where the liquid turbine is off-line.
In situations where the bypass valve 223 is closed, the compressed boiler air stream 145 will be withdrawn from the main heat exchanger 60 at an intermediate location and directed to the cold booster compressor 221 via control valve 226. The intermediate location of the main heat exchanger 60 generally corresponds to the location where the temperature of the boiler air stream 222 is near the boiling temperature for subcritical boiling applications. The further compressed stream 228 exiting the cold booster compressor 221 is then re-introduced to the main heat exchanger 60 for product boiling purposes and to the higher pressure column 72.
The liquid turbine bypass circuit includes bypass valve 253 that directs cooled compressed, boiler air stream 146 to the higher pressure column 172 while bypassing the liquid turbine circuit. The liquid turbine circuit preferably comprises valve 251, liquid turbine 250, and valve 252 which directs the resulting expanded air stream 254 to the higher pressure column 172. The liquid turbine 250 is generally bypassed concurrently with bypassing the cold booster compressor 221. When bypassing the liquid turbine 250, bypass valve 253 is set in the ‘open’ position while valves 251 and 252 are set in the ‘closed’ position.
Because the air stream is cold compressed in the embodiment of FIG. 4, the pressure ratio across the dedicated cold booster compressor may be higher than the pressure ratio across the dedicated warm booster compressor in the embodiment of FIG. 2 while still maintaining the aerodynamic and speed match between the turbine and the coupled gas compressor. Using a booster loaded liquid turbine coupled to a cold compressor the pressure ratio across the cold compressor was about 1.15 enabling the air separation unit to operate with a lower inlet boiler air pressure which provides further power savings compared to the warm booster compression arrangement depicted in FIG. 2.
Alternatively, one can maintain the discharge pressure of the primary boiler air compressor at design conditions and use the dedicated cold booster compressor driven by the liquid turbine expansion to provide a net higher pressure of the boiler air stream directed to the main heat exchanger which in turn allows a reduction in the air flow split to the boiler air circuit. Such net higher pressure of the boiler air stream translates to improved performance of the air separation unit in in the form of additional refrigeration capacity and/or power savings during operation of the air separation unit as more of the incoming air flow can be directed to the turbine air circuit and less of the incoming air flow to the boiler air circuit. In addition, there would be a capital cost savings by avoiding the capital costs associated with the generator and associated controls, partially offset by the added costs of the booster loaded liquid turbine arrangement. Specifically, computer modeling suggests that using the cold booster loaded liquid turbine arrangement of FIG. 4 yields a power savings of about 1.8% compared to a comparably sized air separation unit without the booster loaded liquid turbine, shown generally in FIG. 1.
The incremental capital costs realized when changing from a warm booster loaded liquid turbine arrangement to a cold booster loaded liquid turbine arrangement is manageable, but it is understood to persons skilled in the art that additional design changes to the dedicated cold booster compressor and to the main heat exchanger cores may be required.

