Method for separating air by cryogenic distillation

Abstract

A method for separating air by cryogenic distillation in a column system comprising a first column (K1) operated at a first pressure and a second column (K2) operated at a second pressure lower than the first column, the temperature T1 of the air stream (11) after cooling, when leaving a heat exchanger by rising to the cold end of the heat exchanger and entering the first column, is at least 1°C, preferably at least 2°C above the dew point of the air stream.

Description

[0001] This invention relates to a method for separating air by low-temperature distillation.

[0002] Separation equipment typically includes: an exchange line in which the air to be distilled is cooled by at least two distillation products; and a column system comprising a first column and a second column, the first column operating at a first pressure and the second column operating at a second pressure lower than that in the first column.

[0003] The top of the first tower is thermally connected to the bottom of the second tower.

[0004] WO 19126927 describes an apparatus for separating air by cryogenic distillation, wherein the main exchange line is located below the distillation column system and the hot end of the exchanger is positioned towards the bottom.

[0005] The air intended for use in the first tower is cooled in the exchange pipeline and then sent to the first tower.

[0006] During cooling, the air partially condenses from bottom to top in the main exchange line. It is essential to ensure that the obtained liquid is properly supplied with gas to prevent liquid stagnation in the exchange line and, consequently, to avoid minor impurities such as oxygen and air (typically C). n H m Any possibility of local enrichment, which poses a security risk.

[0007] Obtaining sufficient gas velocity (including during reduced operation) involves creating a significant pressure drop in the air intended for use in the first column, which is costly in terms of energy.

[0008] In the air separation unit, the bottom liquid (RL) from the first column expands and is sent to the midpoint of the second column. The top liquid (PL) from the first column expands and is sent to the top of the second column. Both liquids are subcooled in a heat exchanger by the top gas from the second column.

[0009] In this exchanger (referred to as a subcooler, specifically in a cross-flow configuration), the nitrogen-rich top liquid, called lean liquid PL, and the oxygen-rich bottom liquid, called rich liquid RL, are cooled in two separate sections, as if two exchangers were connected in series. This means that the oxygen-rich liquid is cooled to a temperature higher than the inlet temperature of the top liquid from the first column. In this configuration, the available coolness from the residual nitrogen is not completely extracted.

[0010] A conventional subcooler is shown on page 96 of Kerry's Industrial Gas Handbook, published by CRC Press in 2007. For the sake of simplicity, precise fluid positioning is not always shown in patents or other documents.

[0011] Furthermore, for simplicity, the exhaust gas from the blower turbine is sent directly into the second tower. The relatively hot gas is then sent into the second tower.

[0012] This results in reduced cooling recovery of the cold fluid and necessitates partial condensation (typically about 1% to 2%) of the air leaving the main exchange line and heading to the bottom of the first tower.

[0013] The invention includes feeding at least one airflow (including during reduced operation of the equipment) to the tower, the temperature of which is 1°C or even 2°C higher than the dew point of the airflow. This implementation allows for the prevention of condensation of air intended for use in the first tower in the main exchange line. Therefore, the air exits the main exchange line at a temperature 1°C or even 2°C higher than its dew point.

[0014] One way to ensure that the air intended for the first column is sufficiently above the dew point is to deeply cool certain fluids entering the distillation system, as well as the fluids inside the system. In this case, exchange lines with very small pressure drops can be designed without considering safety-related liquid delivery standards.

[0015] This relates to recovering as much cold as possible from the fluids produced by distillation (residual nitrogen, oxygen, and nitrogen that is purer than the residual nitrogen under a second pressure).

[0016] A variation that ensures the air intended for the first tower is sufficiently above the dew point involves modifying the supercooling to lower the temperature of the liquid as it enters the second tower.

[0017] In the subcooler, the oxygen-enriched liquid is further cooled so that it exits at a temperature lower than its inlet temperature. Therefore, a shared region exists within the subcooler where both liquids are simultaneously cooled by at least one nitrogen stream from the second column.

[0018] According to another variant, in the exchange line, exhaust gas from the turbine is sent back to the exchange line to cool residual nitrogen (and optionally purer nitrogen from the second tower) and oxygen.

