Development and Application of the Zero Length Column (ZLC) Technique for Measuring Adsorption Equilibria

This thesis reports the results of an experimental study aimed at developing the zero length column (ZLC) method as a useful technique for measuring adsorption equilibria. In a ZLC experiment a small sample of adsorbent is pre-equilibrated with the sorbate, at a known partial pressure, in a He carrier. Information concerning the sorption kinetics and equilibria can be obtained fkom the desorption curve when the flow is switched to a pure He purge under carefully controlled conditions. This technique has been widely used to measure intraparticle diffusivities but, when operated at sufficiently low flow rate, it provides a simple and convenient way to obtain equilibrium data, including both Henry\u27s Law constants and complete isotherms. The practical viability and limitations of this approach have been explored by measuring the adsorption isotherms for CO2 on several different zeolites on the same adsorbents and under the same conditions at which the isotherms had been measured, at the Air Products Laboratory, by a conventional volumetric/piezometric method. It was shown that; with careful attention to the details of the experiment, the ZLC measurement replicates the volumetric/piezometric isotherms within a few percent. The ZLC method has advantages of speed and simplicity and thus provides a useful tool for adsorbent screening studies. The ZLC method was used to measure the isotherms for CO2 (and some other sorbates) on a range of different cationic forms of zeolite X. The data, when analyzed using localized adsorption models, suggest adsorption site densities that are reasonably consistent with structural information concerning the distribution of the cations in these materials. The ZLC approach has been extended to the measurement of separation factors in binary adsorption systems and the separation factors measured in this way are shown to be consistent with the values derived from standard binary isotherm measurements. In a further development it is shown that the ZLC method can be easily coupled with a temperature programmed desorption (TPD) measurement. This allows determination of any residual irreversibly adsorbed sorbate and provides a simple way to study the effect of a strongly held adsorbate on the isotherm for a weaker species. The effect of water on the isotherm for CO2 on several type X zeolites has been investigated in this way. The advantages and limitations of the ZLC method are reviewed and experimental precautions needed to obtain reliable results are discussed in detail