EP0789208A1 - Process and installation for the production of gaseous oxygen under high pressure - Google Patents

Abstract

A process for producing a gas at a high pressure of at least 30 bars, using an air distillation unit (1) comprising a low pressure column (11) and a medium pressure column (10) where fractions of the air are delivered to the columns at different pressures after cooling, comprises dividing the air to be distillated into three streams: (i) a first medium pressure stream cooled to its dew point and delivered in the medium pressure column; (ii) a high pressure stream above at least 60 bars, being cooled and liquified (in 17), and after expansion (in 18), delivered in the double column (1); and (iii) an intermediate pressure stream, at least part of that stream being at an intermediate cooling temperature, expanded to medium pressure in a turbine (6) before being directed to the medium pressure column (10). The intermediate pressure is chosen such that the air is near its dew point before entering the turbine. Also claimed is apparatus for producing purified and pressurised gas.

Description

The present invention relates to a process for producing a gas at a high pressure of at least about 30 bars, of the type in which: air is distilled in a double column installation comprising a distillation column which operates under low pressure and a column which operates under medium pressure; pumping a liquid withdrawn from a column of the installation; the compressed liquid is vaporized, by heat exchange, in a heat exchanger of the brazed plate type, with the air being cooled and / or liquefied; and at least one liquid product is withdrawn from the installation.

The invention applies in particular to the production of large quantities, typically of the order of at least 500 tonnes per day, of gaseous oxygen under high pressure.

The pressures in question are absolute pressures.

Many methods of the aforementioned type, called "pump methods", have been proposed, and the invention aims to provide a method of the same type which is particularly advantageous from the point of view of the specific energy expenditure.

• un premier flux d'air sous la moyenne pression, qui est refroidi jusqu'au voisinage de son point de rosée puis introduit dans la colonne moyenne pression;
• un deuxième flux d'air sous une haute pression supérieure à 60 bars environ, ce deuxième flux d'air étant refroidi et liquéfié puis, après détente, introduit dans la double colonne; et
• un troisième flux d'air sous une pression intermédiaire, une partie au moins de ce troisième flux d'air étant, à une température intermédiaire de refroidissement, détendu à la moyenne pression dans une turbine avant d'être introduit dans la colonne moyenne pression, la pression intermédiaire étant choisie de façon que l'air se trouve au voisinage de son point de rosée à l'entrée de la roue de la turbine.

To this end, the subject of the invention is a process of the aforementioned type, characterized in that the air to be distilled is divided into three streams:

• a first air flow under medium pressure, which is cooled to the vicinity of its dew point and then introduced into the medium pressure column;
• a second air flow under high pressure greater than approximately 60 bars, this second air flow being cooled and liquefied then, after expansion, introduced into the double column; and
• a third air flow under an intermediate pressure, at least part of this third air flow being, at an intermediate cooling temperature, expanded at medium pressure in a turbine before being introduced into the medium pressure column, the intermediate pressure being chosen so that the air is in the vicinity of its dew point at the inlet of the turbine wheel.

• ledit produit liquide est au moins en partie de l'argon liquide produit à partir d'une colonne additionnelle de séparation oxygène/argon couplée à la double colonne;
• la totalité dudit produit liquide est constituée d'argon liquide;
• on assure la compression dudit deuxième flux d'air de la pression intermédiaire à la haute pression uniquement au moyen de l'énergie mécanique fournie par la turbine;
• ladite température intermédiaire est voisine de la température de vaporisation de l'oxygène sous la haute pression d'oxygène;
• la haute pression d'oxygène est voisine de 40 bars, et le débit de produit liquide soutiré de l'installation est sensiblement défini par : $${\text{D}}_{\text{L}}\text{= -0,22 P + 22,}$$
  
  où D_L est, en %, le rapport du débit de produit liquide soutiré au débit total d'oxygène produit, et où P est la haute pression d'air en bars absolus;
• le débit de produit liquide soutiré est compris entre 2 et 12% environ du débit total d'oxygène produit;
• lesdits deuxième et troisième flux d'air représentent respectivement environ 20 à 25% et environ 10 à 30% du débit total d'air à distiller.

