Modeling and experimental verification of models of liquid-solid fluidized bed and semifluidized-bed adsorbers: A reaction engineering approach

The axial dispersion model (ADM) and the compartments-in-series with backflow model (CISM) have been adapted to characterize a fluidized-bed adsorber for the adsorption of phenol from its aqueous solution with granular activated carbon (GAC). These models take into account the axial mixing in the solid and liquid phases, the mass transfer resistance in the laminar boundary layer surrounding each adsorbent particle, and the intraparticle diffusional resistance. The models have been solved numerically to simulate the performance of a laboratory-scale adsorber. The results of simulation closely represent experimental observations over the wide ranges of the influent flow rate, fluidized-bed height and adsorbent particle size. Furthermore, a comparative study of the two models has been performed. In the CISM, the number of compartments to represent each of the phases in a heterogeneous system can be chosen individually; this flexibility can be exploited to significantly reduce the computing requirements. Also, the CISM gives rise to mathematical expressions which are readily applicable to the design of a modern control system by means of the state-space matrix approach.; A novel operation, namely, semifluidized-bed adsorption, has been proposed, which combines the advantages of fluidized-bed and packed-bed operations; it has a relatively low pressure drop of the former, and a relatively long breakthrough time of the latter. The new system has been modeled as a series combination of fluidized-bed and packed-bed sections. The CISM has been adapted to represent both sections. The model has been solved numerically to simulate the adsorber performance. The results of simulations indicate that the adsorber performance is sensitive to the axial mixing in the solid phase when the adsorption bed is relatively deep or when the concentration of phenol in the feed solution is low. In contrast, the effect of axial solid mixing is negligible in a shallow bed or when the concentration of phenol in the feed solution is high. These anomalous mixing effects have been explained on the basis of the form of the isotherm equation and the length of the zone in which most of the mass transfer occurs in the adsorber relative to the depth of the adsorption bed.