METHOD FOR SEPARATING AIR BY MEANS OF CRYOGENIC DISTILLATION

Abstract

In an air-distillation method, purified air is cooled in an exchange line and then sent to a distillation column of a system of columns, and oxygen- and nitrogen-rich fluids are extracted from a column of the system of columns only during the repressurization phase. A purified airflow, constituting between 3% and 20% of the air compressed in the compressor, is used to at least partially pressurize the adsorber completing the regeneration phase thereof, and the airflow compressed in the compressor during the adsorption phase is substantially equal to the airflow compressed in the compressor during the pressurization of the adsorber. A portion of the purified air is sent to a turbine where it is decompressed and then sent into the atmosphere an as to ensure that it is kept at least partially cold during the entire cycle, and the amount of decompressed airflow sent into the air during the pressurization of an adsorber is less than the amount sent into the air during the adsorption phase of the same adsorber.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 of International PCT Application PCT/FR2012/050587, filed Mar. 21, 2012, which claims the benefit of FR 1152733, filed Mar. 31, 2011, both of which are herein incorporated by reference in their entireties.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a method for separating air by distillation of air, in particular intended for producing oxygen and/or nitrogen and/or argon, of the type wherein the air to be distilled is purified beforehand by means of at least two adsorbers which each follow, offset, a cycle wherein succeed an adsorption phase, at a pressure of the cycle, and a regeneration phase ending with a pressurisation of the adsorber.

BACKGROUND

The pressures of which it is question here are absolute pressures.

In this type of installation, the distillation of the air, compressed beforehand by a compression apparatus, is carried out cryogenic temperatures and therefore requires that the air be purified in order to remove from therein the constituents of which the solidification temperatures are higher than the distillation temperature of the air, i.e. primarily water and carbon dioxide. The main objective of the distillation of the air is to provide, in liquid and/or gaseous form, oxygen and/or nitrogen and/or argon. This production generates the coproduction of fluids with a low oxygen content, such as, for example, impure nitrogen, called residual nitrogen, and nitrogen of the highest purity, in liquid or gaseous form.

The purification of the air to be distilled is commonly carried out by adsorption of the disturbing constituents, by means in general of two bottles containing adsorbent substances arranged on a bed and operating in alternating cycles. While one bottle is in adsorption phase, (i.e., it is purifying the air that is to be distilled) the other bottle is in regeneration phase, (i.e., it is flushed with a dry regeneration gas, such as residual nitrogen) desorbing the impurities fixed on the adsorbent during its preceding adsorption phase. The regeneration of the adsorbent is increasingly effective when it is applied at a high temperature and at a low pressure in relation to that maintained during the adsorption, which requires that a bottle terminating its regeneration phase be pressurised, in order to restore a satisfactory condition of pressure for its upcoming adsorption phase.

For this, the state of the art consists in sampling a fraction of purified air at the outlet of the bottle in adsorption phase and to decompress it to the bottle at the end of the regeneration phase, in order to increase the pressure of the latter. During this operation, it is however indispensable to maintain the air flow to be sent to the distillation constant in order to prevent any fluctuation in the supply of the distillation apparatus and in order to maintain the production of oxygen and/or nitrogen and/or argon. As such, during each repressurisation, the air compression apparatus must provide this surplus of air which is used for the pressurisation. However, this additional air flow implies oversizing, and therefore an extra cost, of the compression apparatus. It is indeed asked to provide an additional compressed air flow of about 5% of the nominal air flow processed by the bottle in adsorption (according to the optimisation of the cycle), during a pressurisation duration of about 15 minutes for a bottle of common size.

Subsequently, beyond the duration of pressurisation, the compression apparatus operations with nominal air flow (i.e., that which corresponds to the separation capacity of the device for separating air).

As an example, for a maximum compressor flow of 100 kNm³/hr, the normal flow rate for use will be 95 kNm³/hr sent to the cold box wherein takes place the distillation and 100 kNm³/hr solely for the single pressurisation phase at the end of regeneration wherein 5 kNm³/hr of air is sent in order to pressurise one of the bottles.

Certain methods for separating air use a lost air system wherein all of the purified air is not sent to the distillation columns. In this case there is generally an expansion turbine which will decompress to a pressure close to the atmospheric pressure the excess air in relation to the oxygen needs.

In this type of method, it is preferable to not apply the conventional method of pressurisation of adsorbers.

SUMMARY OF THE INVENTION

The purpose of certain embodiments of the invention is to avoid the oversizing of compressors by reducing, even by eliminating, the increase in the air flow to be compressed in order to provide the additional gas required for the pressurisation of the bottles of adsorbent.

