Cryogenic air separation with dual product boiler

Abstract: A cryogenic air separation system wherein pressurized feed air (1) is at least partially condensed (31, 32) to vaporize elevated pressure liquid nitrogen (15) and elevated pressure liquid oxygen (14) to produce elevated pressure nitrogen (65) and oxygen gas (18) eliminating or reducing the need for product compression.

Description: Technical Field This invention relates generally to the field of cryogenic air separation and more particularly to the cryogenic separation of air to produce oxygen and nitrogen. Background Art The cryogenic separation of air to produce oxygen and nitrogen is a well established industrial process. Liquid and vapor are passed in countercurrent contact through one or more columns and the difference in vapor pressure between the oxygen and nitrogen cause nitrogen to concentrate in the vapor and oxygen to concentrate in the liquid. The lower is the presure in the separation column, the easier is the separation into oxygen and nitrogen due to vapor pressure differential. Accordingly the final separation into product oxygen and nitrogen is generally carried out at a relatively low pressure, usually just a few kPa (pounds per square inch (psi)) above atmospheric pressure. Often the product oxygen and nitrogen is desired at an elevated pressure. In such situations the product is compressed to the desired pressure in a compressor. This compression is costly in terms of energy costs as well as capital costs for the product compressors. A method for the cryogenic separation of air to produce oxygen and nitrogen comprising: (A) providing feed air into a higher pressure column and separating the feed air in the higher pressure column into nitrogen-enriched vapor and oxygen-enriched liquid; (B) passing oxygen-enriched liquid from the higher pressure column into a lower pressure column; (C) condensing nitrogen-enriched vapor to produce nitrogen-enriched liquid and passing nitrogen-enriched liquid into the lower pressure column; (D) separating the fluids passed into the lower pressure column into nitrogen-rich vapor and oxygen-rich liquid; and (E) removing oxygen-rich liquid from the lower pressure column, increasing the pressure of said removed oxygen-rich liquid and passing the elevated pressure oxygen-rich liquid in indirect heat exchange in a product boiler with feed air to produce product oxygen gas is known from EP-A-0 024 962. In this known method the entire nitrogen-enriched liquid removed from the higher pressure column is subcooled for subsequent pressure reduction and recovery of liquid nitrogen. US-A-3 210 950 discloses a double column air separation process in which liquid oxygen and liquid nitrogen are boiled by heat exchange with a fraction of the feed air in a lower part of a heat exchanger, and in which another fraction of the feed air is turboexpanded. The feed air, before division thereof into the two fractions, is cooled in an upper part of the heat exchanger by indirect heat exchange with oxygen and nitrogen which were evaporated in the lower part of the heat exchanger. It is an object of this invention to provide an improved cryogenic system for the production of oxygen and nitrogen. It is a further object of this invention to provide an improved cryogenic system for the production of oxygen and nitrogen wherein oxygen and nitrogen may be produced at elevated pressure and thereby eliminate or reduce the need for product gas compression. Summary Of The Invention The above and other objects which will become apparent to one skilled in the art upon a reading of this disclosure are attained by the present invention one aspect of which is a method as defined in claim 1. Another aspect of this invention is an apparatus as defined in claim 8. The term; "column", as used in the present specification and claims means a distillation or fractionation column or zone, i.e., a contracting 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 or 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. Parry 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 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. Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Distillation is the separation process whereby heating of a liquid mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is adiabatic and can include integral or differential contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. The term "indirect heat exchange", as used in the present specification and claims, 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 "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 "condenser/reboiler" means a heat exchange device wherein vapor is condensed by indirect heat exchange with vaporizing column bottoms thus providing vapor upflow for the column. 