US20240068746A1

US20240068746A1 - Method for restarting an air separation unit

Info

Publication number: US20240068746A1
Application number: US18/269,367
Authority: US
Inventors: Benoit Davidian; Jean-Marc Peyron
Original assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2020-12-23
Filing date: 2021-12-16
Publication date: 2024-02-29
Also published as: FR3118145B1; EP4267900A1; FR3118145A1; WO2022136088A1; CN116685822A

Abstract

A method and apparatus for restarting an air separation unit comprising a system of columns including at least a first distillation column and a heat exchanger positioned below the first column with its main axis vertical and with its cold end adjacent to the bottom of the first column, in which, during the restart, air is sent to the exchanger and at least one portion of the cooled air is sent in gaseous form to the atmosphere.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a § 371 of International PCT Application PCT/EP2021/086108, filed Dec. 16, 2021, which claims the benefit of FR2014066, filed Dec. 23, 2020, both of which are herein incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a method for restarting an air separation unit. It also relates to a cryogenic-distillation air separation unit capable of being started by the method according to the invention.

BACKGROUND OF THE INVENTION

Air is often separated using a double distillation column involving a first column operating at a first pressure and a second column operating at a second pressure, lower than the first pressure. The bottom of the second column is heated by the overhead gas from the first column, which condenses there in a condenser.
The air intended for distillation is cooled in a heat exchanger by exchanging heat with at least one gas coming from at least one of the first and second columns. This main exchanger is installed vertically, hot end up and cold end down. Thus, during a shutdown, the cold gas, which is denser, remains in the lower part, thus avoiding the migration of cold toward the top, which can be troublesome during a cold restart.
The disadvantage is that this dictates choices in architecture, generally with the exchanger next to the column, and this increases the size of the cold box, posing problems associated with transport limitations or difficulties if seeking to create a vacuum cold box.
Siting this supply air cooling heat exchanger below the first column is known from WO2019/126927. Here the exchanger is placed upside down compared to the usual arrangement, with its cold end up and its hot end down.
This allows the exchanger to be installed under the system of columns, while minimizing the route of the piping: the piping at the cold end is as close as possible to the system of columns, and the piping at the hot end is located at the bottom, as close as possible to the point of interconnection with the hot part (purification, production). This makes it possible to create a cold box with a small cross section, which is advantageous in terms of transport and in the case of vacuum insulation.
During a shutdown, there is the risk of the cold migrating toward the hot part. This is mitigated by the fact that the outlets from the exchanger to the purification or production stage have valves that are closed in a shutdown.
A “cold restart” is a restart in circumstances in which the unit has not warmed up much, either because the shutdown was short or because the unit was kept cold by adding cold energy.
During a cold restart, these outlet valves from the exchanger to the purification or production stage have to open, and there is a risk of the migration of cold (cryogenic embrittlement) in the transient phase, during the time that it takes for the exchanger to stabilize thermally. In this case, in order to mitigate this risk, stainless steel must be used as the material for the valves and piping, and this is expensive.

SUMMARY OF THE INVENTION

In certain embodiments, the invention can include adding a vent to atmosphere at the cold end of the exchanger on the air entering the distillation system, so as to temporarily unbalance the exchange of heat in the exchanger, and thus to push the cold front back towards the top (cold side) of the exchanger.
In certain embodiments, the invention proposes to force the rapid thermalization of the exchanger upon a cold restart, by temporarily unbalancing it thermally by having an overall flow-rate of hot gases that is much greater than the overall flow-rate of cold gases. This is done by adding a vent to atmosphere at the cold end of the exchanger on at least one gas that is to be heated or cooled, so as to “dump” cold into the atmosphere, at a flow-rate of at least 5%, preferably 10% of the overall flow-rate of gas that is to be heated (or cooled).
In certain embodiments, the invention also relates to the case where the system of columns used for the distillation is not a double column but a single column.
According to one subject of the invention, there is provided a method for restarting an air separation unit comprising a system of columns comprising at least a first distillation column and a heat exchanger by indirect exchange of heat for cooling the air purified of water and of carbon dioxide and intended to be separated in the first column by sending it to one end of the heat exchanger called the hot end and extracting it at the other end of the heat exchanger called the cold end, in gaseous form, the exchanger also serving to heat at least two gases coming from the system of columns, the heat exchanger being sited below the first column with its main axis vertical and its cold end adjacent to the bottom of the first column

