Medium for photocatalytic filter

MEDIA FOR ALYTIC PHOTOCAT FILTER

The invention relates to a filter medium with a fibrous structure, the fibers of which are coated with a coating with photocatalytic action, for the purification of ambient air and more particularly the elimination of organo-volatile compounds contained in ambient air.

The so-called Advanced Oxidation techniques allow the oxidation of organo-volatile compounds (VOCs). The most effective Advanced Oxidation Techniques (TAO) are those that lead to the formation of OH- hydroxyl radicals, which have an oxidizing power superior to that of traditional oxidants. This is the case of heterogeneous photocatalysis. The fundamental principle of the phenomenon is the absorption of a photon by a solid semiconductor, leading to the promotion of an electron from the valence band to the conduction band by liberating a gap, and thus conferring on the solid properties oxidizer and reducer. Most organovolatile compounds as well as many pesticides, herbicides, surfactants and dyes are completely oxidized to less toxic products by this technique.

A PCO (photocatalysis oxidation) reactor for the purification of ambient air generally includes a pre-filter to trap dust and particles, a UV source, a PCO filter. The UV source is generally placed between the pre-filter and the PCO filter. The air to be purified is generally drawn or sucked through the PCO filter using a turbine or a fan.

To be operational, a PCO filter must be optimized on the following points: - UV power received,

- purifier flow,

- speed of passage of pollutants in the media,

- inertia of the PCO medium and coating to the action of UV and hydroxyl radicals, - pressure drop caused by the PCO medium,

- limitation of the creation of potentially toxic intermediate compounds also known as by-products or by-products.

In air treatment applications, the dimensioning of the different elements, fans, ducts, motor power are directly linked to the pressure drop, pressure drop depending on the different filtration elements of the system including the PCO media. This point is fundamental both from a cost point of view of the air treatment unit and its cost of energy exploitation. It was the present inventors who noted the importance of the question of the pressure drop caused by the filtering medium.

The filters already proposed for this type of application often cause too great a pressure drop, so that they require the use of more powerful and more energy-efficient fans. To overcome this drawback, it was then proposed to lower the density of the filter by inserting elements such as honeycomb, fabric with high porosity rate, mosquito net or ceramic foam, but then, real Preferred channels were created and the efficiency of the filter for the oxidation of organo-volatile compounds was reduced due to the lack of "effective" material in contact with the air flow.

WO03010106 teaches the deposition of photocatalytic coating on the surface of veils or silica felts with a specific surface at least equal to 10 m²/ g, in particular at least 30 m²/ g. This document does not suggest the concept of low pressure drop combined with sufficient efficiency in the intended application.

As documents of the prior art, mention may also be made of US4732879A1. This document teaches the deposition of porous catalytic coating on flexible fibrous substrate composed of glass or ceramic fibers. This document suggests the use of such a substrate in “filter bag” applications. EP1 132133 teaches a photocatalytic TiU reactor₂ on pure sintered silica. Such a sintering gives a high rigidity to the structure, which is not desired since it may be desired to fold it. In addition, it can be seen in FIG. 3 of this document that a high pressure drop is exerted. Indeed, at 1 m / second (or about 3600 m³/ h), the pressure drop is 200 Pa in the best of cases. Finally, such a product is very brittle and very fragile if it is fine. Because of its fragility, one cannot reduce its thickness to reduce the pressure drop which it opposes.

WO00 / 25919 and WO00 / 76660 teach the use of a mat of needled cut threads as a support for a photocatalytic coating. The choice of these fibers requires the use of an organic binder and leads to a mat of high density (150 to 600 g / m²). It is not possible with such fibers to have at the same time a low density and a low pressure drop.

WO99 / ​​64364 (or EP1084086) teaches an adhesion promoter for photocatalytic coating. This adhesion promoter is organic.

US6241856 teaches an organo volatile compound analyzer and a pump which is used to circulate the gas but which is in no case controlled by the analyzer. The pump is therefore not used to regulate the gas passing through the purifier according to the result supplied by the analyzer. As other prior art documents, mention may be made of

US6358374 and WO03 / 037389.

The invention relates to an essentially mineral, stable filter medium, insensitive to UV and PCO oxidation, this medium combining a pressure drop compatible with the requirements of air treatment systems and a high efficiency towards atmospheric pollutants due of its active surface and of a PCO effect in its volume and not only a surface effect as is usually encountered according to the prior art. This filter media is preferably essentially mineral (ie inorganic), which means that its loss on ignition is less than 0.1% by weight and even less than 0.01% by weight, or even zero.

Such a filtering medium can in particular be obtained by the use of a felt produced by stretch-blowing of its fibers, which makes it possible to dispense with the use of binder and even of any mechanical binding action (needling, sewing).

The filter medium according to the invention is obtained after depositing a coating with photocatalytic action on a nonwoven of the felt type.

The invention relates firstly to a filter medium with photocatalytic action at least 2 mm thick, homogeneous and without any opening visible to the naked eye, comprising a felt of inorganic fibers, the fibers of which are coated with a coating comprising a catalyst with photocatalytic action, said felt having a surface mass of between 30 and 80 g / m², said coating representing 5 to 80% of the mass of said media, said media having a gas pressure drop of less than 150 Pa at 1 m / s in the non-pleated condition.

The invention also relates to the use of this medium in different applications, its shaping in order to maximize the "front" active surface, its shaping in order to minimize the problems of pressure drop.

The invention also relates to a method and a device for modulating the UV intensity and the speed at the level of the substrate at the start of a scrubber and / or in the event of a pollution peak, so as to reduce the formation of reaction intermediate compounds which may be toxic.

Finally, other objects of the invention are the applications of the PCO media and of the PCO purifier system in the treatment of gaseous ozone effluents in indoor, domestic or industrial air atmospheres, alcohol and solvent vapors in strong industries using these products (composites, perfume manufacturing, etc.).

The medium according to the invention can be used to purify the atmosphere of premises for domestic use (housing), or premises in the tertiary sector (building containing offices). Generally, a felt with a surface mass of up to 300 g / m can be used². However, the felt used in the context of the present invention, because of its basis weight of between 30 and 80 g / m², offers a very low pressure drop to the gas passing through it. The felt and the media according to the invention are rather suitable for purification in a domestic environment. Note that for the tertiary or industrial sector, substrates with a greater basis weight may be necessary, such as 200 to 300 g / m². An alternative for the tertiary and industrial sectors is the use of several media according to the invention placed in series one behind the other. For these sectors, it is also possible to use a single filter media comprising a felt to which the catalytic coating has been applied as explained in the present application, except that the felt has a surface mass greater than 80 g / m², for example 80 to 300 g / m². We can also put several of these media in series one behind the other.

