Treatment of exhaust gas pollution in steel drum coating production
Yang Wenliang
In China, since the emergence of steel drum packaging, its coating and coating technology has been continuously developed along with the development of steel drums. For many years, coatings have changed from vegetable oils and natural resins to modern synthetic polymers. A mixture of oil or synthetic oil, organic or inorganic chemical pigments, organic solvents, etc., most of which are toxic. People have continuously used advanced science and technology to improve the pollution of paints and coatings on the production environment and the health hazards to producers. In the past few decades, they have also achieved some fruitful technical achievements, such as a large number of low-pollution replacement coatings and high Solid coatings, slurry coatings, non-aqueous dispersion coatings, water emulsion coatings, water-soluble coatings, solvent-free coatings, solvent-free powder coatings, radiation-curing coatings and plastic coatings, as well as developments in coating production technology The high-pressure airless spraying method and equipment, electrophoretic spraying method and equipment, electrostatic spraying method and equipment, powder coating method and equipment, etc., have improved environmental pollution and health hazards from various aspects, and initially realized Efficient, low-cost, energy-saving development goals. However, looking at the production structure of modern coatings, the production and use of organic solvent-based coatings still dominates. In the environmental pollution caused by steel drum coating production, exhaust gas is the most serious pollution. Today, as global environmental protection calls are rising, the pollution of exhaust gas in steel drum coatings is also becoming more serious, and measures are taken actively.
First, the production of exhaust gas in the production of steel drums.
The exhaust gas generated in the production of steel drum is mainly a large amount of organic solvent volatilized gas and scattered paint mist; the surface treated acid gas and the trichloroethylene volatilized gas before painting.
Different types and varieties of paint thinners used in steel drum coating have their own coating characteristics and coating methods, and the polluting exhaust gas is also very different. The whistle-based paint used in the spraying method, the perchloroethylene-based paint, the nitro-based paint used in the different primer coating methods, the different primers, and other flux coatings with low synthetic film solids. There are a lot of organic solvents and paint mist scattered in the air, which is harmful. In general, the solvent-based coatings are less polluted by the electrostatic spraying method, and the exhaust gas pollution caused by all the spraying methods is very serious. Although some steel drum plants have adopted good ventilation and other equipment, such as spray booths or water curtain spray booths, many effective emission and recycling treatments. However, due to the poor effect of the organic solvent and the overspray mist due to the poor recycling process, there are still considerable levels of organic solvents and aerosols such as flying paint to pollute the environment and endanger the health of the operator. What's more, some steel drum manufacturers have poor conditions, coating methods, equipment tools, painting environment and operation level are not high, and the environmental protection measures and effects are not good, which will cause serious environmental pollution and harm.
In the drying process of the steel drum, due to the different coatings used, the drying equipment is different, and the exhaust gas with different properties is volatilized during drying. The volatile organic solvents that enter the air during the coating and drying process are mainly harmful toxic gases such as toluene, xylene, fats, ketones, alcohols, small amounts of aldehydes and amines.
Before the coating, the surface treatment liquid needs to be in a certain temperature to react with the oil stain, rust and dirt on the surface of the steel drum to complete the treatment requirements. The phosphating film, the passivation film, and the oxide film are also required to be processed at a certain temperature. At the high, medium and bottom temperatures, the treatment reaction and action will have inorganic waste gas discharged, accompanied by the evaporation of the treatment liquid, which pollutes the environment and endangers health.
Second, the treatment of waste gas in steel drum coating production
The exhaust gas in the steel drum coating has the exhaust gas generated during the surface treatment before the coating and the exhaust gas generated during the coating process, but mainly the exhaust gas during the coating process. The exhaust gas in the coating process is mainly an organic solvent which is largely volatilized or evaporated during the drying process and the spraying process.
1. Adsorption governance method
The adsorption treatment method is suitable for the treatment of exhaust gas generated during the spraying process.
Due to the unbalanced and unsaturated molecular gravitational or chemical bond forces on the solid surface, when it is in contact with the gas, it can attract the gas molecules to concentrate on the solid surface and maintain it. This phenomenon is called adsorption. . The adsorption method is a method that utilizes the property of the solid surface to make the exhaust gas contact with the large surface porous solid substance, adsorbing the harmful components in the exhaust gas on the solid surface, and separating it from the gas to achieve the purpose of purification. The solid substance having adsorption is called an adsorbent, and the adsorbed gas component is called an adsorbate.
The adsorption process is a reversible process. While the adsorbate is adsorbed, some of the adsorbed adsorbate molecules can also be separated from the solid surface and returned to the gas phase due to the thermal motion of the molecule. This phenomenon is called desorption. When the adsorption rate is equal to the desorption rate, the adsorption equilibrium is reached, the adsorption performance process stops, and the adsorbent loses its adsorption capacity. Therefore, when the adsorption is carried out to a certain extent, in order to recover the adsorbate and restore the adsorption capacity of the adsorbent, it is necessary to adopt a certain method to release the adsorbate from the adsorbent, which is called regeneration of the adsorbent. Adsorption treatment of exhaust gas should include the entire process of adsorption and regeneration of the adsorbent.
