EP2712419B1 - Method for separating air by means of cryogenic distillation - Google Patents

Description

The present invention relates to a process for air separation by air distillation, in particular for producing oxygen and / or nitrogen and / or argon, of the type in which the air to be distilled is previously purified by means of at least two adsorbers each of which, in offset, follow a cycle in which an adsorption phase is followed, at a pressure of the cycle, and a regeneration phase ending in a pressurization of the adsorber, in accordance with in the preamble of claim 1 and known FR-A-2,896,860 and WO-2007/033838 . The pressures here are absolute pressures.

In this type of installation, the distillation of air, previously compressed by a compression apparatus, is carried out cryogenically and therefore requires purifying the air to remove constituents whose solidification temperatures are higher. at the distillation temperature of the air, namely mainly water and carbon dioxide. The main purpose of the distillation of air is to provide, in liquid and / or gaseous form, oxygen and / or nitrogen and / or argon. This production leads to the coproduction of fluids with low oxygen content, such as, for example, impure nitrogen, called residual nitrogen, and higher purity nitrogen, in liquid or gaseous form.

The purification of the air to be distilled is commonly carried out by adsorption of the troublesome constituents, generally by means of two bottles containing adsorbent substances arranged in a bed and operating in alternating cycles. While a bottle is in adsorption phase, ie it purifies the air to be distilled, the other bottle is in regeneration phase, that is to say that it is swept by a dry regeneration gas, such as residual nitrogen, desorbing the impurities fixed on the adsorbent during its previous adsorption phase. The regeneration of the adsorbent is all the more effective when it is applied at high temperature and at a low pressure compared with that maintained during the adsorption, which makes it necessary to pressurize a bottle ending its regeneration phase, in order to restore it to a satisfactory pressure condition for its future adsorption phase.

For this, the state of the art consists in taking a fraction of purified air at the outlet of the bottle in the adsorption phase and relaxing it towards the bottle at the end of regeneration phase, in order to increase the pressure of the latter. During this operation, it is however essential to keep constant the flow of air to be sent to the distillation in order to avoid any fluctuation of supply of the distillation apparatus and to maintain the production of oxygen and / or nitrogen and / or argon. Also, during each repressurization, the air compression device must provide the excess air that is used for pressurization However, this additional air flow implies oversizing, and therefore an additional cost, of the compression device. It is indeed required to provide an additional compressed air flow of the order of 5% of the nominal air flow treated by the adsorption bottle (depending on the optimization of the cycle), during a period of pressurization of the 15 minutes for a standard size bottle.

Consequently, outside the pressurization time, the compression apparatus operates at the nominal air flow rate, that is to say that which corresponds to the separating capacity of the air separation apparatus.

Thus for a maximum compressor flow rate of 100 kNm3 / h, the normal flow rate of use will be 95 kNm3 / h sent to the cold box where the distillation takes place and 100 kNm3 / h only for the only phase of pressurization at the end of regeneration where 5 kNm3 / h of air is sent to pressurize one of the bottles.

Some air separation processes use a lost air system in which not all of the purified air is sent to the distillation columns. In this case, an expansion turbine is generally found which will depressurise the excess air relative to the oxygen requirement to a pressure close to atmospheric pressure.

In this type of process, it is preferable not to apply the conventional method of adsorber pressurization.

FR-A-2896860 and WO-2007/033838 describe the adjustment of the flow rates inside a cold box of an air separation apparatus, in order to maintain the production flow rates during the repressurization phase, but do not concern a lost air system.

The object of the invention is to avoid over-sizing the compressors by reducing or even eliminating the increase in the air flow to be compressed in order to supply the additional gas necessary for the pressurization of the adsorbent bottles.

For this purpose, the subject of the invention is a process for the distillation of air, in particular intended to produce oxygen and / or nitrogen and / or argon, of the type in which the air to be distilled is compressed beforehand in a compressor, purified by means of at least two adsorbers each of which, in offset, follow a cycle in which an adsorption phase is followed, at a high pressure of the cycle (P _ads ), and a regeneration phase at a low pressure P _atmos ending in a phase of repressurization of the adsorber, purified air is cooled in an exchange line and then sent to a distillation column of a system of columns and fluids enriched with Oxygen and nitrogen are withdrawn from a column of the column system, only during the repressurization phase a flow of purified air, constituting between 3 and 20% of the compressed air in the compressor, is used to pressurize, at least partially , the adsorber ending its regeneration phase ration and the compressed air flow in the compressor during the adsorption phase is substantially equal to the compressed air flow rate in the compressor during the pressurization of the adsorber, characterized in that a portion of the purified air is sent to a turbine where it is expanded and then sent to the atmosphere to at least partially maintain the cold during the entire cycle and in that the expanded air flow sent to the air during the pressurization of an adsorber is less than that sent to air during the adsorption phase of the same adsorber, or even during the rest of the cycle outside the pressurization phase.

