Inner Works Of Molecular Sieve

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Inner Works Of Molecular Sieve

Everyone knows Silica Gel as a desiccant (a moisture adsorbent), but there are other desiccants out there – that depending on the situation, it can be even more useful! In today’s Desiccant Story, we take a look at other desiccants that may not be as well-known, but still, play a big part in our everyday life! Today, we look into Molecular Sieve.

Molecular Sieves are standard sieves that separate wanted elements from unwanted materials. Molecular Sieves are used to remove unwanted water or gases. Coming in different pore sizes, Molecular Sieve can be used to filter out specific substances. These pore sizes are uniform (same size) throughout the entire Molecular Sieve and are measured in angstroms (Å), with standard sizes being 3Å, 4Å, 5Å, and 13X (10Å).

To give you an idea of how small Angstroms are, 1A is 10−10 m, one ten-billionth of a meter! Even the smallest molecule in the world, hydrogen (H2), is only 0.74Å, with water (H20), one of the smaller particles, being 2.75Å.

If you wanted to filter out water (molecule size of 2.75Å) from ethanol (molecule size of 4.4Å), you would use size 3Å or 4Å, as those sizes of Molecule Sieve would adsorb the water molecules, being small enough to enter through the pores, while leaving the ethanol molecules untouched. You would not use 5Å as that size of Molecular Sieve would adsorb both ethanol and water molecules, resulting in no filtering.

The preciseness of the pores allows Molecular Sieve to be a very effective desiccant, as the user can choose a size that removes what he wants to be removed, leaving the rest undisturbed.
At the start, we mentioned that Molecular Sieve could be better than Silica Gel as a desiccant. And it is true! Too often, people only consider the current relative humidity (RH) level; and forget to consider what is their desired RH level.

For bringing the RH level to deficient levels or almost to 0%, Molecular Sieve is the preferred choice. Other desiccants such as silica gel are mostly only able to reduce the RH level to 40%.

The final aspect of what makes Molecular Sieve such an amazing desiccant is its huge surface area: Its 3-Dimensional matrix structure allows for its high internal surface area – almost 1’000 m2/g – making it highly adsorbent.

Does every molecular sieve of different pore sizes have the same adsorption capabilities? Absolutely not! Due to the different sizes of the pores of molecular sieves, the adsorption capabilities also differ. For example, a 3Å sized molecular sieve would be able to adsorb 20% of its weight while a 4Å sized molecular sieve can adsorb at least 21% of its weight. The adsorption capabilities of molecular sieves rely on factors such as relative humidity and surrounding temperature.

 

Molecular Sieve has many uses, with the most common uses being

1) Gas or liquid drying, purification, or separation

2) Drying of air in confined spaces

3) Hydrocracking, catalytic cracking

4) Detergent builder and water softener.

Besides, unlike many desiccants, Molecular Sieve is effective under very high temperatures in the range of 100 to 300 degrees, making it suitable for machinery applications.

And that’s that! Today, you’ve learned about another desiccant: one that has many interesting properties and uses. Next time you’re looking for a desiccant, consider – Molecular Sieve.

 

References:

Paulus, L., Walker, D. and Thompson, K. (2003). A COMPARISON OF DESICCANT MATERIALS USED FOR MONITORING ATMOSPHERIC TRITIUM CONCENTRATIONS IN A SEMI-ARID CLIMATE. Health Physics, 85(3), pp.348-356.

Trusell, F. and Diehl, H. (1963). Efficiency of Chemical Desiccants. Analytical Chemistry, 35(6), pp.674-677.

Wang, W., Wu, L., Li, Z., Fang, Y., Ding, J. and Xiao, J. (2013). An Overview of Adsorbents in the Rotary Desiccant Dehumidifier for Air Dehumidification. Drying Technology, 31(12), pp.1334-1345.

Wahby, A., Ramos-Fernández, J., Martínez-Escandell, M., Sepúlveda-Escribano, A., Silvestre-Albero, J. and Rodríguez-Reinoso, F. (2010). High-Surface-Area Carbon Molecular Sieves for Selective CO2 Adsorption. ChemSusChem, 3(8), pp.974-981

Ratner, M.A. and Ratner, D. 2003.: Nanotechnology: A Gentle Introduction to the Next Big Idea. Prentice Hall Professional. E-book in https://0x9.me/9so5D. Accessed on 24 October 2015

Gupta, R. and Demirbas, A. (2010). Gasoline, diesel and ethanol biofuels from grasses and plants. Cambridge: Cambridge University Press.