The felt to be used as a substrate may have a density of less than 30 kg / m³. The felt to be used as a substrate generally has a density ranging from 0.5 to 60 kg / m³ and more generally from 1 to 30 kg / m³.

The photocatalytic coating formed according to the invention on the surface of the fibers of the felt used as substrate, represents 5 to 80% and generally 10 to 50% of the mass of the filtering medium. The catalyst with photocatalytic action generally comprises at least one oxide from the group of the following oxides: TiO₂, ZnO, CeO₂. It preferably comprises at least partially crystallized titanium oxide.

The felt is a fibrous structure with mineral fibers. These fibers can be based on silica like glass (generally containing at least 30% by weight of silica, glass can be of type E, C, R, S, D, AR), washed glass (leached fiberglass chemically then possibly thermally stabilized generally containing more than 90% by weight of silica, and in a standard manner between 96% and 99% by weight of silica), ceramic (mention may be made of fibers based on mullite, of which UNIFRAX.THERMALS CERAMICS are well known suppliers, NEXTEL fibers from 3M, pure alumina fiber marketed under the trade name SAFFIL) or pure silica (also called quartz and containing at least 99% SiO₂ amorphous).

Some glass compositions suitable in the context of the present invention are given in Table 1 below: <img file="WO2009019387A2_D0001.tif" />

Table 1

One can also use metallic fiber (generally based on stainless steel 316 or 316 L whose main suppliers are Bekaert and UGITECH). The material used is preferably glass and more preferably silica in order to be as transparent as possible for UV illumination in the application, since UV then penetrates better into the heart of the filtering medium to make it more active.

The felt is preferably unsintered and without organic matter, which is in particular possible by the use of the stretch-blow-molding process described below, applied to mineral matter.

To make a felt of mineral fibers, in particular comprising silica (glass or pure silica), one can for example proceed by stretching rods of the material considered (as in silica or glass as the case may be), of diameter generally less than 7 mm in a burner (in particular oxy-propane) in order to bring them to a filament diameter of less than 0.5 mm. This filament can then be stretched by stretch-blown flame in a second burner and projected onto a moving surface like a moving belt or a rotating receiving drum. The filaments thus obtained generally have a diameter of less than 50 μm, and optimally centered on 9 μm, for example between 7 and 15 μm. Larger filaments generally cause the felt to lose flexibility. By this process, several tens of filaments can be drawn simultaneously. The drawn material can be of quartz, silica, glass, and more generally any type of heat-fusible mineral material, which includes alumina and mullite. This nonwoven (or mat) manufacturing process by stretch-blown flame followed by projection onto a moving surface (moving belt or rotating receiving drum) leads to a particularly homogeneous structure free of orifices. visible to the naked eye, even at very low surface mass. This technique produces fibers that are substantially crimped, which means that they bond naturally to each other by interlacing to form a coherent non-woven mat without the need to use a binder or to perform a mechanical binding. such as needling or sewing. This crimp is more easily obtained by adjusting the flame producing the stretch in turbulent condition.

A person skilled in the art knows, moreover, that a needling of mineral fibers produces holes visible to the naked eye and that the needled mats are not homogeneous for a surface mass of less than 150 g / m². Conventional topping techniques do not make it possible to make homogeneous mats which stand well below a surface mass of 200 g / m². Spinning techniques through dies lead to fibers requiring the use of organic binders to give consistency to the nonwoven. However, these organic binders decompose under the action of UV, which degrades the mat and is capable of generating VOCs.

Pure silica fiber (at least 99% SiO₂) is particularly preferred because it is particularly transparent to UV, which allows it to convey in the manner of optical fibers and with a minimum of absorption UV in all the media.

The felt obtained by this process (flame stretch-blowing followed by projection onto a moving surface) is a nonwoven whose surface mass is adjustable according to the speed of the receiving system (like a rotating drum) . The reception system is adjusted so as to obtain a surface mass of between 30 and 80 g / m². These felts have a thickness ranging from 1 to 200 mm and a density of less than 60 kg / m³. The fibers of this nonwoven have a length generally ranging from 3 cm to 100 cm. This stretch-blow molding process allows the production of pure silica or glass mats (at least 60% SiO₂ in the case of glass). These mats are excellent and preferred because they are flexible (because not sintered) and free from any organic matter.

The final medium has a thickness generally less than that of the felt used, and generally ranging from 1 to 50 mm and more generally between 2 and 30 mm.

As an alternative to this stretching process, it is possible to start with already existing fibers of diameter 7 to 14 μm which are cut to lengths less than 150 mm and generally greater than 45 mm. The cut fibers are then formed into a sheet either by pneumatic lapping or by carding-lapping. The sheet thus formed then undergoes a pre-needling followed by a needling at around 100 strokes / m². By this methodology, sheets of areal mass between 60 g / m² and 2000 g / m² can be done. In the PCO applications concerned by the present invention, weights of less than 80 g / m² will be privileged. These products have thicknesses generally less than 30 mm, and generally densities less than 70 kg / m³ and even less than 60 kg / m³. The density and the thickness are adjustable by a person skilled in the art according to the number of strokes / m² of substrate that the needling exerts, which more or less densifies the felt.

The media can also be prepared by the papermaking process (dispersion of the fibers in a pulper followed by shaping by the wet papermaking process) using, in order to keep an essentially mineral structure, a ceramic precursor binder preferably of the soil type. gel, in particular with a precursor for example of the TEOS type (Tetraethyl orthosilicate), MTES (Methyltriethoxysilane) which after calcination will ceramize. This binder can be deposited locally by point or according to a predefined pattern in order to save the flexibility of the felt.

With the processes which have just been mentioned, a felt is obtained without a hole visible to the naked eye.

The preferred fiber as support felt for the photocatalytic catalyst is a quartz fiber (at least 99% silica) because it supports well the ceramization of the silica sol-gel (between 400 and 600 ° C), it is very pure, free of alkalis, and therefore particularly inert vis-à-vis the catalyst, and moreover, it conducts UV very well without absorbing them.

