A method for separating air by cryogenic distillation.

Abstract

In a method for separating air by cryogenic distillation in a system of columns comprising a first column (K1) operating at a first pressure and a second column (K2) operating at a second pressure lower than the first column, an air stream (11) rises towards the cold end of a heat exchanger such that the temperature T1 at which it leaves said heat exchanger after cooling is at least 1° C., preferably at least 2° C., higher than the dew point of the air stream.

Description

Technical Field

[0001] The present invention relates to a method for separating air by cryogenic distillation.

Background Art

[0002] A separation apparatus generally includes an exchange line for cooling the air to be distilled with respect to at least two products of distillation, and a column system including a first column operating at a first pressure and a second column operating at a second pressure lower than the first pressure.

[0003] The top of the first column is thermally coupled to the bottom of the second column.

[0004] International Publication No. 19126927 describes an apparatus for separating air by cryogenic distillation in which the main exchange line is positioned below the system of distillation columns and the high temperature end of the exchanger is positioned towards the bottom of the column.

[0005] The air entering the first column is cooled in the exchange line before being sent to the first column. [[ID=2*]]

[0006] During cooling from bottom to top in the main exchange line, a part of this air condenses. The resulting liquid must be reliably and properly transported by gas in order to avoid pooling of the liquid in the exchange line which causes safety risks and thus local enrichment of oxygen and associated impurities (typically C

[0008] , , , H "<00*0002>). <00*00025> To obtain a sufficient gas velocity, a significant drop in the pressure of the air intended for the first column is involved, including during reduced operation, which is costly in terms of energy.

[0008] In the air separation unit, the bottom liquid (RL) from the first column is expanded and sent to the center of the second column. The top liquid (PL) from the first column is expanded and sent to the top of the second column. The two liquids are supercooled in a heat exchanger relative to the top gas from the second column.

[0009] In particular, in this cross-flow type exchanger, also known as a subcooler, the nitrogen-enriched top liquid, known as the poor liquid (PL), and the oxygen-enriched bottom liquid, known as the rich liquid (RL), are cooled in two different sections, as if there were two exchangers in series. This means that the oxygen-rich liquid is cooled to a temperature higher than the inflow temperature of the top liquid from the first column. In this configuration, the cooling available from residual nitrogen is not completely removed.

[0010] A conventional subcooler is illustrated on page 96 of Kerry's Industrial Gas Handbook, CRC Press, 2007. The precise location of the fluid is not always shown in patents or other documents for the sake of simplifying the diagrams.

[0011] In addition, the exhaust gas from the blower turbine is sent directly to the second tower for simplification. Relatively high-temperature gas is sent to the second tower.

[0012] This has the effect of reducing cooling recovery from the low-temperature fluid and makes it possible to condense a portion (about 1-2%) of the air that leaves the main exchange line and heads towards the bottom of the first tower. [Overview of the project] [Problems that the invention aims to solve]

[0013] The present invention comprises supplying at least one airflow to the tower at a temperature 1°C and 2°C above its dew point, including during the device's reduction operation. This implementation makes it possible to avoid condensation in the main air exchange line intended for the first tower. The air therefore exits the main exchange line at a temperature 1°C and 2°C above its dew point.

[0014] One way to ensure that the intended air for the first column is well above the dew point is to cool the specific fluid entering the distillation system and the internal fluid of the distillation system more deeply. In this case, it is possible to design exchange lines with very small pressure drops, and safety standards for liquid transport do not need to be considered.

[0015] This applies when trying to recover as much low-temperature (residual nitrogen, oxygen, and second-pressure nitrogen of higher purity than residual nitrogen) as possible from the fluid obtained from distillation.

[0016] One variation of ensuring that the intended air in the first tower is well above the dew point involves adjusting the subcooling to lower the temperature of the liquid entering the second tower.

