Process and apparatus for the separation of air by cryogenic distillation

Abstract

Process and apparatus for the separation of air by cryogenic distillation In a process for the separation of air by cryogenic distillation, air is separated by distillation in a column system comprising at least a first column (K1) operating at a pressure of at least 4 bar abs, the air (1) arriving in gaseous form in the first column, nitrogen originating from the top of the first column is condensed, at least partially, in a top condenser of the column against a liquid which is vaporized by heat exchange with the top condenser (C) of the first column and a gaseous oxygen stream (21) from an external source cooled in the heat exchanger is sent to a bottom reboiler (V) of the first column in order to be condensed by vaporizing bottom liquid of the first column.

Description

[0001] Description

[0002] Title of the invention: Process and apparatus for the separation of air by cryogenic distillation

[0003] The present invention relates to a process and to an apparatus for the separation of air by cryogenic distillation. It is common to separate air in a double column, comprising a first column operating at a first pressure and a 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.

[0004] An electrolyser of water under pressure produces hydrogen and oxygen. The oxygen under pressure is generally not upgraded, the product being hydrogen. It is proposed to use the oxygen under pressure in an appliance for the separation of air by cryogenic distillation in order to increase the nitrogen yield and / or the argon yield, without increasing the energy consumption.

[0005] The pressure energy contained in the gaseous oxygen at the electrolyser outlet under pressure (typically between 8 and 30 bar) is used to produce a cycle within the air separation appliance. The aim is not to upgrade the oxygen molecules as such, even if they are not lost for all that: the energy vector which this represents is mainly used.

[0006] This makes it possible to increase the reboiling and the refluxes in some columns and thus makes it possible to extract more nitrogen, in particular under pressure, and / or argon.

[0007] The oxygen under pressure can also be used to vaporize a liquid fluid under pressure in the main exchange line.

[0008] It is known to use oxygen resulting from an electrolyser in an air separation apparatus for its molecules, either in order to liquefy it (FR 3 131 588) and to sell it as a liquid product, or as source of relatively pure oxygen (and for example devoid of CnHm) in order to finish purifying it in the separation apparatus and to sell the purified product as product.

[0009] According to a subject-matter of the invention, a process is provided for the separation of air by cryogenic distillation, in which: i. purified air cooled in a heat exchanger is separated by distillation in a column system comprising at least a first column operating at a pressure of at least 4 bar abs, the air arriving in the gaseous form in the first column; nitrogen originating from the top of the first column is condensed at least partially in a top condenser of the column against a liquid, ii. condensed nitrogen is returned at the top of the column, iii. the liquid is vaporized by heat exchange with the top condenser of the first column, iv. optionally, a stream of gaseous nitrogen is withdrawn from the column and heated in the heat exchanger, characterized in that: v. a gaseous oxygen stream from an external source is cooled in the heat exchanger, without being mixed with air intended for the distillation, and is sent to a bottom reboiler of the first column in order to be condensed by vaporizing bottom liquid of the first column, and vi. the condensed oxygen stream is expanded and sent to the top condenser or to a second column (K2) operating at a lower pressure than the first column and thermally connected to the first column through the top condenser so that the liquid vaporized by the exchange of heat is a bottom liquid of the second column.

[0010] According to other optional characteristics:

[0011] • the reboiler is positioned at least 3 theoretical plates below the gaseous air inlet in the first column,

[0012] • the gaseous oxygen stream is at a pressure between 5 and 30 bar abs,

[0013] • the gaseous oxygen stream is at a pressure between 8 and 18 bar abs,

[0014] • the gaseous oxygen stream originates from an electrolysis unit,

[0015] • the first column operates at between 4 and 6 bar abs,

[0016] • the column system comprises a second column operating at a lower pressure than the first column and thermally connected to the first column by the top condenser so that the liquid vaporized by the heat exchange is a bottom liquid of the second column, a bottom liquid of the first column is expanded and sent to the second column, a liquid enriched in nitrogen from the first column is expanded and sent to the top of the second column, a nitrogen-rich gas is withdrawn at the top of the second column and heated in the heat exchanger and an oxygen-rich fluid is withdrawn at the bottom of the second column and heated in the heat exchanger, the oxygen stream condensed in the reboiler being sent to the second column, preferably at the bottom of the second column,

