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    Home > Coatings News > Paints and Coatings Market > Analysis of rail vehicle coating VOCs exhaust gas treatment technology!

    Analysis of rail vehicle coating VOCs exhaust gas treatment technology!

    • Last Update: 2020-10-27
    • Source: Internet
    • Author: User
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    In recent years, rail transit has been developing rapidly because of its fast, convenient and effective characteristics of relieving urban traffic pressure. But at present, China's rail transit vehicle coating or solvent-based coating, with increasing demand, the corresponding production of coating process brought about by environmental problems have also attracted more and more attention, with the national requirements for air pollution prevention and control more and more stringent, the rail vehicle coating paint room, drying room exhaust emissions control is imminent.
    1, rail transit vehicle manufacturing resulting from the coating of exhaust gas characteristics
    rail transit vehicle coating coating includes rust-proof primer, putty, medium paint and finish, the vehicle's inner surface and substrate surface spray damping noise reduction paint, some of the bottom of the train spray fire protection paint, the current coating is still mainly used solvent-based paint . Among them, the solvent parts in the coating to benzene (toluene, xylene more), esters, alcohols, ethers, ketones-based, the general proportion of organic solvents in the paint is 40% to 60%. During the spraying process, each time the vehicle body completes a coating spray, it will enter the paint room and drying room, and the VOCs production ratio of the paint room and drying room is about 5:1 to 6:1.
    main pollutants in the paint room are VOCs from paint mist and work surface coatings that are not attached to the surface of the work piece. In order to ensure the surface quality of the work parts after spraying, the air flow of the painting room should refer to the relevant design standards of our country, the spray chamber usually uses the air flow organization mode of the upper and lower rows, the airborne wind speed in the spraying operation area is not less than 0.35m/s, and the air speed is not less than 0.5m/s when there is load. The temperature conditions are 18 to 28 degrees Celsius. Therefore, the paint chamber exhaust gas is characterized by high air volume, low VOCs concentration (approximately 100 to 500 mg/m3), and contains certain fog droplets (where the paint fog does not exceed 1 mg /m3).
    drying room because to maintain a certain temperature, so the exhaust gas air volume is not large, its components and the paint solvent components are basically the same (alcohols, esters, ethers, ketones, benzene, etc.), because the drying room temperature is relatively high, so the solvent will be in this process a large number of wave, VOCs concentration is generally around 2500mg / m3, the resulting exhaust gas for the medium concentration of VOCs, high temperature exhaust gas.
    2, rail transit vehicles spray VOCs exhaust gas treatment scheme choice
    vehicle spray workshop production is generally spray room and drying room at the same time, although the concentration and temperature of drying room exhaust gas are high, but due to the small amount of air in the drying room (about spray paint) Room 1 /8 to 1 /12), so the VOCs exhaust gas generated by the vehicle spraying workshop is generally mixed and centralized with 2 sections of exhaust gas, and the exhaust gas characteristics are basically low temperature, low concentration, atmospheric volume, easy to absorb VOCs exhaust gas. Based on the VOCs exhaust gas characteristics produced by vehicle coating production and the current high emission standard requirements in China, such as the Beijing area industrial coating exhaust emission standards require non-methane total hydrocarbons ≤50mg /m3, total VOCs treatment efficiency of ≥90%. Comprehensive one-time investment in the management system, long-term operating costs, environmental protection emissions and other performance, vehicle coating exhaust gas treatment more reasonable treatment technology for the "exhaust gas adsorption concentrated purification and de-attached exhaust gas high temperature thermal oxidation" combination of treatment program.
    Adsorption concentrated use of adsorbents generally have molecular sieve and activated carbon 2 kinds, that is, low concentration of exhaust gas, adsorption to achieve the standard emissions, dynamic adsorption saturation of the area, after re-angry regeneration, gas volume decreased several times, but then angry pollutant concentration increased several times, in order to achieve the system treatment of energy consumption reasonable goal.
    adsorption capacity of activated carbon is stronger than that of molecular sieve, but with the increase of use time and regeneration, the adsorption capacity of activated carbon equipment will decrease significantly. Although activated carbon can be effectively improved in the de-attachment regeneration treatment efficiency, but in the 7 d of the operating cycle of the rapid decline, it is likely that in the last few days of substandard conditions, and even if the regeneration desorption, adsorption efficiency is also significantly reduced, generally around 300d must be replaced. Therefore, in order to ensure stable adsorption treatment efficiency, the spray industry VOCs exhaust gas treatment commonly used adsorption agent to zeolite molecular sieve-based, while hydrophobic modified zeolite molecular sieve application further increased its use value, the enrichment process to wheel adsorption enrichment process."
    treatment scheme for high concentrations of VOCs exhaust gases is usually combustion or thermal oxidation. At present, the mainstream combustion schemes have catalytic oxidation (CO), heat storage thermal oxidation (RTO) and heat storage catalytic oxidation (RCO)
    3, zeolite wheel concentration and catalytic oxidation (CO) system
    3. 1 exhaust gas dehumidification, pre-filtering treatment subsyssor
    high humidity exhaust gas has a direct impact on the adsorption efficiency of zeolite wheel. According to the application in actual production and many tests, when the relative humidity of the exhaust gas is less than 80%, the adsorption efficiency of the zeolite wheel to VOCs can be stable at more than 90%, but when the relative humidity of the exhaust gas is greater than 90%, for some VOCs, the adsorption efficiency of the zeolite wheel is reduced to about 80%.
