1. Formation and main components of spray paint waste gas
Painting process is widely used in machinery, automobile, electrical equipment, home appliances, ships, furniture and other industries.
Paint raw material —— paint is composed of non-volatile and volatile, non-volatile including film substance and auxiliary film substance, volatile dilution agent is used to dilute the paint, to achieve the purpose of smooth and beautiful paint surface.
The paint spray process mainly produces paint mist and organic waste gas pollution, paint under the action of high pressure into particles, when spraying, part of the paint did not reach the spray surface, diffusion with the airflow to form the paint mist; organic waste gas from the volatilization of the diluent, organic solvent is not attached to the paint surface, the paint and curing process will release organic waste gas (reported hundreds of volatile organic compounds, respectively belong to alkane, alkanes, olefin, aromatic compounds, alcohol, aldehyde, ketones, ester, ether, and other compounds).
2. Source and characteristics of automobile coating exhaust gas
Automobile painting workshop should conduct paint pre-treatment, electrophoresis and spray paint on the workpiece. Paint process includes spray painting, flow and drying, in these processes will produce organic waste gas (VOCs) and spray spray, so these processes need to spray paint room waste gas treatment.
(1) Waste gas from the spray paint room
In order to maintain the working environment of spraying, according to the provisions of the Labor Safety and Health Law, the air should be changed continuously changed in the spraying room, and the air change speed should be controlled within the range of (0.25~1) m/s. The main composition of the air exhaust gas is the organic solvent of spray paint, its main components are aromatic hydrocarbons (three benzene and non-methane total hydrocarbon), alcohol ether, ester organic solvent, because the exhaust volume of the spray room is very large, so the total concentration of the organic waste gas discharged is very low, usually about 100 mg/m3. In addition, the exhaust of the paint room often contains a small amount of completely untreated paint fog, especially the dry paint spray capture spray room, the paint fog in the exhaust, may become the obstacle to waste gas treatment, waste gas treatment must be pretreatment.
(2) Waste gas from the drying room
Face paint after spraying before drying, want to flow air, wet paint film organic solvent in the process of drying the volatile, in order to prevent air indoor organic solvent aggregation explosion accident, air room should be continuous air, change air speed generally control around 0.2 m/s, exhaust exhaust composition and paint room exhaust composition, but does not contain paint mist, the total concentration of organic waste gas than spray room, according to the exhaust volume, generally in the spray room exhaust gas concentration about 2 times, can reach 300 mg/m3, usually mixed with spray room exhaust after centralized treatment. In addition, paint room, surface paint sewage circulation pool should also discharge similar organic waste gas.
(3) Drying exhaust gas
The composition of the drying waste gas is more complex, in addition to the organic solvent, part of the plasticizer or resin monomer and other volatile components, but also contains thermal decomposition products, reaction products. Electrophoretic primer and solvent type topcoat drying have exhaust gas discharge, but its composition and concentration difference is large.
※Hazards of spray paint exhaust gas:
It is known from the analysis that the waste gas from the spray room, drying room, paint mixing room and topface paint sewage treatment room is low concentration and large flow, and the main components of the pollutants are aromatic hydrocarbons, alcohol ethers and ester organic solvents. According to the “Comprehensive Emission Standard for Air Pollution”, the concentration of these waste gas is generally within the emission limit. In order to cope with the emission rate requirements in the standard, most automobile factories adopt the method of high-altitude emission. Although this method can meet the current emission standards, but the waste gas is essentially untreated diluted emission, and the total amount of gas pollutants discharged by a large body coating line may be as high as hundreds of tons, which causes very serious harm to the atmosphere.
Paint fog in the organic solvent —— benzene, toluene, xylene is a strong toxic solvent, operating to the air in the workshop, workers after respiratory tract inhalation can cause acute and chronic poisoning, mainly cause the damage of the central nervous and hematopoietic system, short-term inhalation high concentration (more than 1500 mg/m3) of benzene vapor, can cause aplastic anemia, often inhaled low concentration of benzene vapor can also cause vomiting, neurological symptoms such as confusion.
