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Spray paint professional exhaust gas treatment plan:

In this paper, the primary pollution factors for spray paints are benzene, toluene, xylene and other harmful gases.

I. Overview

In the professions of cars, motorcycles, bicycles, ships, airplanes, household equipment, machinery and equipment, building materials and furniture, it is necessary to paint or paint the exterior to achieve the effects of decoration, beauty and anti-corrosion. During the construction and curing process, many organic solvents are required. These volatile organic compounds are released during use and discharged into the atmosphere. Volatile organic compounds in organic waste gases are called VOCs (Volatile Organic Compounds), most organic substances. Air pollutants that are toxic risks not only pose a direct harm to people's health, but also cause damage to the atmosphere on which people depend.

Second, the characteristics and harms of professional organic waste gas from spray paint

1. Characteristics of exhaust gas

The exhaust gas of the paint baking profession has a large amount of exhaust gas and a high concentration, and contains many kinds of organic substances. The exhaust gas of the general painting profession contains organic substances such as triphenyl, alcohols, ketones, ethers and esters.

2. Hazard of exhaust gas

There are many kinds of organic substances in the exhaust gas of spray paint, in which the content of "triphenyl" is high and the damage is high. The benzene, toluene and xylene are colorless and aromatic, and the boiling point is 80.1 oc, which is volatile organic matter. They are all volatile and flammable.

Benzene is considered a highly toxic carcinogen by the International Organization for Health Research (WHO) International Center for Cancer Research (IARC). Acute toxic effects, primarily for the central nervous system, chronic primary effects on hematopoietic tissue and nervous system, but if the hematopoietic function is completely damaged, it can cause fatal granuloblastic leukopenia and cause leukemia. It also has a stimulating effect on the skin. Benzene, toluene and xylene are present in the air in a vapor state, and the poisoning effect is generally caused by inhalation of vapor or skin absorption. Because of the benzene aromatic hydrocarbons, it is not easy to be alert to its toxicity. If long-term exposure to a certain concentration of toluene, xylene can cause chronic poisoning, there may be neurasthenia such as headache, insomnia, energy wilting, memory loss. Toluene and xylene also have a certain influence on reproductive function and lead to congenital defects (ie, distortion) of the fetus. Irritating to the skin and mucous membranes, it is more harmful to the nervous system than benzene, and long-term touch may also cause bladder cancer.

At the same time, because the paint used in the spray paint industry has adhesiveness, such as high concentration paint mist, if it is not managed, it spreads with the wind, adheres to the building, affects its beauty, and settles in the flowers and trees, which will affect the plants. Photosynthesis.

Third, the organic waste gas source is strong

According to the relevant skill data supplied by a spray paint professional enterprise, the amount of exhaust gas discharged from the discharge point and the concentration of benzene, toluene and xylene in the exhaust gas are shown in Table 1:

Fourth, the planning policy

After the management of the organic waste gas of this project, it can reach the “Specifications for Volatile Organic Compounds Emissions from Exterior Painting (Car Manufacturing Industry)” (DB44/816—2010), specific indicators of emission standards.

Fifth, the selection of professional waste gas treatment process for spray paint

1. Activated carbon adsorption method

Activated carbon adsorption is now the most widely used recovery technique. It was originally a porous structure using adsorbents (granular activated carbon and activated carbon fibers) to capture VOCs in the exhaust gas. The organic waste gas containing VOCs is passed through the activated carbon bed, and the VOCs are adsorbed by the adsorbent, and the exhaust gas is purified and discharged into the atmosphere. When the carbon adsorption reaches fullness, the full carbon bed is desorbed and regenerated; the water vapor is heated to pass through the carbon layer, and the VOCs are blown off and discharged, and form a vapor mixture with the water vapor, and together with the carbon adsorption bed. The vapor mixture is cooled by a condenser to condense the vapor into a liquid. If the VOCs are water soluble, the liquid mixture is purified by rectification; if it is water insoluble, the VOCs are directly withdrawn by a decanter. Carbon adsorption skills are primarily used when VOCs are less than 5000 ppm. Suitable for painting, printing and adhesives where the temperature is not high, the humidity is not large, and the exhaust volume is large.

