Waste Gas Treatment

Coating refers to covering metal and non-metal surfaces with protective or decorative layers, and 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.Pretreatment generally includes processes such as degreasing, derusting, coating, washing, and drying.In the pre-treatment rust removal process, sandblasting, shot blasting or grinding processes are sometimes used.Coating waste gas mainly comes from pretreatment, spraying, drying (curing) processes, and is mainly concentrated in the painting production line, of which the paint spraying room and drying room are the main sources of waste gas.The pollutants emitted during the painting process are mainly: dust or acid mist generated during the pretreatment process, paint mist and organic solvents generated during painting, and organic solvents generated during drying and volatilization.The paint mist mainly comes from the part of the solvent-based paint scattered in the air spraying operation, and its composition is consistent with the paint used.Organic solvents mainly come from solvents and thinners during the use of paint, and most of them are volatile emissions. The main pollutants are benzene, toluene, xylene, etc., these three are organic compounds, commonly known as 'triphenyl '.In addition, there is a certain concentration of organic solvents such as alcohol ethers and esters.

Benzene is a sweet, flammable, carcinogenic and toxic colorless transparent liquid at room temperature with a strong aromatic odor.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 enters the human body mainly through respiratory inhalation (47-80%), gastrointestinal and skin absorption.A part of benzene can be excreted through urine, and the metabolites of unexhausted benzene enter cells and combine with deoxyribonucleic acid (DNA) in the nucleus, causing chromosome mutations and leading to cancer.Long-term exposure to benzene can damage bone marrow, reduce the number of red blood cells, white blood cells, and platelets, and cause chromosomal aberrations, leading to leukemia and even aplastic anemia.Benzene can cause profuse bleeding, thereby suppressing the function of the immune system and making it possible for the disease to take advantage.Some research reports point out that the incubation period of benzene in the body can be as long as 12-15 years.Inhalation of benzene by pregnant animals can lead to underweight larvae, delayed bone development, and bone marrow damage.On October 27, 2017, the list of carcinogens released by the International Agency for Research on Cancer of the World Health Organization was initially compiled for reference, and benzene was included in the list of category 1 carcinogens.

Toluene, liquid at room temperature, colorless, with special aroma, volatile.Has an anesthetic effect and is irritating to the skin.When inhaling toluene vapor, 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.Long-term inhalation of low-concentration toluene vapor can cause chronic poisoning, loss of appetite, fatigue, leukopenia, and anemia.Toluene can also be absorbed through the skin to 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 list of carcinogens released by the International Agency for Research on Cancer of the World Health Organization was initially compiled for reference, and toluene was included in the list of category 3 carcinogens.

Xylene is also a colorless transparent liquid with a special smell 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, and at high concentrations, it has an anesthetic effect on the central system.Short-term inhalation of higher concentrations of this product may cause obvious eye and upper respiratory tract irritation, conjunctival and pharyngeal congestion, dizziness, headache, nausea, chest tightness, weakness of limbs, confusion, unsteady steps, and in severe cases, restlessness, convulsions or coma.Long-term exposure to xylene can cause neurasthenia, and women may have abnormal menstruation.Skin contact may cause skin dryness, chapping, and dermatitis.On October 27, 2017, the list of carcinogens released by the International Agency for Research on Cancer of the World Health Organization was initially compiled for reference, and xylene was included in the list of three types of carcinogens.

Treatment method of painting exhaust gas

1.Treatment of normal temperature waste gas from spray painting

The waste gas from the spray booth, drying room, paint mixing room and finish paint sewage treatment room is low-concentration, high-flow normal-temperature waste gas. The main components of pollutants are aromatic hydrocarbons, alcohol ethers and ester organic solvents. If high-altitude The method of emission does not exceed the air pollution emission standard in the short term. However, in the long run, some large-scale coating production lines may emit hundreds of tons of gas pollutants every year, which will cause very serious harm to the atmosphere. .Therefore, when designing a production line, for the sake of environmental protection and the long-term development of human beings, it is necessary to provide a scientific and reasonable waste gas treatment plan.At present, the relatively mature method in foreign countries is to first concentrate the organic waste gas (concentrate the total amount by about 15 times with an adsorption-desorption runner) to reduce the total amount of organic waste gas to be treated, and then use destructive methods to treat the concentrated waste gas. to process.There is also a similar method in China. First, the adsorption method (activated carbon or zeolite as the adsorbent) is used to adsorb low-concentration, room-temperature spray paint exhaust gas, and high-temperature gas is used for desorption. The concentrated exhaust gas is processed by catalytic combustion or regenerative thermal combustion. deal with.The biological treatment method of low-concentration, room-temperature spray paint exhaust gas is under research and development. The current domestic technology is not yet mature, but it is worthy of attention.In order to truly reduce the pollution caused by painting exhaust, it is necessary to solve the problem from the source, such as using electrostatic rotary cups and other means to improve the utilization rate of paint, and developing environmentally friendly paint such as water-based paint.