[0017] In the subcooler, the cooling of the oxygen-enriched liquid is deepened so that it exits at a temperature lower than the inflow temperature of the enriched liquid. Thus, within the subcooler, there is a shared area where both liquids are simultaneously cooled for at least one nitrogen stream flowing in from the second tower.

[0018] In another variant, within the exchange line, exhaust from the turbine is sent back towards the exchange line and cooled against residual nitrogen (and optionally, higher-purity nitrogen from a second tower) and oxygen.

[0019] According to one object of the present invention, a column system comprising a first column operating at a first pressure and a second column operating at a second pressure lower than the first pressure, wherein the top of the first column is thermally coupled to the bottom of the second column, and a method for separating air by cryogenic distillation in the column system, i) The airflow from which water and carbon dioxide have been removed is cooled in a heat exchanger and sent in gaseous form to the first tower. ii) The oxygen-enriched liquid is drawn from the bottom of the first tower, supercooled in a subcooler, and sent to the second tower. iii) The nitrogen-enriched liquid is drawn from the top of the first tower, supercooled in a subcooler, and sent to the second tower. iv) The nitrogen-rich gas and oxygen-rich fluid are drawn from the second tower and heated in the heat exchanger. v) In a method in which the heat exchanger is located below the first tower, and the first tower is similarly located below the second tower, and the airflow is cooled by rising within the heat exchanger, vi) The temperature T1 at which the airflow exits the heat exchanger and enters the first tower is at least 1°C, preferably at least 2°C, higher than the dew point of the airflow. A method characterized by the above is provided.

[0020] According to other optional embodiments, • Another airflow from which water and carbon dioxide have been removed is cooled in a heat exchanger, exits the heat exchanger at temperature T2, is expanded in a turbine, returns to the heat exchanger at temperature T3, and is cooled in the exchanger to temperature T4 before being sent to the second tower in gaseous form, where temperature T2 is higher than T1. T4 is at least 1°C, preferably at least 2°C, higher than the dew point of the expanded flow. T4 is less than or equal to T1, • Another airflow, from which water and carbon dioxide have been removed, at substantially a second pressure, is cooled in the heat exchanger to a temperature at least 1°C, preferably at least 2°C, above its dew point, and is sent to the second tower without expansion. · In the subcooler, the oxygen-enriched liquid is cooled to a temperature lower than the temperature at which the nitrogen-enriched liquid enters the subcooler, and the two subcooled liquids are each expanded by their respective valves before being sent to the second column. · The first column and potentially the second column are supplied with air by only a gaseous air stream. · The first column and the second column produce only a gaseous stream as the final product. · The heat exchanger and the subcooler are composed of a single unit of aluminum plates welded together. · The pressure drop of the air stream from which water and carbon dioxide have been removed, intended for the first column by cooling in the heat exchanger, does not exceed 120 mbar or even more than 100 mbar. · The liquid stream is drawn out at an intermediate height of the first column, supercooled in the subcooler to an intermediate temperature between the temperature of the oxygen-enriched liquid at the outlet of the subcooler and the temperature of the nitrogen-enriched liquid at the outlet of the subcooler, and sent to the second column.

[0021] The present invention will be described in more detail with reference to the drawings.

Brief Description of the Drawings

[0022] [Figure 1] Figure 1 shows the method according to the present invention.

Mode for Carrying Out the Invention

[0023] This method uses a column system including a first column K1 operating at a first pressure and a second column K2 operating at a second pressure lower than the first pressure. The first column K1 is thermally coupled to the second column K2 by the bottom evaporator of the second column.

[0024] The first column is positioned below the second column K2, and the top of the first column is thermally coupled to the bottom of the second column. The brazed aluminum plate heat exchanger E is positioned below the first column K1.

[0025] The airflow 1 is compressed to a first pressure by the compressor 3, cooled by the cooler 5, and then water and carbon dioxide are removed in the purification unit 7. To cool, the air is sent to the high-temperature end of the heat exchanger E at the bottom of the exchanger and rises towards the top of the tower, as the low-temperature end of the exchanger is at the top of the tower.

