AIR SEPARATION APPARATUS

Abstract

A method and apparatus for separating air in which production of the liquid products can be selectively varied between high and low production rates by varying the pressure ratio across a turboexpander used in imparting refrigeration with the use of a branched flow path. The branched flow path has a system of valves to selectively and gradually introduce a compressed refrigerant air stream into either a booster compressor branch having a booster compressor to increase the pressure ratio during high modes of liquid production or a bypass branch that bypasses the booster compressor to decrease the pressure ratio during low modes of liquid production. A recycle branch is connected to the booster compressor branch to allow compressed air to be independently recycled from the outlet to the inlet of the booster compressor during turndown from the high to the low liquid mode of liquid production to prevent surge.

Claims

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11. An air separation apparatus comprising: an air separation plant having a main air compressor, a purification unit connected to the main air compressor, a main heat exchanger in flow communication with the purification unit to cool the air, a distillation column system connected to the main heat exchanger and configured to rectify the air and thereby to produce at least one liquid product and a constant speed turboexpander connected to the distillation column system so that an exhaust stream generated by the constant speed turboexpander is introduced into the distillation column system, thereby to impart refrigeration to the air separation plant, the constant speed turboexpander not directly coupled to a single compressor of the air separation plant on a common pinion; the air separation plant also having a branched flow path positioned between the pre-purification unit and the constant speed turboexpander to receive a compressed refrigerant air stream to vary production of the at least one liquid product and having a booster compressor branch including a constant speed booster compressor to further compress the compressed refrigerant air stream and thereby obtain a higher pressure ratio across the constant speed turboexpander and a higher rate of production, a bypass branch, bypassing the booster compressor, thereby to obtain a lower pressure ratio across the constant speed turboexpander and a lower rate of production, a recycle branch connecting an outlet of the booster compressor to an inlet of the booster compressor and connected at opposite ends to the booster compressor branch for flow of a recycle stream from the outlet to the inlet of the booster compressor thereby to prevent surge within the booster compressor, and a system of valves to permit selective introduction of the compressed refrigerant air stream into either the booster compressor branch or the recycle branch; the system of valves including a first flow control valve located within the booster compressor branch upstream of the inlet of the booster compressor, a second flow control valve located within the bypass branch, a third valve located in the recycle branch and two valves located in the booster compressor branch and the bypass branch, respectively positioned downstream of the outlet of the compressor and the recycle branch and upstream of the second control valve and configured to prevent a reversal of flow in the booster compressor branch when bypass branch pressure within the bypass branch exceeds that of booster compressor branch and the reversal of flow in the bypass branch when booster compressor branch pressure at the outlet of the booster compressor exceeds that of the bypass branch; and a programmable control system configured to generate control signals to control valve opening of the first flow control valve, the second flow control valve and the third valve and to activate the booster compressor and responsive to selective user input to selectively introduce the compressed refrigerant air stream into the booster compressor branch and the bypass branch, the programmable control system programmed such that: when the compressed refrigerant air stream is introduced into the booster compressor branch, the first flow control valve gradually opens and the second flow control valve gradually closes to gradually divert the compressed refrigerant air stream from the bypass branch to the booster compressor branch and thereby introduce the compressed refrigerant air stream into the booster compressor branch, the booster compressor is activated, the third valve initially is set in an open position to allow flow of the recycle stream and thereafter, is reset from an open position to a closed position when the booster compressor pressure exceeds the bypass pressure; and when the compressed refrigerant stream is introduced into the bypass branch, the first flow control valve gradually closes and the second flow control valve gradually opens to gradually divert the compressed refrigerant air stream from booster compressor branch to the bypass branch and thereby introduce the compressed refrigerant air stream into the bypass branch, the third valve is reset in from the closed position to the open position and the booster compressor is deactivated when the bypass pressure exceeds the booster compressor branch pressure.

12. The apparatus of claim 11, wherein: the constant speed turboexpander is positioned between a location of a main heat exchanger having an intermediate temperature between warm and cold ends thereof and the distillation column system; and the branched flow path is positioned between the pre-purification unit and the main heat exchanger upstream of the constant speed turboexpander to receive a compressed refrigerant air stream.

13. The apparatus of claim 11, wherein the branched flow path has means for passing a purge air stream, composed of purified air, through the booster compressor after the booster compressor is deactivated to prevent ambient air from entering the booster compressor.

