Sorption diffusional resistance

The intrinsic rate of physical adsorption is very rapid so the overall sorption rate is generally controlled by the diffusional resistances associated with mass transfer to the adsorption site. Commercial zeoUte-based catalysts and adsorbents consist of small (micron-sized) zeolite crystals formed into macro-porous (milUmeter-sized) particles, generally with the aid of a clay binder. Such materials offer at least three and in some cases four distinct mass transfer resistances (see Fig. 10) [35] ... 

Because the heat effects associated with adsorption are comparatively large, the assumption of isothermal behavior is a valid approximation only when uptake rates are relativeily slow. The problem of nonisothermal sorption under conditions typical of a gravimetric uptake rate experiment has been analyzed by Lee and Ruthven. ) Two distinct situations were considered nonisothermal sorption in a single particle under conditions such that intraparticle diffusional resistance is the dominant resistance to mass transfer and sorption in a bed of particles under conditions such that the mass transfer rate is controlled by diffusion into the particle bed rather than by intraparticle diffusion. 

If the rate constants for the sorption-desorption processes are small equilibrium between phases need not be achieved instantaneously. This effect is often called resistance-to-mass transfer, and thus transport of solute from one phase to another can be assumed diffusional in nature. As the solute migrates through the column it is sorbed from the mobile phase into the stationary phase. Flow is through the void volume of the solid particles with the result that the solute molecules diffuse through the interstices to reach surface of stationary phase. Likewise, the solute has to diffuse from the interior of the stationary phase to get back into the mobile phase. 

FIGURE 3.1 Concentration profiles in a passive sampling device. The driving force of accumulation is the difference in chemical potentials of the analyte between the bulk water and the sorption phase. The mass transfer of an analyte is governed by the overall resistance along the whole diffusional path, including contributions from the individual barriers (e.g., aqueous boundary layer, biofilm layer, and membrane). 

As an alternative to conventional sorption rate measurements it is also possible to derive diffusional time constants from the dynamic response of a packed column to a change in sorbate concentration. In a chromatographic system the broadening of the response peak results from the combined effects of axial dispersion and mass transfer resistance. By making measurements over a range of gas velocities it is possible to separate the dispersion and mass transfer effects and so to determine the effective overall mass transfer coefficient or the diffusional time constant. Further details are given in Section 8.5. 

---