[0026] The purified air 9 is cooled in a heat exchanger E and split into two at a temperature T2, which is an intermediate temperature in the exchanger E. One portion of the air 11 is further cooled in the exchanger to a temperature T1 that is at least 1°C, preferably at least 2°C, higher than the dew point of the air fraction 9.

[0027] At this temperature T1, this gas exits the exchanger E and is sent to the bottom of the tower K1 as a feed gas stream.

[0028] The pressure drop of the air 9 and 11 passing through the exchanger E shall not exceed 120 or even 100 mbar.

[0029] Air is separated in the first column to form an oxygen-enriched bottom liquid 10 and a nitrogen-enriched top liquid 12. Liquids 10 and 12 are sent to a subcooler S, where at least one nitrogen gas stream 15 from the second column K2 is heated. Liquid 10 enters the subcooler from its hot end and is cooled to a temperature lower than the temperature at which liquid 12 enters the subcooler S. Liquid 12 exits from the cold end of the subcooler. Each of the supercooled liquids 10 and 12 is expanded by a valve, and liquid 10 is sent to a certain height in the second column K2, and liquid 12 is sent to a position in the second column K2 higher than the height at which liquid 10 enters. The subcooler therefore includes a section in which both liquids 10 and 12 are cooled to nitrogen 15. The subcooler S is installed next to column K1, and its hot end can be positioned toward the bottom of the column.

[0030] One portion 13 of the air at temperature T2 is expanded uncompressed by turbine T downstream of the exchanger and sent to exchanger E at temperature T3, where it is cooled to temperature T4 before being sent in gaseous form to the 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 expanded flow 13. The portion 13 exits from the low-temperature end of exchanger E and enters directly into the second tower K2 in gaseous form at a position lower than the arrival of the expanded liquid 10. For air 11, the portion 13 is cooled in the exchanger by rising.

[0031] Column K2 separates flows 10 and 12 by distillation, forming nitrogen-enriched gas 15 at the top and oxygen-enriched gas 17 at the bottom. Gas 17 is heated by descent in exchanger E and then used as the product of this method.

[0032] The gas 15, which is heated by descent in the subcooler and then in the exchanger E, is split into two parts: one part is used to regenerate the purification unit 7, and the other is used as a product or residual nitrogen.

[0033] Here, the first tower K1 is supplied with air only by a gaseous airflow.

[0034] Air can also be supplied to the second tower K2, in which case it is only a feed of gaseous air.

[0035] This method produces only gaseous flows 15 and 17 as final products. The purge flow from the bottom condenser of column K2 is not considered a final product.

[0036] The subcooler S can be integrated into the main exchange line E. This further maximizes the decomposition of the low-temperature fluids 15, 17 obtained from distillation (and possibly pure nitrogen from the second column K2 as well) and maximizes the cooling of the rich and poor liquids 10, 12 and the second-pressure air from the blower turbine.

[0037] In addition to fluids 15 and 17, the liquefied airflow drawn from tower K1 can be supercooled in subcooler S before being sent to tower K2.

[0038] Figure 1 shows one possible method for supplying air to the method. Other variations are also available, such as the one described in French Patent No. 30890831, which includes a first tower operating at a first pressure and a second tower operating at a second pressure lower than the first, with the top of the first tower thermally coupled to the bottom of the second tower.

[0039] In this case, three airflows are used. The air is compressed to the pressure of the second tower and then purified. Next, the air is split into two, one portion of which is cooled to the pressure of the second tower and sent to the second tower by rising through an exchanger. The final temperature upon exiting the exchanger is at least 1°C, preferably at least 2°C, higher than its dew point at the second pressure.

[0040] The other portion is pressurized to the pressure of the first tower. One fraction of this portion is cooled by rising within the exchanger and exits at a temperature at least 1°C, preferably at least 2°C, higher than the dew point of the air fraction.