14. The apparatus of claim 11, wherein: a conduit having an intermediate outlet connects the distillation column system to the main heat exchanger so that a liquid stream is removed from the distillation column system, is divided into a first subsidiary liquid stream discharged from the intermediate outlet and a second subsidiary liquid stream introduced into the main heat exchanger; the at least one liquid product comprises the first subsidiary liquid stream; the at least on liquid flow control valve is connected to the intermediate outlet; the main heat exchanger is configured to heat the second subsidiary liquid stream to form a heated product stream; and the main air compressor has inlet guide vanes that are able to be adjusted to control air flow rate through the main air compressor and thereby decrease the air flow rate during the low mode of production to in turn maintain product flow rate of the heated product stream constant.

15. The apparatus of claim 14, wherein: the distillation column system comprises a higher pressure column and a lower pressure column operating at a lower pressure than the higher pressure column, configured to further refine a crude liquid oxygen column bottoms produced in the higher pressure column and connected to the higher pressure column in a heat transfer relationship so that a nitrogen-rich vapor column overhead produced in the higher pressure column is condensed through indirect heat exchange with an oxygen-rich liquid produced in the lower pressure column, thereby providing liquid nitrogen reflux to the higher pressure column and the lower pressure column; the liquid stream is an oxygen-rich liquid stream composed of an oxygen-rich liquid column bottoms produced in the lower pressure column; the oxygen-rich liquid stream is divided into the first subsidiary liquid stream and the second subsidiary liquid stream; a pump is positioned within the conduit to pressurize the second subsidiary liquid stream and thereby to produce a pressurized liquid product stream that is warmed within the main heat exchanger to produce the heated product stream; means for forming a further compressed air stream is positioned between the pre-purification unit and the main heat exchanger; the main heat exchanger is configured to liquefy the further compressed air stream and thereby form a liquid air stream; the main heat exchanger is in flow communication with at least the lower pressure column to introduce at least part of a liquid air stream into the lower pressure column; and an expansion valve is positioned between the main heat exchanger and the lower pressure column to reduce pressure of the at least part of the air stream prior to introduction into the lower pressure column.

16. The apparatus of claim 15, wherein the constant speed turboexpander is connected to the higher pressure column such that the exhaust stream is introduced into the higher pressure column.

17. The apparatus of claim 15, wherein: the main heat exchanger is positioned in flow communication with the pre-purification unit so that part of the air, after having been compressed and purified, is cooled within the main heat exchanger and introduced into the higher pressure column; and the constant speed turboexpander is connected to the lower pressure column so that the exhaust stream is introduced into the lower pressure column.

18. The apparatus of claim 15, wherein: first and second booster compressors are in flow communication with the pre-purification unit so that first and second subsidiary streams, formed from at least part of a compressed and purified air stream discharged from the pre-purification unit, are further compressed in the first and second booster compressors, respectively and thereby respectively form the compressed refrigerant stream and the further compressed air stream; the further compressed air stream forming means is the second booster compressor; and the booster compressor within the booster compressor branch is a third booster compressor.

19. The apparatus of claim 15, wherein: a first booster compressor is in flow communication with the pre-purification unit so that at least part of a compressed and purified air stream is further compressed; a second booster compressor and the branched flow path are connected to the first booster compressor so that a first subsidiary stream discharged from the first booster compressor forms the compressed refrigerant air stream and a second subsidiary stream discharged from the first booster compressor is further compressed in the second booster compressor to form the further compressed air stream; the further compressed air stream forming means is the second booster compressor; and the booster compressor within the booster compressor branch is a third booster compressor.

20. The apparatus of claim 15, wherein: a first booster compressor is in flow communication with the pre-purification unit so that at least part of a compressed and purified air stream is further compressed; a second booster compressor is situated between the first booster compressor and the branched flow path and the main heat exchanger is in flow communication with the first booster compressor so that a first subsidiary stream discharged from the first booster compressor is further compressed in the second booster compressor and forms the compressed refrigerant air stream and a second subsidiary stream flow to the main heat exchanger and forms the further compressed air stream; the first compressed air stream forming means is the second booster compressor; and the booster compressor within the booster compressor branch is a third booster compressor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] While the specification 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:

[0027] FIG. 1 is a schematic of a process flow diagram of an air separation plant designed to carry out a method in accordance with the present invention;

[0028] FIG. 2 is an alternative embodiment of FIG. 1;

[0029] FIG. 3 is a detailed schematic drawing of a control system that is used in controlling a bypass system of the present invention that is used in the air separation plants shown in FIGS. 1 and 2; and

[0030] FIG. 4 is a graphical representation of the efficiency of a typical turboexpander on the basis of specific diameter versus specific speed.

