Non-Fickian diffusion of methanol in mesoporous media: Geometrical restrictions or adsorption-induced?

Effect of Catalyst (de)activation on Reagent Diffusion in ZSM-5/alumina Extruded Pellet for the Methanol-to-hydrocarbons Conversion

A pelletized ZSM-5/alumina catalyst was prepared by the extrusion technique. The catalyst was activated by ion-exchange with NH4NO3 aqueous solution. The activated catalyst was trained in the methanol-to-hydrocarbons reaction which caused the catalyst deactivation due to coke deposition (6.5 % wt.). Coke deposition resulted in a two-time decrease in the micropore volume. The methane, benzene, and methanol transport through ZSM-5/alumina pellet were consequently studied prior to activation, after activation, and after catalyst deactivation. A slight decrease in the diffusion rate after catalyst activation is observed. After deactivation, the diffusion rate increases insignificantly. The diffusion regime remains unchanged with respect to either activation or deactivation procedure. Contrary, for the methanol, the diffusion rate through a deactivated catalyst pellet remarkably increases due to micropore blockage by coke deposition. The obtained results reveal that the micropores blockage during the catalyst deactivation enhances the methanol mass transfer.

Non-Gaussian subdiffusion of single-molecule tracers in a hydrated polymer network

Single molecule tracking experiments inside a hydrated polymer network have shown that the tracer motion is subdiffusive due to the viscoelastic environment inside the gel-like network. This property can be related to the negative autocorrelation of the instantaneous displacements at short times. Although the displacements of the individual tracers exhibit Gaussian statistics, the displacement distribution of all the trajectories combined from different spatial locations of the polymer network exhibits a non-Gaussian distribution. Here, we analyze many individual tracer trajectories to show that the central portion of the non-Gaussian distribution can be well approximated by an exponential distribution that spreads sublinearly with time. We explain all these features seen in the experiment by a generalized Langevin model for an overdamped particle with algebraically decaying correlations. We show that the degree of non-Gaussianity can change with the extent of heterogeneity, which is controlled in our model by the experimentally observed distributions of the motion parameters.

Time-fractional characterization of brine reaction and precipitation in porous media

Brine reaction and precipitation in porous media sometimes occur in the presence of a strong fluid flowing field, which induces the mobilization of the precipitated salts and distorts their spatial distribution. It is interesting to investigate how the distribution responds to such mobilization. We view these precipitates as random walkers in the complex inner space of the porous media, where they make stochastic jumps among locations and possibly wait between successive transitions. In consideration of related experimental results, the waiting time of the precipitates at a particular position is allowed to range widely from short sojourn to permanent residence. Through the model of a continuous-time random walk, a class of time-fractional equations for the precipitate's concentration profile is derived, including that in the Riemann-Liouville formalism and the Prabhakar formalism. The solutions to these equations show the general pattern of the precipitate's spatiotemporal evolution: a coupling of mass accumulation and mass transport. And the degree to which the mass is mobilized turns out to be monotonically correlated to the fractional exponent α. Moreover, to keep the completeness of the model, we further discuss how the interaction among the precipitates influences the precipitation process. In doing so, a time-fractional non-linear Fokker-Planck equation with source term is introduced and solved. It is shown that the interaction among the precipitates slightly perturbs their spatial distribution. This distribution is largely dominated by the brine reaction itself and the interaction between the precipitates and the porous media.

The effects of MTG catalysis on methanol mobility in ZSM-5

QENS shows that methanol is immobile in a fresh ZSM-5 catalyst and one used for methanol conversion at 623 K, but undergoes isotropic rotation within the mesopores generated in a catalyst used for methanol conversion at 673 K.

Non-Isothermal Effectiveness Factor for Catalytic Particles with Non-Fickian Diffusion

In this note, the effects of the non-Fickian diffusion on the prediction of effectiveness factor in non-isothermal porous catalytic slab considering external transport resistances are studied. A Green’s function formulation is used to solve the fractional diffusion-reaction model assuming non-Fickian diffusion to describe the internal mass transport in the porous catalytic particle. Evaluation of the effectiveness factor considering linear and nonlinear reaction rates was developed under isothermal and non-isothermal conditions. In both cases, numerical simulations show the relation existent between the anomalous diffusion with the performance of the catalytic slabs.