Cryogenic air separation process

Abstract: A cryogenic air separation process and apparatus in which air is cooled in heat exchanger 16 after compression in compressor 10 and purification in a purifier 12. The air is rectified in a rectification column 26 to produce an oxygen rich liquid. An argon-oxygen stream containing liquid lean in nitrogen is separated in column 50 to form oxygen and argon streams. Argon vapour is condensed in condenser 59 to supply reflux to the argon column 50. An oxygen rich liquid stream is expanded through valve 64 to a pressure at which the oxygen rich liquid is below the condensation temperature of the argon vapour and is then vaporized against the condensing argon vapour. The vaporized oxygen rich liquid is then introduced into a nitrogen stripper column 42 and nitrogen is stripped therefrom by a stripper gas to produce the argon-oxygen liquid which is introduced into the argon column 50. The nitrogen stripper column 42 is regulated to operate in a predetermined pressure range so that the entry level at which oxygen enters the nitrogen stripper column 42 is at a pressure no greater than the pressure of the oxygen rich liquid after expansion. Argon is removed from the top of the argon column 50 as a product. The process and apparatus can be operated to produce high purity argon vapour or liquid very lean in nitrogen and oxygen with the use of trays and/or structured packing as liquid contacting mass transfer elements in the columns. Additionally, high purity oxygen and nitrogen products can also be produced by such process and apparatus.

Description: The present invention relates to a process for cryogenically separating air to produce high purity argon. More particularly, the present invention relates to such a process employing a three column distillation system in which argon is produced in an argon column having a sufficient number of theoretical stages to produce the high purity argon as a product. Conventionally, argon is separated from air in a three column distillation system which consists of a high pressure column, a low pressure column and an argon column. In such a system, the high pressure column produces an oxygen rich liquid, the low pressure column further refines the oxygen rich liquid to produce an argon enriched mixture as a vapour, and the argon column refines the argon enriched mixture to produce crude argon. In order to provide reflux for the argon column, a stream of the crude argon is condensed in a head condenser by a subcooled and expanded stream of the oxygen rich liquid from the high pressure column. The crude argon contains oxygen and nitrogen which must be removed to produce high purity argon. Therefore, the crude argon is upgraded, generally through catalytic combustion to remove the oxygen followed by adsorbers to remove formed water and further distillation to remove nitrogen. Theoretically, it is possible to increase the number of stages of separation within the argon column to enhance the separation of argon and oxygen. However, at least with argon columns employing trays or plates, this is not practical because the resultant pressure drop would lower the condensation temperature of the crude argon and therefore raise the degree of expansion required of the oxygen enriched liquid such that the oxygen enriched liquid would be at too low a pressure to flow into the low pressure column. The operating pressure range of the low pressure column cannot not be reduced to accommodate such a highly expanded oxygen enriched liquid because the crude argon feed flows from the low pressure column to the argon column under impetus of the pressure of the low pressure column. Alternative column arrangements are known. For example, DE-B-1 922 956 discloses an arrangement of a high pressure column and two low pressure columns for separating air into oxygen and nitrogen but not argon products. A first low pressure column is fed from the high pressure column and is reboiled by air. A second low pressure column operates at a pressure a little below that of the first low pressure column, is fed from the first low pressure column with argon-enriched oxygen and is reboiled by heat exchange with nitrogen separated in the high pressure column. There are known three column plants that are designed with a sufficient number of theoretical stages in their argon column to separate oxygen from the argon to an extent that catalytic combustion is not required in the upgrading of the crude argon. An example