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6 major problems of activated carbon used to adsorb VOCs exhaust gas

Views: 7     Author: Xicheng Ep Ltd     Publish Time: 2021-09-30      Origin: Xicheng Ep Ltd

The activated carbon adsorption process is the most widely used and simple treatment process in the VOCs treatment industry. It is mainly used to adsorb exhaust gas through the natural adsorption capacity of activated carbon. When the adsorption is saturated, the activated carbon is desorbed and regenerated or handed over to professional hazardous waste companies for treatment. Although the activated carbon adsorption process is very simple and practical, we still need to face up to the existing problems. However, returning to the original point, any VOCs treatment process has its scope of application.

  • The molecular diameter is larger than the diameter of the pore. Due to steric hindrance, the molecules cannot enter the pore and therefore do not adsorb;

  • The molecular diameter is equal to the diameter of the pore, and the adsorbent has a strong catching power, which is very suitable for low concentration adsorption;

  • The molecular diameter is smaller than the diameter of the hole, capillary condensation occurs in the hole, and the adsorption capacity is large;

  • The molecular diameter is much smaller than the pore diameter, the adsorbed molecules are easily desorbed, the desorption rate is high, and the adsorption capacity at low concentrations is small.

The adsorption of activated carbon is mainly related to the structure of activated carbon. The surface atoms of activated carbon combine to act in a variety of ways such as complexation, hydrogen bonding, and ion exchange. Although activated carbon has a fast adsorption rate, it has low selectivity for organic gas adsorption. At the same time, the adsorption process of activated carbon for organic gas is also affected by many factors, mainly including temperature, working environment humidity, water mist, acidity, dust and adsorbed Interaction between gases, etc.

1. The adsorption capacity is small.

Physical adsorption has the problem of adsorption saturation. As the adsorbent is consumed, the adsorption capacity becomes weak. After a period of use, the adsorption capacity may be small or the adsorption function may be lost. It is not suitable for high-concentration exhaust gas. When adsorbing, there is the problem of specificity of adsorption. For mixed gas, the adsorbability will be weakened. At the same time, there is also a mismatch between the molecular diameter and the pore size of activated carbon, which causes desorption. What is more obvious is that the adsorption of activated carbon is only in principle. The transfer of toxic and harmful gases does not achieve the effect of decomposing harmful gases.

2. Temperature influence.

Under normal circumstances, activated carbon adsorption equipment generally requires the temperature of the exhaust gas to be lower than 40°C, and the adsorption conditions at 25°C are better. In principle, the VOCs gas source needs to be cooled to reach this temperature, and in the actual working environment It is difficult to achieve constant temperature adsorption of VOCs. If the temperature of the exhaust gas exceeds 400℃, the adsorption efficiency of activated carbon will drop rapidly. Moreover, when activated carbon adsorbs a certain amount of VOCs and then suspends work, the activated carbon that has adsorbed VOCs will release desorbed VOCs due to changes in temperature or air pressure.

3. The influence of relative humidity.

Relative humidity will also affect the adsorption efficiency of activated carbon adsorption equipment.

The temperature of the VOCs produced in the production process of the enterprise has been on the high side for a long time, and the moisture content has exceeded 50%. Therefore, under the combined action of outdoor environmental conditions and exhaust gas environmental conditions, the adsorption efficiency of activated carbon is greatly reduced. The overall adsorption efficiency of activated carbon adsorption equipment is less than 50% of its design standard. It is conceivable that, in this case, the VOCs contained in the exhaust gas must exceed the national emission standards, which directly pollutes the atmosphere.

4. The influence of air pressure.

In the process of adsorbing VOCs by activated carbon, if it is adsorbed from a low concentration to a high concentration, it is necessary to ensure that the activated carbon can be carried out under a negative pressure environment. In the actual application of the factory production, the equipment cannot be operated continuously. When the work is stopped and no negative pressure is formed, the VOCs that the activated carbon has adsorbed will be desorbed and released, polluting the environment again.

5. The influence of acid gas.

The surface acidity of activated carbon is closely related to the adsorption balance. As the surface acidity of activated carbon increases, the adsorption capacity of acidic and neutral organics is greatly reduced, and most VOCs are acidic.

6. Influence of dust.

The adsorption of activated carbon to VOCs is mainly affected by the physical properties of activated carbon such as specific surface area and pore size. Because the adsorption of activated carbon is non-selective, in addition to adsorbing VOCs, it will also adsorb dust. As the amount of dust on the surface of activated carbon increases, the micropores of activated carbon are blocked and the specific surface area decreases, which reduces the adsorption capacity of activated carbon for VOCs, resulting in activated carbon. Poisoning" inactivation.

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