※Selection of waste gas treatment method for spray paint and coating:
In choosing organic treatment methods, the following factors should be considered generally: the type and concentration of organic pollutants, the organic exhaust temperature and discharge flow rate, the particulate matter content, and the pollutant control level that needs to be achieved.
1 Spray paint at room temperature treatment
The exhaust gas from the painting room, drying room, paint mixing room and the topcoat sewage treatment room is the room temperature exhaust gas of low concentration and large flow, and the main composition of pollutants is aromatic hydrocarbons, alcohol and ethers and ester organic solvents. According to the GB16297 “Comprehensive Emission Standard for Air Pollution”, the concentration of these waste gas is generally within the emission limit. In order to cope with the emission rate requirements in the standard, most automobile factories adopt the method of high-altitude emission. Although this method can meet the current emission standards, but the waste gas is essentially diluted emission without treatment, and the total amount of gas pollutants discharged by a large body coating line may be as high as hundreds of tons, which causes very serious harm to the atmosphere.
In order to fundamentally reduce the emission of exhaust gas pollutants, several exhaust gas treatment methods can be jointly used for treatment, but the cost of exhaust gas treatment with high air volume is very high. At present, the more mature foreign method is to first concentrate (with the adsorption-desorption wheel to concentrate the total amount of about 15 times), in order to reduce the total amount to be treated, and then use the destructive method to treat the concentrated waste gas. There are similar methods in China, the first use adsorption method (activated carbon or zeolite as adsorbent) for low concentration, room temperature spray paint waste gas adsorption, with high temperature gas desorption, concentrated waste gas using catalytic combustion or regenerative thermal combustion method for treatment. Low concentration, normal temperature spray paint waste gas biological treatment method is being developed, the domestic technology at the present stage is not mature, but it is worth paying attention to. In order to really reduce the public pollution of coating waste gas, we also need to solve the problem from the source, such as the use of electrostatic rotary cups and other means to improve the utilization rate of coatings, the development of water-based coatings and other environmental protection coatings.
2 Drying waste gas treatment
Drying waste gas belongs to the medium and high concentration of high temperature waste gas, suitable for combustion method treatment. The combustion reaction has three important parameters: time, temperature, disturbance, that is, the combustion of 3T conditions. The efficiency of waste gas treatment is essentially the sufficient degree of the combustion reaction and depends on the 3T condition control of the combustion reaction. RTO can control the combustion temperature (820~900℃) and the stay time (1.0~1.2s), and ensure that the necessary disturbance (air and organic matter are fully mixed), the treatment efficiency is up to 99%, and the waste heat rate is high, and the operating energy consumption is low. Most Japanese automobile factories in Japan and China usually use RTO to centrally treat the exhaust gas of drying (primer, medium coating, top coat drying). For example, Dongfeng Nissan passenger car Huadu coating line using RTO centralized treatment of coating drying exhaust gas effect is very good, fully meet the requirements of emission regulations. However, due to the high one-time investment of RTO waste gas treatment equipment, it is not economical for waste gas treatment with small waste gas flow.
For the completed coating production line, when the additional waste gas treatment equipment is needed, the catalytic combustion system and the regenerative thermal combustion system can be used. The catalytic combustion system has small investment and low combustion energy consumption.
Generally speaking, the use of / platinum as a catalyst can reduce the temperature of oxidizing most organic waste gas to about 315℃. Catalytic combustion system can be used for the general drying waste gas treatment, especially suitable for the drying power supply using electric heating occasions, the existing problem is how to avoid the failure of catalyst poisoning. From the experience of some users, for the general surface paint drying waste gas, by increasing waste gas filtration and other measures, can ensure that the life of the catalyst is 3~5 years; electrophoretic paint drying waste gas is easy to cause catalyst poisoning, so the treatment of electrophoretic paint drying waste gas should be careful using catalytic combustion. In the process of waste gas treatment and transformation of Dongfeng commercial vehicle body coating line, the waste gas of electrophoretic primer drying is treated by RTO method, and the waste gas of top paint drying is treated by catalytic combustion method, and the use effect is good.
