The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Carl I Steefel - One of the best experts on this subject based on the ideXlab platform.
-
Microbially mediated kinetic sulfur isotope fractionation: Reactive Transport modeling benchmark
Computational Geosciences, 2020Co-Authors: Yiwei Cheng, Sevinc S şengor, Bhavna Arora, Christoph Wanner, Jennifer L. Druhan, Boris M. Breukelen, Carl I SteefelAbstract:Microbially mediated sulfate reduction is a ubiquitous process in many subsurface systems. Isotopic fractionation is characteristic of this anaerobic process, since sulfate-reducing bacteria (SRB) favor the reduction of the lighter sulfate isotopologue (S^32O_4^2−) over the heavier isotopologue (S^34O_4^2−). Detection of isotopic shifts has been utilized as a proxy for the onset of sulfate reduction in subsurface systems such as oil reservoirs and aquifers undergoing heavy metal and radionuclide bioremediation. Reactive Transport modeling (RTM) of kinetic sulfur isotope fractionation has been applied to field and laboratory studies. We developed a benchmark problem set for the simulation of kinetic sulfur isotope fractionation during microbially mediated sulfate reduction. The benchmark problem set is comprised of three problem levels and is based on a large-scale laboratory column experimental study of organic carbon amended sulfate reduction in soils from a uranium-contaminated aquifer. Pertinent processes impacting sulfur isotopic composition such as microbial sulfate reduction and iron-sulfide reactions are included in the problem set. This benchmark also explores the different mathematical formulations in the representation of kinetic sulfur isotope fractionation as employed in the different RTMs. Participating RTM codes are the following: CrunchTope, TOUGHREACT, PHREEQC, and PHT3D. Across all problem levels, simulation results from all RTMs demonstrate reasonable agreement.
-
a Reactive Transport benchmark on heavy metal cycling in lake sediments
Computational Geosciences, 2015Co-Authors: Bhavna Arora, Sevinc S şengor, Nicolas Spycher, Carl I SteefelAbstract:Sediments are active recipients of anthropogenic inputs, including heavy metals, but may be difficult to interpret without the use of numerical models that capture sediment-metal interactions and provide an accurate representation of the intricately coupled sedimentological, geochemical, and biological processes. The focus of this study is to present a benchmark problem on heavy metal cycling in lake sediments and to compare Reactive Transport models (RTMs) in their treatment of the local-scale physical and biogeochemical processes. This benchmark problem has been developed based on a previously published Reactive-diffusive model of metal Transport in the sediments of Lake Coeur d’Alene, Idaho. Key processes included in this model are microbial reductive dissolution of iron hydroxides (i.e., ferrihydrite), the release of sorbed metals into pore water, reaction of these metals with biogenic sulfide to form sulfide minerals, and sedimentation driving the burial of ferrihydrite and other minerals. This benchmark thus considers a multicomponent biotic reaction network with multiple terminal electron acceptors (TEAs), Fickian diffusive Transport, kinetic and equilibrium mineral precipitation and dissolution, aqueous and surface complexation, as well as (optionally) sedimentation. To test the accuracy of the Reactive Transport problem solution, four RTMs—TOUGHREACT (TR), CrunchFlow (CF), PHREEQC, and PHT3D—have been used. Without sedimentation, all four models are able to predict similar trends of TEAs and dissolved metal concentrations, as well as mineral abundances. TR and CF are further used to compare sedimentation and compaction test cases. Results with different sedimentation rates are captured by both models, but since the codes do not use the same formulation for compaction, the results differ for this test case. Although, both TR and CF adequately capture the trends of aqueous concentrations and mineral abundances, the difference in results highlights the need to consider further the conceptual and numerical models that link Transport, biogeochemical reactions, and sedimentation.
-
Reactive Transport codes for subsurface environmental simulation
Computational Geosciences, 2015Co-Authors: Carl I Steefel, K.u. Mayer, C. A. J. Appelo, B. Arora, D. Jacques, T. Kalbacher, O. Kolditz, V. Lagneau, P. C. Lichtner, J. C. L. MeeussenAbstract:A general description of the mathematical and numerical formulations used in modern numerical Reactive Transport codes relevant for subsurface environmental simulations is presented. The formulations are followed by short descriptions of commonly used and available subsurface simulators that consider continuum representations of flow, Transport, and reactions in porous media. These formulations are applicable to most of the subsurface environmental benchmark problems included in this special issue. The list of codes described briefly here includes PHREEQC, HPx, PHT3D, OpenGeoSys (OGS), HYTEC, ORCHESTRA, TOUGHREACT, eSTOMP, HYDROGEOCHEM, CrunchFlow, MIN3P, and PFLOTRAN. The descriptions include a high-level list of capabilities for each of the codes, along with a selective list of applications that highlight their capabilities and historical development.