According to the different adsorption forces, adsorption can be divided into physical adsorption and chemical adsorption. The former is the result of the interaction of intermolecular forces, while the latter is the result of the formation of chemical bonds between solid surface molecules and gas molecules. Most of the current adsorption treatments use physical adsorption.
The adsorption purification method has high purification efficiency, and in particular, it has a strong purification ability for low concentration gases. If the degree of purification is simply stated, as long as the amount of adsorbent is sufficient, any desired degree of purification can be achieved. However, due to the need for repeated regeneration of the adsorbent and the limitation of the adsorption capacity of the adsorbent, the application of the adsorption method is limited. For the treatment of exhaust gas in steel drum coating, the adsorption method is more suitable because the concentration is not high, the service life of the adsorbent is long, the operation cost is not high, and the procedure is relatively simple.
How to properly select and utilize high-efficiency adsorbents plays a key role in improving the effect of the adsorption method. Generally, the adsorbent used in the adsorption method should have the following characteristics:
· Large specific surface and porosity:
·Good selectivity;
· Strong adsorption capacity and large adsorption capacity;
· Easy to regenerate:
·High mechanical strength, strong chemical stability, good thermal stability, wear resistance and long life:
· Cheap and easy to get.
Some adsorbents with high adsorption efficiency, such as activated carbon and molecular sieves, used in the adsorption method are generally expensive, so the spent adsorbent must be regenerated and reused to reduce the cost of the adsorption method. Commonly used regeneration methods include thermal regeneration (or temperature-rise desorption), depressurization regeneration (or decompression desorption), purge regeneration, chemical regeneration, and the like. The operation of regeneration is troublesome, and steam or hot air must be specially supplied to meet the needs of adsorption regeneration, which increases the equipment cost and operation cost, and limits the wide application of the adsorption method.
2. Absorption and treatment method
The absorption method is suitable for treating the exhaust gas generated during the spraying process of the steel drum.
When the gas and the liquid are in contact, the solubility of different components in the gas in the same body fluid is different, and one or several components with high solubility in the gas can be introduced into the liquid phase to make various groups in the gas phase. When the relative concentration changes, the gas can be separated and purified. This process is called absorption. According to this principle, the absorption method is a purification method in which an appropriate liquid is used as an absorbent to bring an exhaust gas containing harmful substances into contact with it, whereby harmful substances in the exhaust gas are absorbed and purified by the absorbent gas. In the absorption method, the liquid after absorption of the absorbed substance is called an absorption liquid.
In the absorption process, the absorption can be divided into physical absorption and chemical absorption depending on whether the absorption of the absorbent and the chemical reaction occurs. The former occurs in the absorption process is a pure physical process, such as water absorption, CO 2 or absorption of S02; and the latter is often accompanied by obvious chemical reactions in the absorption process, such as absorption of CO 2 by lye absorption, absorption of ammonia by acid solution, etc. . The existence of the chemical reaction increases the absorption driving force of the mass transfer coefficient and increases the absorption rate. Therefore, when the gaseous pollutant is treated by the absorption method, the chemical absorption method is often used.
The choice of absorbent in the absorption method directly affects the absorption effect. In general, the absorbent selected should have the following characteristics.
· The absorption capacity is large, that is, the amount of harmful gas absorbed in the unit volume of the absorbent is large;
·High selectivity, that is, strong absorption capacity for harmful gases;
· The saturated vapor pressure is low to reduce the loss of the absorbent due to volatilization;
• The boiling point should be suitable, especially when it is necessary to remove impurities accumulated in the absorbent by distillation, too high a boiling point will bring operational difficulties to the distillation.
· Some other properties. If the viscosity is small, the thermal stability is high, the corrosion is small, and it should be cheap and easy to obtain.
According to the above principle, the absorbent commonly used in the absorption method includes water, which is suitable for removing harmful gases dissolved in water, such as hydrogen chloride, ammonia, sulfur dioxide, hydrogen fluoride, etc.: alkali liquid (such as caustic soda solution, lime milk, ammonia water, etc.) is suitable for Removal of acid gases such as sulfur dioxide, nitrogen oxides, hydrogen sulfide, etc.; acid solutions (such as sulfuric acid solution) are mainly used for the removal of basic gases such as ammonia; and organic solvents such as propylene carbonate, N-methylpyrrolidone and cold Methanol is used as an absorbent to effectively remove carbon dioxide and hydrogen sulfide from the exhaust. The use of a mixture of diesel and water as an absorbent can largely treat the dimethyl solvent and the like in the exhaust gas.
The absorption method generally adopts a countercurrent operation, that is, in the absorption device, the absorbed gas flows from the bottom to the top, and the absorbent flows from the top to the bottom, and the mass transfer process is completed in the gas-liquid countercurrent contact. It can be divided into two types: non-circulating process and cyclic process. The former absorbent is not regenerated, and the latter absorbent is closed and used. Commonly used equipment for absorption methods include packed towers, plate towers, spray towers, venturi scrubbers, and rotary spray towers.