of this can be found in US-A-5,019,145 in which 150 theoretical stages are employed in an argon rectification column utilizing low pressure drop packings. The use of such packings prevents the excessive pressure drop that would otherwise occur with plates or trays. US-A-5,133,790 is an example of cryogenic rectification process in which both oxygen and nitrogen concentrations are directly reduced so that a high purity argon product can be withdrawn directly from the argon column without subsequent catalytic and distillation stages. In this patent, the low pressure column is operated with a sufficient number of theoretical stages (provided by structured packing) such that the nitrogen concentration in the feed to the argon column is less than 50 parts per million. Since less nitrogen is being fed to the argon column, there will be a lower concentration of nitrogen in the argon produced in the argon column. In order to remove the oxygen, the argon column can be fabricated with structured packing to provide approximately 150 theoretical stages, as called for in US 5,019,145, to effect the degree of oxygen separation required for the production of the high purity argon product. The prior art patents, discussed above, both depend on the use of a low pressure drop packing in at least the argon column to prevent excessive pressure drop. As will be discussed, the present invention provides a process for producing a high purity argon product directly from the argon column that does not depend on structured packing for its operability. In fact, both the argon and low pressure columns can be conventionally designed with sieve trays, a low pressure drop packing or any other type of liquid-gas contact device or any combination thereof. Further advantages of the present invention will become apparent from the following discussion. In accordance with the present invention, a cryogenic air separation process is provided to produce high purity argon and has the features set out in claim 1. As mentioned previously, the columns of the present invention can utilize packing, sieve trays, or any other liquid-gas mass transfer device, all at the option of the designer because the present invention does not depend on structured packing for its operation. Rather, the present invention utilizes a nitrogen stripper column in lieu of a low pressure column that is not coupled to the argon column in a manner contemplated in the prior art. In the prior art the argon column must be operated over a pressure range that is less than the pressure of the argon enriched draw pressure of the low pressure column. Since in the present invention the feed to the argon column is a liquid, the operating pressure range of the nitrogen stripper column can be set at or less than the pressure of the argon column feed point because in order to feed the liquid into the argon column the head of the feed can be raised either by pumping or more simply, by setting the nitrogen stripper column at a sufficient height above the entry point of the feed into the argon column. It should be noted that in order to raise the pressure of a vapour, the vapour is compressed. This is not normally done with an oxygen containing vapour such as the argon enriched vapour because of the expense of such compressors as well as the dangers inherent in their use. Since the nitrogen stripper column can be regulated to operate over a lower pressure range than the argon column, the argon column can have a sufficient number of theoretical stages to effect an oxygen separation from the feed without the use of structured packing. Moreover, since nitrogen is being stripped from the oxygen enriched liquid in the nitrogen stripper column, the liquid feed to the argon column will be produced with very low concentrations of nitrogen. Hence, a high purity argon product can be taken directly from the argon column. The term "column" as used herein means a column in which an ascending vapour stream is intimately contacted in a heat and mass transfer relationship with a descending liquid stream by conventional mass transfer elements such as trays, plates or packing elements, either random or structured packings, any combination of the above, or any other type of liquid-gas mass transfer device. Furthermore, by a high purity argon product as used herein is meant one containing by volume, less than about 1000 ppm of oxygen and less than about 1000 ppm nitrogen. The present invention is capable of producing a high purity argon