Simply understand what steps are involved in the physical adsorption process of activated carbon.

The phenomenon that substances accumulate on the solid surface or in the micropore volume is called adsorption. After the mixture passes through some equipment, some substances in it are removed, which is called filtration. As far as indoor air is concerned, some harmful substances are removed after passing through activated carbon, which plays a filtering role; As far as some harmful substances are concerned, activated carbon plays an adsorption role. The adsorption process can be divided into physical adsorption and chemical adsorption. Physical adsorption only relies on intermolecular attraction to adsorb adsorbate on the surface of adsorbent. Physical adsorption is reversible. If the partial pressure of adsorbate in gas phase is reduced and the adsorption temperature is increased, the adsorbate will desorb quickly without changing its chemical composition. Chemical adsorption selectivity is high, and an adsorbent can only adsorb certain substances. Chemical adsorption is irreversible, and the adsorbate has changed after adsorption, changing its original characteristics.

The physical adsorption process can be divided into the following steps: (a) When the polluted gas passes through the adsorption boundary layer, the molecules of the polluted gas may be adsorbed or taken away from the surface of the activated carbon, which depends on the concentration difference of components in the carrier gas and the boundary layer gas, and determines the adsorption intensity. When the polluted air passes through the activated carbon, some harmful gases are absorbed due to the great concentration difference, while the inherent components in the air pass normally due to the basically zero concentration difference, while some particles (such as smoke and dust) are left directly in the macropores and mesopores because they are too large. When the concentration difference of harmful gas is zero, the activated carbon fails and needs to be reactivated. (b) The adsorbed molecules diffuse into the micropores. (c) The molecules are firmly bound to the surface of the adsorbent. The above three steps will inevitably occur in chemical reactions. In chemical adsorption, the combination of adsorbent and adsorbate is relatively firm and must be desorbed at high temperature. Chemical adsorption has stronger driving force and binding force than physical adsorption, so chemical adsorption is safer for highly toxic pollutants. Generally speaking, when adsorbate contacts the surface of adsorbent, whether physical adsorption or chemical adsorption occurs depends on the reactivity of adsorbent surface, the nature of adsorbate, temperature and other factors. In fact, these two kinds of adsorption are not completely separated, which depends on molecular force and chemical bond. Under dynamic conditions, the removal of steam requires activated carbon not only to have a certain adsorption capacity, khd3yd, but also to have a certain adsorption speed. If there is only a large adsorption capacity without a certain adsorption speed, it will be too late for steam to be adsorbed and infiltrated when it contacts the activated carbon layer briefly. On the contrary, if there is only a large adsorption speed without a certain amount of adsorption, the removal amount is also very small. Generally speaking, the removal of water vapor from air by activated carbon includes one or more stages as follows: (1) external diffusion. The gas or steam in the air diffuses to the whole surface of the particles; (2) Internal diffusion. Gas or steam molecules diffuse into the interior (or along the surface) of macroporous activated carbon; (3) Adsorption of molecules on the inner surface of particles; (4) The reaction between adsorbed steam and activated carbon or adsorbed oxygen, water or solvent.