WO2022247973A1

WO2022247973A1 - Method of degradation of volatile organic compounds in waste air

Info

Publication number: WO2022247973A1
Application number: PCT/CZ2021/050057
Authority: WO
Inventors: Luboš Zápotocký; Radim Žebrák; Martin RELI; Tomáš PROSTĚJOVSKÝ; Kamila Kočí
Original assignee: Dekonta A.S.; Vysoká Škola Báňská - Technická Univerzita Ostrava
Priority date: 2021-05-28
Filing date: 2021-05-28
Publication date: 2022-12-01

Abstract

The invention relates to a method of degradation of volatile organic compounds in waste air, comprising the steps of supplying waste air to a first reactor (1), where it is irradiated with UV radiation at a wavelength below 200 nm to form photolytic decomposition intermediates, wherein ozone is produced; removing the waste air and the photolytic decomposition intermediates from the first reactor (1) and introducing them into a second reactor (2), where they are brought into contact with an aqueous solution of hydrogen peroxide; irradiating the aqueous solution of hydrogen peroxide, the waste air and the photolytic decomposition intermediates with UV radiation at a wavelength above 200 nm to form photochemical decomposition products, wherein hydroxyl radicals are produced; and removing the purified waste air including the photochemical decomposition products and being at least partially free of volatile organic compounds from the second reactor (2). The concentration of ozone produced is in the range 185-240 ppm at a first reactor outlet (4) and the concentration of hydrogen peroxide in the aqueous solution is in the range of 0.80-1.15 wt. %.

Description

Method of degradation of volatile organic compounds in waste air

The present invention relates to the field of environmental technologies for purification of air contaminated with specific contaminants, in particular volatile organic compounds (VOCs). The technology concerns photochemical degradation of volatile organic compounds (VOCs) in waste gases, in which VOCs are degraded into simpler and less harmful chemicals, in particular carbon dioxide and short-chained hydrocarbons.

The issue of pollutants present in the environment is increasingly being studied. Studies show that volatile organic compounds (VOCs) play a major role in reactions that lead to increased concentrations of tropospheric ozone, especially in recent years. As part of photochemical smog (Los Angeles type), VOCs have a negative impact not only on human health, but also on the environment and urban buildings. Needless to say, transport is not the only source of volatile organic compounds. Many of them are also well known and widely used as industrial solvents and also as raw materials for the production of polymers. For example, xylene (as a mixture of its three isomers) is a solvent used in printing, rubber, leather and petrochemical industries. The main symptoms of short-term xylene exposure are eye, nose and throat irritation and breathing and nervous system problems. Long-term exposure can lead to serious problems such as lung cancer, anaemia, leukaemia, etc.

Governments around the world are generally lowering exposure limits for volatile organic compounds. Czech legislation states that the amount of volatile organic compounds in waste air should not exceed 150 mg.m-3 (around 35 ppmv). These low limits are beginning to cause a problem, as VOC removal technologies are nowadays becoming insufficient in terms of the availability and size of apparatuses as such. One of these technologies includes biofilters, which can provide high VOC removal efficiency combined with low process costs. The disadvantage of biofilters is their large dimensions. Another way to remove VOCs can be by catalytic combustion of VOCs. High efficiency can be achieved with this method, but there is a higher energy consumption and possible problems with the leakage of the catalyst into the environment. It is also quite common to use an adsorption process, which is often carried out on activated carbon. The disadvantage of this technology is the need for subsequent desorption of VOCs and the necessary regeneration of activated carbon.

The Czech utility model CZ 31903 U1 discloses a device for waste air purification and a related technological process of a two-stage photolytic and photochemical reaction. The device comprises a photolytic reactor provided with an inlet of waste air with contaminants, an outlet of gas phase contaminant intermediates and at least one source of UV-C radiation at 185 nm, and further comprises a photochemical reactor provided with an inlet of gas phase contaminant intermediates connected to the outlet of gas phase contaminant intermediates, an outlet of purified air, an inlet of oxidizing agent based on an aqueous solution of hydrogen peroxide and at least one source of UV-C radiation at 254 nm. The photochemical reactor is further provided with a circulation circuit for circulating the liquid phase, comprising a liquid phase storage tank, a liquid phase feeding line and at least one nozzle for atomizing the liquid phase in the photochemical reactor.

According to the preamble of claim 1, said disclosure of the device implies a related technological process of supplying waste air to a photolytic reactor; irradiating the waste air with UV radiation at 185 nm to form gas phase contaminant intermediates and ozone; removing the waste air and the gas phase contaminant intermediates from the photolytic reactor and introducing them into a photochemical reactor; bringing the waste air and the gas phase contaminant intermediates into contact with an aqueous solution of hydrogen peroxide in the photochemical reactor; irradiating the aqueous solution of hydrogen peroxide, the waste air and the gas phase contaminant intermediates with UV radiation at 254 nm to form hydroxyl radicals and photochemical decomposition products; and removing purified waste air that is at least partially free of volatile organic compounds. However, this technological process does not describe any specific process parameters for carrying out the most efficient two-stage photolytic and photochemical degradation of VOCs.

