WO2017055094A1 - Air cleaning by photocatalytic oxidation system - Google Patents

Air cleaning by photocatalytic oxidation system Download PDF

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Publication number
WO2017055094A1
WO2017055094A1 PCT/EP2016/071850 EP2016071850W WO2017055094A1 WO 2017055094 A1 WO2017055094 A1 WO 2017055094A1 EP 2016071850 W EP2016071850 W EP 2016071850W WO 2017055094 A1 WO2017055094 A1 WO 2017055094A1
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Prior art keywords
air
photo catalyst
hybrid
air cleaning
cleaning system
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PCT/EP2016/071850
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French (fr)
Inventor
Joakim Reimer THØGERSEN
Fei Chen
Chetan Prakash Mittal
Berit HINNEMANN
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Haldor Topsøe A/S
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Publication of WO2017055094A1 publication Critical patent/WO2017055094A1/en

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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
    • A61L9/00—Disinfection, sterilisation or deodorisation of air
    • A61L9/16—Disinfection, sterilisation or deodorisation of air using physical phenomena
    • A61L9/18—Radiation
    • A61L9/20—Ultraviolet radiation
    • A61L9/205—Ultraviolet radiation using a photocatalyst or photosensitiser
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/86—Catalytic processes
    • B01D53/8668—Removing organic compounds not provided for in B01D53/8603 - B01D53/8665
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/86—Catalytic processes
    • B01D53/88—Handling or mounting catalysts
    • B01D53/885—Devices in general for catalytic purification of waste gases
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/50—Catalysts, in general, characterised by their form or physical properties characterised by their shape or configuration
    • B01J35/56—Foraminous structures having flow-through passages or channels, e.g. grids or three-dimensional monoliths
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
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    • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
    • A61L2209/00—Aspects relating to disinfection, sterilisation or deodorisation of air
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    • B01D2259/00—Type of treatment
    • B01D2259/45—Gas separation or purification devices adapted for specific applications
    • B01D2259/4508—Gas separation or purification devices adapted for specific applications for cleaning air in buildings
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B01J35/39—Photocatalytic properties

Definitions

  • the invention relates to an air cleaning system comprising a hybrid photo catalyst arranged for photocatalytic oxidation of pollutants in air, the invention further relates to a method for cleaning air.
  • Air quality in enclosed spaces is often five or more times worse than outdoor air and new technologies will be required to effectively treat the full range of indoor air pollutants.
  • Indoor air contamination is a complex problem and can be classified in three groups:
  • particles as for instance PM2.5 (diameter less or equal to 2.5 ⁇ ) and PM10 (diameter less or equal to 10 ⁇ ) such as very small liquid or solid substances in suspension in the air.
  • PM2.5 diameter less or equal to 2.5 ⁇
  • PM10 diameter less or equal to 10 ⁇
  • particles can include mists, dust, pollen, cigarette smoke, viruses, bacteria, and molds;
  • VOCs volatile organic compounds
  • NOx Oxides of nitrogen e.g. nitric oxide (NO), nitrogen dioxide (NO2)), carbon monoxide (CO), carbon dioxide (CO2) etc.
  • radioactive gases and its progeny such as radioactive gases.
  • PM2.5 and PM10 particles are typically removed by particulate material filters.
  • HEPA High efficiency particulate air
  • ULPA Ultra low particulate air
  • VOCs are the most abundant air pollutants in the indoor air that we breathe.
  • technologies for removal or reduction of VOCs including formaldehyde are still evolving.
  • the field of the present invention is within the field of VOCs removal from indoor air.
  • VOCs released from building materials and furniture are known to be major indoor air contaminants and may cause the well-known 'sick building syndrome' such as headaches, dizziness, nausea, or various allergic reactions.
  • HVAC ven- tilating and air-conditioning
  • VOCs present in smaller amounts can be acetal- dehyde, acetone, 2-butanone, ethanol, n-hexane, limonene, dichloromethane, naphthalene, 2-propanol, propionaldehyde, tetrachloroethylene and others.
  • the level of VOCs in indoor air depends significantly on the sources for VOCs and their emission rates, and can vary widely.
  • the emission of VOCs from building materials has been recognized as the largest source of indoor air pollutants.
  • contributions from outdoor air can also add significantly to the VOCs levels through leakages and makeup air in ventilation systems (see Wang, S., H.M. Ang, and M.O. Tade, Volatile organic compounds in indoor environment and photocatalytic oxidation: State of the art. Environment Interna- tional, 2007. 33(5): p. 694-705).
  • Photocatalytic oxidation is an emerging technology in the HVAC industry and is increasingly being used for oxidative removal of VOCs from indoor air. It has attracted a large amount of attention over the last few decades.
  • PCO In addition to the improvement of Indoor Air Quality (IAQ), PCO has the added potential for limiting the introduction of unconditioned air to the building space, which saves energy.
  • IAQ Indoor Air Quality
  • PCO typically uses short-wave ultraviolet light (UVC), also commonly used for sterilization to energize a catalyst (usually titanium dioxide (T1O2)) and oxidize bacteria, viruses and VOCs.
  • UVC short-wave ultraviolet light
  • T1O2 titanium dioxide
  • PCO units can be mounted to an existing forced-air HVAC system. As PCO itself is not a filtering technology, it is often combined with other filtering technologies for air purification. The most commonly used photocatalyst is T1O2, and to some extent zinc oxide (ZnO).