- i) during normal operation, at least part of the air cooled in the heat exchanger and intended to be distilled is sent in gaseous form to the system of columns, for example to the first column, and at least two fluids from the system of columns are sent to the heat exchanger
- ii) while the unit is shut down, air cooled in the heat exchanger is no longer sent to the system of columns, and fluids are no longer sent from the system of columns to the heat exchanger and
- iii) during the restart, air is sent to the hot end of the heat exchanger and from the cold end of the heat exchanger to the system of columns, where it is separated, and at least one fluid extracted from the system of columns is sent to the heat exchanger, characterized in that during the restart:
- a) the at least one part of the gaseous air coming from the cold end is divided into two, a first portion thereof is sent to the system of columns, and a second portion thereof is vented to the atmosphere and/or
- b) at least part of a gas extracted from the system of columns is vented to the atmosphere.

According to other optional aspects:

- during the restart, a flow of cooled air with a flow-rate
- corresponding to at least 5%, preferably to at least 10% of the overall flow-rate of gases cooled in the heat exchanger during normal operation is vented to the atmosphere at the cold end of the exchanger.
- during the restart, a flow of gas coming from the system of columns and having a flow-rate corresponding to at least 5%, preferentially to at least 10% of the overall flow-rate of gas to be heated in the heat exchanger during normal operation is vented to the atmosphere at the cold end of the exchanger.
- the gas extracted from the system of columns is enriched in nitrogen or in oxygen or in argon.
- the heat exchanger is a brazed plate and fin heat exchanger.
- in normal operation, a first flow of air at a first pressure and a second flow of air at a second pressure lower than the first pressure are cooled in the heat exchanger and are sent respectively to the first column of the system of columns and to a second column of the system of columns operating at a lower pressure than the first column.
- during the restart, the flow-rate of the first flow is smaller compared to that sent during normal operation, or even zero and the second flow is divided in two, a portion being sent to the second column and the other portion being vented to the atmosphere.
- the system of columns comprises a single column which is the first column, and in normal operation all the air that is to be separated is sent to the first column, in a shutdown, no flow of air is sent to the first column and during the restart, the air coming from the cold end is divided in two, and a first portion thereof is sent to the first column and a second portion thereof is vented into the air.
- during normal operation and/or during a restart and/or during a shutdown, part of the air or of a fluid extracted from the system of columns is vented to the atmosphere if, and preferably only if, the pressure in a column of the system of columns exceeds a threshold.

Thus the gas line serves two different purposes.
Preferably during the restart, the first portion thereof is sent to the second column.
During the restart, the at least one part of the gaseous air coming from the cold end can be divided into two, a first portion thereof is sent in gaseous form to the system of columns and a second portion thereof is vented in gaseous form to the atmosphere.
According to another subject of the invention, there is provided a cryogenic-distillation air separation unit comprising a system of columns comprising at least a first distillation column, a heat exchanger using indirect exchange of heat to cool the air intended to be separated in the first column, means for sending air to one end of the heat exchanger called the hot end, means for extracting the cooled air from the other end of the heat exchanger called the cold end, these means being connected to the system of columns, means for sending at least two fluids coming from the system of columns to be heated in the exchanger, the heat exchanger being sited below the first column with its main axis vertical and its cold end adjacent to the bottom of the first column, characterized in that it comprises means for venting to the atmosphere, in gaseous form:

- a. air cooled in the heat exchanger, the venting means for venting to the atmosphere being connected to the means for extracting the cooled air from the other end of the heat exchanger and/or
- b. at least a portion of one of the at least two fluids produced by the system of columns upstream of the heat exchanger.

