Commonly used industrial adsorbent-zeolite molecular sieve

At present, in the treatment of environmentally friendly VOCs, people are more and more like to use zeolite adsorbents for treatment, because conventional activated carbon can no longer meet some process requirements. For example, styrene with a small molecule is difficult to be adsorbed by activated carbon. Disadvantages such as unstable combustion, adsorption and desorption, short life span, and high maintenance cost. The zeolite adsorbent has been widely used in many fields due to its advantages such as large adsorption capacity, less usage and higher removal.
The earliest molecular sieves were discovered in nature, and 36 species have been discovered since 1756. This kind of natural molecular sieve is a kind of crystalline aluminosilicate, because it can "boiling" when heated and melted, so it is also called zeolite or zeolite, and because its internal micropores can be sieved to different sizes It is also called molecular sieve or zeolite molecular sieve.
1. The principle of adsorption
Zeolite molecular sieve has a porous framework structure, Mex/m【(Al2O3)x(SiO2)y】·mH2O, where Me is mainly metal cations such as K+, Na+, Ca2+, and x/n is exchangeable metal cation Me with valence n The number of, m is the number of molecules of crystal water.
The principle of adsorption is: a kind of "external force" caused by molecular gravity acting on the solid surface. When the fluid flow expires, some molecules in the fluid collide with the surface of the adsorbent due to irregular motions, and molecular concentration occurs on the surface, causing The reduction in the number of such molecules in the fluid has reached the intention of separation and eradication. The use of molecular sieve and activated alumina is better when used in adsorption dryers.
2. Commonly used molecular sieve classification
At present, there are more than one hundred kinds of synthetic zeolite molecular sieves. The most commonly used types are Type A, Type X, Type Y, Type M and Type ZSM. The crystal structure of A, X, Y molecular sieve is shown in Figure 2-1:
Picture A, X and Y type molecular sieve crystal structure
(A) Type A; (b) Type X, Type Y
The molecular sieve has many pores with uniform pore size and neatly arranged caves in the structure, and these caves are connected by the pores. The cave not only provides a large specific surface area, but also allows only molecules with a smaller diameter to enter, thereby sieving molecules of different sizes and shapes. Depending on the pore size and the molecular ratio of SiO2 to Al2O3, molecular sieves have different models. Such as 3A (potassium A type), 4A (sodium A type), 5A (calcium A type), 10X (calcium X type), 13X (sodium X type), Y (sodium Y type), sodium mordenite type, etc.
3. Important parameters of commonly used molecular sieves
1. Several important technical parameters of molecular sieve
(1) Particle size: The appearance of the microspheres made of molecular sieve adsorbents. The particle size is planned to be controlled between the upper and lower limits, and the upper, lower and upper limits cannot exceed 5% if it is greater than 95%.
(2) Compressive strength: Because the working conditions of the microspheres are likely to have a large pressure difference, if the compressive strength does not meet the requirements, it is easy to cause damage to the molecular sieve. In addition to affecting the service life, it may also cause congestion of the equipped pipeline Form a severe effect.
(3) Bulk density: The bulk density of the molecular sieve adsorbent is basically proportional to the compressive strength. Under the condition of constant adsorption capacity, the larger the bulk density, the better.
(4) Packed water content: the smaller the packaged water content, the less pre-adsorption of microspheres during storage and transportation.
2. The characteristics of commonly used zeolite molecular sieves are shown in Table 3-1:
Table 3-1 Commonly used adsorbent characteristics
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4. Compared with other adsorbents, molecular sieves have the following advantages:
1. Strong adsorption selectivity
This is because the molecular sieve has a uniform pore size and is an ionic adsorbent, so it can selectively adsorb according to the size and polarity of the molecules. If it can effectively remove ethylene and propylene from saturated hydrocarbons, it can also effectively remove acetylene from ethylene. This is determined by its strong polarity.
2. Strong adsorption capacity
Even if the gas composition concentration is very low, it still has a large adsorption capacity.
3. At a higher temperature, there is still a large adsorption capacity, but other adsorbents are greatly affected by temperature, so under the same temperature conditions, the adsorption capacity of molecular sieve is large
It is precisely because of the above advantages that molecular sieves have become a very good adsorbent, which is widely used in the production of basic organic chemicals and petrochemicals. In the treatment of polluting gases, it is also commonly used in SO2, NO, CO, CO2, CCl4, Purification of water vapor and gaseous hydrocarbon waste gas. At present, zeolite molecular sieves have been widely used in the field of volatile organic compounds treatment. Zeolite molecular sieve runners are playing an increasingly important role in the treatment of large air volume and low concentration of volatile organic compounds. It can concentrate VOCs with a concentration of only tens of milligrams per cubic meter by 10-20 times, making it Gas that can be recycled or burned.