What are the characteristics of the pore structure of woody powdered activated carbon?
The microcrystalline structure of activated carbon determines the developed microporous structure of woody powdered activated carbon. These micropores are the voids created by the carbon and non-organic components of the carbon and the partial carbon in the microcrystalline structure during the activation process. The shape has a capillary shape, two inter-plane cracks, sharp-cut cracks (v-shaped), and an imported ink bottle shape. For ease of calculation, it is usually simplified to a cylindrical or ink bottle shape regardless of the actual pore shape.
The powdery activated carbon has a wide pore size distribution ranging from 10 nm to more than 10,000 nm. Generally, it is divided into three components: an emblem hole, a transition hole (middle hole) and a large (coarse) hole according to the size of the pore radius:
The micropore radius is 2 nm or less
Transition hole radius is 2-1OOnm
Large hole radius is 100-2000nm
Microporous
The effective radius of the micropores is quite small, comparable to the adsorbed molecules. The volume of the micropores is 0.20-0.6 cm 3 /g, which accounts for more than 90% of the total specific surface area of ​​the activated carbon, that is, several hundred m 2 /g, and even more than 1000 m 2 /g. Adsorption depends mainly on it.
2. Transition hole
The effective radius of the transition hole is much larger than that of the adsorbed molecule. Generally, the activated carbon transition hole can be distributed at 4.0-20 nm. Its volume is 0.02-0.1Ocm3/g, accounting for the total
5% of the specific surface area, generally 20-70 m2 / g. Its function is to capture organic vapor, which is a channel for adsorbing molecules into micropores, which is used to adsorb macromolecular substances and is often used for decolorization of solutions. The chemical process has a transition pore volume of up to 0.7 cm 3 /g and a surface area of ​​200-450 m 2 /g.
3. Large hole
The large pore volume of activated carbon is usually 0.2-0.8 cm 3 /g, and its specific surface area is only 0.5-2 m 2 /g. The value of the surface is so small that it does not play a significant role in the adsorption, and is generally undesirable because it reduces the specific gravity and strength of the activated carbon. However, these large pores act as transport channels, and when activated carbon is used as a catalyst support, larger pores may be useful as a site for catalyst deposition.
Physical adsorption and chemisorption characteristics of woody powdered activated carbon:
The physical adsorption of woody powdered activated carbon is the adsorption of molecular attraction between the adsorbate and the adsorbent, and is characterized by:
(l) No chemical reaction occurs, and it can be carried out at a low temperature.
(2) The surface energy is reduced during adsorption, so it is an exothermic reaction, and generally the amount of heat released per mole is less than 20.9 kj.
(3) Due to thermal motion, the adsorbate can be freely transferred on the surface of the adsorbent, so it is easier to desorb.
(4) The adsorption selectivity is poor, and for various substances, the magnitude of the intermolecular force is different. Molecular attraction increases with increasing molecular weight. For the same compound, adsorption increases with increasing molecular weight, which is called the Traube rule.
(5) The physical adsorption may be a single layer adsorption or a multilayer adsorption.
The chemical adsorption of wood powdered activated carbon is a chemical reaction between the adsorbate and the adsorbent due to chemical bond forces, which changes the chemical properties. The zinc particles adsorb mercury in the water to form a zinc amalgam alloy. The characteristics of chemisorption are:
(1) The adsorption heat is large, which is equivalent to the energy of the chemical bond, that is, the heat of chemical reaction;
(2) Chemical adsorption is often carried out at high temperatures;
(3) Adsorption is selective, and when the chemical bond force is large, the adsorption is irreversible;
(4) Generally, monolayer adsorption.
Activated carbon is often used to adsorb molecules, and adsorption determines the applicability, while adsorption is related to the pore size distribution of various carbon types. Take the steam-activated peat-based, lignite-based and coconut-shell-based powdered activated carbon as examples:
Peat-based activated carbon has micropores and mesopores for a variety of applications;
The lignite-based carbon material has more mesopores and a larger mesopores, providing excellent accessibility;
The coconut shell-based carbon is mainly microporous and is only suitable for low molecular removal.
Chemically activated activated carbon is very porous, mostly in the microporous and mesoporous range, however, compared to water vapor activated activated carbon, chemically activated activated carbon has a less hydrophobic surface and more negative charge.
Take extruded and broken granular activated carbon as an example:
Peat-based extruded activated carbon can be made into a variety of different pore size distribution varieties. Microporous-based varieties are mainly used for gold recovery in gas phase applications. Most of the microporous and mesoporous varieties are used in liquid phase applications, such as adsorption of small molecules and macromolecular impurities in water purification.
Broken coal-based carbon combines micropores and mesopores for a variety of applications.
The lignite-based or coconut-shell-based granular activated carbon has the same microporous and mesoporous structure as the powdered charcoal.
What are the performance indicators of activated carbon?
The performance indicators of activated carbon products can be divided into physical performance indicators, chemical performance indicators, and adsorption performance indicators. Three performance indicators play a very important role in the selection and application of activated carbon.
The main physical performance indicators are: shape, appearance, specific surface area, pore volume, specific gravity, mesh number, particle size, wear resistance, and floating rate.
The main chemical performance indicators are: PH value, ash, moisture, ignition point, uncarbide, sulfide, chloride, cyanide, sulfate, acid soluble, alcohol soluble, iron content, zinc content, lead content, arsenic content, Calcium and magnesium content, heavy metal content, phosphate, etc.
The main adsorption performance indicators are: methylene blue adsorption value, iodine adsorption value, phenol adsorption value, carbon tetrachloride adsorption value, caramel adsorption value, quinine sulfate adsorption value, saturated sulfur capacity, sulfur penetration capacity, water capacity, Ethyl chloride vapor protection time, ABS value, etc.
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