[0017] • the second column operates at between 1 .4 bar abs and 2 bar abs,

[0018] • the first column is not thermally connected to another column through the condenser and the bottom liquid of the first column is expanded and then sent to be vaporized by heat exchange with the top condenser,

[0019] • the liquid vaporized by the top condenser is heated in the heat exchanger, exits from the heat exchanger at an intermediate temperature of the latter, is expanded in a turbine and then is returned to be heated in the heat exchanger,

[0020] • the column system includes an argon separation column,

[0021] • at least 90% of the cold, indeed even substantially 100% of the cold, of the process originates from the expansion of vaporized liquid in the turbine,

[0022] • the vaporized liquid contains between 60 mol% and 100 mol% of oxygen,

[0023] • the oxygen stream contains at least 95 mol%, indeed even at least 99 mol%, of oxygen.

[0024] According to another aspect of the invention, there is provided an apparatus for the separation of air by cryogenic distillation comprising a heat exchanger, a column system comprising at least a first column having a top condenser and a bottom reboiler, a pipe for sending gaseous air cooled in the heat exchanger to the first column, a pipe for sending nitrogen originating from the top of the first column to the condenser in order to be condensed at least partially in the top condenser against a liquid, a pipe for sending condensed nitrogen from the condenser to the top of the column, a pipe for exiting liquid vaporized by heat exchange with the top condenser of the first column and means for sending a gaseous oxygen stream from an external source to be cooled in the heat exchanger, without being mixed with air intended for the distillation, means for sending the gaseous oxygen stream to the bottom reboiler of the first column in order to be condensed by vaporizing bottom liquid of the first column, a valve for expanding the oxygen stream condensed in the bottom reboiler and means for sending the expanded condensed oxygen stream to the top condenser.

[0025] Preferably, the external source is a water electrolysis unit. The column system may comprise only a single column, the apparatus comprising means for sending the bottom liquid of the column to the top condenser in order to be vaporized therein.

[0026] The column system may comprise two columns, including the second column, and comprising means for sending a bottom liquid from the first column to the second column and means for sending a liquid enriched in nitrogen from the top of the first column to the top of the second column.

[0027] The invention will be described in more detail with reference to the figures: [FIG. 1] represents a process according to the invention using a double column. [FIG. 2] represents a process according to the invention using a single column. The invention is described for [FIG. 1] in the case of a double column with argon but can be extrapolated to other cases.

[0028] The double column comprises a first column K1 operating at a first pressure between 4 and 6 bar abs and a second column K2 operating at a second pressure between 1 .4 and 2 bar abs, lower than the first pressure, the top of the first column being thermally connected to the bottom of the second column K2.

[0029] Air 1 air purified in order to remove the CO2 and the water which it contains is cooled in a heat exchanger. The cooled air is sent in essentially gaseous form (this might also be air with liquid which exits from a Claude turbine) to the first column K2 and is separated therein to form a liquid enriched in oxygen 3 and a gas enriched in nitrogen. Gas enriched in nitrogen is sent to a bottom condenser C of the second column K2 in order to be condensed therein and the condensed nitrogen is returned to the first column K1 .

[0030] The liquid is divided into two parts 7 and 9, the part 7 being sent to an intermediate level of the second column K2 and the part 9 being sent to a top condenser C1 of a third column K3.

[0031] Gas enriched in argon 11 is withdrawn from the second column at an intermediate level and feeds the bottom of the third column K3, which is a single column surmounted by the top condenser C1 . An argon-rich gas or liquid is withdrawn at the top of the third column K3. This fluid can be used as product or, if not, sent to be mixed with nitrogen exiting from the second column K2.

[0032] A liquid 13 enriched in nitrogen is withdrawn at the top of the first column K1 and sent to the top of the second column K2 as reflux.

[0033] A nitrogen-rich gas 15 is withdrawn at the top of the second column K2 and is heated in the heat exchanger.

[0034] A nitrogen-rich gas 17 is withdrawn at the top of the first column K1 and is heated in the heat exchanger.

[0035] Otherwise, a nitrogen-rich liquid 17 is withdrawn at the top of the first column K1 and is vaporized and heated in the heat exchanger, optionally after pressurization in a pump. An oxygen-rich gas 19 is withdrawn at the bottom of the second column K2 and is heated in the heat exchanger.

