Moving Reaction Zone Research Articles

Revealing the Electrochemical Kinetics of Electrolytes in Nanosized LiFePO4 Electrodes

Lithium-ion battery rate performance is ultimately limited by the electrolyte, yet the behaviors of electrolytes during high-rate (dis)charge remain elusive to electrochemical measurement. Herein, we develop and study a nanosized LiFePO4 model system in which the electrolyte completely controls the electrochemical kinetics of the porous electrode. Impedance spectroscopy, cyclic voltammetry, and rate performance testing prove that ion transport in the electrolyte is the sole rate-limiting process, even in thin electrodes. A novel pseudo-steady-state extrapolation (S3E) method for Tafel analysis shows that LiFePO4 obeys Butler-Volmer kinetics with a transfer coefficient of 3. The combination of these unexpectedly rapid interfacial kinetics and an activation barrier for phase transformation causes extreme reaction heterogeneity, which manifests as a moving reaction zone. Resistance versus capacity analysis enables direct measurement of electrolyte resistance growth during high-rate (dis)charge, revealing how the interaction between concentration polarization and a moving reaction zone controls electrolyte rate performance in LiFePO4 electrodes. This work elucidates the profound impacts of the electrolyte on electrochemical measurements in porous battery electrodes: when the active material is not rate limiting, it is impossible to directly measure the intrinsic kinetics of the active material, but conversely, it becomes possible to directly measure the kinetics of the electrolyte.

Shrinking Core Model Formulation for the Electrochemical Performance Analysis of a Lithium/Carbon Monofluoride Cell

A shrinking core model formulation has been developed based on lithium-ion diffusion through the product layer of carbon monofluoride cathode active material being the predominate mechanism controlling the cell discharge behavior. The formulation expresses the cell discharge time; speed of the moving reaction zone towards the center of a spherical active material, carbon monofluoride, particle; cell current, and the fractional amount of charge involved in the cell electrochemical reaction in terms of the fractional conversion of the cell cathode limiting reactant, carbon monofluoride. An experimental data-based correlation between the required, lithium-ion effective diffusivity and cell voltage loss is also provided to explain the cell discharge behavior under the constant current condition.

Single-chirality nanotube synthesis by guided evolutionary selection.

Bringing to fruition the tantalizing properties, foreseen since the discovery of carbon nanotubes, has been hindered by the challenge to produce a desired helical symmetry type, single chirality. Despite progress in postsynthesis separation or somewhat sporadic success in selective growth, obtaining one chiral type at will remains elusive. The kinetics analysis here shows how a local yet moving reaction zone (the gas feedstock or elevated temperature) can entice the tubes to follow, so that, remotely akin to proverbial Lamarck giraffes, only the fastest survive. Reversing the reaction to dissolution would further eliminate the too fast-reactive types so that a desired chirality is singled out in production.

Photothermal Initiation of Frontal Polymerization Using Carbon Nanoparticles

Frontal polymerization (FP) involves the rapid conversion of monomer into polymer within a moving reaction zone. The front is typically initiated with a thermal stimulus, but noncontact, remote ini...

Emergence of traveling wave endothermic reaction in a catalytic fixed bed under microwave heating

This paper presents a new phenomenon in a packed bed catalytic reactor under microwave heating - traveling wave (moving reaction zones) endothermic chemical reaction. A two-phase model is developed to simulate the nonlinear dynamic behavior of the packed bed catalytic reactor with an irreversible first-order chemical reaction. The absorbed microwave power was obtained from Lambert's law. The structure of traveling wave endothermic chemical reaction was explored. The effects of the gas velocity and microwave power on performance of the packed bed catalytic reactor were presented. Finally, the effects of the change in the location of the microwave source at the packed bed reactor was demonstrated.

Experimental quantification and modelling of reaction zones in a cyclic watergas shift reactor

The cyclic watergas shift reactor (CWGSR) is a cyclically operated fixed bed reactor for the production of hydrogen. It is based on the alternating reduction of an iron oxide fixed bed with synthesis gas and its subsequent oxidation with steam. We show experimental data of moving reaction zones in a tubular CWGSR. Based on this data and to help further design of these reactors, we propose a dynamic, one-dimensional model of the reactor. The formulated process model was fitted to experimental data by adjusting only two parameters describing the catalytic activity and the oxygen capacity of the fixed bed. Exemplary simulation results are shown.

