Activated carbon air purification belongs to the field of adsorption. Adsorption is caused by the force between the molecules of the adsorbent and the adsorbate. According to the different forces, it can be divided into physical adsorption and chemical adsorption.
Physical adsorption mainly relies on van der Waals forces between molecules. Physical adsorption simply relies on the attraction between molecules to adsorb the adsorbate on the surface of the adsorbent.
Physical adsorption is reversible. By lowering the partial pressure of the adsorbate in the gas phase and increasing the adsorption temperature, the adsorbate will desorb quickly without changing its chemical composition.
Chemical adsorption relies on the chemical bonding force between the solid surface and the adsorbed gas molecules.
It is the result of chemical action. Its force greatly exceeds the van der Waals force of physical adsorption. It is often an irreversible process. Chemical adsorption has high selectivity. It is a kind of adsorption. The agent only adsorbs specific substances.
After adsorption, the adsorbate has changed and its original characteristics have been changed. Adsorbing adsorbate on the surface of the adsorbent is a reversible process, which can only temporarily block pollution but cannot eliminate it. Moreover, the rate of chemical adsorption increases with increasing temperature. It is worth noting that the same substance may undergo physical adsorption at lower temperatures, while chemical adsorption may occur at higher temperatures, or both adsorption methods may occur at the same time.
The physical adsorption process can be divided into the following steps:
(1) The pollutant gas passes through the adsorption boundary layer. The molecules of the pollutant gas may be adsorbed or taken away from the surface of the activated carbon, depending on the concentration difference of the component in the carrier gas and the gas in the boundary layer.
This value determines The strength of adsorption. When polluted air passes through activated carbon, some harmful gases have a large concentration difference, so they are adsorbed. However, the inherent components in the air pass normally because the concentration difference is basically zero, while some particles (such as smoke) are too large. They are left directly in the macropores and mesopores. When the concentration difference of harmful gases reaches zero, the activated carbon fails and needs to be reactivated.
(2) The adsorbed molecules diffuse into the micropores.
(3) The molecule is firmly bound to the adsorbent surface.
The chemical adsorption of activated carbon mainly relies on the chemical components on the material to react with pollutants to generate solid components or harmless gases. Activated carbon can also be subjected to some chemical treatments, such as adding some catalysts, which can help decompose harmful gases such as formaldehyde, VOCs, etc.
During use, the adsorption capacity will continue to weaken. When it weakens to a certain extent, the filter will be scrapped. The pore diameter of activated carbon used to purify the air must be slightly larger than the diameter of toxic and harmful gas molecules to have the ability to adsorb toxic and harmful gases. The key factors that affect the service life of air purification activated carbon: the total amount of harmful substances in the environment and the frequency of desorption.
Since the quality of activated carbon adsorbing harmful gases can be close to or even reach its own quality, the quality of harmful gases in the air of ordinary household spaces is far less than the amount of activated carbon used.
Therefore, activated carbon can be used for a long time as long as it is regularly exposed to the sun. Of course, dust blocking the micropores of activated carbon will also reduce its adsorption capacity.
(1) Adsorption capacity: The maximum amount of pollutants that a unit of activated carbon can adsorb is called adsorption capacity. The adsorption capacity of different materials will be different; the adsorption capacity of the same material for different gases will also be different; the adsorption capacity will also change when the temperature and background concentration change.
(2) Residence time: The time that air stays in the activated carbon layer is called residence time. The longer the residence time, the more complete the adsorption. In order to maintain sufficient retention time, the carbon layer must be thick enough and the filtration wind speed must be as low as possible.
(3) Service life: New activated carbon has high adsorption efficiency, but the efficiency continues to decrease during use. When harmful gases downstream of the filter approach the allowable concentration limit, the filter is scrapped. The usage time before scrapping is the service life, also known as the effective protection time.
(4) Selectivity: Generally speaking, the gases that are easily adsorbed in physical adsorption include: gases with large molecular weights, gases with high boiling points, and volatile organic gases. If activated carbon is chemically impregnated, it can also remove gases that are usually difficult to deal with, or enhance the adsorption capacity for certain types of gases.
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