  • photocatalyst are antimony trioxide (Sb20s), bismuth oxide (B12O3), vanadium oxide (V2O3), ferric oxide (Fe20s), zirconium dioxide (Zr02), tungsten trioxide (WO3), tin dioxide (Sn02), aluminum oxide (AI2O3), cerium oxide (Ce02), zinc sulfide (ZnS), cadmium sulfide (CdS), T1O2 doped with metal ions, T1O2 combined with Zr02 and/or silicon dioxide (S1O2) (see Mo, J., et al., Photocatalytic purification of volatile organic compounds in indoor air: A literature review. Atmospheric Environment, 2009. 43(14): p. 2229-2246).
  • Air humidity may be sufficient or water may need to be added (see Mo, J., et al., Photocatalytic purification of volatile organic compounds in indoor air: A literature review. Atmospheric Environment, 2009. 43(14): p. 2229-2246).
  • NO nitric oxide
  • N2 nitrogen dioxide
  • O2 oxygen
  • the hybrid photo catalyst needs to be able to operate even though the air to be purified contains a mixture of pollutants.
  • PCO photocatalytic oxidation
  • Transient effects may be important both as the air purification system reaches steady- state operation and as the building environment changes diurnally and seasonally.
  • Extended hybrid photo catalyst life is a key parameter to make the photocatalytic oxidation (PCO) technology viable.
  • the adsorbent material should have a good adsorption capacity as well as it should be effective for various ranges of VOCs.
  • One well-known drawback of PCO systems is that the UV sterilization bulbs must be replaced about once a year; in fact, manufacturers may require periodic replacement as a condition of warranty. Therefore, PCO systems often have high commercial costs due to the cost of the UV bulbs, and it is desirable to find ways to decrease this cost component to further improve the competitiveness of the PCO technology.
  • Hybrid systems are needed, because T1O2, which is the most widely used photo catalyst for PCO systems, exhibits low adsorption ability, especially for non-polar substances due to its polar structure. Therefore, the low adsorption ability of non-porous T1O2 particles could be improved by making composites of T1O2 with adsorbents.
  • the adsorbents would adsorb the compounds on the adsorbent support, forming a high concentration environment of the compounds around the T1O2. This results in an increase in the photoreaction rate.
  • adsorbents such as zeolum, alumina, silica, mordenite, ferrierite, and activated carbon, and it was demonstrated in several studies that the hybrid photo cata- lysts were effective in achieving high decomposition rates of for instance propionalde- hyde in air. Not only T1O2, but also other photocatalysts such as ZnO were found to have higher photodegradation efficiency when used as a compound system with activated carbon as the adsorbent (see Mo, J., et al., Photocatalytic purification of volatile organic compounds in indoor air: A literature review. Atmospheric Environment, 2009. 43(14): p. 2229-2246).
  • the invention relates to an air cleaning system comprising: a hybrid photo catalyst arranged for photocatalytic oxidation of pollutants in air, said hybrid photo catalyst comprising a porous filter impregnated with photo catalyst material as well as adsorbent material, and a source of ultraviolet (UV) light arranged to periodically irradiate said hybrid photo catalyst in order to decompose said pollutants and to regenerate said adsorbent material.
  • a hybrid photo catalyst arranged for photocatalytic oxidation of pollutants in air, said hybrid photo catalyst comprising a porous filter impregnated with photo catalyst material as well as adsorbent material, and a source of ultraviolet (UV) light arranged to periodically irradiate said hybrid photo catalyst in order to decompose said pollutants and to regenerate said adsorbent material.
  • UV ultraviolet
  • the UV light source In a traditional PCO system consisting of a UV light source and photo catalyst material, the UV light source is in continuous operation, as adsorbed VOCs need to be removed continuously in order to ensure effective removal.
  • VOCs can be adsorbed up to a certain capacity. This allows for periodic operation of the UV light source, which ensures efficient utilization of the expensive UV lamp by decomposition of concentrated pollutants on hybrid photocatalyst surface thereby regenerating hybrid photocatalyst for further adsorption.
  • the UV light is used in a more efficient manner, as by VOCs adsorption, a high local concentration of VOCs on the hybrid PCO system is created, which then can be removed efficiently by UV.
  • PCO photocatalytic oxidation
  • a hybrid PCO and adsorption solution is proposed. The gist of the idea is a combined adsorbent/photo catalyst system to which the pollutants are continuously adsorbed and that the adsorbent is periodically regenerated by UV light.
  • the proposed solution advantageously has the following features to overcome the problems: ⁇ Sufficient adsorption capacity and effective mass transfer rate between gas phase and the adsorbent;
  • a system of panels with a deep porous filters (possibly resembling open filters used for automotive applications) impregnated with high surface area hybrid photocatalyst is proposed.
  • the photo catalyst is e.g. T1O2
  • the adsorbent is e.g. a zeolite
  • the deep filter media is e.g. quartz material.
  • the filter is relatively open in order to secure good penetration of UV light, which means that the filtration mechanism will be deep filtration in the filter media result- ing in low pressure drop.
  • This design allows for a high adsorption surface area, further making it possible to reduce the required photocatalyst area and to ensure a compact design.
  • the hybrid photocatalytic system deep filtration and effective adsorption has the possibility to allow for higher air flow rate, while still maintaining sufficient catalytic activity, due to the added adsorption capacity.
