Reactors for photocatalytic waste-water purification with the use of multi-layered cross-braced plates as solar elements

The invention relates to reactors made of plastic for the photocatalytic purification of waste waters. The term photocatalytic wastewater treatment is meant by the action of light while both the chemical cleaning and decontamination and disinfection of waste water by means of a photocatalyst.

State of the art

The field of wastewater treatment is of enormous ecological importance. In addition to purification by biological methods in sewage treatment chemical, especially photochemical, solutions for cleaning and decontamination of waste water were increasingly discussed in recent years.<i>Bahnemann, D</i>, (Msg Chem Tech Lab 42 (1994) No. 4, p 378 -..... 388) describes in a review article the current status of solar wastewater detoxification.

A principle of the photocatalytic wastewater purification due to the photocatalytic oxidation of pollutants in aqueous solution, or in waste water. As photocatalyst currently mainly titanium dioxide is used here. High-energy photons of sunlight are able, in materials such as TiO₂ to produce so-called electron / hole pairs. By photon energy electrons pass from the valence band of the semiconductor into the conduction band and leave the valence band defect electrons or holes. The resulting electron holes have a high Oxidationskratt for virtually any organic contaminant molecules such as phenol, chloroform, dichloroacetic acid or propanol, etc., which can be converted to the carbon dioxide in the ideal case. A particularly effective action is expected from the combination of solar and conventional biological wastewater treatment.

Another principle of solar wastewater treatment uses organic dyes, such as. For example, methylene blue, and their ability to generate singlet oxygen upon exposure, which also is an effective oxidizing agent and in particular has an antimicrobial effect (See <i>Archer, A</i>, et al. (1990) Photochemical disinfection of effluents - pilot plant studies. Wat. . Res. Vol 24, pp 837-843). Such photocatalysts are also referred to as photosensitizers. With the aid of appropriate solar reactors this process can be used for. B. to to eliminate harmful to health, germs in sewage which are used in water-scarce areas for cereals irrigation or other land.

The practical application of the principle of photokatalflischen wastewater can be done by sewage flowed through reactors. the parabolic trough reactor, the thin film fixed bed reactor and the thin-film slurry reactor are known (see, eg. B .: TNO, Applied Research, December 1993 52, p.4). In parabolic trough reactor, the solar radiation is concentrated by parabolic mirrors on sewage flowed through the reactor tubes may be made of borosilicate glass for better utilization of the UV component of sunlight, for example. The solar elements are tracked solar radiation. Several solar panels for example may be joined together by polyethylene pipes, the aperture area of ​​a larger pilot plant can for example with a length of 100 m 200 m² be. The photocatalyst TiO₂ is used here in suspension and must be separated by the wastewater by filtration.

When thin film fixed bed reactor of the photocatalyst can be fixed on a glass or on a steel plate. The consuming step for the separation of TiO_2-therefore particles and treated wastewater eliminated. The effluent to be purified can be fed over the glass plate in a thin layer. The solar element is placed at an angle to the solar radiation. An advantage over the parabolic trough reactor, which must be tracked as a light-concentrating reactor of the sun, since it only uses the direct solar radiation, results in the fact that the thin film fixed bed reactor can by its flat solar panels to absorb and diffuse solar radiation.

Despite its advantages over the parabolic trough reactor the use of thin-film fixed bed reactors have not are still a number of solved problems contrary. Thus, the hitherto unrealized thin film fixed bed reactors have so far only experimental reactors small size of eg 0.7 m² Solar element surface. In addition, the construction cost for the production, in particular for joining and sealing of today realized solar elements is außordentlich high. A simple transfer of the design to a larger scale, therefore, does not seem favorable. Solutions for large-scale, easy to manufacture and thus cost-effective solar elements are still missing.

Multiple web panels, in particular polycarbonate or polymethylmethacrylate, are widely used especially as a transparent or translucent roof elements in the construction sector. Furthermore, the use of multi-compartment plastic plates as solar panels, for example from DE 4234947 is known. The chambers can be used as liquid flowed through rooms and locked the end faces by means of a, connecting the chambers, collecting adapter. The adapter each have a supply or drain opening. The German utility model G 94,055,157 describes a multiple web plate of polymethyl methacrylate, which can be used after partially opening the web ends and bonding the end faces as flüssigkeitsdurchströmbares solar collector element.