This process can include one or more of the following characteristics:

• said liquid product is at least in part liquid argon produced from an additional oxygen / argon separation column coupled to the double column;
• all of said liquid product consists of liquid argon;
• compressing said second air flow from intermediate pressure to high pressure only by means of mechanical energy supplied by the turbine;
• said intermediate temperature is close to the vaporization temperature of oxygen under the high oxygen pressure;
• the high oxygen pressure is close to 40 bars, and the flow rate of liquid product withdrawn from the installation is substantially defined by: $${\text{D}}_{\text{L}}\text{= -0.22 P + 22,}$$
  
  where D _L is, in%, the ratio of the flow rate of liquid product withdrawn to the total flow rate of oxygen produced, and where P is the high air pressure in absolute bars;
• the flow rate of liquid product withdrawn is between 2 and 12% approximately of the total flow rate of oxygen produced;
• said second and third air flows represent respectively about 20 to 25% and about 10 to 30% of the total air flow to be distilled.

The invention also relates to an installation intended for the implementation of a method as defined above. This installation for producing gaseous oxygen under a high oxygen pressure of at least about 30 bars, of the type comprising: a double air distillation column comprising a column operating under a low pressure and a column operating under an average pressure; a liquid compression pump withdrawn from a column of the installation; means for compressing the incoming air; a heat exchanger of the brazed plate type for bringing the air to be distilled into a heat exchange relationship with the compressed liquid; and a line for withdrawing at least one liquid product from the installation, is characterized in that the compression means comprise means for creating three air flows, respectively at medium pressure, at intermediate pressure and at high air pressure; in that the heat exchanger comprises passages for cooling the medium-pressure air from its hot end to its cold end, passages for partially cooling the air under the intermediate pressure, and passages for cooling the high pressure air from its hot end to its cold outlet; and in that the installation comprises an expansion turbine at medium pressure of at least part of the air under the partially cooled intermediate pressure, as well as a column for producing liquid argon coupled to the double column.

In one embodiment of this installation, the installation includes an additional heat exchanger for sub-cooling the liquid withdrawn from the bottom of the medium pressure column by vaporization of liquid oxygen withdrawn from the bottom of the low pressure column.

• la Figure 1 représente schématiquement une installation de production d'oxygène gazeux conforme à l'invention;
• la Figure 2 est un diagramme d'échange thermique correspondant;
• la Figure 3 est un diagramme qui montre la variation de la production d'oxygène liquide de l'installation en fonction de la haute pression d'oxygène, à l'optimum économique; et
• la Figure 4 représente schématiquement une variante de l'installation de la Figure 1.

Examples of implementation of the invention will now be described with reference to the accompanying drawings, in which:

• Figure 1 schematically shows an installation for producing gaseous oxygen according to the invention;
• Figure 2 is a corresponding heat exchange diagram;
• FIG. 3 is a diagram which shows the variation in the production of liquid oxygen of the installation as a function of the high oxygen pressure, at the economic optimum; and
• Figure 4 schematically represents a variant of the installation of Figure 1.

The installation shown in Figure 1 is intended to produce gaseous oxygen at a pressure at least equal to around 30 bars. It essentially comprises a double distillation column 1, a main heat exchange line 2 consisting of at least one exchanger body of the brazed plate type, a sub-cooler 3, an air compressor 4, an apparatus 5 purification by adsorption of air into water and CO2, a first air booster 6 a second air booster 7, an expansion turbine 8 and a liquid oxygen pump 9. The double column is made up, conventionally, a medium pressure column 10 operating at about 5 to 6 bars and surmounted by a low pressure column 11 operating slightly above atmospheric pressure, with, in the tank of the latter, a vaporizer-condenser 12 which puts in relation of heat exchange liquid oxygen from the bottom of the low pressure column with nitrogen from the top of the medium pressure column.

In operation, the air to be distilled, fully compressed by the compressor 4 at medium pressure and purified at 5, is divided into two streams.

The first stream is cooled under this medium pressure in passages 13 of the exchange line 20 which extend from the hot end to the cold end of the latter. This medium pressure air emerges from the exchange line near its dew point and is introduced at the base of the medium pressure column 10.

