WO2006134149A1

WO2006134149A1 - Method and system for photocatalytically cleaning air and waste water

Info

Publication number: WO2006134149A1
Application number: PCT/EP2006/063248
Authority: WO
Inventors: Detlev MÖLLER; Siegfried Müller
Original assignee: Brandenburgische Technische Universität Cottbus
Priority date: 2005-06-15
Filing date: 2006-06-15
Publication date: 2006-12-21
Also published as: DE102005028660A1

Abstract

The invention relates to a method for oxidising and removing ecologically harmful components from gaseous and/or aqueous phases using the photocatalytical effects of titanium dioxide. The gaseous and/or aqueous phases which are to be cleaned in a reaction chamber are brought into contact with a photocatalytically active titanium oxide layer and is fed simultaneously to an active oxygen produced in a gas phase. The photocatalytically active titanium dioxide-layer is continuously sprayed with a rinsing fluid or with the aqueous phase which is to be cleaned. Said method is particularly suitable for purifying air, preferably for producing pure internal air, but also for used air and/or waste gases, for removing odours and cleaning waste water. The invention also relates to a cleaning system for carrying out said method.

Description

METHOD AND SYSTEM FOR PHOTOCATALYTIC AIR AND WASTE CLEANING

The invention relates to a process for oxidizing and removing environmentally harmful substance components from gaseous and / or aqueous phases using the photocatalytic effect of titanium dioxide. The method is particularly suitable for the purification of air, preferably for the production of pure internal air, but also exhaust air or exhaust gases, for the elimination of odors and purification of waste water. The invention also provides a cleaning system for carrying out the method.

Areas of application are preferably the cleaning of indoor air in private and public building areas, the production of sterile air in hospital, especially operating areas and the production of clean air for special production processes. In addition, an application for exhaust air purification is possible. The process principle is also suitable for the purification of waste water, in particular those that are contaminated with organic pollutants and germs.

For many concerns, the production of clean air, particularly germ-free and containing no organic pollutants, is of great importance. This concerns medical areas with increased hygiene requirements, private and public living areas in areas of high outdoor air pollution. Many production and research areas require either sterile conditions and / or increased air quality requirements. The Abluftbzw. In contrast to indoor air, exhaust gas cleaning is largely demanded by the legislator in order to avoid or reduce harmful effects on humans and nature. Therefore, there are a variety of methods and applications. The same applies to wastewater treatment.

For the reasons presented, there is a great deal of economic and hygienic interest in methods for inexpensively and effectively purifying air, exhaust air and wastewater in a wider range of physico-chemical environmental conditions.

Ozone generators, filter systems, air ionizers, air scrubbers and UV rays have long been used to clean indoor air. This procedure are based on mechanical-physical effects (particle separation by filters with partially preceding electrostatic charging), the washing out of air components by absorption liquids (scrubbers) or a chemical transformation (oxidation), initiated by ozone, ionized oxygen and UV light.

Filter systems are basically only for the separation of particulate coarser air components (dust, pollen, microorganisms). They have the disadvantage that the filters must be regularly replaced or regenerated, but are characterized by high separation efficiency (the

Grain size characteristics must be matched to the filter material, i. it is never possible to deposit all the airborne particle sizes of the particulate material). Air ionizers, which are based on the formation of excited oxygen molecules and possibly "active" oxygen (eg O radicals), do not show a high chemical efficiency even for theoretical reasons, but can possibly charge and precipitate dusts.Ozone generators and UV irradiation have in The air purification is not proven since ozone in the gas phase reacts only with a few organic compounds (olefins) and photochemical conversions remain low in the available times scrubbers are basically effective methods for

Air purification (they are industrially used mainly for the purification of exhaust air); they can be selectively designed for individual trace substances. The prerequisite for effective air purification is always a good absorption of the trace substance to be removed in the washing liquid. The disadvantage is the treatment of the wastewater, which requires separate additional facilities.

Filters and scrubbers are used in particular for the purification of exhaust air. In addition, there are a variety of technical equipment and methods for thermal and catalytic afterburning.