-
a large column analog experiment of stable isotope variations during Reactive Transport i a comprehensive model of sulfur cycling and δ34s fractionation
Geochimica et Cosmochimica Acta, 2014Co-Authors: Jennifer L. Druhan, Carl I Steefel, Donald J. Depaolo, Mark E ConradAbstract:Abstract This study demonstrates a mechanistic incorporation of the stable isotopes of sulfur within the CrunchFlow Reactive Transport code to model the range of microbially-mediated redox processes affecting kinetic isotope fractionation. Previous numerical models of microbially mediated sulfate reduction using Monod-type rate expressions have lacked rigorous coupling of individual sulfur isotopologue rates, with the result that they cannot accurately simulate sulfur isotope fractionation over a wide range of substrate concentrations using a constant fractionation factor. Here, we derive a modified version of the dual-Monod or Michaelis–Menten formulation ( Maggi and Riley, 2009 , Maggi and Riley, 2010 ) that successfully captures the behavior of the 32S and 34S isotopes over a broad range from high sulfate and organic carbon availability to substrate limitation using a constant fractionation factor. The new model developments are used to simulate a large-scale column study designed to replicate field scale conditions of an organic carbon (acetate) amended biostimulation experiment at the Old Rifle site in western Colorado. Results demonstrate an initial period of iron reduction that transitions to sulfate reduction, in agreement with field-scale behavior observed at the Old Rifle site. At the height of sulfate reduction, effluent sulfate concentrations decreased to 0.5 mM from an influent value of 8.8 mM over the 100 cm flow path, and thus were enriched in sulfate δ34S from 6.3‰ to 39.5‰. The Reactive Transport model accurately reproduced the measured enrichment in δ34S of both the reactant (sulfate) and product (sulfide) species of the reduction reaction using a single fractionation factor of 0.987 obtained independently from field-scale measurements. The model also accurately simulated the accumulation and δ34S signature of solid phase elemental sulfur over the duration of the experiment, providing a new tool to predict the isotopic signatures associated with reduced mineral pools. To our knowledge, this is the first rigorous treatment of sulfur isotope fractionation subject to Monod kinetics in a mechanistic Reactive Transport model that considers the isotopic spatial distribution of both dissolved and solid phase sulfur species during microbially-mediated sulfate reduction.
-
Fluid-rock interaction: A Reactive Transport approach
Reviews in Mineralogy and Geochemistry, 2009Co-Authors: Carl I Steefel, Kate MaherAbstract:Fluid-Rock Interaction: A Reactive Transport Approach Carl I. Steefel and 2 Kate Maher Earth Sciences Division Lawrence Berkeley National Laboratory Berkeley, California 94720, USA Dept. of Geological & Environmental Sciences School of Earth Sciences Stanford University Stanford, California 94305, USA INTRODUCTION Fluid-rock interaction (or water-rock interaction, as it was more commonly known) is a subject that has evolved considerably in its scope over the years. Initially its focus was primarily on interactions between subsurface fluids of various temperatures and mostly crystalline rocks, but the scope has broadened now to include fluid interaction with all forms of subsurface materials, whether they are unconsolidated or crystalline (“fluid-solid interaction” is perhaps less euphonious). Disciplines that previously carried their own distinct names, for example, basin diagenesis, early diagenesis, metamorphic petrology, Reactive contaminant Transport, chemical weathering, are now considered to fall under the broader rubric of fluid-rock interaction, although certainly some of the key research questions differ depending on the environment considered. Beyond the broadening of the environments considered in the study of fluid-rock interaction, the discipline has evolved in perhaps an even more important way. The study of water-rock interaction began by focusing on geochemical interactions in the absence of Transport processes, although a few notable exceptions exist (Thompson 1959; Weare et al. 1976). Moreover, these analyses began by adopting a primarily thermodynamic approach, with the implicit or explicit assumption of equilibrium between the fluid and rock. As a result, these early models were fundamentally static rather than dynamic in nature. This all changed with the seminal papers by Helgeson and his co-workers (Helgeson 1968; Helgeson et al. 1969) wherein the concept of an irreversible reaction path was formally introduced into the geochemical literature. In addition to treating the reaction network as a dynamically evolving system, the Helgeson studies introduced an approach that allowed for the consideration of a multicomponent geochemical system, with multiple minerals and species appearing as both reactants and products, at least one of which could be irreversible. Helgeson’s pioneering approach was given a more formal kinetic basis (including the introduction of real time rather than reaction progress as the independent variable) in subsequent studies (Lasaga 1981; Aagaard and Helgeson 1982; Lasaga 1984) . The reaction path approach can be used to describe chemical processes in a batch or closed system (e.g., a
Donald J. Depaolo - One of the best experts on this subject based on the ideXlab platform.
