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Product Specification

Painting waste gas and its hazards

Coating refers to covering the metal and non-metal surfaces with a protective layer or decorative layer. It is the most basic technical means for product surface protection and decoration. The coating process can be simply summarized as: pre-treatment → spraying → drying or curing. Pre-treatment generally includes degreasing, rust removal, film, water washing, drying and other processes. In the pre-treatment rust removal process, sandblasting, shot blasting or grinding processes are sometimes used. Coating waste gas mainly comes from the pre-treatment, spraying, drying (curing) process, mainly concentrated in the spray paint production line, among which the spray paint room and drying room are the main sources of waste gas. The pollutants emitted during the coating process are mainly: dust or acid mist generated during the pre-treatment process, paint mist and organic solvents generated during spraying, and organic solvents generated during drying and volatilization. Paint mist mainly comes from the part of solvent-based paint flying in the air spraying operation, and its composition is consistent with the paint used. Organic solvents mainly come from solvents and diluents in the use of paints, most of which are volatile emissions. The main pollutants are benzene, toluene, xylene, etc. These three are organic compounds, commonly known as "triphenylbenzenes". In addition, there are organic solvents such as alcohol ethers and esters at certain concentrations. Benzene is a sweet, flammable, carcinogenic, colorless, transparent liquid at room temperature with a strong aromatic smell. It is slightly soluble in water, easily soluble in organic solvents, and can also be used as an organic solvent itself. Due to the high volatility of benzene, it is easy to spread when exposed to air. Benzene mainly enters the human body through respiratory inhalation (47-80%), gastrointestinal and skin absorption. A part of benzene can be excreted through urine. After the unexcreted benzene metabolites enter the cells, they combine with deoxyribonucleic acid (DNA) in the cell nucleus, causing chromosome mutations and leading to cancer. Long-term exposure to benzene can damage the bone marrow, reduce the number of red blood cells, white blood cells, and platelets, and cause chromosome abnormalities, leading to leukemia and even aplastic anemia. Benzene can cause heavy bleeding, thereby inhibiting the function of the immune system and giving the disease an opportunity to take advantage. Research reports have pointed out that the incubation period of benzene in the body can be as long as 12-15 years. When pregnant animals inhale benzene, it will cause the young to be underweight, delayed bone development, and bone marrow damage. On October 27, 2017, the World Health Organization's International Agency for Research on Cancer published a preliminary list of carcinogens for reference, and benzene is on the list of Class 1 carcinogens. Toluene is a liquid at room temperature, colorless, with a special aromatic smell, and volatile. It has an anesthetic effect and irritates the skin. When toluene vapor is inhaled, it has a strong stimulating effect on the central nervous system. When a certain concentration of toluene vapor is inhaled for a short time, it will cause excessive fatigue, intense excitement, nausea, headache, etc. When a low concentration of toluene vapor is inhaled for a long time, it will cause chronic poisoning, loss of appetite, fatigue, leukopenia, and anemia. Toluene can also be absorbed through the skin and dissolve the fat in the skin, so direct contact with the skin should be avoided. Toluene has the characteristics of acute toxicity, irritation, chronic toxicity, mutagenicity, and teratogenicity. On October 27, 2017, the World Health Organization's International Agency for Research on Cancer published a preliminary list of carcinogens for reference, and toluene is on the list of Class 3 carcinogens. Xylene is also a colorless and transparent liquid with a special odor of aromatic hydrocarbons. It is widely used as a solvent in coatings, resins, dyes, inks and other industries. It is an important raw material for organic chemicals. Xylene is irritating to the eyes and upper respiratory tract. At high concentrations, it has an anesthetic effect on the central nervous system. Inhalation of high concentrations of this product in a short period of time may cause obvious irritation of the eyes and upper respiratory tract, conjunctival and pharyngeal congestion, dizziness, headache, nausea, chest tightness, limb weakness, confusion, unsteady steps, and in severe cases, agitation, convulsions or coma. Long-term exposure to xylene can cause neurasthenia and women may suffer from menstrual abnormalities. Skin contact can cause dry skin, chapped skin, and dermatitis. On October 27, 2017, the World Health Organization's International Agency for Research on Cancer published a preliminary list of carcinogens for reference, and xylene is on the list of Class 3 carcinogens.

Treatment methods for painting waste gas

1. Treatment of normal temperature exhaust gas from painting

The waste gas from the spray painting room, airing room, paint mixing room and topcoat wastewater treatment room is low-concentration, high-flow room-temperature waste gas. The main components of pollutants are aromatic hydrocarbons, alcohol ethers and ester organic solvents. If high-altitude discharge is adopted, it will not exceed the atmospheric pollution emission standard in the short term. However, in the long run, according to the total amount, some large-scale coating production lines may emit hundreds of tons of gaseous pollutants each year, which will cause very serious harm to the atmosphere. Therefore, when designing the production line, for the sake of environmental protection and the longer-term development of mankind, a scientific and reasonable waste gas treatment plan must be provided. At present, the more mature method abroad is to first concentrate the organic waste gas (using the adsorption-desorption wheel to concentrate the total amount by about 15 times) to reduce the total amount of organic waste gas to be treated, and then use a destructive method to treat the concentrated waste gas. There are similar methods in China. First, the adsorption method (activated carbon or zeolite as adsorbent) is used to adsorb the low-concentration, room-temperature spray painting waste gas, and the high-temperature gas is used for desorption. The concentrated waste gas is treated by catalytic combustion or regenerative thermal combustion. The biological treatment method for low-concentration, room-temperature spray paint waste gas is under development. The technology is not yet mature in China at this stage, but it is worth paying attention to. In order to truly reduce the pollution of painting waste gas, it is necessary to solve the problem from the source, such as using electrostatic spinning cups and other means to improve the utilization rate of paint, and developing environmentally friendly paints such as water-based paints.

