Gas separation: processes – Compressing and indirect cooling of gaseous fluid mixture to... – And liquid contact
Reexamination Certificate
1998-10-02
2001-03-20
Chiesa, Richard L. (Department: 1724)
Gas separation: processes
Compressing and indirect cooling of gaseous fluid mixture to...
And liquid contact
C062S640000, C062S642000, C095S069000, C095S070000, C096S055000, C096S057000, C422S122000, C422S217000, C423S215500
Reexamination Certificate
active
06203596
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for removing carbon monoxide and/or hydrogen from an air stream in which the air stream is subjected to a catalytic oxidation.
BACKGROUND OF THE INVENTION
To produce semiconductor elements, ultra high purity nitrogen having a very low content (for example below 1 Vppm) of reducing components such as hydrogen and carbon monoxide is required. It is known, for example, in the preparation of such ultra high purity nitrogen, to remove impurities such as carbon monoxide and hydrogen from the intake air of an air fractionation plant by catalytic oxidation of carbon monoxide to carbon dioxide and/or hydrogen to water on catalyst beds using solid catalysts. The catalyst material can consist of hopcalite (mixture of oxides and/or peroxides of manganese, silver, cobalt and/or copper), or of a bed of aluminium oxide, in/on which palladium or platinum or else ruthenium, osmium, iridium or rhodium is incorporated or applied. The catalyst bed can be disposed directly downstream of the air compressor and can be operated at an elevated temperature of about 160° C. As an alternative thereto, it is possible to dispose the catalyst bed in the vessel in which the molecular sieve for drying the air is also situated.
Examples of the purification of feed air for an air fractionation plant for generating an ultra pure gas are found in EP-0 438 282-A1 and EP-0 454 531-A1. According to EP-0 438 282-A1, water is initially removed from the feed air stream by adsorption; thereafter, carbon monoxide and hydrogen are converted at ambient temperatures of 0 to 50° C. in one layer each of hopcalite and palladium-activated aluminium oxide and finally residual water, carbon dioxide and hydrocarbons are removed by adsorption. Using the known processes, typically, depletions of carbon monoxide and hydrogen by a factor of 20 are possible.
SUMMARY OF THE INVENTION
The object underlying the invention is to Specify a process and an apparatus of the type mentioned at the outset which effect a particularly high degree of depletion of carbon monoxide and/or hydrogen in the air stream.
This increase of the catalyst activity is effected in the context of the invention by means of the fact that dusts and/or aerosols of a particle size of less than 20 &mgr;m are removed from the air stream upstream of the catalytic oxidation.
Particle size is taken to mean here the particle diameter. In the case of particles whose shape deviate. from the spherical form, the hydraulic diameter is meant. The hydraulic diameter is defined as six times the ratio of volume to external surface area.
As does the older European Patent Application 0 799 633 which does not have an earlier priority date than the present application, the present invention begins from the finding that pollutant components which are present in the air stream decrease the activity of catalyst material. However, it has proven that only a restricted action can be achieved by the measures mentioner in EP 0 799 633 for removing pollutants in a gaseous state. In the context of the present invention it was fecund that the air contaminated with solid and liquid pollutants in the form of aerosols and dusts, in particular of laden dusts and pollutant-impregnated soot, plays an important role for the catalyst activity.
This is because harmful dusts and aerosols of a particle size of less than 20 &mgr;m flow through the customary mechanical filters and a molecular sieve bed essentially unhindered. They can thus deposit their pollutant load on the catalyst surface. A particularly harmful substance for the catalyst activity here is dust, which inter alia consists of soot (for example from combustion processes and/or diesel engines) which itself is enriched up to about 50% with acidic pollutants such as SO
2
, SO
3
, H
2
S, COS, NO, NO
2
as well as Cl, HCl, F, HF and halogenated hydrocarbons and has a particle spectrum with a mean size around 1 &mgr;m. (These and all following percentages are by weight.)
According to the invention, such dusts and/or aerosols of a particle size of less than 20 &mgr;m, preferably less than 10 &mgr;m, are removed upstream of the catalytic oxidation by suitable measures. It is particularly important for increasing the catalyst activity to remove particles of a particle size of 0.05 to 5 &mgr;m, in particular in the range 0.1 to 3 &mgr;m. “Removal” is taken to mean here decreasing the content of the corresponding dusts and/or aerosols by at least 50%, in particular a decrease in the content of dusts and aerosols of a particle size between 0.1 and 3 &mgr;m by at least 50%, preferably by at least 80%.
In principle, all known methods for particle removal can be used. Preferably, dusts and/or aerosols of a particle size of less than 20 &mgr;m are removed by a scrubbing process. A scrubbing process in this case describes the direct contact of the air stream with a liquid; an example is countercurrent scrubbing. The scrubbing apparatus, depending on requirements, can be operated with an open or closed circulation of scrubbing medium, with or without sludge removal respectively.
Although a conventional trickling cooler, which is frequently used to precool feed air for a low-temperature air fractionation plant, can also be considered as a device for carrying out a scrubbing process, a cooler of this type in no way represents an apparatus for removing dusts and/or aerosols in the context of the invention. It is, for example, unable by far to decrease the content of particles of a size between 0.1 and 3 &mgr;m by at least 50%.
As scrubbing media, use can be made of water, in particular customary service water, or other suitable scrubbing liquids having suitable pHs.
Particular preference is given to the use of an overflow Venturi scrubber or of a jet scrubber or spray-zone scrubber. An overflow Venturi scrubber is able, for example, to separate off 50% or more of dusts having a particle size between 0.1 and 3 &mgr;m; depending on the setting of the pressure drop, degrees of separation of 90 to 97% or greater can be achieved. The degree of separation for aerosols is somewhat higher than that for dusts.
Alternatively, or additionally, for particle removal, an electrostatic precipitator or a mechanical particle separator, in particular a cyclone separator or a filter having a fine fibrous filter material, can be used. The last mentioned can consist, for example, of a fabric or felt material which is fabricated from long thin fibres. obviously, in the invention, a plurality of methods for particle removal can also be used in combination.
In addition to the removal according to the invention of dusts and/or aerosols of a particle size of less than 20 &mgr;m, it is expedient if particles of a particle size of at least 20 &mgr;m are removed from the air stream. This can be achieved using customary filter mats which are disposed, for example, upstream of a compressor for compressing the air stream.
The invention further relates to an apparatus for removing carbon monoxide and/or hydrogen from an air stream. In particular, there is provided an apparatus for removing carbon monoxide and/or hydrogen from an air stream having means for catalytic oxidation of carbon monoxide and/or hydrogen, characterized in that means for removing dusts and/or aerosols of a particle size of less than 20 &mgr;m are provided upstream of the means for catalytic oxidation. The means for removing dust and/or aerosols preferably comprises scrubbing apparatus such as an overflow Venturi scrubber, a jet scrubber, or a spray-zone scrubber.
REFERENCES:
patent: 4537748 (1985-08-01), Billiet
patent: 5030428 (1991-07-01), Dörr et al.
patent: 5649985 (1997-07-01), Imamura
patent: 0 416 140 (1991-03-01), None
patent: 0 716 274 (1996-06-01), None
O.C. Leite, “Cleaning up Incineration Exhaust”,Environmental Engineering World, pp. 6-11, Jul.-Aug. 1996.
Chiesa Richard L.
Linde Aktiengesellschaft
Millen White Zelano & Branigan P.C.
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