The influence of particle size (d(p)) and solid concentration (C-s) on the hydrodynamics of a slurry bubble column were studied experimentally using ultrafast electron beam X-ray tomography. Experiments were carried out for d(P)=50-150 mu m at C-s=0-0.20 and superficial gas velocity U-G=0.02-0.05 m/s in a slurry bubble column of diameter D-T=0.07 m. The real time 3D gas flow structure, average bubble diameter and averaged gas hold-up were found to be independent of d(P) and C-s at small d(P) <= 100 mu m and low C-s <= 0.03, however, at large d(P) and high C-s, strong dependency on d(P) and C-s were observed. Corresponding three-phase simulations were also performed using the Euler-Euler multi-fluid model implemented in CFX 14. The bubble size distribution was calculated using the MUSIG model. The predicted gas hold-up and bubble size distribution also shows dependence on d(P) and agreed quite well with the measurements and previous studies. (c) 2013 Elsevier Ltd. All rights reserved.