Direct characterization of phase transformations and morphologies in moving reaction zones in Al/Ni nanolaminates using dynamic transmission electron microscopy

Phase transformations and transient morphologies are examined as exothermic formation reactions self-propagate across Al/Ni nanolaminate films. The rapid evolution of these phases and sub-micrometer morphological features requires nanoscale temporal and spatial resolution that is not available with traditional in situ electron microscopy. This work uses dynamic transmission electron microscopy to identify intermetallic products and phase morphologies, as exothermic formation reactions self-propagate in nanolaminate films grown with 3:2, 2:3 and 1:1 Al/Ni atomic ratios. Single-shot diffraction patterns with 15 ns temporal resolution reveal that the NiAl intermetallic forms within ∼15 ns of the reaction front’s arrival in all three types of films and is the only intermetallic phase to form, as the reactions self-propagate and quench very rapidly. Time-resolved imaging reveals a transient cellular morphology in the Al-rich and Ni-rich foils, but not in the equiatomic films. The cellular features in the Al-rich and Ni-rich films are attributed to a cooling trajectory through a two-phase field of liquid + NiAl.

Regeneration of coked catalysts—modelling and verification of coke burn-off in single particles and fixed bed reactors

The regeneration of a coked naphtha reforming catalyst (Pt/Re– Al 2 O 3 ) was studied by kinetic investigations on the effective rate of coke burn-off. For temperatures of industrial relevance for the catalyst, i.e., below 550 ∘ C (deactivation), the coke burn-off within the cylindrical particles ( d P ≈ 2 mm ) is determined by the interplay of chemical reaction and pore diffusion; limitation by external mass transfer can be excluded for T < 750 ∘ C . Based on the parameters of the intrinsic kinetics and of the structure of the catalyst (porosity, tortuosity), the regeneration process is modelled and discussed both on the level of a single particle and in a technical fixed bed reactor. The results of modelling are compared with data from lab-scale investigations (coke profiles within the particles) and the performance data of the regeneration in an industrial fixed bed reactor (moving reaction zone); the agreement of calculation and measurement is in both cases complete.

Does a large coal particle in a hot fluidised bed lose its volatile content according to the shrinking core model?

Near-spherical particles (diameter ∼ 14 mm ) of six different coals (ranging in rank from an anthracite to a lignite) were immersed in turn in an electrically heated bed of sand fluidised by nitrogen. Such a bubbling fluidised bed ( U / U mf = 3.0 ) was held at a fixed temperature between 750 and 950 °C. A coal particle in such a situation decomposes in the overall reaction: coal → volatile matter + char , under conditions of pyrolysis, i.e., in the absence of oxygen. Particles of coal were extracted from the hot bed and quenched in gaseous nitrogen. It was found that, apart from the lignite, a coal particle fragmented. Thus an anthracite shattered into pieces soon after entering the hot bed, but a bituminous coal fractured into small lumps of char just after devolatilisation had finished. On each occasion the particles removed from the bed were individually weighed and counted. Also, bituminous and lignite particles were withdrawn from the hot bed before any fragmentation had occurred; they were then quenched in N 2, weighed, sectioned and examined in a microscope. Apart from the final stages of devolatilisation, these particles were seen in this way to have a central core of virgin coal, surrounded by char. The boundary between these two regions was sharp, indicating a shrinking core. The velocity of the boundary was found to be in effect constant; this conclusion was confirmed by measurements of a coal particle's mass at increasing times. All these observations indicate that pyrolysis of these relatively large particles proceeds with a shrinking core. A model for the process involves heat transfer from the fluidised bed to the coal particle's exterior, followed by heat conduction through the outer layer of char to provide the enthalpy required for endothermic thermal decomposition in the moving reaction zone. This model predicts that, apart from the very beginning and end of devolatilisation, there is an almost constant velocity for the shrinking core of raw coal. The quantitative agreement of the model with the observations is not perfect in every respect, but indicates that a bituminous coal in effect loses its volatile matter at ∼ 550 ± 100 ° C . The apparent enthalpy change for the thermal decomposition of a bituminous coal is deduced to be in the range 2.4 to 3.1 MJ/kg of coal and the thermal conductivity of the resulting char is 1.7 ± 0.9 W m −1 K −1 . Also, t d , the time for complete devolatilisation, depends on a coal particle's radius, a, according to t d = constant 1 × a 2 + constant 2 × a .

