Long-Range Water Transport and Self-Diffusion in Zeolite 4A Powder Beds

The diffusion of water in industrial-grade sodium zeolite 4A powder beds has been visualized using broad-line gradient-echo magnetic-resonance imaging. Profiles of the equilibration of hydrated beds contacting dehydrated beds have been compared with hydrated beds contacting beds hydrated with D₂O in order to compare hydration-gradient-driven water-transport diffusion with self-diffusion. Although the equilibration rates are very different in the two cases, they are both in agreement with a model of coupled intraparticle liquid and interparticle vapor diffusion extended to incorporate the tracer experiments. The extended model is further tested by studies at elevated temperature, reduced hydration levels, and in ammonium-substituted material.