More realistic dynamic bed-expansion experiments using a three-phase anaerobic fluidized bed reactor (AFBR) with and without internal biogas production were conducted for the establishment of correlation equations for the mean volume ratio of wakes to bubbles (k). A predictive model was also developed for the expansion characteristics of the three-phase AFBR with internal biogas production. The predicted bed-expansion heights (H-GLS) deviated by only +/- 10% from the experimental measurements for the three-phase AFBR. According to the modeling results, if a three-phase AFBR is loaded into a carrier with low specific gravity (dry density of carrier, rho(md) = 1.37 g cm(-3); wet density of carrier, rho(mw) = 1.57 g cm(-3)) and operated at a high superficial liquid velocity (u(1) = 4.0 cm s(-1)), the ratio of H-GLS to H-LS at a high superficial gas velocity (ug = 1.5cm s(-1)) can reach as high as 271%. A higher fluidized-bed height has a greater effect on the bed-expansion behavior because of the decrease in liquid pressure (surrounding gas bubbles) along the fluidized-bed height. From parametric sensitivity analyses, HGLS is most sensitive to the parameter reactor width (X), especially within a small Delta X/X-0 range of +/- 10%; sensitive to rho(mw), diameter of the carrier, rho(md) and total mass of carrier and least sensitive to u(1), biofilm thickness and ug. (c) 2005 Society of Chemical Industry.