This ejects particles on to the freeboard region. In this study, the units are operated in bubbling fluidization regime and hence elutriation of particles is minimized significantly if not eliminated. Figure shows instantaneous snapshot from the 4-inch unit at different flow rates. Red lines represent heights determined using the procedure outlined previously where the threshold value is set to 10%. The resulting time series data are used to derive bed height statistics plotted in Figures 8, 9 and 10. Abscissa in these plots correspond to threshold value or fraction of pixels occupied by particle-phase, which has been used to avoid a subjective definition of interface. As the superficial velocity of gas is increased, bed expands which causes the average height of interface to increase monotonically in all the units. The mean values drop with increase in threshold as expected. At higher fluidization velocities, excess air velocity U -U mf is higher which increases the size of bubbles. As a result, interface oscillations are more vigorous leading to an increase in the standard deviation of bed height.