Reactor and method for treating fluids by using photocatalysts coupled with phosphorescent solids

Description of the Prior Art

Photocatalysis is an effect that occurs when an electrical semiconductor is brought into contact with reactive substances. As a result of the irradiation, electrons are promoted into an energetically higher conductive band. A "hole" remains behind. The excited electron and / or the hole can be formed with molecules or radicals on the surface of the semiconductor, eg redox reactions. Thus, in the presence of oxygen, most organic molecules, bacteria and viruses can be completely oxidized.

There are already applications for cleaning of <nplcit id="ncit0001" npl-type="b"><text>Water and gases (Bahnemann, Detlef: "Photocatalytic Detoxification of Polluted Waters", in the Handbook of Enviromental Chemistry, O. Hutzinger (eds.): Reactions and Processes, Part L: Enviromental Photochemistry, P. Boule (Ed .), Springer Verlag Heidelberg, 1999, 285-351</text></nplcit>). The most widely used photocatalyst is TiO₂. With an energy gap of 3.2 eV it can be activated with ultraviolet light with a wavelength less than 385 nm. However, there are also many other photocatalysts with partial lower energy levels. These can be activated with light of greater wavelength. Recently we have been working on the development of photocatalysts with various properties. In particular, in this context, the area of ​​visible light (<nplcit id="ncit0002" npl-type="s"><text>Lettmann, Christian: "Conventional and combinatorial development of mixed oxides for photocatalytic water purification with visible light", Diss. Univ. Of the Saarland, 2001</text></nplcit>) And the use of sunlight (<patcit id="pcit0001" dnum="EP0812619A1"><text>EP 0 812 619 A1</text></patcit>) to mention.

Various types of reactors are described in the literature.

The "multi-plate reactor" is widely used in which the fluid to be treated flows in a meander-shaped manner over a photocatalyst-coated surface (<patcit id="pcit0002" dnum="EP0738686A1"><text>EP 0 738 686 A1</text></patcit>). The catalyst is irradiated through the fluid, for which purpose, in the described case of a waste water purification, sunlight is used as an exciter and TiO.sub.2₂ As a catalyst. As a variant it is described to suspend the catalyst in the fluid and to separate it again after passing through the device. These devices have an extremely high space requirement.

Cartridges are described which contain the catalyst and are flowed through by the fluid to be treated. The lamps are illuminated with lamps placed on the side of the cartridges (<patcit id="pcit0003" dnum="WO9636565A"><text>WO 96/36565</text></patcit>). This apparatus has a comparatively high space requirement as the "multi-plate multi-plate reactor".

The "ball heap reactor" consists of glass spheres coated with catalyst and through which the fluid flows (<patcit id="pcit0004" dnum="WO9511751A"><text>WO 95/11751</text></patcit>). The irradiation is effected by lamps which are introduced into the bed. Most common application as a fixed bed, but also as a fluidized bed. The disadvantage is that an increased packing density with only small penetration depths of the radiation must be purchased.

In suspension reactors, a finely divided catalyst with variously arranged lamps is irradiated (<patcit id="pcit0005" dnum="EP0233498B1"><text>EP 0 233 498 B1</text></patcit>). As a result of the strong shading by other catalyst or reagent particles, only a very small part of the catalyst surface present is activated in each case if the catalyst solution is not diluted with very dilute catalyst solutions and correspondingly low conversion.

Arrangements are described in which the light is transported through glass plates to the photocatalyst (<patcit id="pcit0006" dnum="WO9737936A"><text>WO 97/37936</text></patcit>). Space requirements and complex design correspond to the above-described bridge plate reactors.

In <patcit id="pcit0007" dnum="WO9817390A"><text>WO 98/17390</text></patcit> An arrangement with many thin glass plates is described. They carry the catalyst on their surface. The irradiation is carried out by lamps which penetrate the annularly arranged stack of glass plates by means of recesses in the glass plates. The construction is very filigree and complicated.

Disadvantages of the prior art and the resulting requirements

All known reactor types have in common that they achieve a very low packing density of the irradiated catalyst surface. This makes the apparatus expensive. In addition, they often have a considerable amount of space, which also makes the application more expensive. The somewhat more compact known types are very filigree and complicated built and therefore correspondingly expensive.

In these circumstances, the actual reason for which photocatalysis has not yet been applied in large scale is the reason for this.

It was therefore the object to develop a method and devices which combine the highest possible packing density of the irradiated catalyst surface with a construction and operation which is as inexpensive as possible.

