Patent Publication Number: US-2009223806-A1

Title: Combined treatment of gaseous effluents by cold plasma and photocatalysts

Description:
The invention relates to a process and a device allowing the effective treatment of gaseous effluents, particularly volatile organic compounds (VOC), by combining cold plasma and photocatalysis techniques. 
     The invention is applicable in a large number of applications, particularly in chemical industry, agriculture and food industry, petrochemical industry, water purification, odor destruction, paint workshops, for both efficiency, economical and environmental reasons. 
     Cold plasma and photocatalysis are two technologies that are, in themselves, known and industrially applicable. 
     Photocatalysis is a chemical process involving a photocatalytic agent i.e. so called photocatalyst that is able to destroy the various organic pollutants present in air or water by a reaction provoked by UV photon excitation. 
     Schematically, the photocatalytic reaction is initiated by activating a semiconducting solid by UV radiation at a wavelength of less than 380 nanometers, provoking electronic changes within the semiconductor and leading to, in the presence of air or water, the creation of oxygen radicals at the surface of the semiconductor. These radicals attack the organic compounds adsorbed on the semiconductor and, by a succession of chemical reactions involving the oxygen from the air or water, degrade the organic compounds until the carbon of the carbon chains is completely transformed into carbon dioxide (CO 2 ). 
     The photocatalytic reaction is capable of destroying, according to the above described process, a large number of air pollutants, particularly NO x , NH 3 , H 2 S, CO, O 3 , chlorinated or non-chlorinated alkenes in C 2 -C 4 , chloromethane, isooctane, benzene, toluene, xylenes, isopropylbenzene, saturated or unsaturated aliphatic alcohols, methylmercaptan, chlorophenols, nitrophenols, methyltertiobutylether, dimethoxymethane, aldehydes in C1-C4, acetone, formic acid, acetic acid, 2-methylpropanoic acid, dichloroacetyl chloride, dimethylformamide, trimethylamine, acetonitrile and pyridine. 
     In practice, titanium dioxide (TiO 2 ) is used as a photocatalyst, activated by UV light. This UV radiation leads to, at the surface of the photocatalyst, the formation of hydroxyl radicals OH •−  and superoxide radicals O 2   •  able to attack the organic compounds adsorbed on the TiO 2  and degrading such until complete mineralization of the organic matter. 
     However, it is possible to implement other photocatalytic agents such as, for example, those chosen from the group of metal oxides, alkaline-earth oxides, actinide oxides and rare earth oxides. 
     The utilization of photocatalysis for the treatment of gaseous effluents is for example described in the document WO 00/72945. 
     However, major limitations of photocatalysis reside in the fact that on the one hand, the photonic yield of this reaction is relatively low and on the other hand, certain compounds, in particular fluorinated compounds, may not be decomposed by this reaction. 
     The principle of cold plasma consists of generating, via a current pulse, very highly reactive primary chemical compounds (radicals, ions, excited species) as well as ultraviolet and visible photons. Therefore, it has been proposed to utilize plasma, and, particularly the very high energy of electrons thus generated, to break the bonds or cycles of molecules in the treatment of gaseous effluents. 
     In practice, cold plasma at atmospheric pressure is obtained by applying a high-voltage pulse between two electrodes in order to ionize the gas situated between the two electrodes, while preventing passage to the electric arc. To avoid the development of an electric arc, the prior art recommends one or two dielectric barriers comprised of an insulating material be placed between the two electrodes. 
     In a particular embodiment, a Dielectric Barrier Discharge (DBD) is implemented, comprised of plasma microfilaments (“streamer”) that are established after propagation in an interelectrode space of an electric charge called a streamer. They have very short durations, on the order of several dozens of nanoseconds. Each microfilament is a narrow cylindrical channel (radius from 50 to 100 μm). The exchange of energy between accelerated electrons and molecules principally takes place in the streamer. The energetic electrons (several eV) excite the molecules and therefore transform a part of their kinetic energy into energy stored in the excited chemical compounds and the free radicals. 
     The utilization of cold plasma technology, in particular associated with DBD, has already been described for destroying VOC type molecules. 
     In addition, the prior art mentions the improvement in the treatment of VOC by practicing cold plasma at atmospheric pressure in the presence of a TiO 2  or MnO 2  type solid, then assimilated to a heterogeneous catalyst (Futamara et al., Catalysis Today 89 (2004) 89-95). 
     These two known technologies i.e. photocatalysis and cold plasma, have already been used in combination. 
     EP-A2-1 086 740 discloses a method of treating harmful substances contained in waste gas and an apparatus for carrying out the method. The problem discussed in the EP document relates to treating harmful substances using materials such as TiO 2  as photocatalytic agents, which need to be excited with ultraviolet radiation. The actual problem discussed in the EP publication is that the application of the photocatalysis requiring UV radiation is limited to outdoor use such as load fences, load surfaces and outer walls of buildings. Therefore, the photocatalysis cannot be utilized for purging waste gases emitted from city type incinerators and industrial waste treating plants. As a solution to the problem the EP document suggests that the treating efficiency can be improved by exciting a photocatalysis material with ultraviolet radiation emitted from the discharge plasma. 
     WO-A1-00/71867 discloses a somewhat similar system for the purification of exhaust gases; and, more particularly, for the purification of exhaust gases of an internal combustion engine of a vehicle using precious metals as a high temperature active catalysts. The purification system comprises a reactor including a honeycomb carrier having a plurality of carrier cells, each of which having a photocatalyst layer coated on a wall surface of each of the carrier cells, and a plasma generating means having a plurality of electrode cells and mounted on an inner end and an outer end of the honeycomb carrier. The photocatalyst layer is activated by the ultraviolet radiation emitted from the discharge plasma. 
     Given the efficiency, economic and environmental issues mentioned already above, an ongoing need exists for technical solutions allowing the performance of the destruction of organic compounds in the gaseous effluents of any origin to be improved. However, the performed experiments show that arranging photocatalyst surfaces in a reactor, and activating the photocatalyst by means of UV radiation originating from the plasma is not efficient enough to be industrially applicable, as the effect of the UV radiation emitted from the plasma seems to be marginal. 
     According to the present invention, combined treatment with cold plasma and photocatalysis with additional source of UV radiation is proposed. It turns out that these two technologies are perfectly complementary and compatible: the plasma allows the organic compounds, including halogens, to be destroyed while generating ozone; photocatalysis, activated by its own source of UV radiation in the presence of ozone, leads to faster mineralization of the organic compounds and to degradation of the ozone in the course of the process. 
     In fact, the Applicant shows a synergistic effect when these two technologies are combined for the treatment of gaseous effluents. 
     More precisely, the Applicant shows that the plasma+TiO 2 +(additional) UV treatment gives better results, in terms of degradation speed and residual VOC concentration, than the photocatalysis alone, plasma alone and plasma+photocatalysis treatment (UV originating from plasma). The high efficiency of the combination treatment of the invention allows the treatment of gaseous effluents at a higher flow rate, and/or at a higher concentration. 
     Therefore, the invention relates to a process for treating gaseous effluents consisting of submitting said effluents to simultaneous treatments with both cold plasma and photocatalysis under additional ultraviolet (UV) radiation. 
     This process presents an obvious interest in the treatment of gaseous effluents containing volatile organic compounds (VOC), notably halogenated organic solvents, generated by numerous human or industrial activities, such as painting. 
     The advantage of the claimed process also resides in the fact that the process allows effluents whose range of VOC concentrations varies from 1 ppbv to several thousand ppmv to be treated. 
     The combination of a homogeneous phase process (plasma) and a heterogeneous phase process (photocatalysis) presents various advantages. The combination allows:
         to preconcentrate the species on the photocatalyst surface and therefore promote their degradation;   to increase the degradation of the molecules having little affinity for the photocatalyst surface;   to increase the degradation of the molecules having a large affinity for the photocatalyst by concentrating the active compounds at the location where these molecules are adsorbed.       

     This combined treatment process may be carried out in different ways. The cold plasma treatment is preferably performed simultaneously (simultaneous coupling) with the photocatalysis treatment, but it may, depending on the substances to be treated, be also performed either prior to or after (sequential coupling) the photocatalysis treatment. 
     In the case of simultaneous coupling, the plasma is generated in contact with the catalyst. The additional UV radiation from a UV source promotes the excitation of the photocatalyst agent. In addition, the ozone generated by the cold plasma promotes the separation of electron/hole pairs in contact with the photocatalyst, and promotes the increased formation of active compounds such as OH •  and O 3   •− . The contribution of UV radiation and ozone therefore accelerates the mineralization process. Simultaneous coupling also has the advantage of regenerating photocatalyst by avoiding its possible contamination. 
     In the sequential approach, three configurations are possible: the plasma may be placed upstream or downstream from the photocatalysis with relation to the gaseous flow, or rather the two modes of treatment are located in the same location, but the treatment is sequential in time, by recirculation. In the case where it is placed upstream, the plasma is then used to create radicals that are then mineralized on the photocatalyst. Ozone, formed by the plasma, participates in the mineralization process by decomposing on the photocatalyst. This has the effect of improving photocatalysis since the electronic affinity of ozone is approximately 5 times greater than that of oxygen. 
     The plasma/photocatalysis sequential combination may be planned in three ways:
         the plasma (electrodes) device is situated upstream of the photocatalysis device (UV lamp+photocatalyst) in the conduit.   the plasma device (electrodes) is situated downstream of the photocatalysis device (UV lamp+photocatalyst) in the conduit.   the elements useful to the two treatments are superimposed in the conduit but the treatment is sequential in time: during a first passage, the effluents are subjected to the plasma then during a later passage, they are subjected to photocatalysis.       

     In all scenarios and without wanting to be linked to any theory, such advantageous results obtained by the Applicant may result in:
         faster mineralization due to the fragmentation of the molecules by the electrons and other active species produced by the plasma and the reaction of these fragments with the OH •  and O 2   •−  species produced by photocatalysis;   initiation of the breaking of the bond or cycle not degradable by photocatalysis due to the energy of the electrons formed in the plasma, the ions or radicals formed may then react with the oxidant species created by photocatalysis, leading to complete mineralization.       

     Therefore this process allows gaseous effluents comprising a VOC concentration between 1 ppbv and several thousand ppmv to be treated. 
     In a second feature, the invention relates to a device adapted for the implementation of such a process. 
     Mainly, such a device comprises:
         a conduit allowing passage of effluents, preferably made of a material ensuring the transmission of UV radiation.   two advantageously metallic opposed electrodes, separated by one or two dielectrics. In one case where the electrodes are separated by a single dielectric, one of the two electrodes is situated inside the conduit, in the other case the two electrodes are situated on the outer surface of the conduit.   In all configurations, the electrodes are connected to a generator delivering alternating voltage less than 50 kV and from 1 to 1000 Hz.   At least one lamp generating ultraviolet radiation (UV) directed towards the inside of the conduit. The lamp advantageously delivers a lighting intensity on the order of 50 mW/cm 2 .   A photocatalyst situated inside the conduit, possibly between the two electrodes. It is advantageously comprised of titanium dioxide (TiO 2 ) and deposited on a fibrous support that may be a fiberglass, synthetic or natural organic fiber, or a non-woven based support. Preferably, the photocatalyst is deposited on the fibrous support by using SiO 2  particles, to form a layer or coating having preferably a basis weight of 5 to 40 g/m 2 . This technology and all its variations are described in detail in document WO99/51345. Alternately, the photocatalyst may be deposited on the fibrous support by using the sol gel technique known to the person skilled in the art.       

     The UV source, normally at least one UV-lamp, positioned outside the conduit may be replaced with an UV source situated inside the conduit. An exemplary source of UV radiation capable of being inserted either inside or outside the conduit or reactor is an ultra violet light emitting diode (UV-LED). Another way of introducing UV radiation inside the conduit is the use of optical fibers. The basic principle is that the optical fibers are arranged in such a manner that they introduce the UV radiation to the best possible location in view of the operation of the photocatalyst. Thus, for instance, the optical fibers may be arranged to be part of the fibrous substrate carrying the photocatalyst, or to terminate at the surface of the photocatalyst layer on any surface it is coated on. 
     When the plasma and photocatalysis are performed simultaneously, it is important that the fibrous support coated with the photocatalyst does not disturb the plasma. Two configurations are then advantageously used: in a first optional configuration, the fibrous support coated with the photocatalyst is disposed perpendicularly to the electrodes and to the flow of the gaseous effluents, and then, it is presented in the form of a permeable support, or in the form of stacked honeycombs. In a second optional configuration, the fibrous support coated with the photocatalyst is disposed parallel to the electrodes and parallel to the flow of gaseous effluents, and is then presented in the form of a succession of layers that are parallel to each other. 
    
    
     
       The invention and resulting advantages stand out better in the following two examples of embodiments in support of the attached figures. 
         FIG. 1  represents a schematic diagram of simultaneous cold plasma/photocatalysis coupling in a plane/plane electrode geometry. 
         FIG. 2  represents the degradation of the model compound C 2 H 2  according to the conditions of treatment from the device of example 1 described in  FIG. 1 . 
         FIG. 3  represents the comparison between the combined treatment of the invention and different treatments known in the prior art. 
     
    
    
     The present study was carried out on an alkyne with a triple carbon-carbon bond of the C 2 H 2  formula. The study illustrates the simultaneous coupling of cold plasma and photocatalysis in a plane/plane geometry. 
     A) Device Utilized: 
     The used reactor  2  is a conduit made of Pyrex glass whose active part  4 , 8 cm in length, presents a rectangular section. The conduit has at the ends of the active part  4  two large sized buffer volumes  6 . Concerning the plasma part  5  of the device, two copper electrodes  8  of 20 cm 2  are affixed to the outside of the conduit  4 . The electrodes  8  are thus positioned at a distance of 10 mm from each other, including 6 mm of gas and two times 2 mm of Pyrex glass. The Pyrex glass walls of the conduit form the dielectric barriers. This is a double dielectric barrier discharge that is initiated with the help of a generator  10  delivering an adjustable sinusoidal voltage of 30 kV at a fixed frequency of 50 Hz. Concerning the photocatalyst, this consists of a fiberglass weaving  12  coated with colloidal silica  14  ensuring the fixation of 20 g/m 2  of titanium dioxide. The photocatalytic media is then placed in the active part  4  of the reactor  2  in the gap between the electrodes  8 . Four PLL40 type Philips UV lamps  16  are disposed symmetrically at a distance of 8 cm around the reactor in order to provide additional UV radiation to the photocatalyst, if necessary. The test arrangement further comprises gas sources shown in upper left corner of the drawing. In other words, for test purposes acetylene is mixed with dry air to result in a gas mixture having a desired acetylene concentration, whereafter the mixture is taken into the reactor  2 . The piping in connection with the reactor is arranged such that the reactor may be run in two modes. In a continuous mode pump  18  is used to draw the gas mixture through the reactor  4  and to the gas chromatograph for analyzing purposes, whereafter the gas is discharged from the system. In a recirculation mode the device works in a similar manner up to the gas chromatograph. However, after the gas chromatograph the gas mixture is recirculated back to the reactor such that a closed circulation is created. 
     B) Conditions of Utilization of the Device: 
     The experiments carried out with the help of this device in the presented example were performed in “recirculation” mode, ensured by a pump  18 . A flow of 180 ml/min continuously flows through the reactor  2  until complete degradation of the pollutant. The initial concentration of the chosen pollutant is 3000 ppm. The destruction of this pollutant, acetylene, was followed under four particular conditions: action of the photocatalyst alone, activated by UV radiation from external lamps; action of plasma alone, without photocatalytic material and additional UV radiation; action of the plasma and the photocatalytic material without additional UV radiation; and lastly, action of the plasma and of the photocatalytic material with additional UV radiation. When the plasma participates the process, the specific energy of plasma is adjusted to 60 J/l. The calculations of the injected specific energy are performed by the conventional method called the Manley method. When the additional UV radiation is used in the process, its average intensity is 3 mW/cm 2  for a 365 nm wavelength. 
     C) Results: 
       FIG. 2  presents the evolution of the acetylene concentration in the reactor described above, as a function of time for the four aforementioned experimental conditions. The comparison of the curve relative to photocatalysis alone (TiO 2 +external UV) with that relative to plasma alone shows very different kinetics that distinguish the two processes. By comparing the curve relative to the plasma process alone and that relative to the plasma+TiO 2  process, a significant improvement in the effectiveness of the destruction of acetylene is observed. This phenomenon is connected on the one hand to the activation of the photocatalytic material by the UV radiation generated in the plasma medium, and on the other hand to the effect of the porous surface introduced in the cold plasma that stabilizes the active species and concentrates the carbon fragments at the surface of the activated material. Lastly, the comparison of the curves relative to the plasma+TiO 2 +external UV process and relative to the plasma+TiO 2  process shows that it is possible to activate the photocatalytic solid in situ with the external UV lamps giving artificial UV radiation and to thus increase its activity in the discharge. 
       FIG. 3  discusses a series of experiments aiming at more clear comparison between the various treatments. This test was carried out such that a volume of 1.5 liter of C 2 H 2  laden gas was circulated by means of the pump  18  in the reactor  4  of  FIG. 1  for a test period of 7.5 minutes. Six different experiments were run. Two experiments relate to conventional methods, i.e. photocatalysis alone (TiO 2 +UV), which requires the use of external/additional artificial UV-light; and plasma alone. The comparative experiments were as follows: plasma together with a fibrous substrate coated with 20 g/m 2  TiO 2  and 20 g/m 2  SiO 2  (Plasma+TiO 2 ), plasma together with a fibrous substrate coated with 40 g/m 2  SiO 2  (Plasma+SiO 2 ), plasma together with a fibrous substrate coated with 40 g/m 2  SiO 2  with additional UV light (Plasma+SiO 2 +UV), plasma together with a fibrous substrate coated with 20 g/m 2  TiO 2  and 20 g/m 2  SiO 2  with UV-light (Plasma+TiO 2 +UV). When using TiO 2  in connection with plasma the titanium dioxide particles having an average diameter of 100 nanometers were wrapped by silicon dioxide particles having an average diameter of 20 nanometers. I.e. the silicon dioxide particles acted as binders for attaching the TiO 2  particles on the substrate. At the end of each test period the acetylene concentration was determined by means of gas chromatography. The table in  FIG. 3  shows the relative amount of acetylene left at the end of each test period. 
     If we take the two known treatments i.e. plasma alone and photocatalysis alone as the starting point of the invention, we see that the degradation rate of plasma alone is 66%, and photocatalysis alone 8%. Thus the theoretical combined effect should be 66+8=74%. For studying the effect of UV radiation created by the plasma the effect of plasma together with both SiO 2  and TiO 2 , 20 g/m 2  each, without external UV light, is compared to plasma together with SiO 2 , 40 g/m 2  alone, and plasma together with both SiO 2  (40 g/m 2 ) and external UV radiation, where there is, in practice, no difference. Since SiO 2  is a mere adsorbent the external UV radiation has no effect on its function (with and without UV give the same 76% degradation rate). However, the result concerning the use of TiO 2  (in fact both SiO 2  and TiO 2 ) together with plasma and without additional UV radiation are interesting. Now that TiO 2  is, by its nature, both a photocatalyst and an adsorbent, the specific areas of the coatings (SiO 2  and TiO 2 ) of the substrate have to be taken into account. Since the SiO 2  particles are, as already discussed above, much smaller than the TiO 2  particles, the specific surface area of the TiO 2  particles is much smaller. Thus the capacity of the TiO 2 /SiO 2  coating to adsorb impurities is significantly smaller than that of a coating having an equal basis weight and being composed merely of SiO 2 . However, the degradation rate of 75% of the TiO 2 /SiO 2  coating is substantially the same as that of 76% of the pure SiO 2  coating. The only explanation for this feature is that the UV radiation created by the plasma is able to penetrate the SiO 2  coating to the TiO 2  particles and initiate photocatalysis that compensates the smaller specific surface area of the adsorbents. Finally, the combination treatment with plasma and photocatalysis provided with external UV radiation give the degradation rate of 89%. When compared with the arithmetically calculated theoretical sum of 74% for the degradation rate (discussed already above) it is easy to see that the simultaneous treatments with both plasma and photocatalysis with additional UV radiation resulting in surprisingly better degradation rate of 89% bring synergistic effects, which were nor foreseeable prior to the performed experiments.