Hydrodynamics of tapered anaerobic fluidized beds for metabolic gas production

A laboratory study using two tapered anaerobic fluidized bed reactors (AFBRs) with taper angles (θ) of 2.5° and 5° was undertaken to explore the hydrodynamic behavior of AFBRs during metabolic gas production. Predictive models were formulated to describe the hydrodynamic behavior of tapered AFBRs in a way that accounted for wake theory, hydrostatic pressure, biofilm thickness distribution, and ambient temperature. When the two tapered AFBRs were maintained in their respective fully fluidized states, both the measured and simulated three-phase bed heights, Hb,GLS, were higher than the corresponding two-phase bed heights, Hb,LS—a bed-expansion effect. In contrast, the measured and simulated three-phase bed-pressure gradient (−ΔPt/Vb)GLS, was lower than the corresponding two-phase value (−ΔPt/Vb)LS, especially at higher superficial liquid (ul) and gas velocities (ug). This helped to thicken the biofilm. Increasing θ from 2.5° to 5°, decreased Hb,GLS sufficiently to offset increases in (1 − ɛf). Consequently, the values of both (−ΔPflu)GLS and (−ΔPt/Vb)GLS decreased. The simulated values for Hb,GLS and (−ΔPt/Vb)GLS were in fairly good agreement with experimental results. Compared to conventional AFBRs, the effect of metabolic gas production on the expansion behavior of tapered AFBRs was smaller. Thus, tapering of AFBRs facilitates the development of thick biofilms.