An earlier developed Discrete Element Model (DEM) for simulating dense gas-solid two-phase flow has been used to study the formation of bubbles at a single orifice. A systematic theoretical study was conducted to study the effect of particle size and injection rate on the bubble growth process for Geldart type D particles. Theoretical results obtained for pseudo-2D beds were compared with predictions from approximate models reported in the literature, the Two-Fluid Model (TFM) and laboratory experiments. The Caram and Hsu model, an approximate model, gives a fairly good description of the trend of bubble size and the superficial gas velocities with time when the change in bubble boundary (defined by the gas voidage) with injection rate is taken into account. The DEM bubble size was found to be sensitive to the value of the gas void fraction defining the contour of the bubble boundary. The bubble behavior is influenced by minimum fluidization velocity, as defined by gas and particle properties, and the injection rate. Higher injection rates give larger bubbles and a small change in leakage rate. The detachment time for the DEM bubbles seem independent of particle size and injection rate when the bed width is small. Predictions from the DEM were consistent with the TFM. (C) 2011 Elsevier Ltd. All rights reserved.