[0036] Otherwise, an oxygen-rich liquid 19 is withdrawn at the bottom of the second column K2 and is vaporized and heated in the heat exchanger, optionally after pressurization in a pump.

[0037] A bottom liquid 23 of the third column K3 is returned to the second column K2. The gaseous oxygen 21 exiting from an external source, for example from a water electrolyser, under pressure, typically between 8 and 30 bar, optionally passes into a purification unit, in order to remove the residual hydrogen and to prevent excessively high concentrations of hydrogen in the process (in order not to exceed the lower explosion limit in oxygen), and then into a dryer, in order to remove the water.

[0038] It is subsequently cooled in the heat exchanger.

[0039] At least a part of the gaseous oxygen is condensed in a reboiler or vaporizer V at the bottom of the first column K1 in order to provide reboiling of the first column K1. The at least a part can be expanded in a turbine after cooling in the heat exchanger and downstream of the vaporizer V.

[0040] Another part of the gaseous oxygen can optionally be liquefied in the heat exchanger or in a dedicated heat exchanger against a liquid product under pressure, for example the liquid nitrogen withdrawn at the top of the first column K1.

[0041] The liquid oxygen downstream of the vaporizer is then used, after expansion and with optional cooling, to generate a reflux in a column, either directly, for example at the top of the second column K2, or indirectly, for example in the first column K1 via the vaporizer V of the first column K1 .

[0042] In the example, it is seen that the liquid oxygen formed in the bottom reboiler or vaporizer V of the first column K1 is expanded in a valve and sent into the bottom liquid bath of the second column K2.

[0043] The liquid oxygen can nevertheless be sent to a higher level of the second column K2.

[0044] The subcoolers are not illustrated in [FIG. 1] but it goes without saying that any liquid can be subcooled before expansion, either in the heat exchanger or in a subcooler, depending on the temperature level targeted.

[0045] This excess of reboiling and of reflux makes it possible to extract, for example: i) more gaseous nitrogen at the top of the first column K1 , ii) more liquid nitrogen at the top of the first column K1 , which will optionally be pressurized upstream of the heat exchanger and then vaporized in the heat exchanger, iii) more gaseous or liquid argon from the third column K3.

[0046] Preferably, all the air is sent to the column K1 in gaseous form.

[0047] According to the alternative form of Fig. 2, the process uses just one column K1 to separate the air 1 . The air 1 was purified to remove the water and the CO2 which it contains and then is cooled in a plate and fin heat exchanger E. The cooled air is sent to the column K1 at an intermediate level of the column and is separated therein to form a liquid enriched in oxygen at the bottom of the column and gas enriched in nitrogen at the top of the column. The gas enriched in nitrogen is condensed in a top condenser C and the liquid formed is returned to the column K1 as reflux. The bottom liquid is heated by a vaporizer V and bottom liquid 7 is withdrawn, expanded and sent to the top condenser C in order to cool it.

[0048] An oxygen stream 3 from an external source, for example an electrolysis unit, is sent at a pressure between 5 and 30 bar abs, indeed even between 8 and 18 bar abs, to the vaporizer V after being cooled in the heat exchanger E. The oxygen stream 3 is condensed in the vaporizer V, forming a liquid 11 which is expanded in a valve and sent to the top condenser C in order to cool it. If the oxygen contains water or other impurities, it is purified upstream of the exchanger E by adsorption by TSA or PSA.

[0049] If the oxygen contains hydrogen, it will be purified upstream of the exchanger E by catalysis. The catalyst for removing the residual hydrogen in the oxygen can be palladium, platinum, cerium or one of their oxides.

[0050] The gas 13 exiting from the condenser C thus originates from the liquids 3 and 7 vaporized in the condenser C and contains between 60 mol% and 100 mol% of oxygen. The gas is heated in the heat exchanger E, exits from the exchanger at an intermediate temperature, is expanded in a turbine T and is then returned to the exchanger E in order to be heated up to a temperature above 0°C. The turbine T preferably provides at least 90% of the cold of the process, indeed even substantially 100%, taking into account the Joule-Thomson cooling in the valves V1 , V2 and V3. Gaseous nitrogen 15 is withdrawn at the top of the column K1 and is heated in the heat exchanger E in order to form the product of the column K1 .

