Effect of solid loading and particle size on the phase holdup distribution and bubble behaviour in a pilot-scale slurry bubble column

Abstract

Complexities in the hydrodynamics of slurry bubble columns lead to noticeable uncertainty in the design and scale-up of these multiphase reactors. Knowledge of local hydrodynamics can alleviate this problem. The effect of solids properties on the local hydrodynamic parameters is a critical subject where the lack of knowledge is undeniable. In the present study, optical fiber probes were used to study the effect of particle size and concentration on the solids and gas distribution inside the bubble column. A pilot-scale cylindrical bubble column 270 cm in height and 29.2 cm in diameter was employed. Glass beads with various mean diameters (35, 71, 156 µm) and 1, 3 and 5 vol% were added to tap water in order to make the slurry phase. Air with a wide range of superficial gas velocities (up to 20 cm/s) was injected into the slurry phase. The results demonstrated that the solid particles' radial and axial distributions were not uniform in the column. Solids concentration showed a local maximum in the near-wall region. Gas holdup had a parabolic shape except for large particles at high solids concentration in which the maximum of the gas holdup was observed at the sides rather than the centerline. The variations in the pressure fluctuations revealed that the particle size decrease led to an increase in the bubble size; however, the initial bubble size was independent of the particle size. Moreover, the increase in the solids concentration resulted in an increase in the gas holdup in the homogeneous flow regime; and a decrease in the heterogeneous one. Finally, a predictive model that can estimate the radial and axial solids distributions in the slurry bubble column is presented.