What are the common waste gas treatment methods?

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Common waste gas treatment methods:

1, absorption purification method

Absorption method is a common method to purify gaseous pollutants Z. Absorption method is defined as: treating waste gas with a suitable liquid absorbent, so that the gaseous pollutants in the waste gas can be dissolved in the absorption solution or chemically react with the active components in the absorption solution to enter the liquid phase, thereby separating the gaseous pollutants from the waste gas; In other words, the process of selective absorption and separation of one or more components (absorbent) in mixed gas by absorbent is called absorption.

Absorption is usually divided into physical absorption and chemical absorption, and the difference is shown in the following table:

2. Adsorption purification method

Adsorption is to treat the fluid mixture with porous solid adsorbent, so that one or more components contained in it can be adsorbed on the solid surface to achieve the purpose of separation. The difference between adsorption process and absorption is that after absorption, the absorption components are evenly distributed in the absorption phase, and after adsorption, the absorption components accumulate or concentrate on the adsorbent, which is only a heterogeneous process.

At present, adsorption operation has been widely used in organic chemical industry, petrochemical industry and other production departments. This method is also widely used in environmental engineering, mainly because of the high selectivity of adsorbent, which can separate the mixture that is difficult to separate in other processes, effectively remove (recover) the harmful substances with low concentration, with simple equipment, convenient operation, high purification efficiency and automatic control.

The adsorption process is a dynamic process. In this process, adsorbate diffuses from the fluid to the surface of adsorbent and the inner surface of micropores, releasing heat and being adsorbed on the surface of adsorbent. Desorption is the opposite of adsorption.

After adsorbate is adsorbed on the surface of adsorbent, the internal energy of adsorbate molecules decreases due to the changes of molecular motion forms, such as diffusion, vibration and rotation, thus releasing energy, which is called adsorption heat. Vaporization heat (or condensation heat) and combination heat are two components of adsorption heat. The adsorption heat is about 1.5 times larger than the vaporization heat, and the existence of special circumstances is not excluded. Generally speaking, the adsorption heat is affected by the adsorption capacity, adsorption temperature, and the empty velocity of fluid during adsorption. If the adsorption heat is not led out in time, part of the heat absorbed by desorption molecules will have a negative impact on the adsorption process.

3, condensation purification method

Condensation purification method is to make use of the different properties of saturated vapor pressure of substances at different temperatures, and use the methods of cooling and pressurizing to condense gas in vapor state and separate it from waste gas, so as to achieve the purpose of purification or recovery.

The removal degree of harmful gases by condensation purification is related to cooling temperature and saturated vapor pressure of harmful components. The lower the cooling temperature, the closer the harmful components are to saturation and the higher the removal degree. It is especially suitable for treating organic solvent vapor with waste gas concentration above 10000* 10-6, but not suitable for treating low-concentration waste gas. The condensation process can be realized by increasing the pressure at constant temperature, or by decreasing the temperature at constant pressure. Exhaust gas can be purified by condensation, but cooling water at room temperature can not meet the high purification requirements. In order to purify Q, it is necessary to cool and pressurize, which is more difficult and costly to treat. Therefore, adsorption, combustion and condensation are usually used as pretreatment of net high concentration organic gas to achieve the purpose of reducing organic load and recovering valuable products. In addition, condensation purification is generally only applicable to the case of high vapor concentration in the air, so the vapor concentration entering the condensation device can be above the explosion limit, and the concentration when the condensation device comes out can be below the explosion lower limit, just between the explosion upper limit and the explosion lower limit during condensation, which is not conducive to A- safety.

4. Catalytic purification method

Catalytic purification method is a purification method that makes gaseous pollutants pass through the catalyst bed and undergo catalytic reaction under the action of catalyst, and transform them into harmless substances or substances that are easy to treat and recycle. There are two kinds of catalytic purification methods: catalytic oxidation and catalytic reduction. Catalytic oxidation method: pollutants in waste gas are oxidized under the action of catalyst. For example, SO2 in exhaust gas can be decomposed into H2O and CO2 through the oxidation process of burning organic compounds in catalytic exhaust gas. Catalytic reduction method is a purification process in which pollutants in waste gas react with reducing gas under the action of catalyst. For example, nitrogen oxides in waste gas react with NH3 under the action of catalyst (copper chromium) to generate harmless gas N2. Catalytic purification is characterized by avoiding secondary pollution that may be produced by other methods, simplifying the operation process and having high conversion rate for pollutants with different concentrations. Its main application is to convert hydrocarbons into carbon dioxide and water, nitrogen oxides into nitrogen, and sulfur dioxide into sulfur trioxide for recycling, catalytic combustion of organic waste gas and odor, and catalytic purification of automobile exhaust. Its disadvantage is that the price of catalyst is high, and the preheating of exhaust gas consumes a certain amount of energy.