Therefore, a need to specify the technological process of purifying waste air from volatile organic compounds in more detail emerges from the prior art.

The present invention is a method that utilizes advanced oxidation processes to remove VOCs in a flow-through configuration. It is a complex two-stage system that uses highly reactive hydroxyl radicals formed by a combination of UV radiation, ozone (O3) and hydrogen peroxide (H2O2). The present invention provides an effective method for removing volatile organic compounds from waste air. Waste air (or exhaust air or flute gas) means a mixture of air (or air and vapours) and volatile organic compounds.

The object of the present invention is to provide a technologically specified method for degradation of volatile organic compounds in waste air according to claim 1, wherein said method is applicable in a device known from CZ 31903 U1.

The underlying idea of the method is that the concentration of ozone produced in the step of irradiating the waste air with UV radiation at a wavelength below 200 nm is in the range 185-240 ppm at a first reactor outlet, and that the concentration of hydrogen peroxide in the aqueous solution is in the range of 0.80-1.15 wt. % at a second reactor inlet.

Preferably, the concentration of ozone at the first reactor outlet is 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235 or 240 ppm. Preferably, the concentration of hydrogen peroxide in the aqueous solution at the second reactor inlet is 0.80, 0.85, 0.90, 0.95, 1.00, 1.05, 1.10 or 1.15 wt. %.

If the concentration of ozone is less than 185 ppm and the concentration of hydrogen peroxide in the aqueous solution is less than 0.80 wt. %, the decomposition of the volatile organic compounds present does not occur to a maximum extent. Therefore, these parameters are the lowest amounts of oxidizing agents for which VOC degradation is most effective. If the concentration of ozone is higher than 240 ppm and the concentration of hydrogen peroxide in the aqueous solution is higher than 1.15 wt. %, the rate of photolysis and photochemical oxidation no longer increases, and the cost-effectiveness ratio of the present process increases.

One aspect of the method of removing VOCs from waste air is a two-stage combination of photochemical treatment of the waste air. The first stage includes a photolytic stainless steel reactor of square cross-section (the first reactor), comprising three parts - an inlet pipe (the first reactor inlet), a reaction chamber with built-in UV lamps and an outlet pipe (the first reactor outlet). In this stage, the UV lamps with a maximum radiation intensity at a wavelength below 200 nm (e. g. at 185 nm) or with 2 maximum radiation intensities at wavelengths below and above 200 nm (e. g. at 185 and 254 nm) are used, which produce ozone at a concentration of 185-240 ppm at the first reactor outlet. Photolytic decomposition refers to photolysis of volatile organic compounds as such and the formation of ozone from air, followed by ozonolysis of volatile organic compounds.

The second stage of the method is a photochemical reactor (the second reactor) conceived as a closed continuously scrubbed column reactor, irradiated in the interior with UV-C radiation at a wavelength above 200 nm (e. g. at 254 nm) or at wavelengths below and above 200 nm (e. g. at 185 and 254 nm), where a circulating medium is water with the addition of 0.80-1.15 wt. % H2O2. The advantage of using both wavelengths (above and below 200 nm, e. g. 185 nm and 254 nm) in both reactors results in better coverage of the absorption maxima of individual contaminants.

In the reactor, nozzles or showers for spraying a solution of hydrogen peroxide are placed in the head, under which nozzles UV lamps and a plastic filling (high density polyethylene, polypropylene and/or unplasticized polyvinyl chloride) for better contact between liquid and air are arranged, and further thereunder a tank with a solution of hydrogen peroxide, in which tank further UV lamps are built.

The principle of oxidation of organic substances in the first stage is the action of ozone, which is generated from the exhaust air by extremely shortwave UV-C radiation (below 200 nm, e. g. at 185 nm), and which leads to primary decomposition of organic substances to form other reactive components - intermediates of photolytic decomposition. Subsequently, the stream of purified air enters the second stage - the photochemical reactor. Here, the inlet contaminants and their partially oxidized decomposition products (modified to generally more polar and thus more soluble compounds) are partially dissolved in the aqueous phase. Compounds in the gaseous and liquid phase are further decomposed by hydroxyl radicals (•OH), which are formed by irradiating hydrogen peroxide by UV-C radiation at a wavelength above 200 nm (e. g. at 254 nm). •OH radicals, which have a very strong oxidizing potential, can oxidize organic contaminants very efficiently to the final oxidation products by a radical mechanism.