The term "substantially equal" covers the case in which the flow rate of compressed air in the compressor during the adsorption phase differs by not more than 5%, preferably not more than 3%, from the compressed air flow rate. the compressor during the pressurization of the adsorber. The two flows are preferably strictly equal.

• le débit d'air comprimé dans le compresseur pendant la phase d'adsorption d'un adsorbeur est égal au débit d'air comprimé dans le compresseur pendant la pressurisation de l'adsorbeur ;
• la réduction du débit d'air envoyé à la turbine et ensuite à l'air pendant la repressurisation est égale au débit d'air utilisée pendant la repressurisation pour pressuriser l'adsorbeur terminant sa phase de repressurisation ;
• la quantité d'air envoyée à la distillation est constante pendant tout le cycle ;
• la réduction du débit d'air envoyé à la turbine et ensuite à l'air pendant la pressurisation d'un adsorbeur est inférieure au débit d'air utilisée pendant la repressurisation pour pressuriser l'adsorbeur terminant sa phase de repressurisation ;
• pendant la phase de repressurisation le débit d'air comprimé dans le compresseur augmente par rapport au débit envoyé pendant le reste du cycle et la quantité d'air envoyée à la distillation reste égale à celle envoyée pendant le reste du cycle ;
• un débit liquide est produit comme produit final ;
• lequel le procédé d'épuration est une adsorption de type PSA, TSA ou TPSA ;
• de l'air est détendu dans une turbine et envoyé à une colonne du système de colonnes :
• le système de colonnes est constitué par une double colonne comprenant une colonne moyenne pression et une colonne basse pression
• on soutire un débit riche en oxygène de la colonne basse pression et on le vaporise dans la ligne d'échange.

According to other characteristics of this process, taken separately or according to the technically possible combinations:

• the flow rate of compressed air in the compressor during the adsorption phase of an adsorber is equal to the compressed air flow rate in the compressor during the pressurization of the adsorber;
• the reduction of the air flow sent to the turbine and then to the air during repressurization is equal to the air flow rate used during the repressurization to pressurize the adsorber ending its repressurization phase;
• the amount of air sent to the distillation is constant throughout the cycle;
• the reduction of the air flow to the turbine and then to the air during the pressurization of an adsorber is less than the air flow rate used during repressurization to pressurize the adsorber completing its repressurization phase;
• during the repressurization phase the compressed air flow rate in the compressor increases with respect to the flow rate sent during the remainder of the cycle and the quantity of air sent for distillation remains equal to that sent during the remainder of the cycle;
• a liquid flow is produced as the final product;
• which the purification process is PSA, TSA or TPSA type adsorption;
• air is expanded in a turbine and sent to a column of the column system:
• the column system consists of a double column comprising a medium pressure column and a low pressure column
• a high oxygen flow rate is withdrawn from the low pressure column and vaporized 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" or "temperature swing adsorption". The term "TPSA" as used herein means "Adsorption at Modulated Pressure and Temperature" or "Temperature and Pressure Swing Adsorption". "

• la figure 1 est une vue schématique d'une installation pour opérer le procédé selon l'invention.

The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the appended drawing, in which:

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

On the figure 1 is represented an air distillation installation 1 according to the invention. This installation is for example intended to produce oxygen gas OG, as well as liquid oxygen OL.

• un compresseur d'air 4 ;
• un appareil 6 d'épuration d'air par adsorption, lequel appareil comporte, d'une part, deux adsorbeurs 7A, 7B sous forme de deux bouteilles contenant chacune des matériaux adsorbants, par exemple du tamis moléculaire avec éventuellement de l'alumine, capable d'adsorber l'eau et le dioxyde de carbone présents dans l'air, et, d'autre part, des conduites et des vannes de raccordement dont la disposition apparaîtra clairement lors de la description du procédé mis en oeuvre dans l'installation 1 et qui permettent de soumettre successivement chaque adsorbeur 7A, 7B au flux d'air à distiller et à un gaz de régénération de l'adsorbant ;
• une turbine d'air perdu 27 ;
• un compresseur froid 3 ;
• une turbine 5 dite Claude envoyant de l'air à la colonne moyenne pression ;
• une ligne principale d'échange thermique 8 ;
• un appareil de distillation d'air sous forme d'une double colonne 10 comportant une colonne moyenne pression 12, une colonne basse pression 14 et un vaporiseur-condenseur 16 couplant ces deux colonnes, ainsi qu'une colonne de séparation d'argon 26 ; et
• un réservoir 18 de stockage d'oxygène liquide.