Problem and Solution

The present invention is to provide a reactor with a strong, lightweight and above all cheapest solar element the task. The object was achieved by a reactor for photocatalytic wastewater treatment, in which the solar element (1) consists essentially of one or more flüssigkeitsdurchströmbaren multiple web panels of thermoplastically extrudable, transparent or translucent plastic, inside which a photocatalyst is.

Essential for the invention is that an existing, industrially produced on a large scale and therefore more cost-effective plastic body, the multiple web plate, is used for the solar element. Multiple web panels, in particular of polymethyl methacrylate, are characterized by a high flexural strength, depending on the version high transparency or translucency and excellent weatherability. Moreover, they are permeable to UV light, which when using z. B. TiO₂ is essential as far gebräuchlichstem catalyst. Moreover, means and methods for partly closing the multilayer cellular plates on the end faces by means of mechanical seals or adhesive bonding are known. Inlet and outlet openings must of course be present in order for the Flüssigkeitsdurchströmbarkeit the multiple web plate is ensured. The invention further relates to a multiple web plate of transparent or translucent thermoplastic, extrudable plastic whose cavities are coated with a photocatalyst.

The invention will erläutet by the figure 1, but should not be limited to the illustrated embodiment,<dl id="dl0001" compact="compact"><dt>Figure 1:</dt><dd>Scheme for an experimental reactor</dd></dl>

The core of the reactor is flowed through the meandering solar element (1) which is at the front sides up to the openings for the inlet (2) and the outlet (3) is closed. (4) denotes a circulation line. (5) means a solid / liquid separation unit (z. B. for separating TiO2 from the purified waste water). (6) referred to vent valves. (7) refers to the pressure gauge for detecting the pressure loss. The circulation and the feed stream can be detected by flowmeters (8). The flow direction is indicated by arrows.

Embodiments of the invention

The photocatalytic hereinafter used term wastewater treatment includes both chemical cleaning or detoxification as well as the disinfection of waste water by means of a photocatalyst upon exposure to light. The term reactor is used for a plant for photocatalytic wastewater treatment in its entirety. The reactor consists in particular of one or more solar elements (1), as well as other conventional reactor components that are required for liquid circulation and / or circuit by the solar elements, such as. For example, lines, pumps, possibly sampling valves, storage tanks, etc. ,

The term solar element specifies the reactor components, in which take place the photokataytischen processes from light. The light source may be both to solar radiation (solar wastewater treatment) as well as to artificial light act. The term photocatalyst both photocatalytic substances such. As TiO₂ and photosensitizers such. as understood methylene blue.

It is essential for the reactor of the invention is the use of a multiple web plate as a solar element (1).

For the solar element preferably commercial multiple web panels of transparent or translucent plastic are used. In these plates, the ridges are perpendicular to the Gurtflächen. Usual dimensions For example with polycarbonate sheets about 5 - 40 mm thickness, web spacing approximately from 5 to 80 mm, width 500 - 2500 mm and lengths of 1000 - be 8000 mm. In principle, however, multi-web plates with different geometries or dimensions can be used.

Examples of plastic materials are polymethyl methacrylate, polycarbonate, polystyrene, polyester, or polyolefins.

When choosing a plastic material which can affect the action of the photocatalyst bandgap or the corresponding radiation spectrum is observed. This is the currently most commonly used catalyst TiO₂ so that permeable transparent or translucent colored plastics are below about 390 nm, the only area in question. are preferred in the use of TiO₂ which consist as a catalyst multilayer cellular plates essentially of polymethyl methacrylate, while, for example polycarbonate sheets are less suitable due to their low permeability for UV light. When using other photocatalysts such as hematite or iron (III) / titanium (IV) mixed oxides or photosensitizers such. As methylene blue, which develop a catalytic effect in the range of visible light, for example, can also plates with low UV transmittance or UV-absorbing plates.

Multiple web panels of transparent or translucent plastics are, for example as a double-skin sheets, Stegdreifach- and quadruple-skin sheets in different versions on the market. Polycarbonate sheets are used. For larger solar panels for example but also Stegdreifach- or quadruple-skin sheets or multi-wall sheets special geometry can be used because of its higher stability. Fully transparent multi-wall sheets are preferred because typically most efficient use of solar energy is desired. Translucent panels can be selected under certain circumstances, if such. As excessive heating of the wastewater to be cleaned should be avoided. Here, for example whitened multiple web panels with a light transmittance of, for example 40 - used 85%.

