Method and apparatus for air separation by cryogenic distillation

Abstract

Title of the invention: Process and apparatus for air separation by cryogenic distillation. In a process for air separation by cryogenic distillation, air is separated by distillation in a column system comprising at least one first column (K1) operating at a pressure of at least 4 bar abs. Air (1) enters the first column in gaseous form. Nitrogen from the top of the first column condenses at least partially in a column top condenser against a liquid that vaporizes by heat exchange with the top condenser (C) of the first column. A flow of gaseous oxygen (21) from an external source, cooled in the heat exchanger, is sent to a tank reboiler (V) of the first column to condense by vaporizing the tank liquid of the first column. Fig. 1

Description

Title of the invention: Method and apparatus for air separation by cryogenic distillation

[0001] The present invention relates to a method and apparatus for separating 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 head of the first column being thermally connected to the vessel of the second column.

[0002] A pressurized water electrolyzer produces hydrogen and oxygen. The pressurized oxygen is generally not utilized, the product being hydrogen.

[0003] It is proposed to use pressurized oxygen in a cryogenic distillation air separation apparatus to increase nitrogen yield and / or argon yield, without increasing energy consumption.

[0004] The pressure energy contained in the gaseous oxygen exiting the pressurized electrolyzer (typically between 8 and 30 bar) is used to cycle within the air separation device. The oxygen molecules themselves are not to be utilized, although they are not entirely lost: the primary focus is on utilizing the energy they represent.

[0005] This allows for increased reboiling and reflux in certain columns and thus allows for the extraction of more nitrogen, particularly under pressure, and / or argon.

[0006] Pressurized oxygen can also be used to vaporize a pressurized liquid fluid in the main exchange line.

[0007] It is known to use oxygen from an electrolyzer in an air separation apparatus for its molecules, either to liquefy it (FR3131588) and sell it as a liquid product, or as a source of relatively pure oxygen (and for example, free of CnHm) to further purify it in the separation apparatus and sell the purified product as a product.

[0008] According to one object of the invention, a process for separating air by cryogenic distillation is provided in which:

[0009] i. Purified and cooled air in a heat exchanger is separated by distillation in a column system comprising at least one first column operating at a pressure of at least 4 bar abs, the air arriving in gaseous form in the first column, nitrogen from the top of the first column condenses at least partially in a column top condenser against a liquid

[0010] ii. Condensed nitrogen is returned to the top of the column

[0011] iii. The liquid vaporizes by heat exchange with the head condenser of the first column

[0012] iv. Optionally, a flow of nitrogen gas is drawn from the column and heated in the heat exchanger

[0013] characterized in that

[0014] v. A flow of gaseous oxygen from an external source is cooled in the heat exchanger, without being mixed with air intended for distillation, and is sent to a first column tank reboiler to condense by vaporizing first column tank liquid.

[0015] According to other optional features: • The reboiler is positioned at least 3 theoretical trays below the gaseous air inlet in the first column. • the flow rate of gaseous oxygen is at a pressure between 5 and 30 bar abs. • the flow rate of gaseous oxygen is at a pressure between 8 and 18 bars abs. • The flow of gaseous oxygen comes from an electrolysis unit. • The first column operates at between 4 and 6 bars absolute • The column system includes 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 tank liquid from the second column, a tank liquid from the first column is expanded and sent to the second column, a nitrogen-enriched liquid from the first column is expanded and sent to the top of the second column, a nitrogen-rich gas is drawn from the top of the second column and heated in the heat exchanger and an oxygen-rich fluid is drawn from the tank of the second column and heated in the heat exchanger, the flow of oxygen condensed in the reboiler being sent to the second column, preferably to the tank of the second column. • The second column operates at between 1.4 bars abs and 2 bars abs • the first column is not thermally connected to another column through the condenser and in which the tank liquid of the first column is expanded and then sent to vaporize by heat exchange with the head condenser. • The liquid vaporized by the head condenser heats up in the heat exchanger, exits the heat exchanger at an intermediate temperature, is expanded in a turbine and then is returned to heat up in the heat exchanger. • The column system includes an argon separation column • at least 90% of the cooling, or even substantially 100% of the cooling of the process comes from the expansion of vaporized liquid in the turbine. • the vaporized liquid contains between 60 and 100% mol of oxygen. • the oxygen flow contains at least 95% mol of oxygen, or even at least 99% mol of oxygen.

[0016] According to another aspect of the invention, an air separation apparatus by cryogenic distillation is provided, comprising a heat exchanger, a column system including at least a first column having a top condenser and a reboiler, a line for sending cooled gaseous air from the heat exchanger to the first column, a line for sending nitrogen from the top of the first column to the condenser to condense at least partially in the top condenser against a liquid, a line for sending condensed nitrogen from the condenser to the top of the column, a line for exiting the liquid vaporized by heat exchange with the top condenser of the first column, and means for sending a flow of gaseous oxygen from an external source to be cooled in the heat exchanger, without being mixed with air intended for distillation.means for sending the flow of gaseous oxygen to the reboiler in the first column of the tank to condense by vaporizing the tank liquid in the first column.

[0017] Preferably the external source is a water electrolysis unit.

