Abstract
Fluid–solid adsorption processes are mostly governed by the adsorbate transport in the solid phase and surface diffusion is often the limiting step of the overall process in microporous materials such as zeolites. This work starts from a concise review of concepts and models for surface transport and variable surface diffusivity. It emerges that the phenomenon of hindered surface diffusion for monolayer adsorption, which is common in zeolites, and models able to fit a non-monotonic trend of surface diffusivity against adsorbate solid phase concentration, have received limited attention. This work contributes to the literature of hindered diffusion by formulating a time-dependent equation for surface diffusivity based on fractal dynamics concepts. The proposed equation takes into account the contributions of both fractal-like diffusion (a time-decreasing term) and hopping diffusion (a time-increasing term). The equation is discussed and numerically analyzed to testify its ability to reproduce the possible different patterns of surface diffusivity vs. time.
Highlights
A typical fluid–solid adsorption process generally consists of three steps when referring to the adsorbate: (i) mass transfer by diffusion from the bulk of the fluid phase to the solid’s external surface, (ii) mass transfer by diffusion into the solid phase, and (iii) adsorption on the solid’s internal surface
To the best of our knowledge, at least in the area of adsorption kinetics modelling, the only equations dealing with both increasing and decreasing trends of variable surface diffusivity are those of Chen and Yang [29] and Marbán et al [33]
A contribution in this sense is given by formulating a time-dependent correlation for the surface diffusivity based on fractal dynamics concepts
Summary
A typical fluid–solid adsorption process generally consists of three steps when referring to the adsorbate: (i) mass transfer by diffusion from the bulk of the fluid phase to the solid’s external surface (film diffusion), (ii) mass transfer by diffusion into the solid phase (intraparticle diffusion), and (iii) adsorption (by physical and/or chemical mechanism) on the solid’s internal surface. These steps are the same in adsorption and ion exchange processes, the difference being related to the stoichiometric nature of the latter [1]. Analytical and/or approximate solutions are mostly based on the works of Crank [6], Boyd et al [7], Paterson [8], Processes 2020, 8, 689; doi:10.3390/pr8060689 www.mdpi.com/journal/processes
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