Molecular Sieve Instructions for Use

Thread Content

Molecular sieve (also known as synthetic zeolite) is a microporous aluminosilicate crystal, which is composed of SiO and AIO tetrahedrons and has a framework structure. Metal cations (such as Na, K, Ca, etc.) exist in the molecular sieve lattice to balance the excess negative charges in the tetrahedrons. Types of molecular sieves are mainly divided into: Type A, Type ； Use Ca2+ to exchange Na+ in the 4A molecular sieve to form a pore size of 5A, which is the 5A (also known as calcium A-type) molecular sieve. ； K+ is used to exchange the Na+ of the 4A molecular sieve to form a pore size of 3A, which is the 3A (also known as potassium A-type) molecular sieve. X-type aluminosilicates have different crystal structures (different silicon-to-aluminum ratios), forming molecular sieve crystals with a pore size of 9-10A, called 13X (also known as sodium X-type) molecular sieves ； Ca2+ is used to exchange Na+ in the 13X molecular sieve to form a molecular sieve crystal with a pore size of 9A, called 10X (also known as calcium Main characteristics of molecular sieves 1. Physical properties: Specific heat: About 0.95KJ/KgXK (0.23Kcal/KgX℃ Thermal conductivity (dehydrated material): 2.09KJ/MXK(0.506Kcal/mX℃ Water adsorption heat: About 3780KJ/Kg (915Kcal/Kg) 2. Thermal stability and chemical stability: Molecular sieves can withstand short-term high temperatures of 600-700°C, but the regeneration temperature is generally below 400°C. Molecular sieves can be used in media with a pH range of 5-10 ； Certain metal cations can be exchanged in salt solutions. 3. Basic characteristics: a) Molecular sieves can reversibly adsorb and desorb water or various gas and liquid compounds. b) Metal cations are easily exchanged. c) The internal cavities and channels of the molecular sieve form a very high internal surface area. The inner surface can be 10,000-100,000 times higher than the outer surface area of ​​the molecular sieve particles. Selective adsorption properties of molecular sieves: 1. Selective adsorption based on different molecular sizes and shapes - Molecular sieve effect. Molecular sieve crystals have a honeycomb structure. The crystal cavities and pores in the crystal communicate with each other, and the pore size is uniform and fixed (the diameter of the molecular sieve cavity is generally between 6-15 Angstroms), which is equivalent to the size of ordinary molecules. Only those molecules with relatively small diameters can be adsorbed by the molecular sieve through the zeolite pores, while molecules with large configurations cannot enter the zeolite pores and are not adsorbed by the molecular sieve. Silica gel, activated alumina and activated carbon do not have uniform pore sizes, and the pore size distribution range is very wide, so they have no screening performance. 2. Selective adsorption molecular sieves based on molecular polarity, degree of unsaturation and polarizability have a high affinity for polar molecules and unsaturated molecules. ； Among non-polar molecules, molecules with a polarizability in the range have a higher selective adsorption advantage. In addition, molecules with lower boiling points are less likely to be adsorbed by molecular sieves. Highly efficient adsorption properties of molecular sieves: Molecular sieves have a high affinity for polar polymers such as H2O, NH3, H2S, and CO2. Especially for water, they still have a high adsorption capacity under very harsh conditions such as low partial pressure (even below 133 Pa) or low concentration, high temperature (even above 100°C). 1. Adsorption under low partial pressure or low concentration. When the relative humidity is 30%, the water absorption capacity of molecular sieve is higher than that of silica gel and activated alumina. As the relative humidity decreases, the advantages of molecular sieves become more and more obvious, while the adsorption capacity of silica gel and activated alumina continues to increase as the humidity increases. When the relative humidity is very low, their adsorption capacity is very small. 2. High-temperature adsorption molecular sieve is the only available high-temperature adsorbent. At 100°C and 1.3% relative humidity, molecular sieve can absorb 15% of water by weight, which is 10 times greater than the water absorption of activated alumina under the same conditions; and more than 20 times greater than that of silica gel. Therefore, at higher temperatures, molecular sieves can still adsorb a considerable amount of moisture, while activated alumina, especially silica gel, * * Lost adsorption capacity. 