GB2279588A

GB2279588A - Microwave and radiofrequency enhanced oxidation

Info

Publication number: GB2279588A
Application number: GB9311295A
Authority: GB
Inventors: David John Harper; Ronald Henson; David John Wheeler
Original assignee: Isambard Services Ltd; GEC Alsthom Ltd
Current assignee: Alstom UK Ltd; Isambard Services Ltd
Priority date: 1993-06-02
Filing date: 1993-06-02
Publication date: 1995-01-11
Anticipated expiration: 2013-06-02
Also published as: GB9311295D0; GB2279588B

Abstract

An apparatus and method for oxidising a chemical substance present in a fluid medium. The chemical substance is oxidised as it passes over a ferroelectric metal or metal compound which is subjected to microwave or radio frequency radiation. Also disclosed in a method of preparing a material for use in microwave or radiofrequency enhanced oxidation where the material comprises any metal or metal compound located in or on a porous solid matrix, the metal preferably being ferroelectric.

Description

MICROWAVE AND RADIOFREQUENCY ENHANCED OXIDATION This invention relates to the microwave and/or radio frequency enhanced oxidation of organic chemical substances and is particularly but not exclusively relevant to the oxidation of ventilated organic compounds (VOC's) and also malodorous organic substances emitted from food preparation equipment and machinery and from certain types of chemical process plant.

VOC's are either vapours or gases of certain organic compounds that are emitted, for example from some types of machinery, mixing vessels, drying ovens, flue stacks and ventilation equipment designed to capture and remove organic pollutants from workroom air and process plant to the atmosphere. Hereafter, VOC is meant to indicate both VOC's and malodorous compounds as described above. The removal of many such materials from large volumes of air is both difficult and costly, and often not very effective.

It is an object of the present invention to provide an apparatus and a method for substantially reducing the amount of VOC's released into the atmosphere from various machinery, apparatus and installations such as any of those referred to above.

According to the first aspect of the present invention there is provided an apparatus for the oxidation of a chemical substance which is present in a fluid medium, said apparatus comprising at least one of a metal or metal compound, and means for exposing the substance and/or the metal or metal compound to at least one of microwave or radio frequency radiation, said apparatus further comprising means to pass the chemical substance within the vicinity of the metal or metal compound, characterised in that the metal or metal compound is ferroelectric.

According to a second aspect of the present invention there is provided a method of oxidising a chemical substance which is present in a fluid medium, said method comprising exposing the substance and/or at least one of a metal or metal compound to at least one of radio frequency or microwave radiation while the chemical substance is passing within the vicinity of the metal or metal compound, characterised in that the metal or metal compound is ferroelectric.

The apparatus and method of the present invention facilitate the oxidation of VOC's without the use of expensive catalysts such as palladium.

The fluid medium may be a gas or a liquid.

Preferably the metal compound is a ferrite of general formula Mx Fe3-xO4' were M is a transition metal other than iron and has an atomic number between 21 and 41, but is preferably selected from manganese, cobalt, chromium, nickel and copper or combinations thereof.

In a preferred embodiment of the invention the metal compound comprises a perovskite of general formula ABO3 where A and B represent metal ions of atomic number between 21 and 41. A particularly preferred metal compound is barium titanate.

The metal or metal ion is preferably located in or on a porous solid matrix. The spaces within the matrix may take any form that permits free passage of the fluid through the matrix whilst exposing the fluid to a large surface area of the solid matrix. A ceramic matrix is particularly preferred. The ceramic matrix may comprise a refractory material such as aluminium oxide, an alumino silicate, or a magnesium aluminium silicate, for example cordierite. The matrix may form a coating on a structural ceramic body or be absorbed or adsorbed into the surface thereof. The metal or metal compounds may form the whole of the substance of the matrix or be present as a surface layer on heat resistant material such as ceramic. The metal or metal compound may form a coating on the ceramic matrix or be adsorbed or absorbed onto the surface thereof.Furthermore, the coating may be coherent and of even thickness or may be in the form of islets scattered randomly on the surface of the ceramic. Such coatings may be applied to the ceramic material by a variety of physical or chemical techniques.

According to a third aspect of the present invention there is provided a method of preparing a material for use in microwave and radiofrequency enhanced oxidation, said material comprising a metal or metal compound located in or on a porous solid matrix, said method comprising the steps of preparing a dispersion of finely divided particles of the metal or metal compound in an organic solvent, a polymer being dissolved in the organic solvent, coating the dispersion onto the surface of the solid matrix so as to form a thin film of the dispersion on the matrix and removing the organic solvent from the film.

The organic solvent is preferably removed by heating. The polymer is preferably a polyalkyl methacrylate such as polymethyl metacrylate.

If the dispersion comprises a metal the porous material may be heated to a high temperature in order to melt the metal such that it forms a coherent film on the surface of the porous matrix and possibly also partly combines with the porous matrix at the interface of the matrix and the film.

This heating step may be achieved in a furnace.

