Commercial ebullated bed hydroprocessors, such as the LC-Finer, are used for the production of synthetic crude oil by upgrading bitumen extracted from the Canadian oil sands. The objectives of this thesis were to experimentally determine bubble characteristics at industrially relevant operating conditions for the design and optimization of the reactor’s recycle pan, as well as to investigate the impact of a catalyst density distribution on the reactor’s fluidization behaviour. High gas holdups have been reported for this type of industrial unit. As a result, high gas holdup conditions were required to assess the commercial unit fluid dynamics. Industrial conditions for the bubble characteristics in the reactor freeboard were simulated in a high pressure gas-liquid bubble column operating at 6.5 MPa using nitrogen and a 0.5 wt.% aqueous ethanol solution. Local bubble characteristics, including gas holdups, bubble rise velocities, and chord lengths, were investigated under various operating conditions using a novel monofibre optical probe designed for high gas holdup and elevated pressure. High gas holdups were achieved (up to 60%) and relatively narrow chord length distributions were observed, where 90% of the bubbles diameters were 1.0 mm or less. The energy dissipated through the distributor plate was shown to have a significant impact on the initial bubble size generated and high gas holdups were also achieved at atmospheric pressure by varying the open-surface area of the distributor. As a result, the impact of catalyst density distribution on local fluidization behaviour was investigated at atmospheric pressure using the previously designed high energy dissipation gas-liquid distributor plate and a 0.5 wt.% aqueous ethanol solution. Fresh and spent hydroprocessing catalysts having relatively narrow and wide density distributions were compared. The introduction of gas greatly impacted the fluidized bed dynamic by rendering the bed-freeboard interface diffuse at low superficial liquid velocity. Bed interface fluctuations were significantly reduced at elevated liquid flow rate due to average bubble size reduction caused by high shearing through the gas-liquid distributor plate. Solid holdup was most affected by the density distribution where bed expansion/contraction was dependent of the liquid flow rate due to varying particle-bubble dynamics.