Cryogenic air separation with dual temperature feed turboexpansion

Abstract: A cryogenic air separation system comprising at least two columns (105, 130) wherein two portions (103, 200) of the feed air are turboexpanded (102, 201) at two different temperature levels to generate refrigeration, a third portion (106) is condensed (107) against vaporizing product (142) from the air separation plant, and all three portions are fed into the same column (105) to undergo separation.

Description: This invention relates generally to cryogenic air separation and more particularly to the production of elevated pressure product gas from the air separation where liquid production may also be desired. An often used commercial system for the separation of air is cryogenic rectification. The separation is driven by elevated feed pressure which is generally attained by compressing feed air in a compressor prior to introduction into a column system. The separation is carried out by passing liquid and vapor in countercurrent contact through the column or columns on vapor liquid contacting elements whereby more volatile component(s) are passed from the liquid to the vapor, and less volatile component(s) are passed from the vapor to the liquid. As the vapor progresses up a column it becomes progressively richer in the more volatile components and as the liquid progresses down a column it becomes progressively richer in the less volatile components. Generally the cryogenic separation is carried out in a main column system comprising at least one column wherein the feed is separated into nitrogen-rich and oxygen-rich components, and in an auxiliary argon column wherein feed from the main column system is separated into argon-richer and oxygen-richer components. WO-A-8805148 discloses a process for cryogenic distillation of compressed air to oxygen product plus optional crude argon byproduct comprising: a) supplying at least an uncondensed portion of a major fraction of the feed air to a high pressure rectifier; b) withdrawing overhead liquid for the high pressure rectifier and feeding at least part of it to a low pressure nitrogen removal column as overhead reflux therefore; c) turboexpanding a minor fraction of the feed air to an intermediate pressure; d) condensing the expanded air by exchanging latent heat with at least one of nitrogen removal column intermediate height liquid and at least part of the high pressure rectifier bottom liquid; and e) splitting the resulting liquid air into at least two fractions and feeding one fraction to an intermediate reflux height of the high pressure rectifier and another to the nitrogen removal column. It is desirable in some situations to produce liquid product which may be used during high demand periods and for purposes other than the uses of the gas product. An object of this invention is to provide for an improved cryogenic air separation system which allows a significant increase in the generation of plant refrigeration without need for additional energy input, and thus has the capability for increasing the production of liquid from the air separation plant. The above object is attained by the present invention which comprises in general the turboexpansion of two portions of compressed feed air at two different temperature levels to provide plant refrigeration, and the condensation of another portion of the feed air against a vaporizing liquid to produce product gas. More specifically one aspect of the present invention comprises: Method for the separation of air by cryogenic distillation to produce product gas comprising: (A) turboexpanding a first portion of compressed feed air, cooling the turboexpanded first portion by heat exchange with return fluid, and introducing the resulting cooled turboexpanded first portion into a first column of an air separation plant, said first column operating at a pressure generally within the range of from (4.1 to 6.9 bar) (60 to 100 psia); (B) cooling a second portion of the compressed feed air, turboexpanding the cooled second portion at a temperature lower than that at which the turboexpansion of step (A) is carried out, and introducing the resulting turboexpanded second portion into said first column; (C) condensing at least part of a third portion of the feed air and introducing resulting liquid into said first column; (D) separating the fluids introduced into said first column into nitrogen-enriched and oxygen-enriched fluids and passing said fluids into a second column of said air separation plant, said second column operating at a pressure less than that of said first column; (E) separating the fluids introduced into the second column into nitrogen-rich vapor and oxygen-rich liquid; (F) vaporizing oxygen-rich liquid by indirect heat exchange with the third portion of the feed air to carry out the condensation of step (C), the vaporized oxygen being used as return fluid in step (A); and (G) recovering vapor resulting from the heat exchange of step (F) as product oxygen gas. Another aspect of the present invention comprises: Apparatus for the separation of air by cryogenic distillation to produce product gas comprising: (A) an air separation plant comprising a first column, a second column, a reboiler, means to pass fluid from the first column to the reboiler and means to pass fluid from the reboiler to the second column; (B) a first turboexpander, means to provide feed air to the first turboexpander, means to pass fluid from the first turboexpander to a heat exchanger, and means to pass fluid from the heat exchanger into the first column; (C) a second turboexpander, means to cool feed air and to provide cooled feed air to the second turboexpander, and means to pass fluid from the second turboexpander into the first column; (D) a condenser, means to provide feed air to the condenser and means to pass fluid from the condenser into the first column; (E) means to pass oxygen-rich liquid from the second column to the condenser and means to pass oxygen product gas from the condenser to the heat exchanger; and (F) means to recover oxygen product gas from the heat exchanger. Often it is also desired to recover product gas from the air separation system at an elevated pressure. Generally this is carried out by compressing the product gas to a higher pressure by passage through a compressor. Such a system is effective but is quite costly. Accordingly it is another object of this invention to provide a cryogenic air separation system for producing elevated pressure product gas while also producing liquid product and reducing or eliminating the need for product gas compression. This further object is reached by increasing, in the above method of the invention, the pressure of the oxygen-rich liquid prior to the vaporization of step (F). The term, "column", as used herein means a distillation or fractionation column or zone, i.e., a contacting column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column or alternatively, on packing elements. For a further discussion of distillation columns see the Chemical Engineers' Handbook, Fifth Edition, edited by R.H. Perry and C.H. Chilton, McGraw-Hill Book Company, New York, Section 13, "Distillation" B.D. Smith, et al., page 13-3 The Continuous Distillation Process. The term, double column is used herein to mean a higher pressure column having its upper end in heat exchange relation with the lower end of a lower pressure column. A further discussion of double columns appears in Ruheman "The Separation of Gases" Oxford University Press, 1949, Chapter VII, Commercial Air Separation. As used herein, the term "argon column" means a column wherein upflowing vapor becomes progressively enriched in argon by countercurrent flow against descending liquid and an argon product is withdrawn from the column. The term "indirect heat exchange", as used herein means the bringing of two fluid streams into heat exchange relation without any physical contact or intermixing of the fluids with each other. As used herein, the term "vapor-liquid contacting elements" means any devices used as column internals to facilitate mass transfer, or component separation, at the liquid vapor interface during countercurrent flow of the two phases. As used herein, the term "tray" means a substantially flat plate with openings and liquid inlet and outlet so that liquid can flow across the plate as vapor rises through the openings to allow mass transfer between the two phases. As used herein, the term "packing" means any solid or hollow body of predetermined configuration, size, and shape used as column internals to provide surface area for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of the two phases. As used herein, the term "random packing" means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. As used herein, the term "structured packing" means packing wherein individual members have specific orientation relative to each other and to the column axis. As used herein the term "theoretical stage" means the ideal contact between upwardly flowing vapor and downwardly flowing liquid into a stage so that the exiting flows are in equilibrium. As used herein the term "turboexpansion" means the flow of high pressure gas through a turbine to reduce the pressure and temperature of the gas and thereby produce refrigeration. A loading device such as a generator, dynamometer or compressor is typically used to recover the energy. As used herein the term "condenser" means a heat exchanger used to condense a vapor by indirect heat exchange. As used herein the term "reboiler" means a heat exchanger used to vaporize a liquid by indirect heat exchange. Reboilers are typically used at the bottom of distillation columns to provide vapor flow to the vapor-liquid contacting elements. As used herein the term "air separation plant" means a facility wherein air is separated by cryogenic rectification, comprising at least one column and attendant interconnecting equipment such as pumps, piping, valves and heat exchangers. Figure 1 is a simplified schematic flow diagram of one preferred embodiment of the cryogenic air separation system of this invention Figure 2 is a graphical representation of air condensing pressure against oxygen boiling pressure. The invention will be described in detail with reference to the Drawings. Referring now to Figure 1 feed air 100 which has been compressed to a pressure generally within the range of from 6.2 to 35 bar (90 to 500 pounds per square inch absolute (psia)) is cooled by indirect heat exchange against return streams by passage through heat exchanger 101. A first portion 200 of the compressed feed air is removed from heat exchanger 101 prior to complete traverse and passed to first turboexpander 201 wherein it is turboexpanded to a pressure generally within the range of from 4.1 to 6.9 bar (60 to 100 psia). Generally first portion 200 will comprise from 10 to 30 percent of feed air 100. Resulting turboexpanded first portion 204 is cooled by indirect heat exchange through heat exchanger 202 and the resulting cooled turboexpanded first portion is passed as stream 206 into first column 105. A second portion 103 of the compressed feed air is cooled by complete traverse of heat exchanger 101 and is provided to second turboexpander 102 and turboexpanded to a pressure generally within the range of from 4.1 to 6.9 bar (60 to 100 psia). The resulting turboexpanded air 104 is introduced into first column 105 which is operating at a pressure generally within the range of from 4.1 to 6.9 bar (60 to 100 psia). Generally second portion 103 will comprise from 40 to 60 percent of feed air 100. Figure 1 illustrates one preferred embodiment wherein the turboexpanded first and second portions are combined and passed into column 105 as a single stream 206. The turboexpansion through turboexpander 201 is carried out at a higher temperature level than the turboexpansion through turboexpander 102. Generally-the temperature difference between these two turboexpansions will be within the range of from 50 to 70 °K. This enables refrigeration to be produced at both high temperature and low temperature levels, allowing for an increase in liquid production over a single turboexpansion system without any additional energy input to the main feed air stream. A third portion 106 of the compressed feed air is provided to condenser 107 wherein it is at least partially condensed by indirect heat exchange with vaporizing liquid taken from the air separation plant. Generally third portion 106 comprises from 5 to 30 percent of feed air 100. Resulting liquid is introduced into column 105 at a point above the vapor feed. In the case where stream 106 is only partially condensed, resulting stream 160 may be passed directly into column 105 or may be passed, as shown in Figure 1, to separator 108. Liquid 109 from separator 108 is then passed Into column 105. Liquid 109 may be further cooled by passage through heat exchanger 110 prior to being passed into column 105. Cooling the condensed portion of the feed air improves liquid production from the process. Vapor 111 from separator 108 may be passed directly into column 105 or may be cooled or condensed in heat exchanger 112 against return streams and then passed into column 105. Furthermore, a fifth portion 113 of the feed air may be cooled or condensed in heat exchanger 112 against return streams and then passed into column 105. Streams 111 and 113 can be utilized to adjust the temperature of the feed air fractions that are turboexpanded. For example, increasing stream 113 will increase warming of the return streams in heat exchanger 112 and thereby the temperature of the feed air streams will be increased. The higher inlet temperatures to the turboexpanders can increase the developed refrigeration and can control the exhaust temperature of the expanded air to avoid any liquid content When the air separation plant indudes an argon column, a fourth portion 120 of the feed air may be further cooled or condensed by indirect heat exchange, such as in heat exchanger 122, with fluid produced in the argon column and then passed into column 105. Within first column 105 the fluids introduced into the column are separated by cryogenic distillation into nitrogen-enriched and oxygen-enriched fluids. In the embodiment illustrated in Figure 1 the first column is the higher pressure column of a double column system. Nitrogen-enriched vapor 161 is withdrawn from column 105 and condensed in reboiler 162 against boiling column 130 bottoms. Resulting liquid 163 is divided into stream 164 which is returned to column 105 as liquid reflux, and into stream 118 which is subcooled in heat exchanger 112 and flashed into second column 130 of the air separation plant. Second column 130 is operating at a pressure