DETAILED DESCRIPTION

[0031] With reference to FIG. 1, an air separation plant 1 in accordance with the present invention is illustrated. As will be discussed, air separation plant 1 is designed to rectify air by compressing and purifying the feed air stream 10, cooling the resulting compressed and purified air within a main heat exchanger 2 and then distilling the air within a distillation column system 3 to produce liquid oxygen and nitrogen product streams 130 and 114, respectively, as well as a pressurized oxygen product stream 136 and a nitrogen product stream 122 as a vapor. However, this is for exemplary purposes only in that the present invention could be used in connection with an air separation plant designed to produce an argon product that would also be taken as a liquid or other product slates of oxygen and nitrogen. Air separation plant 1 is provided with a bypass system 4 in accordance with the present invention to varying the pressure ratio across a turboexpander 64 and thereby vary the refrigeration imparted to the air separation plant 1 during high and low rates of production of the liquid products.

[0032] More specifically, feed air stream 10 is compressed by a main air compressor 12 having inlet guide vanes 13 to produce a compressed air stream 14. Compressed air stream 14 is then introduced into a prepurification unit 16 to produce a compressed and purified air stream 18. As known in the art, the prepurification unit 16 is designed to remove higher boiling impurities from the air such as water vapor, carbon dioxide and hydrocarbons. Such prepurification unit 16 can incorporate adsorbent beds operating in an out of phase cycle that is a temperature swing adsorption cycle or a pressure swing adsorption cycle or combinations thereof.

[0033] The compressed and purified air stream 18 is than introduced into a booster compressor 20 and then divided into first and second subsidiary streams 22 and 24. First subsidiary stream is further compressed in a booster compressor 26 of the bypass system 4 to form a compressed refrigerant stream 28 and second subsidiary stream 24 is further compressed in a booster compressor 30 to form a further compressed air stream 32 for purposes that will be discussed hereinafter.

[0034] It is to be noted that various arrangements of booster compressors are possible in accordance with the present invention. In this regard, only two booster compressors of the type mentioned above are possible in embodiments of the present invention. For instance, an embodiment is possible in which booster compressor 20 is absent. In such case, a first of the booster compressors 26 further compresses the first subsidiary stream, formed from part of the compressed and purified air stream 18, to produce the compressed refrigerant air stream 28 and a second of the booster compressors 30 further compresses the second subsidiary stream, formed from another part of the compressed and purified air stream 18, to produce the further compressed air stream 32, albeit at a lower pressure than the further compressed air stream 32 discussed above. Another possibility is to delete booster compressor 30. In such case, the compressed and purified air stream 18 would be compressed in a first of the booster compressors, booster compressor 20, the first subsidiary stream would be compressed in a second of the booster compressors, booster compressor 26, to form the compressed refrigerant stream 28 and the second subsidiary stream 24 would be the further compressed air stream. In yet another embodiment, booster compressor 26 would not be present and therefore, the compressed and purified air stream 18 would be compressed in a first of the booster compressors, booster compressor 20, the first subsidiary stream would form the compressed refrigerant stream and the second subsidiary stream 24 would be compressed in a second of the booster compressors to, booster compressor 30 to form the further compressed air stream 32.

[0035] As will be discussed, the further compressed air stream 32 is necessary in the illustrated embodiment to heat part of an oxygen-rich liquid stream 128 that is pumped to produce a pressurized liquid product stream 136. However, embodiments of the present invention are possible in which there is no such pressurized product; and therefore, the further compressed air stream 32 would not be necessary. In such case, a possible embodiment could entail the use booster compressor 20 alone to create a compressed refrigerant stream from part of the compressed and purified air stream 18. Another part of the compressed and purified air stream would be introduced into the distillation column system 3 for rectification.