To this effect, an embodiment of the invention has for purpose a method for distilling air, in particular intended to produce oxygen and/or nitrogen and/or argon, of the type wherein the air to be distilled is compressed beforehand in a compressor, purified by means of at least two adsorbers which each follow, offset, a cycle wherein succeed an adsorption phase, at a high pressure of the cycle (P_ads) and a regeneration phase at a low pressure P_atmos ending with a repressurisation phase of the adsorber, the purified air is cooled in an exchange line and then sent to a distillation column of a system of columns and oxygen-rich and nitrogen-rich fluids are withdrawn from a column of the system of columns, only during the repressurisation phase a purified air flow, constituting between 3 and 20% of the air compressed in the compressor, is used to pressurise, at least partially, the adsorber completing its regeneration phase and the air flow compressed in the compressor during the adsorption phase is substantially equal to the air flow compressed in the compressor during the pressurisation of the adsorber, characterised in that a portion of the purified air is sent to a turbine wherein it is expanded and then sent to the atmosphere so as to provide at least partially the refrigeration requirements during the entire cycle and in that the expanded air flow sent to the air during the pressurisation of an adsorber is lower than that sent to the air during the adsorption phase of the same adsorber, even during the rest of the cycle beyond the pressurisation phase.

The term “substantially equal” covers the case wherein the air flow compressed in the compressor during the adsorption phase differs by at most 5%, more preferably by at most 3%, from the air flow compressed in the compressor during the pressurisation of the adsorber. The two flows are more preferably strictly equal.

According to other characteristics of this method, taken individually or according to the technically permissible combinations:

• • the air flow compressed in the compressor during the adsorption phase of an adsorber is equal to the air flow compressed in the compressor during the pressurisation of the adsorber;
  • the reduction of the air flow sent to the turbine and then to the air during the repressurisation is equal to the air flow used during the repressurisation in order to pressurise the adsorber completing its repressurisation phase;
  • the quantity of air sent to the distillation is constant during the entire cycle;
  • the reduction in the air flow sent to the turbine and then to the air during the pressurisation of an adsorber is lower than the air flow used during the repressurisation in order to pressurise the adsorber completing its repressurisation phase;
  • during the repressurisation phase the air flow compressed in the compressor increases in relation to the flow sent during the rest of the cycle and the quantity of air sent to the distillation remains equal to that sent during the rest of the cycle;
  • a liquid flow is produced as a final product;
  • said method of purification is an adsorption of the PSA, TSA or TPSA type;
  • air is expanded in a turbine and sent to a column of the system of columns:
  • the system of columns is constituted by a double column comprising a medium-pressure column and a low-pressure column
  • an oxygen-rich flow is withdrawn from the low-pressure column and it is vaporised in the exchange line.

The term “PSA” used in this document means “Pressure swing adsorption”. The term “TSA” used in this document means “Temperature swing adsorption”. The term “TPSA” used in this document means “Temperature and pressure swing adsorption”.

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 diagrammatical view of an installation for a method according to an embodiment of the invention an embodiment of the present invention.

DETAILED DESCRIPTION

The invention shall be better understood when reading the following description, provided solely by way of example and in reference to the annexed drawing, wherein:

• • FIG. 1 is a diagrammatical view of an installation for operating the method according to the invention.

FIG. 1 shows an installation 1 for the distillation of air according to the invention. This installation is for example intended to produce gaseous oxygen OG, as well as liquid oxygen OL.

The installation 1 substantially comprises:

• • an air compressor 4;
  • an apparatus 6 for purifying air via adsorption, said apparatus comprises, on the one hand, two adsorbers 7A, 7B in the form of two bottles each containing adsorbent materials, for example molecular sieve possibly with alumina, capable of adsorbing the water and the carbon dioxide present in the air, and, on the other hand, ducts and connection valves of which the arrangement shall appear clearly during the description of the method implemented in the installation 1 and which makes it possible to successively submit each adsorber 7A, 7B to the air flow to be distilled and to a regeneration gas of the adsorbent;
  • a lost air turbine 27;
  • a cold compressor 3;
  • a Claude turbine 5 sending air to the medium-pressure column;
  • a main thermal exchange line 8;
  • an apparatus for distilling air in the form of a double column 10 comprising a medium-pressure column 12, a low-pressure column 14 and a vaporiser-condenser 16 coupling these two columns, as well as an argon separation column 26; and
  • a reservoir 18 for storing liquid oxygen.

