APPARATUS FOR THE TREATMENT OF AIR

Abstract

A gas treatment apparatus, suitable for use in an air purifying apparatus for the production of breathable air, includes a catalyst including palladium and iron oxide and a source of a volatile nitrogen-containing compound. The apparatus is useful in gas masks, emergency escape hoods and static air treatment apparatus.

Claims

1. An apparatus for the treatment of air, wherein air having a first concentration of carbon monoxide enters the apparatus and breathable air having a second concentration of carbon monoxide exits the apparatus, said first concentration being higher than said second concentration, the apparatus comprising a gas treatment device, comprising: i) a catalyst for the oxidation of carbon monoxide comprising palladium and iron oxide and ii) a source of a volatile nitrogen-containing compound, the volatile nitrogen-containing compound being selected to remove one or more toxins from the air, wherein the catalyst and source of volatile nitrogen-containing compound are either mixed or separated from one another by a gas-permeable barrier, and wherein the catalyst is resistant to poisoning by any nitrogen-containing compound that comes into contact with the catalyst in the gas treatment device.

2. The apparatus according to claim 1, wherein the catalyst comprises from 0.5 to 10% of palladium, by weight.

3. The apparatus according to claim 1, wherein said catalyst is made by a method in which a mixed oxide and hydroxide containing iron and palladium is precipitated from an acid solution containing soluble iron and palladium compounds.

4. The apparatus according to claim 1, wherein said iron oxide and palladium is supported on a porous support material.

5. The apparatus according to claim 1, wherein the catalyst comprises particles having a size in the range from 300-1000 m.

6. The apparatus according to claim 1, wherein the catalyst is present as a bed of particles and said catalyst bed has a maximum thickness in the direction of the air flow of less than 10 mm.

7. The apparatus according to claim 1, wherein the catalyst is present in a coating.

8. The apparatus according to claim 1, wherein said source of volatile nitrogen-containing compound comprises an absorbent material impregnated with amine, or ammonia.

9. The apparatus according to claim 8, wherein said absorbent material comprises an activated carbon in the form of a bed of particles, a cloth or foam.

10. The apparatus according to claim 1, containing no hopcalite.

11. The apparatus according to claim 1, wherein no guard bed for the removal of catalyst poisons is located between the catalyst and the source of volatile nitrogen-containing compound.

12. The apparatus according to claim 1, in the form of a filter assembly.

13. The apparatus according to claim 1, in the form of a gas filter, filter cartridge, gas mask, self-contained self rescuer, escape hood, personal breathing apparatus or air scrubbing system.

14. The apparatus according to claim 1, capable of reducing the concentration of carbon monoxide in an oxygen-containing gas from 3600 ppm to less than 500 ppm over a continuous period of 10 minutes at 20 C. at a linear air flow rate of 9 cm/second.

15. A method of treating air to form breathable air comprising the step of passing a stream of air containing a first concentration of carbon monoxide through a gas treatment means comprising: i) a catalyst for the oxidation of carbon monoxide comprising palladium and iron oxide and ii) a source of a volatile nitrogen-containing compound such that at least a portion of carbon monoxide contained in said air is oxidised and that the air downstream of said gas treatment means contains a second concentration of carbon monoxide which is less than said first concentration, the volatile nitrogen-containing compound being selected to remove one or more toxins from the air, wherein the catalyst and source of volatile nitrogen-containing compound are either mixed or separated from one another by a gas-permeable barrier, and wherein the catalyst is resistant to poisoning by any nitrogen-containing compound that comes into contact with the catalyst in the gas treatment device.

16. The method according to claim 15, wherein when the stream of air containing said first concentration of carbon monoxide is passed through said gas treatment means at 20 C. for 15 minutes and said first concentration of carbon monoxide is at least 3600 ppm the maximum instantaneous value of said second concentration of carbon monoxide is less than 500 ppm.

17. The method according to claim 16, wherein, when the first concentration of carbon monoxide is 3600 ppm and the air stream is passed through the gas treatment apparatus for a continuous period of 15 minutes at a linear velocity of 9 cm per second, the CT value of CO in the air downstream of the gas treatment means is less than 6000 ppm minutes.

