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Packed bed reactors at the pellet scale: magnetic resonance studies of hydrodynamics, mass transport and reaction

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Elgersma, Scott 


Packed bed reactors are widely used in the chemical and petrochemical industries to facilitate heterogeneous catalytic reactions. Despite the widespread use of packed bed reactors in industry, the flow, transport, and reaction phenomena occurring at the scale of individual catalyst pellets is poorly understood due to a lack of experimental techniques capable of non-invasively probing these phenomena at the pellet scale. This thesis presents the development and implementation of magnetic resonance (MR) techniques to investigate the hydrodynamics, mass transport and reaction occurring at the pellet scale within packed bed reactors. The aim of this work is to gain physical insight into the processes occurring at the pellet scale, which can subsequently be applied to optimize the catalyst and reactor technology used in industry.

A novel MR method is developed and implemented to quantify the liquid-solid mass transfer coefficient in packed beds. The method, which utilizes MR relaxation exchange and a magnetization transport model, enables the true mass transfer process at the pellet scale to be probed without the confounding effect of molecular dispersion. The MR method is applied to measure the mass transfer coefficient for single phase liquid flow and two-phase gas-liquid flow in beds of SiO2 and TiO2 pellets. The limiting mass transfer coefficient at zero flow is directly measured for the first time, helping to resolve a debate in literature regarding its value.

An MR velocity imaging methodology is developed to acquire 3D images of the time-averaged velocity and turbulent kinetic energy for turbulent flows in packed beds of commercially relevant α-Al2O3 pellets. The method is subsequently used to investigate the effect of pellet shape on hydrodynamics, revealing that the hydrodynamics at both the bed and pellet scale are influenced by pellet shape. Further, MR velocity imaging is applied to investigate the effect of the tube-to-pellet diameter ratio on the hydrodynamics. For the beds studied, the pellet scale hydrodynamics are found to be independent of the tube-to-pellet diameter ratio, but some differences in the near wall hydrodynamics are observed.

Finally, MR based chemical shift imaging (CSI) is implemented to map the intra-pellet chemical composition at operando conditions during the hydrogenation of styrene to ethylbenzene over a Pd/Al2O3 catalyst. Inhomogeneous partial wetting of the catalyst, representative of that known to occur in commercial reactors, is found to cause substantial heterogeneity in chemical composition across a single catalyst pellet. Commonly used 1D reaction-diffusion models are inadequate to describe the observed reaction heterogeneity. The pellet-scale composition maps provide novel insight regarding the coupling between transport and reaction at the pellet scale and have important implications for pellet scale catalyst models.





Gladden, Lynn


Catalyst Pellet, Flow Imaging, Magnetic Resonance Imaging, Mass Transfer, Nuclear Magnetic Resonance, Operando, Packed Bed Reactors


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge