In the Fluid Coking (TM) process, heavy oil is contacted with hot fluidized coke particles. If the local concentration of liquid is too high, fluidization is poor, a condition commonly known as bogging. The objective of this study is to identify practical methods for early bogging detection, simulating heavy oil at coker temperatures with a lighter oil at room temperature and determining the impact of the oil concentration on bubble properties, on fluctuations of the bed pressure gradient, and on the transmission of sound through the bed. Bogging, as determined from"changes in bubble properties, occurred when the oil mass fraction was increased from 0.25 to 0275 wff. A Kolmogorov-Smirnov test of the wavelet coefficients of pressure fluctuations, optimised with a genetic algorithm, can detect early bogging more effectively than other methods using pressure fluctuations. A major advantage of this method is that its results are not affected by moderate variations in fluidization gas velocity. The success of this new bogging detection method is explained by studying the transmission of sound of different frequencies through dry and wet fluidized beds, which could also be used to detect bogging: bogging sharply increases the speed of sound from below 30 m/s to well above 40 m/s. A theoretical model confirmed that the changes in bubble properties caused by bogging affect the transmission of sound through the fluidized bed. The geometry of the gas bubbles and their distance from the wall were measured with capacitance sensors to understand how bubble properties affect sound transmission. The propagation of sound with bubbles of various geometries was simulated with Comsol. (C) 2017 Elsevier B.V. All rights reserved.