Effective Reaction Rate Research Articles

A robust upscaling of the effective particle deposition rate in porous media

In the upscaling from pore to continuum (Darcy) scale, reaction and deposition phenomena at the solid–liquid interface of a porous medium have to be represented by macroscopic reaction source terms. The effective rates can be computed, in the case of periodic media, from three-dimensional microscopic simulations of the periodic cell. Several computational and semi-analytical models have been studied in the field of colloid filtration to describe this problem. They typically rely on effective deposition rates defined by complex fitting procedures, neglecting the advection-diffusion interplay, the pore-scale flow complexity, and assuming slow reactions (or large Péclet numbers). Therefore, when these rates are inserted into general macroscopic transport equations, they can lead to several conceptual inconsistencies and significant errors. To study more accurately the dependence of deposition on the flow parameters, in this work we advocate a clear distinction between the surface processes (that altogether defines the so-called attachment efficiency), and the pore-scale processes. With this approach, valid when colloidal particles are small enough, we study Brownian and gravity-driven deposition on a face-centred cubic (FCC) arrangement of spherical grains, and define a robust upscaling based on a linear effective reaction rate. The case of partial deposition, defined by an attachment probability, is studied and the limit of perfect sink is retrieved as a particular case. We introduce a novel upscaling approach and a particularly convenient computational setup that allows the direct computation of the asymptotic stationary value of effective rates. This allows to drastically reduce the computational domain down to the scale of the single repeating periodic unit. The savings are ever more noticeable in the case of higher Péclet numbers, when larger physical times are needed to reach the asymptotic regime and thus, equivalently, much larger computational domain and simulation time would be needed in a traditional setup. We show how this new definition of deposition rate is more robust and extendable to the whole range of Péclet numbers; it also is consistent with the classical heat and mass transfer literature.

Particle-based modeling of heterogeneous chemical kinetics including mass transfer.

Connecting the macroscopic world of continuous fields to the microscopic world of discrete molecular events is important for understanding several phenomena occurring at physical boundaries of systems. An important example is heterogeneous catalysis, where reactions take place at active surfaces, but the effective reaction rates are determined by transport limitations in the bulk fluid and reaction limitations on the catalyst surface. In this work we study the macro-micro connection in a model heterogeneous catalytic reactor by means of stochastic rotation dynamics. The model is able to resolve the convective and diffusive interplay between participating species, while including adsorption, desorption, and reaction processes on the catalytic surface. Here we apply the simulation methodology to a simple straight microchannel with a catalytic strip. Dimensionless Damkohler numbers are used to comment on the spatial concentration profiles of reactants and products near the catalyst strip and in the bulk. We end the discussion with an outlook on more complicated geometries and increasingly complex reactions.

Iminodiacetic acid synthesis over Cu/ZrO2 catalyst in a microchannel flow reactor

Flow microchannel reactor was used to study the synthesis of iminodiacetic acid (IDA) via the catalytic oxidative dehydrogenation of diethanolamine (DEA) over catalyst Cu/ZrO2 (containing copper in amount of 10mass%) at various residence times, reaction temperature ranging within 160–190°C. IDA was shown to form in the course of two consecutive reactions through intermediate compound N-(2-hydroxyethyl)glycine (HEG). DEA conversion and IDA yield were found to increase as residence time and reaction temperature grow, meanwhile HEG concentration maximum on kinetic curves shifts towards shorter residence times. Effective reaction rate constants were determined according to the suggested scheme of IDA synthesis. Calculated activation energies for two consecutive stages of IDA synthesis were found to be 73.7 and 67.2kJ/mol, respectively.

Detailed Kinetic Investigations on the Selective Oxidation of Biomass to Formic Acid (OxFA Process) Using Model Substrates and Real Biomass

A detailed kinetic analysis of the selective catalytic oxidation of different biogenic model substrates to formic acid (OxFA process) is presented using two different polyoxometalate catalysts, HPA-2 (H5PV2Mo10O40) and HPA-5 (H8PV5Mo7O40). By using high oxygen pressures (30–60 bar) and temperatures of 80–90 °C in aqueous solution, we were able to investigate the rate-determining substrate oxidation step catalyzed by the oxidized form of the polyoxometalates (POMs) by keeping the concentration of the catalytic active species constant. Under these conditions, kinetic parameters like effective reaction order and reaction rate constants could be determined by the differential method for model substrates like glucose, fructose, sorbitol, and gluconic acid. Furthermore, kinetic experiments using the two different catalysts showed a different behavior for various functionalities within the carbon framework of the substrate with the tendency of higher reaction rates with the HPA-5 catalyst. This effect was more p...