The finished felt is then impregnated with a solution comprising an organic silica precursor (such as TEOS, MTES see a mixture of several alkoxysilane precursors of chemical formula R'χSi (OR)_4-x in which R and R 'are organic radicals and x an integer ranging from 0 to 3), and of a dispersion of a compound with photocatalytic action, such as TiO₂ with photocatalytic action or Zinc oxide (ZnO), titanium oxide nonetheless remaining the preferred catalyst because of its high efficiency in PCO applications. The invention also relates to a method of manufacturing a media comprising a step of impregnating the felt of inorganic fibers with a composition comprising Tetraethyl orthosilicate (TEOS) and at least one alkoxysilane of formula R '_xIf (OR)_4-x in which R and R 'are organic radicals and x is an integer ranging from 0 to 3, the amount of alkoxysilane representing 10 to 40% and preferably 15 to 25% by weight of TEOS.

The impregnation solution can be prepared according to the indications contained in WO9710186 and WO03087002. By way of example, the impregnation solution can be prepared by premixing a solution A (silica precursor) and a solution B (surfactant), a dispersion of titanium oxide then being added to said premix. By way of example, it can be prepared on the basis of the ingredients indicated in table 2 below:

<img file="WO2009019387A2_D0002.tif" />

Table 2

The amount of water is adjusted to obtain a volume of 100 liters at the level of the final solution. Two solutions A and B are therefore prepared which are mixed, then a suspension of the TiO catalyst is added.₂ in water to this mixture of A + B. The 33.33 kg of C is the mass of suspension of the 19.3% catalyst (and not of pure catalyst).

Table 3 below gives a second example of the preparation of an impregnation solution:

<img file="WO2009019387A2_D0003.tif" />

Table 3

A composition particularly suitable for depositing the coating uses a mixture of MTES and TEOS as a silica precursor. Indeed the sol-gel obtained from this mixture is more flexible and less subject to dusting if we compare to a 100% TEOS or 100% MTES precursor. Preferably, a mixture of 15-30% of MTES is used for 85-70% of TEOS.

The felt is impregnated in the full bath with the impregnation solution, which is sucked through the felt, which is then expressed (which means: pressed to remove the impregnation juice) and dried.

The felt obtained is then calcined at a temperature ranging from room temperature to 550 ° C., in particular at approximately 450 ° C., which then makes it possible to transform the precursor from silica to silica. Preferably, the rise in temperature to the maximum temperature is carried out with a moderate speed, preferably less than 6 ° C. per minute. As an example, this heat treatment can be that indicated in table 4 below:

<img file="WO2009019387A2_D0004.tif" />

Table 4

The cooling can be natural cooling in the ambient air.

In the case of the use of Si and Ti precursors for producing the coating with catalytic action, preference is given to the ratio of the Si and Ti precursors so that the molar ratio Si / Ti in the coating with catalytic action is understood. between 0.25 and 1.35 and more preferably between 0.5 and 1.3.

The filter medium with photocatalytic action according to the invention is thus obtained. This medium can be sucked up by a suction table in order to remove the particles (micron and sub micron) of coating having low adhesion. This avoids significant dusting of the PCO media and generation of particles during the first starts of the PCO purifier.

It can be mounted in cassette and photocatalytic reactor systems.

Mass of catalyst (as TiO₂) is generally less than or equal to 40% by weight and if possible less than 30% by weight, optimally about 15% by weight on the final product (media) obtained. Preferably, the mass of catalyst is greater than or equal to 1% by weight and more preferably greater than or equal to 5% by weight relative to the final medium.

In the case of a TiO catalyst₂, this preferably contains as much anatase as possible.

In certain types of application (bactericidal application, destruction of ozone and sulfur-containing compounds of type H₂S or DMDS (Dimethyl disulfide of formula CH₃-SS-CH₃)) the composition can be doped with at least one compound such as MnO, Mn₂O₃, dicyanoanthracene (DCA) or a compound comprising at least one of the elements from the group of elements V, Cr, Mn, Mo, In, Sn, Fe, Ce, Co, Cu, Nd, Zn, W, Nb, Ta, Bi, Ni, Ru, Ag, said compound being in a concentration of less than 0.5% by mass of the mass of catalyst, in order to accentuate the effectiveness of the medium. The felts prepared as indicated above have the following properties: they are essentially of mineral nature; they have a pressure drop of less than 150 Pa at 1 m / s, and more generally less than 50 Pa at 1 m / s and even less than 20 Pa at 1 m / s of gas; they are homogeneous and therefore have no preferential path for the gas passing through it; they do not have a hole visible to the naked eye.

The production of the photocatalytic coating can sometimes pose problems of adhesion on the fiber, in particular when the latter is subjected to mechanical stresses, even weak, for example during handling. The detachment of the coating results in the formation of an undesired powder. This separation is also called "dusting".

To reduce this powder formation, the thickness of the coating can be reduced. The coating can also be fixed using a polymer. The latter must however resist oxidation under the combined effect of UV and PCO effect of the catalyst, the powers received very often being between 2 and 40 mW / cm² UVA, UVB or UVC.

It has been discovered that polymers comprising fluorine such as polytetrafluroethylene (PTFE) or a fluorinated silane (such as the fluorinated silanes marketed under the reference F8820, F8810, F8263 by the company DEGUSSA), and to a lesser extent a polysiloxane polymer (such as a silicone ), have sufficient UV and PCO reaction stability in this type of application. The polymer can be deposited on the fibers of the medium in the state of dispersion in a liquid, in particular using an aqueous dispersion. An aqueous dispersion of the polymer which is free from surfactant or contains the least possible surfactant is preferably used. Indeed, with certain type of surfactant used for these dispersions, for example to disperse PTFE, a strong odor may appear in the application at the start of the use of the filtering medium, due to the degradation of the surfactant. under the PCO action. Suitable polymer dispersions are, for example, the PTFE dispersions sold under the references TEFLON 30

B, TEFLON 304 A₁TEFLON B, TEFLON-3823, sold by the company

DUPONT DE NEMOURS. Mention may also be made of the silicone polymers of Rhodia silicone such as Rhodorsil Resin 20 B at 83%, Rhodorsil Resin 6405, the siloxane polymers of Wacker such as SILRES H62 C. PTFE is a preferred polymer.