More recently, photocatalytic processes have been used for air purification. About 30 years ago, it was discovered that catalytic reactions occur on the surface of titanium dioxide (TiO 2) under the influence of light, based on the formation of free electrons (Fujishima, A., K. Hashimoto, and T. Watanabe (1999) Photocatalysis, Fundamentals and Applications, BKC Inc., Tokyo). Meanwhile, this effect is in the form of so-called self-cleaning surfaces exploited, whereby the surface coatings are obtained with special nanotechnologies.

This process, which belongs to the Advanced Oxidation Processes (AOPs), is becoming increasingly important for mineralizing toxic or biologically poorly degradable organic water and air constituents with the aid of hydroxyl radicals (Schiavello, M., ed. (1997) Heterogeneous photocatalysis. Vol. 3 Wiley Series in Photosciences and Photoengineering, Wiley, Chichester).

These processes, which are not all based on the photocatalytic TiO ₂ effect, especially in wastewater treatment, generate highly reactive radicals, which in the end reduce the pollutants. The generation of radicals can take place in different ways: with irradiation of different wavelengths (but especially UV), by other photocatalytically active materials, by ozonization and by hydrogen peroxide.

In this case, all organic trace gas molecules are completely oxidized in the presence of atmospheric oxygen, ie there is a quasi-cold combustion reaction to form carbon dioxide (CO2), water (H ₂ O) and possibly other mineral, non-toxic products instead. This direct pollutant removal does not result in a shift of the problem into another phase, as is the case, for example, with absorption processes or stripping. In the case of photocatalysis, irradiation of the TiO 2 catalyst converts both water and atmospheric oxygen to the mentioned reactive OH radicals. Since TiO _{2 is} a semiconductor material with a band gap of 3.2 eV, light in the wavelength range between 200 nm and about 400 nm can initiate these photochemical reactions, ie, in addition to artificial light sources, the UV-A radiation of the sun (300 - 300 nm). 400 nm) can be effectively used. This special form of photocatalysis is called "solar-catalytic water or air purification".

Various commercial products are now available (for example: Matrix Photocatalytic, Inc. TiO ₂ Water & Air Organic Pollutant Destruction Systems, PHOTOX Bradford Ltd., UK, Purifics, Ontario, Canada) which exploit this effect. The air to be cleaned flows through a container which contains plates coated with TiO ₂ in a different constructional arrangement, on their surfaces, upon additional irradiation, photooxidants are formed which react with the trace substances of the air. In this case, the known phenomenon is exploited that the OH radical reacts quickly with almost all organic and a variety of inorganic trace substances.

However, these radical chain oxidations are - which is not or barely taken into account by the providers of such systems - at different speeds and only proceed through many intermediate stages to complete oxidation to CO ₂ and H ₂ O. The international knowledge gained from long-term research projects on air-chemical reactions can be presumed (Möller, D. (2003) Air De Gruyter, Berlin, New York, 750 pp.) That in the short residence times of only a few seconds to be cleaned air in the offered photocatalytic air purifiers only a fraction of the pollutants is implemented, so that possibly a cleaning effect only in the course of many hours or multiple cycles in a closed room is possible. Also, as a result of incomplete oxidative degradation of the trace substances, partially toxic intermediates are obtained which can only be degraded to harmless products in the course of a slower further oxidation.

The decisive disadvantages of previously known methods for photocatalytic air purification are their low effectiveness; they can be summarized as follows: a) low reaction area in the reaction space, b) high residence time in the reaction space, c) incomplete oxidation and discharge of primary products with the reaction vessel

Airflow.

The invention therefore an object of the invention to provide a method by which the purification of air and water can be carried out quickly and environmentally friendly and environmentally harmful substance components can be almost completely removed under extensive Ausoxidieren.

This object is achieved by a method for oxidizing and removing environmentally harmful substance components from gaseous and / or aqueous phases using the photocatalytic effect of titanium dioxide. According to the invention, the gaseous and / or aqueous phases to be purified are reacted in a reaction space with a photocatalytically active titania Surface be brought into contact. In the reaction space are preferably plates and / or packing, which have titanium dioxide coated surfaces and are shown in a system so constructive that they ensure a high surface / volume ratio. They are available in any geometric shapes. To ensure a high

Surface / volume ratio, they are constructive z. B. arranged meandering in horizontal channels or as shelves in columns. A high or high surface / volume ratio (which serves as a measure of the amount of a material that is used per volume of space) is preferably 10 m ² / m ³ to 1000 m ² / m ³ , preferably from 500 m ² / m ³ in front.