Development of continuous-energy sensitivity analysis capability in OpenMC

The iterated fission probability (IFP) method and the Contributon-Linked eigenvalue sensitivity/Uncertainty estimation via Track length importance CHaracterization (CLUTCH) method have been implemented in several Monte Carlo codes to perform sensitivity calculations. In this work the unified adjoint and generalized adjoint calculation framework combining the IFP method and the CLUTCH method is proposed, and corresponding capabilities for calculating nuclear data sensitivity to the effective multipilication factor and reaction rate ratios are implemented in OpenMC. Results are presented on Godiva, Jezebel and a PWR fuel pin system which demonstrate that OpenMC results agree well with reference direct perturbation sensitivity coefficients. In addition, two main problems of the IFP method related to the number of latent generations, namely convergence and statistical fluctuations are investigated theoretically and numerically. This paper further demonstrates that the fission matrix approach is better suited to generate the adjoint and generalized adjoint source distribution in the CLUTCH method to overcome the drawbacks of the IFP method.

Thermodynamic and kinetic investigation of a technical grade manganese-iron binary oxide for thermochemical energy storage

Thermochemical energy storage (TCS) based on gas-solid reactions constitutes a promising concept to exploit reaction enthalpies for thermal energy storage. This concept facilitates the development of efficient storage solutions with higher energy densities compared to widely investigated sensible and latent thermal energy storage systems. Multivalent metal oxides are capable of undergoing a reversible redox reaction at high temperatures, which is why those storage materials are considered particularly suitable for the operating temperature range of concentrated solar power plants with central receiver systems to increase the total plant efficiency and ensure dispatchability of electricity.In the scope of this work a granular manganese-iron oxide with a Fe/Mn molar ratio of 1:3has been selected as a potentially suitable storage material, which is non-toxic, abundant and economical. For this reason a preparation route from technical grade raw materials has been chosen. The reversible redox reaction is investigated with respect to the thermodynamic and kinetic characteristics by means of simultaneous thermal analysis in dynamic and isothermal series of measurements. Those revealed that the observed presence of a strong divergence of the reactive temperature range from the actual thermodynamic equilibrium can mainly be attributed to kinetic limitations. Expressions for the effective reaction rates are deduced from experimental data for the reduction and oxidation step, describing the dependence of the reaction rate on temperature and oxygen partial pressure, respectively. The expressions are valid for the temperature ranges in proximity to the equilibrium, which are relevant for the targeted operating conditions of the storage reactor in air. The storage material provides good cycling stability in terms of reversibility and widely maintained reactivity throughout 100 redox cycles in air. Future work comprises material modifications, which are expected to further enhance the mechanical stability of the particles. Overall, the manganese-iron oxide of the chosen composition exhibits a redox reactivity practical for regenerator-type storage systems combining a high temperature TCS zone and a lower temperature non-reactive zone merely used for sensible thermal energy storage.

Upscaling multicomponent transport in porous media with a linear reversible heterogeneous reaction

The upscaling process of multicomponent mass transfer and reaction in a rigid and homogeneous porous media was carried out using the method of volume averaging. The first-order reversible reaction which occurs at the solid-fluid interface was considered in this paper. The corresponding macroscopic governing equations were derived from the system dynamics at the pore scale. The effective coefficients were obtained by solving the associated closure problems. This study shows that if the backward reaction rate constant at the microscale is small enough, i.e. k-→0, the obtained upscaled model is in accordance with the macroscopic model derived from the first-order irreversible heterogeneous reaction case which was extensively investigated in the literature. The influence of reaction rates on the effective parameters in the macroscopic equations was also investigated. It has been found that both forward and backward reaction rates have significant influence on the effective diffusivities and effective reaction rates in the macroscopic equations. The established equations were successfully verified by the comparison of the direct numerical calculations.

