Research on the classification of adsorption process

The diffusion process of a gas into a solid can be simply divided into three categories [22]: Bulk diffusion, Knudsen diffusio or Molecular diffusion, and Actiwated diffusion.
The difference between the three is based on the size of the average free path of the gas molecules relative to the pore size of the adsorbent. Physical adsorption is usually a very rapid process, reaching equilibrium within a few hours. If adsorption undergoes a long equilibrium time, this will mean that active diffusion occurs. This is due to the necking of the nanospace and the lower adsorption temperature and lower adsorption partial pressure, the adsorbate molecules do not have enough kinetic energy to penetrate the entire nanospace. At this time, the interaction between the adsorbate and the pore wall of the adsorbent is particularly important. This interaction is the superposition of the dispersion force and the repulsive force. When the size of the adsorption space is very close to the diffusion molecule, the repulsion is dominant, and the adsorbate must have a certain kinetic energy to enter the nanospace, that is, there is an activation energy. In the active diffusion range, the diffusion rate is the controlling factor, and small changes in the size of the adsorbate molecules will cause dramatic changes in the activation energy. Therefore, the difference in the adsorption rate of different adsorbates with small size difference on the adsorbent with the size of the adsorbent molecule can lead to a macroscopically broad screening effect [22]. The separation of oxygen and nitrogen by PSA-Pressure Swing Adsorption is a full manifestation of this principle [23,24]. The microporous carbon with adjusted pore size allows relatively small O 2 molecules (0.346 nm in diameter) to diffuse quickly into the nanospace and block relatively large N 2 molecules (0.364 nm in diameter). Since the kinetic selectivity of O_2 is better than that of N_2, when CMS is in contact with air, an oxygen-rich adsorption phase is formed, which is the basis of air separation by CMS. Kinetic selectivity and adsorption capacity are time-dependent. In terms of thermodynamic selectivity, O_2 has no obvious advantage over N_2. If the CMS is allowed to be in contact with air for a long time, N 2 can eventually diffuse into the pores of the CMS, forming an adsorption phase with the same composition as air.

Fig. 1-6 The effect of activated diffusion upon the DR micropore volume of
a wide pore carbon (A) and a narrow pore carbon (B)
Figure Effect of active diffusion on DR micropore volume (A) Wide pore distribution carbon (B) Narrow pore distribution carbon

The figure above shows the effect of adsorption temperature on micropore volume (micropore volume is calculated by the Dubinin-Radushkevich equation). Since sample A has wider micropores, its micropore volume has nothing to do with the adsorption temperature, but for sample B with a narrow pore distribution, the micropore volume decreases as the adsorption temperature decreases [22,25,26]. At low adsorption temperatures, the adsorbate cannot penetrate the entire pore wall, thereby obtaining an abnormally low pore volume, which is also the result of active diffusion.
The behavior of molecular sieves is essentially the macroscopic manifestation of the selective adsorption of smaller molecules in narrow micropores or the separation of molecules of different sizes through certain conditions.

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