CN102179145B - Plasma catalytic reactor for cooperative governance of VOCs (Volatile Organic Compounds) - Google Patents

Abstract

The invention discloses a reactor for plasma catalysis and cooperative treatment of VOCs, which comprises a cylinder body, the cylinder body is a quartz glass tube, the outer wall of the quartz glass tube is wrapped with a metal conductor, and the metal conductor is connected to an AC high-voltage power supply as a dielectric barrier discharge. External electrode; a stainless steel tube is set in the middle of the cylinder, and the stainless steel tube is grounded as the internal electrode of the dielectric barrier discharge; the stainless steel tube wall is opened and connected to an external air source; a discharge air gap is formed between the quartz glass tube and the stainless steel tube, and the discharge The electric gap is filled with adsorbents and catalysts; one end of the cylinder is the air inlet end, the other end is the air outlet end, and the air inlet end and the air outlet end are respectively equipped with airflow uniform distribution plates. The reactor of the present invention combines the functions of plasma oxidation, adsorption and catalysis, can exert the synergistic effect among oxidation, adsorption and catalysis, greatly improves the degradation of VOCs, increases the carbonization rate of VOCs degradation, and effectively inhibits harmful by-products generation.

Description

Plasma catalytic reactor for synergistic treatment of VOCs

technical field

The invention relates to a reactor for VOCs waste gas treatment, in particular to a one-stage plasma catalysis and co-treatment reactor for VOCs.

Background technique

Volatile organic compounds (VOCs) are the second largest category of air pollutants after dust, which not only cause air pollution problems such as ozone holes and photochemical smog, but also are mostly toxic substances, causing great harm to human health . The VOCs in the exhaust gas are not only of various types, but also have a concentration of several hundred ppm, which is extremely difficult to deal with. The traditional control method of VOCs is to use activated carbon to adsorb it, but the activated carbon after adsorption is often directly discarded to landfill, which is easy to cause secondary pollution. Therefore, in recent years, the research on the control of VOCs has gradually turned to methods that can convert VOCs into harmless substances (CO ₂ and H ₂ O, etc.). The low-temperature plasma VOCs control technology utilizes gas breakdown under a strong electric field to generate high-energy electrons, ions, free radicals (such as O, OH, etc.) and various molecular excited states with strong chemical activity. Molecular dissociation and accelerated chemical reactions have high efficiency, and can deeply oxidize low-concentration VOCs in flue gas to generate harmless CO ₂ and H ₂ O. Therefore, it has become a hot spot in the research and development of VOCs control technology at home and abroad. In particular, dielectric barrier discharge has gradually become the most effective way to degrade VOCs due to its combination with adsorption, catalysis and other technologies.

