A molecular sieve is a porous material that has very small, uniform-sized holes. It works like a kitchen sieve, except on a molecular scale, separating gas mixtures that contain multi-sized molecules. Only molecules smaller than the pores can pass through; whereas, larger molecules are blocked. If the molecules you want to separate are the same size, a molecular sieve can also separate by polarity. Sieves are used in a variety of applications as moisture removing desiccants and help prevent the degradation of products.
Types of Molecular Sieves
Molecular sieves come in different types such as 3A, 4A, 5A and 13X. The numeric values define the size of the pore and the chemical composition of the sieve. The ions of potassium, sodium, and calcium are altered in the composition to control the size of the pore. There are different numbers of meshes in different sieves. A molecular sieve with a smaller number of meshes is used to separate gases, and one with more meshes is used for liquids. Other important parameters of molecular sieves include the form (powder or bead), bulk density, pH levels, regeneration temperatures (activation), moisture, etc.
Molecular Sieve vs. Silica Gel
Silica gel can also be used as a moisture removing desiccant but is very different from a molecular sieve. The different factors that can be considered while choosing between the two are assembly options, changes in pressure, moisture levels, mechanical forces, temperature range, etc. The key differences between a molecular sieve and silica gel are:
- The rate of adsorption of a molecular sieve is greater than that of silica gel. This is because the sieve is a fast-drying agent.
- A molecular sieve functions better than silica gel in high temperatures, as it has a more uniform structure that binds water strongly.
- At low Relative Humidity, the capacity of a molecular sieve is far better than that of silica gel.
- The structure of a molecular sieve is defined and has uniform pores, while the structure of silica gel is amorphous and multiple irregular pores.
How to Activate Molecular Sieves
To activate molecular sieves, the basic requirement is exposure to super-high temperatures, and heat should be high enough for the adsorbate to vaporize. The temperature would vary with the materials being adsorbed and the type of adsorbent. A constant temperature range of 170-315oC (338-600oF) would be required for the types of sieves discussed earlier. Both the material being adsorbed, and the adsorbent are heated up at this temperature. Vacuum drying is a quicker way of doing this and requires relatively lower temperatures compared to flame drying.
Once activated, the sieves can be stored in a glass container with a double wrapped parafilm. This will keep them activated for up to six months. To check if the sieves are active, you can hold them in your hand while wearing gloves and add water to them. If they are completely active, then the temperature rises significantly, and you will not be able to hold them even while wearing gloves.
The use of safety equipment like PPE kits, gloves, and safety glasses is recommended as the process of activation of the molecular sieves involves dealing with high temperatures and chemicals, and the associated risks.
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