The operation of the installation 1 of FIG. 1 is as follows.

The air to be distilled, compressed beforehand by the compressor 4, is purified by one of the adsorbers 7A, 7B of the apparatus 6, then cooled by the main thermal exchange line 8 to an intermediate temperature. The adsorption can be of the TSA, PSA or TPSA type. A portion 25 of the air is sent to a lost air turbine 27 and the expanded air is sent to the atmosphere after reheating in the exchanger 8. The rest of the air continues to be cooled. Another portion 29 of the air is sent to the cold compressor 3, sent back to the exchange line 8. A portion of the supercharged flow is expanded in a turbine 5 to the medium pressure in order to form the expanded flow 7. The expanded flow 7 in the vicinity of its dew point is introduced into the tank of the medium-pressure column 12. The rest of the supercharged air 9 continues to be cooled in the exchange line 8, is expanded in a valve V then is sent to an intermediate level of the medium-pressure column 12.

The vaporiser-condenser 16 vaporises liquid oxygen, for example having a purity of 99.5%, of the tank of the low-pressure column 14, by condensation of gaseous nitrogen at the head of the medium-pressure column 12.

“Rich liquid” LR (oxygen-rich air), sampled in the tank of the medium-pressure column 12, is injected, after expansion, at an intermediate level of the low-pressure column 14, while liquid nitrogen NL, substantially pure, is sampled at the head of the medium-pressure column 12 in order to supply the reservoir 22 and the head of the low-pressure column 14. Liquid nitrogen and/or liquid oxygen is produced as a final product, sent to the client in liquid form.

Impure or “residual” nitrogen NR, withdrawn from the top of the low-pressure column 14, is sent back to the main thermal exchange line 8, where it causes the cooling of the air to be distilled.

Liquid oxygen OL is withdrawn from the tank of the low-pressure column 14 and supplies the storage reservoir 18. After pressurisation in the pump P, it is vaporised in the main thermal exchange line 8 and distributed by a production pipe 32 in order to form pressurised gaseous oxygen.

An argon production column 26 is supplied from the low-pressure column 14.

The operation of the installation that has just been described can be implemented continuously, except for the operation of the purification apparatus 6, which follows over time a pressure cycle of FIG. 2. However, it is possible for all of the fluids to not be produced constantly, according to the needs of the client, the cost of electricity, etc.

The cycle of FIG. 2, of which the period is, by way of example, equal to about 360 minutes for an adsorption pressure substantially equal to 20 bars, comprises 4 successive steps I to IV. These four steps shall now be described successively for the adsorber 7A, with the understanding that the adsorber 7B follows these same steps with a time delay of substantially

$\frac{T}{2},$

by means of open or closed connection valves designated by the same upcoming references as those of the adsorber 7A, with the letter A to be replaced with the letter B and the state of each valve (open/closed) to be inverted (closed/open).

During the step 1 (i.e., t=0

$t=\frac{T}{2}),$

to the adsorber 7A is in adsorption phase under a high operating pressure noted as P_ads, while the adsorber 7B is in regeneration phase. The air compressed by the compressor 4 supplies the adsorber 7A, via an open valve 40A. The outlet of the adsorber 7A is connected to the exchange line 8, via an open valve 42A.

During the steps II, III and IV, the adsorber 7A is in regeneration phase, while the adsorber 7B is in adsorption phase. More precisely, during the step II, a valve 44A for venting the adsorber 7A to the air is open in such a way that the pressure inside the bottle of the adsorber 7A is brought to a pressure substantially equal to the atmospheric pressure, noted as P_atmos in FIG. 2.

During the step III, the valve 44A remains open and residual nitrogen NR withdrawn at the head of the low-pressure column 14 then heated in the exchanger 8 supplies, via an open valve 46A, the adsorber 7A in order to circulate therein against the current. This is the effective phase of the regeneration during which the impurities are desorbed and the beds are regenerated. During the step IV, the valves 44A and 46A are closed, in order to allow for the pressurisation of the adsorber. In a first step, i.e. during a first sub-step IV′, the pressurisation of the adsorber is provided by a purified air flow, via the open valve 42A, this purified air flow coming from the bottles 7A, 7B. The sub-step IV′ is continued by the sub-step IV″ until the pressure inside the adsorber 7A is substantially equal to the high pressure P_ads, by opening the valve 50.

