WO2007061295A1

WO2007061295A1 - Method and apparatus for the treatment of a waste gas stream comprising contaminant molecules

Info

Publication number: WO2007061295A1
Application number: PCT/NL2006/000592
Authority: WO
Inventors: Gerardus Petrus Adrianus Leenders
Original assignee: Aerox B.V.
Priority date: 2005-11-28
Filing date: 2006-11-24
Publication date: 2007-05-31
Also published as: NL1030535C2

Abstract

The present invention relates to a method for treating a waste gas stream comprising gaseous contaminant molecules, wherein a reactive mixture with radicals is injected into the waste gas stream, and wherein the thus obtained waste gas stream is passed through at least one layer of a zeolite containing material. The present invention also relates to a system for treating a waste gas stream comprising gaseous contaminant molecules.

Description

Method and apparatus for the treatment of a waste gas stream comprising contaminant molecules

The present invention relates to a method and system for treating a waste gas stream comprising contaminant molecules.

In some branches of industry, such as, for instance, feed and food industry, slaughterhouses and rendering industry, chemical industry and industrial water and sewage water treatment centers, production is often accompanied by associated odour emission. Although, as a rule, these odour molecules have no detrimental effect on public health, they are experienced as unpleasant and undesirable by those living in the proximity of such factories. It is therefore desired, and possibly even compulsory for plants emitting odour molecules, to reduce the odour emission of the plant when this odour emission is too great an inconvenience for those living in their proximity.

Also, with production in the above-cited industries, gaseous volatile organic components (VOCs) can be released that can detrimentally affect the public health. Methods for reducing odour emissions and the emissions of VOCs are known, such as, for instance, the Non-Thermal Plasma (NTP) unit called "Aerox Injector", commercially manufactured by applicant. Non-Thermal Plasma is also called cold plasma. Another apparatus and method for treating odour molecules and other contaminants in gas emissions is known from US patent application US-A-2005/0023128.

A Non-Thermal Plasma is a high-voltage electric discharge between two electrodes that generates high-energy electrons in the air. Then, the electrons collide with gas molecules present in the air, and create free radicals. Thereupon, these free radicals oxidize a portion of the gaseous odour molecules and VOCs in the gas stream. Most free radicals will come from oxygen, and the obtained mixture comprising oxygen radicals is called active oxygen. Most NTP odour and VOC reducing apparatus, as also described in the above-mentioned US patent application, utilize an NTP unit placed in the gas stream. Contrary to this, with its known "Aerox-Injector", applicant utilizes a unit that is placed outside the gas stream to be treated, but injects a reactive mixture (active oxygen) into the gas stream from the outside. The current odour and VOC reducing methods, among which applicants' "Aerox-Injector", can reduce the odour of the gas stream and the amount of VOCs present therein to a considerable extent. This is, however, not always sufficient, so that there is a need for a method for further reducing odour molecules and VOCs present in the gas stream. To this end, for instance, a downstream second odour and VOC reducing apparatus will be required.

Placing such a second odour and VOC reducing apparatus in series is possible, in principle, but is undesired from a practical viewpoint, due to high costs of such apparatus and the relatively small improvement in reduction in odour and VOCs to be realized. Further, from WO-A-2004/014439, a Non-Thermal Plasma air treatment system is known, suitable for reducing airborne contaminations in a living space. The apparatus described in this patent application utilizes two steps, wherein in the first step, an NTP-unit is placed in the gas stream and, in the second step, an adsorbing material is utilized. The air present in the space is circulated through this apparatus. The apparatus described in this patent application is not suitable for industrial use.

The object of the present invention is to provide a method for treating a waste gas stream, preferably an industrial waste gas stream, comprising contaminant molecules, wherein the amount of contaminant molecules present can be significantly reduced with respect to conventional systems.

Surprisingly, it has now been found that this object can be realized when a reactive mixture with radicals is injected into a waste gas stream, whereupon the thus obtained waste gas stream is passed through at least one layer of a zeolite containing material. The present invention therefore relates to a method for treating a waste gas stream comprising gaseous contaminant molecules, wherein a reactive mixture with radicals is injected into the waste gas stream, and wherein the thus obtained waste gas stream is passed through at least one layer of a zeolite containing material.

Injecting a reactive mixture with radicals into the gas stream enables oxidation of at least a portion of the gaseous contaminant molecules present. Then, the gas stream with partly oxidized gaseous contaminant molecules can be guided into a second unit, comprising at least one layer of a zeolite containing material, whereby the gas stream is retarded and a further oxidation of the gaseous contaminant molecules present can take place.

It is preferred that the gaseous contaminant molecules to be treated and present in the waste gas stream are odour molecules and/or VOCs. Many waste gas streams comprise both odour molecules and VOCs. It is therefore preferred that the invention relates to a method for treating a waste gas stream comprising odour molecules and VOCs.

Utilizing the present invention, waste gas streams, preferably industrial waste gas streams, containing several thousands of ppms of odour molecules or VOCs, can be treated. According to a preferred embodiment of the invention, the method is utilized for treating waste gas streams comprising odour molecules in a concentration of less than 100 ppm, preferably less than 50 ppm, still more preferably in a concentration of 5 — 30 ppm. The skilled person will understand that the desired concentration of odour molecules will depend on the use in question. The skilled person will also understand that, as a rule, the concentration of odour molecules will be higher than 1 ppb. The present invention therefore also relates to a method for reducing the amount of gaseous odour molecules present in a waste gas stream, wherein a reactive mixture with radicals is injected into the waste gas stream, and wherein the thus obtained waste gas stream is passed through at least one layer of a zeolite containing material. If the gaseous contaminant molecules to be treated and present in the gas stream are VOCs, these are preferably present in a concentration of less than 1000 ppm, more preferably less than 500 ppm, and still more preferably less than 100 ppm. The present invention therefore also relates to a method for reducing the amount of VOCs present in a waste gas stream, wherein a reactive mixture with radicals is injected into the waste gas stream, and wherein the thus obtained waste gas stream is passed through at least one layer of zeolite containing material.

In a further preferred embodiment, the reactive mixture contains active oxygen, i.e. a mixture comprising oxygen radicals and radicalized water, which is produced in a Non-Thermal Plasma (NTP) unit. It is preferred that this NTP-unit be the "Aerox-Injector" provided by applicant as it has the advantage that the NTP-unit is not present in the gas stream, but on the outside thereof, and is therefore not exposed to the contaminated gas stream. The term "zeolite" is known as such by the skilled person and refers to crystalline materials containing aluminum oxide and silicon oxide.

In a further preferred embodiment, several layers of the zeolite containing material are utilized. The advantage of utilizing several layers is that a fairly constant odour and/or VOC reduction can be effected. The first layer will become saturated first, and any gaseous contaminant molecules that may be present in the gas stream after the first layer can be treated further in the second layer.

Through the provision, in a further embodiment, of sensors between the layers of the zeolite containing material for detecting particles in the gas stream, the efficiency of the odour and/or VOC reduction can be evaluated. When this quality decreases, i.e., when more and more gaseous contaminant molecules are present in the gas stream after the first layer of zeolite containing material, this may suggest that this first layer has become saturated and needs to be replaced. The second layer of the zeolite containing material, which is exposed to fewer gaseous contaminating molecules and will therefore, with a similar duration of use, be less saturated than the first layer, can replace the first layer of zeolite containing material having, in the meantime, become saturated. The place of the second layer of the zeolite containing material which has become vacant can be occupied by a new layer of zeolite containing material. In the different layers, the same zeolite containing material can be used, but it is also possible that in the respective layers to be used mutually different zeolite containing materials can be used. It is preferred that in the different layers, the same zeolite containing material is used. In a further preferred embodiment, the zeolite containing material comprises a zeolite which is provided on a carrier of activated carbon. Such material has improved properties as regards, for instance, the suitability for dealing with high operating temperatures. The zeolite to be used can be a natural or a synthetic zeolite, this material is also called molecular sieve. In a further preferred embodiment of the method, the activated carbon is pretreated for obtaining hydrophobic properties. As a result, the sensitivity to moist gas stream will decrease, so that the applicability is increased. The hydrophobic nature of an activated carbon can be obtained by, for instance, thermally treating an activated carbon. The zeolite to be used can be provided on, for instance, the activated carbon by means of impregnation, by means of, inter alia, vacuum impregnation. If desired, the zeolite containing material can comprise other components such as, for instance, metal ions which may have been provided on the material by means of, for instance, impregnation. The activated carbon can be extrudates with a diameter of between 1 and 6 mm and a length of between 1 and 20 mm, preferably a diameter of approximately 4 mm.

In a still further preferred embodiment of the method, the supply of the at least partly reactive gas stream to the zeolite containing material will regenerate this material at least partly. This reactive gas stream comprises radicals and rest ozone, and oxidizes organic components further that remain behind in one or more layers of the zeolite containing material, so that these or this can be regenerated and the life span of the zeolite containing material can be prolonged. In a further preferred embodiment, the process temperature is regulated by means of the supply of ambient air. In this manner, the process can be maintained at an optimal temperature range for thus obtaining an optimal operation of the system.

In a still further preferred embodiment, the present method degrades substantially all ozone present in the gas stream. A drawback of the NTP-systems is that often, oxidation by-products are formed such as, for instance, ozone. Ozone is harmful and the emission thereof should therefore be prevented. The gas stream is retarded in the layers of the zeolite containing material to such an extent that a further oxidation not only reduces the odour or the amount of VOCs further, but also advantageously effects the reduction of the by-product ozone.

The invention further relates to a system for treating a waste gas stream, preferably an industrial waste gas stream, comprising gaseous contaminant molecules, comprising a supply for supplying a waste gas stream comprising gaseous contaminant molecules, a first apparatus for injecting a reactive mixture with radicals into the gas stream, and a second apparatus comprising at least one layer of a zeolite containing material through which the gas stream treated in the first apparatus is passed.

In a further preferred embodiment of the system, the apparatus for injecting a reactive mixture into the gas stream and the apparatus comprising at least one layer of zeolite containing material are accommodated in separate modules. Owing to this modular structure, a flexible system is provided, offering users the possibility of, first, only deploying an odour and/or VOC reducing apparatus for injecting a reactive mixture into the gas stream for odour and/or VOC reduction, and, when the results thereof do not meet the imposed requirements, offer the user the possibility to further expand the system with an apparatus comprising at least one layer of zeolite containing material through which the gas stream is passed.

In a still further preferred embodiment of the system, the system further comprises an apparatus for separating particles from the gas stream, such as, preferably, a cyclone or other filter. In this manner - if necessary - dust and other particles can be removed from the gas stream before the gas stream reaches the odour and/or VOC reduction step.

In the following description, exemplary embodiments are further explained on the basis of Figs. 1 and 2, wherein:

Fig. 1 shows a general outline of a system of a practical application of the method according to the invention; and

Fig. 2 shows a detailed representation of the odour and/or VOC reduction step of the system shown in Fig. 1. The preferred embodiment of the system as shown in Fig. 1 comprises a waste gas treating system 2 comprising one or more gas stream supplies 4, a cyclone 6, an NTP-unit 8, which is provided by means of a fan 10 and filter/heater 12 with ambient air 14, while the NTP-unit injects a reactive mixture into the gas stream 16, and wherein the gas stream 16 is guided through to a sieve unit 18, the sieve unit comprising at least one layer of modified zeolite provided on activated carbon 20, and a sensor 22 for detecting gaseous contaminant molecules in the gas stream, and a fan 24.

