Hydrodynamics of liquid fluidized beds including distributor effects
One of the key factors influencing the hydrodynamics of two phase liquid fluidized beds is the liquid distributor which can cause severe flow non-uniformities making the modeling process extremely difficult. This is particularly true for liquid fluidized bed bioreactors which contain a low density solid phase in which biocatalyst is immobilized. The influence of important variables associated with the design of distributors was studied experimentally and theoretically with particular emphasis on characterizing the flow behaviour existing in the distributor region. In order to examine the important effect of the density of solid particles present in the liquid fluidized bed on the distributor region behaviour, particles of widely differing densities were used. For fluidized beds containing very low density particles (p s = 1.05 g/mL), undesirable dead zones were found to be responsible for distorting the hydrodynamics of the fluidized bed for poorly designed distributors. Detailed design information is given on the proper distributor design to avoid this dead-zone problem. In this case, the dispersion coefficients (D ac) for the bed region varied from 1.2 to 10.0 cm2 Is for liquid superficial velocities (U 0) ranging from 1.1 to 3.2 emfs. Secondly, stirring effects of high velocity orifice jets issuing forth from distributor holes caused an intense mixing in the distributor region of a fluidized bed with medium density particles (p s = 1.61 g/mL). For this case, a new and more realistic two-region CSTR-Dispersion model (i.e. the CD Model) was proposed which modeled the distributor region as a CSTR and the rest of the fluidized bed using the dispersion model. Here, the variation of liquid superficial velocity from 2.3 to 9.0 cm/s caused a variation of the axial dispersion coefficient in the bed region from 2.8 to 10.5 cm2 Is and the distributor region was found to vary from 0. 0 to 26. 0 cm in height. Thirdly, no distributor effects were observed in liquid fluidized beds containing high density solid particles (p s = 2.46 g/mL). Here, the dispersion model provided good prediction of the fluidized bed behaviour with the dispersion coefficient varying from 4.0 to 33.0 cm2/s for liquid superficial velocities ranging from 4.0 to 16.0 cm/s. Finally, the present study systematically examines the validity of the assumption of constant dispersion coefficient m carefully designed RTD experiments. Results prove conclusively that the dispersion model is valid only after tracer particles are exposed to the fluidized bed for a sufficiently long time. A quantitative criterion is proposed for the time constraint (r f) required for dispersion coefficients to reach the true values actually existing m the fluidized bed in a residence time distribution experiment.
Bibliography: p. 156-162.
Asif, M. (1991). Hydrodynamics of liquid fluidized beds including distributor effects (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/22093