Dielectric barrier discharge is a form of discharge in which at least one electrode surface is covered by an insulating dielectric, or an insulating dielectric is inserted in the discharge space. Microdischarge is the core of dielectric barrier discharge. The microdischarge process not only plays the role of energy storage, limits the free growth of current density, stabilizes the discharge and generates pulses with extremely short delays, but also prevents local sparks or arc discharges. Dielectric barrier microdischarge can generate a large number of electrons, ions and free radicals with strong chemical activity, and oxidize organic pollutants into harmless substances such as CO ₂ and H ₂ O. However, studies have shown that simple dielectric barrier microdischarge has high removal efficiency for benzene, toluene, trichlorethylene, etc., but there are problems such as low energy utilization rate and low carbonization rate. Magureanu et al. (M.Maureanu, NBMandache, et al.Applied Catalysis B-Environmental, 2007, 74:270-277) found that although a removal efficiency of 90% was achieved, the energy The density has reached 500J/l, and the selectivity of _CO2 in the by-product is only 25%, and more CO is generated in the process of treatment. Recently, in order to overcome the shortcomings of pure dielectric barrier microdischarge, the combination of dielectric barrier discharge and chemical catalysis to treat organic pollutants has been gradually developed, and the removal rate, energy utilization rate and _CO2 selectivity have all been improved (THammer, T.Kappes , et al. Catalysis Today, 2004, 89: 5-14; RBSun, ZGXi, et al. Atmospheric Environment, 2007, 41: 6853-6859). Delagrange et al. (S.Delagrange, L.Pinard, et al.Applied CatalysisB-Environmental.2006:68:92-98) found that dielectric barrier discharge combined with MnO ₂ /γ-Al ₂ O ₃ catalyst can make toluene conversion rate from single The discharge rate increased from 36% to 88%, and the ratio of CO ₂ /CO increased from 0.75 to 1.3. Meanwhile, MnO ₂ can effectively suppress the formation of O _{3 ,} a by-product. In addition, studies have shown that the combination of dielectric barrier discharge and chemical catalysis can produce a synergistic effect. Dielectric barrier discharge can promote the formation of active substances on the surface of the catalyst, and the addition of catalysts can strengthen ionization and generate gas discharge in the micropores of the catalyst, thereby promoting the formation of active substances. produce. The combination of dielectric barrier discharge and chemical catalysis improves the removal efficiency of organic pollutants to a certain extent, reduces energy consumption, and suppresses the formation of harmful by-products. However, because the concentration of VOCs in the exhaust gas is generally low (hundreds of ppm), and there are various forms, resulting in high energy consumption and low _CO2 selectivity, there is still a long way to go before real industrialization. Therefore, how to improve the degradation efficiency of VOCs, reduce energy consumption, and effectively control the generation of harmful degradation by-products will become the key to the combined treatment of VOCs with plasma catalysis.

Plasma co-catalyst reactors are generally divided into two-stage and one-stage. The catalyst in the two-stage reactor is generally placed after the discharge plasma reactor, and the plasma chemical reaction and the induced catalytic reaction are carried out in stages. For two-stage reactors, it is generally difficult for short-lived active species to reach the catalyst stage downstream of the discharge zone, mainly due to the reaction of short-lived active particles with N ₂ , O ₂ , H ₂ O and other molecules in the atmosphere to form long-lived species. Active species O ₃ , H ₂ O ₂ , etc. react with VOCs molecules on the surface of the catalyst, which cannot fully reflect the synergistic effect between the non-thermal equilibrium plasma and the catalyst. In the one-stage reactor, the catalyst is placed between two electrodes, and the discharge occurs on the surface of the catalyst or in the micropores of the catalyst. In addition to forming long-lived active species to react with VOCs molecules, short-lived active species such as O ( ¹ D), O( ³ P), OH ^- , etc. undergo free radical reactions on the surface of the catalyst to form new long-lived active substances. During this process, the presence of the catalyst in the discharge air gap can promote the development of the discharge, and the discharge process makes an important contribution to the improvement of the physical properties of the catalyst and the chemical properties of the catalyst surface. Because of this, the plasma catalytic reactors currently used for VOCs treatment are mainly one-stage reactors.

Contents of the invention

Aiming at low-concentration VOCs waste gas which is difficult to treat at present, the invention provides a reactor for treating VOCs with plasma synergistic chemical catalysis.

A reactor for plasma catalysis and cooperative treatment of VOCs, comprising a cylinder, the cylinder is a quartz glass tube, the outer wall of the quartz glass tube is wrapped with a metal conductor, and the metal conductor is connected to an AC high-voltage power supply as the outer wall of the dielectric barrier discharge. Electrode; stainless steel tube is set in the middle of the cylinder, and the stainless steel tube is grounded as the inner electrode of dielectric barrier discharge; the wall of the stainless steel tube is opened and connected to an external gas source, and additional gas can be injected into the reactor; quartz glass tube and stainless steel tube A discharge air gap is formed between them, and the discharge air gap is filled with adsorbents and catalysts to form a micro-discharge filling layer; one end of the cylinder is the air inlet end, and the other end is the air outlet end, and the air inlet end and the air outlet end are respectively set There is an air distribution plate.