Understanding of Transition from Gas-Phase to Material-Kinetic Limitations for Nonstoichiometric Oxides

Oxygen nonstoichiometric oxides are important materials in technologies such as fuel cells, permeation membranes, and solar-driven thermochemical fuel production owing to their reversible redox characteristics. The material kinetic properties of the oxides, the surface reaction rate constant at the solid/gas interface and the chemical diffusion in the bulk, have been widely investigated for the understanding of chemical redox kinetics. However, under certain conditions, insufficient oxidant/reductant supply hinders the overall redox kinetics in which the oxides accompany relatively large nonstoichiometry (δ) deviations during the reaction. Furthermore, since the oxides are usually structured as mesoporous or microporous forms to enhance the effective surface reaction rate and thereby enhance kinetics, understanding the correlation among the material kinetic properties, the morphology of the oxides, and the oxidant/reductant gas supply is essential for establishing the strategy for maximizing global reaction rates. We will present the effects of morphology, temperature and gas flow rate on the global redox kinetics of porous undoped ceria (CeO2-δ) as a model system. Using the reticulated porous ceramic (RPC) approach, and the morphology of the samples was controlled by modifying the starting ceria slurry with/without pore-formers. Kinetic behavior was examined as a function of temperature and gas flow rate by the electrical conductance relaxation technique, which is attractive due to its ease for extracting the kinetic parameters. The validity of the electrical conductance relaxation technique for assessing the kinetic behavior of the porous oxides in the limits of the gas-phase supply and the surface reaction of material will be discussed, and the theoretically anticipated transition behavior from the gas-phase to the material kinetic limits will be shown in comparison to the experimental result.

Reducing Uncertainties in the Production of the Gamma-emitting Nuclei 26Al, 44Ti, and 60Fe in Core-collapse Supernovae by Using Effective Helium Burning Rates

Abstract We have used effective reaction rates (ERRs) for the helium burning reactions to predict the yield of the gamma-emitting nuclei 26Al, 44Ti, and 60Fe in core-collapse supernovae (SNe). The variations in the predicted yields for values of the reaction rates allowed by the ERR are much smaller than obtained previously, and smaller than other uncertainties. A “filter” for SN nucleosynthesis yields based on pre-SN structure was used to estimate the effect of failed SNe on the initial mass function averaged yields; this substantially reduced the yields of all these isotopes, but the predicted yield ratio 60Fe/26Al was little affected. The robustness of this ratio is promising for comparison with data, but it is larger than observed in nature; possible causes for this discrepancy are discussed.

Reaction Rates in Chemically Heterogeneous Rock: Coupled Impact of Structure and Flow Properties Studied by X-ray Microtomography.

We study dissolution in a chemically heterogeneous medium consisting of two minerals with contrasting initial structure and transport properties. We perform a reactive transport experiment using CO2-saturated brine at reservoir conditions in a millimeter-scale composite core composed of Silurian dolomite and Ketton limestone (calcite) arranged in series. We repeatedly image the composite core using X-ray microtomography (XMT) and collect effluent to assess the individual mineral dissolution. The mineral dissolution from image analysis was comparable to that measured from effluent analysis using inductively coupled plasma mass spectrometry (ICP-MS). We find that the ratio of the effective reaction rate of calcite to that of dolomite decreases with time, indicating the influence of dynamic transport effects originating from changes in pore structure coupled with differences in intrinsic reaction rates. Moreover, evolving flow and transport heterogeneity in the initially heterogeneous dolomite is a key determinant in producing a two-stage dissolution in the calcite. The first stage is characterized by a uniform dissolution of the pore space, while the second stage follows a single-channel growth regime. This implies that spatial memory effects in the medium with a heterogeneous flow characteristic (dolomite) can change the dissolution patterns in the medium with a homogeneous flow characteristic (calcite).