The polymer dispersion is applied to the media after the ceramization heat treatment which has led to the formation of the coating with photocatalytic action. Generally from 0.1 to 5% by weight of polymer relative to the mass of the final media is deposited on the media. The polymer can be deposited either by spraying on one side or on both sides of the suspension, or by immersion-soaking in the suspension followed by expression. These impregnations can generally be carried out at room temperature, in particular at a temperature between 10 and 40 ° C. In the case where the dispersion of the polymer comprises a surfactant, preferably, a heat treatment is generally carried out between 45 ° C and 250 ° C (rather between 150 and 250 ° C in the case of a fluoropolymer (not polysiloxane type), for example a few minutes at 230 ° C and rather between 100 and 180 ° C in the case of a fluorinated polysiloxane) or UV (in this case at high UV intensity, in particular between 15 and 100 mW / cm²) after the application of the polymer dispersion and before actual use in order to better eliminate the surfactants or surfactants used to disperse the polymer in the dispersion, in particular fluorinated polymers in aqueous dispersion. Incidentally, it has also been discovered that PCO substrates, the coating of which comprises a hydrophobic polymer, in particular fluorinated or of the polysiloxane type, can float on the surface of water. Such media are of great interest for purifying the atmosphere emanating from a settling tank, a treatment plant, a lagoon (of factories), etc. Indeed, the media can be cut into pieces of surface ranging from a few mm² a few cm², poured on the surface of the water to be treated. Due to its floating nature, the media is easily distributed on the surface of the water (without adding significant additional costs such as floating tarpaulins mounted on floats). The media then adsorbs the pollutants emanating from the polluted water and oxidizes them under the action of solar UV. This principle is very economical in order to very significantly reduce the nauseating and sometimes dangerous fumes in terms of chemical compounds of this type of installation. Thus, the invention also relates to a method of purifying the air above a water containing impurities and generating organo volatile compounds in the air above said water, thanks to the placement on the surface of the water from a self-floating photocatalytic filtering medium (according to the invention), by means of a suitable coating allowing it to float. This coating preferably comprises a hydrophobic polymer applied to the fibers of said media.

The medium according to the present invention combines an active and homogeneous surface (with no apparent preferential path for air) over the entire surface of the medium with a very low pressure drop, in particular due to its very low grammage (or areal mass) and of its associated low apparent density. In addition, the medium having a high thickness, it makes it possible to have a photocatalytic oxidation efficiency (PCO efficiency) throughout its thickness. A simple way to characterize the photocatalytic activity of a medium is to test in cross mode the percentage of degradation of a specific pollutant, such as methanol, in a laboratory reactor. To this end, a reactor developed by the team of Professor PICHAT of the Ecole Centrale de Lyon is commonly used in academia and laboratories. This reactor generally consists of a stainless steel body inside which is placed a media disc, for example with a diameter of 47 mm. In the upper part of the reactor, UV illumination from an HPK 125 W lamp is carried out through a silica slot. The illumination power is adjusted by adjusting the distance between the Media lamp. A power of 5 mW of UVA per cm² of media, measured at 365 nm measured at media level, is generally used. Upstream of the reactor, a constant flow of filtered air containing 300 ppm of the pollutant (especially methanol) is introduced at the rate of 350 ml / min. Downstream of the reactor after PCO treatment, the pollutant concentration is then measured, generally by chromatography. It is important to ensure mineralization (conversion to CO₂ and H₂O) of the pollutant by checking the chemical equilibrium so as not to have only pure adsorption phenomena as can be found with systems of the activated carbon type. This test applied to methanol is called “methanol test” below.

The filtering medium according to the invention, in a non-pleated condition, causes a pressure drop of less than 150 Pa at 1 m / s of gas, and even generally less than 50 Pa at 1 m / s and even less than 20 Pa at 1 m / s of gas, which is remarkably low, while providing excellent purification.

In the event of a high concentration of pollutant in the atmosphere to be treated (for example during pollution peaks or at the start of the depollution treatment with the filtering medium according to the invention, which can easily be detected by sensors of organovolatile compounds) , the formation of intermediate products such as formaldehyde, acetaldehyde or acetone is all the more marked as the flow rate and / or the UV power is important. These derivatives being for some particularly toxic, in order to palliate their formation, it is then recommended (if these particular conditions occur), momentarily, to:

- greatly reduce UV intensity and / or

- greatly reduce the flow rate of the purifier, that is to say the speed of passage of the gas to be treated through the filtering medium. By combining these two actions, a photocatalytic purifier allows efficient air purification without increasing the rates of harmful intermediate products during the first minutes of the start of the device or of the pollutant peak flow. Thereafter, the UV intensity and the flow rate of the device can be raised to their nominal value in order to ensure a maximum PCO effect.

This reduced operating regime is to be followed at the start of operation when the room or the atmosphere is heavily loaded with VOCs. In this case, the VOC in high concentration at the start generates other VOCs by decomposition in contact with the filtering media, said other VOCs themselves being in relatively high concentration, and the combination of a high concentration of VOC and d 'a high gas speed then perhaps does not allow the filtering medium to' mineralize 'all the VOCs in the purifier. In other words, there would be so much VOC in the scrubber that a high proportion of these VOCs could pass through the scrubber. This risk decreases the lower the initial VOC concentration because then the surface of the filter media becomes sufficient to mineralize all the intermediate VOCs or not. It is therefore the same chemical reactions of decomposition of VOCs in the purifier that take place, but in the case of a high concentration of VOCs in the atmosphere to be purified, the mineralization process (following chemical reactions) risks not being able to go all the way due to saturation of the purifier.

As an example of a chemical decomposition reaction in the scrubber, a methanol decomposition process is: methanol - ^ Formaldehyde -ï Formic acid -ï CO₂. As an example of a series of chemical reactions, a process for decomposing ethanol is: Ethanol - ^ Acetaldehyde -ï Acetic acid -ï Formaldehyde - ^ Formic acid + CO₂ -ï 2 CO₂ . Depending on the characteristics of the photocatalytic purifier (type of media, air speed, UV illumination level) and the concentration of VOC at the inlet (pollutant) of the purifier, the intermediate reaction products may or may not be completely mineralized during of the passage on the photocatalytic medium of the initial molecule of initial pollutant.

In order to further increase the efficiency of the photocatalytic reactor system according to the invention, it is possible to act on the increase in the surface of active media and the reduction in the pressure drop. If the installation of several filter media in series makes it possible to increase the active surface, the pressure drop is however correlatively increased. In order to achieve this objective, it is possible to combine the increase in surface area of ​​active photocatalytic media with an increase in the frontal area, the pressure drop being all the smaller the greater the frontal area in contact with the air flow. . To this end, the media according to the invention can be placed in a filtration cassette so as to provide a longer surface for the gas to be treated. Thus, instead of being arranged in a simple linear thickness transversely with respect to the direction of the gas, it can be given a shape comprising at least one angle, like a shape in V, W, etc. (until forming a true "Accordion") or give it a pleated structure. The media can also be placed in cassettes articulated according to the preceding designs allowing the cassette to be placed in a reactor with the minimum of wasted space. This point is particularly interesting in domestic air conditioning systems.

