Electrical capacitance tomography, based on the neural network multicriteria optimization image reconstruction technique, developed by this research group, is used to examine the column size effect on the real time, cross-sectional flow variation during the choking transition in a circulating fluidized bed. Two sizes of columns, 0.05 and 0.1 m i.d., are employed in this study for two types of particles, sand particles (group B) and fluid catalytic cracking catalysts (group A). For group B particles, the formation of the square-nosed slugs (0.05 m i.d. column) or the wall slugs (0.1 m i.d. column) is observed to occur in the regime transition when the gas velocity Ug is below the transport gas velocity Utr or when the solids circulation rate Gs is below the transport solids velocity Gs,tr. When Ug > Utr or Gs > Gs,tr, the formation of open slugs for both 0.05 and 0.1 m i.d. columns is observed in the regime transition. Clearly, for group B particles, these regime transitions accompanied by a distinct bed s...