Transient diffusivity measurements in catalyst pellets with two zones of differing diffusivities



Transient pulse diffusivity measurements were carried out in a pellet string reactor. The outer skin of the catalyst spheres was partially obstructed by contaminants, and the objective was to determine the diffusivity both in this obstructed skin and also in the unobstructed core.

Conservation equations were formulated which describe the transient diffusion process in catalyst pellets with two interconnecting regions of differing diffusivities. The shape of the elution curve from a packed bed with axial dispersion was computed by numerical inversion of the Laplace transformed solutions for a Dirac delta function input.

The calculations indicated that if an appropriate residence time is chosen, the elution curve shape should be sensitive to the differing diffusivities of the two zones in the catalyst pellets.

The experimental peak shapes were analyzed by the Fourier analysis technique and the diffusivities were extracted by the Constrained Simplex optimization routine. The results showed good resolution of the different diffusivities in the skin and the core of the catalyst pellets.