Features
Molecular sieves have extremely strong hygroscopicity (so they are widely used as desiccants). They are used for the purification of gases. Avoid direct exposure to air during storage. Molecular sieves that have been stored for a long time and have been hygroscopic should be regenerated before use. Molecular sieve avoids oil and liquid water. Avoid contact with oil and liquid water when using. Gases for drying treatment in industrial production are air, hydrogen, oxygen, nitrogen, argon, etc. Two adsorption dryers are connected in parallel and one works while the other can be regenerated. Alternate work and regeneration to ensure continuous equipment operation. The dryer works at 8-12 ° C, and it is regenerated by insufflation when heated to 350 ° C. Different specifications of molecular sieve regeneration temperature are slightly different. Molecular sieves have a good catalytic effect on certain organic gas phase reactions.
Also known as zeolite or zeolite, it is a crystalline aluminosilicate. Its crystal structure has regular and uniform pores. The pore size is on the order of molecular size. It only allows molecules with a diameter smaller than the pore size to enter. The molecules in the mixture are sieved by size. It is called molecular sieve. As early as 200 years ago, B. Kronstedt was the first to name aluminosilicate as zeolite. The general chemical composition is: where M and n are metal ions and their valence; x is of silica Number of molecules; y is the number of molecules of water; p is the number of atoms of aluminum; q is the number of atoms of silicon. Molecular sieves are used as solid adsorbents in the chemical industry. The substances adsorbed by them can be desorbed, and the molecular sieves can be regenerated after use. It is also used for the drying, purification, separation and recovery of gases and liquids. Beginning in the 1960s, it was used as a cracking catalyst in the petroleum refining industry, and now a variety of molecular sieve catalysts suitable for different catalytic processes have been developed.
kind
There are two types of molecular sieves, natural zeolite and synthetic zeolite.
①Most natural zeolites are formed by the reaction of volcanic tuff and tuff sedimentary rocks in marine or lacustrine environments. At present, there are more than 1,000 kinds of zeolite ores, more important ones are 35, common are clinoptilolite, mordenite, erionite and chabazite. Mainly distributed in the United States, Japan, France and other countries, China also found a large number of mordenite and clinoptilolite deposits, Japan is the country with the largest amount of natural zeolite mining.
② Because natural zeolite is limited by resources, since the 1950s, a large number of synthetic zeolites have been used (see table).
Commercial molecular sieves often use prefix numbers to classify molecular sieves with different crystal structures, such as 3A, 4A, and 5A molecular sieves. Type 4A is the type A in the table and the aperture is 4Å. A + molecular sieve containing Na + is recorded as Na-A. If Na + is replaced by K +, the pore size is about 3Å, which is 3A molecular sieve. It is 5A molecular sieve.