The catalytically active surface is designed in particular as a packing of different geometric shapes. Geometric shapes can e.g. Be balls or calipers.

According to the invention, an aqueous liquid film is produced on the photocatalytically active surface. The photocatalytically active surfaces are sprayed continuously with a rinsing liquid (for cleaning gaseous phases) or directly with the aqueous phase to be purified. The photochemically catalyzed radical reaction is initiated with a light source. Light source according to the invention may be the sunlight or any other radiation sources that are capable of generating radicals.

According to the invention, ozone generated at the same time in a gas phase is supplied for the purpose of intensifying radical formation, which can be produced with an upstream ozone generator. Alternatively, ozone and the radiation necessary for photocatalysis can also be generated by a common radiation source in the reaction space.

The rinsing liquid used, preferably water, is preferably recycled. To increase the efficiency of the process, both the

Rinsing liquid and the wastewater to be treated additional chemical substances are added, which influence the pH and thus the reaction process and / or generate other reactive radicals that react with the trace substances or can initiate further radical processes. Preferably, chemical substances are added which lead to an increase in the oxidation capacity. These are preferably oxidants which are selected as a function of the pH. Thus, in acidic pH Areas (pH <6) preferably used substances that promote H ₂ θ ₂ formation, such as metal ions (eg, Fe ²⁺ or Mn ²⁺ , etc.), in alkaline areas at pH values> 7, for example, ozone is added.

In a preferred embodiment, the reaction space further preferably comprises at least one UV lamp in addition to the photocatalytic surface.

According to the invention, the process is carried out in such a way that the liquids are preferably introduced into the reaction space via a spray device. This can be located outside the reaction space, or in another embodiment, the reaction space for the purification of gaseous phases, the catalytically active surfaces have designed as contact surfaces of various types in a spray absorber.

In addition, the method can be carried out with the supply of sound waves, wherein sound generators are used, which initiate an increased mass transport of the reactive and degradable species to the photocatalytically active surfaces and tribochemical effects, in particular due to acoustic coagulation.

Preferably, the process is carried out under ultrasonic conditions. To further increase efficiency, the process can take place under pressure (high pressure). As a result, in particular the concentration of ozone and the rate of pollutant oxidation dependent thereon are increased. In addition, an increase in pressure facilitates the overcoming of process-related flow resistance.

The process according to the invention has proven to be suitable for oxidizing and removing environmentally harmful components in the form of organic compounds, sulfite, reduced heavy metals, sulfur dioxide, reduced sulfur compounds, nitrogen oxides, bacteria or germs. By means of modular adaptation, special pollutants can be isolated, such as organic pollutants in the neutral pH range with addition of ozone, sulfur dioxide eg in the alkaline range with ozone and nitrogen oxides eg in the pH range 6 to 8 with Fe ²⁺ ions. As is known, electrons are activated by light on a Tiθ ₂ surface. These can form superoxide anions (O ₂ ^" ) with either surface-adsorbed or dissolved oxygen (O ₂ ) in the aqueous surface film These ions represent the anion of the hydroperoxide radical (HO ₂ ) with which they react in a pH-dependent protolytic equilibrium Hydrogen peroxide (H2O2) is slowly formed from both species, which is significantly accelerated in the presence of transition metal ions (eg Fe, Cu, Mn): O2 ^" + HO2 → HO ₂ ^" (+ O ₂ ), HO ₂ ^" + H ⁺ → H ₂ O ₂ . Hydrogen peroxide is converted to OH radicals (by means of the so-called Fenton reaction). Only the OH radicals are the actual reaction partners of the pollutants; H ₂ O ₂ reacts in the aqueous phase with some inorganic trace substances (preferably sulfite ions) but not with organic substances.

The inventive method combines in a simple manner known per se phenomena of independent primary formation of two oxidants - O ₂ ^" on the photocatalytic TiO ₂ surface and O ₃ in the gas phase by UV radiation.