[0036] The compressed refrigerant air stream 28 is then introduced into a branched flow path 34 of the bypass system 4 that has a bypass branch 38, a booster compressor branch 40 having a booster compressor 42 and a recycle branch 44. The branched flow path 34 discharges a compressed output stream 46, composed of the compressed refrigerant air stream 28, that has a pressure that is dependent upon whether the compressed refrigerant air stream is introduced into the bypass branch 38 or the booster compressor branch 40. When the refrigerant stream 28 is introduced into the booster compressor branch 40, it is further compressed by booster compressor 42 to further compress the compressed refrigerant stream 28 and thereby allow production of the pressure of compressed output stream 46 at an increase in pressure over that obtained when the compressed refrigerant air stream is introduced into the bypass branch 38. When the compressed refrigerant stream 28 is introduced into the bypass branch 38, the booster compressor 42 is bypassed and therefore, the compressed output stream 46 is at a pressure, less piping and valve losses, that is about equal to that of the incoming compressed refrigerant stream 28 which of course is less than when such stream is further compressed by the booster compressor 42. The recycle branch 44 allows a pressure ratio to be maintained across the booster compressor 42 independently of any redirection of the compressed refrigerant air stream 28 between the bypass branch 38 and the booster compressor branch 40 to prevent the booster compressor 42 from encountering surge operational conditions.

[0037] In a manner that will be discussed in more detail hereinafter, diversion of the compressed refrigerant air stream 28 between the booster compressor branch 40 and bypass branch 38 is actively controlled by first and second flow control valves 48 and 50, situated in booster compressor branch 40 and bypass branch 38, respectively and passively by check valves 52 and 54 located in such branches. A third valve 56 in the recycle branch 44 actively controls flow of the recycle stream within the recycle branch 44. Valves 58, 60 and 62 control introduction of flow of a purge stream, composed of purified air, into booster compressor 42 when the same is in a deactivated condition.

[0038] The compressed output stream 46 is then introduced into main heat exchanger 2 where it is partially cooled to an intermediate temperature, between temperatures of the warm and cold ends of the main heat exchange to produce a partially cooled stream 63 that is introduced into a turboexpander 64 that generates an exhaust stream 66. Exhaust stream 66 is introduced into distillation column 3 to impart the refrigeration generated by the expansion. As could be appreciated by those skilled in the art, although the compressed output stream 46 is partially cooled within the main heat exchanger 2, in a possible embodiment of the present invention, compressed output stream 46 could bypass the main heat exchanger 2 and be directly introduced into turboexpander 64, in which case the turboexpander 64 would be a warm expander and an additional turboexpander could be provided to impart a base load of refrigeration in or to maintain the air separation plant of such embodiment in heat balance.

[0039] In the illustrated embodiment, the work of expansion generated by turboexpander 64 is dissipated in producing electricity by being coupled to an electric generator 67. The pressure ratio across the turboexpander 64 and therefore, the refrigeration generated thereby will be dependent upon the pressure of the compressed output stream 46 which, as described above, is dependent upon whether compressed refrigerant air stream 28 was introduced into bypass branch 38 and is thereby generated at a lower pressure or introduced into booster compressor branch 40 and is thereby generated at a higher pressure. When compressed output stream 46 is at a higher pressure, the pressure ratio across turboexpander 64 will increase to in turn increase the refrigeration generated and the rate at which liquid products are able to be produced. Alternatively, when compressed output stream 46 is at a lower pressure, the pressure ratio across turboexpander 64 will decrease to in turn decrease the refrigeration generated and the rate at which the liquid products are produced.

[0040] During both high and low rates of liquid production, the air to be distilled within distillation column system 3 is cooled in main heat exchanger 2. In this regard, the compressed refrigeration air stream 28, after passage through bypass branch 38 or booster compressor branch 40 is partially cooled, as compressed output stream 46, prior to being introduced into the turboexpander 64. The further compressed air stream 32 is fully cooled within the main heat exchanger 2 and is condensed to produce a liquid air stream 68. Main heat exchanger 2 can be of brazed aluminum construction and although illustrated as a single unit, could be a series of such units operated in parallel. Further, banked instruction is also possible in which the high pressure streams, such as further compressed air stream 32 and pumped liquid oxygen stream 134, to be discussed, are subjected to indirect heat exchange within a separate high pressure unit.

[0041] Distillation column system 3 has a higher pressure column 70 and a lower pressure column 72 thermally linked in a heat transfer relationship by a condenser reboiler 74 and operating at a lower pressure than the higher pressure column 70. The exhaust stream 66 is introduced into the higher pressure column 70 and the liquid air stream is expanded to a pressure of the higher pressure column by means of an expansion valve 76 and divided into first and second subsidiary liquid air streams 78 and 80. First subsidiary liquid air stream is introduced into the higher pressure column 70 and second subsidiary air stream 80 after expansion in an expansion valve 82 to a pressure of the lower pressure column 72 is introduced into the lower pressure column 72.

