US20240230222A9

US20240230222A9 - Method for separating air by cryogenic distillation

Info

Publication number: US20240230222A9
Application number: US18/277,794
Authority: US
Inventors: Benoit Davidian
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: 2021-02-18
Filing date: 2022-02-14
Publication date: 2024-07-11
Also published as: WO2022175194A1; FR3119884B1; EP4295096B1; CA3206388A1; KR20230147063A; CN116848365A; JP2024508391A; FR3119884A1; US20240133624A1; EP4295096A1; EP4295096C0

Abstract

In a method for separating air by cryogenic distillation in a system of columns comprising a first column operating at a first pressure and a second column operating at a second pressure which is lower than the first column, the temperature T1 at which an airflow leaves, after cooling, the heat exchanger by rising towards the cold end of said heat exchanger and enters the first column is at least 1° C., preferably at least 2° C., higher than the dew point of the airflow.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a § 371 of International PCT Application PCT/EP2022/053473, filed Feb. 14, 2022, which claims the benefit of FR2101594, filed Feb. 18, 2021, both of which are herein incorporated by reference in their entireties.

FIELD OF THE INVENTION

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

BACKGROUND OF THE INVENTION

A separation device generally comprises an exchange line wherein the air to be distilled cools 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 in the first column.
The top of the first column is thermally coupled to the bottom of the second column.
WO19126927 describes a device for separating air by cryogenic distillation in which the main exchange line is situated below the system of distillation columns, and with the hot end of the exchanger positioned toward the bottom.
The air intended for the first column cools in the exchange line before being sent to the first column.
During cooling, from the bottom up in the main exchange line, this air partially condenses. It must be ensured that the liquid obtained is properly transported 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 air (typically CnHm), which poses a safety risk.
Obtaining a sufficient gas speed, including during reduced operation, involves significant pressure drops in the air intended for the first column, which is costly in terms of energy.
In an air separation device, the bottom liquid (RL) from the first column is expanded and sent to an intermediate point of the second column. The overhead liquid (PL) from the first column is expanded and sent to the top of the second column. The two liquids are subcooled in a heat exchanger against an overhead gas from the second column.
In this exchanger, known as a subcooler, in particular in a cross-flow configuration, the nitrogen-enriched overhead liquid, known as poor liquid, PL, and the oxygen-enriched bottom liquid, known as rich liquid, RL, are cooled in two distinct sections, as if there were two exchangers in series. This means that the oxygen-rich liquid is cooled to a temperature higher than the entry temperature of the overhead liquid from the first column. In this configuration, the cold available in the residual nitrogen is not completely stripped.
The conventional subcooler is illustrated on page 96 of Kerry's Industrial Gas Handbook, CRC Press, 2007. The exact positioning of the fluids is not always illustrated in patents or other documents for reasons of simplification of the FIGURES.
In addition, 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.
This has the effect of reducing the cooling recovery from the cold fluids and makes it necessary to partially condense (typically around 1 to 2%) the air leaving the main exchange line and going toward the bottom of the first column.

SUMMARY OF THE INVENTION

In certain embodiments, the invention may include feeding the column with at least one air flow at a temperature 1° C., or even 2° C., higher than its dew point, including during reduced operation of the device. This implementation makes it possible to avoid condensing the air intended for the first column in the main exchange line. The air thus leaves the main exchange line at a temperature 1° C., or even 2° C., higher than its dew point.
One way of ensuring that the air intended for the first column is sufficiently higher than the dew point is to deepen the cooling of certain fluids entering the distillation system, as well as of the fluids internal to the distillation system. In this case, an exchange line with very small pressure drops can be designed, without having to take into account a liquid transport criterion with respect to safety.
It is a case of recovering as much cold as possible from the fluids resulting from distillation (residual nitrogen, oxygen, nitrogen at the second pressure, which is purer than the residual nitrogen).
One variant for ensuring that the air intended for the first column is sufficiently higher than the dew point consists of modifying the subcooling in order to reduce the temperature of a liquid entering the second column.
In the subcooler, the cooling of oxygen-enriched liquid is deepened so that it leaves at a temperature below the entry temperature of the enriched liquid. There is therefore a shared area in the subcooler where both liquids are cooled at the same time against at least one nitrogen flow coming from the second column.
According to another variant, in the exchange line, the exhaust from the turbine is sent back toward the exchange line in order to cool against the residual nitrogen (and optionally the purer nitrogen coming from the second column) and against the oxygen.
According to one object of the invention, a method is provided for 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 top of the first column being thermally coupled to the bottom of the second column, in which:

- i) an air flow purified of water and carbon dioxide is cooled in a heat exchanger and 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 having been subcooled in the subcooler
- iii) a nitrogen-enriched liquid is withdrawn from the upper part of the first column and sent to the second column having been subcooled in the subcooler
- iv) a nitrogen-rich gas and an oxygen-rich fluid are withdrawn from the second column and heat in the heat exchanger
- v) the heat exchanger is positioned below the first column which in turn is positioned below the second column, the air flow cooling in the heat exchanger by rising, wherein the temperature T1 at which the air flow leaves the heat exchanger and enters the first column is at least 1° C., preferably at least 2° C., higher than the dew point of the air flow.

According to other optional aspects:

- another air flow purified of water and carbon dioxide is cooled in the heat exchanger, leaves the heat exchanger at a temperature T2, is expanded in a turbine, 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 in gaseous form, the temperature T2 being 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 higher than, lower than or equal to T1
- 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., preferably at least 2° C., higher than its dew point, and sent to the second column without having been expanded
- the oxygen-enriched liquid is cooled in the subcooler to a temperature lower than the temperature 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
- the first column and potentially the second column are fed with air solely by gaseous air flows
- the first column and the second column only produce gaseous flows as final products
- the heat exchanger and the subcooler are made up 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 by 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 an intermediate temperature between the temperature at the outlet of the subcooler of the oxygen-enriched liquid and the temperature at the outlet of the subcooler of the nitrogen-enriched liquid, and sent to the second column.

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 drawings. It is to be noted, however, that the drawings 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 shows a method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In certain embodiments, the method 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 coupled to the second column K2 by a bottom evaporator of the second column.
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. An aluminum brazed plate heat exchanger E is positioned below the second column K1.
An air flow 1 is compressed by the compressor 3 to the first pressure, cooled by the cooler 5 and purified of water and carbon dioxide in the purification unit 7. To be cooled, the air is sent to the hot end of the heat exchanger E at the bottom of the exchanger and rises toward the top, as the cold end of the exchanger is located at the top.
The purified air 9 cools in the heat exchanger E and is divided in two at a temperature T2 that is an intermediate temperature of the exchanger E. One portion 11 of the air continues to cool in the exchanger to a temperature T1 at least 1° C., preferably at least 2° C., higher than the dew point of the air fraction 9.
At this temperature T1, it leaves the exchanger E and is sent to the bottom of the column K1 as a feed gas flow.
The pressure drop of the air 9, 11 passing through the exchanger E does not exceed 120, or even 100 mbar.
The air separates in the first column to form an oxygen-enriched bottom liquid 10 and a nitrogen-enriched overhead liquid 12. The liquid 10 and the liquid 12 are sent to a subcooler S where at least one nitrogen gas flow 15 coming 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 the temperature 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 one level of the second column K2 and the liquid 12 is sent to a level of the second column K2 higher than the level at which the liquid 10 enters. The subcooler thus comprises a section in which the liquids 10 and 12 both cool against the nitrogen 15. The subcooler S can be placed next to the column K1 and its hot end can be positioned toward the bottom.
One portion 13 of the air at the temperature T2 is expanded in a turbine T without having been compressed downstream of the exchanger, is sent 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 can 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 leaves 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 rising.
The column K2 separates the 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. The gas 17 heats by descending in the exchanger E and is then used as a product of the method.
The gas 15 heated in the subcooler and then by descending in the exchanger E is divided in two, one portion being used to regenerate the purification unit 7 and the rest being used as a product or as residual nitrogen.
Here, the first column K1 is fed with air solely by gaseous air flows.
The second column K2 could also be fed with air, and in this case it would be a gaseous air feed only.
The method only produces gas flows 15, 17 as final products. The purge flow from the bottom condenser of the column K2 is not considered to be a final product.
The subcooler S can be incorporated into the main exchange line E. This makes it possible to further optimize the stripping of the cold fluids 15, 17 coming from the distillation (potentially also of the pure nitrogen coming from the second column K2) in order maximize the cooling of the rich and poor liquids 10, 12 and the air at the second pressure coming from a blower turbine.
In addition to the fluids 15, 17, a liquefied air flow withdrawn from the column K1 can be subcooled in the subcooler S before being sent to the column K2.
FIG. 1 shows one possibility for feeding the method with air. Other variants are also possible, for example the variant in FR30890831, 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 pressure, the top of the first column being thermally coupled to the bottom of the second column.
In this case, three air flows are used. The air is compressed to the pressure of the second column and then purified. Next, the air is divided in two, one portion being cooled at the pressure of the second column and sent to the second column by rising in the exchanger. The final temperature on leaving the exchanger is at least 1° C., preferably at least 2° C., higher than its dew point at the second pressure.
The other portion is boosted to the pressure of the first column. One fraction of this portion cools in the exchanger by rising and leaves it at a temperature T1 at least 1° C., preferably at least 2° C., higher than the dew point of the air fraction.
The rest of the portion is cooled in the exchanger to a temperature T2, expanded in a blower turbine, returned to the exchanger at a temperature T3, cooled up 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.
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