less than that of first column 105 and generally within the range of from 1.0 to 2.1 bar (15 to 30 psia). Liquid nitrogen product may be recovered from stream 118 before it is flashed into column 130 or, as illustrated in Figure 1, may be taken directly out of column 130 as stream 119 to minimize tank flashoff. Oxygen-enriched liquid is withdrawn from column 105 as stream 117, subcooled in heat exchanger 112 and passed into column 130. In the case where the air separation plant indudes an argon column, as in the embodiment illustrated in Figure 1, all or part of stream 117 may be flashed into condenser 131 which serves to condense argon column top vapor. Resulting streams 165 and 166 comprising vapor and liquid respectively are then passed from condenser 131 into column 130. Within column 130 the fluids are separated by cryogenic distillation into nitrogen-rich vapor and oxygen-rich liquid. Nitrogen-rich vapor is withdrawn from column 130 as stream 114, warmed by passage through heat exchangers 112 and 101 to about ambient temperature and recovered as product nitrogen gas. For column purity control purposes a nitrogen-rich waste stream 115 is withdrawn from column 130 at a point between the nitrogen-enriched and oxygen-enriched feed stream introduction points, and is warmed by passage through heat exchangers 112 and 101 before being released to the atmosphere. Nitrogen recoveries of up to 90 percent or more are possible by use of this invention. As mentioned the embodiment illustrated in Figure 1 includes an argon column in the air separation plant. In such an embodiment a stream comprising primarily oxygen and argon is passed 134 from column 130 into argon column 132 wherein it is separated by cryogenic distillation into oxygen-richer liquid and argon-richer vapor. Oxygen-richer liquid is returned as stream 133 to column 130. Argon-richer vapor is passed 167 to argon column condenser 131 and condensed against oxygen-enriched fluid to produce argon-richer liquid 168. A portion 169 of argon-richer liquid is employed as liquid reflux for column 132. Another portion 121 of the argon-richer liquid is recovered as crude argon product generally having an argon concentration exceeding 96 percent. As illustrated in Figure 1, crude argon product stream 121 may be warmed or vaporized in heat exchanger 122 against feed air stream 120 prior to further upgrading and recovery. Oxygen-rich liquid 140 is withdrawn from column 130 and preferably pressurized to a pressure greater than that of column 130 by either a change in elevation, i.e. the creation of liquid head, by pumping, by employing a pressurized storage tank, or by any combination of these methods. In the embodiment illustrated in Figure 1, oxygen-rich liquid 140 is pumped by passage through pump 141 to produce elevated pressure liquid stream 142. The elevated pressure liquid is then warmed by passage through heat exchanger 110 and throttled into side condenser or product boiler 107 where it is at least partially vaporized. Gaseous product oxygen 143 is passed from condenser 107, warmed through heat exchanger 101 and recovered as product oxygen gas. As used herein the term "recovered" means any treatment of the gas or liquid induding venting to the atmosphere. Liquid 116 may be taken from condenser 107, subcooled by passage through heat exchanger 112 and recovered as product liquid oxygen. The oxygen content of the liquid from the bottom of column 105 is lower than in a conventional process which does not utilize an air condenser. This changes the reflux ratios in the bottom of column 105 and all sections of column 130 when compared to a conventional process. High product recoveries are possible with the invention since refrigeration is produced without requiring vapor withdrawal from column 105 or an additional vapor feed to column 130. Producing refrigeration by adding vapor air from a turbine to column 130 or removing vapor nitrogen from column 105 to feed a turbine would reduce the reflux ratios in column 130 and significantly reduce product recoveries. The invention is able to easily maintain high reflux ratios, and hence high product recoveries and high product purities. Oxygen recoveries of up to 99.9 percent are possible by use of the system of this invention. Oxygen product may be recovered at a purity generally within the range of from 95 to 99.95 percent. Additional flexibility could be gained by splitting the feed air before it enters heat exchanger 101. The air could be supplied at two different pressures if the liquid production requirements don't match the product pressure requirements. Increasing product pressure will raise the air pressure required at the product boiler, while increased liquid requirements will increase the air pressure required at the turbine inlets. The embodiment illustrated in Figure 1 illustrates the condensation of air feed to produce product oxygen gas. Figure 2 illustrates