In order to ensure that a uniform, diffuse and stable micro-discharge can be produced in the discharge air gap of the dielectric barrier discharge, and to ensure that there is a large space filled with adsorbents and catalysts to improve the adsorption and catalytic effects, the quartz glass in the reactor is The distance between the tube and the stainless steel tube (that is, the distance between the dielectric barrier discharge air gap) is generally preferably 5-10mm, and the discharge voltage is about 20kV-40kV at this time.

In order to generate a more uniform and stable micro-discharge, the metal conductor is generally made of metal foil or metal coating, and the metal foil or metal coating is wrapped on the outer wall of the quartz glass tube. The thickness of the metal foil or metal coating is preferably 0.5-1mm. At the same time, there are also certain requirements for metal materials, usually copper, stainless steel and silver are used more often, and according to the experimental results, it is more ideal to use silver as the external electrode discharge effect, so the metal conductor in the present invention is best to use silver foil or silver coating.

The stainless steel tube not only serves as the inner electrode of the dielectric barrier discharge, but also serves as the gas passage for the added gas. The additive gas is generally moist oxygen. During the discharge process, the wet oxygen can be decomposed to produce oxidative free radicals such as O, O ₃ , OH, HO _{2 ,} etc. These active free radicals play an important role in the deep oxidation of VOCs. At the same time, after the reactor has been running for a period of time, reaction by-products will adhere to the surface of the adsorbent and catalyst, which will reduce the adsorption and catalytic performance, so it needs to be cleaned regularly. Injection of moist oxygen can promote the degradation of these reaction by-products, which has the effect of regeneration of adsorbent and catalyst. From the perspective of the uniformity of gas injection and the convenience of processing, the preferred size of the stainless steel pipe is set as follows: a diameter of 5-10mm, four small holes of 1-2mm are evenly opened along each circumference, and the interval between each circumference is 5mm.

The adsorbent is diatomite or bentonite particles. Adsorbents such as diatomaceous earth or bentonite particles have the characteristics of large specific surface area, high porosity, and strong chemical stability, and have good adsorption effects on various organic gases. At the same time, these adsorbents can play a good stabilizing effect on the active components of the catalyst, and are suitable as the carrier of the catalyst in the plasma synergistic chemical catalytic reactor. Since a large number of highly oxidizing free radicals (such as O, O ₃ , HO ₂ , OH, etc.) are produced during dielectric barrier discharge, which will oxidize carbon-based adsorbents, it is necessary to choose mineral-based adsorbents as plasma Sorbent for VOCs treatment by body synergistic chemical catalysis.

The catalyst is one or more of silver, manganese, titanium, aluminum, cobalt and their oxides. Silver, manganese, titanium, aluminum, cobalt and their oxides can be used as catalysts to react at lower temperatures (100-300°C), avoiding the high-temperature state of VOCs degradation, and helping oxidative free radicals in these The recombination of the catalyst surface prolongs the life of free radicals and improves the degradation effect of VOCs. At the same time, economy and availability should be considered in the selection of catalysts. Therefore, the commonly used catalysts in plasma catalytic synergistic reactions are: TiO ₂ , Al ₂ O ₃ , Ag/TiO ₂ , Ag/Al ₂ O ₃ , TiO ₂ /Al ₂ O ₃ , Ag/TiO ₂ , MnO ₂ , CoO ₃ , WO ₃ , etc.