This object is solved by the features of the main claims and is promoted by the subclaims.

Transport of energy by phosphorescent substances

The invention is based on the novel principle the necessary energy by means of phosphorescent substances in the vicinity of the photocatalytically active surface (in the following photocatalyst <b>PK</b> ), Where the phosphorescent particles emit light of appropriate wavelength and activate the PK. The phosphorescent particles (in the following micro-radiators <b>MR</b> ) Must be "charged" at a suitable light source. They are then transported to the photocatalytically active layer where they emit their stored electromagnetic energy completely or partially, in order to return to the UV light source. etc. This has the advantage that, upon selection of suitable MR, the stored energy is passed on to the PC with a half-life of a few seconds to minutes in the interior of the reaction space, Whereas, due to the short half-times of the active state of the PK, they only react in the vicinity of the energy source.

Suitable reactors are various types of reactants which can be used for reactions and / or for mass transport processes, preferably:<ul><li>Fluidized bed, fluidized bed cascade, fluidized bed</li><li>Spouted Bed, Cascade from Spouted Beds</li><li>Loop reactor, cascade of loop reactors</li><li>Agitator, agitator cascade</li><li>pipe reactor </li><li>Fixed bed (coated with PK) or all open, ordered structures such as platelets, honeycombs, etc.</li></ul>

In all of these methods, the MRs are activated at a light source, physically mixed with the reaction solution and the PC, returned to the light source after the energy is delivered to the PC, are charged, recharged and re-supplied to the catalyst.

The mixing can be effected on the one hand by flows and particle diffusion in the apparatus. If these transport mechanisms are sufficient, the light source, for example a UV lamp (s), can be installed directly on the wall of the apparatus or in the apparatus, thereby charging the near-MR MRs with a high energy density, and then flowing back into the interior through the flow D of the reactor.

In addition, these transport mechanisms can be improved by baffles, baffles, agitators, etc.

Alternatively or additionally, the MR can be passed through an external circuit at suitable lamps. For this purpose, the MR is preferably separated from the fluid to be treated and from the PK in order to increase its concentration at the light source and to prevent shading by PK and substrate particles. Preferably, the MR is convectively transported convectively with a small amount of the fluid to be treated.

The most widely used photocatalyst is TiO₂. With an energy gap of 3.2 eV it can be activated with ultraviolet light with a wavelength less than 385 nm. However, many other photocatalysts are known which can be activated with light of a wavelength greater than 385 nm. Examples which may be mentioned here are ZnO and the oxides of other transition elements (<patcit id="pcit0008" dnum="WO9511751A"><text>WO 95/11751</text></patcit>) And CdS [<patcit id="pcit0009" dnum="EP0234875B1"><text>EP 0 234 875 B1</text></patcit>) And SnO₂, SrTiO₃, WHERE₃, Fe₂O₃ ( "<patcit id="pcit0010" dnum="WO9636565A"><text>WO 96/36565</text></patcit>). The series of examples could be continued even further. Recently, photocatalysts for visible light (Lettmann, Christian: "Conventional and combinatorial development of mixed oxides for photocatalytic water purification with visible light", Diss. Univ., Saarland, 2001) and the use of sunlight (<patcit id="pcit0011" dnum="EP0812619A1"><text>EP 0 812 619 A1</text></patcit>).

It is preferred to use relatively hard and abrasion-resistant PC, with inorganic substances which are not oxidized being preferred. Depending on the application, the PK can have very different particle sizes and structures.<dl id="dl0001" compact="compact"><dt>Suspension Catalysts:</dt><dd>Particle diameter: 1 nm to 100 μm</dd><dt>Fluidized Bed Reactor:</dt><dd>Particle diameter: 1 μm to 1 mm</dd></dl>

When using fixed beds (or all reactors with ordered structures such as platelets, honeycombs, etc.), the photocatalyst is fixed as a more or less thin layer on the stationary support.

The micro-radiator MR

The micro-radiator MR is a phosphorescent solid which is used in the form of particles. It must have a sufficiently long afterglow time (at least in the seconds range, preferably a few minutes or longer, preferably 5 seconds to 30 minutes) and emit in the wavelength range in which the photocatalyst can be activated.

