In the production process of spray painting and baking paint for some products, there will be a large amount of process exhaust emissions, which pose a significant threat to the natural environment and the health of operators. Specifically, processes such as spray painting and baking require the use of a large amount of volatile compounds such as benzene, toluene, ethylbenzene, and ethyl acetate as coating solvents and diluents. These organic solvents will not adsorb onto the surface of the workpiece during spraying, but will evaporate completely into the air and become organic waste gas. These exhaust gases have characteristics such as low boiling point and easy volatilization at room temperature, which have a significant impact on the surrounding environment and the health of operators.
The content of volatile organic compounds in the air of some large cities in China is several times higher than that of cities in the United States, and the organic waste gas emitted from industrial production has become the main factor of urban air pollution in China. When these gases evaporate, they produce irritating odors that pose a great threat to the health of operators. Short term exposure can cause nausea, dizziness, and damage to the internal organs and nervous system of the human body when absorbed in large quantities. Therefore, in industrial production activities, in addition to adopting necessary protective measures, it is also necessary to avoid the emission of organic waste gas as much as possible and comprehensively collect and purify organic waste gas.
1 Basic principle
This scheme uses honeycomb shaped activated carbon as an adsorbent, and achieves the goal of air purification by adsorbing, purifying, desorbing, regenerating, and concentrating volatile organic compounds (VOCs), as well as catalytic combustion. The high air volume and low concentration organic waste gas are adsorbed by honeycomb shaped activated carbon to achieve the goal of air purification. After saturation of activated carbon adsorption, the activated carbon is regenerated by hot air desorption. The concentrated organic matter obtained by desorption is sent to the catalytic combustion bed for catalytic combustion, and the internal organic matter is oxidized into harmless CO2 and H2O. The hot exhaust gas after combustion is heated by a heat exchanger to produce cold air. After heat exchange, the cooled gas is partially discharged and partially used for the desorption and regeneration of honeycomb activated carbon, achieving the goal of energy conservation. The entire equipment includes pre filters, adsorption beds, catalytic combustion beds, and fans.
Compared with other organic waste gas treatment methods, this method is a comprehensive treatment mode that draws on the advantages of other modes. The technology is more mature and reliable, and it has significant advantages in treating high air volume and low concentration organic waste gas.
Process design for 2 treatments
2.1 Preprocessing
For organic waste gas, people should first carry out water spraying to remove impurities and soluble organic matter inside the waste gas. After spraying, the gas contains a large amount of moisture and a small amount of dust inside. To avoid the influence of moisture and dust on the effective operation of the activated carbon adsorption bed, people need to use high-efficiency filters for filtration during treatment.
2.2 Adsorption operation
The pre treated organic waste gas is introduced into the adsorption bed under the action of a fan and evenly distributed on the surface of activated carbon. According to the van der Waals forces between molecules, activated carbon will adsorb organic waste gas on the surface. This process takes less time, but the longer the time, the more thorough the adsorption. There is no significant chemical reaction between the two, while the organic waste gas achieves a high purification effect. After purification, the clean exhaust gas can meet the emission standards of relevant atmospheric pollutants. With the action of the fan, it can meet the emission standards of a 15m high exhaust pipe. Each exhaust gas purification treatment system contains adsorption beds at different levels, with two sets for adsorption and one set for desorption. The three sets of equipment can be operated alternately.
2.3 Desorption and Catalytic Combustion
The specific reaction equation is:
After the activated carbon adsorption reaches saturation, switch to the desorption bed, which requires additional heat. The heating device is installed inside the catalytic oxidation bed, and the catalyst is preheated simultaneously when turned on. After the catalytic oxidation bed reaches the set temperature, hot air is introduced into the desorption bed, and the organic waste gas is completely desorbed from the surface of the activated carbon under the heating effect.
High concentration organic waste gas enters the oxidation bed under external force, and is burned and decomposed into H2O and CO2 through the catalytic action of platinum metal. The waste gas is purified through this operation. The characteristics of this combustion process are low temperature, fast and flameless, and generate a large amount of heat. People can reuse activated carbon in the desorption and combustion oxidation of organic waste gas, thereby reducing energy consumption.
When the concentration of organic waste gas is high, excessive heat generated by combustion can lead to a higher temperature in the catalytic oxidation bed, which in turn affects the safety of the entire waste gas treatment system. Therefore, the system designed in this article includes a cold air supplement device, which can introduce fresh air to reduce the reaction temperature and ensure the safety of system operation.
2.4 Sewage treatment
After water spray pretreatment, wastewater containing impurities and organic matter is introduced into the sewage treatment tank of the paint spraying workshop for sedimentation, removing large particles of impurities. Wastewater containing organic matter exhibits weak acidity. To reduce the corrosion of equipment caused by wastewater reuse, a small amount of caustic soda needs to be added to carry out neutralization reactions and adjust its acidity and alkalinity. After thorough purification, the wastewater can be recycled and reused in the water spray pretreatment module, and there is basically no discharge of sewage from the sewage treatment unit. The products of sewage centrifugal analysis and the paint residue sediment generated by sedimentation are considered hazardous waste. According to the relevant national requirements for hazardous waste management, qualified hazardous waste disposal units should be commissioned to carry out safe disposal operations.
