Powder Technology, Vol.266, 45-60, 2014
Particle agglomeration in gas-liquid-solid fluidized beds with a dispersed immiscible liquid: Study on particle size, shape and material
The formation of a denser and more viscous secondary liquid phase may impact the fluid dynamic behaviour of industrial ebullated bed reactors such as hydroprocessors. This study investigates the effects of particle size, shape and material on the global fluid dynamic behaviour of gas-liquid-liquid-solid fluidized beds subject to particle agglomeration. Ebullated bed experiments were carried out in a 152.4 mm diameter column at atmospheric conditions with biodiesel as the continuous liquid, 5 wt.% of glycerol as the denser and more viscous dispersed liquid, and nitrogen. Glass spheres with diameters of 4 and 1.5 mm were compared to aluminum cylinders with equivalent volume to surface area ratios, where the sphericity of both larger and smaller cylinders was approximately 0.8. In a liquid-solid fluidized bed, the previous particles were in the intermediate settling flow regime (0.2 < Re-LT infinity <500) in biodiesel; nonetheless, coalescing and dispersed bubble flow regimes were obtained with the smaller and larger particles, respectively, at the introduction of gas. Liquid-liquid-solid fluidized bed results established that particle size, shape and material had considerable impacts on agglomeration behaviour. In the gas-liquid-liquid-solid ebullated bed, the 1.5 mm glass beads transitioned from coalesced to dispersed bubble flow due to increased particle inertia from agglomeration. Larger glass beads experienced a reduced bed expansion due to agglomeration since the bubble flow regime remained constant The studied aluminum cylinders did not agglomerate to the same extent as the glass beads due to differing material wetting properties, where negligible clustering occurred with the larger cylinders and an axial agglomerate size distribution was observed with the smaller cylinders. Preliminary experiments in a slurry bubble column using 100 to 150 mu m glass beads were inoperable at a relatively low glycerol concentration of 0.7 wt.% due to considerable sedimentation on the distributor. Interparticle forces relevant to gas-liquid-liquid-solid fluidized beds are discussed, with an emphasis on the relation between fluid and particle properties with respect to attractive forces due to liquid bridging. (C) 2014 Elsevier B.V. All rights reserved.