A Beginner’s Guide to Molecular Sieves

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Moisture along with humidity are probably some of the major impediments to keeping things fresh and decay-free. Most larger companies purchase desiccants in bulk in order to maintain their goods moisture-proof throughout the process of manufacture and shipping.A molecular sieve, in simple words, is a tiny bead-like structure that has pores in it. The volume and capacity of the pore are the two most important factors when it comes to determining the size and quantity of quality molecular sieves. You will definitely need more sieves to absorb moisture if the pore size is tiny. 

Generally, there are three sizes of molecular sieve desiccants available that are 3A, 4A, and 5A. Molecular sieves are used for a variety of applications in various industries. Desiccant manufacturers produce them synthetically in large quantities.

The Science Behind Molecular Sieve

Zeolites are the main component used to make molecular sieves (also known as aluminosilicates). Zeolites are synthesized by heating alumina together with silica solutions with sodium hydroxide. These are porous structures, meant to operate as a sieve with the help of a selective filtering process. The size of the pore determines the entire absorption mechanism. Larger pores will be capable of capturing larger molecules.

Molecular sieves are basically three-dimensional interconnected networks of silica and alumina, which is further a tetrahedron in crystalline metal aluminosilicates. Heat is used to create consistent cavities in this unique network that selectively absorb molecules of a given size, removing natural water of hydration.In the case of gas-phase applications, a 4 to 8-mesh sieve is often utilized. On the other hand, an 8 to 12-mesh sieve is best in liquid-phase applications. The 3A, 4A, 5A, along with the 13X sieves are available in powder form and they are used for particular applications.

Long known for their drying capability (even as highest as 90 °C), molecular sieves have lately been very useful in synthetic organic operations. They often permit the separation of desirable products from strong condensation reactions that are regulated by normally unfavorable equilibria. Water, HCl, and alcohols (for example methanol and ethanol) have been demonstrated to be removed by these quality synthetic zeolites from powerful systems. Common examples are ester condensations, ketimine and enamine syntheses, and the conversion of unsaturated aldehydes to polyenals.

Types of Molecular Sieves

There are three material types when we talk about Molecular sieves; microporous, mesoporous, or macroporous.

1. Microporous material (<2 nm)

  • Zeolites (aluminosilicate minerals, not to be confused with aluminum silicate). For example Zeolite LTA: 3–4 Å
  • Porous glass: 10 Å (1 nm)+
  • Active carbon: 0–20 Å (0–2 nm)+
  • Clays

 

2. Mesoporous material (2–50 nm)

  • Silicon dioxide (used to make silica gel): 24 Å (2.4 nm)

 

3. Macroporous material (>50 nm)

  • Mesoporous silica, 200–1000 Å (20–100 nm)

 

Applications

Molecular sieves are used in many industries such as:

  • They are often used to purify ammonia in the massive food businesses that often utilize palletizing robot arm for food processing.
  • Molecular sieves are often utilized in the petroleum industry, notably for drying gas streams.
  • They are widely employed in the gas and oil industries to dry gas streams.
  • Zeolites are molecular sieves that are used in a range of catalytic applications. For example, alkylation, hydrocracking, fluid catalytic cracking to catalyze isomerization, and epoxidation are large-scale industrial processes.
  • To prevent ice jams in the liquid natural gas (LNG) sector, for example, the water content of the gas should be less than 1 ppmv. Molecular sieves are the greatest way to prevent this.
  • Molecular sieves are also often used for aerosol purification. Mercaptan is removed from aerosols using molecular sieves. Mercaptans are pungent-smelling chemicals that are not eliminated during distillation and must be removed using molecular sieves.
  • In the laboratory, molecular sieves are employed to dry the solvent. Traditional drying methods, which sometimes require the use of powerful desiccants, have been demonstrated to be weaker compared to “sieves.”
  • They are also used to filter the air supply for breathing equipment used by scuba divers and firefighters. An air compressor supplies the air in this sort of application. Before it can be used to charge breathing air tanks, it goes through a cartridge filter that contains a molecular sieve or some activated carbon (depending on the use). This kind of filtration may remove particulates as well as compressor exhaust products from the breathing air supply.
  • To eliminate water created during reactions, the chemical industry extensively depends on molecular sieves.
  • Many different chemicals and compounds are used to create goods in pharmaceutical industries. When the products are created, the residue is often left behind. Within them, molecular sieves absorb undesirable compounds and moisture. This increases both procurement and ultimate product quality.

Regeneration of Molecular Sieves

Pressure change (as in oxygen concentrators), heating and purging with a carrier gas (as in ethanol dehydration), or heating under a high vacuum are all methods used exclusively for regenerating molecular sieves.Depending on the molecular sieve type, regeneration temperatures vary anywhere from 175°C to 315°C. In contrast, silica gel can also be regenerated by heating it in a standard oven. This can be done for two hours at 120 °C (250 °F). When exposed to enough water, certain varieties of silica gel will start to pop. This is due to the silica spheres breaking when they come into touch with water.

How to Select the Best Molecular Sieve

Quality molecular sieves by any top molecular sieve manufacturer come in a variety of forms and different sizes. However, spherical beads have an advantage over other forms. This is because they have a smaller pressure drop, and they are more attrition resistant due to the lack of sharp edges. Not only this but they also have superior strength, i.e. the crushing force needed per unit area is much greater. 

Certain beaded molecular sieves come with a smaller heat capacity so they need less energy during regeneration as compared to others. Another super benefit of employing beaded molecular sieves is that they have a bulk density which is generally greater than that of other forms. As a result, less molecular sieve volume for the same adsorption demand. 

Thus, during the process of de-bottlenecking, beaded molecular sieves are considered as the best option to load more adsorbent in the same volume, at the same time, avoiding vessel alterations.

Content supported by Kastnerlabs UK design specialists

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