The rest of the air leaving the device 5 is boosted at 6 to an intermediate pressure and is in turn divided into two streams.

The first flow, at this intermediate pressure, is cooled in passages 14 of the exchange line to an intermediate temperature T1. Part of this flow eventually continues to cool, and is liquefied, until the cold end of the exchange line, then is expanded at medium pressure in an expansion valve 15 and divided into two streams: a first stream sent to the base of the column 10, and a second sub-cooled stream at 3, expanded at low pressure in an expansion valve 16 and sent to the column 11. The rest of the first stream left the exchange line at the intermediate temperature T1, expanded in the turbine 6 at medium pressure and introduced at the base of the column 10.

The second flow of supercharged air is again supercharged, up to a second high pressure of the order of 60 to 80 bars, by the supercharger 7, then cooled and liquefied in passages 17 of the exchange line, until 'at the cold end of it. The liquid thus obtained is expanded in an expansion valve 18 and combined with the liquefied current coming from the expansion valve 15.

Liquid oxygen withdrawn from the tank of column 11 is brought by pump 9 to the desired high production pressure, then vaporized and heated in passages 18 of the exchange line before being evacuated from the installation via a production line 19.

In the installation in FIG. 1, we also find the usual pipes and accessories for double column installations: a pipe 20 for raising in column 11 of the "rich liquid" (oxygen-enriched air) collected in the tank of column 10 , with its expansion valve 21, a pipe 22 for raising to the top of column 11 of the "lean liquid" (almost pure nitrogen) withdrawn at the top of column 10, with its expansion valve 23, as well as the pipes following: a line 24 for producing liquid oxygen, inserted in the bottom of the column 11, a line 25 for producing liquid nitrogen, inserted in the line 22 and provided with an expansion valve 26, and a line 27 of drawing off impure nitrogen, constituting the waste gas from the installation, tapped at the head of column 11. This impure nitrogen is heated in the sub-cooler 3 then in passages 28 of the exchange line before being evacuated via a pipe 29. In the sub-cooled sseur 3, the liquid air coming from the valves 15 and 18, the lean liquid and the rich liquid are sub-cooled, by about 2 ° C. for the rich liquid.

To obtain a specific energy expenditure (the specific energy is the energy necessary to produce a unit quantity of gaseous oxygen under high pressure) as low as possible, the exchange diagram must be heat exchange line 2 is as tight as possible, this in order to approach reversible heat exchange conditions. In particular, it is necessary, on the diagram of Figure 2, where the enthalpies H are plotted on the abscissa and the temperatures on the ordinate, that the temperature differences between the air being cooled (curve C1) and the products being heating (curve C2) are as low as possible at the hot end and at the cold end of the exchange line as well as at the start of the oxygen vaporization level 30.

• La haute pression d'air est choisie aussi élevée que possible compte-tenu de la technologie de réalisation de l'échangeur 2 à plaques brasées. Cette haute pression est typiquement comprise entre 60 et 80 bars environ.
• La température intermédiaire T1, qui est la température d'admission de la turbine 8, est voisine de la température de vaporisation de l'oxygène, et de préférence supérieure de 1°C à cette température de vaporisation.
• La pression intermédiaire est choisie de façon que l'air turbiné soit au voisinage de son point de rosée à l'entrée de la roue de la turbine.

We were able to obtain an average temperature difference close to 5 ° C, with a minimum temperature difference of 1.5 ° C at the start of stage 30, from simulation calculations, under the following conditions:

• The high air pressure is chosen to be as high as possible, taking into account the technology for producing the heat exchanger 2 with brazed plates. This high pressure is typically between approximately 60 and 80 bars.
• The intermediate temperature T1, which is the inlet temperature of the turbine 8, is close to the vaporization temperature of the oxygen, and preferably 1 ° C. above this vaporization temperature.
• The intermediate pressure is chosen so that the turbined air is in the vicinity of its dew point at the inlet of the turbine wheel.