Practice has proved that the absorption method has the characteristics of high efficiency, simple equipment, wide application range and low one-time investment. It can be widely used in the production of steel drums to transfer harmful gas substances into the liquid phase. Therefore, the absorption liquid must be treated. Otherwise it is easy to cause secondary pollution. At the same time, the absorption method is not suitable for the treatment of flue gas at high temperatures, so when treating harmful gases in the drying furnace, the exhaust gas must be cooled and pretreated. Commonly used cooling means include indirect cooling, direct cooling, and setting of pre-scrubbers.
3. Catalytic combustion treatment
The exhaust gas generated by the drying chamber (furnace) is mainly subjected to catalytic combustion. The catalytic combustion method is an organic combination of a catalytic method and a combustion method.
Catalytic purification of gaseous contaminants is a method that utilizes the catalytic action of a catalyst to convert a chemical reaction of harmful components in the exhaust gas into a harmless substance or a substance that is easy to handle and recycle. When a chemical reaction is carried out, a small amount of a certain substance is added to the reaction system, the rate of the reaction is accelerated, and at the end of the reaction, the amount and nature of the substances hardly change, and the added small amount of the substance is called For Cuihua, this effect of the catalyst is called catalysis.
The catalyst is generally a complex system composed of a variety of substances. According to the different functions of the components, the catalyst is mainly divided into: 1. The active component is a key component of whether the catalyst has an acceleration effect on a certain reaction; It is a support for dispersing and supporting active components, which can be added to the reaction surface of the catalyst and improve the performance of the catalyst. 3. The promoter is an additive for improving the activity and thermal stability of the catalyst.
The activity of the catalyst is manifested as a significant change in the reaction rate, and has the following characteristics. 1. The catalyst can only shorten the time when the reaction reaches equilibrium, and it can not make the equilibrium move, and it is even less likely that the thermodynamically unreactive reaction can proceed.
2. Catalyst performance is selective, ie a specific catalyst can only catalyze a specific reaction.
3. Each catalyst has its specific active temperature range, below the lower limit of the activity temperature, the reaction rate is slow, or does not work.
4. Each catalyst has the characteristics of poisoning and aging. Usually, the strong adsorption bond of a certain amount of building materials is preferentially adsorbed on the active surface, and the strong adsorption bond of this small amount of impurities is preferentially adsorbed on the active surface. And this process is often irreversible, leading to poisoning. Aging is mainly due to the loss of surface sintering and mechanical damage of low-melting active components.
Therefore, screening suitable catalysts is the key to the catalytic process. Generally, the selected catalysts have good activity and selectivity, sufficient mechanical strength, good thermal stability and chemical stability. The catalyst is used to treat the exhaust gas in the drying chamber of the steel drum. The commonly used catalysts are Pt, Pd, Rh, Cu0, Cr203, Mn203 rare earth metal oxides.
Catalytic methods include both catalytic oxidation and catalytic reduction. The method has high purification efficiency, and the purification efficiency is less affected by the concentration of pollutants in the exhaust gas. Moreover, in the process of treatment, it is not necessary to separate the pollutants from the main gas stream, and the harmful substances of the main gas stream can be directly converted into harmless substances. Avoid secondary pollution. However, the price of the catalyst used is often relatively expensive, the operation requirements are high, and the harmful substances in the exhaust gas are difficult to be recovered as useful substances, which is a disadvantage of the method.
The combustion method is a violent chemical reaction process accompanied by light and heat. In the presence of oxygen, when the concentration of combustible components in the mixed gas is within the limits of combustion, the combustible component can be burned by an open flame. That is, after the mixed gas containing combustible harmful components is heated to a certain temperature, the components react with oxygen to be burned, or oxidatively decompose at a high temperature, thereby converting these harmful components into harmless substances. The method is mainly applied to the purification treatment of harmful substances such as hydrocarbons and carbon monoxide.
The catalytic combustion method is that in the presence of a catalyst, the combustible component in the exhaust gas can be subjected to a combustion reaction at a lower temperature, which can save fuel preheating, increase the reaction speed, and reduce the volume of the reactor. Increasing the relative conversion of one or more reactants to another reactant or reactants.
The main advantages of catalytic combustion are low operating temperature, low fuel consumption, less stringent insulation requirements, and reduced tempering and fire hazards. However, the catalyst is more expensive, requires regeneration, and has a high capital investment.
In short, the catalytic combustion process is relatively simple, easy to operate, can recover the heat after combustion, but can not recover useful substances, and easily cause secondary pollution.
The environmental pollution of exhaust gas in steel drum production is serious and it is very harmful to operators. We must take all kinds of effective measures to comply with the state's laws on environmental protection, industrial enterprise health law and labor safety and health law, and carry out comprehensive governance. The environmental pollution of the painted exhaust gas and the harm to human health are minimized or eliminated fundamentally. This is not only the only way for China's packaging industry to become green, but also a major event related to the national economy and modernization and human survival. It is also an important guarantee for the smooth progress and continuous development of steel drum coating production. One.
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