Dynamic reservoir-condition microtomography of reactive transport in complex carbonates: Effect of initial pore structure and initial brine pH

We study the impact of brine acidity and initial pore structure on the dynamics of fluid/solid reaction at high Péclet numbers and low Damköhler numbers. A laboratory μ-CT scanner was used to image the dissolution of Ketton, Estaillades, and Portland limestones in the presence of CO2-acidified brine at reservoir conditions (10MPa and 50°C) at two injected acid strengths for a period of 4h. Each sample was scanned between 6 and 10 times at ∼4μm resolution and multiple effluent samples were extracted. The images were used as inputs into flow simulations, and analysed for dynamic changes in porosity, permeability, and reaction rate. Additionally, the effluent samples were used to verify the image-measured porosity changes.We find that initial brine acidity and pore structure determine the type of dissolution. Dissolution is either uniform where the porosity increases evenly both spatially and temporally, or occurs as channelling where the porosity increase is concentrated in preferential flow paths. Ketton, which has a relatively homogeneous pore structure, dissolved uniformly at pH=3.6 but showed more channelized flow at pH=3.1. In Estaillades and Portland, increasingly complex carbonates, channelized flow was observed at both acidities with the channel forming faster at lower pH. It was found that the effluent pH, which is higher than that injected, is a reasonably good indicator of effective reaction rate during uniform dissolution, but a poor indicator during channelling. The overall effective reaction rate was up to 18 times lower than the batch reaction rate measured on a flat surface at the effluent pH, with the lowest reaction rates in the samples with the most channelized flow, confirming that transport limitations are the dominant mechanism in determining reaction dynamics at the fluid/solid boundary.

Tracer‐based characterization of hyporheic exchange and benthic biolayers in streams

AbstractShallow benthic biolayers at the top of the streambed are believed to be places of enhanced biogeochemical turnover within the hyporheic zone. They can be investigated by reactive stream tracer tests with tracer recordings in the streambed and in the stream channel. Common in‐stream measurements of such reactive tracers cannot localize where the processing primarily takes place, whereas isolated vertical depth profiles of solutes within the hyporheic zone are usually not representative of the entire stream. We present results of a tracer test where we injected the conservative tracer bromide together with the reactive tracer resazurin into a third‐order stream and combined the recording of in‐stream breakthrough curves with multidepth sampling of the hyporheic zone at several locations. The transformation of resazurin was used as an indicator of metabolism, and high‐reactivity zones were identified from depth profiles. The results from our subsurface analysis indicate that the potential for tracer transformation (i.e., the reaction rate constant) varied with depth in the hyporheic zone. This highlights the importance of the benthic biolayer, which we found to be on average 2 cm thick in this study, ranging from one third to one half of the full depth of the hyporheic zone. The reach‐scale approach integrated the effects of processes along the reach length, isolating hyporheic processes relevant for whole‐stream chemistry and estimating effective reaction rates.

Mathematical Modeling of Chemical Agent Removal by Reaction with an Immiscible Cleanser

When a hazardous chemical agent has soaked into a porous medium, such as concrete, it can be difficult to neutralize. One removal method is chemical decontamination, where a cleanser is applied to react with and neutralize the agent, forming less harmful reaction products. There are often several cleansers that could be used to neutralize the same agent, so it is important to identify the cleanser features associated with fast and effective decontamination. As many cleansers are aqueous solutions while many agents are immiscible with water, the decontamination reaction often takes place at the interface between two phases. In this paper, we develop and analyze a mathematical model of a decontamination reaction between a neat agent and an immiscible cleanser solution. We assume that the reaction product is soluble in both the cleanser phase and the agent phase. At the moving boundary between the two phases, we obtain coupling conditions from mass conservation arguments and the oil--water partition coefficient of the product. We analyze our model using both asymptotic and numerical methods, and we investigate how different features of a cleanser affect the time taken to remove the agent. Our results reveal the existence of two regimes characterized by different rate-limiting transport processes, and we identify the key parameters that control the removal time in each regime. In particular, we find that the oil--water partition coefficient of the reaction product is significantly more important in determining the removal time than the effective reaction rate.