Indeed very often, the suction ducts are bent very quickly behind the dust filtration elements. Since the PCO scrubber systems are placed in the dust filtration zone, the size of the PCO reactor must be limited. Such an articulated cassette system makes it possible to dispense with the bulk of an angle system (s) for its installation while taking advantage of the angle form (s) once in the working position which allows decrease the system pressure drop. The cassette can therefore be inserted straight (without angle) into the slot allowing it to be introduced into the PCO reactor and the angle form (s) is taken automatically inside the reactor at the time of insertion. FIGS. 3a and 3b show PCO filters inside which the PCO medium has a V or W shape. Thus, the invention also relates to a filter cassette comprising at least one angle and containing a PCO medium, said angle being if necessary articulated.

The PCO purifier according to the invention is in particular intended for domestic air purification applications. One of the major applications is the reduction of ozone in the domestic environment. Thanks to the invention, an ozone reduction efficiency of 90% can be achieved.

The PCO purifier according to the invention is also used for purifying the air in a tertiary, commercial or industrial environment. In this type of application, it is recommended to use media according to the invention in series to generate a higher surface mass compared to what is suitable for the domestic environment. For example, in the context of use in an industrial catering environment equipped with a grease cleaning system (from ozone), a media at 50 g / m² generally does not make it possible to cut down sufficiently high ozone levels up to 300 ppb in a pass at 1 m / s. It is then necessary to use media according to the invention in series (that is to say one after the other, if necessary in contact), generally between 2 to 10 media, more particularly 3 to 6 media, which allows in one pass to oxidize for example 150 ppb of the 300 ppb of ozone present in the inlet gas, the UV power received by filter being 50 mW / cm² UVC illumination. This type of process can be envisaged in particular at the level of industrial kitchen executories in which ozone is generated to eliminate the greases deposited at the level of the hoods for capturing kitchens. It follows a strong odor and concentration of ozone which can be destroyed by the PCO system according to the invention.

The PCO system according to the invention can also be used in industrial applications such as hangars or refrigerators for storing fragile plant products (such as fruits, vegetables, flowers). In this context, it is important to decrease the concentration of ethylene in the shed to slow the ripening of the fruits or the fading of the flowers. The invention also relates to the use of the media or the purifier or the device according to the invention for purifying the air of a shed or a refrigerator containing a plant, in particular a fruit or vegetable or a flower.

The PCO system according to the invention is also very effective in degrading alcohols (methanol, ethanol, propanol) and solvents, for example used in the resins, composites or perfume manufacturing industries. In this type of application, it is necessary to work under ATEX regulated conditions (the expression ATEX coming from "Explosive ATmospheres") in order to avoid any risk of explosion. One of the issues is the nature of the PCO filter media, which must be essentially mineral in order to avoid any risk of ignition. In particular, preferably, for ATEX applications, polymer post-impregnation is not applied to the media to limit dusting. The media of the present invention is then composed of a mineral substrate (felt) with a mineral coating doped with titanium oxide and perfectly meets the specifications of PCO applications in ATEX medium.

The PCO system according to the invention contains a light source necessary for the catalytic activation of the titanium oxide coating. This source can be a UVA, UVB or UVC lamp with mercury or Xenon vapor.

Furthermore, the invention also relates to devices for particular illumination of a PCO medium, suitable for the PCO medium according to the invention or any other PCO medium. These devices are of particular interest from the point of view of energy savings, reduction in maintenance costs or ATEX compliance. The fibrous media lighting device can be realized with an LED

UVA, UVB or UVC with a lighting power of at least 1 mW / cm². Such a system makes it possible to combine minimal energy consumption with notable efficiency. In addition, this type of lighting system allows very specific designs with optimization of the lighting of the media. Thus, the invention also relates to a gas purifier comprising a filter medium with photocatalytic action (according to the invention or not) and a system for UV illumination of said medium, said illumination system comprising an LED, preferably UV, generating an intensity received by the media at least equal to 1 mW / cm² of media. The device for illuminating the fibrous medium can be produced by a light guide, for example an optical fiber: this system makes it possible to move the source away from the illumination zone and thus relatively easily create an ATEX zone in the PCO reactor . Thus, the invention also relates to a gas purifier comprising a filtering medium with photocatalytic action (according to the invention or not) and a UV illumination system of said medium, said illumination system comprising at least one light guide ( like an optical fiber) to bring light to said media. In industrial applications, it is not uncommon to find ATEX zones (outlets, capture). To date conventional UV lamps are not ATEX approved due to the fragility of their envelope. A light guide lighting system allows the lamp to be taken out of the ATEX zone and thus makes the system conform to the requirements of the ATEX zones. The following systems can be considered:

• from a UV source, several light guide strands (for example of the optical fiber type) can be brought into the reactor. These strands serve as a guide for bringing the light energy into the reactor. For example, we can consider a strand every cm.² substrate so as to distribute UV energy as homogeneously as possible in the reactor. The principle of such a system is shown in Figure 14. • from a source, we can bring a single strand of light guide

(like optical fiber) in the reactor. This strand then illuminates a reflector or a mirror provided with a suitable curvature and the UV light is reflected by this mirror or this reflector as homogeneously as possible in the reactor. The principle of this system is shown in Figure 15. The use of any other diffuser system can also achieve this objective.

The fibrous media illumination device can also be centralized with respect to several PCO reactors. This makes it possible to have a single UV generation area relayed by a lighting system using a light guide (for example of the optical fiber type) from different PCO media. This system provides a significant energy gain by avoiding the need to have multiple sources, by reducing the consumption of ballasts and the inevitable losses due to multi-source systems. The principle of this system is shown in FIG. 16. Thus, the invention also relates to a device for purifying the air comprising several air purifiers each comprising a filtering medium with photocatalytic action (according to the invention or not), and including a single source of illumination from the scrubber media. Thus, the invention also relates to a gas purifier comprising a filtering medium with photocatalytic action (according to the invention or not) and a UV illumination system of said medium, said illumination system comprising light guides (for example fiber optic type) and / or cold light. The device for illuminating the fibrous medium can be produced by a flat lamp.

