Process and apparatus for the separation of air by cryogenic distillation

Abstract

 An apparatus for the separation of air by cryogenic distillation comprises a column system including a double column (100, 101), an intermediate pressure column (103), operating a pressure between that of the high pressure column and that of the low pressure column, and an argon column (102), a heat exchanger (200), a conduit for sending purified compressed air to be cooled in the heat exchanger, a conduit for sending cooled purified compressed air from the heat exchanger at least in part to the high pressure column, means for pressurizing and vaporizing oxygen rich liquid (30) from the low pressure column, means for pressurizing and vaporizing nitrogen rich liquid (40), a conduit for sending argon enriched gas (50, 54) from the low pressure column to the argon column, said argon column having a top condenser (105), a conduit for removing argon rich fluid (80) from the top of the argon column, a conduit for sending a liquid air stream (4, 20, 22) at least in part to a top condenser (107) of the intermediate pressure column where it is partially vaporized to form a vapor and a liquid, a conduit for sending the vapor (123) formed in the top condenser of the intermediate pressure column to the low pressure column and a conduit for sending the liquid (24) from the top condenser of the intermediate pressure column to the intermediate pressure column to be separated.

Description

[0001] The present invention relates to a process and apparatus for the separation of air by cryogenic distillation.

[0002] Most oxygen plants are based on the LOX pumped cycle wherein liquid oxygen is pressurized by pump and vaporized by condensing pressurized air and then warmed to form the pressurized gaseous oxygen product. Usually about 25 to 35% of the feed air is liquefied by vaporizing the oxygen product. The liquid pumped cycle is not only applied for liquid oxygen but some liquid nitrogen can also be vaporized as well based on the same concept. When both liquid oxygen and liquid oxygen are pumped and vaporized by condensing air to form pressurized oxygen and nitrogen, an important quantity of air as high as 50% of total feed air must be liquefied.

[0003] By producing gaseous oxygen and nitrogen products by pumping liquid, costly product compressors can be avoided resulting in significant cost reduction. However, by liquefying important quantity of feed air, the high pressure column of the double column process is deprived of gaseous feed air such that its ability to provide the liquid reflux for the low pressure column is adversely affected. Reduction of feed air also reduces the condensing gas at the top of the high pressure column and this translates into less reboiler duty for the low pressure column. The distillation performance will suffer when both liquid oxygen and liquid nitrogen in significant quantities are pumped by condensing air. A loss of oxygen recovery will therefore occur. When all liquid oxygen and about the same molar flow of liquid nitrogen are vaporized, as much as 7-10% loss oxygen recovery can be expected. More power and more feed air, i.e. larger plant size, are needed to produce the same quantity of oxygen.

[0004] In cryogenic air separation technology, an intermediate pressure column can be added to the double column process to improve the distillation performance. The main function of the intermediate pressure column is to distil the rich liquid bottom of the high pressure column to yield additional nitrogen rich liquid reflux for the low pressure column. The intermediate pressure column is usually bottom heated or reboiled by condensing the nitrogen rich gas from the top of the high pressure column. Double column process could have a side-arm column for argon extraction. Sometimes, the reboil of the intermediate pressure column can be provided by feed gas to the argon side-arm column or by some gases derived from the argon column itself. The intermediate pressure column operates at a pressure in between the pressures of the low pressure column and the high pressure column.

[0005] The argon and intermediate pressure columns can be used with the double column process, for example, to produce argon and to maximize the high pressure nitrogen extraction from the high pressure column. Good process efficiency can be achieved. In those processes, oxygen enriched liquid at the bottom of the high pressure column is fed to the intermediate pressure column and the resulting liquid extracted from the bottom of the intermediate pressure column is then partially vaporized in the top condensers of the intermediate and argon columns to provide the needed refluxes.

[0006] However, in situations where both liquid oxygen and liquid nitrogen are vaporized, and sometimes with a liquid production requirement, too much liquid air is formed such that the quantity of rich liquid at the bottom of the high pressure column is sharply reduced. Because of this effect, the use of intermediate pressure column with argon column becomes less effective or not practical for the simple reason that there is not sufficient liquid rich to drive the argon and intermediate pressure columns. In some cases, in order to assure a positive temperature difference in the condenser of the intermediate pressure column, some liquid air or liquid rich in nitrogen must be injected or mixed with the bottom liquid to lower its boiling temperature. This mixing can introduce irreversibility in the distillation system and cause a loss of efficiency. The new process addresses the above shortcomings by providing an alternative technique to process the additional liquid air efficiently.