SQUID detection of magnetic fields produced by chemical reactions

We report on the detection of chemomagnetic fields produced by solid state combustion and liquid-solid reactions. Magnetic time series and noise spectra are recorded with high-T/sub c/ SQUID magnetometers. The moving reaction zone, ion convection, ion diffusion all cause macroscopic ionic currents, which generate electric and magnetic fields. A simple electromagnetic model describes the relation between a moving charge distribution associated with the combustion wave and the resulting magnetic field. Certain reactions exhibit power law behavior in the noise spectra and probability distributions of magnetic field peak amplitudes, consistent with self-organized criticality.

Use of dynamically adaptive grid techniques for the solution of electrochemical kinetic equations: Part 4: The adaptive moving-grid solution of one-dimensional fast homogeneous reaction-diffusion problems with extremely thin reaction zones away from the electrodes

The adaptive moving-grid strategy suggested in Parts 1–3 for the solution of electrochemical kinetic partial differential equations in one space dimension has been subject to further evaluation and tuning. The strategy has been applied to two kinetic examples of fast homogeneous reaction-diffusion systems involving extremely thin moving reaction zones away from the electrodes. One example, the simulation of the double-potential step transient for a simple mechanism of electrogenerated chemiluminescence, has been considered in much detail. As the second example the simulation of the linear potential scan voltammetry for a double-electron transfer with “nuances” has been briefly discussed. The adaptive grid strategy provides effective, satisfactorily accurate and complete solutions to these difficult problems, which would not be possible to obtain by traditional fixed-grid finite difference methods, except at much higher computational cost or with radical simplifications.

Moving reaction zone chemistry: The hydroxylamine, sodium thiocyanate, nitric acid system

It is possible to initiate an oxidation reaction in a solution of hydroxylamine and sodium thiocyanate in aqueous nitric acid in a spectrophotometer cell at the meniscus. The thiocyanate and hydroxylamine are oxidised, and nitric acid is reduced to nitrous acid. A narrow well-defined reaction zone develops which then moves at a constant rate downwards through the solution. One of the reaction intermediates, nitrosyl thiocyanate, absorbs at 460 nm so the reaction zone is coloured red and is easily observed. The distribution of nitrosyl thiocyanate within the reaction zone has been studied.

Periodic Interfacial Precipitation in Polymer Films

Interfacial precipitation of silver halides in water-swollen polymer films occurred in complex, multilayered patterns if the concentrations of counterdiffusing reactants were unequal or decreased at different rates. Development of the rapidly forming Liesegang rings, which extend the phenomenon of periodic precipitation to the submicrometer range, is attributable to the combined effect of a moving reaction zone and periodic immobilization of colloidal silver halide.

Asymptotic analysis of single pore gas-solid reactions

The single-pore model for gas-solid reactions proposed by Ramachandran and Smith[1] is used to develop exact, closed-form solutions for the concentrati pore radius and conversion profiles in a long, narrow pore for both cylindrical and slab geometries. For relatively large values of the Thiele modulus, the solutions portray the asymptotic behavior of the reaction scheme in a finite pore. The cases of incomplete conversion associated with pore closure low initial porosities and complete conversion associated with a moving reaction zone at high initial porosities are analyzed. It is shown that the asy results provide very accurate estimates of the overall conversion in applications involving large pore diffusional resistance or fast reactions. The se of the conversion to various physicochemical parameters is also examined. The asymptotic results show good agreement with experimental data for gas-sol

A THEORETICAL INVESTIGATION OF THE EFFECTS OF VARIABLE POROSITY AND DIFFUSIVITY ON THE MANUFACTURE OF REACTION-BONDED SILICON NITRIDE

Transient behaviour of the exothermic, irreversible silicon-nitrogen reaction has been simulated using a particle-pellet model which incorporates a sharp 'cut-of T reaction into the moving reaction zone in the heat mass transfer Held. Variations of certain physical parameters connected with the micro-structural changes during reaction, with respect to temperature and/or solid concentration are taken into account. The solution procedures to the resulting system or equations are based on an explicit finite difference scheme. Effects of the Thiele modulus and Biot number for heat transfer on the solid conversion and fluid concentration are examined, together with the effect of a zero heat of reaction on the temperature profiles in the compact. The results are graphically presented and interpreted.

