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

Effective squirmer models for self-phoretic chemically active spherical colloids

Various aspects of self-motility of chemically active colloids in Newtonian fluids can be captured by simple models for their chemical activity plus a phoretic slip hydrodynamic boundary condition on their surface. For particles of simple shapes (e.g., spheres) -- as employed in many experimental studies -- which move at very low Reynolds numbers in an unbounded fluid, such models of chemically active particles effectively map onto the well studied so-called hydrodynamic squirmers [S. Michelin and E. Lauga, J. Fluid Mech. \textbf{747}, 572 (2014)]. Accordingly, intuitively appealing analogies of "pusher/puller/neutral" squirmers arise naturally. Within the framework of self-diffusiophoresis we illustrate the above mentioned mapping and the corresponding flows in an unbounded fluid for a number of choices of the activity function (i.e., the spatial distribution and the type of chemical reactions across the surface of the particle). We use the central collision of two active particles as a simple, paradigmatic case for demonstrating that in the presence of other particles or boundaries the behavior of chemically active colloids may be \textit{qualitatively} different, even in the far field, from the one exhibited by the corresponding "effective squirmer", obtained from the mapping in an unbounded fluid. This emphasizes that understanding the collective behavior and the dynamics under geometrical confinement of chemically active particles necessarily requires to explicitly account for the dependence of the hydrodynamic interactions on the distribution of chemical species resulting from the activity of the particles.Comment: 26 pages, 11 figure

Regime mapping and the role of the intermediate region in wall-coated microreactors

Operation of a wall-coated microreactor can occur in several mass transfer-reaction regimes. We define these regimes analytically in several planes of a multi-parametric map, taking into account the different degrees of concentration profile development, as well as the influence of non-unity orders of reaction and reactant inhibition in the kinetic law. It was found that the regions where conversion can be calculated from simplified mass transfer models are not discriminated by common results for entrance-length. We also illustrate the trade-offs that exist across this operating map concerning the catalyst design (costs associated with loading and volume) and overall system performance (evaluated in terms of reactant conversion, flow efficiency and microreactor effectiveness). It is shown that under certain conditions, the existence of moderate mass transfer resistance can be advantageous (even if internal limitations cannot be avoided), clarifying the role of the intermediate transport-reaction region

Phoretic Motion of Spheroidal Particles Due To Self-Generated Solute Gradients

We study theoretically the phoretic motion of a spheroidal particle, which generates solute gradients in the surrounding unbounded solvent via chemical reactions active on its surface in a cap-like region centered at one of the poles of the particle. We derive, within the constraints of the mapping to classical diffusio-phoresis, an analytical expression for the phoretic velocity of such an object. This allows us to analyze in detail the dependence of the velocity on the aspect ratio of the polar and the equatorial diameters of the particle and on the fraction of the particle surface contributing to the chemical reaction. The particular cases of a sphere and of an approximation for a needle-like particle, which are the most common shapes employed in experimental realizations of such self-propelled objects, are obtained from the general solution in the limits that the aspect ratio approaches one or becomes very large, respectively.Comment: 18 pages, 5 figures, to appear in European Physical Journal

Ion and mixed conducting oxides as catalysts

This paper gives a survey of the catalytic properties of solid oxides which display oxygen ion or mixed (i.e. ionic + electronic) conductivity. Particular consideration is given to the oxidation-reduction reactions of gas phase components, but attention is also devoted to oxygen exchange between gas and oxide. An attempt has been made to relate and explain the observed phenomena such as catalytic activity and selectivity in terms of the electrical conducting properties of the oxides, which depend on their crystal and defect structures.\ud \ud In a number of cases possible applications of these materials in (electro)catalytic reactors, sensors, fuel cells, oxygen pumps, etc. are indicated

Modeling of non-uniform hydrodynamics and catalytic reaction in a solids-laden riser