The superoxide radicals initially formed on the catalytic surface react rapidly with ozone in the aqueous phase. On the other hand, a radical formation occurs as a result of O ₃ -fallfall in aqueous solutions. The radical chains formed thereby enable a radical regeneration, in particular by the conversion OH → HO ₂ in the OH-attack on organic substances. From the HO ₂ O ₂ ^" is again formed (pH equilibrium) which in turn reacts with O ₃ , etc.

The process represents a chemical two-phase system and allows a significant acceleration of the trace gas oxidation and thus purification of the air or water. Essential to the invention is the presence of an aqueous liquid film on the photocatalytic surface, in (and at) the pollutant degradation takes place with simultaneous ozone supply. In the case of wastewater treatment, the waste water i.d.R. even the liquid phase.

The primary oxidation products formed from the air pollutants are much more water-soluble than the starting materials, so that a large proportion of the surface remains or is dissolved and further to mineral Residues can be implemented or removed by water renewal with the sewage flow.

The process is a multiple combination. In addition, radical chemical reactions take place in the gas phase as a result of ozone formation and UV radiation, which leads to the neutralization of trace substances, exhaust gas and wastewater components. As a result, bacteria, viruses u.a. Killed microorganisms. In addition, all soluble trace substances can be washed out by water sprinkling and oxidized in the liquid phase. The method has the following advantages over previously described methods:

1. The conversion of impurities (degree of purification) is much higher due to a multi-phase chemical process.

2. A large part of the oxidation products is transferred into the aqueous phase and thus removed from the air to be purified. 3. In particular, the process of air purification after the combination process is extremely fast. 4. The process can be used in combination with other air purification and treatment systems (eg filters, air conditioners). 5. The method can also be used on a mobile basis, so that a

Can be used in non-constantly cleaning areas.

6. The method can also be used for effective exhaust air purification with appropriate dimensioning.

7. The process is suitable for both air and water purification in the same system.

8. The method can be adapted to the removal of specific trace substances in the case of modular adaptation.

The inventive method thus allows environmentally harmful substances to be removed quickly and almost completely. By the inventive principle of

Using a photocatalytic multiphase system, all three phases act

(Gas-liquid-solid) interactively simultaneously. Only this principle ensures a high mass transfer (ozone and / or gaseous pollutants in the liquid phase) and a high radiation yield at the catalyst surface (formation of the liquid as a film or drops). The simultaneous presence of ozone increases the reactivity (by OH yield increase). Thus, it is possible a) alternative to the reaction of the free electrons with O ₂ (oxygen) instead ozone (O ₃ ) to offer and b) the primary reaction product of the irradiated TiO ₂ catalyst (O _2- ) to react with ozone to thereby more OH radicals directly compared to the sole use of oxygen (O ₂ ).

In addition, the method according to the invention can also be used only with sunlight (no special UV light is necessary), which recommends the method, in particular in a special variant for rapid water treatment.

The invention also provides a cleaning system which comprises the following devices and can also be configured as a mobile device: a) at least one reaction space with a photocatalytically active titanium dioxide surface which consists of a system of differently or identically arranged plates and / or random packings b) a spraying device and c) an ozone generator, d) optionally a light source and / or e) optionally a sound generator.

In a preferred embodiment variant, the cleaning system can be coupled to an air conditioning system and / or a further air entrainment system and be operated stationary.

Preferably, it is used for exhaust air and water purification.

In the following the invention with reference to embodiments will be described in more detail.

example 1

A container is provided with catalytically active TiO ₂ -coated plates in such a way that the incoming air stream to be cleaned has to travel as far as possible in relation to a small reactor volume. The Catalyst plate surfaces are continuously wetted from above via a distribution system with an aqueous rinsing liquid in the form of a thin film, wherein the rinsing liquid flows in the circuit. The container surface is internally provided with a system of UV lamps, which both generate ozone and trigger the photocatalytic effect. The oxidation of the air pollutants takes place in the liquid film, wherein the products are largely absorbed and oxidized using the cycle principle over a long period of time completely harmless mineral products and discharged as such. After a certain period of time, which depends on the degree of air pollution, the rinsing liquid enriched with the dissolved mineral residues must be replaced. This point in time can be determined by a simple salinity measurement and the detergent change is fully automatic. The waste water is no further environmental hazard because it consists of natural inorganic substances that are diluted rapidly in the municipal sewage system.