1-14. (canceled)

15. A method for 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 top of the first column being thermally coupled to the bottom of the second column, in which:

i) cooling an air flow that has been purified of water and carbon dioxide in a heat exchanger and then sending said air flow to the first column in gaseous form;

ii) withdrawing an oxygen-enriched liquid from the bottom of the first column, subcooling the oxygen-enriched liquid in a subcooler, and then sending said oxygen-enriched liquid to the second column;

iii) withdrawing a nitrogen-enriched liquid from the upper part of the first column, subcooling the nitrogen-enriched liquid in a subcooler, and then sending said nitrogen-enriched liquid to the second column;

iv) withdrawing a nitrogen-rich gas and an oxygen-rich fluid from the second column and then heating in the heat exchanger;

wherein the heat exchanger is positioned below the first column, which in turn is positioned below the second column,

wherein the air flow cooling in the heat exchanger traversing the heat exchanger from bottom to top,

wherein the temperature T1 at which the air flow leaves the heat exchanger and enters the first column is at least 1° C. higher than the dew point of the air flow.

16. The method as claimed in claim 15, wherein the temperature T1 at which the air flow leaves the heat exchanger and enters the first column is at least 2° C. higher than the dew point of the air flow.

17. The method as claimed in claim 15, wherein another air flow that has been purified of water and carbon dioxide is cooled in the heat exchanger, leaves the heat exchanger at a temperature T2, is expanded in a turbine, 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 in gaseous form, the temperature T2 being higher than T1.

18. The method as claimed in claim 17, wherein T4 is at least 1° C. higher than the dew point of the expanded flow.

19. The method as claimed in claim 18, wherein T4 is at least 2° C. higher than the dew point of the expanded flow.

20. The method as claimed in claim 15, 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.

21. The method as claimed in claim 20, 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.

22. The method as claimed in claim 15, wherein the oxygen-enriched liquid is cooled in the subcooler to a temperature lower than the temperature 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.

23. The method as claimed in claim 15, wherein the first column and potentially the second column are fed with air solely by gaseous air flows.

24. The method as claimed in claim 15, wherein the first column and the second column only produce gas flows as final products.

25. The method as claimed in claim 15, wherein the heat exchanger and the subcooler are made up of a single body of aluminum plates brazed together.

26. The method as claimed in claim 15, wherein the pressure drop of the air flow purified of water and carbon dioxide intended for the first column by cooling in the heat exchanger does not exceed 120 mbar.

27. The method as claimed in claim 26, wherein the pressure drop of the air flow purified of water and carbon dioxide intended for the first column by cooling in the heat exchanger does not exceed 100 mbar.

28. The method as claimed in claim 15, wherein a liquid flow is withdrawn at an intermediate level of the first column, is subcooled in the subcooler to an intermediate temperature between the temperature at the outlet of the subcooler of the oxygen-enriched liquid and the temperature at the outlet of the subcooler of the nitrogen-enriched liquid, and sent to the second column.