The important factors are the permeability of the environment to UV radiation, the flow rate of the purified air via the first and second reactors (e. g. 500, 800, 1000, 1200, 1400, 1500, 2000 or 5000 m³/hour), the intensity of UV lamp radiation (e. g. 60, 70, 80, 90 or 100 W), the number of active UV lamps (e. g. 5, 6, 7, 8, 9 or 10), the concentration of ozone of 185-240 ppm at the first reactor outlet and the concentration of hydrogen peroxide of 0.80-1.15 wt. % in the circulating solution in the second stage.

The present method is preferably designed in a continuous flow arrangement, which is suitable for industrial use. The rectangular shape of the first stage reactor (direct photolysis by UV radiation below 200 nm) and the cylindrical shape of the second stage reactor (photochemical oxidation using hydroxyl radicals generated by the decomposition of hydrogen peroxide by UV radiation above 200 nm) also correspond to this use.

The present method is not as space consuming compared to the use of biofilters.

In contrast to catalytic combustion of VOCs, the use of this method achieves a significantly lower energy consumption, there is no risk of catalyst leakage into the environment and there is no load on the atmosphere by combustion products, especially nitrogen oxides (NOx).

Compared to adsorption processes, a very demanding regeneration of activated carbon is eliminated.

This method can be segmented in terms of its capacity according to the input pollution of the treated air, especially by using only a certain number of UV lamps (according to the concentration of organic contaminants in the purified air). This modular arrangement allows the connection of a different number of UV lamps (with two different wavelengths) in both stages of the oxidation process. This achieves the necessary modulation of the intensity of both types of oxidation processes based on the concentration of undesirable organic substances (contaminants) in the purified air stream.

The advantage of this method is the use of a combination of photolytic and photochemical oxidation process, which allows higher efficiency of decomposition of organic contaminants achieved by the synergistic effect of combining these two oxidation processes, which allows the decomposition of such compounds (contaminants). Thus, air purification is achieved by a synergistic effect of a combination of two different forms of advanced oxidation processes: 1) photolysis on the principle of UV radiation of wavelength below 200 nm causing direct photolytic decomposition of organic substances in combination with ozonolysis (occurring in the first reactor) and 2) photochemical oxidation on the principle of UV radiation at wavelengths above 200 nm, which occur in the second reactor with a continuous scrubbing of the purified air stream with an aqueous solution of hydrogen peroxide, which results in the generation of hydroxyl radicals formed by the decomposition of hydrogen peroxide by this type of UV radiation. The hydroxyl radicals thus formed then act as a strong oxidizing agent decomposing even those organic substances which are not decomposed by photolysis or ozonolysis in the first reactor. Also, the photolysis and ozonolysis in the first step helps to polarize non-polar VOCs, which in the second step then dissolve better in the aqueous hydrogen peroxide solution and are in better contact with the oxidant within one (liquid) phase.

The present method of degradation of volatile organic compounds in waste air by a combination of photolysis and photochemical oxidation can be used for effective removal of the following contaminants: o-xylene, m-xylene, p-xylene, toluene, acetone, styrene, ethyl acetate, methanethiol, organic thiols, dimethyl disulfide, organic disulfides, organic polysulfides and ammonia. The present method is especially useful for removing styrene from waste gases.

The underlying idea of the invention is further elucidated on the basis of examples of its implementation, which are described with the aid of the accompanying drawings, where:

Fig.1

shows an apparatus for the degradation of volatile organic compounds in waste air.

Examples

The invention will be further elucidated on the basis of exemplary embodiments with reference to the corresponding drawings, in particular showing an apparatus suitable for carrying out the present method.

In the first step, the exhaust air for purification is fed to a first (photolytic) reactor 1 through an inlet 3. In the reaction chamber of the first reactor 1, UV lamps 9 are arranged at a distance from one another, e. g. parallel to the direction of the waste air flow. The UV lamps 9 are a source of UV radiation at a wavelength of 185 nm, which causes both photolysis of VOCs to form photolytic decomposition intermediates and ozone production from the oxygen present in the air, with ozone further reacting with VOCs and photolytic decomposition intermediates. The intensity of the UV lamps 9, the number thereof, and the flow rate of the waste air in the first reactor 1 are set such that ozone is present at an outlet 4 of the first reactor 1 at a concentration of 185-240 ppm, e. g. 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235 or 240 ppm. For example, the concentration of ozone of 200 ppm at the first reactor outlet 4 can be achieved by means of six UV lamps 9 at an intensity of 80 W and at a flow rate of the waste air of 1000 m³/hour.