As is well known, cryogenic turbines have an inlet distributor followed by a wheel. The distributor produces a first enthalpy expansion or fall, which is a characteristic of the turbine. The third condition above therefore makes it easy to determine the intermediate pressure, which is the pressure at which air must enter the turbine to be at the near its dew point at the entrance of the wheel. This intermediate pressure is between 30 and 40 bars approximately.

D_L étant, en %, le rapport du débit d'oxygène liquide soutiré au débit total d'oxygène produit.In addition, a certain flow of liquid must be drawn off at 24. This liquid reduces the quantity of products to be heated in the heat exchange line all the more, and its flow is a function of both the high oxygen pressure and high air pressure. Figure 3, established for a high oxygen pressure of 40 bars, shows that the liquid flow rate leading to the economic optimum decreases appreciably linearly when the high air pressure P varies by a value slightly greater than 60 bars up to '' at 80 bars, according to a law of the type: $${\text{D}}_{\text{L}}\text{= -0.22 P + 22,}$$

D _L being, in%, the ratio of the flow of liquid oxygen withdrawn to the total flow of oxygen produced.

As can be seen, this flow rate D _L could be canceled if it were possible to choose a high air pressure clearly greater than 80 bars and, according to the calculation, of the order of 100 bars.

In the example described above, the mechanical energy produced by the turbine 8 is recovered to contribute to the drive of the booster 7, but the latter also has an external source of drive energy. If one wishes, as a variant, to couple the turbine 8 and this booster, to simplify the installation, it is necessary to increase the intermediate pressure as well as the temperature T1, and the calculation shows that this leads to an increase in the flow rate D _{L as} well as specific energy.

By way of example, the air flows at the intermediate pressure and at the high pressure may represent approximately 20% and approximately 25%, respectively, of the flow of treated air.

Returning to FIG. 3, it can be seen that, when oxygen is produced at 40 bars, the flow rate D _L is of the order of 4.5% when the high air pressure is around 80 bars. However, this percentage is the ratio of argon to oxygen in atmospheric air. Consequently, by adding to the double column an additional column 31 of argon / oxygen separation followed by means 31A for elimination of the last traces of oxygen then by means 31B for denitrogenation, as represented in FIG. 4, the withdrawal of liquid product. necessary to reach the economic optimum can be constituted solely by the production of pure liquid argon from the installation.

This is of particular interest since the process described above, due to the relative complexity of the installation, is above all suitable for use in high capacity installations, in which the specific energy is the most important parameter. , and these installations are precisely those which justify the addition of an argon production column.

Conventionally, in the diagram of Figure 4, the tank of the column 31 is connected to the "argon tapping" of the column 11 via two supply and return lines 32, while its head is equipped with a condenser 34 in which rich liquid, expanded at 35 to near atmospheric pressure, is vaporized and then returned to column 11 via a line 36. The impure gaseous argon withdrawn at the top of the column 31 via a line 37 is purified in 31A then 31B, and the pure argon is withdrawn from the installation in liquid form via a production pipe 37A.

As a variant, as indicated in FIG. 4, the sub-cooling of the rich liquid before its expansion in 21 and possibly in 35, can be produced in an additional heat exchanger 38 vaporizing liquid oxygen withdrawn from the tank of column 11. This makes it possible to sub-cool from 4 to 5 ° C. the large quantities of rich liquid which circulate during the use of 'a "pump" process and, consequently, to improve the extraction yield of oxygen and, if necessary, of argon.

Alternatively also, as indicated by dotted lines in Figures 1 and 4, the installation can also produce nitrogen gas under pressure, this nitrogen being taken in the liquid state in line 22, pumped at the desired pressure by a pump 39, vaporized and then reheated in passages 40 of the exchange line 2, and withdrawn via a production line 41.

It is understood that, in the process of the invention, all or part of the liquid withdrawn can also consist of liquid nitrogen (line 25).

The liquid vaporized after pumping can be oxygen, nitrogen or argon.