the air condensing pressure required to produce oxygen gas product over a range of pressures for product boiling delta T's of 1 and 2 degrees K. There will be a finite temperature difference (delta T) between streams in any indirect heat exchanger. Increasing heat exchanger surface area andlor heat transfer coefficients will reduce the temperature difference (delta T) between the streams. For a fixed oxygen pressure requirement, decreasing the delta T will allow the air pressure to be reduced, decreasing the energy required to compress the air and reducing operating costs. Net liquid production will be affected by many parameters. Turbine flows, pressures, inlet temperatures, and efficiencies will have significant impact since they determine the refrigeration production. Air inlet pressure, temperature, and warm end delta Twill set the warm end losses. The total liquid production (expressed as a fraction of the air) is dependent on the air pressures in and out of the turbines, turbine inlet temperatures, turbine efficiencies, primary heat exchanger inlet temperature and amount of product produced as high pressure gas. The gas produced as high pressure product requires power input to the air compressor to replace product compressor power. Recently packing has come into increasing use as vapor-liquid contacting elements in cryogenic distillation in place of trays. Structured or random packing has the advantage that stages can be added to a column without significantly increasing the operating pressure of the column. This helps to maximize product recoveries, increases liquid production, and increases product purities. Structured packing is preferred over random packing because its performance is more predictable. The present invention is well suited to the use of structured packing. In particular, structured packing may be particularly advantageously employed as some or all of the vapor-liquid contacting elements in the second or lower pressure column and, if employed, in the argon column. The high product delivery pressure attainable with this invention will reduce or eliminate product compression costs. In addition, if some liquid production is required, it can be produced by this invention with relatively small capital costs. The system of this invention enables a significant increase in the generation of plant refrigeration without need for additional energy input. This results in the capability for increasing the production of liquid from the air separation plant enabling the plant to operate more effectively under both lower demand and higher demand conditions relative to its design point. The increased refrigeration is generated in part by the higher temperature turboexpansion coupled with the subsequent cooling to produce lower temperature turboexpansion. High temperature turboexpansion and subsequent cooling enable more refrigeration to be recovered from the warming streams at a high temperature level. This results in a smaller cold end temperature difference at heat exchanger 202 and thus improves the cyde's overall efficiency. This Is because the two stage two temperature level turboexpansion can produce the refrigeration more efficiently than a single low temperature level turboexpansion.

Claims: Method for the separation of air by cryogenic distillation to produce product gas comprising: (A) turboexpanding a first portion (200) of cooled, compressed feed air, cooling the turboexpanded first portion (204) by heat exchange with return fluid, and introducing the resulting cooled turboexpanded first portion (206) into a first column (105) of an air separation plant, said first column operating at a pressure generally within the range of from 4.1 to 6.9 bar (60 to 100 psia); (B) cooling a second portion of the compressed feed air, turboexpanding the cooled second portion (103) at a temperature lower than that at which the turboexpansion of step (A) is carried out, and introducing the resulting turboexpanded second portion (104) into said first column; (C) condensing at least part of a third portion (106) the feed air and introducing resulting liquid (109) into said first column; (D) separating the fluids introduced into said first column into nitrogen-enriched and oxygen-enriched fluids and passing said fluids into a second column (103) of said air separation plant, said second column operating at a pressure less than that of said first column; (E) separating the fluids introduced into the second column into nitrogen-rich vapor (114) and oxygen-rich liquid; (F) vaporizing oxygen-rich liquid (140) by indirect heat exchange with the third portion (106) of the feed air to carry out the condensation of step (C), the vaporized oxygen being used as return fluid in step (A); and (G) recovering vapor resulting from the heat exchange of step (F) as product oxygen gas (143). The method of daim 1 wherein the liquid (109) resulting from the condensation of step (C) is