product having even lower oxygen and nitrogen impurity concentrations. The phrase "lean in nitrogen" as used herein means a concentration of impurities by volume of less than about 30 ppm. The process according to the invention will now be described by way of example with reference to the drawing which is a schematic flow diagram of a cryogenic air separation apparatus and process in accordance with the present invention. Referring to the accompanying drawing, air is compressed by compressor 10 and is then purified by a purifier 12 to remove carbon dioxide, moisture and hydrocarbons from the air. Purification unit 12 can be formed of alumina or zeolite molecular sieve beds operating out of phase so that while one bed is in use the other bed is regenerated. An after cooler 14 is provided to remove the heat of compression. After cooler 14 can use water or hydro-chloro-fluorocarbon as refrigerant to remove heat from the compressed and purified air stream. Thereafter, the air is cooled to a temperature suitable for rectification, conventionally, at or near its dew point, by a main heat exchanger 16 of plate and fin construction having first, second, third, and fourth passes designated by reference numerals 18, 20, 22 and 24. The air passes through pass 18 and then is introduced into the bottom of a rectification column 26. A nitrogen rich vapour is produced at the top of rectification column 26 (designated by reference numeral 27) and an oxygen enriched liquid column bottom is produced in the bottom thereof (designated by reference numeral 28). The nitrogen rich vapour after condensation is in part re-introduced into top 27 of rectification column 26 as reflux and is also formed into a stream 32. An oxygen enriched liquid stream 34 is removed from the bottom of rectification column 26 and is then sub-cooled in a sub-cooler 39 which is of conventional construction, again, preferably of plate and fin type. Oxygen enriched liquid stream 34 is then divided into first and second partial streams 36 and 38. The second partial stream 38 is then fed into a nitrogen stripper column 42 at a level thereof having a concentration compatible with that of second partial stream 38. It is to be noted that second partial stream could be expanded to a lower pressure or as illustrated, simply allowed to flash into nitrogen stripper column 42. Although not illustrated, in the case of a packed column a flash separator would have to be used to introduce both gas and liquid components into the column. Within nitrogen stripper column 42, the oxygen enriched liquid is then stripped by a stripper gas to produce an argon-oxygen containing liquid lean in nitrogen at bottom 44 of nitrogen stripper column 42. A high purity nitrogen tower overhead forms at the top of nitrogen stripper column 42, designated by reference numeral 46. The argon-oxygen liquid is then fed as a stream 48 into argon column 50. The argon-oxygen liquid thus introduced into argon column 50 is in part vaporized and is also separated so that liquid oxygen collects in the bottom of argon column 50, designated by reference numeral 52, and high purity argon collects in the top of argon column 50, designated by reference numeral 54. The vaporized argon-oxygen is then introduced into bottom 44 of nitrogen stripper column 42 as an argon-oxygen vapour stream 56 to serve as the stripper gas. The oxygen collecting in the bottom 52 of the argon column 50, is vaporized against condensing nitrogen in a condenser re-boiler 58. The vaporization of the oxygen initiates the formation of an ascending vapour stream. This vapour stream becomes progressively leaner in oxygen until a high purity argon vapour is formed at top 54 of argon column 50. The argon vapour is condensed and re-introduced into top 54 of argon column 50 as reflux to initiate the formation of a descending liquid stream which becomes progressively leaner in argon as it descends within argon column 50. This is done through the use of a head condenser 59, again of conventional construction, and connected to argon column 50 so that an argon vapour stream 60 is removed from argon column 50, is condensed, and returned as a condensed argon liquid stream 62 back into argon column 50 as reflux. Such condensation occurs in head condenser 59 through indirect heat exchange with first partial stream 36 which, prior to entering head condenser 59, is expanded by an expansion valve 64 to a pressure at which the oxygen enriched liquid containing the first partial stream 