Claims (13)

Method for producing a gas at a high pressure of at least about 30 bars, of the type in which: air is distilled in a double distillation column installation (1) comprising a column (11) which operates under a low pressure and a column (10) which operates under medium pressure; pumping (at 9) a liquid withdrawn from a column of the installation (11); the compressed liquid is vaporized, by heat exchange, in a heat exchanger (2) of the brazed plate type, with the air being cooled and / or liquefied; and at least one liquid product is withdrawn from the installation (at 24, 25; 37A), characterized in that the air to be distilled is divided into three flows: - A first air flow under medium pressure, which is cooled to the vicinity of its dew point and then introduced into the medium pressure column (10); - A second air flow under a high pressure greater than about 60 bars, this second air flow being cooled and liquefied (in 17) then, after expansion (in 18), introduced into the double column (1); and a third air flow under an intermediate pressure, at least part of this third air flow being, at an intermediate cooling temperature, expanded at medium pressure in a turbine (6) before being introduced into the medium pressure column (10), the intermediate pressure being chosen so that the air is in the vicinity of its dew point at the inlet of the turbine wheel. Method according to claim 1, characterized in that said liquid product is at less in part of the liquid argon produced from an additional column (31) of oxygen / argon separation coupled to the double column (1). Method according to claim 2, characterized in that the whole of said liquid product consists of liquid argon. Method according to any one of Claims 1 to 3, characterized in that the compression of the said second air flow from the intermediate pressure to the high pressure is ensured only by means of the mechanical energy supplied by the turbine (8) . Process according to any one of Claims 1 to 3, characterized in that the said intermediate temperature is close to the vaporization temperature of the liquid under the high pressure. Process according to any one of Claims 1 to 3 or 5, characterized in that the high pressure is close to 40 bars, and in that the flow rate of liquid product withdrawn from the installation is substantially defined by: $${\text{D}}_{\text{L}}\text{= -0.22 P + 22,}$$

where D _L is, in%, the ratio of the flow rate of liquid product withdrawn to the total flow rate of oxygen produced, and where P is the high air pressure in absolute bars. Process according to any one of Claims 1 to 6, characterized in that the flow rate of liquid product withdrawn is between 2 and 12% approximately of the total flow rate of oxygen produced. Process according to any one of Claims 1 to 7, characterized in that the said second and third air flows represent approximately 20 to 25% and approximately 10 to 30% of the total air flow to be distilled, respectively. Method according to one of the preceding claims, in which the vaporized liquid is oxygen, nitrogen or argon. Installation for producing a gas under a high pressure of at least about 30 bars, of the type comprising a double air distillation column (1) comprising: a column operating under a low pressure (11) and a column operating under medium pressure (10); a pump (9) for compressing a liquid withdrawn from a column of the installation (11); means for compressing (14, 30, 31) the incoming air; a heat exchanger (2) of the brazed plate type for bringing the air to be distilled and the compressed liquid into heat exchange relation; and a pipe (24, 25; 37A) for withdrawing at least one liquid product from the installation, characterized in that the compression means comprise means (4, 6, 7) for creating three air flows, medium pressure, intermediate pressure and high air pressure, respectively; in that the heat exchanger (2) comprises passages (13) for cooling the medium pressure air from its hot end to its cold end, passages (14) for partial cooling of the air under the intermediate pressure , and passages (17) for cooling the high pressure air from its hot end to its cold end; and in that the installation comprises a turbine (8) for expansion at medium pressure of at least part of the air under the partially cooled intermediate pressure, as well as a column (31) for producing liquid argon coupled to the double column (1). Installation according to claim 10, characterized in that it comprises an additional heat exchanger (38) for sub-cooling the liquid withdrawn from the tank of the medium pressure column (10) by vaporization of the liquid withdrawn from the column (11). Method for producing a gas under high pressure of the type in which: air is distilled in a double-distillation plant (1) comprising a column (11) which operates under low pressure and a column (10 ) which operates at medium pressure in which part of the air is expanded to medium pressure in a turbine (6) before being introduced into the medium pressure column, the air pressure being chosen so that l air is near its dew point at the entrance to the turbine wheel. The method of claim 12, wherein the air to be expanded in the turbine is cooled before it is expanded by gas from the double column, optionally after a pressurization and vaporization step.

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