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 "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. Brief Description Of The Drawings Figure 1 is a schematic representation of one preferred embodiment of the method and apparatus of this invention. Figure 2 is a schematic representation of another preferred embodiment of the method and apparatus of this invention. Detailed Description The method and apparatus of this invention will be described in detail with reference to the Drawings. Referring now to Figure 1, clean, cool, compressed feed air 1 is cooled by indirect heat exchange in main heat exchanger 30 against return streams. The feed air is at a pressure sufficient to vaporize liquid to produce elevated pressure product gas as will be more fully described below. Generally the feed air will be at a pressure within the range of from 6.2 to 34.5 bar (90 to 500 pounds per square inch absolute (psia)). The feed air is divided into two portions. The first portion 4, which may be from 5 to 40 percent of the feed air, is passed through heat exchange means which is a dual product side boiler 31. Air portion 4 is at least partially condensed in dual product side boiler 31 and it may be totally condensed. Air portion 4 is then passed through conduit means to heat exchanger or subcooler 32 wherein it is subcooled and then through valve 33 and as stream 6 into first or higher pressure column 34 which is the higher pressure column of a double column system of an air separation plant. Higher pressure column 34 is generally operating at a pressure within the range of from 4.1 to 6.9 bar (60 to 100 psia). The second portion 5 of the feed air, which may comprise from 50 to 90 percent of the feed air, is turboexpanded through turboexpander 35 to develop refrigeration for the cryogenic separation. Expanded air portion 36 is then passed into higher pressure column 34. A portion 3 of the feed air may be cooled by indirect heat exchange through heat exchanger 37 against low pressure nitrogen, passed through valve 38 and passed into higher pressure column 34 as part of stream 6. Alternatively, if the feed air portion 4 is only partially condensed by passage through dual product side boiler 31, the uncondensed part may be used to carry out the heat exchange in heat exchanger 37 instead of or in addition to portion 3. Within higher pressure column 34 the feed air is separated by cryogenic rectification into oxygen-enriched liquid and nitrogen-enriched vapor. Oxygen-enriched liquid is passed 9 through conduit means to heat exchanger 66 wherein it is cooled by indirect heat exchange with low pressure nitrogen and then passed into second or lower pressure column 39, which is operating at a pressure less than that at which higher pressure column 34 is operating, and generally within the range of from 1.0 to 2.1 bar (15 to 30 psia). Nitrogen-enriched vapor is passed 40 through conduit means from higher pressure column 34 to condenser/reboiler 41 wherein it is condensed by indirect heat exchange with column 39 bottoms. Condenser/reboiler 41 is preferably within lower pressure column 39 although it may also be outside the column. Resulting nitrogen-enriched liquid 42 is passed out of condenser/reboiler 41 and a portion 43 is returned to higher pressure column 34 as reflux. Nitrogen-enriched liquid is passed 8 from higher pressure column 34 through heat exchanger 66 and into lower pressure column 39. Alternatively, a portion of liquid 42 could be passed as reflux to lower pressure column 39 instead of stream 8 from higher pressure column 34. Within lower pressure column 39 the fluids fed into the column are separated into nitrogen-rich vapor and oxygen-rich liquid by cryogenic rectification. Nitrogen-rich vapor is removed 10 from lower pressure column 39 and this lower pressure nitrogen is warmed by sequential passage through heat exchanger 66, 37 and 30 and may be recovered as lower pressure nitrogen gas product 11. Oxygen-rich liquid serves to condense the nitrogen-enriched vapor in stream 40 and thus provides vapor upflow for lower pressure column 39. A portion 13 of the oxygen-rich liquid is removed from lower pressure column 39 and is passed to dual product side boiler 31. The oxygen-rich liquid is pressurized and thus is vaporized at elevated pressure in the dual product side boiler to produce elevated pressure oxygen gas product. Referring back to Figure 1, oxygen-rich liquid 13 is passed through valve 44 into at least one tank. As illustrated in Figure 1, the oxygen-rich liquid is passed into either or both of tanks 45 and 46 through valves 47 and 48 respectively and then through valves 49 and 50 respectively and through valve 51 and as stream 14 to subcooler 32. The tank or tanks serve to store product liquid oxygen for later delivery as product oxygen. The tank or tanks may be equipped with a pressure building coil or other means to raise the pressure of the oxygen-rich liquid. Alternatively the pressure of the oxygen-rich liquid may