※Spray paint coating waste gas treatment process:
Spraying industry waste gas treatment scheme is mainly used for spray painting room waste gas treatment, furniture factory waste gas treatment, machinery manufacturing industry waste gas treatment, guardrail factory waste gas treatment, automobile manufacturing and automobile 4S shop spray paint room waste gas treatment. At present, there are a variety of treatment processes, such as: condensation method, absorption method, combustion method, catalytic method, adsorption method, biological method and ion method.
1. Water spray method + activated carbon adsorption and desorption + catalytic combustion
Using spray tower to remove paint mist and the soluble in water material, after the dry filter, in activated carbon adsorption device, such as activated carbon adsorption full, then stripping (stripping method with steam stripping, electric heating, nitrogen stripping), after the stripping gas (concentration increased dozens of times) by stripping fan into the catalytic combustion device combustion, combustion into carbon dioxide and water, after discharge.
2. Water spray + activated carbon adsorption and desorption + condensation recovery method
Using spray tower to remove paint mist and the soluble in water material, after the dry filter, in activated carbon adsorption device, such as activated carbon adsorption full, then to stripping (stripping method with steam stripping, electric heating, nitrogen stripping), after processing the waste gas adsorption concentration condensation, condensate by separation recovery of valuable organic matter. This method is used for the waste gas treatment with high concentration, low temperature and low air volume. But this method investment, high energy consumption, operating cost, spray paint exhaust gas “three benzene” and other exhaust gas concentration is generally lower than 300 mg/m3, low concentration, large air volume (automobile manufacturing paint workshop air volume often in above 100000), and because the automobile coating exhaust organic solvent composition, recycling solvent is difficult to use, and easy to produce secondary pollution, so coating in the waste gas treatment generally do not use this method.
3. Waste gas adsorption method
Spray paint waste gas treatment adsorption can be divided into chemical adsorption and physical adsorption, but “three benzene” waste gas chemical activity is low, generally do not use chemical absorption. The physical absorbing fluid absorbs less volatile, and it absorbs the components with higher affinity for heating, cooling and reuse for analyzing saturation absorption. This method is used for air displacement, low temperature, and low concentration. The installation is complex, the investment is large, the choice of absorption fluid is more difficult, there are two pollution
4. Activated carbon adsorption + UV photocatalytic oxidation equipment
(1): directly through the activated carbon direct adsorption of organic gas, to achieve the purification rate of 95%, simple equipment, small investment, convenient operation, but need to often replace the activated carbon, low concentration of pollutants, no recovery.(2) Adsorption method: organic gas in the activated carbon adsorption, activated carbon saturated air desorption and regeneration.
5. Activated carbon adsorption + low-temperature plasma equipment
After activated carbon adsorption first, then with low temperature plasma equipment processing waste gas, will treat the standard of gas discharge, ion method is to use plasma Plasma (ION plasma) degradation of organic waste gas, remove stench, kill bacteria, viruses, purify the air is a high-tech international comparison, experts at home and abroad are called one of the four major environmental science technology in the 21st century. The key to the technology is through the high voltage pulse medium block discharge in the form of a large number of active ion oxygen (plasma), the gas activation, produce all variety of active free radicals, such as OH, HO2, O, etc., benzene, toluene, xylene, ammonia, alkane and other organic waste gas degradation, oxidation and other complex physical and chemical reactions, and by-product non-toxic, avoid secondary pollution. The technology has the characteristics of extremely low energy consumption, small space, simple operation and maintenance, and is especially suitable for the treatment of various component gases.
Brief summary:
Now there are many kinds of treatment methods on the market, in order to meet the national and local treatment standards, we will usually choose several treatment methods combined to treat the waste gas, to choose in line with their own actual treatment process for treatment.
Post time: Dec-28-2022
