Molecular Sieves as drying agents in industrial production

Molecular sieves have a particularly high affinity for water and have strong water absorption properties, thus they are widely used for gas drying and are a relatively ideal drying agent. Experiments have shown that argon gas continuously dried by Mg(CIO4)2, P2O3 and metallic sodium is not as effective as that dried by molecular sieves. Meanwhile, molecular sieves are stable in nature, not afraid of heat, not afraid of water, not eroded by various solvents, and can be regenerated multiple times while still maintaining good adsorption performance, and can be used for a long time. The advantages of using molecular sieves for gas drying are as follows:

(1)The drying degree is extremely high. After gas is dried by molecular sieves, products with extremely low dew point can be obtained, and no other freezing auxiliary equipment is required. The dew point of air after molecular sieve drying can be as low as -60 to -90℃, while using other drying agents such as alumina or silica for air drying, the dew point can only reach about -60℃.

(2)Strong drying capacity for gases with low relative humidity. When treating compressed gases with low relative humidity or gas that is not fully dried, the adsorption efficiency of molecular sieves is far greater than that of other adsorbents. The lower the water vapor content, the more significant the characteristics of molecular sieves. For example, at a relative humidity of 1%, the adsorption capacity of molecular sieves can reach 18% of its own weight, which is 10 times higher than that of activated alumina and 20 times higher than that of silica gel. Therefore, using molecular sieves to remove trace water from gases is very effective.

(3) Strong water absorption performance at high temperatures. For the drying of high-temperature gases, using molecular sieves for dehydration is the most excellent. For example, at 100℃, for air with a relative humidity of 1.3%, molecular sieves can adsorb up to 15% of the equivalent weight of water, which is 10 times higher than activated alumina and 20 times higher than silica gel.

When adsorbents adsorb water, they all release latent heat. This adsorption heat causes the bed temperature to rise, thereby reducing the adsorption capacity of the adsorbent. However, the adsorption capacity of molecular sieves is less affected by changes in bed temperature. Therefore, sometimes it is not necessary to wait until the bed layer is completely cooled before using it after regeneration.

(4) Good drying efficiency at high gas velocities

At high gas velocities, molecular sieves also have good adsorption capacity for gas drying. For example, at low gas velocities, the drying capacity of silica gel and molecular sieves is similar; when the gas velocity increases, the adsorption capacity of silica gel drops sharply, while that of molecular sieves only slightly decreases.

(5) Can simultaneously adsorb other impurity molecules

Molecular sieves can remove impurities other than water from the gas. Although the adsorption capacity of molecular sieves for water is much stronger than for other gas impurities, as long as the design is appropriate, water and other impurities can be removed simultaneously using molecular sieves.

(6) Selective adsorption

In many drying operations, the components in the raw materials are also often adsorbed during dehydration. Therefore, in certain adsorption drying processes, this co-adsorption problem must be considered. Using molecular sieves can control this phenomenon because molecular sieves have various sieves with different pore diameters, and by selecting a suitable molecular sieve with appropriate pore diameters, the raw material components cannot enter but only adsorb water to control the co-adsorption problem. For example, in the deep cold separation of raw gas for petroleum gas olefins, 3A type molecular sieves can be used to remove water while the olefins are not adsorbed.

Using a fixed bed of molecular sieves for gas drying is a typical adsorption process, and it can be designed relatively reliably using the data of the mass transfer section length.

The adsorption process of gas drying by molecular sieves is generally operated alternately with two or several adsorption columns. When the molecular sieve bed layer of this adsorption column reaches saturation for water adsorption, the adsorption column should be replaced and the regenerated saturated molecular sieve should be adsorbed. The higher the regeneration temperature, the more complete the regeneration, but the energy consumption is also greater, and the lifespan of the molecular sieve will also be shortened. Therefore, the regeneration temperature should be as low as possible to reduce energy consumption and shorten the regeneration cycle, which is beneficial for industrial production. Generally, the regeneration temperature is suitable at 200 - 350℃. The water absorption capacity of molecular sieves will decrease after multiple regenerations. For instance, after 200 regenerations, the general water absorption capacity will drop by approximately 30%, but if further regenerations are carried out, the decrease in water absorption capacity will slow down.

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