18. A method of treating air to form breathable air comprising the step of contacting a stream of air containing a first concentration of carbon monoxide with a catalyst for the oxidation of carbon monoxide comprising palladium and iron oxide such that at least a portion of carbon monoxide contained in said air is oxidised and that the air downstream of said gas treatment means contains a second concentration of carbon monoxide which is less than said first concentration, wherein said air is also brought into contact with a volatile nitrogen-containing compound, the volatile nitrogen-containing compound being selected to remove one or more toxins from the air, wherein the catalyst and source of volatile nitrogen-containing compound are either mixed or separated from one another by a gas-permeable barrier, and wherein the catalyst is resistant to poisoning by any nitrogen-containing compound that comes into contact with the catalyst in the gas treatment device.

19. The apparatus according to claim 2, wherein said catalyst is made by a method in which a mixed oxide and hydroxide containing iron and palladium is precipitated from an acid solution containing soluble iron and palladium compounds.

20. The apparatus according to claim 2, wherein said iron oxide and palladium is supported on a porous support material.

21. An apparatus for the treatment of air, wherein air having a first concentration of carbon monoxide enters the apparatus and breathable air having a second concentration of carbon monoxide exits the apparatus, said first concentration being higher than said second concentration, the apparatus comprising a gas treatment device, comprising: i) a catalyst for the oxidation of carbon monoxide comprising palladium and iron oxide and ii) a source of a volatile nitrogen-containing compound, the volatile nitrogen-containing compound being selected to remove one or more toxins from the air, the catalyst being resistant to poisoning by any nitrogen-containing compound that comes into contact with the catalyst in the gas treatment device, wherein the catalyst and source of volatile nitrogen-containing compound are provided in the absence of a guard bed for the removal of catalyst poisons.

22. A method of treating air to form breathable air comprising the step of passing a stream of air containing a first concentration of carbon monoxide through a gas treatment means comprising: i) a catalyst for the oxidation of carbon monoxide comprising palladium and iron oxide and ii) a source of a volatile nitrogen-containing compound such that at least a portion of carbon monoxide contained in said air is oxidised and that the air downstream of said gas treatment means contains a second concentration of carbon monoxide which is less than said first concentration, the volatile nitrogen-containing compound being selected to remove one or more toxins from the air, the catalyst being resistant to poisoning by any nitrogen-containing compound that comes into contact with the catalyst in the gas treatment device, wherein the catalyst and source of volatile nitrogen-containing compound are provided in the absence of a guard bed for the removal of catalyst poisons.

23. A method of treating air to form breathable air comprising the step of contacting a stream of air containing a first concentration of carbon monoxide with a catalyst for the oxidation of carbon monoxide comprising palladium and iron oxide such that at least a portion of carbon monoxide contained in said air is oxidised and that the air downstream of said gas treatment means contains a second concentration of carbon monoxide which is less than said first concentration, wherein said air is also brought into contact with a volatile nitrogen-containing compound, the volatile nitrogen-containing compound being selected to remove one or more toxins from the air, the catalyst being resistant to poisoning by any nitrogen-containing compound that comes into contact with the catalyst in the gas treatment device, wherein the catalyst and source of volatile nitrogen-containing compound are provided in the absence of a guard bed for the removal of catalyst poisons.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0027] The CO-oxidation catalyst may be used in the gas filter either as a separate catalyst bed, e.g. as a layer of catalyst in a multi-layered arrangement, or alternatively mixed with other components of the air-purification system such as activated carbon materials, optionally absorbents impregnated with a nitrogen-containing compound.

[0028] The invention will be further described in the following examples, with reference to the accompanying drawings, which are:

[0029] FIG. 1: A plot of CO and CO.sub.2 concentration vs time using catalyst of invention;

[0030] FIG. 2: A plot of CO and CO.sub.2 concentration vs time using comparative catalysts;

[0031] FIG. 3: A plot of CO and CO.sub.2 concentration vs time using comparative catalyst;

[0032] FIG. 4: A plot of CO concentration vs time using a catalyst of the invention;

[0033] FIG. 5: A plot of CO concentration vs time using a catalyst of the invention.