As a preference, the unit comprises means for preventing any transfer of fluid between the heat exchanger and the air processing means and between the heat exchanger and the means for venting at least one product from the system to the atmosphere in the event of a distillation shutdown.
The means for venting to the atmosphere air cooled in the heat exchanger can be connected to the first column or to the second column.
The unit may comprise means for processing the air that is to be distilled and comprising a purification unit, means for sending at least one product of the system of columns and that has been heated in the heat exchanger to the outside, the heat exchanger being connected to the air processing means and to the means for sending at least one product from the system to the outside and means for preventing any transfer of fluid between the heat exchanger and the air processing means and between the heat exchanger and the means for sending at least one product of the system to the outside in the event of a distillation shutdown.
The unit may include a subcooler for cooling at least one liquid extracted from the system of columns against a gas from the system of columns.
This sub-cooler can be sited between the heat exchanger and the first column or next to the heat exchanger in order to reduce the overall height.
In instances in which the air to be distilled is sent partly at medium pressure into the first column (MP column), partly directly at low pressure into the second column (LP column), it is beneficial to vent to the atmosphere only the low-pressure air without starting the distillation system (i.e. without sending MP air into the first column). This makes it possible to accentuate the thermal imbalance and therefore to accelerate the restoring of the temperature of the exchanger. For example, if the low-pressure air represents 30% of the total air flow (=overall flow of hot fluids), if 10% of the total air flow is dumped into the atmosphere, this makes it possible to have 30% of the overall hot flow passing through the exchanger, for 20% of the overall cold flow, i.e. a cold/hot ratio of ⅔, instead of 9/10 if the system of columns is started. This makes it possible to proceed more quickly and reduce the amount of venting.
In this configuration, venting can also be used to maintain a maximum permissible pressure in the second column, due to the introductions of heat which vaporize part of the liquid stored in the bottom of the LP column. This avoids stressing the valves.
Note that the diagrams are simplified: they may notably include a turbine, a sub-cooler or a compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawing(s). It is to be noted, however, that the drawing(s) illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

FIG. 1 represents a unit according to an embodiment of the invention illustrated schematically.