The VOCs waste gas to be treated enters from the air inlet, and plasma oxidation, adsorption and catalysis occur in the dielectric barrier discharge gap. During the dielectric barrier discharge process, many micro-discharge channels are formed in the discharge air gap, and micro-discharges can also be generated on the surface of the adsorbent and catalyst as well as in their pores, which greatly improves the dielectric barrier discharge effect and produces a large number of high-energy electrons and activity. Free radicals, these active substances react with VOCs to achieve efficient degradation of VOCs. At the same time, the effective adsorption of VOCs by the adsorbent greatly prolongs the residence time of VOCs in the discharge area, and the concentration of VOCs in the reactor after adsorption is greatly increased, which is conducive to the efficient degradation of VOCs and improves energy utilization efficiency. In addition, the plasma interacts with the adsorbent and the catalyst to improve the pore structure of the adsorbent and the catalyst, improve the adsorption and catalytic performance, and at the same time generate a longer-lived active substance on the surface of the catalyst, improve the catalytic oxidation performance, and overcome the simple dielectric barrier The problem of short lifetime of discharge radicals. Under the synergistic effect of plasma oxidation, adsorption, and catalysis, VOCs undergo a deep oxidation reaction in the discharge air gap, and are finally converted into harmless _CO2 and _H2O .

The reactor of the present invention has a wire barrel structure, integrates plasma oxidation technology, adsorption technology and chemical catalysis technology, fully utilizes the advantages of each technology, improves the energy utilization rate of VOCs degradation, and effectively controls the generation of harmful by-products at the same time. Efficiently degrade VOCs into harmless CO ₂ and H ₂ O.

Description of drawings

Fig. 1 is a structural representation of the reactor of the present invention;

Fig. 2 is a schematic diagram of the cross-sectional structure along the line A-A of Fig. 1 .

Explanation of each reference sign in the figure:

1-quartz glass tube 2-metal conductor 3-stainless steel tube 4-external air source

5-discharge air gap 6-adsorbent 7-catalyst 8-air inlet

9-air outlet 10-air distribution plate 11-AC high voltage power supply

Detailed ways

The reactor of the present invention will be further described below in conjunction with the accompanying drawings.

As shown in Fig. 1 and Fig. 2, a reactor for co-treatment of VOCs by plasma catalysis, the reactor is a wire cylinder structure, the cylinder body is a quartz glass tube 1, and the outer wall of the quartz glass tube 1 is wrapped with a metal conductor 2, The metal conductor 2 is connected to the AC high-voltage power supply 11 as the external electrode of the dielectric barrier discharge. In order to produce a more uniform and stable micro-discharge, silver is used as the metal conductor material, and the quartz glass is wrapped with silver foil or silver coating. The thickness of the silver foil or silver coating is generally 0.5-1mm; a stainless steel tube is installed in the middle of the cylinder 3 , the stainless steel tube 3 is grounded as the inner electrode of the dielectric barrier discharge. The wall of the stainless steel pipe 3 is perforated and connected to an external gas source 4 to inject additional gas into the reactor. The additional gas is generally moist oxygen. Adding gas not only promotes the oxidation of VOCs, but also provides an oxidation gas source for the regeneration of adsorbents and catalysts. The diameter of the stainless steel pipe is 5-10mm, and four small holes of 1-2mm are evenly opened along each circumference, and the interval between each circumference is 5mm. A discharge air gap 5 is formed between the quartz glass tube 1 and the stainless steel tube 3 . The discharge air gap 5 is filled with an adsorbent 6 and a catalyst 7 to form a discharge filling layer. Because the surface and pores filled with adsorbent 6 and catalyst 7 in the discharge air gap 5 can form micro-discharges during the discharge process and promote the development of streamer corona, so the distance between the discharge air gap 5 in this reactor is larger than that of a simple dielectric barrier. The discharge form can be increased, but considering the stability of the discharge, 5-10mm is generally used. Adsorbent 6 is diatomaceous earth or bentonite particles. The catalyst 7 is one or more of silver, manganese, titanium, aluminum, cobalt and their oxides. One section of the cylinder is the air inlet port 8, and the other end is the air outlet port 9. The air inlet port 8 and the air outlet port 9 are respectively equipped with airflow uniform distribution plates 10, the aperture diameter of which is less than 3mm, and the aperture ratio is 30%~ 50%.

