WO2003013700A1

WO2003013700A1 - Method and device for the production of dry compressed gas

Info

Publication number: WO2003013700A1
Application number: PCT/FR2001/002577
Authority: WO
Inventors: Jacques Jacob
Original assignee: Cami Haute Technologie
Priority date: 2001-08-08
Filing date: 2001-08-08
Publication date: 2003-02-20
Also published as: EP1425082A1

Abstract

The invention relates to a method of producing a dry compressed gas or gas mixture. The inventive method is characterised in that it comprises the following steps: the wet gas mixture or gas is compressed; the wet compressed gas mixture or gas is made to flow from the inlet of at least one hollow tube having a wall made from a water vapour-permeable membrane to the tube outlet, said hollow tube being disposed inside a housing and the water vapour passing through the membrane from the inside of the tube to the outside thereof in order to collect in the housing; the dry compressed gas mixture or gas is recovered at the tube outlet; the water vapour that collects in the housing is periodically flushed by forcing part of the dry gas mixture or gas to flow from inside the housing to the outside thereof and outside the tube; after drying, the compressed gas mixture or gas is filtered in order to remove the carbon and sulphur oxides; the carbon dioxide filter of the filter is regenerated periodically by means of reverse scavenging with a neutral gas.

Description

PROCESS AND DEVICE FOR THE MANUFACTURE OF DRY COMPRESSED GAS

The present invention relates to a process for manufacturing dry compressed gas, in particular dry compressed air, and the device for implementing such a process. Dry compressed air can be used for human respiration.

Devices are known for compressing air at pressures varying from 3 to 15 bar. This compressed air is then dried for industrial use.

In a known manner, a compressed gas or compressed air is dried by one of the following methods:

The refrigeration drying process involves cooling the gas or air containing water vapor to a temperature at which the water can condense. In this continuous process, the condensed water is then removed from the system in liquid form. By this process, the dew point that can be obtained is 4 ° C. The device necessary to carry out this process on an industrial scale is very bulky and consumes a lot of energy. In addition, this process does not make it possible to obtain air having dew points below 0 ° C.

Another method used is the adsorption method. To do this, certain types of zeolites or other adsorbents are used, such as silica gel or activated alumina, which are materials in the form of fine particles and having a large active surface.

According to this process, the gas, the gas mixture or the air which must be dried, is introduced into a first column or a tower containing the adsorbent material. The water in the air is adsorbed and the material becomes charged with water vapor. At the same time, a second column of the same type is regenerated. Pipes, valves and control systems allow the circulation between the two columns to be interrupted or resumed. Then, we repeat the process. Because two pressure columns, valves and control systems are used, adsorption dryers are complex and expensive. The regeneration of the adsorbent material can be done in several ways. A first method consists in increasing the temperature of the adsorbent bed, during the regeneration phase, so as to desorb the humidity. Another method for regenerating the adsorbent material is to subject it to an abrupt low pressure during the regeneration phase.

Dew points can be obtained with these processes under pressure from -20 ° C to -40 ° C, with zeolites or other adsorbents. However, this results in energy losses.

The air can also be dried by cooling it by circulation in a refrigeration circuit, for example using Freon or C.F.C.

Thus, the methods of drying compressed gas, mixing compressed gases or compressed air, of the prior art, have many drawbacks. In addition, dry compressed air is commonly used in hospitals, clinics, laboratories, etc., in particular for human respiration. This compressed air is obtained by mixing oxygen and liquid nitrogen under pressure, in proportions close to those of air 78/22. However, it is not a natural mixture and therefore the proportions of the various constituents of the air are not respected. In addition, to make the mixture, it is necessary to have special and expensive facilities.

It is known from patent FR 2 751 894 of which the applicant is the holder, a process for manufacturing dry compressed gas, in particular dry compressed air, which overcomes the drawbacks presented above. In this patent, the filtration of carbon dioxide is sufficient but the filter is quickly saturated with carbon dioxide and must therefore be removed from the installation for cleaning.

The invention aims to overcome these drawbacks. A first object of the invention is to provide a simple method for obtaining dry compressed air, in which it is not necessary to remove the carbon dioxide filter to clean it.

A second object of the invention is to provide a compressed air drying device which does not require periodic maintenance either. A third object of the invention is to provide a device for drying compressed air which does not require refrigerating gas such as Freon or C.F.C.

