(225ac) Equilibrium Isotherms and Diffusion Mechanism of Ammonia in Zeolite 5A Pellets

The majority of research on NH₃ adsorption focuses on measuring the uptake capacity [1], while relatively few data are available on equilibrium isotherms and kinetics of NH₃ uptake on adsorbents. However, the shape of the isotherm and adsorption kinetics are key factors in designing and developing large-scale adsorption separation units. This suggests that further research is required to better understand the NH₃ adsorption process.

Among commercial zeolites, 5A is a well-established adsorbent utilized in diverse industrial applications [2]. However, the equilibrium data for NH₃-5A zeolite are limited [3–5], and the adsorption kinetic data are rare [6]. NH₃-5A zeolite shows a highly nonlinear type â…  isotherm, similar to water in hydrophilic zeolites [7]. In these systems, accurate isotherm measurement using a gravimetric or volumetric technique is time-consuming [8]. Moreover, the measuring apparatus should be chemically compatible to utilise NH₃.

The adsorption equilibrium and diffusion mechanism of NH₃ gas in commercial zeolite 5A pellets were studied using a zero-length column (ZLC) system. The experiments were conducted under both equilibrium and kinetic-controlled conditions, with 1%NH₃ in helium on small quantities of zeolite 5A. The equilibrium isotherms were measured over a wide range of temperatures (25 – 200 °C) and fitted to the Toth equation. The isotherm parameters were then used to develop a nonlinear diffusion model to predict the ZLC dynamic experiments, including full loading, partial loading and temperature-programmed desorption (TPD). Kinetic experiments with different pellet sizes revealed that the rate-controlling mechanism is macropore diffusion. The ZLC technique offered two main advantages over other techniques for studying the NH₃-5A system: 1) it enabled quick isotherm measurements for a highly nonlinear system, and 2) it allowed for kinetic studies under isothermal conditions with minimized external mass transfer resistances.

Acknowledgements

This work was supported by UK Research and Innovation GCRF South Asian Nitrogen Hub- Grant NE/S009019/1.

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