[0019] According to one object of the present invention, a method for separating air by cryogenic distillation in a column system is provided, the column system comprising a first column and a second column, the first column operating at a first pressure, the second column operating at a second pressure lower than the first pressure, the top of the first column being thermally connected to the bottom of the second column, wherein:

[0020] i) Cool the air stream purified into water and carbon dioxide in a heat exchanger and send it to the first tower in gaseous form.

[0021] ii) Oxygen-enriched liquid is drawn from the bottom of the first column and supercooled in a supercooler before being sent to the second column.

[0022] iii) Nitrogen-rich liquid is drawn from the top of the first column and subcooled in a supercooler before being sent to the second column.

[0023] iv) Extract nitrogen-rich gas and oxygen-rich fluid from the second tower and heat them in a heat exchanger.

[0024] v) A heat exchanger is positioned below a first tower, which in turn is positioned below a second tower, and the airflow is cooled by rising within the heat exchanger, characterized in that...

[0025] vi) The temperature T1 of the air stream as it leaves the heat exchanger and enters the first tower is at least 1°C higher, preferably at least 2°C higher, than the dew point of the air stream.

[0026] According to other optional aspects:

[0027] Another stream of air, purified into water and carbon dioxide, is cooled in a heat exchanger, leaves the heat exchanger at temperature T2, expands in a turbine, returns to the heat exchanger at temperature T3, and is cooled to temperature T4 in the heat exchanger. It is then sent to the second tower in gaseous form at a temperature T2 higher than T1.

[0028] • T4 has a dew point at least 1°C higher than that of the expanding flow, preferably at least 2°C higher.

[0029] • T4 is higher than, lower than or equal to T1

[0030] Another stream of air, purified into water and carbon dioxide under a second pressure, is cooled in a heat exchanger to a temperature at least 1°C, preferably at least 2°C, above its dew point and is then sent to the second tower without expansion.

[0031] The oxygen-rich liquid is cooled in a subcooler to a temperature lower than that of the nitrogen-rich liquid when it enters the subcooler. Both subcooled liquids expand in their respective valves and are then sent to the second tower.

[0032] The first tower and the potential second tower are fed air only through gaseous airflow.

[0033] The first and second towers only produce gaseous streams as the final product.

[0034] The heat exchanger and subcooler consist of individual bodies made of aluminum plates brazed together.

[0035] • The pressure drop of the air stream intended for use in the first column, which is purified into water and carbon dioxide by cooling in a heat exchanger, is not to exceed 120 mbar, or even 100 mbar.

[0036] • The liquid stream drawn from the middle height of the first tower is subcooled to an intermediate temperature in a subcooler and then sent to the second tower. This intermediate temperature is between the temperature of the oxygen-rich liquid at the outlet of the subcooler and the temperature of the nitrogen-rich liquid at the outlet of the subcooler.

[0037] The present invention will be described in more detail with reference to the accompanying drawings.

[0038] [ Figure 1 The method according to the present invention is shown.

[0039] The method uses a tower system comprising a first tower K1 and a second tower K2, the first tower operating at a first pressure and the second tower operating at a second pressure below the first pressure. The first tower K1 is thermally connected to the second tower K2 via a bottom evaporator.

[0040] The first tower is positioned below the second tower K2, and the top of the first tower is thermally connected to the bottom of the second tower. An aluminum brazed plate heat exchanger E is positioned below the first tower K1.

[0041] Airflow 1 is compressed to a first pressure by compressor 3, cooled by cooler 5, and purified into water and carbon dioxide in purification unit 7. To be cooled, the air is sent to the hot end of heat exchanger E (at the bottom of the exchanger) and rises to the top, since the cold end of the exchanger is located at the top.

[0042] The purified air 9 is cooled in heat exchanger E and divided in two at temperature T2, which is the intermediate temperature of exchanger E. One portion of the air 11 continues to be cooled in the exchanger to temperature T1, which is at least 1°C, preferably at least 2°C, higher than the dew point of the air portion 9.

[0043] This portion of air leaves the exchanger E at temperature T1 and is fed to the bottom of tower K1 as a feed gas stream.

[0044] The pressure drop of the air 9 and 11 passing through exchanger E shall not exceed 120 mbar or even 100 mbar.