[0007] The process of EP-A-0828123 utilizes the intermediate pressure column to improve the argon recovery when both liquid oxygen and liquid nitrogen are pumped and vaporized. In order to remedy the issue of lack of oxygen enriched liquid, some liquid air is fed to the intermediate pressure column to produce additional bottom liquid of the intermediate pressure column. Intermediate liquid with composition similar to air is mixed with intermediate pressure column's bottom liquid to provide cooling of the top condenser of the intermediate pressure column. The top condenser of the argon column is also cooled by vaporizing bottom liquid of the intermediate pressure column.

[0008] In Technical Disclosure IPCOM000019394D, Figure 2 discloses the use of the intermediate pressure column to enhance argon recovery. The process is similar to that of EP-A-0828123 but more or almost all liquid air extracted from the high pressure column is sent to the intermediate pressure column to yield additional liquid nitrogen reflux. The bottom stream of the intermediate pressure column is partially vaporized in its top condenser for cooling. The liquid fraction is fed to the top condenser of the argon column and vaporized to supply the needed cooling. In this arrangement, the two top condensers of the intermediate and argon columns are in series in terms of receiving vaporizing liquid from the bottom of the intermediate pressure column.

[0009] According to an object of the invention, there is provided a process for the separation of air by cryogenic distillation in a column system including a high pressure column, a low pressure column, the bottom of the low pressure column being thermally coupled with the top of the high pressure column, an intermediate pressure column, operating a pressure between that of the high pressure column and that of the low pressure column, and an argon column wherein:

i) purified compressed air is cooled in a heat exchanger and sent at least in part to the high pressure column,

ii) nitrogen enriched liquid is sent from the top of the high pressure column to the top of the low pressure column,

iii) oxygen rich liquid is removed from the low pressure column, pressurized and vaporized in the heat exchanger or another heat exchanger,

iv) nitrogen rich liquid is removed from the column system, pressurized and vaporized in the heat exchanger or another heat exchanger,

v) argon enriched gas is sent from the low pressure column to the argon column, said argon column having a top condenser, and argon rich fluid is removed from the top of the argon column,

vi) oxygen enriched liquid from the bottom of the high pressure column is partially vaporized in the top condenser of the argon column and the gas thereby formed is sent to the low pressure column,

vii) an intermediate stream is removed at an intermediate point of the high pressure column and sent at least in part to a top condenser of the intermediate pressure column where it is partially vaporized to form a vapor and a liquid,

viii) the vapor formed in the top condenser of the intermediate pressure column is sent to the low pressure column,

ix) the liquid from the top condenser of the intermediate pressure column is sent to the intermediate pressure column to be separated,

x) a liquid from the bottom of the intermediate pressure column is sent to the low pressure column, and

xi) a liquid from the top of the intermediate pressure column is sent to the top of the low pressure column.

[0010] Preferably:

• the intermediate pressure column has a bottom reboiler and wherein argon enriched gas from the low pressure column is condensed in the bottom reboiler.
• the liquid sent from the top condenser of the intermediate pressure column is the only feed to the intermediate pressure column.
• all the oxygen enriched liquid from the high pressure column is sent to argon column top condenser or to the low pressure column and the argon column top condenser, without passing through the intermediate pressure column top condenser.
• all the bottom liquid of the intermediate pressure column is sent to the low pressure column without passing through the intermediate pressure column top condenser.
• part of the liquid from the intermediate pressure column top condenser is sent to the low pressure column.
• the liquid from the top condenser is sent to the intermediate pressure column between 2 and 5 theoretical trays above the bottom of the intermediate pressure column.
• at least part of the air is cooled to an intermediate temperature of the heat exchanger, compressed in a compressor, further cooled in the heat exchanger and sent to at least the high pressure column.
• the bottom liquid of the intermediate pressure column contains at least 70% mol. oxygen.
• at least one of the bottom reboiler and the top condenser of the intermediate pressure column is a falling film reboiler.
• at least one part of the feed air is compressed from a first pressure to a second pressure in a warm booster, fed to the heat exchanger at the second pressure, cooled, expanded in a first turboexpander and sent to the high pressure column and at least another part of the feed air is sent to the heat exchanger at the first pressure, divided into three portions, the first portion being compressed from the first pressure to a third pressure in a cold booster, cooled, expanded and sent to the high pressure column, the second portion being expanded in a second turboexpander and sent to the high pressure column and the third portion being cooled to the cold end of the heat exchanger and sent to the high pressure column.

