Molecular Sieve

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Molecular Sieve

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Molecular Sieve 3A, 4A, 5A, and 13X… What’s the difference?

Note: An angstrom (A) = 1/10 nm (nanometer) = 1/10,000,000,000 m (meter)

Why would someone chose 3A over 4A? –-> The answer depends on what you are trying to accomplish with your molecular sieve.
See example: Purifying methanol and ethanol

Molecular Sieve Basics: Crystals Help Determine the Pore Size of Molecular Sieve

Molecular sieve are crystalline metal aluminosilicates that belong to the zeolite family.  
=Alumina, silicon, and oxygen.

Molecular sieves specialize in separating very small molecules and atoms apart from one another. 
Being part of the zeolite family, molecular sieve has a three dimensional network of pores which can adsorb molecules of a specific size.

Four standard pore sizes that a molecular sieve can have:

==> 3A, 3 Angstrom pore size
==> 4A, 4 Angstrom pore size
==> 5A, 5 Angstrom pore size
==> 13X, 10 Angstrom pore size (depending on the manufacturer the pore size may be either 8 or 9 A)

Two structure types:

==>A structure (3A, 4A, and 5A are made from an A structure)
==>X structure   (13X is made from an X structure. )

Molecular Sieve A Structure

Molecular Sieve X Structure

Aluminum Hydroxide, Sodium Hydroxide, Sodium Bicarbonate, and clay are used in the sieve manufacturing process, when the process is created this combination of material will make 4A molecular sieve when created with a type A structure or 13X molecular sieve when created with a type X structure.3A and 5A molecular sieve are made once they are ion exchanged with the originally created 4A sieve. 
==> 4A molecular sieve is ion exchanged with potassium to create 3A sieve, the potassium molecules are larger than the sodium molecules they were exchanged with shrink the pore size.  
==> 5A sieve is created when 4A sieve is ion exchanged with calcium, calcium molecules are exchanged in a 1:2 ratio.  Every calcium molecule removes two sodium molecules thus increasing  the size of the pore.

7 Properties That Determine the Quality of Your Molecular Sieve

-Density – Knowing the density (when coupled with water adsorption) allows you to figure out the overall water capacity of a vessel in terms of volume or mass. Higher capacity = more water adsorbed.  A more valuable sieve has a higher volumetric capacity.

-Particle size and distribution – Allows for the calculation of pressure drop, fluidization parameters, and critical velocity through the bed which ultimately effects flow rate.  A higher quality sieve has a tight distribution with less “tails.”

-Static water adsorption – This refers to the overall capacity of the sieve to adsorb water.  (Do not confuse with working capacity which is much less than static capacity and varies with the operation as well as the sieve).  For more information on working capacity see my previous article on calculating working capacity.  A sieve with a higher static water adsorption capacity is always better.

-CO2 adsorption – This measures how much ethanol is being adsorbed with the water in your dehydration beds.   Water and (sometimes ethanol) can be adsorbed by 3A sieve because 3A is made from 4A sieve and as a result the sieve bed will not entirely be made up of 3A.  Some of the left over 4A sieve adsorbs CO2 and ethanol therefore the higher the CO2 adsorption rate is the higher the ethanol co-adsorption rate in the bed is.  This ultimately reduces the overall working capacity per cycle in an ethanol plant, look for low CO2 adsorption rates.

-Crush strength – This one’s simple, the higher the crush strength the higher the durability of the molecular sieve beads in operation.  A higher number here means a higher quality sieve.

-Attrition – This refers to fryability, which is the tendency of the sieve beads to grind up, which produces dust, thus lowering the overall capacity of the bed.  A lower attrition number is better.

-Ethanol ΔT (Methanol Delta T) – This is a measurement of the ability of sieve to adsorb ethanol, or a measurement of the co-adsorption characteristics of water and ethanol.  If capacity is being taken up by ethanol then the water capacity suffers, which is why a lower number is better.

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