[0051] In both examples, all the air is sent in gaseous form to the column K1 , which is the medium-pressure column of the double column in Figure 1 and the single column in Figure 2. In this case, all the products of the process are gaseous. A small amount of liquid can also be made, if necessary.

[0052] Provision can also be made to send a part of the air in liquid form to the column K1 in both figures and / or in liquid form to the column K2 in Figure 2. In this case, a liquid product can be withdrawn from one of the columns K1 and K2. It will be understood that the heat exchanger of Figure 1 , which is not illustrated, is similar to that of Figure 2, making possible the cooling of the air and of the gaseous oxygen from the external source against the cold fluids originating from the column system K1 , K2 and K3, including at least the fluids 15, 17 and 19.

Claims

AMENDED CLAIMS received by the International Bureau on 23 January 2026 (23.01.2026)Claims1 . Process for the separation of air by cryogenic distillation, in which:

1. purified air (1 ) cooled in a heat exchanger is separated by distillation in a column system comprising a first column (K1 ) operating at a pressure of at least 4 bar abs, the air arriving in the gaseous form in the first column; nitrogen originating from the top of the first column is condensed at least partially in a top condenser of the column against a liquid, ii. condensed nitrogen is returned at the top of the column, iii. the liquid is vaporized by heat exchange with the top condenser (C) of the first column, iv. optionally, a stream of gaseous nitrogen (17) is withdrawn from the column and heated in the heat exchanger, v. a gaseous oxygen stream (21 ) from an external source is cooled in the heat exchanger, without being mixed with air intended for the distillation, and is sent to a bottom reboiler (V) of the first column in order to be condensed by vaporizing bottom liquid of the first column, vi. the condensed oxygen stream is expanded and sent to the top condenser or to a second column (K2) operating at a lower pressure than the first column and thermally connected to the first column through the top condenser so that the liquid vaporized by the exchange of heat is a bottom liquid of the second column and vii. the first column (K1 ) not being thermally connected to another column through the condenser and in which the bottom liquid of the first column is expanded and then sent to be vaporized by heat exchange with the top condenser (C)..

2. Process according to Claim 1 , in which the reboiler (V) is positioned at least 3 theoretical plates below the gaseous air inlet (1 ) in the first column (K1 ).

3. Process according to either of Claims 1 and 2, in which the gaseous oxygen stream (21 ) is at a pressure between 5 and 30 bar abs, indeed even between 8 and 18 bar abs.

4. Process according to one of the preceding claims, in which the gaseous oxygen stream (21 ) originates from an electrolysis unit.

5. Process according to Claim 1 , in which the liquid vaporized by the top condenser (C) is heated in the heat exchanger (E), exits from the heat exchangerat an intermediate temperature of the latter, is expanded in a turbine (T) and then is returned to be heated in the heat exchanger.

6. Process according to Claim 5, in which at least 90% of the cold, indeed even substantially 100% of the cold, of the process originates from the expansion of vaporized liquid in the turbine (T).

7. Process according to Claim 5 or 6, in which the vaporized liquid (13) contains between 60 mol% and 100 mol% of oxygen.

8. Apparatus for the separation of air by cryogenic distillation comprising a heat exchanger (E), a column system (K1 , K2, K3) comprising at least a first column (K1) having a top condenser (C) and a bottom reboiler (V), a pipe for sending gaseous air cooled in the heat exchanger to the first column, a pipe for sending nitrogen originating from the top of the first column to the condenser in order to be condensed at least partially in the top condenser against a liquid, a pipe for sending condensed nitrogen from the condenser to the top of the column, a pipe for exiting liquid vaporized by heat exchange with the top condenser (C) of the first column and means for sending a gaseous oxygen stream (21 ) from an external source to be cooled in the heat exchanger, without being mixed with air intended for the distillation, means for sending the gaseous oxygen stream to the bottom reboiler (V) of the first column in order to be condensed by vaporizing bottom liquid of the first column, a valve for expanding the oxygen stream condensed in the bottom reboiler and means for sending the expanded condensed oxygen stream to the top condenser or to a second column (K2) capable of operating at a lower pressure than the first column and thermally connected to the first column by the top condenser so that the liquid vaporized by the heat exchange is a bottom liquid of the second column, the column system comprising only a single column (K1 ), the apparatus comprising means for sending the bottom liquid of the column to the top condenser in order to be vaporized therein.

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