[0042] Higher pressure column 70 is provided with mass transfer contacting elements 84 and 86, such as structured packing or trays or a combination of packing and trays to contact descending liquid and ascending vapor phases of the air that is introduced into the higher pressure column 70 by means of the first subsidiary liquid air stream 78 and the exhaust stream 66. Due to such contact, as the descending liquid phase will be evermore enriched in oxygen as it descends and the ascending vapor phase will become ever more enriched in nitrogen as it ascends to produce a nitrogen-rich vapor column overhead 88 and a crude liquid oxygen column bottoms 90, also known as kettle liquid. A crude liquid oxygen stream 92 is withdrawn from the higher pressure column 70, valve expanded in expansion valve 94 to the pressure of the lower pressure column 72 and then introduced into the lower pressure column 72 for further refinement. The crude liquid oxygen stream 92 can be subcooled prior to such introduction in an embodiment of the present invention.

[0043] The Lower pressure column 72 is also provided with mass transfer contacting elements 96, 98, 100 and 102 to again contact descending liquid and vapor phases to produce an oxygen-enriched liquid column bottoms 104 and a nitrogen-rich vapor column overhead 106. The condenser reboiler 74 partly vaporizes the oxygen-enriched liquid column bottoms 104 through indirect heat exchange with a nitrogen-rich vapor stream 105 composed of the nitrogen-rich vapor column overhead 88 of the higher pressure column 70. The vaporization initiates formation of the ascending vapor phase within the lower pressure column 72 and condenses the nitrogen-rich vapor to produce a nitrogen-rich liquid stream 106. Nitrogen-rich liquid stream 106 is divided into first and second subsidiary nitrogen-rich liquid streams 108 and 110. First subsidiary nitrogen-rich liquid stream 108 is introduced into the top of the higher pressure column 70, as reflux, to initiate formation of the descending liquid phase. Second subsidiary nitrogen-rich liquid stream 110 is then subcooled in a subcooling heat exchanger 112 and optionally divided into a liquid nitrogen product stream 114 and a liquid nitrogen reflux stream 116 that after expansion in valve 118 to a compatible pressure is introduced into the top of the lower pressure column 72 to initiate formation of the descending liquid phase.

[0044] A nitrogen-rich vapor stream 120 composed of the nitrogen-rich vapor column overhead 106 is withdrawn from the top of the lower pressure column 72, partly warmed in subcooling heat exchanger 112 and then fully warmed in the main heat exchanger to produce a nitrogen product stream 122. Additionally, a waste nitrogen stream 124 can be removed from the lower pressure column 72, at a level below that at which the nitrogen-rich vapor stream 120 is withdrawn, partly warmed in the subcooling heat exchanger 112 and then fully warmed in the main heat exchanger 2 to form a warmed waste nitrogen stream 126. The warming of such streams in the subcooling heat exchanger 112 provide the indirect heat exchange necessary to subcool the second subsidiary nitrogen-rich vapor stream 110. The further warming of such streams in the main heat exchanger 2 help to cool the incoming air. The warmed waste nitrogen stream 126 can be used to regenerate adsorbents within adsorbent beds of the pre-purification unit 16.

[0045] An oxygen-rich liquid stream 128, composed of residual oxygen-rich liquid column bottoms 104, can be removed from the lower pressure column 72 and then divided into a liquid oxygen product stream 130 and a remaining stream is pressurized by a pump 132 to produce a pumped liquid oxygen stream 134. Pumped liquid oxygen stream 134 is then fully warmed in the main heat exchanger 2 to produce a pressurized oxygen product stream 136. Depending upon the degree of pressurization, pressurized oxygen product stream 136 is vaporized in the main heat exchanger or is heated to produce such product stream as a supercritical fluid. The heat exchange for such heating is provided by the further pressurized air stream 32. As can be appreciated, if the oxygen product stream as a vapor were to be taken without further pressurization, the further pressurized air stream 32 would not be necessary at a pressure suitable to provide the necessary heat exchange duty. Also, it is to be mentioned that the liquid oxygen product stream 130 could be the only liquid product taken or the nitrogen liquid product stream 114 could be the only liquid product stream taken. In this regard, if a nitrogen stream were desired at pressure, part of the liquid nitrogen product stream 114 could similarly be pressurized by means of a pump.