Stability Phenomena in Chemical Reactors

AbstractSome important types of non‐linear phenomena occuring in chemically reacting systems are explained by means of some single examples of chemical reactors. Multiple steady‐states exhibiting a distinct behavior of ignition and extinction are in the category of these phenomena. In addition all dynamic instabilities, of which there are a wide variety must also be included. These instabilities might be of thermal or purely kinetic nature, or they may result from the interaction of chemical reaction and physical transport processes. Generally these instabilities result in increasing oscillations which finally converge in stable limit cycles. The moving reaction zone which can appear in fixed‐bed reactors is described in greater detail in order to demonstrate that such a dissipative structure may be considered a prototype of an active transport process. This kind of active transport process can be observed in many systems, which are not limited just to chemical engineering. Furthermore, it is emphasized that a moving reaction zone is characterized by a certain feature which here is called “form stability”. This property might be used in order to considerably reduce the order of the non‐linear equations, describing the reactor behavior. Model reduction is of great importance with respect to both a detailed analysis of the dynamic behavior of a reactor and to the design of control concepts.

Transport effects in the manufacture of reaction‐bonded silicon nitride

AbstractThe transient behaviour of the exothermic, irreversible, silicon‐nitrogen reaction is analysed, using mixed type boundary conditions to represent external resistances. The mathematical model incorporates a sharp cut‐off reaction into the moving reaction zone, in the heat mass transfer field, and the resulting system of equations is solved numerically using an explicit finite difference scheme. Effects of varying the Thiele modulus, Prater temperature and Biot number are examined, together with the different geometrical effects. The results are graphically presented and interpreted.

Zur dynamik des katalytischen festbettreaktors bei katalysator-desaktivierung—I.

The influence of catalyst deactivation on the dynamic behaviour of a catalytic fixed-bed-reactor is investigated. Using the spatially one-dimensional two-phase-model and an additional differential equation for the activity, profiles of temperature and activity are shown and compared to experimental results. It turns out that, due to deactivation, moving reaction zones can occur even at constant inlet conditions and that, in case of rapid deactivation, serious overheating of the reactor may result.

In-place leaching of primary sulfide ores: Laboratory leaching data and kinetics model

Experimental results obtained in laboratory leaching studies of primary copper sulfide ore in sulfuric acid systems pressurized with oxygen are interpreted by a computerized geometric model involving the movement of a reaction zone through the ore fragments. Physical properties of the ore, including size, shape, and mineral content, are considered. The leaching mechanism involves mixed kinetics and includes a surface reaction within a moving reaction zone plus pore diffusion of dissolved oxygen through the reacted portion of the ore fragment to the reaction zone. The results are applicable to conditions that would exist innuclear solution mining or similar processes in which the ore is converted into rubble and then inundated by a leach solution adequately supplied with oxidants. Experimental results at 90°C are correlated with the model, which includes temperature-dependent parameters.

Quasi-stationäres verhalten von wandernden brennzonen

During the performance of a fast exothermic reaction in a catalyst bed, the complete chemical reaction can occur in a very short moving reaction zone, which frequently only ranges over a few pellet diameters. Experimental examinations, made by Wicke and Vortmeyer, on the behavior of such a moving reaction zone resulted in the following fact: through variation of the flow velocity u in axial direction of the bed, this zone can optionally be moved. If the conditions of the reaction are time constant, and the reactor is sufficiently long on both sides, a quasi-stationary axial movement of this reaction zone appears. This movement is characterized through a constant velocity w. For these quasi-stationary dynamics, the diffusion model's equations are approximately solved. From these, the velocity w of the moving reaction zone can be determined as a function of all parameters of reaction and reactor. This can be done in a very distinct way by means of a graph.