  • hybrid photocatalyst material is meant to denote a material comprising both a photocatalyst and an adsorbent.
  • the air cleaning system further comprises a particulate matter filter upstream of the hybrid photo catalyst.
  • a particulate matter filter is e.g. a coarse pre-filter that can be cleaned on a regular basis,
  • the air cleaning system further comprises a prefilter upstream of the particulate matter filter and/or the hybrid photo catalyst.
  • the prefilter is e.g. an electrostatically charged filter, an activated carbon filter, a particulate matter filter or a combination thereof.
  • An electrostatically charged filter is useful for particulate matter removals, an activated carbon filter for odour removal, and a particulate matter (PM) filter (e.g. HEPA, ULPA) is useful for reduction of the particulate matter (PM) before entering the hybrid PCO system.
  • PM particulate matter
  • the air cleaning system further comprises air conditioning equipment downstream the hybrid photo catalyst.
  • the air conditioning equipment e.g. comprises activated carbon equipment or filter to eliminate traces of interme- diate and unreacted pollutants and/or an ion generation unit to release ions to keep the air fresh.
  • the UV light irradiates said hybrid photo catalyst in less than 50% of the time, preferably less than 40% of the time, more preferably less than 25% of the time.
  • a source of UV light viz. a UV lamp is typically quite expensive; however, its lifetime depends on the number of operation hours.
  • a UV lamp is typically quite expensive; however, its lifetime depends on the number of operation hours.
  • the photo catalyst material comprises one or more of the following: Titanium dioxide (T1O2), zink oxide (ZnO), antimony trioxide (Sb20s), bismuth oxide (B12O3), vanadium oxide (V2O3), ferric oxide (Fe20s), zirconium dioxide (ZrC>2), tungsten trioxide (WO3), tin dioxide (SnC>2), aluminium oxide (AI2O3), cerium oxide (CeC>2), zinc sulphide (ZnS), cadmium sulphide (CdS), T1O2 doped with metal ions, T1O2 combined with ZrC>2 and/or silicon dioxide (S1O2).
  • the adsorbent material comprises activated carbon, zeolite, zeolum, alumina, silica, mordenite, ferrierite, clinoptilolite, ZSM-5.
  • Figure 1 a shows a schematic drawing of a system of the invention
  • Figures 1 b-1 d show two side views and a top view of an air cleaning unit
  • Figures 2a-2c show a typical operation cycle of the hybrid photo catalyst.
  • FIG 1 a shows a schematic drawing of an air cleaning system 100 of the invention.
  • the air cleaning system 100 comprises an air cleaning unit 20 comprising a hybrid photo catalyst arranged for photocatalytic oxidation of pollutants in air.
  • the hybrid photo catalyst comprises a porous filter 26 impregnated with photo catalyst material as well as adsorbent material.
  • the air cleaning unit 20 moreover comprises sources 22 of ultraviolet (UV) light arranged to periodically irradiate the hybrid photo catalyst in order to regenerate the adsorbent material.
  • the air cleaning system 20 comprises a number of UV lamps arranged to periodically irradiate a hybrid photo catalyst 26. In figure 1 , an array of two times two UV lamps is arranged within cylinders of hybrid photo catalyst 26.
  • the overall system 100 moreover comprises a PM filter 10, such as e.g. a HEPA or ULPA filter.
  • PM filter 10 such as e.g. a HEPA or ULPA filter.
  • further filters upstream the PM filter and/or upstream the air cleaning system 20 may be present, such as a pre-filter, for example an ESP filter.
  • the air cleaning system may comprise further air conditioning units downstream the air cleaning unit 20, such as an activated carbon filter to eliminate traces of intermediate and unreacted pollutants and/or an ion generator.
  • polluted air 1 enters the system 100.
  • the polluted air may already have undergone some purification in a pre-filter prior to entering the system 100.
  • the polluted air 1 reaches the HEPA or ULPA filter 10 for removal of particulate matter.
  • the resultant air 2 comprising VOCs/NOx , particulates and/or bioaerosols, from which the major part of particulate matter has been removed, is led to the air cleaning system 20 having a hybrid photo catalyst and adsorbent 26.
  • the air 2 enters the air cleaning system between hybrid photo catalyst or adsorbent 26 and UV lamps 22.
  • the adsorbent 26 adsorbs polluting compounds. Thereby a high concentration environment of the compounds is formed around the photocatalyst, e.g. T1O2, resulting in an increase in the photoreaction rate upon radiation with UV light.
  • the UV lamps 22 are turned on to irradiate the adsorbent with UV light, thereby regenerating the adsorbent.
  • FIG. 1 b-1 d show two side views and a top view of an air cleaning unit 20.
  • the side view of Figure 1 B is a view of the left end (as seen in figure 1 A) of the air cleaning unit 20, whilst the side view of Figure 1 B is a view of the right end (as seen in figure 1A) of the air cleaning unit 20 and figure 1 C is a top view of the air cleaning unit 20.
  • the air cleaning unit 20 has a front plate 21 (shown hatched) and four units, each comprising a UV lamp 22, surrounded by an annular space and a cylindrical hybrid photocatalyst 26 (shown as concentric circles in figure 1 b).
