Ebullated bed fluid dynamics relevant to industrial hydroprocessing

Abstract

This study investigates the overall fluid dynamics of an ebullated bed operating at high gas holdup conditions to provide relevant observations for industrial residue hydroprocessors. Scaling approaches for three-phase fluidized beds were compared specifically for the scale-down of the industrially observed high gas holdup conditions. Five dimensionless groups, a binary approach for bubble coalescence behaviour in multi-component liquids, and geometric considerations are proposed to achieve dynamic similitude. Experiments were carried out in a 101.6mm diameter co-current gas–liquid–solid fluidized bed operating at 0.1 and 6.5MPa with liquids that do (e.g., 0.5wt% aqueous ethanol) and do not (e.g., tap water) significantly inhibit bubble coalescence. A comparison of the overall phase holdups for two sizes of cylindrical particles (dSV of 1.6 and 3.9mm) at matching dimensionless groups provided a preliminary verification of the proposed scaling approach when bubble coalescence was sufficiently and consistently inhibited. The impacts of increased liquid viscosity (e.g., greater vacuum distillation tower residue feed fraction), varying superficial gas velocity (e.g., inlet gas flow rate and gas entrainment in the liquid recycle line), and varying superficial liquid velocity (e.g., liquid recycle pump speed) were experimentally studied due to their relevance for industrial ebullated bed hydroprocessors. For the studied operating conditions, gas holdups in the ebullated bed were much greater when bubble coalescence was sufficiently inhibited (up to approx. 45vol%) compared to a coalescing system at equivalent conditions (up to approx. 25 and 35vol% for 0.1 and 6.5MPa, respectively). When increasing the liquid viscosity of the 0.5wt% aqueous ethanol, a fraction of the gas was entrained in the liquid recirculation, increasing gas holdups and exhibiting operational similarities to industrial hydroprocessors. After adjusting the gas and liquid flow rates based on the gas recirculation, the increased liquid viscosity mainly reduced the solid holdups while gas holdups were approximately unchanged. Enhanced bubble break-up or bubble coalescence due to particles resulted in an overestimation or underestimation, respectively, of the freeboard gas holdups from a solids-free estimate based on the bed region phase holdups, where this model could not capture the complex bubble–particle interactions. Experimental results at high gas holdups conditions were used to correlate the bed and freeboard phase holdups based on the proposed dimensionless groups.