further cooled prior to being introduced into the first column (105). The method of daim 1 wherein the oxygen-rich liquid (104) is warmed prior to the vaporization of step (F). The method of daim 1 wherein the oxygen-rich liquid (140) is increased In pressure prior to the vaporization of step (F). The method of daim 1 wherein the air separation plant further comprises an argon column (132), a stream (134) is passed from the second column (130) to the argon column and separated into argon-richer vapor (167) and oxygen-richer liquid (133), the argon-richer vapor is condensed and at least some is recovered. The method of daim 5 wherein the argon-richer vapor (167) is condensed by indirect heat exchange with oxygen-enriched fluid (117) to produce argon-richer liquid (168, 121, 169). The method of claim 6 wherein argon-richer liquid (121) is vaporized by indirect heat exchange with a fourth portion (120) of the cooled, compressed feed air and the resulting condensed fourth portion is passed into the first column (105). The method of claim 1 wherein the third portion (106) of the feed air is partially condensed, the resulting vapor (111) is subsequently condensed and is then introduced into the first column (105). The method of daim 1 comprising with-drawing liquid (116, 119) from the air separation plant and recovering said liquid as product liquid. The method of daim 9 wherein said product liquid is nitrogen-enriched fluid (119). The method of daim 9 wherein said product liquid is oxygen-rich liquid (116). The method of claim 1 wherein the liquid (109) resulting from step (C) is introduced into the first column (105) at a point higher than the vapor (206) resulting from step (A) or the vapor (206) resulting from step (B). The method of claim 1 further comprising cooling a fifth portion (113) of the feed air having a pressure higher than that of either the turboexpanded first portion (206) or the turboexpanded second portion (104) by indirect heat exchange with fluid (114, 115) taken from the air separation plant and passing the resulting fifth portion into the first column (105). The method of daim 1 further comprising recovering nitrogen-rich vapor (114) as product nitrogen gas. Apparatus for the separation of air by cryogenic distillation to produce product gas comprising: (A) an air separation plant comprising a first column (105) a second column (130), a reboiler (162), means to pass fluid (161) from the first column to the reboiler and means to pass fluid (163, 118) from the reboiler to the second column; (B) a first turboexpander (201), means to provide feed air (200) to the first turboexpander, means to pass fluid (204) from the first turboexpander to a heat exchanger (202), and means to pass fluid (206) from the heat exchanger into the first column; (C) a second turboexpander (102), means (101) to cool feed air and to provide cooled feed air (103) to the second turboexpander, and means to pass fluid (104, 206) from the second turboexpander into the first column; (D) a condenser (107), means to provide feed air (106) to the condenser-and means to pass fluid (109) from the condenser into the first column; (E) means to pass oxygen-rich liquid (104, 142) from the second column (130) to the condenser and means to pass oxygen product gas (143) from the condenser (107) to the heat exchanger (202); and (F) means to recover oxygen product gas (143) from the heat exchanger (202). The apparatus of claim 15 further comprising means (141) to increase the pressure of the fluid (140, 142) passed from the air separation plant to the condenser (107). The apparatus of claim 15 further comprising means (110) to increase the temperature of the fluid (140, 142) passed from the air separation plant to the condenser (107). The apparatus of daim 15 wherein the air separation plant further comprises an argon column (132) and means to pass fluid (134) from the second column (130) into the argon column. The apparatus of claim 18 further comprising an argon column condenser (131), means to provide vapor (167) from the argon column (132) to the argon column condenser, means to pass liquid (121) from the argon column condenser to an argon column heat exchanger (122), means to provide feed air (120) to the said argon column heat exchanger and from the said argon column heat exchanger into the first column (105). The apparatus of claim 15 wherein the first column (105) contains vapor-liquid contacting elements comprising structured packing. The apparatus of daim 15 wherein the second column (130) contains vapor-liquid contacting elements comprising structured packing. The apparatus of daim 18 wherein the argon column (132) contains vapor liquid contacting elements comprising structured packing.

Inventors: DRAY JAMES ROBERT, PARSNICK DAVID ROSS, DRAY, JAMES ROBERT, PARSNICK, DAVID ROSS
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 1991
Main classification: F25J3
IPC codes: F25J3