This system allows an extremely homogeneous illumination of the entire surface of the PCO media and at the same time increases the oxidation yield and therefore the efficiency of the system. Thus, the invention also relates to a gas purifier comprising a filter medium with photocatalytic action (according to the invention or not) and a UV illumination system of said medium, said illumination system comprising a flat lamp.

When the PCO system is operated in an atmosphere polluted with VOCs, the concentration of these VOCs can be quite high at the start of operation. This high concentration of VOCs can result in the undesired formation of intermediate compounds (formaldehyde, acetaldehyde, acetone) also harmful following the functioning of the PCO system according to the invention. This is why it is recommended, in the event of a concentration of VOC assumed to be high, to start operation of the PCO system according to the invention in attenuated mode, either by reducing the UV power, or by reducing the gas flow rate, or both. . After a while, when the VOC concentration is lower, the operating power can be increased. An attenuated UV illumination (for a start of operation) is for example less than 8 and even less than 7.5 mW / cm². An attenuated gas flow is for example less than 60% and even 50% of the nominal gas flow. Table 5 below gives the concentrations of certain VOCs commonly measured in living rooms. These values ​​are the results of several hundred measurements. The right column "critical concentration" in Table 5 indicates, for information only, the concentrations from which it is recommended to reduce the operating regime of the PCO purifier according to the invention, by reducing the gas flow through it and / or by reducing the UV intensity illuminating the PCO media. The invention therefore relates to a method of purifying gas using a purifier comprising a filter medium with photocatalytic action (according to the invention or not) and a system for UV illumination of said medium, so that when the concentration of a compound in the gas is greater than a value V1, the operating regime of the purifier is less than its regime when the concentration of the compound in the gas is less than a value V2, V2 being less than or equal to V1 . In the case of formaldehyde (very frequent impurity), it is recommended to reduce the operating regime of the purifier when the formaldehyde concentration is greater than 30 μg / m³. The operating regime can be increased when the formaldehyde concentration is less than 30 μg / m3. More generally, the purifier can be operated with a reduced operating regime when the formaldehyde concentration is greater than a V1 value of between 0.3 and 80 μg / m3 and then increase the operating regime when the formaldehyde concentration is lower. at a value V2 between 0.3 and 80 μg / m3, V2 being less than or equal to V1.

<img file="WO2009019387A2_D0005.tif" />

Table 5

In order to be able to detect whether the gas to be purified (generally the air) exceeds the values ​​for which it is recommended to reduce the power of the purifier, the purifier according to the invention is advantageously provided with an analyzer of organovolatil compound. The purifier can operate in a fully automated manner according to the levels of organovolatil compound transmitted by the analyzer: high speed when the content is below a certain value, low speed when the content is above a certain value.

Thus, the invention also relates to a gas purifier (generally air) comprising a filtering medium with photocatalytic action (according to the invention or not) comprising a means of varying the gas flow rate passing through it or (which signifies and / or) variation of the intensity of UV illumination. The purifier may include an organo volatile compound analyzer and means for automatically adjusting the speed of the gas passing through it or adjusting the intensity of the UV illumination as a function of the content of organo volatile compound analyzed by the analyzer. . The scrubber can analyze incoming gas or outgoing gas, but generally analyzes incoming gas. As an example of a suitable device operating according to this principle, the following operating mode can be given, according to which above a C1 content of a VOC, the device operates in reduced mode. Above a C1 content of a VOC, the analyzer gives a signal 1 which enters a regulator, which is programmed to transform this signal 1 into output setpoint 1 in accordance with a mathematical formula, said setpoint then attacking a variator speed which controls the speed (reduced) of the drive motor of the purifier fan, said fan causing a flow D1 (moderate) of gas through the purifier.

After a certain period of operation at this reduced speed and when the concentration in the gas in said VOC drops below C2 (C2 being less than C1), then the analyzer gives a signal 2 to the regulator, which is programmed to transform this signal 2 as an output instruction 2 in accordance with a mathematical formula, said instruction then attacking the speed controller which controls the speed (high) of the drive motor of the purifier fan, said fan causing a flow D2 (high) of gas through the purifier.

The setting of the gas speed and or of the light intensity can also be simply modulated as a function of time. For example when starting a purifier in a room containing pollutants, it is recommended to work with a low gas speed and / or a low UV illumination for example during 2h, the time to have sufficiently purified the room, then to switch to nominal speed. Such a system avoids forming by-products, for example when starting the purifier. The purifier can therefore include a time delay means (that is to say a means measuring or determining the time or triggering a device after a certain time) making it possible to control the moderate or stronger speed of the purifier . The purifier can therefore include a delay means and an automatic adjustment means for the speed of the gas passing through and / or for adjusting the intensity of the UV illumination as a function of the time determined by the delay means. This time-based system can be used, when a pollution peak is detected by a system independent or not of the purifier, such as information communicated by radio, to put the system in reduced regime (low gas speed and / or low UV illumination) for a predefined time, the stronger purification regime then activating automatically at the end of the predefined time. In such a device, by way of example, after starting the purifier, the control chain including a timed relay gives a reduced operating instruction 1 for a period 1 to a variable speed drive, which gives the speed 1 at the fan turbine drive motor to drive a reduced gas flow D1 through the purifier. After a predefined duration at this reduced speed, the control chain including the time delay relay gives a setpoint 2 for higher speed operation to the variable speed drive, which gives the speed setpoint 2 to the drive motor of the fan turbine for cause a flow D2 (stronger) of gas through the purifier

FIG. 1 very schematically represents the structure of a PCO purifier according to the invention, containing two layers 2 of PCO media. The gas flow is represented by thick arrows, the left arrow representing the incoming gas, the right arrow representing the outgoing gas. Between the two layers 2 of PCO media are placed UV lamps 3. A fan 5 ensures the circulation of air. All these elements are placed in a stainless steel box 1. The distance 4 between the UV lamp and the media can be 20 mm. Such a purifier could moreover include a pre-filter with particles as soon as the air arrives, that is to say placed to the left of the first PCO medium.

FIG. 2 represents the structure of a PCO purifier according to the invention, containing a single layer of PCO 2 media. The air flow is represented by thick arrows. A UV lamp 3 illuminates the media 2. A fan 5 ensures the circulation of air. All these elements are placed in a stainless steel box 1. The distance 4 between the UV lamp and the media can be 20 mm. Such a purifier could moreover include a pre-filter with particles as soon as the air arrives, that is to say placed to the left of the first PCO medium.