[0018] The invention will be described in more detail with reference to the figures:

[0019] [Fig-1] represents a method according to the invention using a double column.

[0020] [Fig.2] represents a method according to the invention using a simple column.

[0021] The invention is described in the case of a double column with argon, but can be extrapolated to other cases.

[0022] The double column comprises a first column Kl operating at a first pressure between 4 and 6 bars abs and a second column K2 operating at a second pressure between 1.4 and 2 bars abs, lower than the first pressure, the head of the first column being thermally connected to the tank of the second column K2.

[0023] Purified air 1, to remove CO2 and water content, is cooled in a heat exchanger. The cooled air is sent in essentially gaseous form (it could also be air with liquid exiting a Claude turbine) to the first column K2, where it separates to form an oxygen-enriched liquid 3 and a nitrogen-enriched gas. The nitrogen-enriched gas is sent to a tank condenser C of the second column K2 to condense, and the condensed nitrogen is returned to the first column KL.

[0024] The liquid is divided into two parts 7, 9, part 7 being sent to an intermediate level of the second column K2 and part 9 being sent to a top condenser Cl of a third column K3.

[0025] Argon-enriched gas 11 is drawn from the second column at an intermediate level and feeds the tank of the third column K3, which is a single column surmounted by the top condenser Cl. An argon-rich gas or liquid is drawn from the top of the third column K3. This fluid can be used as a product or otherwise sent to mix with nitrogen from the second column K2.

[0026] A nitrogen-enriched liquid 13 is drawn off at the top of the first column K1 and sent to the top of the second column K2 as reflux.

[0027] A nitrogen-rich gas 15 is drawn from the top of the second column K2 and heated in the heat exchanger.

[0028] A nitrogen-rich gas 17 is drawn from the top of the first column Kl and heated in the heat exchanger.

[0029] Otherwise, a nitrogen-rich liquid 17 is drawn from the top of the first column Kl and vaporizes and heats up in the heat exchanger, possibly after pressurization in a pump.

[0030] An oxygen-rich gas 19 is drawn from the tank of the second column K2 and heated in the heat exchanger.

[0031] Otherwise, an oxygen-rich liquid 19 is drawn from the tank of the second column K2 and vaporizes and heats up in the heat exchanger, possibly after pressurization in a pump.

[0032] A tank liquid 23 from the third column K3 is returned to the second column K2.

[0033] The gaseous oxygen 21 exiting an external source, for example from a water electrolyzer, under pressure, typically between 8 and 30 bar, possibly passes into a purification unit, to remove residual hydrogen and avoid excessively high concentrations of hydrogen in the process (in order not to exceed the lower explosion limit in oxygen), then into a dryer to remove the water.

[0034] It is then cooled in the heat exchanger.

[0035] At least part of the gaseous oxygen is condensed in a reboiler or vaporizer V in the tank of the first column Kl to ensure reboiling of the first column KL. At least part can be expanded in a turbine after cooling in the heat exchanger and downstream of the vaporizer V.

[0036] Another portion of the gaseous oxygen may optionally be liquefied in the heat exchanger or in a dedicated heat exchanger against a pressurized liquid product, for example, liquid nitrogen drawn off at the top of the first column KL

[0037] The liquid oxygen downstream of the vaporizer is then used, after expansion and possibly under 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 Kl via the vaporizer V of the second column K2.

[0038] In the example, we see that the liquid oxygen formed in the reboiler or vaporizer V of the first column K1 is expanded in a valve and sent into the liquid bath of the second column K2.

[0039] Liquid oxygen can nevertheless be sent to a higher level of the second column K2.

[0040] Subcoolers are not shown in [Fig.1], but it is understood that any liquid can be subcooled before expansion, either in the heat exchanger or in a subcooler depending on the target temperature level.

[0041] This excess reboiling and refluxing makes it possible to extract, for example, i. more gaseous nitrogen at the top of the first column Kl ii. more liquid nitrogen at the top of the first column Kl, which we will Optionally pressurize upstream of the heat exchanger, then vaporize in the heat exchanger. iii. to extract more gaseous or liquid argon from the third column K3.

[0042] Preferably all the air is sent to column K1 in gaseous form.

[0043] According to the embodiment shown in [Fig. 2], the process uses a single column Kl to separate the air 1. The air 1 has been purified to remove the water and CO2 it contains and is then cooled in a plate and fin heat exchanger E. The cooled air is sent to the column Kl at an intermediate level of the column and separates there to form an oxygen-enriched liquid in the column tank and a nitrogen-enriched gas at the top of the column. The nitrogen-enriched gas is condensed in a top condenser C, and the resulting liquid is returned to the column Kl as reflux. The tank liquid is heated by a vaporizer V, and some of the tank liquid 7 is drawn off, expanded, and sent to the top condenser C for cooling.

[0044] A flow of oxygen 3 from an external source, for example an electrolysis unit, is sent at a pressure between 5 and 30 bar ans, or even between 8 and 18 bar abs, to the vaporizer V after being cooled in the heat exchanger E. The flow of oxygen 3 condenses in the vaporizer V forming a liquid 11 which is expanded in a valve and sent to the head condenser C for cooling.