  • the number of units in an air cleaning unit can be any appropriate number, and that the diameter of the cylindrical hybrid photocatalyst 26 could be different than shown in the figures. If the diameter of the cylindrical photocatalyst 26 is somewhat larger, the total area of the front plate 21 would be smaller and the effective area of the air cleaning system would be increased.
  • FIG 1 c a side view of the end of the air cleaning unit 20 is shown.
  • the cylindrical photocatalyst 26 ends in a part 28, which is either not penetrable by air, or is made from hybrid photocatalyst material. Hereby, it is ensured that the air entering the air cleaning unit 20 has to pass through the hybrid photocatalyst material 26.
  • Struts or braces 27 support the end of the cylindrical hybrid photocatalyst 26 of the four units.
  • FIG 1 d a top view of the air cleaning unit 20 is shown.
  • air comprising pollutants is arranged to enter the air cleaning unit 20 from the left, and clean(ed) air is arranged to exit the air cleaning unit to the right or along the sides of the units, through the hybrid photocatalyst material 26.
  • the air cleaning unit 20 shown in figures 1 a-1 d is an open system, where the cleaned air exits through the walls of hybrid photocatalyst material 26 or from the ends 28 of the cylindrical hybrid photocatalyst. This is shown by the curved arrows in figure 1 d.
  • the system is a closed system where side walls enclose the hybrid photocatalyst material 26.
  • air to be cleaned enters into the annular space between the UV lamp 22 and the hybrid photocatalyst material 26 and exits the air cleaning unit via the end structure 25 or the end parts 28.
  • an end structure 25 comprising struts or braces 27 is shown.
  • the end structure 25 could alternatively be an end plate, which is impenetrable to gas, except from the end parts 28.
  • FIGs 2a to 2c show a typical regeneration cycle of a hybrid photocatalyst of an air cleaning system according to the invention.
  • FIGs 2a-2c only a small part of the hybrid photo catalyst 26 is shown.
  • the operation cycle includes three stages illustrated in the three figures.
  • the hybrid photocatalyst 26 has recently been regenerated and it is ready for adsorption of pollutants.
  • the photocatalyst 26 is shown with a few particles 4 of pollutants adsorbed on the surface thereof.
  • Air 2 comprising VOCs, NOx, particulates and/or bioaerosols is passed along the surface of the hybrid photocatalyst 26, and clean or cleaner air 3 leaves the photo- catalyst 26.
  • one or more sources of UV light are turned on to irradiate the hybrid photocatalyst 26 with UV light in order to decompose the adsorbed pollutants and thereby regenerate the hybrid photocatalyst.
  • the primary end products from the oxidation of VOCs, NOx, particulates and/or bioaerosols in the air 2 are CO2 and H2O. These primary end products are denoted by "7" in figure 2c.
  • the photocatalyst is ready for another cycle, starting with the first stage.
  • the UV source(s) is/are turned on simultaneously with an air flow passing along the surface of the photocatalyst 26.

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Abstract

The invention relates to an air cleaning system comprising a hybrid photo catalyst arranged for photocatalytic oxidation of pollutants in air, said hybrid photo catalyst comprising a porous filter impregnated with photo catalyst material as well as adsorbent material, and a source of ultraviolet (UV) light arranged to periodically irradiate said hybrid photo catalyst in order to regenerate said adsorbent material. The invention further relates to a method for operating an air cleaning system of the invention.

Description

Air cleaning by photocatalytic oxidation system
FIELD OF THE INVENTION
The invention relates to an air cleaning system comprising a hybrid photo catalyst arranged for photocatalytic oxidation of pollutants in air, the invention further relates to a method for cleaning air.
BACKGROUND
Air quality in enclosed spaces is often five or more times worse than outdoor air and new technologies will be required to effectively treat the full range of indoor air pollutants. Indoor air contamination is a complex problem and can be classified in three groups:
1 ) particles as for instance PM2.5 (diameter less or equal to 2.5 μηη) and PM10 (diameter less or equal to 10 μηη) such as very small liquid or solid substances in suspension in the air. These particles can include mists, dust, pollen, cigarette smoke, viruses, bacteria, and molds;
2) gaseous pollutants such volatile organic compounds (VOCs) including formaldehyde, NOx (Oxides of nitrogen e.g. nitric oxide (NO), nitrogen dioxide (NO2)), carbon monoxide (CO), carbon dioxide (CO2) etc.;
3) radioactive gases and its progeny such as radioactive gases.
(see e.g. Guieysse, B., et al., Biological treatment of indoor air for VOC removal: Potential and challenges. Biotechnology Advances, 2008. 26(5): p. 398-410).