[0041] The remainder of this portion is cooled to temperature T2 in the exchanger, expanded by the blower turbine, returned to the exchanger at temperature T3, rises to the low-temperature end of the heat exchanger to cool to temperature T4, and sent to the second tower. T4 is at least 1°C, preferably at least 2°C, higher than the dew point. The following is a direct reproduction of the claims as originally filed. [1] A column system comprising a first column (K1) operating at a first pressure and a second column (K2) operating at a second pressure lower than the first pressure, wherein the top of the first column is thermally coupled to the bottom of the second column, a method for separating air by cryogenic distillation in a column system, i) The airflow (11) from which water and carbon dioxide have been removed is cooled in a heat exchanger (E) and sent in gaseous form to the first tower. ii) The oxygen-enriched liquid (10) is drawn from the bottom of the first tower, supercooled in the subcooler (S), and sent to the second tower. iii) The nitrogen-enriched liquid (12) is drawn from the top of the first tower, supercooled in the subcooler, and sent to the second tower. iv) The nitrogen-rich gas (15) and the oxygen-rich fluid (17) are drawn from the second tower and heated in the heat exchanger. v) In a method in which the heat exchanger is located below the first tower, the first tower is similarly located below the second tower, and the airflow is cooled by rising within the heat exchanger, vi) The temperature T1 at which the airflow (11) exits the heat exchanger and enters the first tower is at least 1°C higher than the dew point of the airflow. A method characterized by the following. [2] The method according to [1], wherein the temperature T1 at which the airflow (11) exits the heat exchanger and enters the first tower is at least 2°C higher than the dew point of the airflow. [3] Another airflow (13) from which water and carbon dioxide have been removed is cooled in the heat exchanger (E), exits the heat exchanger at temperature T2, is expanded in the turbine (T), returns to the heat exchanger at temperature T3, and is cooled in the exchanger to temperature T4 before being sent in gaseous form to the second tower (K2), wherein the temperature T2 is higher than T1, the method according to [1] or [2]. [4] The method according to [3], wherein T4 is at least 1°C higher than the dew point of the expanded flow (13). [5] The method according to [4], wherein T4 is at least 2°C higher than the dew point of the expanded flow (13). [6] The method according to [1], wherein another airflow, substantially at the second pressure, from which water and carbon dioxide have been removed, is cooled in the heat exchanger to a temperature at least 1°C above its dew point and sent to the second tower without expansion. [7] The method according to [6], wherein another airflow, substantially at the second pressure, from which water and carbon dioxide have been removed, is cooled in the heat exchanger to a temperature at least 2°C above its dew point and sent to the second tower without expansion. [8] The method according to any one of [1] to [7], wherein the oxygen-enriched liquid (10) is cooled in the subcooler to a temperature lower than the temperature at which the nitrogen-enriched liquid enters the subcooler, and the two supercooled liquids are each expanded by their respective valves before being sent to the second tower (K). [9] The method according to any one of [1] to [8], wherein the first tower (K1) and potentially the second tower (K2) are supplied with air solely by a gaseous airflow.

[10] The method according to any one of [1] to [9], wherein the first column (K1) and the second column (K2) produce only a gaseous flow as the final product.

[11] The method according to any one of [1] to

[10] , wherein the heat exchanger (E) and the subcooler (S) are made of a single aluminum plate brazed together.

[12] The method according to any one of [1] to

[11] , wherein the pressure drop in the airflow (11) from which water and carbon dioxide have been removed, as intended for the first tower, due to cooling in the heat exchanger, does not exceed 120 mbar.

[13] The method according to

[12] , wherein the pressure drop in the airflow (11) from which water and carbon dioxide have been removed, as intended for the first tower, due to cooling in the heat exchanger, does not exceed 100 mbar.