EXAMPLE 1: PREPARATION OF PD-FeOx CATALYST BY PRECIPITATION

Detailed Description of the Invention

[0034] 1577.86 g K.sub.2CO.sub.3 was dissolved in 5 L demineralised water in a 10 L glass vessel equipped with overhead stirrer and reflux condenser and the solution was heated to 60 C. with the stirrer set to 300 rpm. Separately, 2529.90 g Fe(NO.sub.3).sub.3.9H.sub.2O and 152.44 g Pd(NO.sub.3).sub.2 were dissolved in 5 L water. The Pd/Fe solution was added beneath the surface of the K.sub.2CO.sub.3 solution via a pump over 60 minutes. After all of the Pd/Fe had been added the resulting brown slurry was held at 60 C. for 1 hour before the solid was collected by filtration. The solid was thoroughly washed with 2 L portions of demineralised water at 60 C. until the conductivity was <50 S. After washing, the solid catalyst was dried on the filter to give a gelatinous brown solid which was then dried in an air oven at 105 C. for 48 hours. The dry solid was crushed and sieved to between 425 and 850 m and then divided into two portions. One portion of the dry particles was calcined in air at 500 C. for 2 hours (10 C./minute heating ramp, 2-hour hold at 500 C., followed by cooling at 30 C./minute to 80 C.) and then reduced in a stream of 5% H.sub.2 in N.sub.2 at 80 C. for 3 hours. The sample was then cooled to <30 C. and passivated by gradual introduction of an oxygen-containing gas. The second portion was not calcined but only reduced at 80 C., after a heating ramp rate of 2 C./minute, and passivated as described above. On analysis of a portion of the calcined particles by XRD, the sample was found to be of low crystallinity. After reduction, analysis by XRD shows some Pd metal and some magnetite peaks, although the material has low crystallinity. Elemental analysis of the material by ICP-AES found the material contains 2.35% Pd and 0.08% K. All other metallic elements were present at less than 0.1%.

EXAMPLE 2: (COMPARATIVE) PREPARATION OF PT-FEOX CATALYST

[0035] A catalyst was made according to the method described in Example 1, using platinum nitrate instead of the palladium nitrate. 47.39 g K.sub.2CO.sub.3 was dissolved in 500 ml demineralised water in a 2 L round bottomed flask equipped with an overhead stirrer and reflux condenser. The resulting solution was then heated to 60 C. with the stirrer set to 300 rpm. Separately 75.90 g Fe(NO.sub.3).sub.3.9H.sub.2O and 2.45 g Pt(NO.sub.3).sub.4 were dissolved in 500 ml water. The Pt/Fe solution was added beneath the surface of the K.sub.2CO.sub.3 solution via a pump over 60 minutes. After all of the Pt/Fe had been added the resulting brown slurry was held at 60 C. for 1 hour before the solid was collected by filtration. The solid was thoroughly washed with 2 L portions of demineralised water at 60 C. until the conductivity was <50 S. After washing, the solid catalyst was dried on the filter to give a gelatinous brown solid which was then dried in an air oven at 105 C. for 48 hours. The dry solid was sieved to between 425 and 850 m, reduced at 80 C., after a heating ramp rate of 2 C./minute, and passivated as described in Example 1. Elemental analysis of the material by ICP-AES found the material contains 2.39% Pt and 0.08% K.

EXAMPLE 3: (COMPARATIVE): PREPARATION OF AU-FEOX CATALYST

[0036] A catalyst was made according to the method described in Example 1, using tetrachloroauric acid instead of the palladium nitrate.