A fourth object of the invention is to provide a device having no electrical connection, which makes it possible to use it in an explosion-proof zone.

Another object of the invention is to provide a method of drying the compressed air which gives immediate obtaining of the dew point.

A final object of the invention is to provide dry compressed air which can be used for human respiration. To this end, the invention relates to a method for manufacturing a gas or mixture of gases, compressed, dry, characterized in that the following steps are carried out:

- the gas or mixture of wet gases is compressed,

the gas or mixture of gases, compressed and wet, is circulated from the inlet of at least one hollow tube, the wall of which is made of a membrane permeable to water vapor, towards the outlet of the tube, the hollow tube being placed in an enclosure, the water vapor passing through the membrane from the inside of the tube towards the outside of the tube, to accumulate in the enclosure, - the gas or mixture of compressed and dry gas is recovered at the outlet of the tube,

- the water vapor accumulated in the enclosure is periodically removed by circulating part of the dry gas or mixture of gas from inside the enclosure to the outside, and outside the tube,

- the compressed gas or mixture of gases is filtered, after drying, to remove the oxides of carbon and sulfur,

- the carbon dioxide filter of the filter is periodically regenerated by sweeping it against the current with a neutral gas. According to another particular feature, the compressed gas or mixture of gases is filtered, before drying, to remove the dust.

According to another particular feature, the compressed gas or mixture of gases is sent, after drying, to a pressure tank and the mixture is homogenized. In another feature, the compressed gas is air.

According to another particular feature, after removing the dust and drying, the air is passed through an ozonator to form ozone.

The second, third and fourth objects are achieved by a device for implementing the method according to one of the preceding claims, characterized in that it comprises a drying chamber in which parallel hollow tubes are placed, the wall tubes consisting of a membrane permeable to water vapor, a wet compressed air inlet at one end of the tubes, a dry compressed air outlet at the opposite end of the tubes, a dry air inlet in the enclosure, the dry air circulating in the enclosure, outside the tubes, a humid air outlet from the drying enclosure, traps of oxides and dioxides of carbon and sulfur, comprising in particular d a carbon dioxide retention filter and means for periodically removing carbon dioxide from the retention filter. According to another particular feature, the means for periodically removing carbon dioxide from the filter consist of counter-current sweeping means of the filter using a neutral gas.

According to another particular feature, the device comprises dust filters, possibly an oil filter, placed upstream of the enclosure, and an antibacterial filter placed downstream of the drying enclosure.

According to another particular feature, the sulfur and carbon oxide and dioxid traps comprise a catalyst filter transforming carbon monoxide into carbon dioxide placed upstream of the carbon dioxide retention filter.

According to another particularity, the catalyst and retention filters consist of a container closed at one end by a perforated grid and a plug, the plug having two orifices and a fixing hole on the associated filter caps, the second end of the container being closed by a perforated grid.

According to another particular feature, the catalyst filter is filled with granules composed of manganese oxide, copper oxide and manganese carbonate and having a particle size between 1 and 3.5 mm. According to another particular feature, the retention filter is filled with a molecular sieve of synthetic zeolites of crystal structure of type X, comprising a network of pores with effective diameter of 10 angstroms and having a particle size between 1, 6 and 2.5 mm.

According to another particular feature, the scanning means consist of a neutral gas inlet pipe into the retention filter, the neutral gas arriving through the pipe and sweeping the screen against the current to drive out the carbon dioxide.

According to another particular feature, the neutral gas is dry compressed air.

According to another particular feature, the neutral gas is dioxygen. According to another particularity, the dry compressed air is that manufactured by the device and used for a hygrometry test.

According to another particular feature, the device comprises an air homogenizer, placed downstream of the drying enclosure. According to another particular feature, the device comprises one or more air compressors, upstream of the dust filters operating without lubricant.

The latter object is achieved by dry compressed air, obtained by the process described above. This air is characterized by the fact that its pressure is between 8 and 12 bar, its oil content is less than 0.01 mg / m ³ , its water vapor content is 0.09 g / m ³ , = - 42 ° C, its CO content is less than 2 ppm, its C0 ₂ content is less than 300 ppm, its S0 ₂ content is less than 16.10 ^ ppm and its monoxide / nitrogen dioxide content is lower at 25.5.10 ³ ppm. According to the invention, this dry compressed air obtained can be used in the food industry and in the field of civil security.

