(221e) On the Use of the Absolute, Excess and Net Amounts Adsorbed in Dynamic Mass Balances
The definition of the amount adsorbed dates back over a century with the vision of W. J. Gibbs and his dividing surface (i.e., partition or interface) between the gas and adsorbed phases. Since that time, numerous articles have devoted significant effort into the interpretation and definitions of the absolute, excess and net amounts adsorbed to describe the equilibrium relationship, and more importantly the partitioning or demarcation, between the gas and adsorbed phases. The excess amount adsorbed has prevailed in the literature as the only true experimental quantity because it is the only one that can be measured unambiguously and universally within all adsorption laboratories. What is interesting, however, is that most dynamic mass balances utilize an absolute amount adsorbed formulation because the volume within the adsorbent accessible to helium is generally not accounted for, even though the isotherms are based on this helium volume, i.e., the excess amount adsorbed. This presentation will sort through some of these issues by addressing one or more of the following: 1) clearly defining these three definitions of the amount adsorbed and discussing their subtle differences (as done by others), 2) showing how to use each definition in a dynamic mass balance, 3) suggesting a universal solution to the definition of the adsorbed phase volume as recently initiated by Brandani et al. [Adsorption, 2016], and 4) treating some of the general issues, potential pitfalls and revelations that arise in modeling gas phase adsorption processes under dynamic conditions like breakthrough curves when using the three definitions of the amount adsorbed.