3. The adsorption rate of high-speed adsorption molecular sieves for polar molecules such as water is much higher than that of silica gel and activated alumina when the partial pressure or concentration is very low. Although the equilibrium water absorption capacity of silica gel is higher than that of molecular sieves when the relative humidity is high, as the linear velocity of the adsorbate increases, the water absorption rate of silica gel becomes less and less efficient than molecular sieves. Ion exchangeability of molecular sieves An important property of molecular sieves is that they can perform reversible ion exchange. Through this exchange, the adsorption and catalytic properties of the molecular sieve are improved, leading to a wide range of applications (such as water softening and wastewater treatment). Catalytic performance of molecular sieves Molecular sieve crystals have a uniform pore structure, and the pore size is comparable to that of ordinary molecules. ； They have a large surface area. And the surface polarity is very high ； Cations that balance the negative charge of the skeleton and can perform ion exchange ； Some catalytically active metals can also be exchanged into crystals and then reduced to their elemental state with extremely high dispersion ； At the same time, the molecular sieve skeleton structure has high stability. These structural properties make molecular sieves not only excellent adsorbents, but also effective catalysts and catalyst carriers. Regeneration of Molecular Sieves In order to obtain good operating performance and the longest possible life, the divided sieves must be regenerated after being used for a certain period of time. Correctly regenerated molecular sieves are the same as fresh ones, and the attenuation and aging of their adsorption properties and mechanical properties are very low. There are two basic methods for regeneration of molecular sieves: 1) Change the temperature, that is, "changing temperature". It removes adsorbed substances by heating molecular sieves. In industry, preheated regeneration gas is generally used to heat, purge the molecular sieve to about 200, and take away the desorbed adsorbates. 2) Change the relative pressure, that is, "pressure change". Generally used in gas phase adsorption processes. The basic method is to keep the temperature of the adsorbent constant, and remove the adsorbate by reducing the pressure and backflushing with inert gas. Regeneration is usually carried out in the opposite direction to adsorption, which prevents most of the adsorbate contained at the inlet of the adsorption bed from passing through the entire bed, and some molecular sieves do not need to be in contact with hot and humid gases, thereby increasing the service life of the molecular sieve. The regeneration gas should be as dry as possible, otherwise it will affect the adsorption efficiency. The main technical indicators of molecular sieves are particle size (mm,%): The appearance index of molecular sieve, the particle size range is controlled within: The upper and lower limits are greater than 95% ； The upper and lower limits and the upper and lower limits shall not exceed 5%. Static water adsorption (mg/g): Main indicators of molecular sieves. The static water adsorption capacity basically reflects the quality of the molecular sieve. The range of static water adsorption is generally between 200-265mg/g. Amount of adsorbed specific medium (mg/g): Main indicators of molecular sieves. Different molecular sieves must be tested with a specific medium according to their main uses and standard pore sizes. The test results will visually indicate the quality of the molecular sieve. Compressive strength (N/piece): Main indicators of molecular sieves. Since the working conditions for using molecular sieves are mostly large pressure differences (especially when switching between adsorption and regeneration), if the compressive strength of the molecular sieve does not meet the requirements, it is easy to cause damage to the molecular sieve. In addition to affecting the service life of the molecular sieve, it may also block the equipment pipeline and cause serious consequences. Compressive strength and adsorption capacity are basically inversely proportional to each other. How to improve the compressive strength while ensuring the adsorption capacity is also the key to improving the quality of molecular sieves. Bulk density (g/ml): Main indicators of molecular sieves. There is basically a proportional relationship between packing density and compressive strength. When the adsorption capacity remains unchanged, the higher the packing density, the better. Wear rate (%): Appearance indicators of molecular sieves. The lower the wear rate, the less dust there is in the molecular sieve, and the less bad things happen during use, the better the quality of the molecular sieve. Packaging moisture content (%): The smaller the moisture content of the packaging, the less pre-adsorption of the molecular sieve during storage and transportation. For users, in addition to saving unnecessary regeneration processing, its value is greater.