The difference between activated alumina and molecular sieve

Both activated alumina and molecular sieves are commonly used adsorbents in industrial production, and both play an irreplaceable role. But there is a question that has been entangled with us, that is, what is the difference between activated alumina and molecular sieve? What are the different effects. Knowing this, I can make rational product choices in industrial production. So today I focus on analyzing their specific differences from the perspective of structure, adsorption performance, and application.

First, the difference in structure

Both activated alumina and molecular sieve are solid materials with high porosity and high dispersion, and both have a large specific surface area, but the difference between activated alumina and molecular sieve can be seen from the distribution of structural pores. Molecular formula of activated alumina: Al2O3 -x(OH)2x, 0<x<0.8. The pore structure distribution of activated alumina is relatively messy, and is generally divided into micropores, mesopores and macropores according to the proportion of pore size distribution. Activated alumina pore distribution is not very uniform.

The molecular sieve is different. Molecular sieve molecular sieve: (M)2/nO·Al2O3·xSiO2·pH2O. Molecular sieves have many channels 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. Therefore, from the structural pore distribution, we can basically understand the difference between activated alumina and molecular sieves.

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, and has a special adsorption polarity for water, so it is used in daily industrial production. Active oxidation is commonly used as a desiccant, and can also be used 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, high specific surface area and is in 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 resurrected, and it can be carried out 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 the molecular sieve. The molecular sieve has a uniform pore distribution, which makes the selective adsorption performance of molecular sieve much better than that of activated alumina. The adsorption of molecular sieve is a process of physical change. The main reason for the adsorption of molecular sieves is a kind of "surface force" generated by the action of molecular attraction 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, resulting in molecular concentration on the surface of the molecular sieve. The number of such molecules in the fluid is reduced 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 depends on the size of the molecules.

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

3. Differences in application

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 normally reach -40 °C. Molecular sieve is only used when the drying depth is relatively high, and the gas of activated alumina desiccant can be used. Mainly: 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.

Activated alumina and molecular sieve product map

Molecular sieve has strong hydrophilicity, because in industrial production, sometimes the water content of 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 sieve can be used in extremely low water content. Then adsorption is carried out at the level of molecular sieve, and the drying depth of molecular sieve can reach -70 °C. At this time, there is a problem. Since the desiccant depth of molecular sieve is so high, why do we not use molecular sieve directly? We cannot understand that the cost of the first molecular sieve is relatively high. Necessary waste. The second is the difference between the two adsorption conditions. 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 has no obvious effect on activated alumina, which is caused by molecular structure. , but at a very low level of water content, it needs to be reduced by another order of magnitude. At this time, the water absorption rate of molecular sieve is stronger than that of activated alumina. In fact, sometimes we can use activated alumina and molecular sieve together, which can give full play to their respective strengths and use them rationally, which not only saves costs but also achieves the effect of twice the result with half the effort.

Both of them have their own advantages and disadvantages in industrial production. I should have a basic understanding of the difference between activated alumina and molecular sieves, so that a more reasonable selection is conducive to giving full play to their respective characteristics.