36 is at a temperature below the condensation temperature of the argon vapour tower overhead contained with argon vapour stream 60. First partial stream 36 is vaporized within head condenser 59 against the condensation of the argon vapour and is then introduced into an appropriate level of nitrogen stripper column 42, that is, a level at which the concentration of oxygen, nitrogen and argon is compatible with the entry of first partial stream 36. It is understood that depending upon process requirements, first stream 36 could be the only oxygen enriched stream removed from rectification column 26 and further, that first stream might only be partially vaporized. In order for first and second partial streams 36 and 38 to flow into nitrogen stripper column 42 at levels of entry 64 and 66, the nitrogen stripper column 42 must have pressures that are no greater than the pressures of first and second partial streams 36 and 38 just prior to their entry. A preferred manner of effecting such control of the operating pressure range of nitrogen stripper column 42 is to control or regulate the pressure of argon-oxygen vapour stream 56, which serves as a stripper gas, upon its entry into bottom 44 of nitrogen stripper column 42. Such pressure regulation is effected through the use of a pressure regulator valve 68 which regulates the pressure of argon-oxygen vapour stream 56 and therefore the operating pressure range of nitrogen stripper column 42. In practice, in most possible embodiments in the present invention, nitrogen stripper column 42 will operate over a lower pressure range than argon column 50. A point worth mentioning here is that the lower pressure range of nitrogen stripper column 42 means that the highest pressure of nitrogen stripper column 42 is lower than the highest pressure found in argon column 50. A further point is that in such possible embodiments, argon column 50 will usually operate over a lower pressure range than rectification column 26. A liquid head is created to produce a flow of argon-oxygen liquid stream 48 into argon column 50. This is preferably accomplished by simply raising the level of nitrogen stripper column 42 so that gravity, provides the requisite head. Argon-oxygen stream 48 could alternatively be supplied with an increased head by pumping the argon-oxygen stream into argon column 50. A liquid argon product stream 70 is removed from head condenser 59. The argon product stream can either be a liquid argon condensate or vapour directly removed from the top of argon column 50 or any combination thereof. An oxygen product stream 72 composed of oxygen vapour is withdrawn from argon column 50 and sent through pass 24 of main heat exchanger 16 to help cool the incoming air. The high purity oxygen can be at least about 99.5%. It is to be understood that high purity argon products can be produced in accordance with the present invention with concomitant production of oxygen at lower purity levels. A product nitrogen stream 74 can be removed from top 46 of nitrogen stripper column 42 as well as a waste nitrogen stream 76 (removed below top 46 of nitrogen stripper column 42). Streams 74 and 76 pass through sub-cooler 39 and in indirect heat exchange with oxygen enriched liquid stream 34 and nitrogen rich stream 32 to sub-cool the same. Thereafter, streams 74 and 76 pass through passes 20 and 22 of main heat exchanger 16 and then out of the air separation apparatus as product and waste streams, respectively. In order to maintain heat balance of the illustrated air separation process and plant design, a partially cooled subsidiary air stream 78 ("partially cooled" because such stream is withdrawn from between the cold and warm ends of main heat exchanger 16) is diverted into a turboexpander 80. The exhaust of turboexpander 80 is then introduced into an appropriate level of nitrogen stripper column 42. As can be appreciated, the exhaust could in part be introduced into nitrogen stripper column 42. As mentioned previously, any of the columns illustrated in the figure could contain either trays or packing or combinations thereof. In the illustrated embodiment, rectification column 26 is provided with trays, nitrogen stripper column 42 and argon column 50 are provided with structured packing. Regardless of the mass transfer element employed, oxygen and argon products could be produced in the illustrated apparatus. It should be noted that in an air separation process and apparatus in accordance with the present invention, the