[lullu]

why physical adsorption is an equilibrium process?

[Stepan]

Because it is reversible.

[lullu]

Because it is reversible.

ok! Can I have another question? In the isotherm is there on the abscissa the solute concentration in the fluid phase and on the ordinate the solid concentration in equilibrium?

[Stepan]

If we are still talking about adsorption, then on ordinate I would expect to find amount of chemical extracted (adsorbed) by the adsorbent in wt/wt or wt/area units.

[Enthalpy]

Adsorption is reversible because, as more molecular layers stick to the solid, the outer layers stick less for being farther. So adsorption stops at some thickness which depends on the physical conditions, which means "equilibrium" and "reversible".

Now, if you want to desorb the first layer, you will have a really difficult time. More precisely: impossible, even on gold. Clean surfaces are prepared by cleaving or by deposition, not by desorption, and last short in the very best vacuum humans achieve.

[Stepan]

Adsorption is reversible because, as more molecular layers stick to the solid, the outer layers stick less for being farther. So adsorption stops at some thickness which depends on the physical conditions, which means "equilibrium" and "reversible".

Now, if you want to desorb the first layer, you will have a really difficult time. More precisely: impossible, even on gold. Clean surfaces are prepared by cleaving or by deposition, not by desorption, and last short in the very best vacuum humans achieve.

Physical adsorption is reversible in mono-layer. If it is non-reversible, it is likely chemical adsorption. The biggest difference between physical and chemical - enthalpy of adsorption. For physical adsorption, the enthalpy is closed to enthalpy of condensation, usually just a little bit more.   Chemical adsorption is often destructive.

[Siddarth]

The adsorption isotherm represents the EQUILIBRIUM relationship between the concentration of fluid phase at some temperature. But actual adsorption cannot occur at equilibrium, because it is the variation from equilibrium that drives mass transfer and separation. The term equilibrium comes into the fray only when saturation of the adsorbent bed occurs. There is no such thing as "irreversible" as far as physical adsorption is concerned, until you apply the necessary temperature required.

[smirf]

i am working on chromium adsorption onto activated carbon prepared from coconut shell, having 3 different particle size. Granular Medium and Powder. ACarbons are loaded in Tapered fixed bed column and process is continuous.

I am having a problem with adsorption capacity. As everyone says Granular has less capacity than other two and capacity should increase with the decrease of particle size. however in my case, breakthrough time of medium sized carbon almost doubles that of powder (which is supposed to be higher). in other words, capacity increases from granular to medium and then decreases from medium to powder. If anyone has explanation for this, please... 

[Stepan]

Most likely you have air pockets in your column and some carbon is not accessible for adsorption. Try ideal mixing. Also, adsorption capacity does not depend on the particle size; adsorption rate does.