What is Molecular Sieve?
Molecular sieve are materials with pores of uniform or equal size. The diameter of these pores is very similar to small-sized molecules. So, the molecules having a diameter larger than the diameter of pores can not enter or be absorbed while smaller size molecules can.
The mixture of different sized molecules will be passing through the sieve or the matrix so that molecules with higher molecular weight will leave the base first followed by smaller molecules.
Molecular sieve will be used in the process of separating the molecules based on their sizes, which is known as chromatography.
Like silica gel or activated alumina, it is also used as a desiccant in industrial procedures.
The diameter of the pores of the sieve is measured in angstroms (Å) or nanometers (nm).
Various types of molecular sieves are available for application in the market.
Molecular sieves are extremely efficient for removing water from liquids and gases. They are even more effective than desiccants like activated alumina or silica gel because of their crystalline composition.
Molecular sieves can create virtually water-free products. They are very useful in cryogenic operations when gas needs to be liquified, or to prevent it from freezing, water needs to be completely absorbed.
The basic applications of molecular sieves consist of drying gas stream on the petroleum industry and drying solvents in the laboratory along with various catalytic applications. They are also used for air interaction and liquid purification assemblies.
Compared to any other systems, using molecular sieves to remove impurities from a liquid or gas is very easy, because they can be designed as per the size impurity one wishes to filter.
Ease of use and super efficiency are the most important aspects which make a molecular sieve a very important solution compared to any other kind of desiccant available in the market.
Molecular Sieve Materials:
Molecular sieves can be microporous, mesoporous, or macroporous materials.
These materials have had a diameter of less than 20 Å which is around 2nm.
- Zeolites: diameter of Zeolite LTA is 3-4 Å.
- Porous Glass: more than 10 Å.
- Activated Carbon: 0-20 Å.
○ 1 nm spacing for hydrated clay
○ 0.7 nm spacing for meta-halloysite or dehydrated clay.
The molecules in this category have a diameter between 2-50 nm (20-500 Å).
- Silicon dioxide which is used to make silica gel has a diameter of around 24 Å.
These materials have a diameter more than 50nm.
- Mesoporous silica: 200-1000 Å (20-100 nm)
Application of Molecular Sieve:
● In the Petroleum Industry:
Molecular sieves are widely used in the petroleum industry.
The main application of molecular sieves is in the drying gas streams assemblies.
For example, in the liquid natural gas (LNG) industry the water content of gas should be less than 1 ppmv. If it is more, then the chances of blockages in the stream due to ice can increase.
● In Laboratory:
In laboratories, molecular sieves are used to dry the solvents. These sieves have proven superior to any traditional drying technique, which is often a desiccant.
● In catalytic operations:
Zeolites or molecular sieves are used in various catalytic operations.
They are used to catalyze processes like isomerisation, alkylation, and epoxidation.
Molecular sieves are used in large scale industrial processes like hydrocracking and fluid catalytic cracking.
● Air and Water Filtration Assemblies:
They are used in air and water supplies in the filtration apparatus, for example, the underwater masks used by scuba divers or firefighters.
In such kinds of applications, the air is supplied by an air compressor and after that, depending on the nature of the application, it is filled with a molecular sieve or activated carbon or a combination of both.
These filters remove particulates and compressor exhaust products from the breathing air supply.
The United States FDA approved molecular sodium aluminosilicate for its direct contact with consumables back in 2012.
Before this approval, Europe used the molecular sieve in different pharmaceutical applications. Different independent testing also suggested that molecular sieve meets all the government requirements. But sieving industry has been unwilling to fund for the more advanced testing.
Regeneration of Molecular Sieve:
There are three different procedures used for the regeneration of the molecular sieve.
- Changing the pressure by using oxygen concentrators.
- Heating and purging with a carrier gas, which is also used while dehydrating ethanol.
- Heating the sieve inside a high vacuum assembly.
Unlike silica gel which can be regenerated by heating it to a temperature of 120°C in a regular oven, the temperature range for regeneration for molecular sieve varies from 175°C to 315°C depending on the type of molecular sieve used.
How to select a Molecular Sieve?
Now that we’re done with the different types, their dimensions, and various applications of molecular sieve, we need to know how to choose the right molecular sieve for your particular application. Molecular sieves come in several shapes and sizes in the market. But the molecular sieves shaped as spherical beads have an advantage over any other shape because they offer a very low-pressure drop.
It will also not reduce the force of stream passing through it since the spherical structure does not have any sharp edges and is also a very strong structure which means that the force required to crush a molecular sieve spherical bead is higher per unit area. A few molecular sieves provide lower heat capacity which will result in lower power requirements.
Another important advantage of using the spherical shape over any other shape is that the bulk density of this shape will be a lot higher than any other shape so for absorption applications, the area required for molecular sieve will reduce.
Thus, while you’re performing debottlenecking, use a beaded molecular sieve, load more absorbent in the same volume, and avoid any kinds of vessel modification.
Molecular Sieves due to its manufacturing process, it is highly customizable and also highly efficient as well as it is easy to use.
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