Examples of suitable phosphorescent solids

Many phosphorescent solids with visible-range emissions are known which have been developed for other purposes but cover all requirements for the application described herein. Instead of an extensive list, reference is made to the following references, the contents of which are hereby incorporated by reference ( "<patcit id="pcit0012" dnum="US6287993P"><text>USP 6,287,993</text></patcit>, <patcit id="pcit0013" dnum="DE19521119A1"><text>DE 195 21 119 A1</text></patcit>, <patcit id="pcit0014" dnum="DE19926980A1"><text>DE 199 26 980 A1</text></patcit>, <patcit id="pcit0015" dnum="DE19934436A1"><text>DE 199 34 436 A1</text></patcit>).

The application of this MR is, for example, in combination with the method described in FIG <nplcit id="ncit0003" npl-type="s"><text>Lettmann, Christian: "Conventional and combinatorial development of mixed oxides for photocatalytic water purification with visible light", Diss. Univ. Of the Saarland, 2001</text></nplcit>, Described photocatalysts (PK).

In <patcit id="pcit0016" dnum="US6287993P"><text>USP 6,287,993</text></patcit>, Long-lasting phosphorescent substances are described. Among other things, glass doped with zinc and praseodymium, which is suitable with an emission of 350 to 450 nm, is described in Example 17, TiO₂ As a photocatalyst (cf. <figref idrefs="f0002">FIG</figref>).

Also in <patcit id="pcit0017" dnum="DE19521119A1"><text>DE 195 21 119 A1</text></patcit>, "Slowly decaying" phosphorescent substances are described which are referred to as TiO 2₂-activators, since they also emit at below 400 nm. These are glasses that are doped with rare earth metals.

In principle, the grain size spectrum of the MR as in the case of the photocatalyst PK is between 1 nm and 1 mm, preferably 1 μm - 0.5 mm, depending on the application.

An efficient and economical solution - preferably for larger devices - is that the particle size of the MR is significantly above that of the PC, so that a "charged" MR particle is irradiated as many PK particles as possible. Furthermore, this has the advantage that the MRs can be easily separated from the fluid and the PK contained therein by filters or sieves and can be supplied to a regeneration.

Another efficient and economical solution - preferably for small devices - is that the particle size of the MR is significantly below that of the PK in order to achieve a simple separation of MR and PK.

In principle, however, no ratio of the particle sizes of PK and MR can be excluded. This is due to the variety of suitable apparatus types.

Massive particles which are particularly easy to produce, but also those in which the phosphorescent material is coated on a carrier core, are suitable. When a magnetic core is used, there are additional possibilities for the separation and transport of the MR particles.

Avoidance of abrasion at the MR and / or corrosion and / or dissolution of the MR

To avoid abrasion, the MR, if it does not consist of a glass anyway, can be coated with a (thin) light-transmissive layer (eg glass). This can also protect against corrosion or dissolution of the MR in the fluid to be treated.

Separation of photocatalyst PK and micro-radiators MR

The separation of PK and MR (also abrasion!) From the fluid can be carried out by means of classical methods such as filters, cyclone, centrifuge, etc., but also with a magnetic separator (see above).

The separation of PK and MR from one another can take place via the particle size (filter, cyclone) but also over the density (cyclone, centrifuge) and other physical differences (eg magnetic core of MR, see above).

A preferred method is to separate larger MR by means of a band filter of fluid and PK, to activate the separated MR at an energetic light source, and to return from the end of the filter band into the fluid.

Particularly suitable as light sources are UV emitters with the appropriate spectrum for excitation of the micro-radiator particles.

In the case of an external circuit, the lamp is preferably to be installed in a special apparatus. Due to its guidance of the MR particles, this ensures that a most efficient illumination of all MR particles takes place; For example by:<ul><li>Movement of the MR particles in a narrow gap around the lamp.</li><li>Flow with good particle transport transversely to the direction of flow (eg in a fluidized bed with built-in lamps or by a turbulent flow, etc.)</li></ul>

Preferably, the photocatalyst PK is separated from the micro-radiator MR in front of the external lamp in order to avoid shading through the PC during the "charging" of the MR.

A (at least partial) separation of the PK from the MR in the vicinity of the radiator (eg via the flow guide or upstream filters or magnetic fields when using a magnetic core of the MR) is to be preferred even with direct irradiation in the apparatus.

It goes without saying that the necessary reactants must be fed to the reactor in addition to the fluid to be treated. (Eg: supply of O₂ For the oxidation of organic impurities in water).

The reaction products (eg CO₂) Are separated from the treated fluid downstream of the reactor.

