Does anyone have experience what is typically the difference between adsorption capacity (in mg/g or meq/g) of powdered and granular adsorbent made of same raw material? For example powdered activated carbon (PAC) and granulated activated carbon (GAC)?
Typically both PAC and GAC are rated by iodine number. You can find both with fairly high iodine numbers made from the same starting material. The main difference is that the PAC is added upstream (generally) and the removed by clarification/filtration. It has a single use, then discarded. The GAC is typically in filter beds or dedicated adsorption columns. The life of GAC is measured in months or years.
In reality, you are probably not adsorbing iodine, so the structure of the carbon as well as your target contaminant will come into play and determine the effectiveness of the carbon as well as the useful life (for GAC).
I 'up vote' response of Dr Orren and would further like to add to this response.
The adsorption capacity of materials is determined by carrying out the experiments with powdered form of the materials. This is important to know that if a material is granulated or pelletized, it retains its capacity. This what Prof Orren ' discussed by quoting that both the PAC and GAC are characterized by high iodine number.
Smaller particle (same material) will have a greater aspect ratio and thereby will be a better adsorbent.
The differences in dynamic behaviour might be bigger than the actual equilibrium adsorption capacity, as diffusion paths are longer and more hindered and equilibrium state is reached after a longer time. Because of this, the difference in actual equilibrium adsorption capacity will also vary with the adsorptive's ability to diffuse. In aqueous applications (that is, with adsorptives that diffuse fast enough) the differences I have seen are usually small, in the 0 - 10% range.
In gas phase applications PAC, as was already mentioned, is added in a downstream mode so you have one theoretical plate only. GAC, on the other hand, is used in a fixed bed, which is a counter current operation, so the working capacity acieved with GAC is higher, but not because GAC is the better adsorbent. The choice is made, as you will know better than I do, purely on what is technically feasible.
I agree with Claudia, the dynamic behavior is the key difference (dynamic capacity). Not to forget the differences in how the columns can be packed (channeling etc.).
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