[0046] With brief reference to FIG. 2, in an air separation plant 1, a main air stream 138 formed from part of the compressed and purified air stream 18 is fully cooled within the main heat exchanger 2 and then introduced into the higher pressure column 70. The exhaust stream 66 produced by the turboexpander 36 is introduced into the lower pressure column 72. The description of the features of air separation plant 1 is otherwise the same as those discussed relative to air separation plant 1.

[0047] As mentioned above, a system of valves is incorporated into the bypass system 4 to control flow within the branches of the branched flow path 34. While manual control is conceivably possible, the control is preferably automated with the use of a controller 140 shown in FIG. 3. Controller 140 could be a programmable logic controller obtainable from a variety of sources or could alternatively be incorporated into the plant control system of the air separation plant 1. Control system 140 is activated by user input 142 to set the plant into modes of production in which the liquid products are produced at higher or lower rates. Control system 140 is designed to control valve operation so that diversion of the compressed refrigerant air stream 28 between the booster compressor branch 40 and the bypass branch 34 is gradual and with independent control of the recycle of the recycle stream within the recycle branch 44, that acts independent of the bypass branch 34, to prevent the booster compressor 42 from entering surge. In turn this allows a far greater range in pressure ratio across the turboexpander 36 than is possible in the prior art and therefore liquid production.

[0048] Specifically, when the air separation plant it to be switched from a low liquid rate of production to the high rate of liquid production, first flow control valve 48 is then gradually opened and second control valve 50 within the bypass branch 38 gradually closes to gradually divert the compressed refrigerant stream 28 from the bypass branch 38 to the booster compressor branch 40. It is to be noted that the term flow control valve as used herein and in the claims means a valve able to control or meter flow. The control signals for the first and second flow control valves 48 and 50 are transmitted through electrical connections 144 and 146, respectively. In this regard, preferably the opening and closing times should be about 5 seconds. As such ramp functions are programmed into the controller 140 to accomplish the opening and closing of such flow control valves. As could be appreciated by those skilled in the art, a certain degree of tuning would be required in practice to completely perfect such ramp functions. Preferably, a purge stream, composed of purified air, has been introduced into the booster compressor 42 previous to the diversion of the compressed refrigerant stream 28 to the booster compressor branch 40. In order to end the introduction of the purge stream, valve 58 is set in the closed position and check valve 60 closes under the increased pressure within the booster compressor branch 40. Thereafter, a valve 62 is set in the closed position. The control system 140 then activates booster compressor 42 through electrical connection 148. During the previous time at which the compressed refrigerant air stream has been diverted to bypass branch 38, third valve 56 was set in the open position. However, even in an embodiment in which the third valve 56 is in a closed position, it would be reset in an open position. This allows compressed gas from the compressed refrigerant air stream 28 to flow from the outlet of the booster compressor 42 to the inlet thereof and thereby prevent surge. When the booster compressor branch pressure, within the booster compressor branch 40, exceeds the bypass branch pressure, within the bypass branch 38, check valve 54 closes to prevent the flow from reversing in the booster compressor branch 38. At the same time, a valve 52 opens. This can be automatic and therefore, valve 52 can be a check valve. It of course, can also be a remotely activated valve that is activated upon the closing of check valve 54. Check valve 54 could of course also be a remotely activated valve. At this point, second flow control valve 50 can preferably be set in a closed position and third valve 56 in the bypass branch 44 is reset into the closed position. This reset occurs from check valve 54 closing and valve 52 opening and the position of such valves being sensed by the controller 140 through electrical connections 150 and 152. Although not illustrated, turboexpander 36 can be provided with inlet guide vanes to allow the turboexpander 36 to be adjusted for stable operation.