[0045] Air is separated in the first column to form an oxygen-rich bottom liquid 10 and a nitrogen-rich top liquid 12. Liquids 10 and 12 are fed to a subcooler S, where at least one nitrogen stream 15 from the second column K2 is heated. Liquid 10 enters the subcooler at its cold end and is cooled to a temperature lower than when liquid 12 enters the subcooler S. Liquid 12 exits the cold end of the subcooler. The subcooled liquids 10 and 12 each expand in a valve, and liquid 10 is fed to a height in the second column K2, while liquid 12 is fed to a height in the second column K2 higher than when it entered. Therefore, the subcooler includes a section in which both liquids 10 and 12 are cooled by nitrogen 15. The subcooler S can be placed next to column K1, and its hot end can be positioned towards the bottom.

[0046] A portion of the air 13 expands uncompressed in a turbine T downstream of the heat exchanger at temperature T2, is sent to heat exchanger E at temperature T3, and is cooled to temperature T4 in heat exchanger E. It is then sent as a gas to a second tower, where temperature T2 is higher than T1. T4 may be higher than, equal to, or lower than T1. T4 is at least 1°C, preferably at least 2°C, higher than the dew point of the expanding stream 13. The portion 13 exits the cold end of heat exchanger E and is sent directly as a gas to a height in the second tower K2, below the height at which the expanding liquid 10 arrives. For the air 11, the portion 13 rises and is cooled within the heat exchanger.

[0047] Column K2 separates streams 10 and 12 by distillation to form nitrogen-rich gas 15 at the top of the column and oxygen-rich gas 17 at the bottom of the column. Gas 17 descends and heats up in exchanger E and is subsequently used as a product of this process.

[0048] Gas 15 is heated in a supercooler and then condensed in exchanger E, where it is split in two. One part is used to renew purification unit 7, while the other part is used as a product or as residual nitrogen.

[0049] Here, the first tower K1 is fed air only through a gaseous airflow.

[0050] The second tower K2 can also be fed with air, and in this case, it can be fed only with gaseous air.

[0051] This method only produces gas streams 15 and 17 as the final product. The purge stream from the bottom condenser of column K2 is not considered as a final product.

[0052] The subcooler S can be integrated into the main exchange line E. This allows for further optimization of the extraction of cold fluids 15, 17 from distillation (and possibly pure nitrogen from the second column K2) in order to minimize the cooling of the rich liquid 10 and lean liquid 12, as well as the air from the blower turbine at a second pressure.

[0053] In addition to fluids 15 and 17, the liquefied air stream drawn from tower K1 can also be subcooled in subcooler S and then sent to tower K2.

[0054] [ Figure 1 This illustrates one possibility for feeding air into the method. Other variations are also possible, such as the variation in FR3090831 which uses a tower system comprising a first tower and a second tower, the first tower operating at a first pressure and the second tower operating at a second pressure below the first pressure, the top of the first tower being thermally connected to the bottom of the second tower.

[0055] In this configuration, three air streams are used. The air is compressed to the pressure of the second column and then purified. Next, the air is split in two; one portion is cooled at the pressure of the second column and sent to the second column via an upward flow in an exchanger. The final temperature exiting the exchanger is at least 1°C, preferably at least 2°C, higher than its dew point at the second pressure.

[0056] Another portion is pressurized to the pressure of the first tower. A small portion of this portion is cooled by rising in an exchanger and leaves the exchanger at a temperature T1 that is at least 1°C, preferably at least 2°C, higher than the dew point of that small portion of air.

[0057] The remainder of this section is cooled to temperature T2 in the heat exchanger, expanded in the blower turbine, returned to the heat exchanger at temperature T3, cooled to temperature T4 at the cold end of the heat exchanger, and then sent to the second tower. T4 is at least 1°C higher than the dew point, preferably at least 2°C higher.

