WHAT IS MOLECULAR SIEVE?

Molecular sieves are synthetic zeolite materials engineered with pores of precise and uniform structure and size. This allows them to preferentially adsorb gases and liquids based on molecular size and polarity. Zeolites are naturally existing, highly porous crystalline solids, belonging to the class of chemicals known as aluminosilicates.

A practical example of the function of molecular sieves is to dry ethanol. Because of the azeotrope formed when it is mixed with water, normal distillation of ethanol can only achieve a purity of 96% ethanol – the remaining 4% being water. For ethanol to be considered fuel grade, it has to be greater than 99% dehydrated. To achieve this level of purity, a 3A molecular sieve, designed specifically with 3 Angstrom-sized pores, is used to adsorb the water molecules, while the larger ethanol molecules are excluded. As there is no competition for adsorption, this process easily dehydrates ethanol to the desired level of purity so that it can be considered fuel grade.

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The size of the pores of both Type A and Type X molecular sieves is closely controlled during the manufacturing process. Sodium, calcium, and potassium ions can be exchanged with one another in the molecule to regulate the size of the pore opening. This allows for preferential adsorption of gas and liquid molecules. To get a sense of how this works, try to imagine a parking garage: the height of your vehicle is 7′, but the roof of the garage is only 6’8″. As hard as you may try, you are not going to get your vehicle into the garage. The same principal applies to the adsorption of molecules in the pores of a molecular sieve. This allows scientists and engineers to design systems that can separate chemicals on a molecular level.

Many people do not realize all of the applications of molecular sieves that help to improve our everyday lives. Almost every imaginable product has been touched by molecular sieve in some way. From steel production, insulated glass windows, fuel ethanol, and oxygen for breathing apparatuses, to the air conditioning filter cores in our cars, molecular sieves are a part of our lives every day.

Different Types of Molecular Sieves?

There are four main types of molecular sieves: 3A, 4A, 5A, and 13X. The type depends on the chemical formula of the molecule and the molecular sieve determines the size of the pore. The molecular sieve works by adsorbing gas or liquid molecules that are effectively diameter than pores, except for those molecules with larger openings

3A Molecular Sieve

3A means that the pore size measures as 3 angstrom.  Anything larger than 3 angstrom won’t be able to be adsorbed. The order of adsorption rate is helium, neon, nitrogen and water.

3Å molecular sieves have a bulk density in the range of 0.60-0.68 g/ml. They’re mainly used in the petroleum industry for desiccating alkenes and purifying petroleum gas, as well as for selective H2O absorption in polyurethane and insulated glass.

3A EDG Molecular Sieve

This Molecular Sieve was made with drying fuel grade ethanol in mind.  Ethanol can only be dried to the azeotropic point of 95.6% purity by traditional distillation.  This is the method of drying that is picked by fuel ethanol producers.

4A Molecular Sieve

4A means that the pore size measures as 4 angstrom.  Anything larger than 4 angstrom won’t be able to be adsorbed. The order of rate of adsorption is argon, krypton, xenon, ammonia, carbon monoxide, C2H4, C2H2, CH3OH, C2H5OH, CH3CN2, CS2, CH3CL, CH3Br, and carbon dioxide.

Having a bulk density of 0.60 to 0.65 g/ml, 4A molecular sieves are used for the following industrial applications:

• Used by pharmaceutical companies in medical containers to protect the contents from moisture contamination.
• Used by food manufacturers as an additive (anti-caking agent)
• Used for static dehydration for closed gas or liquid systems, i.e. in drug packaging.
• Used for scavenging water in plastics and printing systems.
• Used for drying saturated hydrocarbon streams.

4A Blue Indicating Molecular Sieve Desiccant

4A is similar to 4A Molecular Sieve however it is injected with an inorganic metal salt moisture indicator. The important thing is that it is visibly noticeable to determine when the mole sieve attains saturation.  The molecular sieve beads turn blue when they becoming active.  The beads are colored beige when they are saturated.

5A Molecular Sieve

5A means that the pore size measures as 5 angstrom.  Anything larger than 5 angstrom won’t be able to be adsorbed. The sequence rate of adsorption is C3-C14, C2H5CL, C2H5Br, CH3L, C2H5NH2, CH2CL2CH2Br2, CHF2CL, CHF3, CF4, (CH3)NH2, B2H6CF2CL2, CHFCL2, and CF3CL.

 Like their 4A counterparts, 5Å molecular sieves have a bulk-density in the range of 0.60 to 0.65 g/ml as well. These are most comply used for:

• Purification and desiccation of air
• Desulfurization and dehydration of LPG (liquefied petroleum gas) and natural gas.
• Hydrogen and oxygen production through the PSA (pressure swing absorption) process.
• Used in oil refineries for separating hydrocarbons and alkenes.
• Used for adsorbing water in non-nitrogen sensitive polyurethane.

13X Molecular Sieve

This Mole Sieve is considerably larger any of the other A type openings.   Its pour size is 10A.  This desiccant is used mainly for modifications of gases and liquid because it offers corresponding absorption for bi-molecule and tri-molecule.  The sequence rate of adsorption is SF6, CHCL3, CHBr3, CHI3, N-C3F8, CCL4, N-C4F10, N-C7H16, CBr4, C6H6, B5H10, (CH3)3N, C(CH4)4, (C2H5)3N, C(CH3)C3CL, C(CH3)3Br, and C(CH3)3CH.