The following description, with reference to the accompanying drawings by way of nonlimiting examples, will make it possible to understand how the invention can be put into practice. Figure 1 is a schematic view of the enclosure comprising the hollow tube or tubes.

Figure 2 is a schematic view of a first embodiment of the device according to the invention.

Figure 3 is a schematic view of a second embodiment of the device according to the invention.

Figure 4 is a schematic view of a third embodiment of the invention.

Figure 5 is a front view of the device according to the invention, according to the second embodiment. Figure 6 is a top view of the device according to the invention, according to the second embodiment.

Figure 7 is a sectional view of a filter.

Figure 8 is a sectional view of a carbon dioxide retention filter.

Figure 9 shows the direction of air passage through the filters.

In Figure 1, there is shown an enclosure (1) in which is placed one or more hollow tubes (2). These hollow tubes (2) or hollow fibers have a wall which is a membrane permeable to water vapor. The tubes (2) are placed parallel to each other. The enclosure (1) comprises a supply line (3) which is connected to the ends (4) of the tubes (2) and an outlet line (5) which is connected to the opposite ends (6) of the tubes (2) . The outlet pipe (5) comprises a branch (7) of reduced size compared to the outlet pipe and which opens into the enclosure (1), outside the tubes or fibers (2). Finally, the enclosure (1) has a lateral outlet (8).

The enclosure (1) shown in Figure 1 is a membrane dryer, the operation of which is as follows:

The compressed gas, or the compressed gas mixture enters through the supply line (3), into the tubes (2). Preferably, the gas mixture is compressed air. This compressed air can for example be at a pressure of between approximately 8 and approximately 12 bar, preferably between 9 and

11 bar.

Compressed air enters the supply line (3) in a direction according to Arrow F1. Compressed air enters the tubes (2) through their ends (4) which are connected to the supply line (3). Compressed air moves from the ends (4) of the tubes (2) to the opposite ends (6) of the tubes (2). During the movement of the compressed air, the water molecules pass through the walls of the tubes (2), these walls being permeable to water vapor and impermeable to other gases. Water vapor is therefore stored inside (9) of the enclosure (1).

The dry compressed air exits through the outlet pipe (5), in the direction of Arrow F2. When the interior (9) of the enclosure (1) is saturated with water vapor, part of the dry air bypassed, through the outlet pipe (5), and the bypass (7), at the inside (9) of the enclosure (1), in the direction of the arrow F3 evacuates the water vapor.

The dry air inside (9) of the enclosure (1), but outside the tubes (2), saturates with water vapor and exits through the lateral pipe (8). Because the walls of the tubes are permeable in one direction to water vapor, that is to say from the inside of the tubes (2), towards the outside of the tubes (2), but impermeable in the opposite direction, that is to say from the outside of the tubes (2), towards the inside, the passage of dry air in the enclosure (9), makes it possible to evacuate the water vapor or the water inside (9) of the enclosure.

FIG. 2 shows an embodiment of the invention, this embodiment being the simplest. The device shown schematically in Figure 2 can be used, for example in a hospital or clinic which is already equipped with an air compressor. Two cases can arise: the compressor in the hospital or clinic is an oil compressor or else a compressor of another kind.

When the compressor is an oil compressor, it supplies compressed air comprising oil. This is the reason why, before reaching the enclosure (1), the compressed air passes through an oil separator (10).

When the compressor is of a different kind than an oil compressor, the compressed air comprising no oil, the device according to the invention does not comprise an oil separator. The device according to the invention comprises one or more dust filters. For example, it includes a micron filter (11) of 1 micron and a submicron filter (12) of 0.01 micron, upstream of the drying chamber (1). At the outlet of the enclosure (1), the device according to the invention comprises a catalyst filter (13) making it possible to transform carbon monoxide into carbon dioxide, then downstream a retention filter (14) making it possible to retain the carbon dioxide and, still downstream, a deodorization filter (15) with activated carbon and an antibacterial filter (16) of 0.01 micron. The catalyst filter (13) is filled with black granules