Complex Coupling of Fluid Transport and Geochemical Reaction Rates: Insights from Reactive Transport Models

Geologic heterogeneity can cause spatially variable fluid and solute transport, leading to important implications for geochemical processes. The impact of heterogeneity on spatially variable and averaged, effective geochemical reaction rates is not yet fully described. Two examples of using reactive transport models as a tool to explore the complex connections between fluid transport and geochemical reaction rates are presented. In one case, changes in heterogeneity structure lead to variations in spatial distribution of anorthite dissolution rate in a meter-scale simulation. In areas where fluid flow is fast, reaction products are removed and anorthite dissolution proceeds at far from equilibrium rates. In contrast, in parts of the domain where fluid flow is slow, concentrations of reaction products increase and the fluid approaches chemical equilibrium with respect to anorthite and the dissolution rate slows. The heterogeneity structure and spatial distribution of reaction rates leads to differences in calculated effective reaction rates over the entire domain. In a second case, simulation of biologic reduction of nitrogen at floodplain scale, physical heterogeneity leads to spatial distribution of reaction rates that are up to 5 orders of magnitude lower than the far from equilibrium rate. It is clear from these simulations that heterogeneity in physical structure can impart spatially variable distributions of geochemical reaction rates. More work is still needed to understand how this variation translates to domain averaged reaction rates and whether physical heterogeneity contributes to some of the long-observed differences between laboratory measured and field calculated geochemical reaction rates.

Electro-Chemical Impedance Spectral (EIS) Study of Patinated Bronze Corrosion in Sulfate Media: Experimental Design Approach

The aim of the present investigation was to model the experimental conditions of tin bronze patination using full factorial experimental design. In this sense, a full factorial design approach was developed to model the corrosion behavior of patinated tin bronze alloy in sulfate electrolyte. Three experimental factors (the immersion time in the chloride electrolyte, the potential limit for the anodic sweep Elim, and the potential scan rate) were chosen to identify the significant factor on the patina growth process at the bronze substrate. The experimental responses were the kinetic parameters extracted from the electro-chemical spectra (EIS) for eight different experiments. An equivalent electrical circuit containing an electrolyte resistance (Re), a double layer capacitance (CPEdl), a charge transfer resistance (Rt) and Gerischer element (G), was developed to model the patinated bronze corrosion process. The electro-chemical spectra (EIS) show that the corrosion process of the patinated bronze alloy occurred from a chemical reaction is followed by an electrochemical one. Analysis of the experimental responses showed that while the scan rate is the most influent factor for the corrosion potential (Ecorr), the electrolyte resistance (Re), and the double layer capacitance CPEdl variation, the potential limit is the significant factor for charge transfer resistance Rt, reciprocal of the admittance parameter Y0 and the effective transfer rate of the chemical reaction k variation.

Thermochemical energy storage with CaO/Ca(OH)2 – Experimental investigation of the thermal capability at low vapor pressures in a lab scale reactor

The reversible reaction of calcium hydroxide (Ca(OH)2) to calcium oxide (CaO) and water vapor is well known in the context of thermochemical energy storage. Cheap material costs, a theoretically very high energy density and the potentially wide temperature range of the reaction imply that the storage system could be beneficial for many high temperature processes. For example the system could be applied to store and reutilize industrial waste heat or as an alternative storage solution in future concentrated solar power plants.In this publication the reaction is experimentally investigated in an indirectly operated fixed bed reactor at different technically relevant but so far not investigated operating conditions. This in particular means the thermal charging and discharging of the storage at low water vapor pressures under different heating and cooling loads induced by a heat transfer fluid. The experiments revealed that the reaction gas handling not only affects the operating range of the storage but has also a significant influence on its thermal capability. Especially at low vapor pressures operational limits of the system have been identified and could be contributed to the effective reaction rate of the reaction material which is in the relevant operating range very sensitive to small changes of the local reaction conditions.

Rovibrationally Resolved Time-Dependent Collisional-Radiative Model of Molecular Hydrogen and Its Application to a Fusion Detached Plasma

A novel rovibrationally resolved collisional-radiative model of molecular hydrogen that includes 4,133 rovibrational levels for electronic states whose united atom principal quantum number is below six is developed. The rovibrational X 1 Σ g + population distribution in a SlimCS fusion demo detached divertor plasma is investigated by solving the model time dependently with an initial 300 K Boltzmann distribution. The effective reaction rate coefficients of molecular assisted recombination and of other processes in which atomic hydrogen is produced are calculated using the obtained time-dependent population distribution.