The pollutant content in waste gas is usually low, and it often has the following characteristics after catalytic purification treatment: 1) Because of the low pollutant content in waste gas, small thermal effect in the process and simple reactor structure, fixed bed catalytic reactor is often used. 2) The amount of waste gas to be treated is often large, which requires that the catalyst can withstand the influence of fluid scouring and pressure drop. 3) Due to high purification requirements, complex exhaust gas components and great changes in some reaction conditions, the catalyst is required to have high selectivity and thermal stability.

5. Biological methods

In Genf-Villette (place name, 1964, S has a biological purification device) D, the waste gas is purified once by the biological purification device. The technology of waste gas biological treatment developed rapidly in 1980s and 1990s, and the Netherlands and Germany became the G countries that applied waste gas biological treatment on a large scale. Subsequently, the application of biotechnology in waste gas treatment has become more and more extensive. At present, there are more than 7,500 biogas purification devices in Europe, half of which are used to treat sewage and compost odor. It has also formed an important system in the purification principle and engineering application experience of biodegradable gases. Biological purification technology makes up for the shortcomings of traditional physical and chemical treatment technology. Traditional methods need special safety operation procedure management (such as chemical absorption), which has high energy consumption and large economic investment. In contrast, biological purification is a clean treatment method, which has become the frontier and hot spot of waste gas treatment, especially biochemical waste gas treatment.

Biological waste gas purification technology is a multi-disciplinary environmental protection high-tech. Specifically, it is a mature method based on microbial treatment of wastewater and a frontier hot technology for purifying low-concentration industrial waste gas. The existing research in China shows that low-concentration industrial waste gas cannot be purified economically and effectively by conventional technology, while biological waste gas purification technology is effective and has obvious technical and economic advantages.

6. Membrane separation and purification

Membrane purification method is that when mixed gas permeates a specific membrane under the action of pressure gradient, different gases have different permeation speeds, so that different components in the gas mixture can be separated. Pressure difference, concentration difference and potential difference promote the membrane separation process. Membrane separation technology is a new separation technology which uses membrane to separate, purify and concentrate the mixture according to the difference of selective permeability of each component in the mixture. There are at least two interfaces between two separated phases or two parts, which can limit and transfer specific forms of fluid substances. These two interfaces are bridges for fluid contact and transfer between the two sides. For fluids, the separation membrane can be translucent or completely transparent to ultraviolet rays, but it cannot be completely impermeable to ultraviolet rays.

The main feature of membrane separation is to realize the separation of the molecular size of mixture and substance, and it will choose osmotic membrane as the separation means. There is no phase change (except pervaporation membrane) in the membrane separation process, so the operation can be carried out at room temperature, avoiding the disadvantage that the properties of concentrated concentrate change due to high temperature. Membrane separation is widely used in food, medicine and other industries because of its many advantages. It has the advantages of low energy consumption, low cost, high efficiency, no pollution and recoverable useful substances. For the separation of isomers, components with similar properties, heat-sensitive components and biological components, membrane separation method is very suitable, and sometimes it can replace chemical unit operations such as distillation, extraction, evaporation and adsorption. Practice shows that membrane separation will become a very useful technology if conventional separation cannot be realized by economic methods. The combination of conventional separation and membrane separation is more economical and effective. Based on the above advantages, membrane science and membrane technology have developed rapidly in the past two or three decades, and have become indispensable separation means in industrial and agricultural production, national defense, science and technology and people's daily life, and are more and more widely used in chemical industry, environmental protection, food, medicine, electronics, electric power, metallurgy, textile, seawater desalination and other fields.

7, combustion purification method

Burning is used to destroy toxic gas, steam or smoke and dust, so that it becomes a non-toxic and harmless substance, which is called combustion purification. Combustion purification can only destroy toxic gases and smoke that are flammable or can be decomposed at high temperature. Its chemical action is mainly combustion oxidation and in some cases thermal decomposition. Combustion purification can be widely used in the purification of organic solvent vapors and hydrocarbons. These toxic substances are flammable and harmful gases with high concentration and high calorific value (mainly gaseous substances containing hydrocarbons) produced during combustion and oxidation, and the combustion temperature is generally 600~800℃. C. The combustion method is simple and easy to operate, and can recover heat energy, but it can't recover harmful gases, which is easy to cause secondary pollution.

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