In the present paper, the extraction of dominant particle concentration fluctuation at various time space frequency levels from reconstructed electrical capacitance tomography (ECT) images by means of the discrete wavelet transform has been proposed. Using this concept, three-dimensional images (time and two-dimensional space) obtained by an ECT system are decomposed into different time and space levels to characterize the intrinsic features of the particle distribution in a down-flow fluidized bed with a newly designed distributor using three-dimensional discrete wavelet multiresolution. As a result, the dominant time and space particle distribution characteristics for a specific frequency level can be visualized. On the basis of the wavelet analysis results, the best operation condition, under which the most stable and uniform particle distribution can be obtained, was clarified. This provides a good reference for the optimization of a downer reactor to qualify the refinery process in the petroleum industry. (C) 2011 Elsevier B.V. All rights reserved.