Molecular Sieve 3A, 4A, 5A, and 13X…

What’s the difference?

So what’s the difference between all of these molecular sieve types?  – The difference is the size of the pores that come on each molecular sieve bead.  The A in 3A stands for Angstrom,a unit of measurement named after Swedish scientist Anders Jonas Angstrom who was looking for a unit of measurement small enough to measure spectral lines (beams of light).  An angstrom is equal 1/10 of a nanometer, or 1/10,000,000,000 of a meter, so when speaking of 3A sieve it refers to the size of the pore on the bead which is 3 angstroms or 3/10,000,000,000 of a meter.  (On a side note here 13X equals 10A).

So why would someone chose 3A over 4A? – The answer depends on what you are trying to accomplish with your molecular sieve.  For example 3A vs 4A, ethanol producers try to make ethanol that is over 99% pure ethanol.  Traditional distillation methods only give them a 95% ethanol purity rate, while the remaining 5% of the substance  is mostly water.  In short they need to separate the final 5% of the water from the ethanol.  Ultimately the choose a 3A molecular sieve here is why.

For this example 3A sieve works best because the size of water molecule is approximately 2.8 angstrom and the size of an ethanol molecule is 3.8 angstrom.  The 3A sieve adsorbs all of the water molecules because they are small enough to fit inside the pores.   The ethanol molecules, which are too large to fit in the pores, are free to pass by thus separating water from ethanol.  If this person were to use 4A, 5A, or 13X sieve it would not work because the pore sizes are large enough to adsorb both the ethanol and water molecules, and thus no separation would occur.

Generally speaking 3A sieve is used for purifying methanol and ethanol.  4A is used for removing C02  and ammonia from natural gas streams as well as being a desiccant for refrigerants, medicines, and electrical components.  5A is used for sweetening natural gas and purifying hydrocarbon gas and liquid streams.  13X (which is really 10 Angstrom) is a multipurpose sieve, it can adsorb the all the particles that previous 3 sieves can adsorb, but it is usually used to sweeten natural gas streams and purify petrochemical liquids and gases.  Ultimately the pore size of sieve can have a very specific use like 3A or it can have wide range of uses like 13X, it all depends on what you wish to accomplish.