Activated carbon adsorption systems are commonly used to treat gases with a gas content of 200 to 100,000 m/h and a concentration ranging from 2 to 5,000 ppm. The scale of the equipment depends on the volume and concentration. The investment cost of the activated carbon adsorption system is not high, the operation is flexible, and the operating cost is generally lower than other methods. Regeneration can be carried out using hot air, steam or hot nitrogen. The concentrated pollutant gas generated by regeneration needs to be further treated by condensation, thermal incineration, catalytic incineration, and the like. Since the concentrated gas stream is processed at a small volume at this time, the cost of the secondary treatment is greatly reduced. Activated carbon will continuously lose its adsorption capacity during the process of application and regeneration, so it needs to be replaced after a certain time.

Fibrous activated carbon exhibits good adsorption and desorption properties because its micropores directly face the gas flow, so a shorter adsorption and desorption cycle can be selected. In addition, because the carbon fiber is lower than the metal content of the general activated carbon (about 50% to 90% less), the catalytic effect on the halogenated hydrogen is small, and it does not constitute a hydrolysis effect which causes corrosion, and there is no carbon bed boring. Burning phenomenon.

2. Absorption method

The absorption method uses the dissolution of a liquid (absorbent) to remove soluble components from the exhaust. In the pollution control, the absorption operation is selected to remove some polluting gas components to achieve the purpose of reducing pollution emissions. The exhaust gas accompanying the solute enters from the bottom of the absorption tower, and the absorbent enters from the top of the tower for absorption touch. The gas flowing out from the top of the tower is a treated clean gas, which can be discharged or guided by other units, and the liquid flowing out from the bottom of the tower is sent to the regeneration unit for regeneration.

The absorption tower can be classified into three types according to the type of gas-liquid interface that occurs, namely, a Film Absorber, a Jet Absorber, a Bubble and a Drop-Absorber. As for the absorption equipment commonly used in industry, there are several types such as a spray tower, a text scrubbing tower, a packed tower, and a plate tower. Generally, a packed tower or a plate column for treating exhaust gas containing organic matter is used. The packed column is generally used to treat liquids that contain corrosive substances or have a tendency to foam/block, or that an overpressure drop occurs when a plate column is used. Plate towers are often used for in-house cooling (Intemal Cooling) or because the absorbent flow is so low that it is not completely wet-filled and is therefore typically used in large-scale absorption operations. After the exhaust gas enters the absorption tower, the absorbent touches the exhaust gas from top to bottom, and absorbs the components soluble in the absorbent in the exhaust gas, and the clean gas that is treated later flows out from the top of the tower. The absorbent flowing out from the bottom of the tower can be recovered by steam stripping and then recycled into the packed tower for application or into the treatment plant.

3. Condensation method

Condensation is a physical procedure that reduces the temperature of the exhaust gas below the boiling point to condense gaseous substances. It can be used as a preliminary manipulation of volatile organic compounds throughout the separation and purification of raw materials or products. The condensation power is between 50% and 85%. Condensation is most suitable for the discharge of large frozen storage tanks, because condensation only changes the phase of the material without affecting its composition, and the procedure is simple, if the organic matter in the discharge airflow High purity, no need for fine control. The upper limit of the discharge gas flow rate is about 55 Nm/min. When the flow rate is too high or the gas stream contains a lot of gas that cannot be condensed, the heat exchange area of the condenser is too large and does not meet the economic value.

The most commonly used condenser for organic matter is a shell-and-tube heat exchanger (Shen-Tube Heat Exchanger). The condensed liquid (refrigerant) passes through the tube, and the gas passes through the shell portion and condenses outside the tube. The non-touch shell-and-tube condenser is the most commonly used equipment for organic vapor condensation. The choice of condensing gas varies depending on the condensation temperature. Cooling water is the lowest-priced coolant, but its use is temperature-constrained. The condenser for organic matter discharge treatment rarely uses cooling water because the pressure of the exhaust gas is about normal pressure (1 atm), and the condensation point of volatile organic compounds is generally lower than the temperature of the cooling water (25 degrees Celsius to 4 JD degrees). ) It is necessary to use ice water, ice brine, or a refrigerant such as a chlorofluorocarbon.

