Molecular sieves are aluminosilicate compounds with cubic lattices. Molecular sieve has a uniform microporous structure, and its pore diameters are uniform. These pores can adsorb molecules smaller than their diameters into the interior of the pore cavity, and have preferential adsorption capacity for polar molecules and unsaturated molecules, so they can adsorb polar molecules and unsaturated molecules. Molecules with different degrees of sexuality, different degrees of saturation, different molecular sizes and different boiling points are separated, that is, they have the function of "sieving" molecules, so they are called molecular sieves. Because molecular sieves have the advantages of high adsorption capacity and strong thermal stability that other adsorbents do not have, molecular sieves have been widely used.
Molecular sieves have high affinity for polymer polarities such as H2O, NH3, H2S, CO2, etc., especially for water, at low partial pressure (even below 133 Pa) or low concentration, high temperature (even above 100 ℃), etc. It still has a high adsorption capacity under harsh conditions.
Comparison of adsorption characteristics of molecular sieve and activated alumina:
1. Adsorption under low partial pressure or low concentration: when the relative humidity is 30%, the water absorption of molecular sieve is higher than that of silica gel and activated alumina. As the relative humidity decreases, the superiority of molecular sieves becomes more and more significant, while silica gel and activated alumina have an increasing adsorption capacity with the increase of humidity. When the relative humidity is very low, their adsorption capacity is very small.
2. High temperature adsorption: Molecular sieve is the only high temperature adsorbent available. At 100°C and 1.3% relative humidity, molecular sieve can absorb 15% of the water by weight, which is 10 times larger than that of activated alumina under the same conditions, and 20 times larger than that of silica gel. Therefore, at higher temperatures, molecular sieves can still absorb a considerable amount of water, while activated alumina, especially silica gel, greatly loses its adsorption capacity.
3. High-speed adsorption: The adsorption rate of molecular sieve 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 of silica gel is higher than that of molecular sieve when the relative humidity is high, with the increase of the linear velocity of the adsorbate, the water absorption rate of silica gel is less and less efficient than that of molecular sieve.
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