[0011] According to another object of the invention, there is provided an apparatus for the separation of air by cryogenic distillation comprising a column system including a high pressure column, a low pressure column, the bottom of the low pressure column being thermally coupled with the top of the high pressure column, an intermediate pressure column, operating a pressure between that of the high pressure column and that of the low pressure column, and an argon column a heat exchanger, means for sending purified compressed air to be cooled in the heat exchanger, means for sending cooled purified compressed air from the heat exchanger at least in part to the high pressure column, a conduit for sending nitrogen enriched liquid from the top of the high pressure column to the top of the low pressure column, a conduit for removing oxygen rich liquid from the low pressure column, said conduit being connected to first pressurization means, a conduit for sending pressurized oxygen rich liquid from the first pressurization means to the heat exchanger or another heat exchanger, a conduit for removing nitrogen rich liquid from the column system connected to second pressurization means, a conduit connecting the second pressurization means to the heat exchanger or another heat exchanger, a conduit for sending argon enriched gas from the low pressure column to the argon column, said argon column having a top condenser, a conduit for removing argon rich fluid from the top of the argon column, a conduit for sending oxygen enriched liquid from the bottom of the high pressure column to the top condenser of the argon column to be partially vaporized, a conduit for sending the gas thereby formed to the low pressure column, a conduit for sending an intermediate stream removed at an intermediate point of the high pressure column at least in part to a top condenser of the intermediate pressure column where it is partially vaporized to form a vapor and a liquid, a conduit for sending the vapor formed in the top condenser of the intermediate pressure column to the low pressure column, a conduit for sending the liquid from the top condenser of the intermediate pressure column to the intermediate pressure column to be separated, a conduit for sending a liquid from the bottom of the intermediate pressure column to the low pressure column and a conduit for sending a liquid from the top of the intermediate pressure column to the top of the low pressure column.

[0012] Preferably:

• the intermediate pressure column has a bottom reboiler and the apparatus comprises a conduit for sending argon enriched gas from the low pressure column to be condensed in the bottom reboiler.
• the liquid from the top condenser of the intermediate pressure column is sent to the intermediate pressure column between 2 and 5 theoretical trays above the bottom of the intermediate pressure column.
• the apparatus comprises a compressor, a conduit for sending at least part of the air cooled to an intermediate temperature of the heat exchanger to the compressor, a conduit for sending air from the compressor to be further cooled in the heat exchanger and a conduit for sending the air from the compressor via the heat exchanger to at least the high pressure column.
• the apparatus comprises a third pressurization means for pressurizing the liquid removed from the top condenser of the intermediate pressure column , before sending it to be separated in the intermediate pressure column.

[0013] Purified air has been treated to remove the water and carbon dioxide which it contains.

[0014] Oxygen rich liquid contains at least 70% mol. oxygen, preferably at least 85% mol. oxygen. It contains less than 100% mol. oxygen.

[0015] Nitrogen rich liquid contains at least 85% mol. nitrogen, preferably at least 90% mol nitrogen. It contains less than 100% moll nitrogen.

[0016] Oxygen enriched liquid contains at least 25% mol oxygen, or at least 30% mol oxygen.

[0017] For the present invention, the high pressure column operates at between 4 and 8 bar, the intermediate pressure column at between 2 and 3 bar, the argon column at between 1 and 2 bar, the low pressure column at between 1 and 2 bar.

[0018] All pressures mentioned are absolute pressures

[0019] The gaseous oxygen produced by pumping and vaporizing can be as low as 2 bar and as high as 80 bar or even 100 bars. The upper limit of the high pressure of pumped oxygen is usually dictated by the maximum allowable working pressure of the brazed heat exchanger.

[0020] The intermediate stream withdrawn from the high pressure column and sent to the intermediate pressure column top condenser contains between 18 and 25 mol% oxygen.