In the system shown in Fig. 1, the gas stream 4 is first passed through a cyclone 6 or other filter for removing the coarse particles and dust present in the gas stream, whereupon the gas stream 16 is guided through for odour and/or VOC reduction. The NTP-unit 8, preferably comprising an "Aerox-Injector", draws in "clean" ambient air 14 by means of a fan 10 into a reactor 8 (a so-called 'high voltage plasma reactor') and is fitted on the outside of a chimney-pot of a factory. The reactor comprises a number of modules which each treat a portion of the ambient air 14. Each module consists of a number of conductive intermediate plates (not shown). Between each pair of conductive plates, a high potential difference is generated so that a process — the so-called plasma - generates free electrons. The oxygen and water molecules of the ambient air 14 will react with the free electrons, thereby forming a reactive mixture of oxygen (O²-), hydrogen (H-) and OH- radicals and OH-ions. This reactive mixture is injected at high speed into the gas stream 16, so that the gaseous contaminant molecules partly oxidize. The gas stream treated via this continuous process has a significantly reduced odour. Some characteristics of this process are that the stream of ambient air 14 is small in relation to the gas stream 16, and that the gas stream 16 does not pass through the NTP-unit 8, which has the advantage that the NTP unit 8 does not become polluted as a result of the pollutants present in the gas stream. This has as an advantage that no sealing of electrodes is required. Through the use of the NTP-unit 8, a considerable reduction of gaseous contaminant molecules (preferably odour molecules and/or VOCs), of for instance 90% or more can be obtained, but there are cases in which even this is insufficient. Particularly in the feed industry, such as in production of, for instance, fish and cat feed, in petrochemistry and other feed and food industries, it frequently happens that a very small number of gaseous contaminant molecules present in the air and to be treated already cause a stench that is quite offensive to those living in the proximity of the industry.

The gas stream 16 already treated by the "Aerox-Injector" is therefore guided through to a sieve unit 18 which reduces the odour even further by means of adsorption; chemisorption; ion exchange and a less rapid oxidation step through remaining oxidizing action of the reactive mixture, which also still contains a small amount of ozone. When leaving the sieve unit 18, the gas stream is drawn in by means of a fan 24 and exhausted into the outside air 26.

In Fig. 2, the odour and/or VOC reducing step according to the invention is represented in more detail. At the location A, the gas stream 16 still comprises all gaseous contaminant molecules present in the waste gas stream. At location B, the number of gaseous contaminant molecules is significantly reduced as a result of the gaseous contaminant molecules oxidizing with the reactive mixture that is injected into the gas steam 16 by the NTP-unit 8. The gas stream, comprising remaining gaseous contaminant molecules and reactive mixture which has not reacted yet, will then be passed through in a first carrier of activated carbon with zeolite provided thereon, during which passing- through the oxidation will continue and the number of gaseous contaminant molecules will reduce further. Also, ozone, which can be a by-product of the oxidation, will be reduced in this carrier of active carbon with modified zeolite 20a. At location B, the number of gaseous contaminant molecules has already considerably reduced, and, at location C, this will have reduced even further, depending on the degree of saturation of the carrier 20a. In a preferred embodiment, a second carrier of activated carbon with zeolite 20b provided thereon is placed behind the first carrier 20a to ensure that the gas stream 26 exhausted by the system comprises a minimum amount of gaseous contaminant molecules. When the first carrier 20a becomes saturated, an increasing amount of gaseous contaminant molecules will be present in the gas stream at location C. A sensor 22 is provided between the first carrier 20a and the second carrier 20b for detecting gaseous contaminant molecules in the gas stream. When the amount of gaseous contaminant molecules increases, this may be an indication that the first carrier 20a has become saturated and is due for replacement. The second carrier 20b however will still guarantee that the exhaust gases that leave the system comprise a minimum amount of gaseous contaminant molecules and can continuously meet strict requirements. When the carrier 20a is saturated, the carrier 20b can be removed from the gas stream and be replaced by a new carrier. Then, the carrier 20a can be taken from the gas stream and be replaced by the (just removed) carrier 20b. Replacing can take place with the system continuing to run so that a continuous operating process is possible. Hence, the process is continuous and requires no desorption/regeneration, which, in conventional system, takes place by means of recirculation. Such a need for recirculation in conventional system makes them unsuitable for continuous use. With the system according to the invention, the process can be switched on 24 hours a day for a long period of time, and be replaced during, for instance, a regularly planned maintenance break — in these factories often, for instance, one week a year. In this manner, an odour and/or VOC-reduction system is provided which is suitable for industrial use.

As there is no desorption/regeneration step, the zeolite containing material also has the advantage that there is no need for a heat source for thermal regeneration, i.e., desorption of adsorbed components (through increase of the temperature of the absorbing material, its retention of components becomes poorer and it will desorb). Such a heat source results in large apparatus, which is undesired for the industrial use of conventional systems. Furthermore, such heat sources consume much energy.