be increased by means of a liquid pump or by liquid head, i.e. the height differential between liquid levels. The pressurized oxygen-rich liquid is warmed by passage through subcooler 32 and resulting stream 52 is passed to phase separator 53. Oxygen-rich liquid 54 is passed from phase separator 53 through dual product side boiler 31 wherein it is partially vaporized and serves to carry out the condensation of the feed air which was discussed above. The two phase stream 17 is returned to phase separator 53 and vapor 55 is passed from phase separator 53 through main heat exchanger 30 and is recovered as high pressure oxygen gas product stream 18. The high pressure oxygen gas product may have a pressure within the range of from 2.7 to 44.8 bar (40 to 650 psia). Additionally, depending on available system refrigeration, some liquid products may be recovered. For example, liquid oxygen 75 and liquid nitrogen 76 can be produced along with the elevated pressure gas products. Nitrogen-enriched liquid is passed from condenser/reboiler 41 to dual product side boiler 31. The nitrogen-enriched liquid is pressurized and thus is vaporized at elevated pressure in the dual product side boiler to produce elevated pressure nitrogen gas product. Referring back to Figure 1, nitrogen-enriched liquid is passed 56 through valve 57 into at least one tank. As illustrated in Figure 1, the nitrogen-enriched liquid is passed into either or both of tanks 58 and 59 through valves 60 and 61 respectively and then through valves 62 and 63 respectively to subcooler 32. The tank or tanks serve to store product liquid nitrogen for later delivery as product nitrogen. The tank or tanks may be equipped with a pressure building coil or other means to raise the pressure of the nitrogen-enriched liquid. Alternatively the pressure of the nitrogen-enriched liquid may be increased by means of a liquid pump or liquid head. The pressurized nitrogen-enriched liquid 15 is warmed by passage through subcooler 32 and then is vaporized by passage through dual product side boiler 31 wherein it serves to carry out the condensation of the feed air which was discussed above. Nitrogen vapor stream 64 is passed through main heat exchanger 30 and is recovered as high pressure nitrogen gas product stream 65. The high pressure nitrogen gas product may have a pressure within the range of from 6.9 to 41.4 bar (100 to 600 psia). The cryogenic system of this invention can produce nitrogen with a purity of at least 99 percent and up to a purity of 99.99 percent or more, and can produce oxygen with a purity within the range of from 95 to 99.95 percent. If desired some liquid oxygen and/or liquid nitrogen may be recovered directly from the columns without vaporization. Also, if desired, some gaseous oxygen or gaseous nitrogen could be recovered directly from the columns. Figure 2 illustrates another embodiment of the invention wherein the first portion of the feed air is turboexpanded prior to passage through the dual product side boiler. The numerals in Figure 2 correspond to those of Figure 1 for the common elements and these common elements will not be described again. In the embodiment illustrated in Figure 2, first portion 70 of the clean, cool, compressed feed air is taken from about the midpoint of main heat exchanger 30 and turboexpanded through turboexpander 71. The resulting first feed air portion 72 is then passed through dual product side boiler 31 and heat exchanger 32 and then combined with the second portion of the feed air downstream of turboexpander 35 and passed into higher pressure column 34 as stream 67. With the embodiment illustrated in Figure 2, the additional feed air turboexpansion provides additional refrigeration to the columns thus enabling the production of more liquid products. However the gaseous products would be produced at lower pressures. The column internals for either or both of the higher and lower pressure columns may comprise trays or packing. If packing is used the packing may be either random or structured packing. However the invention is particularly suited for use with structured packing column internals. This is because packing will reduce the operating pressures in the columns, helping to improve product recoveries and increase liquid production. Additional stages can be added to packed columns without significantly increasing the operating pressure of the column. Structured packing is preferred over random packing because its performance is more predictable and more stages can be attained in a given bed height. This is important to the first cost and complexity of the system. Table I lists a summary of a computer simulation of the invention carried out with the embodiment illustrated in Figure 1. The data in Table I is presented for illustrative purposes and is not intended to be limiting. The stream numbers in Table I correspond to those of Figure 1.