[14] The method according to any one of [1] to

[13] , wherein the liquid flow is drawn out at an intermediate height in the first tower (K1), supercooled in the subcooler (S) to an intermediate temperature between the temperature of the oxygen-enriched liquid at the outlet of the subcooler and the temperature of the nitrogen-enriched liquid at the outlet of the subcooler, and then sent to the second tower (K2).

Claims

1. A column system comprising a first column (K1) operating at a first pressure and a second column (K2) operating at a second pressure lower than the first pressure, wherein the top of the first column is thermally coupled to the bottom of the second column, and a method for separating air by cryogenic distillation in such a column system, i) The airflow (11) from which water and carbon dioxide have been removed is cooled in a heat exchanger (E) and sent to the first tower in gaseous form. ii) The oxygen-enriched liquid (10) is drawn out from the bottom of the first tower, supercooled in the subcooler (S), and sent to the second tower. iii) The nitrogen-enriched liquid (12) is drawn from the top of the first tower, supercooled in the subcooler, and sent to the second tower. iv) A nitrogen-rich gas (15) and an oxygen-rich fluid (17) are drawn from the second tower and heated in the heat exchanger. v) In a method in which the heat exchanger is located below the first tower, the first tower is similarly located below the second tower, and the airflow is cooled by rising within the heat exchanger, vi) The temperature T1 at which the airflow (11) exits the heat exchanger and enters the first tower is at least 1°C higher than the dew point of the airflow. A method characterized by the following.

2. The method according to claim 1, wherein the temperature T1 at which the airflow (11) exits the heat exchanger and enters the first tower is at least 2°C higher than the dew point of the airflow.

3. The method according to claim 1 or 2, wherein another airflow (13) from which water and carbon dioxide have been removed is cooled in the heat exchanger (E), exits the heat exchanger at temperature T2, is expanded in the turbine (T), returns to the heat exchanger at temperature T3, and is cooled in the heat exchanger to temperature T4 before being sent in gaseous form to the second tower (K2), the temperature T2 being higher than T1.

4. The method according to claim 3, wherein T4 is at least 1°C higher than the dew point of the expanded other airflow (13).

5. The method according to claim 4, wherein T4 is at least 2°C higher than the dew point of the expanded other airflow (13).

6. The method according to claim 1, wherein another airflow from which water and carbon dioxide have been removed, substantially at the second pressure, is cooled in the heat exchanger to a temperature at least 1°C above its dew point and sent to the second tower without expansion.

7. The method according to claim 6, wherein another airflow from which water and carbon dioxide have been removed, substantially at the second pressure, is cooled in the heat exchanger to a temperature at least 2°C above its dew point and sent to the second tower without expansion.

8. The method according to any one of claims 1 to 7, wherein the oxygen-enriched liquid (10) is cooled in the subcooler to a temperature lower than the temperature at which the nitrogen-enriched liquid enters the subcooler, and the supercooled oxygen-enriched liquid and nitrogen-enriched liquid are each expanded by their respective valves before being sent to the second tower (K).

9. The method according to any one of claims 1 to 8, wherein the first tower (K1) and the second tower (K2) are supplied with air solely by a gaseous airflow.

10. The method according to any one of claims 1 to 9, wherein the first tower (K1) and the second tower (K2) generate only a gas flow as the final product.

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

12. The method according to any one of claims 1 to 11, wherein the pressure drop in the airflow (11) from which water and carbon dioxide have been removed, as intended for the first tower, due to cooling in the heat exchanger, does not exceed 120 mbar.

13. The method according to claim 12, wherein the pressure drop in the airflow (11) from which water and carbon dioxide have been removed, as intended for the first tower, due to cooling in the heat exchanger, does not exceed 100 mbar.

14. The method according to any one of claims 1 to 13, wherein the liquid flow is drawn out at an intermediate height in the first tower (K1), supercooled in the subcooler (S) to an intermediate temperature between the temperature of the oxygen-enriched liquid at the outlet of the subcooler and the temperature of the nitrogen-enriched liquid at the outlet of the subcooler, and then sent to the second tower (K2).

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