Physical Properties of Adsorbents

Physical Properties of Adsorbents

The good adsorption performance of the adsorbent is due to its dense pore structure. The physical properties related to adsorbent pores are:

a. Pore volume (VP): The volume of the micropores in the adsorbent is called the pore volume. 

Usually, the ultrasonic level meter is expressed by the volume of the adsorbent micropores in the unit weight of the adsorbent (cm3/g). 

The pore volume is the adsorbent. It is the value calculated from the saturated adsorption capacity, that is, the volume of the adsorbent that can accommodate the adsorbate, 

so it is better to have a larger pore volume. The pore volume (Vk) of the adsorbent is not necessarily equal to the pore volume (VP), and only the micropores in the adsorbent can adsorb, 

so the macropores are not included in the VP. And Vk includes the volume of all pores, which is generally larger than VP.

b. Specific surface area: that is, the surface area per unit weight of the adsorbent, and the commonly used unit is m2/g. 

The surface area of the adsorbent is hundreds to thousands of square meters per gram. 

The surface area of the adsorbent is mainly the surface of the micropore wall, and the outer surface of the adsorbent is very small.

c. Pore size and pore size distribution: In the adsorbent, the shape of the pores is extremely irregular, 

and the size of the pores is also different to reduce the pressure difference of the medium between the tube and shell sides. 

Pores with a diameter of several angstroms (A0) to tens of angstroms are called fine pores, and pores with a diameter of hundreds of angstroms or more are called coarse pores. 

The more pores, the larger the pore volume, and the larger the specific surface of the ultra-clean workbench, which is conducive to the adsorption of adsorbates. 

The role of the macropores is to provide access for the adsorbate molecules to enter the adsorbent. 

The relationship between coarse pores and fine pores is like the streets and alleys, and foreign molecules can quickly reach the depths of the adsorbent through the coarse pores. 

Therefore, the coarse holes should also occupy an appropriate proportion. Coarse and fine pores in adsorbents such as activated carbon and silica gel are formed during the manufacturing process. 

Zeolite molecular sieves form crystals with a diameter of several micrometers during synthesis, in which only uniform fine pores are formed, and coarse pores between crystals are formed during molding.

Pore size distribution is the relationship between the pore size and the corresponding pore volume. From this, the pore characteristics of the adsorbent were characterized.

d. Apparent severity (dl): also known as apparent severity.

The volume (Vl) of the adsorbent particles consists of two parts: the volume of the solid framework (Vg) and the pore volume (Vk), namely:

Vl= Vg+ Vk

The apparent gravity is the ratio of the weight (D) of the adsorbed particle to the volume occupied by it (Vl).

The pore volume (Vk) of the adsorbent is not necessarily equal to the pore volume (VP), only the micropores in the adsorbent have an effect, 

so the macropores are not included in the VP. And Vk includes the volume of all pores, which is generally larger than VP.

e. True weight (dg): also known as true weight or the weight of the adsorbent solid, that is, the ratio of the weight (D) of the adsorbent particles to the volume Vg of the solid skeleton.

Assuming that the weight of adsorbed particles is based on one gram, according to the definition of apparent weight and true weight of HEPA filter:

dl==l/Vl ; dg=l/Vg

The pore volume of the adsorbent is then:

Vk=l/dl – l/dg

f. Bulk weight (db): also known as filling weight, that is, the weight of adsorbent filled per unit volume. 

This volume also includes the voids between the adsorbed particles, and the stacking weight is an important parameter for calculating the volume of the adsorbent bed.

The above weight units are usually expressed in g/cm3, kg/l and kg/m3.

g. Porosity (εk): the ratio of the pore volume in the adsorbed particle to the particle volume.

εk=Vk/(Vg+Vk)=(dg-dl)/ dg=1-dl/dg

h. Porosity (ε): the ratio of the voids between the adsorbed particles to the bulk volume of the entire adsorbent.

ε=(Vb-Vl)/Vb=(dl-db)/dl=1-db/dl