The installation 1 essentially comprises:

• an air compressor 4;
• an apparatus 6 for purifying air by adsorption, which apparatus comprises, on the one hand, two adsorbers 7A, 7B in the form of two bottles each containing adsorbent materials, for example molecular sieve with optionally alumina, capable of adsorbing the water and carbon dioxide present in the air, and, on the other hand, pipes and connection valves whose arrangement will be clearly apparent when describing the process implemented in the installation 1 and which make it possible successively to subject each adsorber 7A, 7B to the flow of air to be distilled and to a regeneration gas of the adsorbent;
• a lost air turbine 27;
• a cold compressor 3;
• a so-called Claude turbine 13 sending air to the medium pressure column;
• a main heat exchange line 8;
• an air distillation apparatus in the form of a double column 10 comprising a medium pressure column 12, a low pressure column 14 and a vaporizer-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 the figure 1 is the next.

The air to be distilled 23, previously compressed by the compressor 4, is purified by one of the adsorbers 7A, 7B of the apparatus 6, and then cooled by the main heat exchange line 8 to an intermediate temperature. The adsorption may be of the TSA, PSA or TPSA type. Part 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 cooling. Another portion 29 of the air is sent to the cold compressor 3 and then returned to the exchange line 8. Part of the supercharged flow is expanded in a turbine 5 to the medium pressure to form the expanded flow 7. The flow the rest of the pressurized air 9 continues to cool in the exchange line 8, is expanded in a valve V and is then sent to a pressure vessel 8 in the vicinity of its dew point. intermediate level of the medium pressure column 12.

The vaporizer-condenser 16 vaporizes liquid oxygen, for example having a purity of 99.5%, of the tank of the low pressure column 14, by condensing nitrogen gas at the top of the medium pressure column 12.

"Rich liquid" LR (air enriched with oxygen), taken from the bottom of the medium pressure column 12, is injected, after expansion, to an intermediate level of the low pressure column 14, while liquid nitrogen NL, substantially pure, is taken at the top of the medium pressure column 12 to feed the reservoir 22 and the head of the low pressure column 14. Liquid nitrogen and / or liquid oxygen is produced as the final product, sent to the customer under liquid form.

Impure nitrogen or "waste" NR, withdrawn from the top of the low pressure column 14, is returned to the main heat exchange line 8, where it causes cooling of the air to be distilled.

OL liquid oxygen is withdrawn from the tank of the low pressure column 14 and feeds the storage tank 18. After pressurization in the pump P, it vaporizes in the main heat exchange line 8 and distributed by a pipe of production 32 to form gaseous oxygen under pressure.

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

The operation of the installation which has just been described can be implemented continuously, with the exception of the operation of the purification apparatus 6, which follows in time a pressure cycle of the figure 2 . However it is possible that not all fluids are produced continuously, depending on the customer's needs, the cost of electricity etc.

The cycle of figure 2 , the period of which is, for example, equal to approximately 360 minutes for an adsorption pressure substantially equal to 20 bars, comprises 4 successive stages I to IV.

au moyen de vannes de raccordement ouvertes ou fermées désignées par les mêmes références à venir que celles de l'adsorbeur 7A, la lettre A étant à remplacer par la lettre B et l'état de chaque vanne (ouverte/fermée) étant à inverser (fermée/ouverte).These four steps will now be described successively for the adsorber 7A, it being understood that the adsorber 7B follows these same steps with a time lag substantially equal to $\frac{\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T</figure-callout>}}{\mathrm{2}},$

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

l'adsorbeur 7A est en phase d'adsorption sous une pression de fonctionnement haute notée P_ads, tandis que l'adsorbeur 7B est en phase de régénération. L'air comprimé par le compresseur 4 alimente l'adsorbeur 7A, via une vanne 40A ouverte. La sortie de l'adsorbeur 7A est reliée à la ligne d'échange 8, via une vanne 42A ouverte.During step I, ie from t = 0 to $\underset{}{\mathrm{t}}=\frac{\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T</figure-callout>}}{\mathrm{2}},$

the adsorber 7A is in the adsorption phase under a high operating pressure marked P _ads , while the adsorber 7B is in the regeneration phase. The compressed air 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 stages II, III and IV, the adsorber 7A is in the regeneration phase, while the adsorber 7B is in the adsorption phase. More specifically, during stage II, a valve 44A for venting the adsorber 7A is opened so that the pressure inside the bottle of the adsorber 7A is reduced to a pressure substantially equal to the atmospheric pressure, noted P _atmo on the figure 2 .