[0037] 47.55 g K.sub.2CO.sub.3 was dissolved in 500 ml demineralised water in a 2 L round bottomed flask equipped with and overhead stirrer and reflux condenser. The resulting solution was then heated to 60 C. with the stirrer set to 300 rpm. Separately 75.90 g Fe(NO.sub.3).sub.3.9H.sub.2O and 0.78 g HAuCl.sub.4 were dissolved in 500 ml water. The Au/Fe solution was added beneath the surface of the K.sub.2CO.sub.3 solution via a pump over 60 minutes. After all of the Au/Fe had been added the resulting brown slurry was held at 60 C. for 1 hour before the solid was collected by filtration. The solid was thoroughly washed, dried, crushed and sieved as described in Example 2. Elemental analysis of the material by ICP-AES found the material contains 2.22% Au and 0.06% K.

EXAMPLE 4: CO OXIDATION TEST

[0038] 0.05 g of sieved granules (850-425 m sieve sizes) made in Example 1 was placed in a tubular plug flow reactor of internal diameter 4 mm at approximately 21 C. and about 90% relative humidity. A flow of gas made up of 3600 ppm CO, 20% O.sub.2 in N.sub.2 was introduced after 50 seconds and maintained through the reactor at 100 ml/min. The outlet gas composition was analysed using an infra-red gas analyser and a graph of CO and CO.sub.2 concentration was recorded over a period of 10 minutes. The results are shown in FIG. 1 and show no breakthrough of CO during the 10 minute test. In FIG. 1, the upper (solid) line represents the concentration of CO.sub.2 and the lower (dotted) line represents the CO concentration in the gas exiting the reactor tube.

EXAMPLE 5: CO OXIDATION TEST

[0039] The Au and Pt catalysts described in Examples 2 and 3 were tested as described in Example 4 with the results shown in FIG. 2. Significant breakthrough of CO was evident during the 10 minute test, demonstrating that Pd/FeOx is superior to these other two catalysts.

EXAMPLE 6: (COMPARATIVE) PREPARATION OF AU/TIO.SUB.2

[0040] 4.05 g HAuCl.sub.4 was dissolved in 4.5 L of demineralised water and 0.1 M NaOH solution was added at room temperature until the pH of the solution was about 10.75. Once the pH was stable, 100 g Degussa P25 TiO.sub.2 was added over about 5 minutes. Addition of the TiO.sub.2 caused the pH to drop to 7.81 so some more 0.1 M NaOH solution was added to increase the pH to 9-10. The resulting mixture was stirred for an hour and 0.1 M NaOH was added as needed to keep the pH between 9 and 10. In total 78 ml 0.1 M NaOH was added to the mixture after the TiO.sub.2 had been added.

[0041] After stirring for an hour, the solid was collected by filtration. The solid was washed with demineralised water until the conductivity of the filtrate was <10 S. The solid was then dried in an oven overnight at 105 C. to give 96.02 g of the product as a light purple powder.

Catalyst Ageing Procedure

[0042] Ageing Procedure A: Catalyst ageing was carried out in the presence of an amine-impregnated activated carbon, of the type typically found in gas mask filters. The carbon used for the ageing studies was a commercially available activated carbon containing 1% by weight triethylenediamine (TEDA). The catalyst was placed in a glass vial with an amount of carbon having a bulk volume equivalent to three times the bulk volume of catalyst. The catalyst layer was separated from the carbon layer by cotton wool. The vial was sealed under reduced pressure in a foil bag which had been flushed with nitrogen. The bag was maintained at 70 C. for 5 weeks. After ageing, the catalyst (without the activated carbon) was removed from the vial for testing.

[0043] In order to identify the effects of the carbon and the ageing process, 2 comparative studies were run:

C1: The ageing procedure described in A was run in the absence of the activated carbon.
C2: A sample of catalyst was sealed into a glass bottle in air and stored at room temperature for 5 weeks.

EXAMPLE 7: (COMPARATIVE) EFFECT OF AGEING WITH ACTIVATED CARBON ON AU/TIO.SUB.2

[0044] A sample of the Au/TiO.sub.2 catalyst prepared in Example 6 was aged according to the procedure described above. Further samples were treated according to the comparative procedures C1 and C2. Following the ageing and comparative procedures, the catalysts were tested for CO oxidation using the test as described in Example 4. A fresh sample of catalyst was also tested. The results are shown in FIG. 3. The results show that, although the fresh and C2 sample gave almost no CO slip, the samples aged at 70 C. showed considerable slip with the A procedure catalyst, aged in the presence of activated carbon, showing almost no CO oxidation activity.

