Molecular sieves adsorb moisture from the air like other desiccants but it is their pore structures and high adsorption capacities that set them apart.
Molecular sieves are adsorptive materials often used as desiccants for the purpose of removing moisture from gases and liquids. They are often made from a type of zeolite, which is a crystalline material with a porous structure that allows for selective adsorption of molecules.
The chemical formula for molecular sieve desiccants is not singular, as there are many different types of molecular sieve materials with varying chemical compositions.
However, molecular sieves are typically composed of metal oxides, such as aluminum oxide, silicon oxide, and titanium oxide, along with other elements, such as sodium, potassium, and calcium.
The specific chemical formula for a particular molecular sieve desiccant will depend on the specific composition of that material, which in turn will depend on the type of molecular sieve and the manufacturing process used to produce it.
Thanks to their ability to selectively adsorb molecules, molecular sieve desiccants support a wide range of applications in various industries, including:
The unique properties of molecular sieve desiccants make them an important component in various industrial processes where selective adsorption or catalytic activity is required.
There are many different types of molecular sieve desiccants, which differ based on the size and shape of their pores, the specific molecules they are designed to adsorb, and the applications for which they are best suited.
The different sizes of molecular sieves are created by swapping out cations (positive ions) in the chemical makeup of the desiccant. In turn, this creates larger or smaller pores in a highly organized crystalline structure, which enables smaller molecules to pass through while trapping larger molecules. In this way, it’s possible to use molecular sieve to selectively capture specific compounds.
3A, 4A, and 5A molecular sieves are organized in a Linde Type A crystal structure, while the 13X utilizes a Type X crystal structure.
Here are some of the most common types of molecular sieve desiccants:
Additionally, each molecular sieve features different mesh sizes, commonly either 4 x 8 or 8 x 12. Learn more about mesh sizes as it pertains to molecular sieve desiccants.
Be aware that there are many types of molecular sieves and this list is not exhaustive. However, the specific type of molecular sieve desiccant used in a particular application will depend on the specific requirements of that application, such as the type of gas or liquid being dried, the purity requirements, and the desired operating conditions.
The amount of water that a molecular sieve can adsorb before it saturates will depend on various factors such as the pore size of the molecular sieve, the relative humidity of the surrounding environment, and the temperature. Generally, molecular sieves are more effective at adsorbing water than silica gel, often adsorbing approximately 21% of their weight in water and at a faster rate.
Know that it is common industry practice to swap out desiccants once the RH of an enclosure (or other system being desiccated) reaches 40%RH, with the goal of maintaining an RH level less than 40%. Therefore, it is at this level of humidity that most desiccant capacities are compared.
It is also important to note that like silica gel, once a molecular sieve becomes saturated with water, it will no longer be effective at adsorbing moisture. In fact, it may start releasing moisture back into the gas or liquid stream. Therefore, it's important to monitor the saturation level of the molecular sieve and replace it or regenerate it as needed to ensure that it continues to be effective.
Molecular sieve desiccants are manufactured through a series of steps that involve the synthesis of the molecular sieve material, shaping the material into the desired form, and activating it to create a highly adsorptive desiccant. Such processes include:
It is worth noting that the specific process for manufacturing molecular sieve desiccants may vary depending on the desired application and the specific type of molecular sieve material being used. Additionally, the manufacturing process may include further steps, such as impregnation with certain metals or chemicals, to improve its performance for certain applications.
While molecular sieve is a highly effective desiccant, its benefit to a given project, as compared to silica gel or activated alumina, will depend on the particular application and its specific requirements. For help choosing, Contact AGM.
Like other desiccants, molecular sieves trap moisture (adsorb moisture); effectively removing it from the surrounding atmosphere. However, molecular sieves feature several unique characteristics.
Molecular sieves are formed from zeolite, which provides them with a highly organized crystalline structure. Different sized molecular sieves are formed through the combination of the zeolite with different cations. These two traits (structure and pore size) enable molecular sieve desiccants to adsorb moisture or various other molecules from ambient air, flowing gas streams, and liquids.
Like many other desiccants, molecular sieve can be regenerated for reuse.
However, due to molecular sieve’s high thermal stability, regenerating temperatures are in the range of 400°C (752°F) to 600°C (1112°F), in order to remove adsorbed water and other impurities. This range is much higher in comparison to silica gel or activated alumina.
The exact regeneration process can vary depending on the specific molecular sieve material and its intended application, but it typically involves heating the desiccant material in a stream of hot gas, such as nitrogen or air, for a period of time.
AGM recommends storing molecular sieve in metal cans at room temperature (between15°C (60°F) and 27°C (80°F)) with humidity between 0%RH and 70%RH.
AGM guarantees its molecular sieves for a minimum 1-year shelf life when stored in the original, unopened packaging.
AGM is developing a sweet of tools to help you determine how much desiccant is necessary to protect your project. AGM’s Desiccant Unit Calculator is a free tool that provides desiccant unit quantities for Molecular Sieve, Silica Gel, and Clay desiccants based on a given volume of space.
Molecular sieve desiccants play a significant role in the moisture protection process for a wide set of industries. Their strengths include high adsorption capacity, great thermal stability, and selectivity of molecule capture.
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