Claims: A method for the cryogenic separation of air to produce oxygen and nitrogen comprising: (A) cooling feed air (1) in a main heat exchanger (30) by indirect heat exchange with oxygen gas (55) and nitrogen gas (64) to produce product oxygen gas (18) and product nitrogen gas (65), dividing the cooled feed air into a product boiler portion (4, 72) and a turboexpansion portion (5), turboexpanding (35) the turboexpansion portion (5), passing the turboexpanded portion (36) of the feed air and the product boiler portion (4, 72) of the feed air into a higher pressure column (34) and separating the feed air in the higher pressure column into nitrogen-enriched vapor (40) and oxygen-enriched liquid (9); (B) passing oxygen-enriched liquid (9) from the higher pressure column (34) into a lower pressure column (39); (C) condensing nitrogen-enriched vapor (40) to produce nitrogen-enriched liquid (42) and passing nitrogen-enriched liquid (8) into the lower pressure column (39): (D) separating the fluids (8, 9) passed into the lower pressure column (39) into nitrogen-rich vapor (10) and oxygen-rich liquid (13); (E) removing oxygen-rich liquid (13) from the lower pressure column (39), increasing the pressure of said removed oxygen-rich liquid, warming the elevated pressure oxygen-rich liquid (14) by passing it in indirect heat exchange in a subcooler (32) with the product boiler portion (4, 72) of the feed air cooled in the main heat exchanger (30) and further cooled in a dual product side boiler (31), passing the resulting warmed oxygen-rich liquid (52) to a phase separator (53) and passing oxygen-rich liquid (54) from the phase separator in indirect heat exchange in the dual product side boiler (31) with the product boiler portion (4, 72) of the feed air cooled in the main heat exchanger (30) to partially vaporize the oxygen-rich liquid (54) from the phase separator and to produce a two phase stream (17), passing the two phase stream (17) to the phase separator (53). separating in the phase separator (53) the two phase stream (17) into oxygen-rich vapor (55) and said oxygen-rich liquid (54); passing oxygen-rich vapor (55) from the phase separator (53) through the main heat exchanger (30) to produce oxygen product gas (18); and (F) removing nitrogen-enriched liquid (56) produced by the condensation of nitrogen-enriched vapor in step (C) against oxygen-rich liquid, increasing the pressure of said removed nitrogen-enriched liquid, warming the elevated pressure nitrogen-enriched liquid (15) by passing it in indirect heat exchange in the subcooler (32) with the product boiler portion (4, 72) of the feed air cooled in the main heat exchanger (30) and further cooled in the dual product side boiler (31), passing resulting warmed nitrogen-enriched liquid in indirect heat exchange in the dual product side boiler (31) with the product boiler portion (4, 72) of the feed air cooled in the main heat exchanger (30) to produce nitrogen gas (64), and passing nitrogen vapor (64) from the dual product side boiler (31) through the main heat exchanger (30) to produce nitrogen product gas (65), wherein the product boiler portion (4. 72) of the cooled feed air is at least partly condensed by the heat exchange in said dual product side boiler (31). The method of claim 1 wherein the product boiler portion (4. 72) of the cooled feed air is totally condensed by the heat exchange in said dual product side boiler (31). The method of claim 1 or 2 wherein the product boiler portion (72) of the cooled feed air is turboexpanded prior to the heat exchange of steps (E) and (F). The method of any one of the preceding claims further comprising recovering nitrogen rich vapor (10) taken from the lower pressure column (39). The method of any one of the preceding claims wherein the nitrogen-enriched vapor (40) is condensed by indirect exchange with oxygen-rich liquid. The method of any one of the preceding claims further comprising recovering some oxygen-rich liquid (75). The method of any one of the preceding claims further comprising recovering some nitrogen-enriched liquid (76). An apparatus for the cryogenic separation of air to produce oxygen and nitrogen by the process of claim 1, comprising: a main heat exchanger (30) for cooling feed air (1) by indirect heat exchange with oxygen gas (55) and nitrogen gas (64) to produce product oxygen gas (18) and product nitrogen gas (65); means for dividing the cooled feed air into a product boiler portion (4, 72) and a turboexpansion portion (5); a turboexpander (35) for turboexpanding the turboexpansion portion (5); a higher pressure column (34) for separating feed air (5. 