Differences in Adsorption Performance between 5A and 4A Molecular Sieves

Size of Adsorbable Molecules

• 5A Molecular Sieve: With an average pore size of approximately 5 Å, it can adsorb molecules with a critical diameter of less than 5 Å. Besides small molecules such as water, carbon dioxide, and hydrogen sulfide, it can also adsorb normal alkanes and some alcohols. For example, in the petrochemical industry, it can adsorb normal alkanes like n – hexane (with a molecular diameter of about 4.3 Å) for the dewaxing process of oil products.
• 4A Molecular Sieve: Having an average pore size of about 4 Å, it can only adsorb molecules with a critical diameter of less than 4 Å. Common adsorbable substances include water, methanol (with a molecular diameter of about 3.8 Å), ethanol, hydrogen sulfide, sulfur dioxide, and other small – molecule substances. Molecules with a diameter larger than 4 Å, such as propane (with a molecular diameter of about 4.3 Å) and butane, cannot be adsorbed by 4A molecular sieve.

Adsorption Selectivity

• 5A Molecular Sieve: It has unique selective adsorption capabilities, especially in the separation of normal and isomeric alkanes. It can preferentially adsorb normal alkanes while having a relatively weak adsorption capacity for isomeric alkanes. This characteristic makes it highly valuable in the alkane separation process in the petrochemical industry, where it can be used to separate normal alkanes from mixed alkanes to meet the requirements for producing high – quality aviation kerosene, diesel, etc.
• 4A Molecular Sieve: It mainly has good adsorption selectivity for polar molecules and unsaturated molecules, with extremely high selectivity for water. Due to the strong polarity and small molecular diameter of water molecules, 4A molecular sieve can adsorb water molecules quickly and efficiently. Therefore, it is often used as a high – efficiency desiccant and is widely applied in the drying process of gases and liquids.

Adsorption Capacity

• 5A Molecular Sieve: Generally, it has a relatively large adsorption capacity for adsorbable molecules. Especially for some larger molecules within its pore – size range, such as normal alkanes, it can accommodate more molecules. Under certain conditions, the adsorption capacity of 5A molecular sieve for normal alkanes is higher than that of 4A molecular sieve because its larger pore size and suitable pore – channel structure are more conducive to the entry and storage of large molecules.
• 4A Molecular Sieve: Although it also has a certain adsorption capacity, its overall adsorption capacity for some larger molecules is relatively small. However, in the adsorption of small molecules like water, the adsorption capacity of 4A molecular sieve can also meet the needs of many practical applications. Moreover, due to its relatively small pore channels, it has a strong adsorption affinity for small molecules and can achieve a high adsorption amount at a lower partial pressure.

Adsorption Speed

• 5A Molecular Sieve: Due to its larger pore size, the diffusion speed of molecules within the pore channels is relatively fast. Therefore, the adsorption speed for some larger molecules is relatively high. When dealing with a system containing larger adsorbable molecules, 5A molecular sieve can reach a high adsorption amount in a relatively short time.
• 4A Molecular Sieve: It has a very fast adsorption speed for small molecules because small molecules can quickly enter its smaller pore channels and be adsorbed. However, for molecules with a diameter close to or slightly larger than its pore size, the adsorption speed will be significantly slower, or they may not be adsorbed at all. For example, when there are both water molecules and slightly larger alcohol molecules in the system, 4A molecular sieve will preferentially and quickly adsorb water molecules.