Patent Application
20240133624
The present invention relates to a method for separating air by cryogenic distillation.
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.
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:
According to other optional aspects:
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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR 2101594 | Feb 2021 | FR | national |
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.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/053473 | 2/14/2022 | WO |