In step III, the valve 44A remains open and residual nitrogen NR withdrawn at the top of the low pressure column 14 and then heated in the exchanger 8 feeds via an open valve 46A, the adsorber 7A to circulate there against a current. This is the actual phase of regeneration during which impurities are desorbed and the beds regenerated. In step IV, the valves 44A and 46A are closed, to allow the pressurization of the adsorber. In a first step, that is to say during a first sub-step IV ', the pressurization of the adsorber is ensured by a stream of purified air, via the open valve 42A, this air flow purified from bottles 7A, 7B. Sub-step IV 'is continued by 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 pressurization of each adsorber no longer requires, during step IV, to increase the flow rate of the compressor 4. In this way, the compressor 4 is optimally sized, that is, - say so that its nominal flow is substantially constant. The investment and operating costs of this compression apparatus are reduced, compared with those of prior art installations.

During the adsorption phase, the compressor 4 compresses 100 kNm ³ / h of air and all the purified air is sent to the exchange line 8. 30 kNm ³ / h of air are sent to the turbine. Air lost 5. 70 kNm ³ / h of air are sent to the system of distillation columns.

During the pressurization phase at the end of the regeneration phase, the compressor 4 compresses 100 kNm ³ / h of air, 95 kNm ³ / h are sent to the exchange line 8 and 5 kNm ³ / h are sent to pressurize an adsorption bottle. 25 kNm ³ / h of air (thus 5 kNm ³ / h less) are sent to the lost air turbine 5 and 70 kNm ³ / h of air are always sent to the distillation column system.

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

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

Claims (13)

1. 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 (4), purified by means of two adsorbers (7A, 7B) which each follow, offset, a cycle with in succession 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, purified air is cooled in an exchange line (8) and is then sent to a distillation column (12) of a system of columns and oxygen- and nitrogen-rich fluids are extracted from a column (14) of the system of columns, only during the repressurisation phase a purified airflow, constituting between 3 and 20% of the compressed air in the compressor, used to at least partially pressurise the adsorber completing the regeneration phase thereof and the airflow compressed in the compressor during the adsorption phase is substantially equal to the compressed airflow in the compressor during the pressurisation of the adsorber, characterised in that a portion of the purified air is sent to a turbine (27) where it is decompressed and then sent into the atmosphere so as to at least partially ensure the maintaining cold of the method during the entire cycle and in that the amount of decompressed airflow sent into the air during the pressurisation of an adsorber is less than the amount sent into the air during the adsorption phase of the same adsorber.
2. Method according to claim 1 wherein the amount of decompressed airflow sent into the air during the pressurisation of an adsorber is less than that sent into the air during the rest of the cycle outside of the pressurisation phase.
3. Method according to claim 1 or 2 wherein the airflow compressed in the compressor (4) during the adsorption phase of an adsorber is equal to the compressed airflow in the compressor during the pressurisation of the adsorber.
4. Method according to claim 1 or 2 or 3 wherein the reduction in the airflow sent to the turbine (27) and then into the air during the repressurisation is equal to the airflow used during repressurisation in order to pressurise the adsorber complete the repressurisation phase thereof.
5. Method according to claim 4 wherein the quantity of air sent to distillation is constant throughout the entire cycle.
6. Method according to one of claims 1 or 2 or 3 wherein the reduction in the airflow sent to the turbine (27) and then into the air during the pressurisation of an adsorber is less than the airflow used during the repressurisation to pressurise the adsorber completing the repressurisation phase thereof.
7. Method according to claim 6 when the latter does not depend on claim 3 wherein during the repressurisation phase the airflow compressed in the compressor (4) increases in relation to the flow sent during the rest of the cycle and the quantity of air sent to distillation remains equal to that sent during the rest of the cycle.
8. Method according to one of the preceding claims wherein a liquid flow (22) is produced as a final product.
9. Method according to one of the preceding claims wherein the method of purification is an adsorption of the PSA, TSA or TPSA type.
10. Method according to one of the preceding claims wherein air is decompressed in a turbine (5) and sent to a column of the system of columns.
11. Method according to one of the preceding claims wherein the amount of decompressed airflow sent into the air during the pressurisation of an adsorber is less than that sent to the air during the rest of the cycle outside any phase of pressurisation.
12. Method according to one of the preceding claims wherein the system of columns is comprised of a double column (10) comprising an average pressure column (12) and a low pressure column (14).
13. Method according to claim 12 wherein an oxygen-rich flow is extracted from the low pressure column (14) and it is vaporised in the exchange line (8).

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