INDUSTRIAL APPLICABILITY

Using computer based simulations and models, it is projected that embodiments of the present system and method for cryogenic air separation units using a booster-loaded liquid turbine improves the cost effectiveness of air separation projects compared to installations employing a generator-loaded liquid turbine. Turning now to FIG. 5, there is shown a graph plotting selected results of the simulations, and particularly the expected total net compression power consumed versus the fraction of the incoming air stream directed to the boiler air stream for comparably sized cryogenic air separation units. The first curve 610 on the graph represents the simulation data for a cryogenic air separation unit having a generated loaded liquid turbine with the primary boiler air compressor is configured to raise the pressure of the boiler air stream to about 1200 psia. The second curve 620 on the graph represents the simulation data for a cryogenic air separation unit having an embodiment of a booster loaded liquid turbine wherein the primary boiler air compressor is configured to raise the pressure of the boiler air stream to only about 1100 psia and the supplemental booster compressor is configured further compress the boiler air stream to a pressure of about 1200 psia. Note the pressure of the boiler air stream exiting the primary boiler air compressor is lower than the baseline case of a cryogenic air separation unit having a generator loaded liquid turbine. Comparing the first curve 610 to the second curve 620, one would notice there is a small power penalty incurred when the booster-loaded liquid turbine is used compared to the generated-loaded liquid turbine at the same boiler air pressure.
However, there is a significant capital cost savings realized by switching to the booster-loaded liquid turbine arrangement which offsets the small power penalty. As discussed above, the construction and operation of a cryogenic air separation plant employing a conventional a generator-loaded liquid turbine arrangement has the disadvantage of incremental capital costs associated with generator-loading the turbine (e.g., the generator itself, structural support, electrical work, and valves/instrumentation required to facilitate safe generator start-up and speed control).
The third curve 630 on the graph of FIG. 5 represents the simulation data for a cryogenic air separation unit having a booster loaded liquid turbine wherein the primary boiler air compressor is configured to raise the pressure of the boiler air stream to about 1200 psia—which is comparable to the pressure of the boiler air stream exiting the primary boiler air compressor in the baseline case—and the supplemental booster compressor is configured to further compress the boiler air stream to a pressure of about 1295 psia. Such an application may be used in retrofit situations of pre-existing air separation units to The beneficial effect of raising the boiler air stream pressure from 1200 psia exiting the primary boiler air compressor to 1,295 psia exiting the supplemental booster compressor (as represented by curve 630) in an upper column turbine cycle means less turbine air stream flow is required to produce the same or similar product slates while using less overall net compression power in the main air compressor (MAC). In addition, such pressure enhancement may allow the same air separation unit to realize better oxygen recovery and/or less incoming feed airflow.
The present system and method for cryogenic air separation units using a booster-loaded liquid turbine may also be used in air separation units where a generator-loaded liquid turbine arrangement is deemed economically unfavorable. The following table presents results of computer simulations comparing a cryogenic air separation project without the booster-loaded liquid turbine arrangement compared to a cryogenic air separation project with the booster-loaded liquid turbine arrangement having a cold booster compressor.

TABLE 1

	Ref #	Baseline ASU	ASU
ASU Parameter	(See Drawings)	w/o BLLT	w/Cold BLLT

Main Air Compression (MAC) Pressure	14/114	179.7	psia	179.7	psia
Main Air Compression (MAC) Flow	14/114	7647	kcfh	7647	kcfh
Boiler Air Compressor (BAC) Pressure	45/145	715.1	psia	625	psia
Boiler Air Compressor (BAC) Flow	45/145	2200	kcfh	2143	kcfh

Main Heat Exchanger U/A	60/160	9.913 MM	9.776 MM
		BTU/hr-k	BTU/hr-k

Gaseous Oxygen Flow	102/202 + 104/204	1445.2	kcfh	1445.2	kcfh
Argon Flow	63/163	68.96	kcfh	68.99	kcfh

Cold Booster Compressor Efficiency	221 (FIG. 4)	N/A	79%
Liquid Turbine Efficiency	250 (FIG. 4)	N/A	79%

Cold Booster Compressor Inlet Pressure	222 (FIG. 4)	N/A	625	psia
Cold Booster Compressor Inlet Temp	222 (FIG. 4)	N/A	144°	K
Cold Booster Compressor Discharge P	228 (FIG. 4)	N/A	717.5	psia
Liquid Turbine Recovered Power	250 (FIG. 4)	N/A	87.29	kW

MAC Power	20/120	23263	kW	23263	kW
BAC Power	44/144	3685	kW	3205	kW
Total Compression Power	N/A	26948	kW	26468	kW
Power Savings (compared to Baseline)	N/A	0	kW	480	kW

The results suggest a power savings of 480 kW for the basic same air separation unit but having the additional capital costs associated with the booster-loaded liquid turbine arrangement and modified main heat exchanger. Such power savings represents a favorable trade-off against the additional capital costs.
While the present system and method for cryogenic air separation using a booster-loaded liquid turbine has been described with reference to one or more preferred embodiments, it is understood that numerous additions, changes and omissions can be made without departing from the spirit and scope of the present system and method as set forth in the appended claims.

Claims

What is claimed is:

1. An air separation unit having a booster loaded liquid turbine arrangement configured to produce one or more oxygen enriched streams and one or more nitrogen enriched streams, the air separation unit comprising:

a main air compression system configured to receive an incoming air stream and produce a compressed air stream at a pressure of between about 5 bar(a) and 15 bar(a);

a pre-purification unit configured to pre-purify the compressed air stream to produce a compressed, pre-purified air stream;

a primary booster compressor configured to receive a portion of the compressed, pre-purified air stream and produce a further compressed, pre-purified air stream at a pressure of between about 25 bar(a) and 90 bar(a);

a supplemental booster compressor configured to further compress the further compressed, pre-purified air stream and produce a still further compressed, pre-purified air stream at a pressure of between about 30 bar(a) and 95 bar(a);