Activated carbon adsorption method

This method mainly uses the excellent adsorption performance of activated carbon to make the organic solvent vapor adsorb on its surface. When the adsorption medium is heated and dried, the adsorbed gas is decomposed, cooled to a liquid state, and then separated to achieve the purpose of solvent recovery.

The activated carbon adsorption method needs to be pretreated with filters and coolers to remove the paint mist in the exhaust gas and reduce the exhaust gas to an appropriate temperature to ensure that the activated carbon will not be blocked and will not burn due to excessive exhaust gas temperature.Since activated carbon absorbs organic solvents, its adsorption force will gradually decrease. In order to ensure the adsorption efficiency, desorption is required to restore the activity of activated carbon.The most commonly used activated carbon regeneration method is the water vapor desorption method. The desorbed mixed gas enters the condenser to be cooled into a liquid, and then enters the separator to separate the solvent and water to achieve the purpose of solvent recovery, and the separated water needs to be treated before it can be used. emission.At present, there is a new development in the activated carbon adsorption method, that is, activated carbon fibers are used instead of the commonly used granular or columnar activated carbon, and its service life is 3-4 times longer than that of ordinary granular activated carbon.A device that uses activated carbon fiber (ACF) as an adsorption medium to recover organic solvents has also appeared in China.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, and can increase equipment and use fractionation by itself or entrust solvent manufacturers to fractionate.

The latter needs to add 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. There are disposal requirements for the separated water during solvent recovery.The latest development in the world is to use activated carbon fiber cloth (ACFC) as the adsorption medium, and to desorb and recover organic solvents by heating with electricity.This method has the advantages: the mass transfer of the organic solvent between the solid phase and the fluid phase is fast, 2-20 times faster than granular activated carbon; the ACFC has a long service life; the ACFC can be heated rapidly and does not require water vapor desorption, so the operation And maintenance is simple, the desorbed organic solvent does not need to remove water, can be directly applied, and there is no problem of wastewater treatment.

2. Drying waste gas treatment

Drying waste gas belongs to high-temperature waste gas with medium and high concentration, which is suitable for treatment by combustion.There are three important parameters in the combustion reaction: time, temperature, and disturbance, that is, the combustion 3T condition.The efficiency of exhaust gas treatment is essentially the adequacy of the combustion reaction, which depends on the 3T condition control of the combustion reaction.At present, as an effective waste gas treatment technology, the RTO combustion system is adopted by many large-scale coating production line manufacturers.

RTO combustion system

RTO combustion system is the abbreviation of Regenerative Thermal Oxidizer Combustion System (renewable thermal oxidation combustion system/heat energy recovery 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 exhaust gas is first heated to close to the thermal oxidation temperature by the heat storage body, and then enters the combustion chamber for thermal oxidation. The temperature of the oxidized gas rises, and the organic matter is basically converted into carbon dioxide and water. The purified gas passes through another heat storage body, the temperature Decrease, and discharge 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, the heat recovery rate is greater than 95%, and no or a small amount of fuel is used to treat VOCs.To deal with low-concentration exhaust gas, an optional enrichment device can reduce fuel consumption.According to customer needs, two-chamber, three-chamber, multi-chamber or single-chamber rotary RTO devices can be designed.

Advantages of RTOs:

The requirements for working conditions are low, and the exhaust gas can contain a variety of organic components;

Large air volume range: 1000~300000Nm³/h;

High purification rate, the purification rate of the two-bed RTO is above 95%, and the purification rate of the three-bed RTO is above 98%;

Fully automatic control, simple operation and low operating cost;

The operating cost is low, and 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 VOCs removal rate of the two-chamber RTO device is 95% to 98%, and the VOCs removal rate of the three-chamber RTO device 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 costs.It is suitable for the same production line, because of different products, the composition of exhaust gas often changes or the concentration of exhaust gas fluctuates greatly.Application industries include the exhaust gas treatment of coating lines in factories such as automobiles and motorcycles, shipbuilding, bicycles, home appliances, containers, etc., petrochemical, rubber, paint, viscose, printing, enameled wire and other production lines, especially for enterprises that need heat recovery. Energy recovery is used in the drying line, thereby achieving the purpose of saving energy.

The combustion medium of RTO combustion system can choose gas, natural gas, diesel oil, etc., and the one-time investment of RTO exhaust gas treatment equipment is relatively high.