FIGS. 3 show PCO filter modules inside which the PCO medium has a V or W shape. The module comprises a generally metallic casing 6 (stainless steel, galvanized steel or aluminum) containing a particulate pre-filter 7, a UV lamp 8, a cassette 9 with catalytic photo medium, articulated or not. The cassette is returned to the assembly through an opening 10. Depending on the space available for opening the envelope, it is decided whether the cassette should be articulated or not. For example, an articulated cassette can be retracted straight into the module through the opening 10 (FIG. 3c) then folded into a V thanks to the articulation 11 (see FIG.

3a). The cassette can include three joints 12 to take a W shape as in FIG. 3b. Figures 4 to 13 give the results of cleaning the air polluted by various molecules using a medium according to the invention. Figures 4 to 11 correspond to the results of Examples 24 to 31 and Figures 12 and 13 correspond to the results of Examples 33 and 34. FIG. 14 represents the principle of the illumination of the filtering medium by means of several light guides (for example optical fibers) brought to the PCO 15 reactor. In FIG. 14 a), it can be seen that from the light source UV 13, several light guide strands 14 bring light into the reactor 15 in order to illuminate the PCO medium. In FIG. 14 b), we see the distribution of the arrival of eight light guides 14 on the section of the reactor 15 so as to distribute the energy

UV as homogeneously as possible in reactor 15.

FIG. 15 represents the principle of the illumination of the filtering medium by means of a single light guide (for example optical fiber) brought to the PCO reactor 21. From the UV source 16, a single strand 17 of light guide brings UV light into the reactor 21. In the case shown, the light guide passes through the media 18 to the point of illumination 19 at the end of the light guide. This strand then illuminates a mirror 20 provided with a suitable curvature and the UV light is reflected (arrows) by this mirror as homogeneously as possible in the reactor towards the medium 18. FIG. 16 represents the principle of the illumination of several PCO media contained in several PCO reactors 22, via light guides (for example optical fibers) 23 connected to a single UV generator 24. The lighting device is therefore centralized with respect to several PCO reactors. This makes it possible to have a single UV generation area relayed by a lighting system using a light guide from different PCO media. This system avoids the need to have multiple sources.

EXAMPLES 1 to 13 (powder evaluation)

Felts are produced in the following manner. Molten silica rods with a diameter of 4.4 mm are drawn out in an oxy-propane burner in order to bring them to a filament diameter of 0.2 mm. This filament is then re-stretched by flame stretch-blowing in a second burner in order to obtain an average diameter of 9 μm and projected onto a carpet or a receiving drum. The speed of the drum is adjusted in such a way as to obtain the surface weights of the felts shown in the table below (2nd table). The felt obtained is then impregnated with the preparation obtained by the recipe indicated in Table 6 below:

<img file="WO2009019387A2_D0006.tif" />

Table 6

Different tests have been carried out with different quantities of TEOS, MTES and TiO catalyst₂ as reported in Table 7 below.

<img file="WO2009019387A2_D0007.tif" />

Table 7

The impregnated felt is then subjected to a heat treatment under the conditions of table 4 already seen above. The final medium obtained has a surface mass after impregnation of approximately 120 g / m², an apparent thickness of 20 mm, and its titanium oxide content is of the order of 20% by weight (except in case N ° 1: no catalyst and N ° 2: less than 4% of catalyst).

Measurements to assess the powder coating tendency are then carried out using 100 x 100 mm samples² PCO media. The samples are placed in a FRITSH / Labogerdebau type sieve for 30 minutes with an amplitude of 4 (machine-specific value). At the end of the 30 minutes, the residue is weighed and the loss of mass is reported at 1 m² of substrate. The values ​​are averaged over 5 samples. The results are reported in a comparative manner in Table 7. The results are expressed in "dusting", that is to say the amount of powder formed per unit of surface area of ​​the media (in mg / m²), and in PCO efficiency on the basis of the “methanol test” already described above. This efficiency is expressed in quantity of methanol oxidized in ppm. In particular, test No. 13 is excellent because it combines very good PCO activity with very low dusting.

EXAMPLES 14 to 23 (methanol oxidation) A reactor consisting of a stainless steel body is used, inside which is placed a media disc with a diameter of 47 mm. In the upper part of the reactor, HPK 125 W UV illumination is carried out through a silica slot. The illumination power is adjusted by adjusting the distance between the Media lamp. A power of 5 mW / cm² at 365 nm (of media) of UV measured at the media level is used. Upstream of the reactor, a constant flow of filtered air containing 300 ppm of methanol is introduced at the rate of 350 ml / min. Downstream of the reactor after PCO treatment, the methanol concentration is measured by gas chromatography. We ensure the mineralization of the pollutant (conversion to CO₂ and H₂O) by checking the chemical balance so as not to have only pure adsorption phenomena as can be found with systems of the activated carbon type. Table 8 gives the more precise operating conditions for producing the media as well as the efficiency observed on various media in the context of the “methanol test” already explained above. In the Millenium S5 300 A TiO₂ was 100% anatase. This is not necessarily the case for all TiO-based catalysts.₂. For example, in Degussa's P25 catalyst, TiO₂ contains about 1/3 of Rutile for 2/3 of anatase.

<img file="WO2009019387A2_D0008.tif" />

Table 8 EXAMPLES 24 to 31 (oxidation of organic molecules at different rates and intensity of illumination)

Molten silica rods with a diameter of 4.4 mm are drawn out in an oxy-propane burner in order to bring them to a filament diameter of 0.2 mm. This filament is then re-stretched by flame stretch-blowing in a second burner to obtain an average diameter of 9 μm and projected onto a receiving drum. The speed of the drum is adjusted so as to obtain a surface mass of the felt of 80 g / m². The felt is then impregnated in accordance with the recipe in table 9 below:

<img file="WO2009019387A2_D0009.tif" />

Table 9

The impregnated felt is then subjected to a heat treatment under the conditions of table 4 already seen above.

The filter medium obtained has an apparent thickness of 20 mm. The felt has a total mass after impregnation of 120 g / m², the titanium oxide content being 20% ​​of the total mass of the media.