[0021] To illustrate the invention, Figure 1 shows the column portion of a process operating according to the invention and Figures 2 and 3 show two alternative corresponding heat exchanger portions, to be used for oxygen pressures above 15 bars abs.

[0022] In Figure 1, gaseous air 2 and liquid air 4 are fed to high pressure column 100. Oxygen enriched liquid 10 formed at the bottom of the high pressure column 100 is divided in two. One portion 12 is expanded and sent to an intermediate level of the low pressure column 101. Another portion 11 is expanded and sent to top condenser 105 of the argon column where it vaporizes to form stream 13 which is sent to the low pressure column 101. Alternatively all the oxygen enriched liquid 10 can be sent to the condenser 105 and partially condensed. In this case, stream 12 is absent and liquid from condenser 105 is sent to the low pressure column 101.The top of the high pressure column 100 is thermally coupled to the bottom of the low pressure column 101 via a condenser-reboiler 104. Nitrogen enriched liquid 40 from the top of the high pressure column 100 is divided in two, one portion 41 being sent to the top of the low pressure column 101 as reflux. Nitrogen enriched gas is removed from the top of the low pressure column 101.

[0023] A side liquid stream 20 with composition similar to air, containing between 18 and 25% mol. oxygen is extracted from column 100. Alternatively the side liquid stream could be replaced or supplemented by a part of liquid air stream 4 or another liquid air stream. A portion 22 of stream 20 (or stream 4, not illustrated) is partially vaporized in the top condenser 107 of intermediate pressure column 103. Condenser 107 could be a falling film vaporizer. The vapor 123 containing around 10% mol. oxygen) is sent to the low pressure column 101. A portion 24 of the liquid fraction 26 of the partially vaporization is then fed to column 103. Column 103 operates at about 2 bar and its condenser 107 at 1.4 bar. Gravity feed or a pump 110 can be used to transfer this liquid from condenser 107 to a position between 2 and 5 theoretical trays above the bottom of intermediate pressure column 103. Oxygen enriched liquid 60 from the bottom of column 103 containing preferably between 70 and 75 mol% oxygen is expanded and sent to the low pressure column. It is useful to note that a liquid air stream formed from the condensation of air for vaporizing liquid oxygen and liquid nitrogen products in the main heat exchanger can be sent to the top condenser of the intermediate column instead of using a part of the liquid stream 20 extracted from the high pressure column.

[0024] Preferably the average temperature difference for condensers 106, 107 should be between 0.8 and 0.9°C. Column 103 produces additional reflux liquid 23 for the top of the low pressure column 101. Column 102 is a typical side-arm argon column for a double column process. A portion 54 of argon enriched feed gas from the low pressure column 101 is separated in the argon column 102 to form argon product 80 in liquid form as shown or in gaseous form. The bottom liquid 52 from the argon column is sent back to the low pressure column 101. A portion 51 of argon enriched feed gas 50 from the low pressure column 101 is condensed in the bottom reboiler 106, preferably of the falling film type, of column 103 to yield liquid 53 which is then fed to column 102 or 101 to be separated. The argon column 102 is equipped with a top condenser 105 which vaporizes a portion 11 of oxygen enriched liquid 10 produced at the bottom of the high pressure column 100.

[0025] Another portion 45 of stream 40 is pumped by pump 121 to high pressure, vaporized and warmed to yield high pressure nitrogen product. Liquid oxygen 30 produced at the bottom of column 101 is pumped by pump 120 to high pressure, vaporized and warmed to yield high pressure oxygen product.

[0026] The embodiment shown in Figure 2 can be used to vaporize efficiently the liquid products 31, 42. The liquid products are vaporized in pumps 120,121, the oxygen being pressurized to a pressure between 15 and 80 bars abs. The cold compression technique is utilized and is described as follows:

Feed air compressed by compressor 201 to an elevated pressure of about between 15 and 25 bar is dried and its CO₂ content is removed in the front end purification unit 208. The resulting dried and CO₂ free stream 80 is divided into several portions. Portion 83 is cooled in heat exchanger 200 to an intermediate temperature thereof, a portion 91 of portion 83 is expanded in turboexpander 204 into the high pressure column 100. Second portion 84 of portion 83 is cold compressed, at a inlet temperature which is an intermediate temperature of the heat exchanger, in cold booster 202 to higher pressure to yield stream 85. Stream 85 is next cooled in exchanger 200 and liquefied to form liquid air stream 4. Another portion 79 of the cooled stream 83 is further cooled and liquefied to yield a second liquid air stream 6. Streams 4 and 6 are fed at least in part to the high pressure column 100 as feeds. A third portion 82 of feed air is further compressed in warm booster 207, cooled in exchanger 200 to yield cooled compressed stream 88 which is then expanded in turboexpander 203 into the high pressure column 100. The power generated by turboexpanders 203 and 204 can be used to drive boosters 202 and 207. Depending upon the pressure levels and the quantities of oxygen and nitrogen to be vaporized in the heat exchanger 200, it is sometime beneficial to also extract a portion of cooled compressed stream 88 and liquefy it in exchanger 200 in a similar fashion as stream 79. The resulting liquid stream (not shown) is then fed to the column system. By generating those auxiliary liquid streams, less liquid, i.e. lower flow, needs to be compressed by the cold compressor to satisfy the refrigeration balance at the cold end of the exchanger. More efficient system can be achieved by reducing the required cold compression flow.

[0027] The embodiment shown in Figure 3 can be used to reduce the plant power consumption. A multi stage booster compressor comprising several stages 209, 210 and 211 is added to further compress the fraction 82 feeding compressor 207. Multiple pressurized streams 95 and 96 can be generated by the booster compressor to vaporize efficiently the liquid products to form liquid air streams 97 and 99.

[0028] For lower oxygen pressures, more conventional vaporization processes such may be used. For very low pressures, the oxygen vaporizes in a dedicated vaporizer like a bath type vaporizer.

[0029] A cold booster has an inlet temperature of below -20°C.

Claims

1. Process for the separation of air by cryogenic distillation in a column system including a high pressure column (100), a low pressure column (101), the bottom of the low pressure column being thermally coupled with the top of the high pressure column, an intermediate pressure column (103), operating a pressure between that of the high pressure column and that of the low pressure column, and an argon column (102) wherein:

i) purified compressed air is cooled in a heat exchanger (200) and sent at least in part to the high pressure column,

ii) nitrogen enriched liquid (40, 41) is sent from the top of the high pressure column to the top of the low pressure column,

iii) oxygen rich liquid (30) is removed from the low pressure column, pressurized and vaporized in the heat exchanger or another heat exchanger,

iv) nitrogen rich liquid (42) is removed from the column system, pressurized and vaporized in the heat exchanger or another heat exchanger,

v) argon enriched gas is sent from the low pressure column to the argon column, said argon column having a top condenser (105), and argon rich fluid (80) is removed from the top of the argon column,

vi) oxygen enriched liquid (11) from the bottom of the high pressure column is partially vaporized in the top condenser of the argon column and the gas (13) thereby formed is sent to the low pressure column,

vii) a liquid air stream (4) or a stream (20, 22) containing between 18 and 25 mol.% oxygen withdrawn from an intermediate point of the high pressure column is sent at least in part to a top condenser (107) of the intermediate pressure column where it is partially vaporized to form a vapor and a liquid,

viii) the vapor (123) formed in the top condenser of the intermediate pressure column is sent to the low pressure column,

ix) the liquid (24) from the top condenser of the intermediate pressure column is sent to the intermediate pressure column to be separated,

x) a liquid (60) from the bottom of the intermediate pressure column is sent to the low pressure column, and

xi) a liquid from the top of the intermediate pressure column (23) is sent to the top of the low pressure column.

2. Process according to Claim 1 wherein the intermediate pressure column has a bottom reboiler (106) and wherein argon enriched gas (51) from the low pressure column is condensed in the bottom reboiler.

3. Process according to Claim 1 or 2 wherein the liquid (24) sent from the top condenser of the intermediate pressure column is the only feed separated in the intermediate pressure column.

4. Process according to any preceding claim wherein all the oxygen enriched liquid (10, 11, 12) from the high pressure column is sent to the argon column top condenser (105) or to the low pressure column and the argon column top condenser, without passing through the intermediate pressure column top condenser.

5. Process according to any preceding claim wherein all the bottom liquid (60) of the intermediate pressure column is sent to the low pressure column without passing through the intermediate pressure column top condenser.

