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The influence of pore structure on fluid flow across large length scales (ca. 104 m) in silica-alumina pellet-based catalysts is significant for understanding their performance. In this project, multiscale tomography (MT) methods were used to acquire images of catalyst pellets from the nanometer to the millimeter scale. The specimens used were sintered/calcined at different temperatures.
The images were then analyzed and segmented into binarized datasets before the nano/microstructure for each length scale was meshed in Simpleware software. Permeability calculations were carried out in ANSYS Fluent on the effect of different pore sizes on flow, enabling new insights to be made into the transport properties of catalysts.
F. Tariq • P.D. Lee • R. Haswell • D.W. McComb:
Imperial College, London; University of Manchester; Shell Global Solutions International B.V
Two pellet-based catalysts were extruded to create trilobed pellets, which were calcined at different temperatures to produce two samples for scanning. Synchrotron X-ray microtomography (XMT), dual beam-focused ion tomography (DBFIB) and electron tomography (ET) were used to obtain 3D images at different length scales.
Following 3D tomographic reconstruction, the data was processed to reduce noise and segment bulk material and voids, resulting in a binarised dataset. The pellet samples were also experimentally measured to determine permeability.
Representation of different tomographic techniques with their typical voxel length scales
For modelling permeability through the catalyst pellets, the tomographic volumes were mirrored using MATLAB®, with Simpleware ScanIP and Simpleware FE then used to convert the nano/microstructure at each length scale into a volume mesh. Regions of interest were selected using flood-fill, with fluid boundary interfaces meshed finer than the bulk, and a buffer region added in upstream and downstream directions for the mesh, which was exported to ANSYS Fluent ready for CFD analysis of permeability. The simulated results from the finer length scales were inputted into the structure for the next length scale until the simulation had considered all relevant scales in the catalysts.
Multi-part mesh in Simpleware FE of the micro-structure of a S2 catalyst pellet (pores in red)
The analysis demonstrated how the catalyst pellets sintered at the lower temperature resulted in larger, open pore structures with better interconnectivity than catalysts sintered at a higher temperature.
The presence of a more open pore structure enables better permeability and flow between active sites, improving catalyst performance. Research into fluid flow through pore structures at different length scales obtained through MT therefore offers a basis for future tailoring of catalyst properties, with wider applications to improving other energy materials.
Porosity across length scales example: scaling the permeability (K) of volume A into the bulk of volume B
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