Process for low temperature gas cleaning with ozone and a catalytic bag filter for use in the process

Abstract

A process for the cleaning of a lean gas stream contaminated with volatile organic compounds (VOCs) and/or sulfur-containing compounds comprises the steps of adding ozone to the contaminated lean gas stream, subjecting the ozone-containing lean gas stream to ultraviolet irradiation, thereby transforming VOCs to particles, maintaining the irradiated gas stream in a stay zone for a sufficient time to allow aerosol particle growth, and passing the gas stream through a catalytic bag filter at a temperature down to room temperature to remove the formed particles and eliminate any remaining ozone. The bag filter has been made catalytic by impregnation with one or more metal oxides in which the metals are selected from V, W, Pd and Pt, supported on TiO.sub.2.

Claims

1. A process for the cleaning of a gas stream contaminated with volatile organic compounds (VOCs) and/or sulfur-containing compounds, said process comprising adding ozone to the contaminated gas stream prior to subjecting to ultraviolet irradiation, subjecting the ozone-containing gas stream to ultraviolet irradiation, thereby transforming VOCs to particles, maintaining the irradiated gas stream in a stay zone for a sufficient time to allow aerosol particle growth, and passing the gas stream through a catalytic bag filter to remove the formed particles and eliminate any remaining ozone, wherein the bag filter has been made catalytic by impregnation with a catalyst substance, wherein the catalytic bag filter comprises two or three layers of filter fabric impregnated with the catalyst substance, and wherein the two or three layers are configured such that a layer for ozone removal is the last layer the gas stream passes through before exiting the bag filter.

2. Process according to claim 1, wherein the catalyst substance contains one or more metal oxides, in which the metals are selected from vanadium, tungsten, palladium and platinum, supported on TiO.sub.2.

3. Process according to claim 1, wherein the catalyst substance contains vanadium supported on TiO.sub.2.

4. Process according to claim 1, wherein the catalyst substance contains vanadium and palladium supported on TiO.sub.2.

5. Process according to claim 1, wherein the catalyst substance contains vanadium and platinum supported on TiO.sub.2.

6. Process according to claim 1, wherein the two or three layers comprise an inner layer and one or two outer layers, and the one or two outer layers capture particulates formed in the stay zone.

7. Process according to claim 6, wherein the inner layer of the catalytic bag filter contains a catalytic substance which is especially efficient in removing ozone, while the one or two outer layers contain catalytic substances which are more efficient for VOC removal.

8. Process according to claim 1, wherein the gas stream is at a temperature between 20 and 200 C. when it is passed through the catalytic bag filter.

9. Process according to claim 6, wherein the gas stream is at a temperature lower than 50 C. when it is passed through the catalytic bag filter.

10. Process according to claim 1, wherein ozone is generated when the ozone-containing gas stream is subjected to ultraviolet irradiation, thereby transforming VOCs to particles.

11. Process according to claim 7, wherein the gas stream passes through the catalytic bag filter first through the one or two outer layers and then through the inner layer.

12. A process for the cleaning of a gas stream contaminated with volatile organic compounds (VOCs) and/or sulfur-containing compounds, said process comprising adding ozone to the contaminated gas stream and subjecting the ozone-containing gas stream to ultraviolet irradiation, thereby transforming VOCs to particles, maintaining the irradiated gas stream in a stay zone for a sufficient time to allow aerosol particle growth, and passing the gas stream through a catalytic bag filter to remove the formed particles and eliminate any remaining ozone, wherein the bag filter has been made catalytic by impregnation with a catalyst substance, wherein the catalytic bag filter comprises two or three layers of filter fabric impregnated with the catalyst substance, wherein the two or three layers are configured such that a layer for ozone removal is the last layer the gas stream passes through before exiting the bag filter, and wherein adding ozone to the contaminated gas stream and subjecting the ozone-containing gas stream to ultraviolet irradiation occur in a same step of the process.