Example 2

A cylindrical container is filled with optically permeable Tiθ ₂ -coated packings, for example Raschig rings, and is wetted with water via a spraying system located on the ceiling. Likewise on the ceiling lamps are installed, which ensure a photocatalytic excitation. The air is supplied via an ozone generator. The ozone-enriched polluted air flows past the packing and reacts on the wet surface with formed OH radicals, resulting in a fast and effective air purification.

Example 3

For the treatment of the waste water, for example of textile dyes, this is added directly instead of the rinsing liquid in the examples described above in the photocatalytic purification reactor and cycled until complete degradation. The devices described in the aforementioned embodiments can be combined with a superior air filter for coarse cleaning of particulate trace substances. Both devices can also be installed in existing systems for air treatment, in particular systems for room and building air conditioning.

Claims

claims

1. Method for oxidizing and removing environmentally harmful

Substance components of gaseous and / or aqueous phases using the photocatalytic effect of titanium dioxide, characterized in that the gaseous and / or aqueous phases to be cleaned in a reaction chamber with a photocatalytically active titanium dioxide surface are brought into contact and simultaneously generated in a gas phase ozone is fed, wherein the photocatalytically active titanium dioxide

Surface is sprayed continuously with a rinsing liquid or with the aqueous phase to be purified and the photochemically catalyzed radical reaction takes place with any light source.

2. The method according to claim 1, characterized in that a titanium dioxide surface is used which has a high surface / volume ratio, which is 10 m ² / m ³ to 1000 m ² / m ³ .

3. The method according to claim 1 or 2, characterized in that the rinsing liquid or the aqueous phase to be purified in addition chemical substances are added, which lead to an increase in the oxidation capacity.

4. The method according to any one of claims 1 to 3, characterized in that the liquids are added via a spray device in the reaction space.

5. The method according to any one of claims 1 to 4, characterized in that the reaction space is constructed with the catalytically active surfaces as a spray absorber.

6. The method according to any one of claims 1 to 5, characterized in that initiated by sound generators, in particular due to an acoustic coagulation an increased mass transport of the reactive and degradable species to the photocatalytically active surfaces and tribochemical effects.

7. The method according to any one of claims 1 to 6, characterized in that the ozone is generated with an upstream ozone generator.

8. The method according to any one of claims 1 to 7, characterized in that ozone and necessary for photocatalysis radiation are generated by a common radiation source in the reaction space.

9. The method according to any one of claims 1 to 8, characterized in that the rinsing liquid is water.

10. The method according to any one of claims 1 to 9, characterized in that the rinsing liquid is circulated.

11. The method according to any one of claims 1 to 10, characterized in that it takes place under pressure or ultrasonic conditions.

12. A cleaning system for oxidizing and removing environmentally harmful substance components from gaseous and / or aqueous phases using the photocatalytic effect of titanium dioxide according to a method of claims 1 to 11, comprising a) at least one reaction chamber with a photocatalytically active titanium dioxide surface, which consists of a System of arbitrarily arranged plates or packing of any geometric shapes to achieve a large surface / volume ratio consists b) a spray and c) an ozone generator.

13. Cleaning system according to claim 12, characterized in that it comprises d) a light source and optionally e) a sound generator.

14. Cleaning system according to claim 13, characterized in that the reaction space comprises at least one UV lamp in addition to the photocatalytic surface.

15. The method according to any one of claims 12 to 14, characterized in that the catalytically active surface is designed as a filling body of different geometric shapes.

16. Use of a cleaning system according to any one of claims 12 to 15 for exhaust air and water purification.

17. Use according to claim 16 for Ausoxidieren and for the removal of environmentally harmful components in the form of organic compounds, sulfite, reduced heavy metals, sulfur dioxide, reduced sulfur compounds, nitrogen oxides, bacteria or germs.

18. Use according to claim 16 or 17 coupled with an air conditioner and / or another Lufteinigungssystem.