Liquid phase adsorption of polar compounds from toluene using zeolites
The liquid phase removal of low concentrations of polar compounds (acetonitrile, acrylonitrile and dioxane) from toluene by adsorption on zeolites reveals very high selectivity factors. Kinetic selectivity factors as high as 16,100 are observed. The selection of the zeolites (MFI, MWW and FAU type) allowed the study of the effects of varying aluminum content, the presence and absence of acidic centers and varying pore volumes. The equilibrium data show that the effectiveness of liquid-phase polar compound adsorption using H-ZSM-5 increases with the number of Bronsted acid sites which are proportional to the alumina content. Also on a per cation basis, the H-ZSM-5 and H-MCM-22 protonic zeolites appear to have higher selectivity for these polar adsorbates than the metal cation. With similar porous structures and alumina content, the H-ZSM-5 and H-MCM-22 show similar adsorption behavior. ^ To assess the relative effectiveness of each adsorbent, equilibrium batch, kinetic batch and continuous flow experiments were conducted. The zeolites H-ZSM-5, H-MCM-22, H-mordenite and Na-X are highly effective in removing the polar compounds from toluene to a concentration level down to less than 20 ppm (detection limit). ^ The liquid-phase effective diffusion coefficients of acrylonitrile, acetonitrile and dioxane were estimated by matching simulations to kinetic batch experimental data using both the Pore Diffusion Control (PDC) model and the Surface Diffusion Control (SDC) model. The PDC described the kinetic behavior better and the diffusivities show the order of Deff(Ace.) > Deff(Acr.) > Deff(Dio.) which is consistent with the literature data on the respective diffusivities in water. ^ The adsorption isotherms were described by various isotherm equations. The best-fit parameters obtained from the Langmuir equation were incorporated into the fixed-bed adsorption simulation. A mathematical model was established to simulate single-solute fixed-bed adsorption. An orthogonal collocation method was employed to solve the partial differential equations. The behaviors of the fixed-bed adsorption systems were successfully described using this model with only the effective diffusion coefficient as the adjustable parameter. The model was then extended to the two-solute adsorption system. By applying the information derived from the single-solute systems, simulation results were obtained that matched the experimental data qualitatively well. ^
Yi-hsiung Sean Lin,
"Liquid phase adsorption of polar compounds from toluene using zeolites"
(January 1, 2002).
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