[0049] When air separation plant is to be switched from the high rate of liquid production to the low rate of liquid production, compressed refrigerant air stream 28 is gradually diverted from the booster compressor branch 40 to the bypass branch 38. To such end, second control valve 56 is gradually opened to gradually increase flow of the compressed refrigerant air stream 28 into the bypass branch 34. At the same time, first flow control valve 48 gradually closes to gradually decrease the flow of the compressed refrigerant air stream 28 within the booster compressor branch 44. At the same time, the third valve 56 in the bypass branch is commanded to open by controller 140 to allow the flow of a recycle stream within the recycle branch 44 from the outlet to the inlet of the booster compressor 42 to prevent surge. Once the bypass branch pressure exceeds the booster compressor branch pressure check valve 54 opens, valve 52 closes, controller 140 closes valve 56 and booster compressor 42 is deactivated. As mentioned above, the term deactivated as used herein and in the claims encompasses either an operation in which booster compressor 42 is turned off or it is set in a low pressure mode of operation. In the low pressure mode of operation the power is reduced and the compressor operates at a very low inlet pressure and at a reduced mass flow rate. In addition to recycle, the low pressure mode of operation would require suitable adjustment of inlet guide vanes to the compressor. In any event, turning off the booster compressor 42 or setting it in a low pressure mode will result in less electricity being consumed during turndown of liquid production.

[0050] At this point, the purge air stream is introduced into booster compressor 42 to prevent the entry of untreated air. The problem with ambient air entry into the booster compressor 42 is that the ambient air has not been purified of the higher boiling contaminants; and without such system, the higher boiling contaminants could enter the main heat exchanger 2 and the distillation column 3 and solidify. The purge air stream is composed of purified air and may be obtained from a bleed stream from an operating compressor that is also used in supplying instrument air to air separation plant. In this regard, as known in the art, booster compressor 42 can be provided with labyrinth seals that surround the outer portion of the compressor impeller to prevent high pressure air from escaping from such region. In such an arrangement, a balance of forces acting on the impeller of the compressor is obtained by balancing compressor eye side forces at the inlet of the compressor and forces acting at the back side of the impeller. The forces on the back side of the impeller are produced by high pressure compressed air acting at an outer, annular region of the impeller, outbound of the labyrinth seals, and at an inner circular region of the back side of the impeller, inbound of the labyrinth seals, by providing air from the inlet of the compressor to such inner region of the impeller. Assuming that the booster compressor 42, when deactivated, is operated in the low pressure mode the pressure at the inlet of the booster compressor 42 will be low, typically about 5 psia. When first flow control valve 48 is set in a fully closed position, check valve 60 opens due to such low pressure and the slightly higher pressure of the instrument air. At this point, valve 62 is set in an open position through control action effectuated through an electrical connection 154 between valve 62 and controller 140. Thereafter, valve 58 is reset into an open position by means of an electrical connection 156 between controller 140 and valve 58. The purge air stream simply escapes from the labyrinth seals to the interior of the compressor and through the volute to the outlet of the compressor to prevent ambient air from entering the booster compressor 42. In lieu of such an operation, it also is possible for the purge air stream to simply escape from the outlet of the compressor and be discharged through valve 58.

[0051] As can be appreciated, the density of air entering air separation plant 1 will vary due to such factors as temperature and humidity. However, it is important that turboexpander 64 be exposed to specific pressure ratios during both high and low liquid rates of production and incoming pressure will have an effect on the pressure of compressed refrigerant air stream 28 and therefore, such pressure ratios. Preferably in order to compensate for variation in air density, the pressure of the compressed output stream 46 can be controlled to in turn control such pressure ratios. The pressure of the compressed refrigerant air stream 28 is regulated by means of a pressure sensor 158 that generates a signal referable to pressure that is sent to a proportional, integral and derivative (PID) controller 160 that in turn generates a control signal to control the opening of a valve 162 to maintain such pressure at a set point. When first control valve 48 is set in the open position and booster compressor 42 is activated, first control valve 48 can be used to regulate entry pressure into booster compressor 42. To such end, a pressure sensor 164 can be provided to generate a signal referable to pressure that is fed to PID controller 166. PID controller 166 has a preprogrammed set point to adjust the opening of first control valve 48 for such purposes.

[0052] In both of the air separation plants described above, during a turndown mode of operation when less liquid is desired, less liquid must be taken. To such end control valves 170 and 172 are provided to control the flow rate of the liquid oxygen and nitrogen product streams 130 and 114, respectively. As can be appreciated if during a turndown mode of production the liquid withdrawal rate of the liquid oxygen and nitrogen products were unchanged, the level of the oxygen-rich liquid column bottoms 104 within the lower pressure column 72 would drop resulting in less boilup in the lower pressure column 74 and less liquid nitrogen reflux in the higher pressure column 70. In this regard, it is preferable to maintain the level of the oxygen-rich liquid column bottoms 104 constant. Therefore, although not illustrated, flow of the liquid could be controlled by local PID controllers reacting to liquid flow and targets set by a master controller for such liquid flow. The master controller would in turn be responsive to a signal from a level detector placed within the lower pressure column 72 to measure the liquid level of the oxygen-rich liquid column bottoms 104. Alternatively, the control could be to reset control valves automatically upon entering high and low modes of liquid production. A yet other alternative is to allow for manual control by plant operation personnel.