exhaust of turboexpander 80 could be returned back into main heat exchanger 16 to provide refrigeration through the lowering of the enthalpy of the incoming air. It should also be noted that structured packing has a distinct advantage of providing a lower pressure drop than trays or plates and thus, a lower cost of operation. The following two examples (labelled "EXAMPLE 1" and "EXAMPLE 2") are computer simulations of plant operation showing the efficacy of the use of either structured packing or sieve trays in both nitrogen stripper column 42 and argon column 50. In EXAMPLE 1, rectification column 26 utilizes 40 trays operating at an efficiency of about 100% and a pressure drop of about 0.04 psia/tray. Structured packing, for instance 700Y manufactured by Sulzer Brothers Limited of Winterthur, Switzerland are used in both nitrogen stripper column 42 and argon column 50. In EXAMPLE 2, rectification column 26 utilizes 50 trays operating at an efficiency of about 100% and a pressure drop of about 275 Pa/tray (about 0.04 psia/tray). Trays are used in both nitrogen stripper column 42 and argon column 50. Such trays operate at an efficiency of about 70% and a pressure drop of about 275 Pa tray (about 0.04 psia/tray). EXAMPLE 1: Table of Flows, Temperatures, Pressures and Composition Stream Flow kg- moles/hr Temp Degree K Pressure Bara % N 2 % Ar %O 2 72 before main heat exchanger 16 105 92.98 1.35 0 0.27 99.73 70 4 89.09 1.23 0.1ppm 99.9992 8.3ppm 48 241.5 92.4 1.342 5ppb 7.9 92.1 56 before valve 68 132.5 92.4 1.342 5.5ppb 11.2 88.8 56 after valve 68 132.5 92.4 1.335 5.5ppb 11.2 88.8 32 after subcooling 208.4 81 5.25 99.97 0.03 1ppm 74 at top of nitrogen stripper column 42 260.5 79.5 1.3 99.985 0.015 0.3ppm 34 after subcooling 241.6 96 5.36 59.26 1.71 39.03 38 99.5 96 5.36 59.26 1.71 39.03 36 after vaporisation 142.1 87.03 1.35 59.26 1.71 39.03 76 at top of nitrogen stripper column 42 130.5 79.55 1.303 99.7 0.3 19ppm 10 prior to compression 500 298 1 78.113 0.931 20.956 10 after compression 500 293 5.8 78.113 0.931 20.956 78 after expansion 50 100.84 1.35 78.113 0.931 20.956 74 after passage through heat exchanger 38 260.5 97.51 1.2 99.985 0.015 0.3ppm 74 after passage through main heat exchanger 16 260.5 291.37 1.1 99.985 0.015 0.3ppm 76 after passage through heat exchanger 38 130.5 97.51 1.2 99.7 0.3 19ppm 76 after passage through main heat exchanger 16 130.5 291.37 1.1 99.7 0.3 19ppm 72 after passage from main heat exchanger 16 104.54 291.37 1.25 0 0.27 99.73 In the example given above, nitrogen stripper column 42 has approximately 60 theoretical stages. Stream 76 is withdrawn at theoretical stage 6 and passed first through heat exchanger 39 and next through main heat exchanger 16. Stream 76 can then be exhausted as waste or used to regenerate purifier 12. Stream 74 is withdrawn at theoretical stage 1 and passed first through heat exchanger 39 and next through main heat exchanger 16. Stream 74 can then be exhausted as waste or taken as product or any division of the two. Stream 34 (after subcooling) is split into streams 36 and 38. Stream 38 is flashed into nitrogen stripper column 42 at theoretical stage 26. Stream 36 is expanded through valve 64 and vaporized in argon column condenser 59. Stream 36 after vaporization is fed into nitrogen stripper column 42 at theoretical stage 30. Argon column 50 has approximately 220 stages of which 195 are rectifying and 25 are stripping. Stream 48 is taken from the bottom of nitrogen stripper 42 and fed to theoretical stage 195 of argon column 50. Stream 56 is withdrawn from argon column 50, reduced in pressure across valve 68 and fed to the bottom of nitrogen stripper 42. The argon product as indicated is produced at a rate of 4 kg-moles/hr and has a concentration of 0.1 ppm nitrogen and 8.3 ppm oxygen with balance argon. EXAMPLE 2: Table of Flows, Temperatures, Pressures and Composition Stream Flow kg- moles/hr Temp Degree K Pressure Bara % N 2 % Ar %O 2 72 before main heat exchanger 16 105.5 97.6 2.08 0 0.5 99.5 70 3.3 88.4 1.15 0.3ppm 99.999 9.3ppm 48 222.15 94 1.56 10ppb 7.6 92.4 56 before valve 68 113.35 96 1.88 12ppb 11.6 88.4 56 after valve 68 113.35 94 1.56 1.2ppb 11.6 88.4 32 after cooling 197.7 81 7.34 99.94 0.06 1ppm 74 at top of nitrogen stripper column 42 261.5 79.5 1.3 99.97 0.03 1.3ppm 34 after subcooling 252.3 101 7.45 61.01 1.62 37.37 38 99.5 101 7.45 61.01 1.62 37.37 36 after vaporisation 142.1 87.35 1.43 61.01 1.62 37.37 76 at top of nitrogen stripper column 42 130 79.73 1.32 