By the method according to the invention, the pressurisation of each adsorber no longer requires, during the step IV, to increase the flow of the compressor 4. In this way, the compressor 4 is sized in an optimum manner, i.e. in such a way that its nominal flow is substantially constant. The investment and operating costs for this compression apparatus are reduced, in relation to those of installations concerning prior art.

During the adsorption phase, the compressor 4 compresses 100 kNm³/hr of air and all of the purified air is sent to the exchange line 8. 30 kNm³/hr of air is sent to the lost air turbine 5. 70 kNm³/hr of air is sent to the system of distillation columns.

During the pressurisation phase at the end of the regeneration phase, the compressor 4 compresses 100 kNm3/h of air, 95 kNm³/hr is sent to the exchange line 8 and 5 kNm³/hr is sent in order to pressurise an adsorption bottle. 25 kNm³/hr of air (therefore 5 kNm³/hr less) is sent to the lost air turbine 5 and 70 kNm³/hr of air is still sent to the system of distillation columns.

It shall be understood that this invention applies to any method involving a lost air turbine, whether there is compression in a cold compressor or not, a double column or not, a production of argon or not, pressurisation and vaporisation of liquid oxygen or not.

It shall also be understood that if the reduction in the lost air flow is less than the flow sent to the pressurisation, either the compressed flow will have to increase during the pressurisation and the distilled air flow remains unchanged or less air will be sent to the distillation and the compressed flow will remain unchanged.

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 a range is expressed, it is to be understood that another embodiment is from the one.

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 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 distilling air to produce an air gas selected from the group consisting of oxygen, nitrogen, argon, and combinations thereof, the method comprising the steps of:

compressing the air to be distilled in a compressor;

purifying the air following compression using at least two adsorbers to form a purified air, wherein which each adsorber follows, offset, a cycle wherein succeed an adsorption phase, at a high pressure of the cycle (Pads), and a regeneration phase at a low pressure Patmos ending with a repressurization phase of the adsorber;

cooling the purified air in an exchange line and then sending the purified air to a distillation column of a system of columns;

withdrawing oxygen-rich and nitrogen-rich fluids a from a column (14) of the system of columns; and

sending a portion of the purified air to a turbine wherein the portion of the purified air is expanded and then sent to the atmosphere so as to provide at least partially the refrigeration requirements during the entire cycle and in that the flow of the expanded air sent to the atmosphere during the pressurization of the adsorber is lower than that sent to the atmosphere during the adsorption phase of the same adsorber,

wherein solely during the repressurization phase a purified air flow, constituting between 3 and 20% of the air compressed in the compressor, is used to pressurize, at least partially, the adsorber completing its regeneration phase,

wherein the air flow compressed in the compressor during the adsorption phase is substantially equal to the air flow compressed in the compressor during the pressurization of the adsorber.

15. The method as claimed in claim 14, wherein the expanded air flow sent to the air during the pressurization of an adsorber is less than that sent to the air during the rest of the cycle beyond the pressurization phase.

16. The method as claimed in claim 14, wherein the air flow compressed in the compressor during the adsorption phase of an adsorber is equal to the air flow compressed in the compressor during the pressurization of the adsorber.

17. The method as claimed in claim 14, wherein the reduction of the air flow sent to the turbine and then to the air during the repressurization is equal to the air flow used during the repressurization in order to pressurize the adsorber completing its repressurization phase.

18. The method as claimed in claim 17, wherein the quantity of air sent to the distillation is constant during the entire cycle.

19. The method as claimed in claim 14, wherein the reduction in the air flow sent to the turbine and then to the air during the pressurization of an adsorber is less than the air flow used during the repressurization in order to pressurize the adsorber completing its repressurization phase.

20. The method as claimed in claim 19, wherein during the repressurization phase the air flow compressed in the compressor increases in relation to the flow sent during the rest of the cycle and the quantity of air sent to the distillation remains equal to that sent during the rest of the cycle.

21. The method as claimed in claim 14, wherein a liquid flow is produced as a final product.

22. The method as claimed in claim 14, wherein the purification method is an adsorption of the PSA, TSA or TPSA type.

23. The method as claimed in claim 14, wherein the air is expanded in a turbine and sent to a column of the system of columns.

24. The method as claimed in claim 14, wherein the expanded air flow sent to the air during the pressurization of an adsorber is less than that sent to the air during the rest of the cycle beyond any pressurization phase.

25. The method as claimed in claim 14, wherein the system of columns is constituted by a double column comprising a medium-pressure column and a low-pressure column.

26. The method as claimed in claim 25, wherein an oxygen-rich flow is withdrawn from the low-pressure column and it is vaporized in the exchange line.