Description

(1) The present invention relates to a novel process for the cleaning of a lean gas stream contaminated with volatile organic compounds (VOCs) and/or sulfur-containing compounds, said process comprising adding ozone to the contaminated lean gas stream, subjecting the ozone-containing lean gas stream to ultra-violet irradiation, thereby transforming VOCs to particles, maintaining the irradiated gas stream in a stay zone for a sufficient time to allow aerosol particle growth, and passing the gas stream through a catalytic bag filter at a temperature down to room temperature to remove the formed particles and eliminate any remaining ozone,

(2) wherein the bag filter has been made catalytic by impregnation with a catalyst substance.

(3) Preferably the catalyst substance contains one or more of the metals vanadium, tungsten, palladium and platinum supported on TiO.sub.2.

(4) A catalytic bag filter consists of a substrate, a carrier and one or more catalytic materials. The substrate of the catalyst substance is a fiber structure having the form of woven glass fibers, and the carrier can be titanium dioxide or another suitable compound. The catalytic material can be vanadium, tungsten, palladium and platinum. The carrier (TiO.sub.2) can itself be catalytically active in the process of the invention.

(5) Bag filters are well suited for the removal of dust and particulate matter from gas streams. Catalytic bag filters have the double utility of being able both to remove particulates from a gas stream and to catalyze one or more desired reactions in the gas. A catalytic bag filter typically comprises two or three layers of filter fabric, each layer containing a tailored catalyst optimized for removal of a specific kind of compound from the gas that passes through it. Dust and other particulate matter will settle on the surface of the outer bag, from where it can easily be removed. The two or three-layer structure provides the flexibility to tailor different catalytic combinations for different purposes.

(6) It is especially preferred to tailor different catalytic combinations for the individual layers of the bag filter, such that the inner layer of the catalytic bag filter contains a catalytic substance which is especially efficient in removing ozone, while the other layers contain catalytic substances which are more efficient for VOC removal. This is because the gas passes through the bag layers in the direction from the outside to the inside and the last step of the cleaning process is the ozone removal.

(7) In the process of the invention, ozone is added to the lean gas stream. Any source of ozone can, in principle, be used in the ozone treatment step, and some ozone is generated during the UV light treatment of the gas in the subsequent process step.

(8) From the ozone addition step, the gas stream proceeds to a step of UV irradiation treatment. Depending on the apparatus design, these two steps can be made into one.

(9) Broad-spectrum UV light with a wavelength between 100 and 330 nm initially breaks down the ozone into oxygen (O.sub.2) and an oxygen radical (O*), but the main feature of the UV irradiation in the context of the invention is that any VOC present in the gas stream is transformed to particles by the UV light.

(10) The irradiated gas stream is then maintained in a stay zone for a sufficient time to allow particles of transformed VOC to grow.

(11) Finally, the gas stream is passed through a catalytic bag filter to remove the particles formed and eliminate any remaining ozone. This is done at a temperature down to room temperature, more specifically at a temperature between 20 and 200 C., preferably lower than 50 C.

(12) The catalyst substance used in the process according to the invention consists of a substrate and a carrier impregnated with one or more metal oxides, in which the metals are selected from vanadium, tungsten, palladium and platinum.

(13) The catalyst carrier is preferably titanium dioxide, and the preferred metal oxide is vanadium oxide.

(14) In addition to removing VOCs and/or sulfur-containing compounds down to very low residual levels, the process of the invention has the important characteristic feature that the specific catalyst used in the process is able to remove any residual ozone. This is very important because, as already mentioned, ozone is very toxic, and therefore any residual ozone from the gas cleaning process has to be thoroughly removed.

(15) Ozone removal catalysts that have so far been used in various embodiments include MgO.sub.2 or aluminum catalysts, carbon-supported metal oxides, carbon fibers coated with copper chloride, carbon-iron aerosol particles, Carulite and metal catalysts.