4. Thermal incineration

The principle of thermal incineration is to incinerate the exhaust gas directly under a high-temperature flame, and the volatile organic matter contained therein is converted into carbon dioxide and water through intense oxidation. The heat incinerator generally has a refractory material and an incinerator at the furnace end. The inflowing gas and the incineration air are thoroughly mixed in the premixing chamber and then incinerated in the incinerator. The set temperature and residence time of the furnace are the primary factors determining the power removal. It varies with the composition and content of the exhaust gas. Generally, the furnace temperature is about 700-800 ° C and the residence time is about 0.5 second. When the oxygen content in the gas to be treated is below 10% or the concentration of carbon monoxide is too high, a treatment temperature of 800 ° C or higher is required.

The biggest advantage of thermal incineration is that it removes volatile organic compounds (up to 98% for normal operation) and is easy to handle. The defects are high temperature operation, large fuel consumption, considerable fuel cost, and the second pollution of nitrogen oxides (NOx) due to high temperature incineration; if the treatment gas contains sulfur, halogen and other components, it is incinerated and oxidized. When harmful gases such as sulfur oxides (SOx) and halides are discharged at the same time, attention should be paid to the planning of exhaust gas treatment equipment and furnaces. It is generally necessary to cooperate with equipment such as scrubbing towers to prevent secondary pollution emissions. For safety reasons, the concentration of VOCs in the discharge fluid is generally limited to 25% of the LEL (low explosion limit). Therefore, when the concentration of VOCs is high, it is necessary to dilute.

5. Biological treatment

The air pollution caused by organic odor and volatile organic substances is treated with biological skills, with high power and low cost, and is highly competitive in the market. Suitable for VOCs-containing exhaust gas treatment

(1) Biological filter bed method

The biological filter bed method is transported from the gas phase to the wet biofilm, and the microorganisms fixed on the surface of the filler undergo oxidative differentiation to convert the pollutant into water, carbon dioxide and harmless salts, so there is no secondary pollution problem. The primary filling material of the filter bed includes the solids to which the biofilm is attached, and further includes a nutrient salt, a dispersibility improver, and a pH buffer. The biological filter bed method has the advantages of low equipment and operation cost, but the filter material is prone to drying and acidification, and must be replaced and durable, which is a defect.

(2) Biological trickling filter method

The biological drip filter method has a biological action principle similar to that of the biological filter bed method, but the filler in the tower is mostly wood, ceramics and plastics, and a circulating water system is also included to control the pH change and remove the generated salts. After the pollutants are absorbed and collected by the circulating liquid and transported into the biofilm, the microorganisms immobilized on the biofilm differentiate. Because the operating conditions of the biological trickling filter method are relatively messy and need to actively monitor the system control, the equipment cost is high. However, the filter bed made of raw materials such as plastics will make the pressure loss lower than the filter bed method, and there is no replacement problem of acidification or drying of the filter material, which is its advantage. However, when the organic load is too high, it is easy to constitute a problem of biological anaerobic conditions and spalling, which is a defect. Therefore, the petrochemical plant can further evaluate the feasibility of biological treatment skills when selecting and treating low-concentration organic waste gas.

Sixth, the process selection

The above various organic waste gas removal skills, equipment costs, possible secondary pollution and excellent defects are listed in Table 6:

According to the actual situation of the project, because the organic waste gas discharged from the spray paint treatment project has a large recovery value, the damage method is not selected. The activated carbon adsorption method has been used for the recovery of many VOCs, and the recovery of organic waste gas in the shoes, painting, printing, electronics and other occupations is very effective.

It can be seen from Table 6 that the processing plan of VOCs depends on many factors, including the nature and concentration of VOCs, the flow rate of incoming logistics, and the recovery value of pollutants such as power recovery. Regarding the sprayed paint exhaust gas with a relatively messy composition and a large amount of air, activated carbon adsorption is now a better method.

The selection of activated carbon adsorption method for the treatment of spray paint organic waste gas has the following characteristics:

1. Activated carbon adsorption method has a wide range of applications, for most organic substances

It has a high adsorption rate and is used for the treatment of exhaust paint with a large amount of exhaust gas.

The power is higher.

2. Investment costs and operating expenses are not high, and maintenance is simple.

3. The operation is simple, safe and stable.

Conclusion

After analyzing the characteristics of the professional exhaust gas of spray paint, compare the principle, application range and excellent defects of various organic waste gas treatment skills. Relatively speaking, activated carbon adsorption has a good treatment effect on the treatment of spray paint professional organic waste gas, which is a kind of comparison. A mature, useful approach.

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Spray paint professional waste treatment plan

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Spray paint professional waste treatment plan

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