    if water-based paint is used, the actual relative humidity in the exhaust gas may be too high, so in order to ensure that the relative humidity in the exhaust gas is less than 80%, the exhaust gas temperature can be increased to reduce the relative humidity of the exhaust gas. Usually the vehicle is coated with VOCs exhaust gas treatment system, the inlet exhaust gas temperature is about 30 degrees C, if the inlet temperature is increased by about 2 degrees C, the relative humidity of the inlet exhaust gas can be reduced by about 15%. Therefore, even if the relative humidity of the workshop exhaust gas reaches 100%, the relative humidity when entering the wheel can be reduced to less than 85%, but in the actual industrial application, it is safe and reliable to raise the exhaust gas inlet temperature into the zeolite wheel to more than 3 degrees C higher than the workshop mixed exhaust gas temperature.
    Second, because the vehicle coating exhaust gas contains a small amount of paint mist and other particulate impurities, in order to avoid affecting the adsorption efficiency of the wheel, usually pre-filtration using primary, medium and high-efficiency combination filtration technology for the removal of particulate impurities, but the pre-filtration design should minimize the frequency of replacement of medium and high-efficiency filters, in order to reduce the operating costs of the equipment.
    3. 2 Zeolite wheel concentrate subsysysty
    wheel is the core component of zeolite wheel enrichment subsysysty, is also a key equipment for low concentration VOCs concentration, it is usually hydrophobic zeolite molecular sieve and ceramic fibers processed into ripple-shaped diaphragms, and then rolled into a beehive-shaped roulette structure, and in the center of the wheel installed rotating bearings. 、。 Usually the wheel is isolated into 3 areas, i.e. adsorption zone, dissoction zone and cooling zone, and the wheel speed is usually 3 to 6r /h continuously and slowly rotating.
    core part of zeolite wheel enrichment subsysys system is re-angry heating and heat exchange system, as well as fan system, automatic control system, etc. At present, the world's wheel manufacturers can provide the largest model of a single zeolite wheel, the maximum can handle 200000m3 / h air volume of exhaust gas, according to the components and concentration of the treatment of exhaust gas, can increase the concentration of VOCs in the exhaust gas 5 to 20 times.
    Zeolite wheel is designed to ensure that the excess VOCs exhaust gas entering the wheel can be discharged directly into the atmosphere after absorption purification in the wheel adsorption zone, and the removed original VOCs components are retained and adsorbed inside the zeolite molecular sieve, when the wheel is adsorbed with When a large number of VOCs are rotated to the de-attachment zone, a small air-volume de-attached fan blows high-temperature gas from about 180 to 240 degrees C into the wheel and dissocents the adsorption VOCs from it, at which point the exhaust gas is concentrated. The dissoction wheel because of the high temperature, at this time does not have a high adsorption performance, so first enter the cooling zone, after the cooling gas blow-off cooling, so that it resumes the adsorption capacity, and then rotate to the adsorption area, restart the next round of work. Each area remains airtable. Each part of the wheel can be rotated continuously from the adsorption zone to the dissoction zone, then to the cooling zone, and finally to the resorption zone to complete a rotation.
    3. The Catalytic Oxidation (CO) subsysyscies
    Catalytic Oxidation (CO) subsysyscies typically consist of burners or heaters, precious metal catalysts, valve packs and safety alarm devices. Catalytic oxidation (CO) is a flame-free combustion technology, the common catalytic oxidation temperature of coating exhaust gas treatment systems is between 300 and 350 degrees C, can also be as high as 600 degrees C, the processing efficiency can reach 99. More than 5%, and no NOx is produced.
    4, Zeolite wheel concentration and catalytic oxidation system technical features
    (1) for high wind volume, low concentration of VOCs exhaust gas, has a more stable purification efficiency, usually the system VOCs processing efficiency can be as high as 95%.
    (2) Zeolite wheel consists of inorganic oxides, which are non-flammable and highly safe.
    (3) zeolite wheel can withstand de-attachment temperatures above 300 degrees C, and VOCs at high boiling points can also be effectively handled. Thermal stability is very high, can be regenerated by repeated heating de-attachment, service life of more than 40000h.
    (4) The entire system start-up time is less than 1h at a time, for the rail vehicle spraying industry non-continuous operation of the operating conditions, the system start/stop fast, flexible operation, can save a lot of running energy consumption.
    5, conclusion
    zeolite wheel concentration and catalytic oxidation (CO) treatment scheme, is a reasonable process for vehicle coating VOCs exhaust gas treatment, processing process system safety, high removal rate, high stability, reasonable and feasible operating costs. Therefore, zeolite concentrated wheel and catalytic oxidation (CO) technology is the best choice for non-continuous operating conditions under the existing technical conditions, the treatment of high wind volume, low concentration VOCs exhaust gas, in the rail vehicle coating exhaust gas treatment will be more and more widely used.
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