8 replies to this topic

[processengineer1998]

Hi
I want to know the criteria for selection of silica gel or molecular sieve for dehydration of gas.
what are advantages and disadvantages of each process in design,operation and cost standpoints.

[Zauberberg]

You can refer to the GPSA Databook for more detailed information - a comparative overview of performance of different adsorbents is given in the "Dehydration" chapter.

In brief, you can see the basic difference from corresponding adsorption isotherms of Alumina, Silica Gel, and Molecular Sieves. Water adsorption capacity of Alumina and Silica Gel increases almost linearly with increasing water content of feed gas, and they have superior performance in high humidity portion of dehydration process (i.e. unit of mass of Alumina or Silica Gel can adsorb more water from the feed gas saturated with water, than Mol Sieves can do - see the PDF link), but Mol Sieves are superior in the so-called "polishing area" of water adsorption process since they can remove moisture from the feed gas down to 0.1ppm which other adsorbents cannot do, or impractically large mass of desiccant would be required. By having such high adsorption capacity in low H2O partial pressure region, and ability to hold moisture more firmly, they require more energy for desorption process.

http://www.airvacuum...t desiccant.pdf
http://www.bosnax.co...ieve&act_al.pdf

Attached Files

•  Adsorbents.JPG   28.19KB   77 downloads

[Art Montemayor]

Additionally to what Zauberberg has noted, I would add the following:

Silica Gel and Mol Sieves are not two different processes. In fact, they are not processes. They are simply adsorbents with significantly different properties and characteristics. The process is the same one: Adsorption.

From a practical standpoint, I have never come across an adsorption process that had the option of using either Silica Gel or Mol Sieves. The performances are unique to each other as well as the applications. You either REQUIRE MOL SIEVES - or you don't. If you don't require the ultra low dewpoint produced by mol sieves, then you are in the silica gel application area. But even then, I would not recommend silica gel if you have the option of obtaining Activated Alumina. Silica gel is a notorious dusting adsorbent. It breaks down in service - especially when agitated or confronted with liquid water droplets. I stopped using silica gel back in 1965 and never bought or applied it ever again. Activated Alumina is much more rugged and durable in service. It performs just as well - or even better than silica gel.

I hope this experience helps.

[ankur2061]

The link provided below gives an excellent analysis and comparison between the various adsorbents. This is worth a read:

http://www.porocel.c... Dehy Paper.pdf

Regards,
Ankur.

[ankur2061]

processengineer1998,

Additionally have a look at the attachment for the comparison of different types of adsorbents.

Regards,
Ankur.

Attached Files

•  Adsorbent_Properties_Comparison.pdf   262.23KB   204 downloads

[Zauberberg]

One additional - and very important - characteristic of Silica Gel is that it adsorbs Hydrocarbons as well (especially C5+), while Alumina doesn't have that affinity.

[ankur2061]

alok,

Have a look at the following link for Englehard sorbead technology (customised silica gel) for removal of hydrocarbons:

http://www.molecular...e-process2.html

This should help in your making a decision for silica gel adsorption bed for removal C5+ removal.

Regards,
Ankur.

[1729]

The link that Ankur gave in #4 seems to be broken; I cannot find this paper on the manufacturer's website.

Could someone please repost it?

[Art Montemayor]

1729:

Attached is the Abstract from BASF on Simultaneous Adsorption.

This is for you and any other interested Forum Member.

Attached Files

•  The Sorbead Quick Cycle Process - LRGCC 2007.pdf   189.39KB   113 downloads