Suitability of problem solving for media and reactions

The process according to the invention is suitable for all chemical reactions which can be carried out on photocatalytically active surfaces in liquids or gases.

The invention is preferably used for the oxidation of dissolved organic molecules, dispersed drops and solid particles, microorganisms and viruses in water and gases (also gas bubbles in the case of water).

The wavelengths of the light for "charging" the MR and the emission by the MR need not be the same. Frequently, the radiated light is shifted long-wave. The light emitted by the MR should be sufficiently energetic (short-wave) to apply the necessary catalyst energy (eg UV light).

The term "light" can be replaced by the term "electromagnetic radiation of suitable wavelength", so also applies to other wavelength ranges when using corresponding photocatalysts (in particular visible light or sunlight).

The use of the micro-radiators MR achieves a packing density of the irradiated catalyst surface, which is not possible with any of the known methods. In addition, quite simple, possibly known devices can be used. They need only be adapted to the new process. Only the devices containing the external lamps may require new designs.

example 1

Laboratory Shake Reactor

According to FIG

The reactor consists of the stirred vessel 1 with blade stirrer 2, a supply line for oxygen (air) 3, a discharge line for exhaust gases 4 and an external lamp (UV radiator) 5 and contains a suspension of a reaction medium 6, the microroradiators 7 As well as the photocatalysts 8 shown.

The suspension consisting of 500 ml of an aqueous solution of an organic substance and photocatalyst as well as micro-radiator MR in a reactor volume of 800 ml is constantly stirred, resulting in a circulating flow ascending in the center and ascending on the wall while a UV- (20 watt power at 350 nm radiation maximum) to the side (irradiated area 50 cm²). Thus, all MR particles arrive at the UV source, where they are activated. Air is passed through in fine beads.

The addition of micro-radiator MR considerably increases the decomposition rate of the organic component by introducing radiation energy into the interior of the reactor.

Example 2

Laboratory Shake Reactor with External Circulation of Micro Radiator MR

According to FIG

The reactor consists of a stirred vessel 1 with a blade stirrer 2, an inlet for oxygen (air) 3, a discharge for exhaust gases 4, a sedimentation space 9 being provided below the stirrer 2, in which the heavier MR 7 collect, 6 together via the pump 10 and line 11 into the outer annular gap 12 of a UV lamp 5 and, after activation, via the line 13, back into the stirred tank 1 from above.

The suspension, consisting of an aqueous solution of an oxidizable substance, photocatalyst and micro-radiator, is constantly stirred and passed through air with oxygen₂ saturated. The micro-radiator (0 to 10 g) is constantly separated from the photocatalyst in the sedimentation chamber and passed on a UV lamp to the laboratory reactor. The rotational speed is, for example, 5 or 10 ml / min. In the outer circuit, for example, there are 10 g, or, for two lamps 20 g, additional MR to the amount in the stirred tank. The addition of micro-radiators increases the decomposition rate of the org. Substance considerably. Increasing the circulating flow also increases the decomposition rate.

Example 3

Tube reactor with meandering ribs and separate external circulation of photocatalyst (PK) and micro-radiators (MR) <figref idrefs="f0002">FIG</figref>.

The reactor consists of a tube reactor 21 with built-in meandering-oriented horizontal ribs 22 from which a mixture consisting of: reaction solution 6 enriched with oxygen and supplied to the mixer via the line 25 from below via the line 23 from the mixer 24, Photocatalyst 8, which is circulated via the pump 27 and line 26/28 and is supplied with micro-radiators 7, which are circulated via the lines 11 and 13, the pump 10 and the annular jacket 12 surrounding the UV lamp 5, Which leaves the reactor via the separator 29, where it is separated into the components. The reaction solution and the exhaust gases formed are discharged via the line 30.

Example 4

Tube reactor with photocatalyst coated honeycombs and external activation loud <figref idrefs="f0003">FIG</figref>.

The reactor consists of the tubular reactor housing 1 with honeycomb inserts 32 in the direction of the tube which are coated with photocatalyst. Reaction solution 6 from the feed line 25 and activated micro-radiators 7 from the circulation line 13 are introduced into the reactor via the mixer 24 and line 23, pass through the honeycomb installations 32 and thereby give their photoenergy to the PK before being separated from the solution 6 in the separator 29 And are fed via line 11 and pump 10 into the annular jacket 12 of the lamp 5, where they are activated by UV light and are fed back into the reactor via the line 13.