METHOD FOR AIR SEPARATION BY CRYOGENS DISTILLATION

Description

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

[0002] A separation apparatus generally comprises an exchange line where the air to be distilled is cooled against at least two products of the 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 column.

[0003] The head of the first column is thermally connected to the tank of the second column.

[0004] WO19126927 describes an air separation apparatus by cryogenic distillation in which the main exchange line is located below the distillation column system, and with the hot end of the exchanger arranged downwards.

[0005] The air intended for the first column cools in the exchange line before being sent to the first column.

[0006] During its cooling, from the bottom to the top of the main exchange line, this air partially condenses. It is necessary to ensure that the liquid obtained is well entrained by the gas, to avoid the liquid stagnating in the exchange line, and therefore any possibility of local enrichment in oxygen and secondary impurities of the air (typically C n H m ) which presents a safety risk.

[0007] Obtaining sufficient gas speed, even at reduced speed, requires significant pressure losses in the air intended for the first column, which is costly in terms of energy.

[0008] In an air separation apparatus, the bottom liquid (LR) from the first column is expanded and sent to an intermediate point in the second column. The overhead liquid (LP) from the first column is expanded and sent to the top of the second column. Both liquids are subcooled in a heat exchanger against an overhead gas from the second column.

[0009] In this exchanger called subcooler, especially in a cross-flow configuration, the cooling of the nitrogen-enriched overhead liquid called lean liquid LP and the oxygen-enriched bottom liquid called rich liquid LR is done in two separate sections, as if there were two exchangers in series. This means that the oxygen-enriched liquid is cooled to a temperature higher than that of the inlet of the overhead liquid of the first column. In this configuration, the cold available in the waste is not completely exhausted.

[0010] The conventional subcooler is illustrated on page 96 of Kerry's "Industrial Gas Handbook", CRC Press 2007. The exact fluid arrangement is not always illustrated in patents or other documents for reasons of simplification of the figures.

[0011] Furthermore, the exhaust from the blower turbine is sent directly into the second column, for reasons of simplicity. A relatively hot gas is sent into the second column.

[0012] This results in reducing the refrigeration recovery of cold fluids and requires partial condensing (typically around 1 to 2%) of the air leaving the main exchange line and going to the tank of the first column.

[0013] EP0342436A2 discloses an air separation method according to the preamble of claim 1.

[0014] The invention consists of supplying the column with at least one flow of air at a temperature higher than 1°C, or even 2°C, compared to its dew point temperature, including in the reduced steps of the device. This implementation makes it possible to avoid condensing the air intended for the first column in the main exchange line. Thus, the air leaves the main exchange line at a temperature higher than 1°C, or even 2°C compared to its dew point temperature.

[0015] One way to ensure that the air for the first column is sufficiently above the dew point is to push the cooling of certain fluids entering the distillation system, as well as the fluids internal to the distillation system. In this case, an exchange line can be sized with very low pressure losses, without having to take into account a liquid entrainment criterion with regard to safety.

[0016] This involves recovering as much cold as possible from the fluids produced by distillation (residual nitrogen, oxygen, nitrogen at the second pressure purer than the residual nitrogen).

[0017] An alternative to ensuring that the air for the first column is sufficiently above the dew point is to modify the subcooling to reduce the temperature of a liquid entering the second column.

[0018] In the subcooler, the cooling of the oxygen-enriched liquid is pushed so that it exits at a lower temperature than the inlet temperature of the enriched liquid. This creates a common area in the subcooler where both liquids are cooled at once against at least one flow of nitrogen from the second column.

[0019] Alternatively, in the exchange line, the turbine exhaust is returned to the exchange line to cool against the waste nitrogen (and possibly the purer nitrogen from the second column) and against the oxygen.

[0020] According to an object of the invention, there is provided a method of separating air by cryogenic distillation in 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, the head of the first column being thermally connected to the tank of the second column in which: i. a flow of air purified of water and carbon dioxide is cooled in a heat exchanger and is sent to the first column in gaseous form ii. an oxygen-enriched liquid is withdrawn from the bottom of the first column and sent to the second column after being subcooled in a subcooler iii. a nitrogen-enriched liquid is withdrawn from the upper part of the first column and sent to the second column after being subcooled in the subcooler iv. a nitrogen-rich gas and an oxygen-rich fluid are withdrawn from the second column and are heated in the heat exchanger v. the heat exchanger is arranged below the first column which is itself arranged below the second column, the air flow being cooled in the exchanger by circulating upwards vi.the temperature T1 at which the air flow leaves the heat exchanger and enters the first column is at least 1°C higher, preferably at least 2°C higher than the dew point temperature of the air flow.