PM2.5 and PM10 particles are typically removed by particulate material filters. The market of these filters is quite established and HEPA (High efficiency particulate air) filters and ULPA (Ultra low particulate air) filters fulfil the market needs in this area. Among rest of the air pollutants, VOCs are the most abundant air pollutants in the indoor air that we breathe. However, technologies for removal or reduction of VOCs including formaldehyde are still evolving. The field of the present invention is within the field of VOCs removal from indoor air. VOCs released from building materials and furniture are known to be major indoor air contaminants and may cause the well-known 'sick building syndrome' such as headaches, dizziness, nausea, or various allergic reactions. In addition, indoor air pollution is further aggravated by recent trends for constructing airtight buildings with heating, ven- tilating and air-conditioning (HVAC) system for reducing energy consumption. These central HVAC systems have the disadvantage that they distribute the contaminants present in the indoor air throughout the whole building. Therefore, HVAC systems should include air purifiers which remove VOCs from indoor air. In general, the concentrations of individual VOCs in indoor air are in the range from ppb to ppm level. Some of the most commonly detected VOCs in indoor air include formaldehyde, toluene, m,p-xy- lenes, opinene, and benzene. Other VOCs present in smaller amounts can be acetal- dehyde, acetone, 2-butanone, ethanol, n-hexane, limonene, dichloromethane, naphthalene, 2-propanol, propionaldehyde, tetrachloroethylene and others. The level of VOCs in indoor air depends significantly on the sources for VOCs and their emission rates, and can vary widely. The emission of VOCs from building materials has been recognized as the largest source of indoor air pollutants. In some cases, contributions from outdoor air can also add significantly to the VOCs levels through leakages and makeup air in ventilation systems (see Wang, S., H.M. Ang, and M.O. Tade, Volatile organic compounds in indoor environment and photocatalytic oxidation: State of the art. Environment Interna- tional, 2007. 33(5): p. 694-705).
Numerous methods are available for removal of indoor VOCs, including microbiological filters, HEPA filters, charcoal filters, ozonization and air ionization, and these have been used for some time. However, none of these methods is sufficiently effective in all cases. For indoor air, traditional thermal catalytic oxidation is out of the question due to the low concentration of the VOCs and especially, as they require operation at elevated temperatures. This is not viable, and it is a typical requirement that any solution operates at ambient temperature (see Yang, L, et al., Performance analysis of a novel Ti02-coated foam-nickel PCO air purifier in HVAC systems. Separation and Purification Technology, 2009. 68(2): p. 232-237). Photocatalytic oxidation (PCO) is an emerging technology in the HVAC industry and is increasingly being used for oxidative removal of VOCs from indoor air. It has attracted a large amount of attention over the last few decades. In addition to the improvement of Indoor Air Quality (IAQ), PCO has the added potential for limiting the introduction of unconditioned air to the building space, which saves energy. The key advantage of this technology is the possibility for complete removal of a broad range of VOCs by conversion to environmentally harmless compounds such as CC>2 and H2O at room temperature and atmospheric pressure. PCO typically uses short-wave ultraviolet light (UVC), also commonly used for sterilization to energize a catalyst (usually titanium dioxide (T1O2)) and oxidize bacteria, viruses and VOCs. PCO units can be mounted to an existing forced-air HVAC system. As PCO itself is not a filtering technology, it is often combined with other filtering technologies for air purification. The most commonly used photocatalyst is T1O2, and to some extent zinc oxide (ZnO). Additional examples of photocatalyst are antimony trioxide (Sb20s), bismuth oxide (B12O3), vanadium oxide (V2O3), ferric oxide (Fe20s), zirconium dioxide (Zr02), tungsten trioxide (WO3), tin dioxide (Sn02), aluminum oxide (AI2O3), cerium oxide (Ce02), zinc sulfide (ZnS), cadmium sulfide (CdS), T1O2 doped with metal ions, T1O2 combined with Zr02 and/or silicon dioxide (S1O2) (see Mo, J., et al., Photocatalytic purification of volatile organic compounds in indoor air: A literature review. Atmospheric Environment, 2009. 43(14): p. 2229-2246).
Irradiation of UV light onto the photocatalyst with adsorbed water molecules creates free OH radicals, which, in turn oxidize pollutants. In the absence of water vapor, the photo- catalytic degradation of some chemical compounds (e.g., formaldehyde, acetone, and toluene) is seriously retarded. However, excessive water vapor on the catalyst surface will inhibit the reaction rate because the presence of water vapor competes with pollutants for adsorption sites on the photo catalyst, thus reducing the pollutant removal rate. This is called "competitive adsorption" between water vapor and pollutant. Therefore, competitive adsorption plays a key role in photocatalytic oxidation. Air humidity may be sufficient or water may need to be added (see Mo, J., et al., Photocatalytic purification of volatile organic compounds in indoor air: A literature review. Atmospheric Environment, 2009. 43(14): p. 2229-2246).
The conversion of nitric oxide (NO) is of importance, NO promotes the conversion of indoor pollutants. The positive effect of NO is due to the hydroxyl radicals generated from the photodegradation of NO into nitrogen dioxide (NO2). However, ultimately NO2 may lead to formation of nitric acid, when NO2 reacts with the air moisture. A T1O2 photo catalyst can possibly be optimized for NO decomposition into nitrogen (N2) and oxygen (O2). The hybrid photo catalyst needs to be able to operate even though the air to be purified contains a mixture of pollutants. Moreover, it is important that the effluent from the photocatalytic oxidation (PCO) air purification system of the invention does not provide hazardous by-products or effluents.
Transient effects may be important both as the air purification system reaches steady- state operation and as the building environment changes diurnally and seasonally.
Extended hybrid photo catalyst life is a key parameter to make the photocatalytic oxidation (PCO) technology viable.
