Modeling VOC adsorption in a multistage countercurrent fluidized bed adsorber

Abstract

The adsorption of 1,2,4-trimethylbenzene (TMB) on beaded activated carbon (BAC) in a six-stage countercurrent fluidized bed adsorber was simulated employing a two-phase model, assuming the gas in particulate phase to be either in plug flow (EGPF model) or in perfectly mixed flow (EGPM model). A rather simple model considering equilibrium state on each stage (Equilibrium model) was also used for comparison. Simulation results were compared with experimental data obtained at different values of adsorbent feed rate, superficial gas velocity, TMB initial concentration and weir height (which influences the effective bed height). The results demonstrate that the Equilibrium model overpredicts the overall removal efficiencies when the adsorbate-adsorbent system is far from equilibrium condition. On the other hand, both EGPF and EGPM show good agreement with the experimental results over industrially relevant operating conditions. Stage-wise removal efficiencies show that the EGPF model tends to predict removal efficiency better than EGPM when the weir height is high. The sensitivity analysis of the EGPM model indicates that internal diffusion within the BAC is rate-limiting for adsorption, while BAC diameter strongly influences the overall removal efficiency and can be optimized for different conditions. The effect of changes in BAC adsorption capacity on overall removal efficiency depends on the number of available adsorption sites, as well as proximity to equilibrium condition. The model developed in this study is also able to predict the effect of the number of stages on overall removal efficiency of the adsorber. The results of this study could pave the way for optimizing the design and operation of fluidized bed adsorbers, leading to cost savings and performance improvements.