[0053] As can be appreciated, during the low rate of liquid production, less oxygen and nitrogen molecules as a liquid will be removed from the air separation plant 1. If nothing further is done; and if the flow rate through the main air compressor 12 is maintained at a constant level, the flow rate of the gaseous products will increase such as the pressurized oxygen productions stream 136. However, it is often desired to maintain such stream at a constant flow rate. In such case, the inlet guide vanes 13 of main air compressor 12 can be adjusted to reduce the flow of the incoming feed air stream 10 entering air separation plant 1 to maintain gaseous production at a constant level.

[0054] With brief reference to FIG. 4, a graphical representation of the efficiency of a typical turboexpander is illustrated as a function of specific speed (Ns) and specific diameter (Ds) with iso-efficiency lines is illustrated. Such a chart illustrates the entire operating range of the turboexpander over a large speed variation, pressure ratio variation and volumetric flow variation. In case of turboexpander 64 or any turboexpander, as turbine inlet pressure changes at constant inlet temperature and constant flow, the volume through the machine increases and the expansion ratio lowers. Typical design practice is to operate the machine near point C, and thus to maximize the design efficiency. In an air separation plant 1 of the present invention, where the turbine inlet pressure is varied to manipulate the liquid production rates, this is not the ideal choice. Instead, the turboexpander should be designed so that the turboexpander is capable of operating at locations of high liquid production, point A and low liquid production, point B, while maintaining a high efficiency at both points, while not necessarily reaching the peak efficiency in either case. By choosing the points to straddle the ideal efficiency, both points are assured to be in the good rather than ideal efficiency region, minimizing the performance penalty in both high and low liquid rates of production. In this regard, it is preferable that the range between peak efficiency and at either the high or low rates of liquid production is not greater than 5 percent. It is, however, possible to operate air separation plant 1 at a mid pressure ratio between points A and B, at constant mass flow, where the ideal efficiency could be reached. Although the present invention will typically be used in connection with a constant speed booster compressor 42, the present invention also encompasses the use of a variable speed booster compressor at which the peak efficiency will be maintained.

[0055] The turboexpander 64 will operate even if the low pressure high volume case has a very poor efficiency. This is due to the nature and thermodynamic favorability of expansion through a turbine as opposed to a compression in a booster. However, across all operating ranges, the turboexpander 64 loading device must be able to absorb the generated power to prevent over speed. This load can be in the form of an electric generator 62, a coupling to a gearbox such as illustrated in U.S. Pat. No. 5,901,579 or to an oil or air brake. However, the work performed by a turboexpander used in connection with the present invention should not be directly dissipated in a single compressor for instance, in a booster loaded turboexpander where a compressor and the turboexpander are mounted on a common pinion. In such case, as the pressure ratio changes across the turboexpander, the speed of turboexpander will change and therefore, the compressor. As a result, the operating range will be narrow because as the speed decreases during period of low liquid production, such compressor will be driven towards surge.

[0056] In a turboexpander, such as turboexpander 64 used in connection with the present invention, since the pressure on turboexpander 56 is variable, the turboexpander 64 must take into account the widely varying rotor thrust conditions caused by the variation in eye and tip pressure's on the stages. If this is not controlled, the impeller used in such a device could contact stationary parts, drive gears could be overstressed or other damage could occur. This thrust loading can be alleviated using several different schemes known in the art such as classical thrust bearings able to bear such loads, integral gear thrust collars should the turboexpander 64 be directly mounted on an integral gear machine as shown in U.S. Pat. No. 5,901,579, balance pistons, impeller mounted balance pistons and dry gas seals. It is to be noted that booster compressor 42 would experience similar variability in loading and as such, could incorporate the means discussed above to counter high variable thrust loadings.

[0057] While the present invention has been described with reference to preferred embodiments, as will occur to those skilled in the art, numerous changes, additions and omission can be made without departing from the spirit and scope of the invention as set forth in the appended claims.