Preferably, the sieve unit 18 is placed after the NTP-unit 8, because, as a result of the NTP-unit 8, a rapid oxidation with radicals (NTP) oxidizes a considerable part of the gaseous contaminant molecules present in the gas stream 16 in a rapid, simple and effective manner and, hence, reduces the number of gaseous contaminant molecules significantly. In the following step, which takes place in the sieve unit 18, the remainder of odour is slowly oxidized by temporarily retarding the gaseous contaminant molecules in the sieve unit 18, while the remaining ozone can also contribute to the oxidation of the remaining gaseous contaminant molecules. According to the present invention, the ozone that is formed as by-product of the Non-Thermal Plasma, or cold plasma, provides a positive contribution to the oxidation of gaseous contaminant molecules.

Although the system also operates in reverse order, this would be less efficient and result in a rapid saturation of the zeolite containing material that is provided on the carriers 20. Furthermore, the technical effect of the radicals regenerating the zeolite containing material would be lost when the sieve unit 18 is placed in front of the NTP-unit 8, so that the zeolite containing material would be saturated much faster and, as a result thereof, would have to be replaced much sooner. A further, additional advantage is that the sieve unit 18, through placement behind the NTP-unit 8, can reduce or even completely remove the by-product ozone that can be formed with the oxidation of the gaseous contaminant molecules in the gas stream 16.

By designing the sieve unit 18 as a modular system, an already existing odour and/or VOC-reduction system which utilizes an NTP-unit 8, 30, can later be expanded with a sieve unit 18. Through separation of the NTP- unit 8 and the sieve unit 18, thus, a compact and flexible system is obtained.

The above-described embodiments - although they represent preferred embodiments of the invention - are merely intended to illustrate the present invention and not to delimit the description of the invention in any manner. The scope of the invention is therefore solely determined by the following claims.

Claims

1. A method for treating a waste gas stream comprising gaseous contaminant molecules, wherein a reactive mixture with radicals is injected into the waste gas stream, and wherein the thus obtained waste gas stream is passed through at least one layer of a zeolite containing material.

2. A method according to claim 1, wherein the waste gas stream is an industrial waste gas stream.

3. A method according to claim 1 or 2, wherein the gaseous contaminant molecules are odour molecules and/or VOCs.

4. A method according to claim 3, wherein the odour molecules are present in a concentration of less than 100 ppm.

5. A method according to claim 3 or 4, wherein the gaseous molecules

VOCs are present in a concentration of less than 500 ppm.

6. A method according to any one of the preceding claims, wherein the reactive mixture comprises active oxygen which is produced in a Non-Thermal Plasma (NTP)-unit.

7. A method according to any one of the preceding claims, wherein several layers of a zeolite containing material are used.

8. A method according to claim 7, wherein between the layers of zeolite containing material, sensors have been provided for the detection, in the gas stream, of gaseous molecules to be treated.

9. A method according to any one of the preceding claims, wherein the zeolite containing material comprises a zeolite that is provided on a carrier of activated carbon.

10. A method according to claim 9, wherein the activated carbon has been pre treated for obtaining hydrophobic properties.

11. A method according to any one of the preceding claims, wherein the supply of the at least partly reactive mixture to the zeolite containing material regenerates this material at least partly.

12. A method according to any one of the preceding claims, wherein the process temperature is regulated by means of the supply of ambient air.

13. A method according to any one of the preceding claims, wherein substantially all ozone present in the gas stream is degraded.

14. A system for treating a waste gas stream comprising gaseous contaminant molecules, comprising:

- a supply for supplying a waste gas stream comprising gaseous contaminant molecules;

- a first apparatus for injecting a reactive mixture with radicals into the gas stream; and - a second apparatus comprising at least one layer of a zeolite containing material through which the gas stream treated by the first apparatus is passed.

15. A system according to claim 14, wherein the first apparatus and the second apparatus are accommodated in separate modules.

16. A system according to claim 14 or 15, further comprising an apparatus for separating particles from the gas stream, preferably a cyclone or other filter.

17. A system according to any one of claims 14 - 16, for carrying out the method according to any one of claims 1 - 13.