"HOPCALITE" of dimensions between 1 and 3.5 mm and whose average composition is 57% manganese oxide, 13% copper oxide, 30% manganese carbonate. The apparent density of the mixture is 1 and this filter is regenerated during the periods when the compressor does not work. The granules are arranged in the body of the filter (13) which is identical from the point of view of constitution to that (140) represented in FIG. 7. The cylindrical body (141) ends at one end by a shoulder (1410) against which comes to bear a perforated grid (145) and above this grid a plug (142) is crimped in the cylindrical body (141) by the upper ends of the cylindrical body folded over the plug (142). This plug has holes (143, 144) and a threaded blind hole allowing it to be fixed on the associated filter caps (139, 149, 159, 169). The other end of the cylindrical body also has a shoulder (1411) against which abuts a perforated grid (146) held against the shoulder by a circlip (148) which is received in a groove in the body (141). The empty space of the filter (147) is filled with granules of either catalysts, molecular sieves or activated carbon depending on the type of filter. The carbon dioxide retention filter (14) is filled with Siliporite G5 molecular sieves which are synthetic zeolites, of crystalline structure of type X, in sodium form, which have a network of pores with an effective diameter of 10 angstroms. These Siliporite G5 beads have a particle size of 1.6 to 2.5 mm. Finally, the filter (15) is filled with granulated activated carbon, based on activated coconut shells, with steam and having a particle size at 90% of between 2 and 5 mm. The size of the filters is calculated according to the flow rate of the installation which can be 8 m ³ / h, 19m ³ / h, 30m ³ / h, 50 m ³ / h, for example. Depending on these flow rates, the catalysis filter, the retention filter or the deodorization filter, consists of a container whose volume is calculated so that it can contain the weights of granules corresponding to each of the properties of filtering according to the desired flow for the installation. The table below gives examples of calculations of the weight of granules to be contained by the respective filters according to the flow rates of the installation.

DEODORIZATION RETENTION CATALYSIS

Q m ³ / h Weight Kg Weight Kg Weight Kg

8 0.112 0.20 0.06

19 0.266 0.475 0.14

30 0.420 0.750 0.225

50 0.700 1.25 0.375

Average 0.014 0.025 0.0075 in gr / m ³

So for 8m ³ / h we will need to have a volume for the catalysis filter to contain a weight of "HOPCALITE" of 112 gr. Given this table, it is necessary to provide a filter element container allowing to contain between 0.010 to 0.017 gr of HOPCALITE constituting the catalyst per m ³ / h of air treated by the installation, 0.02 to 0 , 03 gr of molecular sieve per m ³ / h of air treated by the retention filter and 0.005 to 0.010 gr of activated carbon per m ³ / h of air treated by the deodorizing filter.

According to the invention, in FIG. 9 is shown the direction (defined by the arrows) of the passage of air through the catalysis filter (13), the filter (14) of retention, the deodorization filter (15) and the antibacterial filter (16), placed one behind the other. According to the invention, the air passing through the retention filter (14) arrives through a grid (140) of larger section than the outlet (141) of the filter (14). In this way, the air remains and takes longer to exit the filter and the carbon dioxide is better retained by the balls.

In Figure 3, there is shown another embodiment of the device according to the invention. This device includes a homogenizer (17). Indeed, the air which is compressed is atmospheric air, and depending on the quality of the ambient air, the components of the compressed air can vary. Thus, thanks to the homogenizer (17), a dry compressed air is obtained which is of substantially constant composition.

The homogenizer (17) includes a pressure switch (18) or an electronic pressure sensor. The device according to the invention is such that when the pressure of dry compressed air drops in the homogenizer (17), the pressure switch trips and starts the compressor. Thus, the device according to the invention operates only according to demand, namely, when no dry compressed air is pumped at the outlet (19), the compressor is stopped and it does not triggers only when dry compressed air is pumped. A non-return valve (20) closes the dry compressed air supply line to the homogenizer (17).