[0021] According to other optional aspects: another flow of air purified of water and carbon dioxide is cooled in the heat exchanger, leaves the heat exchanger to a temperature T2, is expanded in a turbine, is returned to the heat exchanger to a temperature T3 and is cooled in the exchanger to a temperature T4 before being sent to the second column in gaseous form, the temperature T2 being higher than T1. T4 is higher by at least 1°C, preferably at least 2°C, than the dew point temperature of the expanded flow T4 is higher, lower or equal to T1 another flow of air purified of water and carbon dioxide substantially at the second pressure is cooled in the heat exchanger to a temperature higher by at least 1°C, preferably at least 2°C, than its dew point temperature and sent to the second column without having been expanded.the oxygen-enriched liquid is cooled in the subcooler to a temperature lower than that at which the nitrogen-enriched liquid enters the subcooler, the two subcooled liquids being expanded each in a respective valve before being sent to the second column. the first column and possibly the second column are supplied with air only by gaseous air flows the first column and the second column produce only gaseous flows as final products. the heat exchanger and the subcooler consist of a single body of aluminum plates brazed together. the pressure drop of the air flow purified of water and carbon dioxide intended for the first column while cooling in the heat exchanger does not exceed 120 mbar, or even 100 mbar.a liquid flow is withdrawn at an intermediate level of the first column, is subcooled in the subcooler to a temperature intermediate between that of the subcooler outlet of the oxygen-enriched liquid and that of the subcooler outlet of the nitrogen-enriched liquid and sent to the second column.

[0022] The invention will be described in more detail with reference to the figure.

[0023] [ Fig.1 ] represents a method according to the invention.

[0024] The process uses 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. The first column K1 is thermally connected to the second column K2 by a bottom vaporizer of the second column.

[0025] The first column is arranged below the second column K2 and the head of the first column is thermally connected to the tank of the second column. A heat exchanger E with brazed aluminum plates is arranged below the first column K1.

[0026] An air flow 1 is compressed by compressor 3 to the first pressure, cooled by cooler 5 and purified of water and carbon dioxide in purification unit 7. To cool, the air is sent to the hot end of exchanger E at the bottom of the exchanger and rises upwards, since the cold end of the exchanger is at the top.

[0027] The purified air 9 cools in the heat exchanger E and is divided into two at a temperature T2 which is an intermediate temperature of the exchanger E. A portion 11 of the air continues its cooling in the exchanger to a temperature T1 higher by at least 1°C, preferably at least 2°C, than the dew point temperature of the air fraction 9.

[0028] At this temperature T1, it leaves the exchanger E and is sent to the tank of column K1 as a feed gas flow.

[0029] The pressure drop of the air 9, 11 passing through the exchanger E does not exceed 120, or even 100 mbar.

[0030] The air separates in the first column to form an oxygen-enriched liquid 10 at the bottom and a nitrogen-enriched liquid 12 at the top. The liquid 10 and the liquid 12 are sent to a subcooler S where at least one flow of gaseous nitrogen 15 from the second column K2 is heated. The liquid 10 enters the subcooler at the hot end thereof and is cooled to a temperature lower than that at which the liquid 12 enters the subcooler S. The liquid 12 leaves the cold end of the subcooler. The subcooled liquid 10 and the subcooled liquid 12 are each expanded in a valve and the liquid 10 is sent to a level of the second column K2 and the liquid 12 is sent to a level of the second column K2 higher than that at which the liquid 10 enters. Thus the subcooler comprises a section where the liquids 10 and 12 are both cooled against the nitrogen 15.Subcooler S can be placed next to column K1 and have its hot end facing downwards.

[0031] A portion 13 of the air at temperature T2 is expanded in a turbine T without having been compressed downstream of the exchanger, is returned to the heat exchanger E at a temperature T3 and is cooled in the exchanger E to a temperature T4 before being sent to the second column in gaseous form, the temperature T2 being higher than T1. T4 may be higher, equal to or lower than T1. T4 is higher by at least 1°C, preferably by at least 2°C, than the dew point temperature of the expanded flow 13. The portion 13 leaves at the cold end of the exchanger E and is sent directly to the second column K2 in gaseous form at a level below the arrival of the expanded liquid 10. As for the air 11, the portion 13 cools in the exchanger by going upwards.

[0032] Column K2 separates flows 10 and 12 by distillation to form a nitrogen-enriched gas 15 at the top of the column and an oxygen-enriched gas 17 at the bottom of the column. Gas 17 is heated as it descends into exchanger E and then serves as a product of the process.

[0033] The gas 15 heated in the subcooler and then descending into the exchanger E is divided into two, one part serving to regenerate the purification 7 and the rest serving as product or waste.

[0034] Here the first column K1 is supplied with air only by gaseous air flows.

[0035] The second column K2 could also be supplied with air, in this case it would be an exclusively gaseous air supply.

[0036] The process produces only gas streams 15, 17 as end products. The blowdown from the bottom condenser of column K2 is not considered an end product.

