Multicomponent adsorption equilibrium and kinetics of gases in heterogeneous activated carbon are theoretically studied in this thesis. To modify the vacancy solution model for adsorption, a new adsorption isotherm is developed on the basis of a mass-action law. It inherits the advantage of the original vacancy solution theory (VST) approach in that the calculation of multicomponent equilibria is in the same framework of the single component case. In addition, the new isotherm maintains the thermodynamic consistency for multicomponent formulation. The heterogeneity of activated carbon is considered in terms of its structural heterogeneity, which is expressed by a pore size distribution function. The heterogeneity of adsorbent is introduced through the fluid-solid interaction potential energy, which is represented by Steele's 10-4-3 potential.
The mathematical model for adsorption kinetics on activated carbon is developed for both single and multicomponent adsorbate. The transport process in the bidisperse carbon is described by the simultaneous diffusion in the macropore and micropore phases. The gaseous adsorptives diffuse in the macropore through the molecule-molecule collision and the molecule-wall collision, which are represented by the molecular and Knudsen diffusivities, respectively. The micropore diffusion is conducted by the hopping of the adsorbate molecules on the adsorption site. The driving force for the micropore diffusion is the chemical potential gradient in the adsorbed phase. The concentration gradient in the microparticle is assumed to be negligible, and the mass balance equations for both phases are developed over the particle scale. The diffusions in the gaseous and adsorbate phases are correlated by a general expression for mass exchange flux between these two phases, which can be applied to any arbitrary isotherm. In considering the connectivity of pore network in activated carbon, the effective medium theory (EMT) approach is applied to estimate the effective adsorbate phase diffusivity for single component adsorption.
On the development of the predictive model for multicomponent adsorption kinetics on activated carbon, the generalized Maxwell-Stefan (MS) formulation is applied to estimate the binary diffusivities in both the macropore and the micropore phases. The expression for the phenomenological diffusivities can account for the diffusive and viscous contribution unambiguously in both phases. The cross-term of the binary adsorbate phase diffusivity is represented by the semi-empirical Vignes correlation, which explicitly relates the diffusivity to the surface coverage and the self diffusivity of each species.
In evaluation of the theoretical model, a wide range of published experimental data has been applied to compare the model calculation with the experimental results. In the application of the mathematical model to the literature data, the isotherm parameters are first extracted from fitting adsorption equilibrium data of single component. The information related to the pore structure of activated carbon is also obtained from this process. The binary adsorption equilibria on the corresponding adsorbents are then 'predicted by the proposed model. The azeotropy phenomena on homogeneous porous adsorbents can be theoretically predicted by the modified vacancy solution model, exhibiting the non-ideality due to adsorbate size differences in binary system. The binary adsorption equilibria on various activated carbons can be accurately predicted for ambient bulk pressures, supporting the reliability of the fitting parameters and the applicability of the equilibrium model. The deviation of model prediction at elevated pressures may be due to the model assumption regarding the porous structure of carbon. The adsorbate-related kinetic parameters are extracted from fitting the single component kinetic data on two different carbons. The adsorption kinetics of binary adsorbates is predicted using the fitting parameters. The present model can successfully predict the co-adsorption, co-desorption and displacement ofC2H6 and C3H8.