6. Process according to any preceding claim wherein part (25) of the liquid from the intermediate pressure column top condenser (107) is sent to the low pressure column.

7. Process according to any preceding claim wherein the liquid (24) from the top condenser is sent to the intermediate pressure column (103) between 2 and 5 theoretical trays above the bottom of the intermediate pressure column.

8. Process according to any preceding claim wherein at least part of the air is cooled to an intermediate temperature of the heat exchanger (200), compressed in a compressor (202), further cooled in the heat exchanger and sent to at least the high pressure column (100).

9. Process according to any preceding claim wherein the bottom liquid (60) of the intermediate pressure column (103) contains at least 70% mol. oxygen.

10. Process according to any preceding claim wherein at least one of the bottom reboiler (106) and the top condenser (107) of the intermediate pressure column (103) is a falling film reboiler.

11. Process according to any preceding claim wherein at least one part of the feed air is compressed from a first pressure to a second pressure in a warm booster (207) , fed to the heat exchanger at the second pressure, cooled, expanded in a first turboexpander (203) and sent to the high pressure column and at least another part of the feed air is sent to the heat exchanger at the first pressure, divided into three portions, the first portion being compressed from the first pressure to a third pressure in a cold booster (202), cooled, expanded and sent to the high pressure column, the second portion being expanded in a second turboexpander (204) and sent to the high pressure column and the third portion being cooled to the cold end of the heat exchanger and sent to the high pressure column.

12. Apparatus for the separation of air by cryogenic distillation comprising a column system including a high pressure column (100), a low pressure column (101), the bottom of the low pressure column being thermally coupled with the top of the high pressure column, an intermediate pressure column (103), operating a pressure between that of the high pressure column and that of the low pressure column, and an argon column (102), a heat exchanger (200), a conduit for sending purified compressed air to be cooled in the heat exchanger, a conduit for sending cooled purified compressed air from the heat exchanger at least in part to the high pressure column, a conduit for sending nitrogen enriched liquid (40,41) from the top of the high pressure column to the top of the low pressure column, a conduit for removing oxygen rich liquid (30) from the low pressure column, said conduit being connected to first pressurization means (120), a conduit for sending pressurized oxygen rich liquid from the first pressurization means to the heat exchanger or another heat exchanger, a conduit for removing nitrogen rich liquid (40) from the column system connected to second pressurization means (121), a conduit connecting the second pressurization means to the heat exchanger or another heat exchanger, a conduit for sending argon enriched gas (50,54) from the low pressure column to the argon column, said argon column having a top condenser (105), a conduit for removing argon rich fluid (80) from the top of the argon column, a conduit for sending oxygen enriched liquid (10, 11) from the bottom of the high pressure column to the top condenser of the argon column to be partially vaporized, a conduit for sending the gas (13) thereby formed to the low pressure column, a conduit for sending a liquid air stream (4) or a liquid stream (20, 22) containing between 18 and 25% oxygen removed at an intermediate point of the high pressure column at least in part to a top condenser (107) of the intermediate pressure column where it is partially vaporized to form a vapor and a liquid, a conduit for sending the vapor (123) formed in the top condenser of the intermediate pressure column to the low pressure column, a conduit for sending the liquid (24) from the top condenser of the intermediate pressure column to the intermediate pressure column to be separated, a conduit for sending a liquid (60) from the bottom of the intermediate pressure column to the low pressure column and a conduit for sending a liquid (23) from the top of the intermediate pressure column to the top of the low pressure column.

13. Apparatus according to Claim 12 wherein the intermediate pressure column (103) has a bottom reboiler (106) and comprising a conduit for sending argon enriched gas (51) from the low pressure column to be condensed in the bottom reboiler.

14. Apparatus according to Claim 12 or 13 wherein the liquid (24) from the top condenser is sent to the intermediate pressure column (103) between 2 and 5 theoretical trays above the bottom of the intermediate pressure column.

15. Apparatus according to any of Claims 12 to 14 comprising a compressor, a conduit for sending at least part of the air cooled to an intermediate temperature of the heat exchanger to the compressor (202), a conduit for sending air from the compressor to be further cooled in the heat exchanger and a conduit for sending the air from the compressor via the heat exchanger to at least the high pressure column.

Drawing