Effective adsorption is a challenge for traditional PCO when it comes to formaldehyde, and the UV radiation requirement probably increases for the light hydrocarbons. Therefore, the adsorbent material should have a good adsorption capacity as well as it should be effective for various ranges of VOCs. One well-known drawback of PCO systems is that the UV sterilization bulbs must be replaced about once a year; in fact, manufacturers may require periodic replacement as a condition of warranty. Therefore, PCO systems often have high commercial costs due to the cost of the UV bulbs, and it is desirable to find ways to decrease this cost component to further improve the competitiveness of the PCO technology. This issue has pre- viously been addressed by development of hybrid systems of a T1O2 photocatalyst dispersed on an adsorbent with the objective to increase photocatalytic efficiency. Hybrid systems are needed, because T1O2, which is the most widely used photo catalyst for PCO systems, exhibits low adsorption ability, especially for non-polar substances due to its polar structure. Therefore, the low adsorption ability of non-porous T1O2 particles could be improved by making composites of T1O2 with adsorbents. The adsorbents would adsorb the compounds on the adsorbent support, forming a high concentration environment of the compounds around the T1O2. This results in an increase in the photoreaction rate. Various adsorbents were used, such as zeolum, alumina, silica, mordenite, ferrierite, and activated carbon, and it was demonstrated in several studies that the hybrid photo cata- lysts were effective in achieving high decomposition rates of for instance propionalde- hyde in air. Not only T1O2, but also other photocatalysts such as ZnO were found to have higher photodegradation efficiency when used as a compound system with activated carbon as the adsorbent (see Mo, J., et al., Photocatalytic purification of volatile organic compounds in indoor air: A literature review. Atmospheric Environment, 2009. 43(14): p. 2229-2246).
It is an object of the present invention to provide an air cleaning system and method with reduced costs of operation. More specifically, it is an object of the invention to provide a PCO air cleaning system and method fulfilling one or more of the following requirements:
• Removal of VOCs and other pollutants as per indoor air quality standards
• Low pressure drop and low energy consumption (low operational expenditure, OP EX)
· Ambient temperature solution
• Acceptable capital expenditure (CAPEX)
• Compact design
• Extended photocatalyst lifetime
• Minimum / no byproduct formation DESCRIPTION OF THE INVENTION
The invention relates to an air cleaning system comprising: a hybrid photo catalyst arranged for photocatalytic oxidation of pollutants in air, said hybrid photo catalyst comprising a porous filter impregnated with photo catalyst material as well as adsorbent material, and a source of ultraviolet (UV) light arranged to periodically irradiate said hybrid photo catalyst in order to decompose said pollutants and to regenerate said adsorbent material.
In a traditional PCO system consisting of a UV light source and photo catalyst material, the UV light source is in continuous operation, as adsorbed VOCs need to be removed continuously in order to ensure effective removal. However, when a hybrid system com- prising a photocatalyst and an adsorbent is used, VOCs can be adsorbed up to a certain capacity. This allows for periodic operation of the UV light source, which ensures efficient utilization of the expensive UV lamp by decomposition of concentrated pollutants on hybrid photocatalyst surface thereby regenerating hybrid photocatalyst for further adsorption. Moreover, the UV light is used in a more efficient manner, as by VOCs adsorption, a high local concentration of VOCs on the hybrid PCO system is created, which then can be removed efficiently by UV. This is a synergistic effect. Heterogeneous photocatalytic oxidation (PCO) is a very promising technology for VOCs removal at room temperature and to some extent for removal of NOx, particulates, and bioaerosols. In order to develop an economical solution avoiding continuous UV irradiation, a hybrid PCO and adsorption solution is proposed. The gist of the idea is a combined adsorbent/photo catalyst system to which the pollutants are continuously adsorbed and that the adsorbent is periodically regenerated by UV light.
The proposed solution advantageously has the following features to overcome the problems: · Sufficient adsorption capacity and effective mass transfer rate between gas phase and the adsorbent;
• Open pore structure to ensure sufficient UV irradiation on catalyst surface;
• Rapid regeneration to minimize the use of UV lamp.
A system of panels with a deep porous filters (possibly resembling open filters used for automotive applications) impregnated with high surface area hybrid photocatalyst is proposed. The photo catalyst is e.g. T1O2, the adsorbent is e.g. a zeolite and the deep filter media is e.g. quartz material.
Preferably, the filter is relatively open in order to secure good penetration of UV light, which means that the filtration mechanism will be deep filtration in the filter media result- ing in low pressure drop. This design allows for a high adsorption surface area, further making it possible to reduce the required photocatalyst area and to ensure a compact design.
Traditional PCO systems have limitations of air flow rate for effective catalytic activity and cannot handle higher airflows while maintaining effective pollutants decomposition. In the present invention, the hybrid photocatalytic system, deep filtration and effective adsorption has the possibility to allow for higher air flow rate, while still maintaining sufficient catalytic activity, due to the added adsorption capacity.
As used herein, the term "hybrid photocatalyst material" is meant to denote a material comprising both a photocatalyst and an adsorbent. In an embodiment, the air cleaning system further comprises a particulate matter filter upstream of the hybrid photo catalyst. Such a particulate matter filter is e.g. a coarse pre-filter that can be cleaned on a regular basis,
In an embodiment the air cleaning system further comprises a prefilter upstream of the particulate matter filter and/or the hybrid photo catalyst. The prefilter is e.g. an electrostatically charged filter, an activated carbon filter, a particulate matter filter or a combination thereof.
An electrostatically charged filter is useful for particulate matter removals, an activated carbon filter for odour removal, and a particulate matter (PM) filter (e.g. HEPA, ULPA) is useful for reduction of the particulate matter (PM) before entering the hybrid PCO system.
In an embodiment, the air cleaning system further comprises air conditioning equipment downstream the hybrid photo catalyst. The air conditioning equipment e.g. comprises activated carbon equipment or filter to eliminate traces of interme- diate and unreacted pollutants and/or an ion generation unit to release ions to keep the air fresh.
In an embodiment, the UV light irradiates said hybrid photo catalyst in less than 50% of the time, preferably less than 40% of the time, more preferably less than 25% of the time.
A source of UV light, viz. a UV lamp is typically quite expensive; however, its lifetime depends on the number of operation hours. Thus, by minimizing the consumption of the UV source by effectively adsorbing the pollutants over a period of time and periodically decomposition of pollutants and regeneration of the hybrid catalyst, the overall price of the system is reduced.
In an embodiment, the photo catalyst material comprises one or more of the following: Titanium dioxide (T1O2), zink oxide (ZnO), antimony trioxide (Sb20s), bismuth oxide (B12O3), vanadium oxide (V2O3), ferric oxide (Fe20s), zirconium dioxide (ZrC>2), tungsten trioxide (WO3), tin dioxide (SnC>2), aluminium oxide (AI2O3), cerium oxide (CeC>2), zinc sulphide (ZnS), cadmium sulphide (CdS), T1O2 doped with metal ions, T1O2 combined with ZrC>2 and/or silicon dioxide (S1O2). In an embodiment, the adsorbent material comprises activated carbon, zeolite, zeolum, alumina, silica, mordenite, ferrierite, clinoptilolite, ZSM-5.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 a shows a schematic drawing of a system of the invention; and Figures 1 b-1 d show two side views and a top view of an air cleaning unit; and Figures 2a-2c show a typical operation cycle of the hybrid photo catalyst. DETAILED DESCRIPTION OF THE DRAWINGS
Figure 1 a shows a schematic drawing of an air cleaning system 100 of the invention. The air cleaning system 100 comprises an air cleaning unit 20 comprising a hybrid photo catalyst arranged for photocatalytic oxidation of pollutants in air. The hybrid photo catalyst comprises a porous filter 26 impregnated with photo catalyst material as well as adsorbent material. The air cleaning unit 20 moreover comprises sources 22 of ultraviolet (UV) light arranged to periodically irradiate the hybrid photo catalyst in order to regenerate the adsorbent material. The air cleaning system 20 comprises a number of UV lamps arranged to periodically irradiate a hybrid photo catalyst 26. In figure 1 , an array of two times two UV lamps is arranged within cylinders of hybrid photo catalyst 26. However, any appropriate number and size of the UV lamps and the photo catalyst 26 are conceivable. Preferably, the overall system 100 moreover comprises a PM filter 10, such as e.g. a HEPA or ULPA filter. Furthermore, further filters upstream the PM filter and/or upstream the air cleaning system 20 may be present, such as a pre-filter, for example an ESP filter. Moreover, the air cleaning system may comprise further air conditioning units downstream the air cleaning unit 20, such as an activated carbon filter to eliminate traces of intermediate and unreacted pollutants and/or an ion generator. During operation, polluted air 1 enters the system 100. The polluted air may already have undergone some purification in a pre-filter prior to entering the system 100. The polluted air 1 reaches the HEPA or ULPA filter 10 for removal of particulate matter. The resultant air 2 comprising VOCs/NOx , particulates and/or bioaerosols, from which the major part of particulate matter has been removed, is led to the air cleaning system 20 having a hybrid photo catalyst and adsorbent 26. The air 2 enters the air cleaning system between hybrid photo catalyst or adsorbent 26 and UV lamps 22. The adsorbent 26 adsorbs polluting compounds. Thereby a high concentration environment of the compounds is formed around the photocatalyst, e.g. T1O2, resulting in an increase in the photoreaction rate upon radiation with UV light. Intermittently or periodically, the UV lamps 22 are turned on to irradiate the adsorbent with UV light, thereby regenerating the adsorbent.
The air 3 exiting from the air cleaning system 20 is cleaned or clean air 3; VOCs, NOx, particulates and/or bioaerosols in air 2 has been oxidized through a procession of reactions and intermediates into CO2 and H2O as primary end products. These primary end products are denoted by "7" in figure 2c. Figures 1 b-1 d show two side views and a top view of an air cleaning unit 20. The side view of Figure 1 B is a view of the left end (as seen in figure 1 A) of the air cleaning unit 20, whilst the side view of Figure 1 B is a view of the right end (as seen in figure 1A) of the air cleaning unit 20 and figure 1 C is a top view of the air cleaning unit 20.
In figure 1 b it is seen that the air cleaning unit 20 has a front plate 21 (shown hatched) and four units, each comprising a UV lamp 22, surrounded by an annular space and a cylindrical hybrid photocatalyst 26 (shown as concentric circles in figure 1 b). It should be noted that the number of units in an air cleaning unit can be any appropriate number, and that the diameter of the cylindrical hybrid photocatalyst 26 could be different than shown in the figures. If the diameter of the cylindrical photocatalyst 26 is somewhat larger, the total area of the front plate 21 would be smaller and the effective area of the air cleaning system would be increased.
In figure 1 c, a side view of the end of the air cleaning unit 20 is shown. The cylindrical photocatalyst 26 ends in a part 28, which is either not penetrable by air, or is made from hybrid photocatalyst material. Hereby, it is ensured that the air entering the air cleaning unit 20 has to pass through the hybrid photocatalyst material 26. Struts or braces 27 support the end of the cylindrical hybrid photocatalyst 26 of the four units.
