An earlier developed hydrodynamic model describing dense gas-solid two phase flow has been used to study the bubble formation process at a single orifice. A systematic experimental and theoretical study has been conducted to investigate the effect of particle properties (i.e. particle diameter and density) on the bubble growth process for Geldart type B powders. Theoretical results, obtained for both two-dimensional and three-dimensional geometries, have been compared with experimental data and with predictions from approximate models reported in literature. A comparison of the theoretical results and experimental data shows that the advanced hydrodynamic model gives a satisfactory good description of the bubble growth process for several particle types which makes this model a useful tool to study the bubble formation process in fluidised beds. It appears that the influence of particle size and particle density on bubble formation can be related to the effect of the minimum fluidisation velocity on this process. At a constant gas injection rate through the orifice higher minimum fluidisation velocities result in larger bubbles and decreased leakage. Further, it has been found that coarse particles give rise to the formation of relatively elongated bubbles. The detachment times, on the other hand, seem to be independent of the particle size used.