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The difference between activated alumina and molecular sieve

2021-02-05
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Activated alumina and molecular sieves are commonly used adsorbents in industrial production, and both play an irreplaceable role. But there is a problem that has always entangled us, that is, what is the difference between activated alumina and molecular sieve? What are the different functions of each. Only when I know these can I make a reasonable product choice in industrial production. So today I will focus on analyzing their specific differences in terms of structure, adsorption performance, and application.


1. The difference in structure


Activated alumina and molecular sieves are both porous and highly dispersed solid materials, and both have a large specific surface area. However, the difference between activated alumina and molecular sieves can be seen from the distribution of structural pores. Activated alumina molecular formula: Al2O3 -x(OH)2x, 0<x<0.8. The pore structure distribution of activated alumina is rather messy, and it is generally divided into micropores, mesopores, and macropores according to the proportion of pore size distribution. The pore distribution of activated alumina is not very uniform.


The molecular sieve is different, molecular sieve molecular sieve: (M)2/nO·Al2O3·xSiO2·pH2O. The molecular sieve has many pores with uniform pore size and neatly arranged pores in the structure. According to the different molecular ratios of SiO2 and Al2O3, molecular sieves with different pore sizes are obtained. Its models: 3A molecular sieve, 4A molecular sieve, 5A molecular sieve, 13X molecular sieve, etc. So from the structure pore distribution, we can basically understand the difference between activated alumina and molecular sieve.


Second, the difference in adsorption performance


Activated alumina has uneven pore size distribution, so its selectivity is not very good, but it has higher mechanical strength than molecular sieves, and at the same time has a high specific surface area. It has a special adsorption polarity for water, so it is used in daily industrial production. Active oxidation is commonly used as a desiccant or as a catalyst carrier, which can make the catalyst have the characteristics of pressure resistance and high temperature resistance. Because activated alumina has a porous structure, a high specific surface area and an unstable transition state, it has greater activity. After the adsorption is saturated, it can be heated at about 175 to 315°C to remove water and resurrect. It can be repeated many times. In addition to being used as a desiccant, it can also absorb the vapor of lubricating oil from oxygen, hydrogen, carbon dioxide, natural gas, etc.


The adsorption and separation performance of molecular sieve depends on the size of the pores and pore volume of molecular sieve. Molecular sieve has uniform pore distribution, which makes molecular sieve have much better selective adsorption performance than activated alumina. The adsorption of molecular sieve is a process of physical change. The main reason for molecular sieve adsorption is a kind of "surface force" generated by molecular gravity acting on the solid surface. When the fluid flows through, some molecules in the fluid collide with the surface of the molecular sieve due to irregular motion, and molecular concentration occurs on the surface of the molecular sieve. Reduce the number of such molecules in the fluid to achieve the purpose of separation and removal. In fact, in layman's terms, it can be understood literally that molecular sieves are like sieves for gas and liquid molecules, and whether they are adsorbed is determined according to the size of the molecules.


It is mentioned here that both molecular sieve and activated alumina can be heated and regenerated after being saturated by adsorption. The adsorption and regeneration can be used for many times until the adsorption performance and activity are reduced to a certain range, and they need to be replaced.


Three, the difference between applications


Activated alumina can actually be called an industrial desiccant. More than 80% of the air pressure drying equipment commonly used in the industry is basically activated alumina, which can reach -40°C normally. Molecular sieves are used only when the drying depth is relatively high. The gas of activated alumina desiccant can be used. Mainly include: acetylene, cracked gas, coke oven gas, hydrogen, oxygen, air, ethane, hydrogen chloride, propane, ammonia, ethylene, hydrogen sulfide, propylene, argon, methane, sulfur dioxide, carbon dioxide, natural gas, helium, nitrogen , Chlorine, etc.


Molecular sieves have strong hydrophilicity, because in industrial production, sometimes the water content of the gas is required to be controlled at a very low level. At this time, the drying depth of activated alumina cannot meet the requirements. Molecular sieves can be used in extremely low water content. Adsorption is carried out at the level of, and the drying depth of molecular sieve can reach -70℃. At this time, there is a problem. Since the desiccant depth of molecular sieves is so high, why don't we use molecular sieves directly? We can’t understand that the cost of a molecular sieve is high, and sometimes using molecular sieves will cause unnecessary Waste. The second is the difference between the adsorption conditions of the two. The water absorption rate of activated alumina is much higher than that of molecular sieve when the gas moisture content is high. Under the same conditions, molecular sieve does not have activated alumina. The result is obvious, which is caused by the molecular structure. , But it needs to be reduced by an order of magnitude when the water content is very low. At this time, the water absorption of molecular sieve is stronger than that of activated alumina. In fact, sometimes we can use activated alumina and molecular sieves together, so that we can give full play to their respective strengths and use them rationally, which saves costs and achieves a multiplier effect.


Both have their own strengths and weaknesses in industrial production. I should have a basic understanding of the difference between activated alumina and molecular sieves, so that a more reasonable selection can help bring out their respective characteristics.

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