[0045] If the oxygen contains water or other impurities, it is purified upstream of the exchanger E by adsorption by TSA or PSA.

[0046] If the oxygen contains hydrogen, it will be purified upstream of the exchanger E by catalysis. The catalyst for removing residual hydrogen from the oxygen can be palladium, platinum, cerium, or one of their oxides.

[0047] The gas 13 exiting condenser C therefore originates from the liquids 3 and 7 vaporized in condenser C and contains between 60% and 100% mol of oxygen. The gas is heated in heat exchanger E, exits the exchanger at an intermediate temperature, is expanded in a turbine T, and then returned to exchanger E to be heated to a temperature above 0°C. Turbine T preferably provides at least 90% of the cooling capacity of the process, or even substantially 100% when taking into account Joule-Thomson cooling in valves VI, V2, and V3.

[0048] Nitrogen gas 15 is drawn off at the top of column Kl and is heated in heat exchanger E to form the product of column KL

[0049] In both examples, all the air is sent in gaseous form to column Kl, which is the medium-pressure column of the double column in [Fig. 1] and the single column in [Fig. 2]. In this case, all the process products are gaseous. A small amount of liquid can also be produced if necessary.

[0050] It may also be provided that a portion of the air is sent in liquid form to column K1 in both figures and / or in liquid form to column K2 in [Fig.2]. In this case, a liquid product can be withdrawn from one of the columns K1, K2.

[0051] It will be understood that the heat exchanger of [Fig.1] not shown is similar to that of [Fig.2], allowing the cooling of air and gaseous oxygen from the external source against the cold fluids from the column system K1, K2, K3 of which at least fluids 15, 17, 19.

Claims

Demands

1. A process for separating air by cryogenic distillation in which: i. Purified and cooled air in a heat exchanger is separated by distillation in a column system comprising at least one first column (Kl) operating at a pressure of at least 4 bar abs, the air (1) arriving in gaseous form in the first column, nitrogen from the top of the first column condenses at least partially in a column top condenser against a liquid ii. Condensed nitrogen is returned to the top of the column iii. The liquid vaporizes by heat exchange with the top condenser (C) of the first column iv. Optionally, a flow of gaseous nitrogen (17) is withdrawn from the column and heated in the heat exchanger characterized in that v.A flow of gaseous oxygen (21) from an external source is cooled in the heat exchanger, without being mixed with air intended for distillation, and is sent to a tank reboiler (V) of the first column to condense by vaporizing tank liquid from the first column.

2. A method according to any one of claims 1 in which the reboiler (V) is arranged at least 3 theoretical trays below the gaseous air inlet (1) in the first column (Kl).

3. A method according to any one of claims 1 or 2 wherein the flow rate of gaseous oxygen (21) is at a pressure between 5 and 30 bar abs, or even between 8 and 18 bar abs.

4. A method according to any one of the preceding claims wherein the flow of gaseous oxygen (21) is from an electrolysis unit.

5. A method according to any one of the preceding claims, wherein the column system comprises a second column (K2) operating at a lower pressure than the first column and thermally connected to the first column by the top condenser such that the liquid vaporized by the heat exchange is a tank liquid from the second column, a tank liquid (3) from the first column is expanded and sent to the second column, a nitrogen-enriched liquid (13) from the first column is expanded and sent to the top of the second column, and a nitrogen-enriched gas (15) is drawn from the top of the second column and heated in the heat exchanger and an oxygen-rich fluid (19) is drawn into the tank of the second column and heated in the heat exchanger, the flow of oxygen condensed in the reboiler being sent to the second column, preferably to the tank of the second column.

6. A method according to any one of claims 1 to 5 wherein the first column (Kl) is not thermally connected to another column through the condenser and wherein the tank liquid of the first column is expanded and then sent to vaporize by heat exchange with the head condenser (C).

7. A method according to claim 6 in which the liquid vaporized by the head condenser (C) heats up in the heat exchanger (E), exits the heat exchanger at an intermediate temperature therefrom, is expanded in a turbine (T) and then is returned to heat up in the heat exchanger.

8. A process according to claim 7 wherein at least 90% of the cooling, or even substantially 100% of the cooling of the process comes from the expansion of vaporized liquid in the turbine (T).

9. A method according to claim 7 or 8 in which the vaporized liquid (13) contains between 60 and 100% mol of oxygen.

10. Cryogenic distillation air separation apparatus comprising a heat exchanger (E), a column system (K1, K2, K3) comprising at least a first column having a top condenser (C) and a tank reboiler (V), a line for sending cooled gaseous air from the heat exchanger to the first column, a line for sending nitrogen from the top of the first column to the condenser to condense at least partially in the top condenser against a liquid, a line for sending condensed nitrogen from the condenser to the top of the column, a line for removing liquid vaporized by heat exchange with the top condenser (C) of the first column, and means for sending a flow of gaseous oxygen (21) from an external source to be cooled in the heat exchanger, without being mixed with air intended for distillation,means for sending the flow of gaseous oxygen to the tank reboiler (V) of the first column to condense by vaporizing the tank liquid of the first column.