a main heat exchanger configured to cool the still further compressed, pre-purified air stream to produce a liquid air stream;

a liquid turbine operatively coupled to the supplemental booster compressor and configured to expand the liquid air stream and produce an expanded air stream at a pressure between 7 bar(a) and 12 bar(a) and wherein the work of expansion is used to drive the supplemental booster compressor; and

a distillation column system comprising a higher pressure column and a lower pressure column linked in a heat transfer relationship via a condenser reboiler, the distillation column system configured to receive all or a portion of the expanded air stream in the higher pressure column for distillation of the expanded air stream into the one or more oxygen enriched streams and the one or more nitrogen enriched streams.

2. The air separation unit of claim 1 wherein the supplemental booster compressor is a cold booster compressor configured to receive further compress the further compressed, pre-purified air stream at a pressure of between about 40 bar(a) and 50 bar(a) that has been partially cooled in the main heat exchanger and produce a still further compressed, pre-purified air stream at a pressure of between about 50 bar(a) and 60 bar(a).

3. The air separation unit of claim 1 wherein the supplemental booster compressor is a warm booster compressor configured to further compress the further compressed, pre-purified air stream at a pressure of between about 70 bar(a) and 90 bar(a) before any cooling in the main heat exchanger and produce a still further compressed, pre-purified air stream at a pressure of between about 75 bar(a) and 95 bar(a).

4. The air separation unit of claim 1 wherein the portion of the compressed, pre-purified air stream received by the primary booster compressor is a boiler air stream comprising between about 25% to 45% by volume of the compressed, pre-purified air stream.

5. The air separation unit of claim 1 further configured such that all of the expanded air stream is directed to the higher pressure column of the distillation column system.

6. The air separation unit of claim 1 further configured such that a first portion of the expanded air stream is directed to the higher pressure column of the distillation column system and a second portion of the expanded air stream is further reduced in pressure and directed to the lower pressure column of the distillation column system.

7. A method of air separation comprising the steps of:

compressing an incoming air stream in a main air compression system configured to a pressure of between about 5 bar(a) and 15 bar(a);

pre-purifying the compressed air stream to produce a compressed, pre-purified air stream;

further compressing all or a portion of the compressed, pre-purified air stream in a primary booster compressor to produce a further compressed, pre-purified air stream at a pressure of between about 25 bar(a) and 90 bar(a);

still further compressing the further compressed, pre-purified air stream in a supplemental booster compressor to produce a still further compressed, pre-purified air stream at a pressure of between about 30 bar(a) and 95 bar(a);

cooling the still further compressed, pre-purified air stream in a main heat exchanger to produce a liquid air stream;

expanding the liquid air stream in a liquid turbine operatively coupled to the supplemental booster compressor to produce an expanded air stream at a pressure between 7 bar(a) and 12 bar(a) and wherein the work of expansion drives the supplemental booster compressor;

separating the expanded air stream into one or more oxygen enriched streams and the one or more nitrogen enriched streams in a distillation column system comprising a higher pressure column and a lower pressure column linked in a heat transfer relationship via a condenser reboiler, the distillation column system configured to receive all or a portion of the expanded air stream in the higher pressure column.

8. The method of claim 7 further comprising the step of partially cooling the further compressed, pre-purified air stream in the main heat exchanger and wherein the supplemental booster compressor is a cold booster compressor configured to further compress the cooled, further compressed, pre-purified air stream at a pressure of between about 40 bar(a) and 50 bar(a) to produce a still further compressed, pre-purified air stream at a pressure of between about 50 bar(a) and 60 bar(a).

9. The method of claim 7 wherein the supplemental booster compressor is a warm booster compressor configured to further compress the further compressed, pre-purified air stream at a pressure of between about 70 bar(a) and 90 bar(a) to produce a still further compressed, pre-purified air stream at a pressure of between about 75 bar(a) and 95 bar(a).

10. The method of claim 7 wherein the portion of the compressed, pre-purified air stream received by the primary booster compressor is a boiler air stream comprising between about 25% to 45% by volume of the compressed, pre-purified air stream.

11. The method of claim 7 wherein all of the expanded air stream is directed to the higher pressure column of the distillation column system.

12. The method of claim 7 wherein a first portion of the expanded air stream is directed to the higher pressure column of the distillation column system and a second portion of the expanded air stream is further reduced in pressure and directed to the lower pressure column of the distillation column system.