Transition from Knudsen to molecular diffusion in activity of absorbing irregular interfaces

We investigate through molecular dynamics the transition from Knudsen to molecular diffusion transport towards 2d absorbing interfaces with irregular geometry. Our results indicate that the length of the active zone decreases continuously with density from the Knudsen to the molecular diffusion regime. In the limit where molecular diffusion dominates, we find that this length approaches a constant value of the order of the system size, in agreement with theoretical predictions for Laplacian transport in irregular geometries. Finally, we show that all these features can be qualitatively described in terms of a simple random-walk model of the diffusion process.Comment: 4 pages, 4 figure

Dynamic Scaling of an Adsorption-Diffusion Process on Fractals

A dynamic scaling of a diffusion process involving the Langmuir type adsorption is studied. We find dynamic scaling functions in one and two dimensions and compare them with direct numerical simulations, and we further study the dynamic scaling law on fractal surfaces. The adsorption-diffusion process obeys the fracton dynamics on the fractal surfaces.Comment: 9 pages, 7 figure

Permeance inhibition of Pd-based membranes by competitive adsorption of CO: Membrane size effects and first principles predictions

Many studies have demonstrated an inhibition of hydrogen permeance through a Pd or Pd-alloy membrane due to competitive adsorption of CO, ammonia and other species. Several corrections to Sieverts’ law were suggested to account for these effects, based on a Langmuir isotherm, but they all suffer from certain drawbacks as detailed below. We derive a new correction, based on the kinetic balances on the adsorbed layer, and on the underlying dissolved H layer from which the diffusing H atoms originate. The new correction is physically reasonable and it predicts that the permeation inhibition declines with increasing membrane size, in agreement with several suggestions in the literature. We test the new expressions against published experimental data of inhibition due to CO co-adsorption, using energetic parameters that were either estimated by DFT studies or derived from experimental data. A fair agreement is obtained, but it depends on assumptions regarding entropy effects. We also derived an approximation and adjusted its parameters. Finally we comment on the observation of inhibition due to H2S and ammonia, which represent more complex cases

Control of Traveling Solutions in a Loop-Reactor

We consider the stabilization of a rotating temperature pulse traveling in a continuous asymptotic model of many connected chemical reactors organized in a loop with continuously switching the feed point synchronously with the motion of the pulse solution. We use the switch velocity as control parameter and design it to follow the pulse: the switch velocity is updated at every step on-line using the discrepancy between the temperature at the front of the pulse and a set point. The resulting feedback controller, which can be regarded as a dynamic sampled-data controller, is designed using root-locus technique. Convergence conditions of the control law are obtained in terms of the zero structure (finite zeros, infinite zeros) of the related lumped model

Activated-sludge system design for species selection: analysis of a detailed multispecies model

NRC publication: Ye

Activated-sludge system design for species selection: analysis of a detailed multispecies model

NRC publication: Ye

Transversal hot zones formation in catalytic packed-bed reactors

Spatiotemporal patterns reported to form in the cross sections of packed-bed reactors (PBRs) may pose severe safety hazard when present next to the reactor wall. Understanding what causes their formation and dynamic features is essential for the rational development of design and control strategies that circumvent their generation. We review the current knowledge and understanding about the formation of these transversal temperature patterns. Simulations and model analysis revealed that the formation of the hot spots and their dynamics are sensitive to the assumed kinetic and reactor models. Under practical conditions, stable symmetry-breaking bifurcation to nonuniform states, from stable, stationary, transversally Uniform states cannot be predicted by common PBR models with a rate expression that depends only on the surface temperature and concentration of the limiting reactant. However, analysis and simulations reveal that transient nonuniform transversal temperatures may emerge in an upstream moving traveling front under practical conditions. Microkinetic oscillatory reactions predict the formation of a plethora of intricate spatiotemporal temperature patterns and temperature front motions that are sensitive to the reactor operating conditions and properties such as diameter and initial conditions. The predicted temperature patterns may be rather intricate as a result of conjugation of several modes. The nonlinear coupling between the states at different axial positions, that is, the interaction among the local temperature and concentrations at different cross-sections of the bed, may explain the intricate conjugation of several modes and modulation of the observed spatiotemporal patterns. While some simulations predicted spatiotemporal pattern evolution in PBRs, there is a need to understand which reaction mechanisms may lead to their formation. Most previous simulations and analysis utilized two-dimensional reactor models. However, hot zones are three-dimensional structures, often very small, and difficult to detect in large reactors. A 3-D simulation, although tedious, is necessary to provide full information about the size, shape and dynamic features of small hot zones. Moreover, common PBR models may have to be modified to account for the impact of local states such as flow distribution and nonuniform packing. Verification of the various model predictions requires in situ measurements of 3-D hot zones