FIG. 2 represents a unit according to an embodiment of the invention illustrated schematically.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a cryogenic-distillation air separation unit comprising a system of columns comprising at least a first distillation column. Here the system comprises a double column 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 bottom of the second column is heated by the overhead gas from the first column, which condenses there in a condenser.
The unit may comprise means for processing the air that is to be distilled and comprising a purification unit. These means make it possible to purify the air of water and of carbon dioxide in a known manner. The unit may also comprise means for sending at least one product of the system of columns and that has been heated in the heat exchanger to the outside, for example to a customer, to storage.
These means are not essential to the unit because they can be added to the separation unit, once it has been installed on site.
An indirect heat exchanger 3 of the plate and fin type is used to cool the air that is to be separated the air 1 here being sent only to the first column K1. The heat exchanger cools the air purified of water and of carbon dioxide by indirect exchange of heat with at least two different fluids coming from the system of columns, which are a nitrogen-enriched gas and an oxygen-enriched gas in this example but which may include other gases.
The heat exchanger is sited below the first column K1 so that its main axis is vertical and preferably corresponds to the main axis of the first column K1.
The exchanger can comprise a single body as illustrated here or several bodies connected in parallel.
As a preference, no other heat exchanger (such as for example a gas condenser of the system of columns) is located between the heat exchanger 3 and the first column K1.
The air, having already been purified of water and CO2 in a purification unit, enters the hot end of exchanger 3. The heat exchanger is sited below the first column K1 with its main axis vertical and its cold end adjacent to the bottom of the first column. The air rises in the heat exchanger in normal operation and cools to exit at the cold end, which is the end corresponding to the bottom of the first column, in gaseous form. It is separated in this column K1, and then fluids from the column K1 are sent into column K2 in a known manner and the distillation in the column K2 produces a nitrogen-enriched gas 13 at the top of the column and an oxygen-enriched gas 9 at the bottom of the column. These two gases are sent to be heated in normal operation in the exchanger 3 to emerge at the hot end having exchanged heat indirectly with the air 1. Obviously the exchanger can be more complex: air can be extracted from the exchanger to be expanded in a turbine, compressed in a cold booster, etc.
When distillation is shut down, valves close the lines to prevent any transfer of fluid between the heat exchanger and the part of the installation upstream of the heat exchanger, in particular the means for processing the air that is to be distilled and which comprise a purification unit, means for sending at least one product of the system of columns and that has been heated in the heat exchanger to the outside.
During this period, the unit can be kept cold simply by the insulation that surrounds it or also by adding liquid nitrogen. The fluids remaining at the cold end in the heat exchanger, that were present there during distillation in normal operation, and that are heavier than the fluids at the hot end, tend to migrate toward the bottom of the heat exchanger, that is to say the hot end.
During the restart, the valves are opened to allow the transfer of fluid (air) to the heat exchanger and the system of columns. Valves are also opened to allow the fluids produced by the distillation to be sent to a customer or for the regeneration of the air purification. The air enters the heat exchanger where it is cooled.
In order to reduce the risk of cold migration, part of the gaseous air cooled in the heat exchanger 3 can be vented to the atmosphere by opening the valve V1 in order to open the line 5 connected to the outside of the insulated space so as to vent the cooled air to the atmosphere. Another part 7 of the gaseous air is fed to the first column K1.
In addition to or instead of this venting of air to the atmosphere, it is possible to vent to the atmosphere at least a part of at least one of the gases produced by the system of columns. Here it is the nitrogen-enriched flow which is divided into two by opening valve V2 so as to open the line 15, leaving the rest of the flow 17 to heat up in the heat exchanger. The oxygen-enriched flow is heated in its entirety in the heat exchanger, but a part of this gas could also be vented to the atmosphere.
In some cases, an entire flow of product gas could be vented to the atmosphere, for example in the case of a flow of impure argon produced only in order to have a purer production of oxygen.
It will be appreciated that due to its orientation, the heat exchanger 3 does not contain any liquid, but only gases.
FIG. 2 differs from [FIG. 1 ] in that two air flows 1, 2 are sent for distillation in normal operation. A first flow 1 at a first pressure is cooled in the heat exchanger 3 and is sent to the first column K1 in gaseous form. A second flow 2 at a second pressure lower than the first pressure is cooled in the heat exchanger 3 and is sent to the second column K2 in gaseous form.
The rest of the normal operation and shutdown happens as for the previous case.
For restarting, only one of the air flows is restored, namely the one with the lowest pressure, which is flow 2. This gas flow is divided in two, one part being vented to the atmosphere by opening the valve V1 in the line 4. The remainder 6 is sent directly to the second column K2.
The line 4 with its valve V1 can also be used to depressurize the column K2 if the pressure therein becomes excessive, during normal operation, during start-up or during shutdown.
As a preference, during normal operation and/or during a shutdown, part of the air or of a fluid extracted from the system of columns (K1, K2) is vented to the atmosphere only if the pressure in a column of the system of columns exceeds a threshold.
While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.
The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.
“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.
“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.
Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.
Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.
All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.

Claims

1-13. (canceled)

14. A method for restarting an air separation unit comprising a system of columns comprising at least a first distillation column and a heat exchanger by indirect exchange of heat for cooling the air purified of water and of carbon dioxide and intended to be separated in the first column by sending it to one end of the heat exchanger called the hot end and extracting it at the other end of the heat exchanger called the cold end, the exchanger also serving to heat at least two gases coming from the system of columns, the heat exchanger being sited below the first column with its main axis vertical and its cold end adjacent to the bottom of the first column, and

i. during normal operation, at least part of the air cooled in the heat exchanger and intended to be distilled is sent to the system of columns, for example to the first column, and at least two gases from the system of columns are sent to the heat exchanger;

