Hydrodynamic comparison of spherical and cylindrical particles in a gas–liquid–solid fluidized bed at elevated pressure and high gas holdup conditions

• Experimental comparison of spherical and cylindrical particles in an ebullated bed
• High gas holdup conditions achieved with elevated pressures and surfactant addition
• Sauter mean diameter used to account for cylindrical shape effects
• Comparable hydrodynamics for L–S and G–L–S (dispersed bubble flow) fluidized beds
• Discrepancies observed with larger coalescing bubbles due to bed contraction

Experiments were carried out to validate the use of spheres in lieu of cylinders when investigating the global hydrodynamic features of a co-current gas–liquid–solid fluidized bed. Two sizes of glass spheres with diameters of 4 and 1.5 mm were compared to aluminum cylinders with equivalent volume/surface area ratios (i.e., matching Sauter mean diameters). Lengths/diameters of the larger and smaller cylinders were 7.5/3.2 mm and 3.1/1.2 mm, respectively, which resulted in equal particle sphericity of 0.8 for both sizes. The particle properties of the larger particles led to the inertial settling flow regime (ReLT∞ > 500) in water while the smaller particles were in the intermediate regime (0.2 < ReLT∞ < 500). High gas holdup conditions were obtained by increasing the system pressure to 6.5 MPa and/or adding a surfactant. Atmospheric conditions were also studied for comparison. Experiments were conducted in a 101.6 mm diameter column with tap water or a 0.5 wt.% aqueous ethanol solution as the liquid phase while the gas phase was a combination of air and nitrogen. Global phase holdups measured from the dynamic pressure profiles characterized the hydrodynamic behavior of the fluidized bed and studied the impact of particle shape. Standard deviations of the mean holdups aided the comparison and also examined the fluctuations of the bed interface. Liquid–solid fluidized bed experiments demonstrated that equivalent Sauter mean diameters resulted in comparable bed porosities. Gas–liquid–solid fluidized bed dynamics of equivalent size spherical and cylindrical particles were similar in the dispersed bubble flow regime whereas differences were observed in the presence of larger coalescing bubbles.

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