After flowing through the first reactor 1, the waste air leaves the outlet 4 and enters a second (photochemical) reactor 2 through an inlet 5. UV lamps 10 are arranged at a distance from each other in the reaction chamber of the second reactor 2, e. g. perpendicularly or obliquely crosswise with respect to the direction of the waste air flow. In a counter-flow arrangement of the second reactor 2, at least one nozzle 7 is arranged at the end of the reaction chamber opposing the inlet 5, the nozzle 7 being suitable for spraying an aqueous solution of hydrogen peroxide at a concentration of 0.80-1.15 wt. %, e. g. 0.80, 0.85, 0.90, 0.95, 1.00, 1.05, 1.10 or 1.15 wt. %. An aqueous solution of hydrogen peroxide is fed to the nozzle 7 from a reservoir by means of a pump 8. The UV lamps 10 are a source of UV radiation at a wavelength of 254 nm which causes photochemical decomposition of hydrogen peroxide into hydroxyl radicals in the presence of waste air and photolytic decomposition intermediates, the hydroxyl radicals being very reactive towards other waste air components, in particular the remaining VOCs and their intermediates from the first reactor 1. Similarly, in the co-flow arrangement of the second reactor 2, at least one nozzle 7 is arranged at the end of the reaction chamber adjacent to the inlet 5. Both arrangements provide sufficient contact area in the reaction chamber of the second reactor 2.

The sequence of both steps (photolysis and photochemical oxidation) leads to the purification of waste air and the removal of volatile organic compounds therefrom to produce simpler and less harmful chemicals, especially carbon dioxide and short-chained hydrocarbons which leave the second reactor 2 through an outlet 6.

The present method can be used mainly in areas of industrial production (paint shops, printing, production of composite and plastic materials, automotive industry, mechanical engineering, etc.), treatment plants such as composting plants and wastewater treatment plants, biogas plants and pharmaceutical and other chemical production plants.

first (photolytic) reactor
second (photochemical) reactor
waste air inlet in the first reactor 1
waste air outlet from the first reactor 1
waste air inlet in the second reactor 2
waste air outlet from the second reactor 2
nozzle for spraying an aqueous solution of hydrogen peroxide
pump for dosing an aqueous solution of hydrogen peroxide
UV lamp of the first reactor 1
UV lamp of the second reactor 2

Claims

A method of degradation of volatile organic compounds in waste air, comprising the following steps:
supplying waste air to a first reactor (1);

irradiating the waste air with UV radiation at a wavelength below 200 nm to form photolytic decomposition intermediates, wherein ozone is produced;

removing the waste air and the photolytic decomposition intermediates from the first reactor (1) and introducing them into a second reactor (2);

bringing the waste air and the photolytic decomposition intermediates into contact with an aqueous solution of hydrogen peroxide in the second reactor (2);

irradiating the aqueous solution of hydrogen peroxide, the waste air and the photolytic decomposition intermediates with UV radiation at a wavelength above 200 nm to form photochemical decomposition products, wherein hydroxyl radicals are produced;

removing the purified waste air including the photochemical decomposition products and being at least partially free of volatile organic compounds from the second reactor (2);
characterised in that the concentration of ozone produced in step b. is in the range 185-240 ppm at a first reactor outlet (4), and in that the concentration of hydrogen peroxide in the aqueous solution in step d. is in the range of 0.80-1.15 wt. %.
The method according to claim 1, wherein the waste air is irradiated with UV radiation at a wavelength above 200 nm in step b.
The method according to claim 1 or 2, wherein the aqueous solution of hydrogen peroxide, the waste air and the photolytic decomposition intermediates are irradiated with UV radiation at a wavelength below 200 nm in step d.
The method according to any of the preceding claims, wherein the UV radiation at a wavelength below 200 nm has a wavelength maximum at 185 nm a/or the UV radiation at a wavelength above 200 nm has a wavelength maximum at 254 nm.
The method according to any of the preceding claims, wherein the waste air and the photolytic decomposition intermediates are brought in contact with the aqueous solution of hydrogen peroxide in a counter-flow arrangement in step d.
The method according to any of claims 1 to 4, wherein the waste air and the photolytic decomposition intermediates are brought in contact with the aqueous solution of hydrogen peroxide in a co-flow arrangement in step d.
The method according to any of the preceding claims, wherein the aqueous solution of hydrogen peroxide is sprayed into the second reactor (2) in step d.
The method according to any of the preceding claims, wherein the waste air and the photolytic decomposition intermediates are at least partially dissolved in the aqueous solution of hydrogen peroxide in step d.
The method according to any of the preceding claims, wherein the waste air and the photolytic decomposition intermediates are brought in contact with the aqueous solution of hydrogen peroxide on a plastic filling.
The method according to any of the preceding claims, wherein the volatile organic compound in the waste air is any compound selected from the group comprising o-xylene, m-xylene, p-xylene, toluene, acetone, styrene, ethyl acetate, methanethiol, organic thiols, dimethyl disulfide, organic disulfides, organic polysulfides and ammonia.