Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Patent
1994-04-25
1996-06-18
Beck, Shrive
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
502349, 502353, B01J 2106, B01J 2316, B01J 2340
Patent
active
055277551
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to catalysts for the catalytic afterburning of exhaust gases containing carbon monoxide and/or oxidizable organic compounds, especially for the purification of exhaust gases from the phthalic anhydride or maleic anhydride production and also relates to processes for the preparation thereof.
BACKGROUND ART
Oxidizable gaseous, vaporous or particulate pollutants in exhaust gases can be removed by combustion. Under conditions which permit a substantially complete oxidation of contaminants, chemically bound carbon and hydrogen are converted into the substances carbon dioxide (CO.sub.2) and water (H.sub.2 O), which are safe with regard to air hygiene. Other exhaust gas constituents, such as sulfur compounds, nitrogen compounds or halogen compounds, are converted in the oxidative treatment into substances which, when they exceed permitted emission limit values, must be eliminated in a purification plant downstream of the combustion, e.g. by an exhaust gas scrubber. In practice, therefore, exhaust gas combustion is preferably employed where contaminants essentially composed of carbon and hydrogen are to be destroyed and where, in addition, utilization of recovered exhaust gas constituents is not possible or other purification processes are unsatisfactory. (Ullmanns Encyklopadie der Technischen Chemie [Ullmanns Encyclopedia of Industrial Chemistry], "Verfahren und Gerate zur Abgasreinigung [Processes and equipment for exhaust gas purification]"; 4th Edition, Verlag Chemie: Weinheim 1981, Volume 6, p. 312).
In engineering practice, a differentiation is made between thermal afterburning (TAB) at temperatures between 600.degree. and 1100.degree. C. and catalytic afterburning (CAB) at temperatures between 250.degree. and 500.degree. C. which is more favorable from the energy viewpoint. For CAB, on the one hand, noble metal catalysts are suitable, which generally comprise palladium, platinum and/or rhodium finely distributed over a heat-resistant support and are particularly sensitive to sulfur- and heavy metal-containing exhaust air streams; on the other hand, the more resistant mixed-oxide catalysts are used which contain, for example, oxides of vanadium, tungsten, copper, manganese, iron and/or cobalt. Depending on the structure, the catalysts are subdivided into unsupported catalysts (homogeneous catalysts) and supported catalysts; preferred usage forms respectively are packed bed and honeycomb catalysts.
A mixed oxide catalyst in the form of cylindrical pellets, comprising 92% by weight of Co.sub.3 O.sub.4 and 8% by weight of CeO.sub.2, which requires temperatures of about 290.degree. C. for the combustion of low xylene contents in exhaust air streams, is described, for example, in DE-A 26 41 317.
Catalysts based on TiO.sub.2, preferably in the anatase modification, and V.sub.2 O.sub.5 can be used for a great number of partial oxidation reactions in the gas phase and, moreover,--when ammonia is added--for flue gas denitration. The gas phase oxidation of o-xylene and/or naphthalene to give phthalic anhydride (PA) with the use of vanadium- and titanium-containing supported catalysts is disclosed, for example, by DE-C 14 42 590 and DE-C 21 06 796. After desublimation of the reaction products and, if necessary, a subsequent scrubbing of the process air, the exhaust gases from PA production plants, in addition to reaction water and carbon dioxide, still contain, inter alia, carbon monoxide, o-xylene and/or naphthalene, o-tolylaldehyde, formaldehyde, phthalic anhydride and maleic anhydride, despite optimized reaction conditions. However, increasingly strict environmental regulations in some countries (e.g. the TA-Luft [Air Pollution Control Regulations] in the Federal Republic of Germany) in many cases necessitate a secondary purification of such exhaust air streams passing into the environment.
The use of a TiO.sub.2 /V.sub.2 O.sub.5 catalyst for the total oxidation of organic compounds is described in DE-A 30 19 879: TiO.sub.2 is impregnated with NH.sub.4 VO.sub.3 s
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Hammer Margit
Wenski Guido
Beck Shrive
Consortium fur Elektrochemische Industrie GmbH
Parker Fred J.
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