Which molecular sieve of 3A and 4A has the smallest pore size?
When purchasing molecular sieves, you do not specifically say which molecular sieve is better, only which molecular sieve is more suitable. The main function of molecular sieve is to sieve a substance molecule through the pore size of the molecular sieve's own crystal structure. When meeting the technical requirements of water removal, is it better to use 3A molecular sieve or 4A molecular sieve? First of all, this problem still needs to be specifically distinguished from the substance to be dehydrated. First, the size of the molecular diameter of the moisture (dry) needs to be removed. If the dynamic molecular diameter of the substance to be dehydrated (dry) is greater than 3nm and less than 4nm, then, If we want to get rid of water from this substance, we can only use 3A molecular sieve, because if we use 4A molecular sieve, it will not only absorb the water in the substance, but also absorb the substance that needs to be dehydrated. This is the first case, and there is another The situation is that if the dynamic diameter of the material that needs to be dehydrated (dried) is greater than 4A, then 4A molecular sieves are generally used, because 4A molecular sieves have a stronger ability to absorb moisture than 3A. If both 3A molecular sieve and 4A molecular sieve can be used to remove water, which molecular sieve should we use? In this case, we generally use 4A molecular sieve, because the water absorption of 4A molecular sieve is more than 22%, and the water absorption capacity of 3A is more than 21%. Generally speaking, the water absorption capacity of 4A molecular sieve is stronger than that of 3A molecular sieve. That is, the price of 4A molecular sieve is cheaper than that of 3A molecular sieve.
Quickly understand the classification of activated carbon
Activated carbon is a kind of black porous solid carbon, which is produced by pulverizing and shaping coal or carbonizing and activating uniform coal particles. The main component is carbon, and contains a small amount of oxygen, hydrogen, sulfur, nitrogen, chlorine and other elements. The specific surface area of ordinary activated carbon is between 500 and 1700 m2/g. It has strong adsorption performance and is an industrial adsorbent with a wide range of uses. Activated carbon is a traditional and modern man-made material, also known as carbon molecular sieve. Classification: According to the different sources of raw materials, manufacturing methods, appearance and shape, and application occasions, there are many types of environmentally friendly activated carbon. Up to now, there are no measurable statistical materials, and there are about thousands of varieties. According to the source of raw materials: 1. Wooden activated carbon; 2. Animal bones, blood charcoal; 3. Mineral raw material activated carbon; 4. Other raw material activated carbon; 5. Regenerated activated carbon. According to the manufacturing method: 1. Chemical activated carbon (chemical carbon); 2. Physical activated carbon; 3. Chemical-physical or physical-chemical activated carbon. According to appearance shape: 1. Powdered activated carbon; 2. Granular activated carbon; 3. Unshaped granular activated carbon; 4. Cylindrical activated carbon; 5. Spherical activated carbon; 6. Activated carbon of other shapes. According to the aperture: Macropore radius>20 000nm; transition pore radius 150-20000nm; micropore radius<150nm The surface area of activated carbon is mainly provided by micropores. Classified by material: Coconut shell activated carbon; nut shell activated carbon (including apricot shell activated carbon, fruit core shell activated carbon, walnut shell activated carbon); wooden activated carbon; coal-based activated carbon.
Activated alumina as a catalyst and carrier for chemical reactions
Activated alumina has a large specific surface area, a variety of pore structures and pore size distributions, and rich surface properties. Therefore, it has a wide range of uses in adsorbents, catalysts and catalyst carriers. Alumina for adsorbent and catalyst carrier is a fine chemical and also a special chemical. Different uses have different requirements for physical structure, which is the reason for its strong specificity and many varieties and grades. According to statistics, the amount of alumina used as catalysts and carriers is more than the total amount of catalysts using molecular sieve, silica gel, activated carbon, diatomaceous earth and silica alumina gel. This shows the pivotal position of alumina in catalysts and carriers. Among them, η-Al2O3 and γ-Al2O3 are the most important catalysts and supports. They are both spinel structures containing defects. The difference between the two is: the tetrahedral crystal structure is different (γ>η), and the hexagonal layer stack The row regularity is different (γ>η) and the Al—O bond distance is different (η>γ, the difference is 0.05～0.1nm).
Carbon molecular sieves is a new type of non-polar adsorbent
The ability of molecular sieve to separate air depends on the diffusion speed of various gases in the air in the pores of Carbon Molecular Sieves, or the adsorption force, or both. Carbon Molecular Sieves PSA air separation nitrogen production is based on this performance. Carbon Molecular Sieves are used to produce nitrogen. The N2 concentration and gas production volume can be adjusted according to the user's needs. When the gas production time and operating pressure are determined, the gas production volume will be lowered, and the N2 concentration will increase, otherwise, the N2 concentration will decrease. Users can adjust according to actual needs.
Influence of molecular sieve in PSA nitrogen generator
Carbon molecular sieve PSA nitrogen generator production relies on van der Waals force to separate oxygen and nitrogen. Therefore, the larger the specific surface area of the molecular sieve, the more uniform the pore size distribution, and the greater the number of micropores or submicropores, the greater the adsorption capacity; , If the pore size can be as small as possible, the van der Waals force field overlaps, and it has a better separation effect on low-concentration substances. Carbon molecular sieve is a non-quantitative compound, and its important properties are based on its microporous structure. Its ability to separate air depends on the different diffusion speeds of various gases in the air in the pores of the carbon molecular sieve, or different adsorption forces, or both effects work at the same time. Under equilibrium conditions, the adsorption capacity of carbon molecular sieve for oxygen and nitrogen is quite close, but the diffusion rate of oxygen molecules through the narrow gaps of the carbon molecular sieve microporous system is much faster than that of nitrogen molecules. Carbon molecular sieve air separation nitrogen production is based on this Performance, before the time to reach equilibrium conditions, the nitrogen is separated from the air through the PSA process.
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.