Numerical and experimental investigations of particle stress and fracture for complex-shaped pellets
Creators: name: Farsi, Ado
Publisher: name: Imperial College London
Year: 2017

description: Reactors with fixed beds of cylindrical particles have a wide application in the chemical industry. Ceramic particles are pelletized and fired to produce high porosity catalyst pellets of complex shapes. These pellets fill cylindrical reactor columns with particulate packing structures that are key to the in-service performance, but will suffer breakages, which impact on catalyst performance. The combined Finite-Discrete Element Method (FEMDEM) implemented in the Solidity code would appear to be ideally suited to capturing both the multi-body pellet interactions and pellet fracture and fragmentation. However, to put to use the Solidity code for this purpose and establish its capabilities and limitations required a substantive research programme, as reported in this PhD thesis. Laboratory experiments were performed to evaluate the elastic and fracture properties of reference ceramic samples, as required for input parameters for computer simulation and to investigate code capability to describe fracture in such high strength and porous media for which no previous such simulations had been reported. Each set of specimens was characterised by means of micro- and nano-indentations, ultrasonic and strength tests. Standard laboratory rigs are generally too compliable for capturing the deformations of stiff and tiny ceramic specimens. For this reason, a novel digital image correlation methodology was developed to obtain both strength and stiffness from three-point bending tests on alumina bars which would have been otherwise impossible. The effects of the catalyst support shapes on their final strength and fragmentation behaviour were investigated through controlled experiments and predominantly 2D plane stress simulations on single pellet shapes. Uniaxial compression tests and high-speed video recordings were employed to estimate the strength and fragment size respectively. The Solidity FEMDEM code was employed to simulate the effects of geometrical features and loading orientation on the pre- and post-failure behaviour of catalyst supports. The Solidity FEMDEM code was also used to simulate the deposition of packs of catalyst supports in cylindrical containers. A post processing tool was implemented to extrapolate the packing density profiles, packing structure, bulk porosity and orientation distributions of the resulting bodies making up the pack of pellets. The numerical results were compared with the corresponding experimental packing density profiles and orientation distributions published in the literature, together with other reported simulation results. The final part of the thesis addresses the goal of this research which is to investigate the effects of pellet shapes on the packing and fragmentation behaviour. The findings suggest that the use of Solidity FEMDEM will have a significant industrial impact by contributing to improvements in the performance of catalysts through understanding of induced packed structures and its associated physical processes including stress and breakages.