According to the invention, the retention filter (14) shown in FIG. 8, comprises means for obtaining the desorption of carbon dioxide and for driving the carbon dioxide out of the filter (14). These means therefore make it possible to clean the retention filter (14) when the balls are or become saturated with carbon dioxide. This cleaning is carried out periodically when the device according to the invention does not work. This cleaning is carried out suddenly by scanning against the current of the retention filter (14) using a neutral gas. In this way the retention filter (14) becomes more efficient and this makes it possible to reduce the amount of carbon dioxide present in the dry compressed air obtained. finally by the device according to the invention. A duct (280) has one end joining the outlet duct (141) of the retention filter (14) leading to the deodorization filter (15). According to the invention, this duct (280) has its other end connected to a device (28) for distributing a neutral gas. The arrival of the neutral gas is controlled by a solenoid valve (283) and the neutral gas is sent against the current in the retention filter (14) to expel the retained carbon dioxide and thus clean the filter (14). The carbon dioxide is discharged through the grid (140) of the filter and through a conduit (281) to the outside of the device according to the invention. According to the invention, the neutral gas could for example be oxygen or air 78/22. According to a variant, the device (28) for distributing neutral gas is unnecessary since the dry compressed air obtained according to the invention is used. As shown in Figure 4, an air sample is taken after passing through the homogenizer (17). This air is analyzed for example using an analyzer (29), the hygrometry rate of the air obtained. The air samples once analyzed are released to the atmosphere. The principle of the invention consists in using this dry compressed air intended solely for hygrometric analysis to sweep against the current the retention filter (14). This air is pure according to the invention and the advantage lies in the fact that it is not necessary to have the external neutral gas distribution device (28). A non-return valve (282) is placed on the neutral gas inlet pipe to prevent the gas from going backwards. Of course, it will also be possible to take air obtained according to the invention and not intended for hygrometric analysis as represented by the path in points in FIG. 4. The dotted path represents the case where the device (28 ) distribution is used and the solid line path the one where the compressed air from the hygrometric analysis is used.

The device shown in Figure 4 is a complete device that can be supplied to a hospital or clinic that is not already equipped with a compressor. This group is compact and autonomous. This device comprises, upstream of the enclosure (1), a micron filter (11), a submicron filter (12), and downstream of the drying enclosure (1), a catalyst filter (13), a retention (14), a deodorizing filter (15) and an antibacterial filter (16), a homogenizer (17), a non-return valve (20). The device further comprises a compressor (21) and possibly a second compressor (22), upstream of the filters (11) and (12). These compressors (21, 22) are preferably chosen without oil, which makes the oil separator unnecessary.

In Figures 5 and 6, there is shown a device according to the invention, in front view and in top view. The homogenizer (17) has at its upper end a safety valve (23) and a pressure gauge (24) for pressure measurement. The drying enclosure (1) is placed so that its longitudinal axis is vertical. The wet compressed gas enters the filters (11, 12), via the pipe (25), according to the arrow F1 and leaves the homogenizer (17) by one or more outlet pipes (26, 27) according to the arrows F2 , F'2.

The device according to the invention can be placed on a support (28) on which the various members are fixed.

With this device, dry compressed air is obtained which meets AFNOR NFS 90140 and NF EN 737-3 standards, and even which may be of superior quality.

We can produce from 8 m ³ / h to 400 m ³ / h of dry compressed air, at pressures from 10 to 12 bar.

If we compare the quality of the dry compressed air obtained with the device according to the invention, with the AFNOR NFS 90140 standard, we have the following results:

AFNOR NFS Standard Device according to

90140 the invention Oil content in mg / m ³ 0.1 <0.01

Water vapor in g / m ³ 0.09 0.09 = - 42 ° C

CO content in ppm <5 <2

C0 ₂ content in ppm <350 <300

S0 ₂ content in ppm 16.10 ^"3 <16.10 ^{" 3}

25.5 content. 10 ³ <25.5. 10 ³ nitrogen monoxide / dioxide in ppm

The dry compressed air obtained with the device according to the invention therefore has a quality superior to the AFNOR standard since the contents of oil, water vapor, carbon monoxide, carbon dioxide, sulfur dioxide, monoxide and dioxide of nitrogen are lower.

According to the invention, an ozonator making it possible to manufacture ozone by electrolysis of oxygen can be placed at the outlet of the drying enclosure (1).

According to the invention, this dry compressed air can be used for example in the field of civil security to fill air bottles. It can also be used in the food industry, for example to carry out the growth of milk powders or the profusion.

It should be obvious to those skilled in the art that the present invention allows embodiments in many other specific forms without departing from the scope of the invention as claimed. Therefore, the present embodiments should be considered by way of illustration, but may be modified in the field defined by the scope of the appended claims, and the invention should not be limited to the details given above.

Claims

1. A method of manufacturing a gas or mixture of gases, compressed, dry, characterized in that the following steps are carried out: - the gas or mixture of wet gases is compressed,

the gas or mixture of gases, compressed and wet, is circulated from the inlet of at least one hollow tube, the wall of which is made of a membrane permeable to water vapor, towards the outlet of the tube, the hollow tube being placed in an enclosure, the water vapor passing through the membrane from the inside of the tube towards the outside of the tube, to accumulate in the enclosure,

- the gas or mixture of compressed and dry gas is recovered at the outlet of the tube,

- the carbon dioxide filter of the filter is periodically regenerated by sweeping it against the current with a neutral gas.

2. Method according to claim 1, characterized in that the gas or compressed gas mixture is filtered, before drying, to remove the dust.

3. Method according to claim 1, characterized in that the gas or compressed gas mixture is sent, after drying, in a pressure tank and the mixture is homogenized.

4. Method according to one of the preceding claims, characterized in that the compressed gas is air.

5. Method according to claim 4, characterized in that, after removing the dust and drying, the air is passed through an ozonator to form ozone.

6. Device for implementing the method according to one of the preceding claims, characterized in that it comprises a drying enclosure (1) in which are placed hollow tubes (2) parallel, the wall of the tubes being consisting of a membrane permeable to water vapor, an inlet (3) of wet compressed air at one end (4) of the tubes (2), an outlet (5) of dry compressed air at the opposite end (6) tubes (2), an inlet (7) of dry air into the enclosure (1), the dry air circulating in the enclosure (1), outside the tubes (2), a outlet (8) of humid air from the drying enclosure (1), traps (13, 14, 15) of carbon and sulfur oxides and dioxides, comprising in particular a carbon dioxide retention filter and means for periodically removing carbon dioxide from this retention filter.

7. Device according to claim 5, characterized in that the means for periodically expelling carbon dioxide from the filter consist of means for sweeping against the current of the filter using a neutral gas.

8. Device according to claim 6 or 7, characterized in that it comprises filters (1 1, 12) to dust, possibly an oil filter (10), placed upstream of the enclosure (1), and an antibacterial filter (16) placed downstream of the drying chamber.

9. Device according to one of claims 6 to 8, characterized in that the sulfur and carbon oxides and dioxides traps comprise a catalyst filter (13) transforming carbon monoxide into carbon dioxide placed upstream of the filter ( 14) retention of carbon dioxide.

10. Device according to claim 9, characterized in that the catalyst filters (13) and retention (14) consist of a container closed at one end by a perforated grid (145) and a plug (142), the plug (142) comprising two orifices (143, 144) and a fixing hole on the associated filter caps, the second end of the container being closed by a perforated grid.

11. Device according to claim 9 or 10, characterized in that the catalyst filter is filled with granules composed of manganese oxide, copper oxide and manganese carbonate and having a particle size between 1 and 3.5 mm .

12. Device according to one of claims 6 to 11, characterized in that the retention filter (14) is filled with a molecular sieve of synthetic zeolites of crystal structure of type X, comprising a network of pores with effective diameter of 10 angstroms and having a particle size between 1, 6 and 2.5 mm.

13. Device according to claim 12, characterized in that the sweeping means consist of a neutral gas inlet pipe in the retention filter, the neutral gas arriving through the pipe and sweeping the sieve against the current to drive out carbon dioxide.

14. Device according to one of claims 6 to 13, characterized in that the neutral gas is dry compressed air.

15. Device according to one of claims 6 to 13, characterized in that the neutral gas is dioxygen.

16. Device according to claim 14, characterized in that the dry compressed air is that produced by the device and used for a hygrometry test.

17. Device according to one of claims 6 to 16, characterized in that it comprises a homogenizer (17) of air, placed downstream of the enclosure (1) for drying.

18. Device according to one of claims 6 to 17, characterized in that it comprises one or more air compressors (21, 22), upstream of the dust filters operating without lubricant.

19. Dry compressed air, obtained by the method according to one of claims 1 to 4, characterized in that its pressure is between 8 and 12 bar, its oil content is less than 0.01 mg / m ³ , its water vapor content is 0.09 g / m ³ , = - 42 ° C, its CO content is less than 2 ppm, its C0 ₂ content is less than 300 ppm, its SO ₂ content is less than 16.10 ^"3 ppm and its nitrogen monoxide / dioxide content is less than 25.5.10 ^{" 3} ppm.

20. Use of the dry compressed air obtained according to claim 19 in the food sector.

21. Use of dry compressed air obtained according to claim 19 in the field of civil security.