On the Variability and Reproducibility of Equilibrium Adsorption Isotherm Measurements From Different Laboratories

The accurate and precise measurement of equilibrium adsorption isotherms of pure gases on commercial adsorbents is perhaps the most important piece of information required in the development of new cyclic adsorption processes based on pressure and/or temperature swing adsorption. In principle, the measurement appears to be straightforward via commercially available or in-house developed volumetric and gravimetric adsorption apparatuses.  In practice, the measurement tends to be difficult, time consuming and error prone, with accuracy and precision always being questioned.

The precision of an instrument can be checked very easily by making the same measurements several times to verify reproducibility. It is much more difficult to check the accuracy of the instrument. Usually an independent measurement must be made with a different instrument.

Teams from the University of South Carolina, Vanderbilt University and the Marshall Space Flight Center set out to measure the equilibrium adsorption isotherm of CO₂ on Grace Davison 13X zeolite at 25 ^oC. This adsorbent is being used by NASA to remove metabolic CO₂ from spacecraft cabins; it has many other commercial and developmental applications. It also represents a stringent adsorbate-adsorbent pair to test different apparatuses because the quadrupole in the CO₂ molecule interacts very strongly with the cations present in the 13X zeolite crystals.

This seemed like a fairly simple and straightforward task for each team. However, it became quite a challenge to try to explain the differences that appeared in the equilibrium adsorption isotherms obtained from the different laboratories. During this presentation details about this round-robin exercise between these three laboratories will be provided, along with compelling and revealing results about the variability and reproducibility of equilibrium adsorption isotherms of common gases on commercial adsorbents.