Claims

1. A method for separating air by cryogenic distillation in a column system, the column system comprising a first column (K1) and a second column (K2), the first column operating at a first pressure, the second column operating at a second pressure below the first pressure, the top of the first column being thermally connected to the bottom of the second column, wherein: i) The first air stream (11), which has been purified by removing water and carbon dioxide, is cooled in a heat exchanger (E) and sent to the first tower in gaseous form; ii) The oxygen-rich liquid (10) is drawn from the bottom of the first column and then subcooled in the supercooler (S) before being sent to the second column; iii) Nitrogen-rich liquid (12) is drawn from the top of the first column and then subcooled in the subcooler before being sent to the second column; iv) Extract nitrogen-rich gas (15) and oxygen-rich fluid (17) from the second tower and heat them in the heat exchanger (E); v) The heat exchanger (E) is positioned below the first tower, which in turn is positioned below the second tower, and the first airflow is cooled by rising within the heat exchanger (E), characterized in that... vi) The temperature T1 of the first air stream (11) when it leaves the heat exchanger (E) and enters the first tower is at least 1°C higher than the dew point of the first air stream (11).

2. The method as described in claim 1, wherein, The temperature T1 of the first airflow (11) when it leaves the heat exchanger (E) and enters the first tower is at least 2°C higher than the dew point of the first airflow (11).

3. The method as described in claim 1, wherein, The second air stream (13), which has been purified by removing water and carbon dioxide, is cooled in the heat exchanger (E), leaves the heat exchanger (E) at a temperature T2, expands in the turbine (T), returns to the heat exchanger (E) at a temperature T3, and is cooled to a temperature T4 in the heat exchanger (E), and is then sent to the second tower (K2) in gaseous form at a temperature T2 higher than the temperature T1.

4. The method of claim 2, wherein, The second air stream (13), which has been purified by removing water and carbon dioxide, is cooled in the heat exchanger (E), leaves the heat exchanger (E) at a temperature T2, expands in the turbine (T), returns to the heat exchanger (E) at a temperature T3, and is cooled to a temperature T4 in the heat exchanger (E), and is then sent to the second tower (K2) in gaseous form at a temperature T2 higher than the temperature T1.

5. The method of claim 3, wherein, Temperature T4 is at least 1°C higher than the dew point of the second airflow (13) that has already expanded in the turbine (T).

6. The method of claim 4, wherein, Temperature T4 is at least 1°C higher than the dew point of the second airflow (13) that has already expanded in the turbine (T).

7. The method of claim 5, wherein, Temperature T4 is at least 2°C higher than the dew point of the second airflow (13) that has already expanded in the turbine (T).

8. The method of claim 6, wherein, Temperature T4 is at least 2°C higher than the dew point of the second airflow (13) that has already expanded in the turbine (T).

9. The method of claim 1, wherein, The purified third air stream, having removed water and carbon dioxide under the second pressure, is cooled in the heat exchanger (E) to a temperature at least 1°C above its dew point and is sent to the second tower without expansion.

10. The method of claim 9, wherein, The purified third air stream, having removed water and carbon dioxide under the second pressure, is cooled in the heat exchanger (E) to a temperature at least 2°C above its dew point and is sent to the second tower without expansion.

11. The method according to any one of claims 1-10, wherein, The oxygen-rich liquid (10) is cooled in the subcooler to a temperature lower than that of the nitrogen-rich liquid when it enters the subcooler. The two subcooled liquids expand in their respective valves and are then sent to the second tower (K2).

12. The method according to any one of claims 1-10, wherein, The first tower (K1) and the second tower (K2) are fed air only by a gaseous airflow.

13. The method of claim 11, wherein, The first tower (K1) and the second tower (K2) are fed air only by a gaseous airflow.

14. The method of claim 12, wherein, The first tower (K1) and the second tower (K2) are fed air only by a gaseous airflow.

15. The method of claim 13, wherein, The first tower (K1) and the second tower (K2) are fed air only by a gaseous airflow.

16. The method according to any one of claims 1-10, wherein, The first tower (K1) and the second tower (K2) produce only gas streams as the final product.

17. The method of claim 11, wherein, The first tower (K1) and the second tower (K2) produce only gas streams as the final product.

18. The method of claim 12, wherein, The first tower (K1) and the second tower (K2) produce only gas streams as the final product.

19. The method according to any one of claims 1-10, wherein, The heat exchanger (E) and the subcooler (S) are composed of a single body of aluminum plates brazed together.

20. The method of claim 11, wherein, The heat exchanger (E) and the subcooler (S) are composed of a single body of aluminum plates brazed together.

