Catalytic process

Details Catalytic process Catalytic process

1995-06-26

1997-01-28

Lewis, Michael

Chemistry of inorganic compounds

Modifying or removing component of normally gaseous mixture

422 4, B01D 4700

Patent

active

055975399

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a catalytic process and in particular to a catalytic process for the removal of odours and/or volatile organic compounds from gas streams.
Discharge of gas streams, such as air, containing volatile organic and/or odoriferous substances into the atmosphere is often objectionable on environmental grounds; not only may the odour be objectionable, but in some cases the odoriferous and/or volatile organic compounds may be carcinogenic and/or toxic, and/or discharge into the atmosphere may be unacceptable for other reasons. Examples of such gas streams include the exhaust gases from operations such as meat processing, eg fat rendering; sewage treatment; foundries; paint spraying and the like. Examples of objectionable odoriferous substances that may occur in such gas streams include aliphatic amines and aldehydes; sulphur compounds such as hydrogen sulphide, mercaptans and organic sulphides; phenols, cresols, and xylenols; while volatile organic compounds include a wide range of aliphatic and aromatic compounds such as short chain aldehydes, ketones, alcohols, esters, and halogenated hydrocarbons. It is thus desired to treat the gas stream to remove the aforesaid substances and/or to convert then to a less objectionable material. Oxidation often results in the conversion of such substances to less odorous and/or less volatile compounds or to inorganic end products.
The gas streams may be treated to remove those substances, and/or render them less objectionable, by passing the gas stream through a scrubber packing in counter-current flow to an aqueous solution containing an oxidant such as hydrogen peroxide or hypochlorite ions. The oxidant effects oxidation of the unwanted components to less objectionable compounds. The addition of a suitable metal salt, eg a nickel salt, such as nickel sulphate, where the oxidant is hypochlorite or an iron salt where the oxidant is hydrogen peroxide, to the aqueous solution catalyses the decomposition of the oxidant rendering it even more effective.
However the presence of such dissolved metal salts presents problems of corrosion of the apparatus and also presents an effluent problem since the discharge of aqueous solutions containing dissolved metal salts into sewage systems or into rivers is often environmentally unacceptable.
In the present invention a fixed bed of a metal oxide catalyst is utilised: not only are the problems of corrosion and effluent disposal avoided but also the amount of oxidant required to effect the desired degree of treatment may be decreased, and the intensity of oxidation enhanced.
It has been proposed in GB 2047217-B to de-odorise waste gases by passing the gas up through a packed column containing catalyst of nickel "peroxide" on an inert support in counter-current to a sodium hypochlorite solution passing down the column. There is thus vapour/liquid contact between the gas and the hypochlorite solution in the presence of the catalyst. Part of the liquor from the base of the column is recycled, together with fresh hypochlorite. One disadvantage of such a system is that to ensure adequate vapour/liquid contact in the presence of the catalyst, a significant resistance to flow of the gas through the tower is presented, necessitating the use of higher power fans to effect the gas flow. Also, as may inevitably happen from time to time, if the liquid flow is interrupted, the catalyst will dry out and allow salt to crystallise in the pores of the catalyst. This results in cracking and eventual break-up of the catalyst. Also there is a rick that any non-volatile, insoluble, organic compounds that may be entrained in the waste gas fed to the column will be deposited on the catalyst and thereby decreasing its effectiveness.
It has been proposed, for example in U.S. Pat. No. 3,944,487, EP 276044-B and EP 211530-A, to decompose organic pollutants in effluents by mixing the effluent containing the pollutant with an aqueous solution containing an oxidising agent such as hypochlorite and passing the mixture over a fixed bed of certain ni

REFERENCES:
patent: 4443342 (1984-04-01), Stas et al.
patent: 4764286 (1988-08-01), Bon et al.

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