The photocatalyst such as TiO₂, May be passed through the multiple web plate in suspension. In this case, z. B., the TiO2 after complete cleaning of the waste water are removed by precipitation and / or filtration. This can, for example, by adjusting a pH, eg pH 6 -.. 8, carried out, in which the TiO2 no longer stable and remains in suspension sedimented.

It is however also possible to fix the photocatalyst on the inside of the multiple web plate. A method for coating the inside of polycarbonate sheets with water-spreading layers are known for example from EP 530 617 A1. In this case, holes are generated in the upper chord of the cooled after extrusion hollow extruded profiles through which the coating agent can be filled by means of a special tool. the hollow profile is extruded in order to achieve complete wetting of the interior spaces behind the tool in the elastic range bent downward and continuously wetted with the coating agent. After passing through this distance the hollow chamber profile is again guided upward so that excess coating material runs back. In an analogous manner, a coating with a colloidal example, 0.1 to 15% TiO₂ Suspension in H₂O, which possibly also contains a water insoluble wetting agent, for example 1 to 10 wt .-% of an oxyethylated fatty alcohol made. The layer can be dried and fixed for example by blowing hot air then. The amount of catalyst in the coated layer is about 0.01 to 5 mg / cm² be.

Furthermore Stegdreifach- or quadruple-plates can also be coated by dipping or internal flooding. Water-soluble photocatalysts can z. B. be applied by dipping or flow coating with a layer of varnish, in which they are fixed, for the interior of the plates. B..

Optionally, an additional Photoplatinierung the catalyst layer be useful to get an increase in the reaction rate of the redox processes. For this example, as with Muradov, NZ (1994, Solar Energy, Volume 52, pp 283-288) describes metallic platinum from an H₂PtCl₆Solution by the action of light on the TiO₂Layer are fixed.

The as solar element (1) used multiple web plate serves as a system for carrying out the to be cleaned or to be detoxified waste water or a sewage / photocatalyst suspension. Therefore, it has at least an inlet and an outlet so that a flüssigkeitsdurchströmbares system exists. Conveniently, most of the end faces, with the exception of the inlet (2) or maturity (3), sealed by metal or preferably plastic parts. The hollow chambers may be combined at the ends in a collection channel, which is formed by the adapter. Likewise, a meandering flow through the hollow chamber to be created, eg, by first milled the web ends alternately or broken and then the end faces are closed, so that the waste water a cavity after another flows through before exiting out of the solar element. Several solar elements can be joined together. The flow rate of the waste water should be selected so that turbulent flow is present.

For closure of numerous solar reactor systems for multiple web panels are known. DE 423 947 describes corresponding collecting adapter which can be flange-mounted by means of pins with seals to multi-compartment plastic plates. EP 381 028 describes the connection of a plastic collecting adapter to wall sheets that serve as heat exchange elements, by welding the plastic. The German utility model G 94,055,157 example describes a life and had spannungsrißfreie bonding for multiple web plates of polymethyl methacrylate. The flüssigkeitsdurchströmbare cavity sheet there described in the example is as useful as a solar element for a reactor for solar wastewater.

EXAMPLE

Use of a double-skin sheet made of polymethylmethacrylate as a solar element of an experimental reactor for solar wastewater.

It is a double-skin sheet made of transparent polymethyl methacrylate with the dimensions, width 980 mm, length 1400 mm, height 16 mm and web spacing 32 mm. The web ends are respectively milled alternately at the ends to about 2 cm. A front end is completely closed by sticking with a Polymethylmethacrylatstreifen. The other face is also sealed to each have an opening as inlet and as an outlet. The liquid flowed through the system is designed so that starting up the liquid emerges from the inlet all hollow chambers are flowed through in succession by the process. Inlet and outlet are connected to the liquid conveying system of the reactor, so that contaminated waste water can be passed through the solar element. A model wastewater artificially with 1 mmol / l dichloroacetic acid (DCA) prepared are about 7.5 g TiO₂/ L (Hombikat UV100®, Sachtleben) was added as a photocatalyst. The model waste water / photocatalyst suspension is used with an initial pH value of 3.6. The flow rate is about 0.7 m³/H. The solar element is illuminated for the simulation of sunlight with a light field consisting of seven lamps of type Phillips TL-40 UVA with an output of 105 W / m2 h.

The irradiated reactor surface is approximately 0.7 m², From the plot of the logarithm of the DCA concentration versus time (plotting the 1st order) a reduction of 0.0124 mmol / l min was determined. This gives a quantum yield of about 2%.