4, 72) introduced into the higher pressure column into nitrogen-enriched vapor (40) and oxygen-enriched liquid (9), said turboexpander (35) being in flow communication with the higher pressure column (34); a lower pressure column (39) for separating the fluids (8, 9) passed into the lower pressure column into nitrogen-rich vapor (10) and oxygen-rich liquid (13); conduit means for passing oxygen-enriched liquid (9) from the higher pressure column (34) into the lower pressure column (39); a condenser/reboiler (41) for condensing nitrogen-enriched vapor (40) against oxygen-rich liquid to produce nitrogen-enriched liquid (42); conduit means for passing nitrogen-enriched liquid (8) into the lower pressure column (39); conduit means for removing oxygen-rich liquid (13) from the lower pressure column (39) and means for increasing the pressure of said removed oxygen-rich liquid; conduit means for removing nitrogen-enriched liquid (56) from the condenser/reboiler (41) and means for increasing the pressure of said removed nitrogen-enriched liquid; a dual product side boiler (31), a subcooler (32) and a phase separator (53); said subcooler (32) being connected to receive said elevated pressure oxygen-rich liquid (14) and said elevated pressure nitrogen-enriched liquid (15), and to receive the product boiler portion (4, 72) of the feed air from the dual product side boiler (31), said subcooler (32) being arranged to pass the elevated pressure oxygen-rich liquid (14) and the elevated pressure nitrogen-enriched liquid (15) in indirect heat exchange with the product boiler portion of the feed air from the dual product side boiler (31) for warming the elevated pressure oxygen-rich liquid (14) and the elevated pressure nitrogen-enriched liquid (15) and for subcooling the product boiler portion (4, 72) of the feed air from the dual product side boiler (31); conduit means for passing the two phase oxygen-rich stream (17) from the dual product side boiler (31) to the phase separator (53); conduit means for passing oxygen-rich liquid (54) from the phase separator (53) to the dual product side boiler (31); conduit means for passing the warmed elevated pressure oxygen-rich liquid (52) from the subcooler (32) to the phase separator (53); said dual product side boiler (31) being connected to receive the product boiler portion (4, 72) of the feed air from the main heat exchanger (30), to receive oxygen-rich liquid (54) from the phase separator (53), and to receive the elevated pressure nitrogen-enriched liquid (15) warmed in the subcooler (32), said dual product side boiler (31) being arranged to pass the oxygen-rich liquid (54) from thephase separator (53) and the elevated pressure nitrogen-enriched liquid (15) in indirect heat exchange with the product boiler portion (4, 72) of the feed air from the main heat exchanger (30) to produce a two phase oxygen-rich stream (17) and nitrogen vapor (64) and to at least partially condense the product boiler portion (4, 72) of the feed air; conduit means for passing the subcooled product boiler portion of the feed air from said subcooler (32) to said higher pressure column (34); conduit means for passing oxygen vapor (55) from the phase separator (53) to the main heat exchanger (30); and conduit means for passing nitrogen vapor (64) from the dual product side boiler (31to the main heat exchanger (30). The apparatus of claim 8 wherein means provided to pass oxygen-rich liquid (13) from the lower pressure column (39) to the dual product side boiler (31) comprises at least one tank (45, 46). The apparatus of claim 8 or 9 wherein means provided to pass nitrogen-enriched liquid (42) from the condenser/reboiler (41) to the dual product side boiler (31) comprises at least one tank (58, 59). The apparatus of any one of claims 8 to 10 wherein the means for increasing the pressure of the oxygen-rich liquid removed from the lower pressure column (39) comprises a liquid pump. The apparatus of any one of claims 8 to 11 wherein the means for increasing the pressure of the nitrogen-enriched liquid removed from the condenser/reboiler (41) comprises a liquid pump. The apparatus of any one of claims 8 to 12 further comprising a turboexpander (71) in flow communication with the dual product side boiler (31). The apparatus of any one of claims 8 to 13 wherein at least some of the internals of the higher pressure column (34) comprise structured packing. The apparatus of any one of claims 8 to 14 wherein at least some of the internals of the lower pressurc column (39) comprise structured packing.

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