This plan has numerous advantages. Firstly, a new and efficient activated carbon is used with a stable and reasonable adsorption bed structure, greatly improving the adsorption effect. Secondly, catalytic combustion is carried out using platinum metal to decompose organic waste gas into non-toxic and harmless H2O and CO2, achieving efficient and thorough purification; Again, the combustion process has a lower temperature and is safe. The heat generated during the combustion process can be recycled, resulting in less heat loss and a significant reduction in energy consumption; Finally, the design and installation of system equipment adopt various measures to eliminate safety hazards, with simple operation and ease of later use and management.
3. Operation methods for each component module
3.1 Paint mist filter
Spray painting exhaust gas mainly appears on the spray painting workbench where the workpiece is applied. Many of the high-pressure air sprayed paint stays on the workpiece, while others are discharged with the exhaust gas and become paint mist. These paint sprays have low dust content and small particles, with the majority having a diameter of less than 10mm. If left untreated, it will quickly clog the micropores of activated carbon, causing it to lose its original function. Therefore, paint exhaust gas must first undergo coarse filtration treatment.
3.2 Selection and parameter setting of adsorbent
Activated carbon has the advantages of large surface area, strong adsorption capacity, and low cost. It is currently a common adsorbent for VOCs pollution. It is inconvenient to replace activated carbon in powder form. Activated carbon fibers contain regular micro crystalline structures, which have a large adsorption capacity and are prone to detachment, resulting in high costs. Honeycomb shaped activated carbon has high wind speed and low resistance, and can be applied to the adsorption of low concentration exhaust gases with high air volume. This article selects honeycomb shaped activated carbon, and the structure of the adsorption bed adopts a drawer style assembly mode, which is convenient for filling and disassembling during use.
3.3 Catalytic combustion design
3.3.1 Heat exchanger
The structure of the heat exchanger is relatively complex. In order to reduce production costs and facilitate subsequent installation, this article adopts a relatively simple fixed tube plate heat exchanger, with cold gas flowing through the shell and hot gas flowing through the tube side.
3.3.2 Electric heating chamber
In this scheme, the heating chamber only provides the heat required for preheating the gas during startup. After catalytic combustion of benzene, a large amount of preheating can be utilized, so the required thermal power is relatively low. It can be heated by electricity without the need for additional heating of natural gas or liquefied petroleum gas.
3.3.3 Treatment of insulation module
The temperature inside the catalytic combustion integrated equipment is much higher than room temperature, and insulation treatment needs to be added to avoid harm to the workers. The insulation cotton used for insulation is made of aluminum silicate fiber felt, designed based on the possible high temperature of 400oC in the combustion chamber. The thickness of the insulation cotton is 64mm.
3.4 Design of flame arrester
A flame arrester is composed of many small channels or gaps, and when the flame enters these small channels, many small flame streams are formed. Due to the large heat transfer area of channels or pores, the temperature of the flame decreases after heat exchange through the channel walls, and the flame extinguishes when it reaches a certain degree. Flame arresters are devices used to prevent the spread of flames caused by the evaporation of burning gases or flammable liquids. In VOCs catalytic combustion reactors, the presence of sparks can trigger gas flames and promote the combustion of the entire pipeline network. Therefore, the role of flame arresters is significant. The size of the shell of the flame arrester is directly related to the fluid resistance, and the diameter of the shell is usually about 4 times the diameter of the pipeline used in conjunction, that is, D=4d. This article is designed according to specifications, using open and closed ends of an open flame for ignition. This scheme adopts a flameless combustion method. If D=4d is used, the flame arrester will be too large. Based on actual operational requirements, the data value designed in this article is D=2d and the angle is 60o. The flame arrester is made of 1mm stainless steel, with a pipe diameter of 500mm × 200mm and a diffusion angle of 60o. The height of the front half of the shell is taken as 250 × sin60o=433mm.
4 Conclusion
This scheme adopts the adsorption catalytic combustion method to treat spray paint exhaust gas. Firstly, a filter is used to remove paint mist. Then, through system control, a honeycomb shaped activated carbon adsorption bed is used to continuously adsorb it, and at the same time, saturated activated carbon is desorbed. By using 80% hot air blowing, high air volume and low concentration organic waste gas is concentrated into low air volume and high concentration organic waste gas. At the same time, the catalytic combustion chamber is used to convert the organic gas into CO2 and H2O, while maintaining stable self combustion. Practice has proved that this treatment mode has the advantages of high purification efficiency, no secondary pollution and low operation cost compared with the traditional process.
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