The exhaust volume of the printing production line is about 40,000 m³/h. According to statistics from the original exhaust pipe inspection data, the concentration of VOCs imported from waste gas treatment equipment is about 100~300mg/m³.
A certain automobile trim coating process in Wuhan uses a large amount of organic solvents for oily paint during the production process. The pollutants are mainly organic pollutants produced by the evaporation of paint and solvents. The main components are ethyl acetate and benzenes (toluene, xylene, etc.). Honeycomb activated carbon adsorption equipment was originally used for purification treatment. Due to the low treatment efficiency (about 60%) and the need to regularly replace honeycomb activated carbon after adsorption saturation, hazardous waste generated also needs to be treated; this time, the environmental protection equipment will be upgraded and replaced with a new purification equipment.
Currently, common VOCs waste gas treatment methods include: combustion method, adsorption method, condensation method, chemical oxidation method, low-temperature plasma method, photocatalytic oxidation method and biodegradation method. In the "Feasibility Technical Guidelines for Pollution Prevention and Control in the Painting Industry" issued by the Ministry of Ecology and Environment, it is recommended to use adsorption, combustion, condensation or a combination of them for treatment. In the actual application process, according to the actual pollution production situation of the enterprise, the enterprise can reasonably select the process for the matching of the project.
The coating exhaust gas first enters the washing tower for washing and dry filtration equipment, and then is adsorbed and processed through fiber felt activated carbon adsorption boxes 1, 2, and 3. The treated exhaust gas is discharged through adsorption fans and exhaust shafts. At this time, the catalytic combustion furnace is preheated. When the catalytic light-off temperature of 260℃ for ethyl acetate is reached, the corresponding valve and desorption fan are opened. The air is heated to about 100 ° C through the heat exchanger to desorb the activated carbon adsorption box 4. The desorbed exhaust gas enters the electric heater of the catalytic combustion furnace and is heated to the light-off temperature of 260 ° C. After the catalytic combustion reaction forms carbon dioxide (CO2) and water vapor (H2O). The burned exhaust gas is heat recovered through a heat exchanger and then discharged to the exhaust pipe (chimney) through a desorption fan.
According to the company's original equipment and feedback on the collection effect, the exhaust volume of the printing production line is about 40,000 m³/h. According to statistics from the original exhaust pipe inspection data, the concentration of imported VOCs in exhaust gas treatment equipment is about 100~300mg/m³.
The working process of this device can be divided into 4 process sections: washing tower, dry filtration stage, fiber felt activated carbon adsorption stage, and fiber felt activated carbon desorption and regeneration stage.
2-1 Washing tower
The waste gas discharged from the workshop enters the washing tower device. When the organic waste gas passes through the washing tower, the particulate matter (paint mist) in the waste gas can be effectively intercepted by the water mist and environmental protection balls in the washing tower. The gas components and particulate matter components are separated and entered into the lower-level processor for processing.
2-2 Dry filtration stage
After the waste gas discharged from the workshop enters the washing tower device, the organic waste gas still contains water mist and trace amounts of paint mist. The water baffle and mist eliminator in the dry filter device are used to remove the water vapor, and then primary effect and bag filter are used to intercept trace amounts of paint mist, separate gas components and particulate matter components again, and enter the lower-level processor for processing.
2-3 Fiber felt activated carbon adsorption stage
The exhaust gas discharged from the workshop is treated by the pre-process and then enters the activated carbon adsorption box. When the organic exhaust gas passes through the activated carbon layer, the organic components in the exhaust gas are attracted into the pores of the activated carbon and concentrated and maintained therein. The organic components are separated from their components, and other component gases (clean gases) are evacuated through the fan.
2-4 Fiber felt activated carbon regeneration stage
After using activated carbon for a period of time, after adsorbing a certain amount of solvent, the adsorption capacity will be reduced or lost due to the gradual increase of adsorption amount. At this time, the activated carbon needs to be desorbed and regenerated. After regeneration, the activated carbon can resume its adsorption function before the activated carbon can continue to be used. During regeneration, the combustion chamber is first warmed up by preheating. When the temperature reaches the light-off temperature, it is switched to the desorption route to carry out desorption and regeneration treatment on the activated carbon adsorption box.
The specific process flow chart is shown in Figure 1.
The project's exhaust gas emission standard complies with the emission standard provisions of DB33/2146-2018 "Emission Standards for Atmospheric Pollutants in Industrial Painting Processes". The emission requirements are as follows: non-methane total hydrocarbons (ethyl acetate, etc.) ≤ 40mg/m³, and the emission rate ≤ 1.5 kg/h.
operation results
1. Pollutant monitoring data
After the project was running normally, the equipment was monitored for several weeks at different operating periods, and a local third-party certification company was entrusted to use standardized detectors to test the concentration indicators of import and export VOCs. The main test values are shown in Table 1.
The removal rate of non-methane total hydrocarbons reaches more than 90%.
2. Investment and operating expenses
The project investment mainly includes the equipment and installation of fiber felt activated carbon adsorption equipment, filters, fans, catalytic combustion beds, ventilation pipes, desorption combustion pipes, automatic control systems, etc. The total investment of the project is about 1.9 million yuan. The operating cost of the system is mainly based on the power consumption of the equipment, and the amount of waste gas processed is 40,000 m³/h, 24 hours a day. The power consumption is about 120KW *h, and the electricity price is 1.0 yuan/(KW*h), that is, the electricity price for treating 1m ³ exhaust gas is 0.003 yuan
conclusion
After the upgrading and renovation of the exhaust gas treatment facilities, the overall removal rate is over 90%. After being put into use in 2018, the exhaust gas pollution emissions from the automotive trim coating have been greatly reduced, and the environmental smell of the factory has been significantly improved. There are no complaints of odor.
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