The efficiency of the photocatalytic medium was measured by placing it in a 130 m PCO purifier.³/ h, the frontal speed being 1 m / s, the received illumination power of 15 mW of UVC per cm² of media (these values ​​were measured by a BIOBLOCK VLX-3W radiometer with a 254 nm probe). The enclosure is made of stainless steel and measures 1 m³. The purifier is made of stainless steel. It is fitted with 3 Philips 36 W TUV lamps placed 20 mm from the PCO media. The media surface is 270X420 mm². The PCO purifier contains one or two media as shown in Figures 1 and 2. The fan is placed behind the PCO filter (s).

Using a permeameter, a mixture of pollutants is introduced into the enclosure at a rate of 1.951 / min. This mixture consists of benzene, toluene, oxylene, decane, limonene and formaldehyde.

New filtered air is introduced at a rate of 21 l / min and a system at the pump outlet 23 l / min from the atmosphere this in order to simulate the rate of renewal of fresh air existing in any construction. In the event of a high concentration of VOCs, and in order to reduce the formation of reaction intermediates which can be harmful such as formaldehyde, acetaldehyde, acetone, it is important to work with UV illumination levels not too much important as well as moderate flow rates this in order to increase the reaction time within the substrate and to allow a more complete oxidation of the organic compounds (which includes these possible undesired intermediate compounds) within the substrate.

The meaning of the different abbreviations is as follows:

1 F: - 1 PCO media 2F: - 2 PCO media on either side of the UV lamp

Dmax: - Maximum flow (130 m³/ h)

D1 / 2: - Maximum flow divided by 2

UV Max: - Maximum UV illumination

UV / 2: - Maximum UV illumination divided by 2 min UV: - UV illumination received 2 mW / cm².

The results are given in FIGS. 4 to 11. The moment for which the purifier operates is indicated in the figures by the double arrow “purifier in operation”.

In view of the different curves, the oxidation of the different benzene, toluene, decane, xylene, limonene compounds using the PCO filter according to the invention is very clear. Correlatively, it appears that the formation of intermediate products such as formaldehyde, acetaldehyde, acetone is all the more marked as the flow rate and the UV power is important.

EXAMPLE 32 (ozone) Molten silica rods with a diameter of 4.4 mm are drawn out in an oxy-propane burner in order to bring them to a filament diameter of 0.2 mm. This filament is then re-stretched by stretch-blow-molding in a flame in a second burner in order to obtain an average diameter of 9 μm and projected onto a receiving drum. The speed of the drum is adjusted so as to obtain a basis weight of the felt of 65 g / m². The product is then impregnated with the preparation obtained according to the recipe in Table 10 below:

<img file="WO2009019387A2_D0010.tif" />

Table 10

The impregnated felt is then subjected to a heat treatment under the conditions of table 4 already seen above.

The filter medium obtained has an apparent thickness of 20 mm. The felt has a total mass after impregnation of 100 g / m², the titanium oxide content being 20% ​​by weight of the total weight of the media.

The tests were carried out in an experimental house. The scrubber was that described in Examples 24 to 31, equipped with 2 media.

The test conditions were as follows:

- 30 m office³

- Air renewal rate (TRA) between 0.6 and 1 volume / h

- Quartzel brand fused silica fiber felts (registered trademark of Saint-Gobain Quartz SAS) and a surface mass of 65 g / m² (media mass 100 gr / m²)

- Nominal flow, purifier 130 m³/ h.

- UV illumination received 15 mW / cm²

The measurements were carried out one week before the installation of the purifier, one week during the operation of the purifier and one week after stopping the purifier. The results of the indoor to outdoor ozone concentration ratio are expressed. Indeed, in life-size, an air renewal rate still existing, fresh air loaded with pollutants enters the room and stale air loaded with pollutants leaves the room. To compare efficiencies, it is therefore useful to be able to work in terms of indoor air pollutant concentration / outdoor air pollutant concentration. Without PCO scrubber, this ratio is 0.14. With the PCO scrubber, this ratio is 0.01.

EXAMPLES 33 and 34

Molten silica rods with a diameter of 5.5 mm are drawn out in an oxy-propane burner in order to bring them to a filament diameter of 0.2 mm. This filament is then re-stretched by stretch-blow-molding in a flame in a second burner in order to obtain an average diameter of 9 μm and projected onto a receiving drum. The speed of the drum is adjusted so as to obtain a surface mass of the felt of 50 g / m²The felt is then impregnated with the preparation made according to the recipe in Table 1 1 below:

<img file="WO2009019387A2_D0011.tif" />

Table 11

The impregnated felt is then subjected to a heat treatment under the conditions of table 4 already seen above. The filter medium obtained has an apparent thickness of 20 mm. The felt has a total mass after impregnation of 85 g / m², the titanium oxide content being 20% ​​of the total weight of the media.

The medium is then placed in a purifier identical to that described for Examples 24 to 31 (a single PCO medium) with the following parameters: Nominal flow rate in the purifier: 130 m³/ h;

UV illumination received: 15 mW / cm².

The purifier to be tested is placed in an enclosure of one m³ in plexiglass. Prior to the test, the enclosure is purged with ultra pure air and humidified in order to eliminate the presence of pollutant before the introduction of the mixture of model molecules. A liquid mixture of the different pollutants is introduced through a septum via a syringe into a glass weighing shoe. Two mixtures were tested, one containing propionaldehyde, heptane, acetone, toluene, acetaldehyde, ethylene, styrene, o-xylene (example 33) and the other containing toluene, heptane, butyraldehyde, acetone, methoxyethanol (example 34) . After evaporation, the concentration of the various compounds is of the order of ppmv.

CO monitoring₂ is done by a gas phase micro-chromatograph equipped with a thermal conductivity detector (μGC-TCD), the other pollutants are analyzed by a gas phase chromatograph equipped with a photo-ionization detector (PID). The PID allows the analysis of ionizable organo-volatile compounds (VOCs) in the ppbv range. The possible presence of degradation by-products in the gas phase is detected by adsorption on an adsorbent cartridge (flow rate 100 ml / min, time 20 minutes) followed by analysis by thermal desorption coupled to a gas chromatograph and detection by spectrometry of mass. The results are reported in Figures 12 and 13. There is a remarkable efficiency of the PCO system according to the invention on the elimination of the various solvents as soon as the UV illumination is triggered (the moment of illumination is indicated by the arrow "UV on"). In the case of Example 34 (FIG. 13), acetaldehyde is formed momentarily upon UV illumination and then is itself oxidized. The methoxyethanol peak between 40 and 70 min corresponds to a second injection of this product into the enclosure. The affinity of methoxyethanol for the media is such that it is immediately adsorbed.