Activated carbon adsorption method

This method mainly utilizes the excellent adsorption performance of activated carbon to make organic solvent vapor adsorb on its surface. When the adsorption medium is heated and dried, the adsorbed gas is analyzed and cooled into liquid, and then separated to recover the solvent. The activated carbon adsorption method requires the installation of filters and coolers for pretreatment to remove paint mist in the exhaust gas and reduce the exhaust gas to an appropriate temperature to ensure that the activated carbon is not blocked and will not burn due to excessive exhaust gas temperature. After the activated carbon adsorbs the organic solvent, its adsorption capacity will gradually decrease. In order to ensure the adsorption efficiency, it is necessary to desorb and restore the activity of the activated carbon. The most commonly used activated carbon regeneration method is the water vapor desorption method. The mixed gas after desorption enters the condenser to cool into liquid, and then enters the separator to separate the solvent and water to achieve the purpose of recovering the solvent. The separated water needs to be treated before it can be discharged. At present, the activated carbon adsorption method has a new development, that is, activated carbon fiber replaces the commonly used granular or columnar activated carbon, and its service life is 3-4 times longer than that of ordinary granular activated carbon. There are also devices in China that use activated carbon fiber (ACF) as an adsorption medium to recover organic solvents. There are two ways to use the recovered solvent: re-fractionation or combustion to generate heat. The former depends on the company's own situation. It can add equipment, self-fractionation or entrust the solvent manufacturer to fractionate. The latter requires adding a catalytic combustion chamber and a hot air circulation system to the equipment to catalytically burn the organic solvent desorbed from the activated carbon fiber at high temperature and provide energy for the operation of the equipment. Therefore, steam desorption of organic solvents is no longer required, and there is no requirement for separation of water during solvent recovery. The latest development in the world is to use activated carbon fiber cloth (ACFC) as an adsorption medium, and desorb and recover organic solvents by electric heating. This method has the advantages of: fast mass transfer of organic solvents between solid phase and fluid phase, 2-20 times faster than granular activated carbon; long service life of ACFC; ACFC can be heated quickly and does not require water vapor desorption, so it is easy to operate and maintain, the desorbed organic solvent does not need to be dehydrated, can be used directly, and there is no wastewater treatment problem.

2. Drying waste gas treatment

Drying waste gas is a medium- and high-concentration high-temperature waste gas, which is suitable for combustion treatment. Combustion reactions have three important parameters: time, temperature, and disturbance, which are also called the 3T conditions of combustion. The efficiency of waste gas treatment is essentially the fullness of the combustion reaction, which depends on the 3T condition control of the combustion reaction. At present, the RTO combustion system is adopted by many large coating production line manufacturers as an effective waste gas treatment technology.

RTO Combustion System

RTO combustion system is the abbreviation of Regenerative Thermal Oxidizer Combustion System (Renewable Thermal Oxidizer Combustion System/Renewable Thermal Oxidizer Combustion System). Its principle is that combustible organic waste gas undergoes thermal oxidation reaction at 760~1000 degrees Celsius to produce carbon dioxide and water. The waste gas is first heated to a temperature close to the thermal oxidation temperature by a heat storage body, and then enters the combustion chamber for thermal oxidation. The temperature of the oxidized gas increases, and the organic matter is basically converted into carbon dioxide and water. The purified gas passes through another heat storage body, and the temperature drops. It is discharged after reaching the national emission standard. The regenerative ceramic packed bed heat exchanger in the RTO device can recover heat energy to the maximum extent, with a heat recovery rate of more than 95%. No or a small amount of fuel is used when treating VOCs. If low-concentration waste gas is to be treated, a concentration device can be selected to reduce fuel consumption. According to customer needs, two-chamber, three-chamber, multi-chamber or single-chamber rotary RTO devices can be designed.

Advantages of RTO:

Low requirements on working conditions, the exhaust gas can contain a variety of organic components; Large range of air volume: 1000~300000Nm³/h; High purification rate, the two-bed RTO purification rate is above 95%, and the three-bed RTO purification rate is above 98%; Fully automatic control, simple operation, low operating cost; Low operating cost, when the VOC concentration reaches 400ppm, no additional fuel consumption is required.

Different combustion chambers

RTO devices include two-chamber, three-chamber and multi-chamber devices. The removal rate of VOCs in two-chamber RTO devices is between 95% and 98%, and the removal rate of VOCs in three-chamber RTO devices can reach more than 98%. Compared with traditional catalytic combustion and direct-fired thermal oxidation furnaces, RTO has the advantages of high thermal efficiency (greater than or equal to 90%) and low operating cost. It is suitable for the same production line, where the exhaust gas composition often changes or the exhaust gas concentration fluctuates greatly due to different products. The application industries include exhaust gas treatment of coating lines, petrochemical, rubber, coating, viscose, printing, enameled wire and other production lines in factories such as automobiles, motorcycles, shipbuilding, bicycles, home appliances, and containers. It is especially suitable for enterprises that need heat recovery, and can use energy recovery for drying lines to achieve the purpose of energy saving. The combustion medium of the RTO combustion system can be selected from gas, natural gas, diesel, etc., and the one-time investment of RTO exhaust gas treatment equipment is relatively high.