EXAMPLE 8: EFFECT OF AGEING WITH ACTIVATED CARBON PD/FEOX

[0045] A series of catalysts containing different amounts of Pd were prepared using the preparation procedure described in Example 1 with variations to change the Pd loading. A sample of each catalyst was calcined before reduction in air at 500 C. for 2 hours (10 C./minute heating ramp, 2-hour hold at 500 C., followed by cooling at 30 C./minute to 80 C.). A second sample of each catalyst was tested without calcining but after reduction in hydrogen as described in Example 1. A sample of each of the pre-calcined and uncalcined catalysts was aged following the A ageing procedure described above. A second sample of each was tested fresh. Fresh and aged catalysts were tested for CO oxidation activity according to the description in Example 4.

[0046] The maximum instantaneous CO slip through the catalyst in ppm was measured and is shown in Table 1. Pd loadings shown are nominal loadings. The data shows that at lower Pd loadings (<4 wt %) the un-calcined catalysts show less CO slip than the calcined catalysts and that below 2 wt % Pd the aged catalysts show considerably more CO slip than the fresh catalyst.

TABLE-US-00001 TABLE 1 Catalyst Calcined Not calcined Pd/FeOx before reduction before reduction Pd % Aged Fresh Aged Fresh 1 N/T 2127 1281 287 2 >3000 486 179 134 2.5 >3000 398 10 60 3 N/T N/T 0 83 4 0 0 0 0 5 0 0 0 0 15 0 0 0 0

EXAMPLE 9: PREPARATION AND MECHANICAL TESTING OF GRANULATED PD/FEOX

[0047] A dried 2.5% Pd/FeOx catalyst which had been made according to Example 1 was pre-crushed on an Alexanderwerk RAN 70 grater/shredder with a 0.40 mm Conidur screen. The crushed particles were then passed through a roll compactor (Alexanderwerk WP120) using a compacting force between 50 and 140 bar. The resulting catalyst particles were sieved to 0.4-0.8 mm particle size and reduced at 80 C. as described in Example 1. The catalyst was then sealed into a gas mask filter assembly, having area dimensions of about 108 mm75 mm, to give a 4.5 mm deep bed together with a 12 mm deep bed of activated carbon containing a TEDA. The filter assembly was then treated to simulate the NIOSH requirement of Escape Hoods for vibration conditioning and the rough handling drop test described in NIOSH Standard Test Procedure: CET-APRS-STP-CBRN-0411: Laboratory Durability Conditioning Process for Environmental, Transportation and Rough Handling Use Conditions on Chemical, Biological, Radiological and Nuclear (CBRN) (Air-Purifying or Self-Contained) Escape Respirator. The filter assembly was then tested for CO oxidation at 25 C. with a gas composition of 3600 ppm CO in air with a relative humidity of 90% at a flow rate of 32 l/min through the filter assembly. The outlet CO concentration shows CO breakthrough of only 4 ppm. If the catalyst particles had broken down during the test then the appearance of voids and channelling would have been expected to result in the passage of CO through the filter. The results show that only minor amounts of CO passed through the filter.

EXAMPLE 10: COMPOSITIONS INCLUDING ORGANIC BINDERS

[0048] 4 different compositions containing dried, unreduced Pd/FeOx and an organic binder selected from acacia, xanthan gum, polyvinylalcohol (PVA) and hydroxypropyl cellulose (HPC) were made into granules. The binders were selected because they do not require treatment at high temperature which may adversely affect the performance of the sample. Each binder was added as an aqueous solution to a separate sample of dried, unreduced Pd-FeOx catalyst powder containing 2.5% Pd, in an amount sufficient to yield a granular composition containing 2.5 wt % of the binder when dried. The catalyst and binder solution were mixed at 3000 rpm in a Speed Mixer for one minute to give a granular product which was then dried at 105 C., sieved to 425-850 m and reduced at 80 C. as described in Example 1 above. A sample of the resulting granular catalyst material was aged using Ageing Procedure A described above, for 3 weeks. Following ageing, the CO oxidation test was run. The equivalent maximum CO slip for all of the binders after ageing is shown in Table 2. These results show that the use of a binder to form a granule is detrimental to the performance of the catalyst after ageing in contact with an activated carbon.