21. The method of claim 12, wherein, The heat exchanger (E) and the subcooler (S) are composed of a single body of aluminum plates brazed together.

22. The method of claim 16, wherein, The heat exchanger (E) and the subcooler (S) are composed of a single body of aluminum plates brazed together.

23. The method according to any one of claims 1-10, wherein, The pressure drop of the first air stream (11) intended for use in the first tower, purified by removing water and carbon dioxide, during cooling in the heat exchanger (E) shall not exceed 120 mbar.

24. The method of claim 11, wherein, The pressure drop of the first air stream (11) intended for use in the first tower, purified by removing water and carbon dioxide, during cooling in the heat exchanger (E) shall not exceed 120 mbar.

25. The method of claim 12, wherein, The pressure drop of the first air stream (11) intended for use in the first tower, purified by removing water and carbon dioxide, during cooling in the heat exchanger (E) shall not exceed 120 mbar.

26. The method of claim 16, wherein, The pressure drop of the first air stream (11) intended for use in the first tower, purified by removing water and carbon dioxide, during cooling in the heat exchanger (E) shall not exceed 120 mbar.

27. The method of claim 19, wherein, The pressure drop of the first air stream (11) intended for use in the first tower, purified by removing water and carbon dioxide, during cooling in the heat exchanger (E) shall not exceed 120 mbar.

28. The method of claim 23, wherein, The pressure drop of the first air stream (11) intended for use in the first tower, purified by removing water and carbon dioxide, during cooling in the heat exchanger (E) shall not exceed 100 mbar.

29. The method according to any one of claims 1-10, wherein, A liquid stream is drawn from the first tower (K1) at an intermediate height. The liquid stream is subcooled to an intermediate temperature in the subcooler (S) and sent to the second tower (K2). The intermediate temperature is between the temperature of the oxygen-rich liquid at the outlet of the subcooler and the temperature of the nitrogen-rich liquid at the outlet of the subcooler.

30. The method of claim 11, wherein, A liquid stream is drawn from the first tower (K1) at an intermediate height. The liquid stream is subcooled to an intermediate temperature in the subcooler (S) and sent to the second tower (K2). The intermediate temperature is between the temperature of the oxygen-rich liquid at the outlet of the subcooler and the temperature of the nitrogen-rich liquid at the outlet of the subcooler.

31. The method of claim 12, wherein, A liquid stream is drawn from the first tower (K1) at an intermediate height. The liquid stream is subcooled to an intermediate temperature in the subcooler (S) and sent to the second tower (K2). The intermediate temperature is between the temperature of the oxygen-rich liquid at the outlet of the subcooler and the temperature of the nitrogen-rich liquid at the outlet of the subcooler.

32. The method of claim 16, wherein, A liquid stream is drawn from the first tower (K1) at an intermediate height. The liquid stream is subcooled to an intermediate temperature in the subcooler (S) and sent to the second tower (K2). The intermediate temperature is between the temperature of the oxygen-rich liquid at the outlet of the subcooler and the temperature of the nitrogen-rich liquid at the outlet of the subcooler.

33. The method of claim 19, wherein, A liquid stream is drawn from the first tower (K1) at an intermediate height. The liquid stream is subcooled to an intermediate temperature in the subcooler (S) and sent to the second tower (K2). The intermediate temperature is between the temperature of the oxygen-rich liquid at the outlet of the subcooler and the temperature of the nitrogen-rich liquid at the outlet of the subcooler.

34. The method of claim 23, wherein, A liquid stream is drawn from the first tower (K1) at an intermediate height. The liquid stream is subcooled to an intermediate temperature in the subcooler (S) and sent to the second tower (K2). The intermediate temperature is between the temperature of the oxygen-rich liquid at the outlet of the subcooler and the temperature of the nitrogen-rich liquid at the outlet of the subcooler.

35. The method of claim 28, wherein, A liquid stream is drawn from the first tower (K1) at an intermediate height. The liquid stream is subcooled to an intermediate temperature in the subcooler (S) and sent to the second tower (K2). The intermediate temperature is between the temperature of the oxygen-rich liquid at the outlet of the subcooler and the temperature of the nitrogen-rich liquid at the outlet of the subcooler.

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