In figure 1 d a top view of the air cleaning unit 20 is shown. As seen in figure 1 d, air comprising pollutants is arranged to enter the air cleaning unit 20 from the left, and clean(ed) air is arranged to exit the air cleaning unit to the right or along the sides of the units, through the hybrid photocatalyst material 26. The air cleaning unit 20 shown in figures 1 a-1 d is an open system, where the cleaned air exits through the walls of hybrid photocatalyst material 26 or from the ends 28 of the cylindrical hybrid photocatalyst. This is shown by the curved arrows in figure 1 d.
However, at an alternative, it is conceivable that the system is a closed system where side walls enclose the hybrid photocatalyst material 26. In this case, air to be cleaned enters into the annular space between the UV lamp 22 and the hybrid photocatalyst material 26 and exits the air cleaning unit via the end structure 25 or the end parts 28. In the embodiment shown in figures 1 a-1 d, viz. an open system, an end structure 25 comprising struts or braces 27 is shown. However, for an open system the end structure 25 could alternatively be an end plate, which is impenetrable to gas, except from the end parts 28.
Figures 2a to 2c show a typical regeneration cycle of a hybrid photocatalyst of an air cleaning system according to the invention. In figures 2a-2c only a small part of the hybrid photo catalyst 26 is shown. The operation cycle includes three stages illustrated in the three figures. In the first stage, shown in figure 2a, the hybrid photocatalyst 26 has recently been regenerated and it is ready for adsorption of pollutants. In figure 2a, the photocatalyst 26 is shown with a few particles 4 of pollutants adsorbed on the surface thereof. Air 2 comprising VOCs, NOx, particulates and/or bioaerosols is passed along the surface of the hybrid photocatalyst 26, and clean or cleaner air 3 leaves the photo- catalyst 26.
In the second stage, shown in figure 2b, pollutants 4 from polluted inlet air have been adsorbed on the surface of the hybrid photocatalyst 26; the concentration of pollutants 4 is higher than in the first stage shown in figure 2a.
In the third stage, shown in figure 2c, after a certain pre-defined time interval, one or more sources of UV light are turned on to irradiate the hybrid photocatalyst 26 with UV light in order to decompose the adsorbed pollutants and thereby regenerate the hybrid photocatalyst. The primary end products from the oxidation of VOCs, NOx, particulates and/or bioaerosols in the air 2 are CO2 and H2O. These primary end products are denoted by "7" in figure 2c. After the regeneration in the third stage, the photocatalyst is ready for another cycle, starting with the first stage. In figure 2c, the UV source(s) is/are turned on simultaneously with an air flow passing along the surface of the photocatalyst 26.

Claims

1 . An air cleaning system comprising:
- a hybrid photo catalyst arranged for photocatalytic oxidation of pollutants in air, said hybrid photo catalyst comprising a porous filter impregnated with photo catalyst material as well as adsorbent material, and
- a source of ultraviolet (UV) light arranged to periodically irradiate said hybrid photo catalyst in order to regenerate said adsorbent material.
2. An air cleaning system according to claim 1 , further comprising a particulate matter filter upstream of said hybrid photo catalyst.
3. An air cleaning system according to claim 1 or 2, further comprising a prefilter upstream of hybrid photo catalyst and/or upstream of said particulate matter filter.
4. An air cleaning system according to claim 3, wherein said prefilter is an electrostatically charged filter, an activated carbon filter, a particulate matter filter or a combination thereof.
5. An air cleaning system according to any of the claims 1 to 4, further comprising further air conditioning equipment downstream said hybrid photo catalyst.
6. An air cleaning system according to claim 5, wherein said further air conditioning equipment comprises activated carbon equipment and/or an ion generation unit.
7. An air cleaning system according to any of the claims 1 to 6, wherein the UV light irradiates said hybrid photo catalyst in less than 50% of the time, preferably less than 40% of the time, more preferably less than 25% of the time.
8. An air cleaning system according to any of the claims 1 to 7, wherein the photo catalyst material comprises one or more of the following: titanium dioxide (ΤΊΟ2), zinc oxide (ZnO), antimony trioxide (Sb20s), bismuth oxide (B12O3), vanadium oxide (V2O3), ferric oxide (Fe2C>3), zirconium dioxide (ZrC>2), tungsten trioxide (WO3), tin dioxide (SnC>2), aluminium oxide (AI2O3), cerium oxide (CeC>2), zinc sulphide (ZnS), cadmium sulphide (CdS), ΤΊΟ2 doped with metal ions, ΤΊΟ2 combined with ZrC>2 and/or silicon dioxide (S1O2).
9. An air cleaning system according to any of the claims 1 to 7, wherein said adsorbent material comprises activated carbon, zeolite, zeolum, alumina, silica, mordenite, ferrier- ite, clinoptilolite, ZSM-5.
10. A method of cleaning air, said method comprising:
- providing a hybrid photo catalyst arranged for photocatalytic oxidation of pollutants in air, said hybrid photo catalyst comprising a porous filter impregnated with photo catalyst material as well as adsorbent material,
- passing air comprising pollutants through said hybrid photocatalyst, and - periodically irradiating said hybrid photo catalyst by means of a source of ultraviolet (UV) light in order to regenerate said adsorbent material.
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