Principles and characteristics of common adsorbents (activated carbon, molecular sieve, silica gel, activated alumina)

1. Overview of the adsorption and separation process

Adsorption means that when a fluid (gas or liquid) is in contact with a solid porous substance, one or more components in the fluid are transferred to the outer surface of the porous substance and the inner surface of the micropores to be enriched on these surfaces to form a monolayer or multiple molecules Layer process. The adsorbed fluid is called adsorbate. Due to the different physical and chemical properties of adsorbate and adsorbent, the adsorption capacity of adsorbent for different adsorbates is also different. Therefore, when the fluid is in contact with the adsorbent, the adsorbent will affect one of the fluids. Or some components have higher adsorption selectivity compared to other components, and the components of the adsorption phase and the resorbance phase can be enriched, so as to realize the separation of substances.

2. The adsorption/desorption process

Adsorption process: It can be considered as a process of concentration or liquefaction. Therefore, the lower the temperature and the higher the pressure, the greater the adsorption capacity. For all adsorbents, the more easily liquefied (the higher the boiling point), the greater the amount of gas adsorbed, and the less likely to liquefy (the lower the boiling point), the lower the amount of gas adsorbed.
Desorption process: It can be considered as a process of gasification or volatilization. Therefore, the higher the temperature and the lower the pressure, the more complete the desorption. For all adsorbents, the gas that is more easily liquefied (the higher the boiling point) is less likely to be desorbed, and the gas that is less likely to be liquefied (the lower the boiling point) is, the easier it is to desorb. Adsorption is divided into physical adsorption and chemical adsorption.
The principle of physical adsorption separation: use the difference in the adsorption force (van der Waals force, electrostatic force) between the atoms or groups on the solid surface and the foreign molecules to achieve separation. The size of the adsorption force is related to the properties of both the adsorbent and the adsorbate.
The principle of chemical adsorption separation: based on the adsorption process that chemical reactions occur on the surface of the solid adsorbent to combine the adsorbate and the adsorbent with a chemical bond, so the selectivity is strong. Chemical adsorption is generally slow, can only form a monolayer and is irreversible.

3. characteristics of different adsorbents

Activated carbon: It has a rich microporous and mesoporous structure, the specific surface area is about 500-1000m2/g, and the pore size distribution is mainly 2-50nm. Activated carbon mainly relies on the van der Waals force generated by the adsorbent to produce adsorption, and is mainly used for adsorption of organic compounds, adsorption and removal of heavy hydrocarbons, deodorants, etc.;
Molecular sieve: It has a regular microporous pore structure with a specific surface area of ​​about 500-1000m2/g, mainly micropores, with a pore size distribution between 0.4-1nm. The adsorption characteristics of molecular sieve can be changed by adjusting molecular sieve structure, composition and type of balance cation. Molecular sieves mainly rely on the characteristic pore structure and the Coulomb force field between the equilibrium cations and the molecular sieve framework to produce adsorption. It has good thermal and hydrothermal stability. It is widely used in the separation and purification of various gas and liquid phases. When used, the adsorbent has the characteristics of strong selectivity, high adsorption depth and large adsorption capacity;
Silica gel: The specific surface area of ​​silica gel adsorbent is about 300-500m2/g, mainly mesoporous, with a pore size distribution of 2-50nm, and the internal surface of the pore channel has abundant surface hydroxyl groups, which are mainly used for adsorption drying and pressure swing adsorption for CO2 production, etc.;
Activated alumina: specific surface area 200-500m2/g, mainly mesoporous, pore size distribution in 2-50nm, mainly used in dry dehydration, acid waste gas purification, etc.