ii. while the unit is shut down, air cooled in the heat exchanger is no longer sent to the system of columns, and fluids are no longer sent from the system of columns to the heat exchanger; and

iii. during the restart, air is sent to the hot end of the heat exchanger and from the cold end of the heat exchanger in gaseous form to the system of columns, where it is separated, and at least one gas extracted from the system of columns is sent to the heat exchanger, wherein during the restart:

a) the at least one part of the gaseous air coming from the cold end is divided into two, a first portion thereof is sent to the system of columns, and a second portion thereof is vented to the atmosphere, and/or

b) at least part of a gas extracted from the system of columns is vented to the atmosphere.

15. The method as claimed in claim 14, wherein, during the restart, a flow of cooled air corresponding to at least 5%, preferably to at least 10% of the overall flow-rate of gases cooled in the heat exchanger during normal operation is vented to the atmosphere at the cold end of the exchanger.

16. The method as claimed in claim 14, wherein, during the restart, a flow of gas coming from the system of columns and having a flow-rate corresponding to at least 5%, preferentially to at least 10% of the overall flow-rate of gas to be heated in the heat exchanger during normal operation is vented to the atmosphere at the cold end of the exchanger.

17. The method as claimed in claim 14, wherein the gas extracted from the system of columns is enriched with nitrogen or with oxygen or with argon.

18. The method as claimed in claim 14, wherein the heat exchanger is a brazed plate and fin exchanger.

19. The method as claimed in claim 14, wherein:

in normal operation, a first flow of air at a first pressure and a second flow of air at a second pressure lower than the first pressure are cooled in the heat exchanger and are sent respectively to the first column of the system of columns and to a second column of the system of columns operating at a lower pressure than the first column, and

during the restart, the flow-rate of the first flow is smaller compared to that sent during normal operation, or even zero and the second flow is divided in two, a portion being sent to the second column and the other portion being vented to the atmosphere.

20. The method as claimed in claim 14, wherein the system of columns comprises a single column which is the first column, and in normal operation all the air that is to be separated is sent to the first column, in a shutdown, no flow of air is sent to the first column and during the restart, the air coming from the cold end is divided in two, and a first portion thereof is sent to the first column and a second portion thereof is vented into the air.

21. The method as claimed in claim 14, wherein, during normal operation and/or during a restart or during a shutdown, part of the air or of a fluid extracted from the system of columns is vented to the atmosphere if, and preferably only if, the pressure in a column of the system of columns exceeds a threshold.

22. The method as claimed in claim 14, wherein, during the restart, the first portion is sent to the second column.

23. The method as claimed in claim 14, wherein, during the restart, the at least one part of the gaseous air coming from the cold end is divided into two, a first portion thereof is sent in gaseous form to the system of columns and a second portion thereof is sent in gaseous form to the atmosphere.

24. A cryogenic-distillation air separation unit comprising a system of columns comprising at least a first distillation column, a heat exchanger using indirect exchange of heat to cool the air intended to be separated in the first column, means for sending air to one end of the heat exchanger called the hot end, means for extracting the cooled air from the other end of the heat exchanger called the cold end, these means being connected to the system of columns, means for sending at least two fluids coming from the system of columns to be heated in the exchanger, the heat exchanger being sited below the first column with its main axis vertical and its cold end adjacent to the bottom of the first column, characterized in that it comprises means for sending to the atmosphere, in gaseous form:

a. air cooled in the heat exchanger, the venting means for venting to the atmosphere being connected to the means for extracting the cooled air from the other end of the heat exchanger; and/or

b. at least a portion of one of the at least two fluids produced by the system of columns upstream of the heat exchanger.

25. The unit as claimed in claim 24 further comprising means for preventing transfer of fluid between the heat exchanger and the air processing means and between the heat exchanger and the means for venting at least one product from the system to the atmosphere in the event of a distillation shutdown.

26. The unit as claimed in claim 24, wherein the means for venting to the atmosphere air cooled in the heat exchanger are connected to the first column or to the second column.