TABLE-US-00002 TABLE 2 Xanthan None Acacia gum PVA HPC Binder aged fresh aged fresh aged fresh aged fresh aged Max CO <50 83 1787 206 1777 322 2778 101 1316 slip (ppm)

EXAMPLE 11: COMPOSITIONS INCLUDING INORGANIC BINDERS

[0049] Separate compositions incorporating dried, unreduced Pd-FeOx catalyst powder containing 2.5% Pd, were formed containing 2.5 wt % of two different inorganic binder mixtures when dried. The binders were added to the catalyst in the form of slurries and mixed, dried and reduced as described in Example 9. The samples were tested fresh and after ageing for one and two weeks. The equivalent maximum CO slip for the samples is shown in Table 3.

TABLE-US-00003 TABLE 3 Portland cement + Ciment fondu + Attagel co-binder Attagel co-binder Aged Aged Aged Binder fresh 1 week 2 weeks fresh 1 week Max CO 175 405 615 115 351 slip (ppm)

EXAMPLE 12: CATALYST PREPARATION BY IMPREGNATION

[0050] 10 g of gamma-alumina powder having a surface area >100 m.sup.2 g.sup.1 was impregnated with a mixed palladium nitrate and iron nitrate solution containing 0.5 g Pd equivalent and 0.25 g Fe as iron nitrate with a further 2 ml water using the incipient wetness technique in which the volume of the impregnation solution is calculated to fill the total pore volume of the alumina10%. The powder was dried in an oven at 105 C. for 12 hours and pelletised to 0.4-0.8 mm. The formed sample was then reduced under 5% H.sub.2 in N.sub.2 for 2 hours. Finally the catalyst was passivated in air. The resulting catalyst contained 5 wt % Pd and 2.5 wt % Fe on gamma-alumina. The catalyst was tested for CO oxidation performance using the test method described in Example 4 above. No breakthrough of CO was evident during a 10 minute test.

EXAMPLE 13: CO OXIDATION TEST AT HIGHER CO INLET CONCENTRATION, FOLLOWING STORAGE

[0051] A gas mask filter assembly was made as described in Example 9 using a catalyst made according to example 1 but with a reduction temperature of 90 C. The assembly was then stored under N.sub.2 in a sealed foil pack for 6 months at ambient temperature. After storage, the pack was opened and the catalyst was extracted from the assembly. A quantity of the catalyst was placed in a tubular plug flow reactor of internal diameter 4 mm at room temperature, (approximately 21-25 C.), and about 90% relative humidity. The amount of catalyst used was sufficient to form a 4 mm long bed in the reactor (0.05 g) or a 6 mm bed (0.075 g). A flow of gas made up of 9600 ppm CO in a 20% O.sub.2 in N.sub.2 gas mixture was introduced after 50 seconds and maintained through the reactor at linear velocity of 13 cm/s. The outlet gas composition was analysed using an infra-red gas analyser and a graph of CO concentration was recorded over a period of one hour. The results are shown in FIG. 4.

EXAMPLE 14: CO OXIDATION TEST FOLLOWING STORAGE

[0052] 0.05 g of the catalyst extracted from the gas filter assembly after storage was placed in a tubular plug flow reactor of internal diameter 4 mm at room temperature, (approximately 21-25 C.), and about 90% relative humidity. A flow of gas made up of 3600 ppm CO in a 20% O.sub.2 in N.sub.2 gas mixture was introduced after 50 seconds and maintained through the reactor at linear velocity of 13 cm/s. The outlet gas composition was analysed using an infra-red gas analyser and a graph of CO and CO.sub.2 concentration was recorded over a period of one hour. The results are shown in FIG. 5.