The atmosphere of the furnace may comprise unreactive gases such as nitrogen or may contain oxidising or reducing gases to achieve particular chemical changes in the transition metal or transition metal compound applied to the surface of the ceramic. For example a metal oxide may be fired in the presence of hydrogen to convert it to the metal.

The metallic coating on the ceramic may consist of more than one metal. In particular it has been found an advantage to apply first a layer of molybdenum to the ceramic and then, by the techniques described above, to apply a second layer of metal or metal compound, either as particles or as film.

The metal coating may also be applied by other means. For example by immersing the metal coated alumina in a solution of the salt of another transition metal, the transition metal having an electrode potential such that it will displace and replace, either partially or in the whole, the metal originally coating the ceramic. For example, an iron coated alumina immersed in copper sulphate can be converted to a copper/iron coated alumina or alternatively to a copper coated alumina.

In a preferred embodiment of the invention the metal or metal compound is a d-block transition metal.

Preferred examples include metals and metal compounds from the first transition series between atomic numbers 21 and 42, inclusive, and their compounds, or mixtures of these. Certain metal irons from groups 1 and 2 of the periodic table may also be present in the metal compounds or mixtures.

The metal used is preferably one that can exist in more than one oxidation state, and the following metals or metal compounds are particularly preferred: chromium, manganese, iron, cobalt, nickel, copper, molybdenum and their oxides and mixed oxides, chlorides, sulphides, carbides and sulphates.

The metal or the metal compound may form part of a semiconducting or ferromagnetic material, exemplified by ferrites and ferrates, and analogous compounds in which some or all of the iron atoms (or ions thereof) are substituted by other of the metal atoms (or ions thereof) mentioned above. The metal compound is ideally a ferroelectric material, such as barium titanate.

The metal or metal ion may be introduced into or onto the ceramic medium in any form, either as the element itself, as an oxide, salt or in any other chemical state. These metals or their ions tend to be capable of existing in two or more oxidation states, any of which will serve for the purposes of the invention.

The VOC's present in the gas stream may include any of the following, including substituted derivatives thereof - dioxins, furans, aliphatic and aromatic alcohols, phenols, esters, saturated and unsaturated hydrocarbons, ketones, aldehydes, acids, and malodorous materials from food preparation and chemical plant and equipment. Examples include: linear and branched chain alkanes containing up to 12 carbon atoms; linear and branched chain alkenes containing up to 12 carbon atoms; benzene, toluene, the three isomers of xylene, polycyclic aromatics such as napthlane and anthracene; methanol, ethanol, isomers of propanol, butanol, pentanol and hexanol, formaldehyde, isomers of propanone, butanone and pentanone in particular 2-butanone and 3-methyl-2-butanone; esters of acetic, benzoic and phthalic acids such as methyl, ethyl, propyl and butyl.

The VOC's may also be present in the gas stream as either liquid or solid aerosols or particulates. Such compounds would be high boiling aliphatic and aromatic hydrocarbons and their derivatives. Examples would be mellitic acid and anhydride, esters of phthalic acid such as dibutyl and dioctyl phthalate, organic esters of inorganic acids such as dialkyl sulphates, trialkyl and triaryl phosphates. The gases treated may also include malodorous gases from animal products factories, food processing, including ventilated emissions from restaurants.

The heating process involved in either the manufacture of the ceramic medium and/or during microwave and/or radiofrequency irradiation may tend to result in the formation of other chemical derivatives of the element.

In order that the invention may be more readily understood, a specific embodiment thereof will now be described by way of example only.

An open matrix based for example on a ceramic material such as aluminium oxide, containing a system comprised of at least one type of transition metal atom or ion, which is sensitive to microwave or radiofrequency radiation. The transition metal ion or atom is adsorbed onto the surface layers of the ceramic matrix, where it is or will become chemically bound.

This will concentrate the microwave energy at the gas/solid interface where it will be most advantageous.

Transition metal ions have been found to be particularly effective. The matrix is located in conjunction with a microwave or radiofrequency generator, in such a way that the electromagentic energy is concentrated in the surface of the matrix. The matrix material may be in or or more of a variety of physical forms that present a high surface area in comparison to its volume, or mass. For example, the physical form can be selected from granules, open ended cylinders, spheres, rods, grids, two or three dimensional lattices or meshes. The exposure of the transition metals or transition metal compounds to microwave radiation, of appropriate frequency and power, causes the transition metal coating to pass into an existed state, which enhances the catalytic oxidation effect of the metal ion.

Gases from flue stacks, exhaust ventilation equipment or the like are passed through the open matrix in the presence of atmospheric oxygen.

By passing the gases containing the pollutants over the microwave activated catalytic material, the facile oxidation of almost all vapours and gases of organic compounds comprised of carbon and hydrogen alone, or including also oxygen, and or sulphur, and or nitrogen, or carbon monoxide.

The radiation may also activate the chemical substance which is to be oxidised. In this case the frequency of the radiation may be separately tuned to specific chemical substances or constituents thereof and to the catalytic system.

The apparatus and method of the present invention have many applications, for example in odour control and elimination, and in the destruction of VOC's and carbon monoxide in effluent gases. In particular, the invention would be useful in reducing VOC's from ventilation control equipment, for example in factories or plant manufacturing and/or using paints, inks, adhesives and degreasing agents.

