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Spray exhaust gas treatment
Release time：2021-04-02 Views：2088
1. Sources and hazards of paint spray exhaust gas
Spraying exhaust gas comes from the spraying process of automobiles, motorcycles, bicycles, ships, airplanes, furniture equipment, mechanical equipment, building materials and other industries, including three steps of spraying, leveling and drying. Due to the addition of organic solvents in the paint, organic waste gas is emitted, mainly containing organic pollutants such as benzene, toluene, and xylene. In addition to organic waste gas, because paint or varnish is atomized into particles under high pressure, some of the paint particles that fail to reach the surface of the spray paint are dispersed with the airflow to form paint mist, so the spray waste gas also contains paint mist particles. The paint mist particles are small, most of which are below 10μm, and the viscosity is large, which is easy to adhere to the surface of the material.
2. Spray paint industry category and exhaust gas composition
The main pollutants are paint mist and volatile organic waste gas.
a. Paint mist: The atomized dust particles formed by the liquid paint under the action of air pressure are viscous, easily adsorbed on the people or objects in contact, and are also potential carcinogens, causing pollution to the entire atmospheric environment.
b. Volatile organic waste gas: organic solvents, mainly triphenyl (benzene, toluene, xylene) are used to dilute the paint to achieve the purpose of smooth and beautiful surface of the object, but the organic solvent is very volatile and cannot be attached to the paint for a long time. The surface of the object will be completely released during the spraying and drying process, thereby forming organic waste gas, which is characterized by colorless and extremely irritating.
Third, the characteristics of paint spray exhaust gas
(1) The characteristics of paint spray exhaust gas; according to different paint formulations, the material composition and content are complicated. Characteristics of spray paint exhaust gas: The poisonous substances of spray paint exhaust gas are.
(2) Benzene, toluene, xylene, glycol ether, toluene diisocyanate, heavy metals, phthalates, VOC, formaldehyde, etc. Toluene, xylene; belong to moderately toxic substances, anesthetize the human body, stimulating effect, high concentration is toxic to nerves. Benzene; a highly toxic substance that accumulates in the nerves and bone marrow, causing damage to the nerves and hematopoietic ancestors, and can cause leukemia for a long time. Glycol ether; can form highly toxic compounds in the body, cause permanent damage to the blood circulation and nervous system, and can cause cancer for a long time. TDI; (toluene diisocyanate) destroys the cornea and damages the liver and kidney function after absorption. Long-term exposure can cause cancer.
(3) Heavy metals; can cause chronic poisoning, remain in the brain and liver organs, and cause permanent damage to the liver and kidneys. Phthalates, which are plasticizers, can cause stunted growth in children.
(4) VOC volatile compounds cause atmospheric pollution, destroy human living environment, and endanger human health.
(5) Formaldehyde; harms and irritates the cornea.
(6) Benzene and benzene series in paint exhaust gas are not easily soluble in water under normal conditions, the benzene ring is quite stable, not easy to oxidize, and has high thermal stability. It will not decompose when heated to 900C. Treat the paint exhaust gas. Causes a certain degree of difficulty.
(7) The fine and viscous paint particles in the paint spray exhaust gas are not easy to dilute and easily cause filter blockage.
(8) The substances in the paint exhaust gas are highly volatile, and the accumulation is not easy to collect, so there must be a closed paint spraying space.
4. Introduction to paint spraying waste gas treatment process
The current mainstream technology with very good effect is activated carbon adsorption desorption + catalytic combustion technology.
Activated carbon adsorption and desorption: The currently widely used organic waste gas treatment technology passes the organic waste gas through the activated carbon bed, where the organic pollutants are adsorbed by the adsorbent, and the waste gas is purified and discharged into the atmosphere. When the carbon adsorption reaches saturation, hot air is introduced to heat the carbon layer, and the saturated carbon bed is desorbed and regenerated. The organic pollutants released by the desorption blowing need to be further processed. The main adsorbents are activated carbon, activated carbon fiber, coke powder and so on. Activated carbon and activated carbon fiber have dense pore structure, large inner surface, good adsorption performance, stable chemical properties, acid and alkali resistance, water resistance, high temperature and high pressure resistance, not easy to break, and low air resistance, so they are widely used.
Catalytic combustion: This method is suitable for the treatment of organic waste gas and the elimination of malodors. The operating temperature of the catalytic combustion method is half lower than that of the ordinary combustion method, usually 200-400°C (generally lower than 800°C). Under the action of the catalyst, the hydrocarbons in the organic waste gas can be rapidly oxidized at a lower temperature to produce carbon dioxide and water, and at the same time emit the heat of combustion. The catalytic combustion method has a purification rate of up to 95%. It is suitable for the treatment of organic waste gas with high concentration, low air volume and high exhaust gas temperature. A suitable design process can maintain combustion with only a small amount of energy supplemented, and can generate rich energy. Thoroughly decompose pollutants and low operating costs. However, the concentration of "triphenyl" in the paint spraying waste gas is generally lower than 300mg/m3, so the catalytic combustion method is not suitable for the spraying waste gas treatment [2~4].
5. Design basis and principles for spray paint exhaust gas treatment
1. Implement "Comprehensive Emission Standard of Air Pollutants" GB16297-96, benzene≤12mg/m3 toluene≤40mg/m3 xylene≤70mg/m3
2. Comply with TJ36-79 "Design Sanitary Standards for Industrial Enterprises". Benzene<40mg/m3, toluene<100mg/m3, xylene<100mg/m3.
3. Comply with GB6514-95 "Safety Regulations for Coating Work, Ventilation of Coating Process".
6. Introduction to the catalytic combustion process of spray paint exhaust gas adsorption and desorption activated carbon
Organic waste gas + air filter + activated carbon adsorption + system fan + activated carbon desorption regeneration-catalytic combustion + standard emission
(1) The organic waste gas is firstly filtered through dry filtration to intercept and remove particulate pollutants in the waste gas, and then enter the adsorption bed for adsorption. The honeycomb activated carbon with a large specific surface area is used to adsorb the waste gas on the surface of the activated carbon.
(2) The treated clean gas is discharged at high altitude through the fan and chimney.
(3) The activated carbon reaches saturation after a period of time of adsorption operation, and the desorption-catalytic combustion process of the system is started. The hot air flow desorbs the activated carbon which has been adsorbed on the surface of the activated carbon, and transforms into CO2 and water vapor through the catalytic combustion reaction. Harmful substances and emit heat.
(4) The heat generated by the reaction is reused in the desorption hot air flow through the heat exchange part. When the desorption reaches the rated level, the exothermic heat and the desorption heat reach a balance, and the system completes the desorption process without external heating.
(5) The exhaust gas composition and index meet the standards for safe discharge.
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