The VOCs waste gas to be treated enters from the inlet port 8 of the reactor, and enters the discharge air gap 5 evenly through the air flow distribution plate 10 . When the AC high-voltage power supply 11 is turned on, gas discharge occurs in the discharge air gap 5 . During the dielectric barrier discharge process, many micro-discharge channels are formed in the discharge air gap 5, and micro-discharges can also be generated on the surfaces of the adsorbent 6 and the catalyst 7 and their pores, which greatly improves the dielectric barrier discharge effect and generates a large amount of high-energy Electrons and active free radicals, these active substances undergo free radical reactions with VOCs to achieve efficient degradation of VOCs. At the same time, under the action of dielectric barrier discharge, the surface structure and pores of the adsorbent 6 and catalyst 7 change, which promotes the adsorption of VOCs gas, greatly prolongs the residence time of VOCs in the discharge area, increases the concentration of VOCs in the discharge area, and contributes to VOCs efficient degradation and improved energy utilization. In addition, the dielectric barrier discharge process works synergistically with chemical catalysis to generate longer-lived active substances on the surface of the catalyst 7, which improves the catalytic oxidation performance and overcomes the problem of short lifetime of free radicals in pure dielectric barrier discharge. Under the synergistic effect of plasma oxidation, adsorption, and catalysis, VOCs undergo a deep oxidation reaction in the discharge air gap, and are finally converted into harmless _CO2 and _H2O .

Application example 1

Adopt the reactor of the present invention to carry out the processing of toluene, waste gas flow rate 500mL/min, toluene concentration 200ppmv, adopt diatomaceous earth particle as adsorbent, _TiO2 as catalyst, the discharge air gap of dielectric barrier discharge is 10mm, and the energy density reaches 200J/ L, the degradation efficiency of toluene reaches 80%, and the selectivity of _CO2 is also above 75%.

Claims (7)

1. a plasma-catalytic is worked in coordination with the reactor of administering VOCs, comprise cylindrical shell, it is characterized in that: described cylindrical shell is quartz glass tube (1), the outside wall surface coated metal conducting objects (2) of quartz glass tube (1), metallic conduction thing (2) connect ac high voltage source (11) as the external electrode of dielectric barrier discharge; The inner barrel middle arranges stainless steel tube (3), and stainless steel tube (3) ground connection is as the interior electrode of dielectric barrier discharge; Stainless steel tube (3) tube wall perforate, and connect external air source (4); Form discharge air-gap (5) between quartz glass tube (1) and stainless steel tube (3), fill adsorbent (6) and catalyst (7) in described discharge air-gap (5), form the micro discharge packed layer; One end of cylindrical shell is air inlet end (8), and the other end is gas outlet end (9), and air inlet end (8) and gas outlet end (9) are respectively equipped with airflow uniform distribution plate (10); Described stainless steel tube (3) diameter is 5～10mm, offers 4 holes along an even circumferential, the diameter 1～2mm in hole, per circle spacing 5mm.

2. reactor according to claim 1, it is characterized in that: described metallic conduction thing (2) thickness is 0.5～1mm.

3. reactor according to claim 2, it is characterized in that: described metallic conduction thing (2) is silver foil or silver coating.

4. reactor according to claim 1, it is characterized in that: the spacing between described quartz glass tube (1) and stainless steel tube (3) is 5～10mm.

5. reactor according to claim 1, it is characterized in that: described adsorbent (6) is diatomite or bentonite clay particle.

6. reactor according to claim 1 is characterized in that: described catalyst (7) is one or more in silver, manganese, titanium, aluminium, cobalt and their oxide thereof.

7. reactor according to claim 1 is characterized in that: the perforate aperture of described airflow uniform distribution plate (10) is less than 3mm, percent opening 30%～50%.

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