The riser reactors are widely used in a variety of industrial applications such as polymerization, coal combustion and petroleum refinery because of the strong mixing of gas and solids that yields high heat and mass transfer rates, and reaction rates. In a Fluid Catalytic Cracking (FCC) process, the performance of riser reactor is strongly dependent on the interaction between the fluid and catalysts, since the reaction takes place on the active surface of the catalysts. This is why, the local coupling between hydrodynamics and reaction kinetics is critical to the development of riser reaction models. The local gas-solids flow structure in riser reactors is highly heterogeneous both in axial and radial direction with back-mixing of catalyst. The radial non-uniform gas-solid flow structure is presented as core-annulus regime, with up-flow of dilute suspension of fresh catalyst and hydrocarbon vapor in the core regime, which is surrounded by dense down-flow of deactivated catalyst in the wall regime. As a result, the reaction characteristics in core and wall regions are strikingly different. The performance of the riser reactor is also strongly dependent on the vaporization and reaction characteristics in the feed injection regime of the riser reactors. From the modeling point of view, to predict the reaction characteristics in riser reactors, there is a need to develop hydrodynamics model, which can predicts both axial and radial nonuniform distribution of hydrocarbon vapor and catalyst and back-mixing of catalyst. There is also need for reasonable description of mechanistic coupling between nonuniform flow hydrodynamics and the cracking kinetics. This dissertation is aimed to develop the mechanistic model for nonuniform hydrodynamics and catalytic reactions in a FCC riser reactor. A mechanistic model for multiphase flow interactions, vaporization of droplets and reactions in the feed injection regime is developed for to decide proper input boundary conditions for FCC riser reaction models. The dissertation is divided into the three major parts: 1) development of governing mechanisms and modeling of the axial and radial nonuniform distribution of the gas-solids transport properties in riser reactors 2) development of mechanistic model that gives a quantitative understanding of the interplay of three phase flow hydrodynamics, heat/mass transfer, and cracking reactions in the feed injection regime of a riser reactor 3) modeling of nonuniform hydrodynamics coupled reaction kinetics in the core and wall regime of the riser reactors. For the modeling of the axial nonuniform distribution of gas-solids transport properties, a new controlling mechanism in terms of impact of pressure gradient along the riser on the particles transport is introduced. A correlation for inter-particle collision force is proposed which can be used for any operation conditions of riser, riser geometry and particle types. For simultaneous modeling of axial and radial nonuniform distribution of the gas-solids phase transport properties, a continuous modeling approach is used. In this dissertation, governing mechanisms for radial nonuniform distribution of gas-solids phase is proposed based on which a mechanistic model for radial nonuniform distribution of the gas and solid phase transport properties is proposed. With the proposed model for radial nonuniform phase distribution, the continuous model can successfully predicts both axial and radial nonuniform distribution of phase transport properties. As the performance of the riser reactor is strongly influence by the vaporization and reactions in the feed injection regime, in this dissertation, a detailed mechanistic model for the multiphase flow hydrodynamics, vaporization and reaction characteristics in feed injection regime is established. To simulate the conditions of industrial riser reactor, the four nozzle spray jets were used, while overlapping of the spray jets is also considered. Finally, in this dissertation, a modeling concept for the reactions in the core and wall regime of the riser reactor is explored. The proposed modeling concept takes into the account very important missed out physics such as, non-thermal equilibrium between the hydrocarbon vapor and the feed, back mixing and recirculation of the deactivated catalyst, activity of catalyst in core and wall regime, and coupling between the flow hydrodynamics and reaction kinetics

Confinement effects on diffusiophoretic self-propellers

We study theoretically the effects of spatial confinement on the phoretic motion of a dissolved particle driven by composition gradients generated by chemical reactions of its solvent, which are active only on certain parts of the particle surface. We show that the presence of confining walls increases in a similar way both the composition gradients and the viscous friction, and the overall result of these competing effects is an increase in the phoretic velocity of the particle. For the case of steric repulsion only between the particle and the product molecules of the chemical reactions, the absolute value of the velocity remains nonetheless rather small.Comment: 18 pages, 4 figures, J. Chem. Phys. (in print; full bibliographic info and DOI to be added once available