Mole Sieve 3A vs. 4A

[Badders]

We have been using 4A mole sieve in our gas plant for years to remove the water from natural gas. We have to change out the beds this year and some companies are recommending changing from 4A to 3A.

What are the advantages/disadvantages of using 3A instead of 4A?

Are there any operational problems that we might encountered by the switch?

 

[aspearin1]

One utilizes sodium exchange ions, while the other utilizes potassium exchange ions. This difference accounts for a different seive pore size (in Angstroms, thus 3"A" and 4"A") At the moment, I cannot recall which cation corresponds to which molecular sieve. But sodium and potassium differ in cation exchange capacity (CEC) which will affect its usefulness as a catalyst surface. You may wish to start you search in such a direction.

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee

 

[Montemayor]

Badders:

If you do your own research on the matter, I'm sure you will confirm my field experience that Mol Sieves (whether 3A or 4A) are a process over-kill when applied to drying Natural Gas. Additionally, they are much more costly for drying Natural Gas to pipeline standards. However, you don't state your scope of work, so I'll assume you are drying the Natural Gas for downstream liquefaction and thus require a product dew point of -100 oF or less. If so, then Mol Sieves is the process of choice.

Another item of practical experience: never take another engineer's advice on which Mol Sieve to employ in your process when you can simply raise the phone and request a free, secure, and expert opinion from the Mol Sieve manufacturer. Some suppliers will even warrant the application. Why you don't do this is beyond my understanding.

What Aspirin writes about may be great Physical Chemistry basics, but the statement that "Sodium and Potassium differ in cation exchange capacity which will affect its usefulness as a catalyst surface" has zero applicability in the Unit Process called Adsorption. Adsorption, as proposed being used in this application, involves the physical surface-binding capability of adsorbents by Van der Waals forces. This physical effect has nothing to do with cation exchange or any catalyst effects at all. The basic adsorbent does not change physically or chemically during the process or adsorption and de-sorption (regeneration).

That's why I stand by my firm recommendation to always refer to the real "experts" in the matter: the manufacturer of the adsorbent(s).


Art Montemayor
Spring, TX

 

[dcasto]

Here's what most of the manufacturers will tell you, if the sieves are upstream of an NGL plant: The 3A will not adsorb as much propane and heavier and those molecules will be reovered in the plant. They wat to sell what is best for them too.

If you regenerate with inlet gas, propane and heavier loss is not an issue. The amount lost using residue gas will not be great enough to offset the higher cost of the 3A. So take the vendors recommendations and do your own lost revenue analysis.