99.35 0.62 270ppm 10 prior to compression 500 298 1 78.113 0.931 20.956 10 after compression 500 293 7.9 78.113 0.931 20.956 78 after expansion 50 123.9 1.43 78.113 0.931 20.956 74 after passage through heat exchanger 38 261.5 101.4 1.2 99.97 0.03 1.3ppm 74 after passage through main heat exchanger 16 261.5 289.6 1.1 99.97 0.03 1.3ppm 76 after passage through heat exchanger 38 130 101.4 1.2 99.35 0.62 270ppm 76 after passage through main heat exchanger 16 130 289.6 1.1 99.35 0.62 270ppm 72 after passage from main heat exchanger 16 105.5 289.6 1.976 0 0.5 99.5 In EXAMPLE 2 given above, nitrogen stripper column 42 has approximately 65 theoretical stages. Stream 76 is withdrawn at theoretical stage 6 and passed first through heat exchanger 39 and next through main heat exchanger 16. Stream 76 can then be exhausted as waste or used to regenerate purifier 12. Stream 74 is withdrawn at theoretical stage 1 and passed first through heat exchanger 39 and next through main heat exchanger 16. Stream 74 can then be exhausted as waste or taken as product or any division of the two. Stream 34 (after subcooling) is split into streams 36 and 38. Stream 38 is flashed into nitrogen stripper column 42 at theoretical stage 20. Stream 36 is expanded through valve 64 and vaporized in argon column condenser 59. Stream 36 after vaporization is fed into nitrogen stripper column 42 at theoretical stage 30. Argon column 50 has approximately 220 stages of which 185 are rectifying and 35 are stripping. Stream 48 is taken from the bottom of nitrogen stripper 42 and fed to theoretical stage 185 of argon column 50. Stream 56 is withdrawn to the bottom of nitrogen stripper 42. The argon product as indicated is produced at a rate of 3.3 kg-moles/hr and has a concentration of 0.3 ppm nitrogen and 9.3 ppm oxygen with balance argon.

Claims: A cryogenic air separation process for producing high purity argon comprising: compressing and purifying the air; cooling the purified and compressed air to a temperature suitable for its rectification; rectifying the cooled air in a rectification column (26) so as to produce an oxygen enriched liquid and a nitrogen vapour; separating a stream of argon-oxygen containing liquid lean in nitrogen within an argon column (50) to form a liquid oxygen and high purity argon vapour; removing a stream composed of the high purity argon vapour from the argon column (50), condensing the argon vapour stream by indirect heat exchange, and introducing the condensed argon stream into the argon column (50) as reflux; removing a stream of oxygen enriched liquid from the rectification column (26), reducing the pressure of the oxygen enriched stream, at least partially vaporizing the oxygen enriched stream by indirect heat exchange with the condensing argon vapour stream, and introducing the at least partially vaporised oxygen enriched stream into a nitrogen stripper column (42); stripping nitrogen from the oxygen enriched stream in the nitrogen stripper column (42) with a stripper gas and thereby producing said argon-oxygen containing liquid lean in nitrogen; taking said stripper gas from the argon column (50); operating the argon column (50) in a higher pressure range than that of the nitrogen stripper column; and removing as product a stream of high purity argon from the argon column (50). A process according to claim 1, wherein the nitrogen vapour produced in the rectification column (26) is condensed by indirect heat exchange vaporising with liquid oxygen separated within the argon column (50) the nitrogen condensate is in part returned to the rectification column (26) as liquid nitrogen reflux and is also formed into a reflux stream which is introduced into the nitrogen stripper column as reflux. A process according to claim 2 wherein: product and waste nitrogen streams are removed from the nitrogen stripper column (42); a product oxygen stream is removed from the argon column (50); the liquid nitrogen reflux stream and the oxygen enriched stream are subcooled through indirect heat exchange with the product and waste nitrogen streams which as a result are warmed; and the product oxygen and product and waste nitrogen streams are further warmed downstream of their indirect heat exchange with the liquid nitrogen reflux stream and the oxygen enriched stream.

Inventors: KRISHAMURTHY RAMACHANDRAN, SWEENEY PAUL A, KRISHAMURTHY, RAMACHANDRAN, SWEENEY, PAUL A.
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1993
Main classification: F25J3
IPC codes: F25J3