The latest process design, namely dry filtration + activated carbon adsorption and desorption + catalytic combustion, is very effective in controlling VOCs pollution and has good practical value for environmental protection, labor protection and human health protection.
As a process design case for the organic waste gas treatment system of a paint shop, paint waste gas is one of the main sources of pollution to the atmospheric environment. Because of its high toxicity, it poses a very serious hazard to human health, especially benzenes and polycyclic aromatic hydrocarbons. They invade human nerve, respiratory, blood and reproductive systems, causing dizziness, nausea, and at worst, aplastic anemia, leukemia, and cancer that threaten life, so there is an urgent need for treatment.
(1)Emission volume：30000Nm3/h;
(2) Operating temperature: Normal temperature;
(3)Exhaust gas transportation pressure：3000Pa;
(4)Processing efficiency：80%;
(5)Inlet concentration：≤200mg/m3
(6)Excess concentration：≤50mg/m3
When gas molecules move to the solid surface, due to the interaction between the gas molecules and the solid surface molecules, the gas molecules temporarily stay on the solid surface, resulting in an increase in the concentration of gas molecules on the solid surface. This phenomenon is called gas adsorption on the solid surface. The substance to be adsorbed is called the adsorbate, and the solid substance of the adsorbate is called the adsorbent. The activated carbon adsorption method uses activated carbon as an adsorbent to adsorb organic compounds in exhaust gas onto the surface of the solid phase for adsorption and concentration, thereby purifying exhaust gas.
Large-air volume and low-concentration organic waste gas passes through a front-end primary three-stage dry filter to remove paint mist and dust, and then passes into an activated carbon adsorption bed, where it is fully contacted with honeycomb activated carbon, and is then desorbed and regenerated before being converted into low-air volume and high-concentration organic waste gas. This part of the gas enters the catalytic combustion and oxidation equipment at the back end for oxidation reaction, so that volatile organic compounds in the exhaust gas are oxidized and decomposed into carbon dioxide and water in the catalytic oxidation chamber, and heat is released. The desorbed gas before entering the catalytic bed is preheated by the heat exchanger. After that, it is discharged high through the chimney. At the same time, the system uses part of the hot gas to regenerate the activated carbon to ensure that the activated carbon is reused.
Organic waste from the spray shop.
The working process of this device can be divided into four stages: exhaust gas collection stage, primary dry filtration stage, activated carbon adsorption stage and activated carbon desorption and regeneration stage.
Exhaust gas collection stage:
A special exhaust gas collection hood and piping system are set up in the spray booth to collect exhaust gas centrally.
Primary dry filtration stage
The exhaust gas from the spray booth first enters the primary dry filter for pretreatment. After three-stage filtration of the primary dry filter, particulate matter in the organic exhaust gas is intercepted, providing powerful conditions for the subsequent activated carbon adsorption process and ensuring the service life of activated carbon.
Activated carbon adsorption process:
The organic waste gas after passing through the primary dry filter enters the activated carbon adsorption device. When the organic waste gas passes through the activated carbon layer, the organic components in the organic waste gas are attracted to the surface of the activated carbon and concentrated and maintained on it. The organic components are thereby separated from other components, and the other components (clean gas) are evacuated by the fan.
Activated carbon desorption process:
After using activated carbon for a period of time, after adsorbing a certain amount of solvent, its adsorption capacity will be reduced or lost. At this time, the activated carbon needs to be desorbed and regenerated. After regeneration, the activated carbon will regain its adsorption capacity and can continue to be used. During regeneration, the power supply of the preheating chamber of the catalytic combustion device is turned on to preheat the air. The preheated gas is sent to the adsorption box. After the activated carbon in the box is heated, the solvent adsorbed by the activated carbon is volatilized, and the solvent is sent to the catalytic combustion box through a fan to burn, decompose to generate hot air such as CO2 and H2O. Part of the hot air returns to the activated carbon adsorption box to continue heating the activated carbon, and the other part is evacuated. The activated carbon can be regenerated by circulating the hot air multiple times.
The main characteristics of this process
(1) The production environment is good and pollution-free.
(2) The process equipment has strong reliability, small failures and maintenance, low maintenance costs, and reduces the workload of maintenance personnel.
(3) Due to the relatively high degree of automation, there are few operators and low labor intensity.
(4) It can fully recover waste heat and save energy.
(5) Small system investment and low operating costs.
General layout of organic waste gas treatment process
(1) Primary dry filter: arranged in the factory building. This plan selects 1 DN1000 primary dry filter;
(2) Activated carbon adsorption box: arranged in the factory building. This plan uses 3 activated carbon adsorption boxes, and the operation mode is: 2 on and 1 on standby;
(3) Catalytic burner: arranged in the factory building. This plan selects a 2500 Nm3/h catalytic burner;
(4) Chimneys: arranged outdoors in the open air. The design diameter of the chimney is DN1000 and the height is 16 meters;
(5) Main exhaust fan: arranged in the factory building. Configure 1 exhaust fan. Model: 4-72-9 Right 45°, flow rate 35700 Nm3/h, full pressure 2914Pa, motor: 45KW;
(6) Desorption fan: arranged in the factory building. Configure 1 desorption fan. Model: 6-35-4 Right 0°, flow rate 2314 Nm3/h, full pressure 2512Pa, motor: 3KW;
(7) Supplementary air cooler: arranged in the factory building. Configure 1 supplementary air cooler. Model: 4-72-2.8 Right 0°, flow rate 1141 Nm3/h, full pressure 963Pa, motor: 1.5KW;
(8) The organic waste gas treatment plant should be arranged separately from other production plants, and the spacing should comply with the relevant provisions of the current national standard "Fire Prevention in Building Design";
(9) The fire lanes in the organic waste gas treatment area shall comply with the relevant provisions of the current national standard "Code for Fire Protection Design of Buildings";
(10) General layout of organic waste gas treatment process. See Figure 4-1 General layout of organic waste gas treatment process for details.
(1) Wide scope of application. Catalytic burners can process almost all hydrocarbon organic waste gases and odorous gases. For low-concentration, multi-component, and non-recycling waste gases emitted by organic chemical, coatings, insulating materials and other industries, the adsorption catalytic burner method has a better treatment effect.
(2) High treatment efficiency and no secondary pollution. The purification rate of organic waste gas treated by catalytic oxidation is generally more than 95%. The final products are harmless CO2 and H2O. Due to the low combustion temperature, the formation of NOx can be greatly reduced, so secondary pollution will not be caused.
(3) The treatment of organic mixed waste gas is controlled by an automatic control system. The operating process is simple, the startup and shutdown of the furnace are simple, the failure rate is low, the operation cycle is long, and the safety is good.
conclusion
As the country's requirements and efforts for environmental protection continue to increase, the management of industrial organic waste gases (VOCs) has developed rapidly. Several relatively mature treatment processes have also emerged on the market. The process design described in this paper uses dry filtration + activated carbon adsorption and desorption + catalytic combustion. This design process is mainly aimed at the treatment of VOCs with large air volume, low concentration, and no need to be recycled. This process has obvious effects and significant advantages in controlling VOCs pollution, and has good practical value for environmental protection and human health protection. This technology has low investment, low operating costs, simple operation, and long service life, which is more conducive to promotion and application in the field of VOCs treatment technology in factory painting workshops.
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