It is to be understood that the above described embodiments of the invention are by way of illustration only. Many modifications and variations are possible.

Claims

1. An apparatus for the oxidation of a chemical substance which is present in a fluid medium, said apparatus comprising at least one of a metal or metal compound, and means for exposing the substance and/or the metal or metal compound to at least one of microwave or radio frequency radiation, said apparatus further comprising means to pass the chemical substance within the vicinity of the metal or metal compound, characterised in that the metal or metal compound is ferroelectric.

2. An apparatus as claimed in claim 1, wherein the metal compound is a ferrite of general formula MxFe3x04 where M is a transition metal other than iron, wherein the atomic number of the transition metal is in the range from 21 to 41.

3. An apparatus as claimed in claim 2, wherein the transition metal comprises any of the following either alone or in combination: manganese, colbalt, chromium, nickel or copper.

4. An apparatus as claimed in claim 1, wherein the metal compound comprises a perovskite of general formula ABO3 where A and B represent metal ions of atomic number between 21 and 41.

5. An apparatus as claimed in any preceding claim, wherein the metal or metal compound is located in or on a porous solid matrix.

6. An apparatus as claimed in claim 5, wherein the matrix comprises a ceramic material, the metal or metal compound being located on the matrix.

7. An apparatus as claimed in claim 6, wherein the ceramic matrix comprises a refractory material, an alumino silicate or a magnesium aluminum silicate.

8. A method of oxidising a chemical substance which is present in a fluid medium, said method comprising exposing the substance and/or at least one of a metal or metal compound to at least one of radio frequency or microwave radiation while the chemical substance is passing within the vicinity of the metal or metal compound, characterised in that the metal or metal compound is ferroelectric.

9. An apparatus as claimed in claim 8, wherein the metal compound is a ferrite of general formula MxFe3x04 where M is a transition metal other than iron, wherein the atomic number of the transition metal is in the range from 21 to 41.

10. An apparatus as claimed in claim 9, wherein the transition metal comprises any of the following either alone or in combination: manganese, colbalt, chromium, nickel and copper.

11. An apparatus as claimed in claim 8, wherein the metal compound comprises a perovskite of general formula ABO3 where A and B represent metal ions of atomic number between 21 and 41.

12. An apparatus as claimed in any of claims 8 to 11, wherein the metal or metal compound is located in or on a porous solid matrix.

13. An apparatus as claimed in claim 12, wherein the matrix comprises a ceramic material, the metal or metal compound being located on the matrix.

14. An apparatus as claimed in claim 13, wherein the ceramic matrix comprises a refractory material, an alumino silicate or a magnesium aluminium silicate.

15. A method of preparing a material for use in microwave and radiofrequency enhanced oxidation, said material comprising a metal or metal compound located in or on a porous solid matrix, said method comprising the steps of preparing a dispersion of finely divided particles of the metal or metal compound in an organic solvent, a polymer being dissolved in the organic solvent, coating the dispersion onto the surface of the solid matrix so as to form a thin film of the dispersion on the matrix and removing the organic solvent from the film.

16. A method as claimed in claim 15, wherein the organic solvent is removed by heating.

17. A method as claimed in claim 15 or claim 16, wherein the polymer comprises a polyalkyl methacrylate.

18. A method as claimed in any of claims 15 to 17, wherein the dispersion comprises a metal which is subjected to heat so as to melt the metal such that it forms a film in the surface of the porous membrane.

19. A method as claimed in any of claims 15 to 18, wherein the heating step is carried out in an inert environment.

20. A method as claimed in any of claims 15 to 19, wherein the heating step is carried out in an oxidising or reducing environment.

21. A method as claimed in any of claims 15 to 20, wherein the coating comprises successive layers of different metals.

22. A method as claimed in any of claims 15 to 21, wherein the coating is applied by immersing metal coated alumina in a solution of the salt of another transition metal, the transition metal having an electrode potential such that it at least partially displaces and replaces the original metal coating.

23. A method as claimed in any of claims 15 to 22, wherein the metal or metal compound is a transition metal.

24. A method as claimed in any of claims 15 to 23, wherein the transition metal has an atomic number in the range from 21 to 41.

25. A method as claimed in, any of claims 15 to 24 wherein the metal or metal compound further comprises a metal ion from group 1 and/or group 2 of the periodic table.

26. A method as claimed in any of claims 15 to 25, wherein the metal can exist in more than one oxidation state.

27. A method as claimed in any of claims 15 to 26, wherein the metal or metal compounds comprises any of the following either alone or in combination: chromium, manganese, iron, colbalt, nickel, copper, molybdenum and their oxides and mixed oxides, chlorides, sulphides, carbides and sulphates.

28. A method as claimed in any of claims 15 to 27, wherein the metal or metal compound is a semiconductor.

29. A method as claimed in any of claims 15 to 28, wherein the metal or metal compound is ferromagnetic.

30. A method as claimed in any of claims 15 to 29, wherein the metal or metal compound is ferroelectric.