Working principle of carbon molecular sieve

Carbon molecular sieve uses the characteristics of sieving to achieve the purpose of separating oxygen and nitrogen.

When the molecular sieve adsorbs impurity gases, the macropores and mesopores only act as channels to transport the adsorbed molecules to the micropores and submicropores. The micropores and submicropores are the volumes that really play the adsorption role.

The carbon molecular sieve contains a large number of micropores, which allow molecules with small kinetic size to diffuse quickly into the pores while restricting the entry of large diameter molecules. Due to the difference in the relative diffusion rate of gas molecules of different sizes, the components of the gas mixture can be effectively separated.

Therefore, when manufacturing carbon molecular sieves, the distribution of micropores inside the carbon molecular sieve should be between 0.28 and 0.38 nm according to the size of the molecules. Within this micropore size range, oxygen can quickly diffuse into the pores through the micropore orifices, while nitrogen is difficult to pass through the micropore orifices, thereby achieving oxygen and nitrogen separation. The size of micropores is the basis for carbon molecular sieves to separate oxygen and nitrogen. If the pore size is too large, oxygen and nitrogen molecular sieves can easily enter the micropores and cannot separate them. If the pore size is too small, oxygen and nitrogen cannot enter the micropores and cannot separate them.

Due to the limitations of conditions, domestic molecular sieves do not have a good control over the pore size. The micropore size of carbon molecular sieves sold on the market is distributed in the range of 0.3 to 1nm, and only the Iwatani molecular sieve has achieved 0.28 to 0.36nm.

The raw materials of carbon molecular sieves are coconut shells, coal, resins, etc. The first step is to process and pulverize them, and then mix them with the base material. The base material is mainly a material that increases strength and prevents crushing and pulverization; the second step is activation and pore formation. The activator is introduced at a temperature of 600-1000℃. Commonly used activators are water vapor, carbon dioxide, oxygen and their mixed gases.

They undergo thermochemical reactions with relatively active amorphous carbon atoms to expand the specific surface area and gradually form pores. The activation pore-forming time ranges from 10 to 60 minutes. The third step is pore structure adjustment, using the vapor of chemical substances such as benzene to deposit on the micropore walls of the carbon molecular sieve to adjust the pore size to meet the requirements.