Johnson, Thomas Francis; (2019) Three dimensional representation and analysis of individual bead and packed bed scale chromatography using X-ray computed tomography and focused ion beam microscopy. Doctoral thesis (Eng.D), UCL (University College London).

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Abstract

Understanding the detailed, internal geometry of chromatography bead and packed bed structure remains a challenge, which is addressed in this thesis by using tomographic techniques to both visualise and quantify microstructural characteristics at both scales. Two main approaches were investigated for the purposes of producing accurate representations: X-ray computed tomography and focused ion beam microscopy, both providing high-resolution capability for imaging geometric features, enabling comparison between material types and techniques when considering suitability for chromatography structural research. At the bead scale, X-ray computed tomography and focused ion beam microscopy were used for imaging and comparison of the three bead types, with optimal cubic pixel sizes of 32nm and 15nm achieved respectively. Despite the superior resolution attainable for focused ion beam microscopy, drawbacks of intensive preparation requirements and the necessity for physical slicing and thus destruction highlighted that pixel dimensions were not the only consideration for sub-micron tomographic imaging. Tortuosity, which impacts important performance metrics such as transfer rates, was found to be below 2 in all cases due to the high porosities exhibited, with average pore size greatly influenced by the overall resolution. At the packed bed scale, X-ray CT was the sole technique selected using two different systems, with one system only capable of sufficiently imaging the harder ceramic samples, albeit achieving an overall superior pixel size of 2.7µm. Optimisation of X-ray conditions was required for each different material and corresponding equipment in order to achieve 3D representations of sufficient quality; to both visually display the packed bed structure in addition to providing the capability of quantifying key metrics relating to chromatography geometries and thus performance. Porosity readings of approximately 35% were in agreement with values obtained using established techniques and values, with radial discrepancies identified that were expected due to wall-effects impacting packing densities. Two industrially relevant chromatography processing considerations were examined using X-ray CT: fouling and packed bed compression. Both scales were investigated for fouling, with individual beads imaged between cycles to measure the change in simulated diffusivity due to foulant impregnation. Compression of packed beds was imaged before, during and after excessive flow through columns, where visual and quantitative changes to aspects such as simulated permeability were compared between states. The values obtained in both of these studies based upon real systems were compared to changes in porosity and tortuosity factor by applying erosion-dilation to structure in an original state.

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