Effect of Rabbit Anti-human Thymocyte Immunoglobulin Combined with Cyclosporin A on Severe Aplastic Anemia

To study the efficacy and safety of anti-human thymocyte immunoglobulin(ATG-F) combined with cyclosporin A(CsA) on patients with severe aplastic anemia (SSA), so as to provide support for clinical work. From January 2010 to December 2015, 78 patients with SAA admitted in our hospital were divided into 2 groups: ATG-F+CsA group(40 cases) and ATG-F group(38 cases). After treatment for 6 months, the effective rate, side reaction rate and time of effect initiation were compared between 2 groups. The follow-up results were compared between 2 groups. The effective rate and side reaction rate in ATG-F+CsA group were 100.00% and 32.50% respectively, those in ATG-F group were 94.74% and 44.74% respectively and without statistical significant difference(P>0.05). In ATG-F+CsA group, the time of effect initiation in cured patients, remission and obvious inprovement were (44.9±15.4) d, (68.8±15.9) d and (85.4±17.6) d; in ATG-F group, patients with those were (59.6±11.5) d, (94.7±17.8) d and (119.8±21.4) d respectively, the difference showed statistical significance(P<0.05). The follow-up results were not statistically significantly different between 2 groups(P>0.05). ATG-F combined with CsA can shorten the time of effect initiation, and demonstrates reliable safety.

The slowly reacting mode of combustion of gaseous mixtures in spherical vessels. Part 2: Buoyancy-induced motion and its effect on the explosion limits

This paper investigates the effect of buoyancy-driven motion on the quasi-steady ‘slowly reacting’ mode of combustion and on its thermal-explosion limits, for gaseous mixtures enclosed in a spherical vessel with a constant wall temperature. Following Frank-Kamenetskii's seminal analysis of this problem, the strong temperature dependence of the effective overall reaction rate is taken into account by using a single-reaction model with an Arrhenius rate having a large activation energy, resulting in a critical value of the vessel radius above which the slowly reacting mode of combustion no longer exists. In his contant-density, convection-free analysis, the critical conditions were found to depend on the value of a Damköhler number, defined as the ratio of the time for the heat released by the reaction to be conducted to the wall, to the homogeneous explosion time evaluated at the wall temperature. For gaseous mixtures under normal gravity, the critical Damköhler number increases through the effect of buoyancy-induced motion on the rate of heat conduction to the wall, measured by an appropriate Rayleigh number . In the present analysis, for small values of , the temperature is given in the first approximation by the spherically symmetric Frank-Kamenetskii solution, used to calculate the accompanying gas motion, an axisymmetric annular vortex determined at leading order by the balance between viscous and buoyancy forces, which we call the Frank-Kamenetskii vortex. This flow is used in the equation for conservation of energy to evaluate the influence of convection on explosion limits for small , resulting in predicted critical Damköhler numbers that are accurate up to values of on the order of a few hundred.

The Impact of Pore Structure Heterogeneity, Transport, and Reaction Conditions on Fluid–Fluid Reaction Rate Studied on Images of Pore Space

We perform direct numerical simulation using a pore-scale fluid–fluid reactive transport model (Alhashmi et al. in J Contam Hydrol 179:171–181, 2015. doi: 10.1016/j.jconhyd.2015.06.004 ) to investigate the impact of pore structure heterogeneity on the effective reaction rate in different porous media. We simulate flow, transport, and reaction in three pore-scale images: a beadpack, Bentheimer sandstone, and Doddington sandstone for a range of transport and reaction conditions. We compute the reaction rate, velocity distributions, and dispersion coefficient comparing them with the results for non-reactive transport. The rate of reaction is a subtle combination of the amount of mixing and spreading that cannot be predicted from the non-reactive dispersion coefficient. We demonstrate how the flow field heterogeneity affects the effective reaction rate. Dependent on the intrinsic flow field heterogeneity characteristic and the flow rate, reaction may: (a) occur throughout the zones where both resident and injected particles exist (for low Peclet number and a homogeneous flow field), (b) preferentially occur at the trailing edge of the plume (for high Peclet number and a homogeneous flow field), or (c) be disfavored in slow-moving zones (for high Peclet number and a heterogeneous flow field).