6 replies to this topic

[A mukherjee]

Hii every one,

Our adsorbent vendor has offered activated alumina of 7.5% dynamic adsorption capacity.I only know that this means to adsorb 100 kg of moisture(that is coming with incoming air) we need 7.5 kg activated alumina.
But the thing is I dont know what is actually dynamic adsorption capacity?On what factors does it depend?Is there anything called static adsorption capacity?
Any information about it will be highly appreciated.
Thnxx in advance..

[Art Montemayor]

A mukherjee:

Please tell us who informed you that “to adsorb 100 kg of moisture (that is coming with incoming air) we need 7.5 kg activated alumina”. This information is totally erroneous. It takes approximately 100 kg of Activated Alumina type H-151 to adsorb 7.5 kg of water moisture before it has to be reactivated.

Dynamic adsortion capacity is, in other words, the steady-state capacity of an adsorbent while it is undergoing steady-state adsorption. I used to use 7 lb water/100 lb of Activated Alumina as my design figure when I was designing and building compressed gas dryers. Adsorption capacity (or sorptive) depends on the adsorbent, the surface area it has, the process condtions imposed on it, the method of reactivation, the degree of reactivation, and the composition of the gas stream to be treated. I have never heard the term, “static adsorption capacity”.

[A mukherjee]

Sorry sir..You are 100% correct...My info is totally wrong...100Kg of activated alumina is required for 7.5kg moisture adsorption.
Actually I wanted to know how do the we define"dynamic adsorption capacity"?
I mentioned the term"static adsorption capacity" thinking that if there is dynamic capacity,then there has to be static capcity also..

[Zauberberg]

Actually, the static adsorption capacity does exist but this term is of very little practical importance: as the name indicates, it is the maximum content of water that can be adsorbed by the unit of desiccant weight, when desiccant is exposed to constant humidity atmosphere for prolonged period of time. According to ALCOA, there are several methods to determine the static adsorption capacity, and the results are normally represented in the form of adsorption isotherms. They all represent equilibrium or maximum adsorption capacity as a function of relative humidity.

The static adsorption capacity represents the true water adsorption capacity when it is used in sealed packages, such as between thermal window panes. For dynamic gas processing, a dynamic adsorption capacity, DAC, must be used since the DAC value is less than the static adsorption capacity. In practical operations, maximum (static) capacity of adsorbent cannot be fully utilized because of mass transfer effects involved in actual fluid-solid contacting processes.

Adsorption capacity of activated alumina decreases with reduction of relative humidity, almost in a linear fashion (see attached product datasheet) - as compared to Molecular Sieve. But, Alumina has higher capacity than Mol Sieve in higher humidity area of adsorption: their capacity increases linearly while Mol Sive capacity remains constant.

Attached Files

•  Activated Alumina F220 Data Sheet.pdf   201.21KB   106 downloads

[Zauberberg]

The following leaflet from Axens gives good information about relative performance of several desiccants (Mol Sieve, Alumina, Silica Gel). I'd suggest you to go through GPSA databook as well. Excellent info has been provided there.

Attached Files

•  Axens Adsorbents.pdf   799.75KB   123 downloads

[A mukherjee]

Sir thnxx a lot..I have gone through both GPSA & Axens...They are very helpful...

[Zauberberg]

One interesting additional information: the same desiccant performs differently when employed in drying different streams, such as Inert gas or air, Saturated hydrocarbons, or Unsaturated hydrocarbons. Dynamic adsorption capacity of a desiccant is highest for Inert gas/Air, and the smallest for Unsaturated hydrocarbon gases. The difference in dynamic adsorption capacity can be as high as 50% of the higher value (the one for Air stream). Co-adsorption of hydrocarbon increases in the same direction as well.