[0037] The subcooler S can be integrated into the main exchange line E. This makes it possible to further optimize the exhaustion of the cold fluids 15, 17 from the distillation (possibly also pure nitrogen from the second column K2) to cool the rich and lean liquids 10, 12 and the air at the second pressure from an insufflation turbine as much as possible.

[0038] In addition to fluids 15, 17, a flow of liquefied air withdrawn from column K1 can be subcooled in subcooler S before being sent to column K2.

[0039] [ Fig.1] shows a possibility for supplying the process with air. Other variants are also possible, for example that of FR3090831, using 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 column, the head of the first column being thermally connected to the tank of the second column.

[0040] In this case, three airflows are used. The air is compressed to the pressure of the second column and then purified. Then the air is split in two, one part being cooled to the pressure of the second column and sent to the second column by rising in the exchanger. The final temperature leaving the exchanger will be at least 1°C, preferably at least 2°C, higher than its dew point temperature at the second pressure.

[0041] The other part is overpressured to the pressure of the first column. A fraction of this part cools in the exchanger as it rises and leaves at a temperature T1 at least 1°C higher, preferably at least 2°C higher than the dew point temperature of the air fraction.

[0042] The remaining part is cooled in the exchanger to a temperature T2, expanded in a blower, returned to the exchanger to a temperature T3, cooled to the cold end of the heat exchanger to a temperature T4 and sent to the second column. T4 is at least 1°C, preferably at least 2°C higher than the dew point temperature.

Claims

1. Method for separating air by cryogenic distillation in 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, the top of the first column being thermally coupled to the bottom of the second column, wherein: i. an air flow (11) purified of water and carbon dioxide is cooled in a heat exchanger (E) and is sent to the first column in gaseous form ii. an oxygen-enriched liquid (10) is withdrawn from the bottom of the first column and sent to the second column after having been subcooled in a subcooler (S) iii. a nitrogen-enriched liquid (12) is withdrawn from the upper part of the first column and sent to the second column after having been subcooled in the subcooler iv. a nitrogen-rich gas (15) and an oxygen-rich fluid (17) are withdrawn from the second column and heat up in the heat exchanger, characterized in that v. the heat exchanger is disposed below the first column, which is itself disposed below the second column, the air flow cooling in the heat exchanger by rising vi. the temperature T1 at which the air flow (11) leaves the heat exchanger and enters the first column is at least 1°C higher than the dew point of the air flow.

2. Method according to Claim 1, wherein the temperature T1 at which the air flow (11) leaves the heat exchanger and enters the first column is at least 2°C higher than the dew point of the air flow.

3. Method according to Claim 1 or 2, wherein another air flow (13) purified of water and carbon dioxide is cooled in the heat exchanger (E), leaves the heat exchanger at a temperature T2, is expanded in a turbine (T), is returned to the heat exchanger at a temperature T3 and is cooled in the exchanger to a temperature T4 before being sent to the second column (K2) in gaseous form, the temperature T2 being higher than T1.

4. Method according to Claim 3, wherein T4 is at least 1°C higher than the dew point of the expanded flow (13).

5. Method according to Claim 4, wherein T4 is at least 2°C higher than the dew point of the expanded flow (13).

6. Method according to Claim 1, wherein another air flow purified of water and carbon dioxide substantially at the second pressure is cooled in the heat exchanger to a temperature at least 1°C higher than its dew point and sent to the second column without having been expanded.

7. Method according to Claim 6, wherein another air flow purified of water and carbon dioxide substantially at the second pressure is cooled in the heat exchanger to a temperature at least 2°C higher than its dew point and sent to the second column without having been expanded.

8. Method according to one of the preceding claims, wherein the oxygen-enriched liquid (10) is cooled in the subcooler to a temperature lower than that at which the nitrogen-enriched liquid enters the subcooler, the two subcooled liquids each being expanded in a respective valve before being sent to the second column (K2).

9. Method according to one of the preceding claims, wherein the first column (K1) and potentially the second column (K2) are supplied with air solely by gaseous air flows.

10. Method according to one of the preceding claims, wherein the first column (K1) and the second column (K2) only produce gaseous flows as final products.

11. Method according to one of the preceding claims, wherein the heat exchanger (E) and the subcooler (S) are constituted by a single body of aluminium plates that are brazed together.

12. Method according to one of the preceding claims, wherein the pressure drop of the air flow (11) purified of water and carbon dioxide intended for the first column by cooling in the heat exchanger does not exceed 120 mbar.

13. Method according to Claim 12, wherein the pressure drop of the air flow (11) purified of water and carbon dioxide intended for the first column by cooling in the heat exchanger does not exceed 100 mbar.

14. Method according to one of the preceding claims, wherein a liquid flow is withdrawn at an intermediate level of the first column (K1), is subcooled in the subcooler (S) to an intermediate temperature between that at the outlet of the subcooler of the oxygen-enriched liquid and that at the outlet of the subcooler of the nitrogen-enriched liquid, and sent to the second column (K2) .