-
a large column analog experiment of stable isotope variations during Reactive Transport i a comprehensive model of sulfur cycling and δ34s fractionation
Geochimica et Cosmochimica Acta, 2014Co-Authors: Jennifer L. Druhan, Carl I Steefel, Donald J. Depaolo, Mark E ConradAbstract:Abstract This study demonstrates a mechanistic incorporation of the stable isotopes of sulfur within the CrunchFlow Reactive Transport code to model the range of microbially-mediated redox processes affecting kinetic isotope fractionation. Previous numerical models of microbially mediated sulfate reduction using Monod-type rate expressions have lacked rigorous coupling of individual sulfur isotopologue rates, with the result that they cannot accurately simulate sulfur isotope fractionation over a wide range of substrate concentrations using a constant fractionation factor. Here, we derive a modified version of the dual-Monod or Michaelis–Menten formulation ( Maggi and Riley, 2009 , Maggi and Riley, 2010 ) that successfully captures the behavior of the 32S and 34S isotopes over a broad range from high sulfate and organic carbon availability to substrate limitation using a constant fractionation factor. The new model developments are used to simulate a large-scale column study designed to replicate field scale conditions of an organic carbon (acetate) amended biostimulation experiment at the Old Rifle site in western Colorado. Results demonstrate an initial period of iron reduction that transitions to sulfate reduction, in agreement with field-scale behavior observed at the Old Rifle site. At the height of sulfate reduction, effluent sulfate concentrations decreased to 0.5 mM from an influent value of 8.8 mM over the 100 cm flow path, and thus were enriched in sulfate δ34S from 6.3‰ to 39.5‰. The Reactive Transport model accurately reproduced the measured enrichment in δ34S of both the reactant (sulfate) and product (sulfide) species of the reduction reaction using a single fractionation factor of 0.987 obtained independently from field-scale measurements. The model also accurately simulated the accumulation and δ34S signature of solid phase elemental sulfur over the duration of the experiment, providing a new tool to predict the isotopic signatures associated with reduced mineral pools. To our knowledge, this is the first rigorous treatment of sulfur isotope fractionation subject to Monod kinetics in a mechanistic Reactive Transport model that considers the isotopic spatial distribution of both dissolved and solid phase sulfur species during microbially-mediated sulfate reduction.
-
the mineral dissolution rate conundrum insights from Reactive Transport modeling of u isotopes and pore fluid chemistry in marine sediments
Geochimica et Cosmochimica Acta, 2006Co-Authors: Kate Maher, Carl I Steefel, Donald J. Depaolo, B E VianiAbstract:Abstract Pore water chemistry and 234U/238U activity ratios from fine-grained sediment cored by the Ocean Drilling Project at Site 984 in the North Atlantic were used as constraints in modeling in situ rates of plagioclase dissolution with the multicomponent Reactive Transport code Crunch. The Reactive Transport model includes a solid-solution formulation to enable the use of the 234U/238U activity ratios in the solid and fluid as a tracer of mineral dissolution. The isotopic profiles are combined with profiles of the major element chemistry (especially alkalinity and calcium) to determine whether the apparent discrepancy between laboratory and field dissolution rates still exists when a mechanistic Reactive Transport model is used to interpret rates in a natural system. A suite of reactions, including sulfate reduction and methane production, anaerobic methane oxidation, CaCO3 precipitation, dissolution of plagioclase, and precipitation of secondary clay minerals, along with diffusive Transport and fluid and solid burial, control the pore fluid chemistry in Site 984 sediments. The surface area of plagioclase in intimate contact with the pore fluid is estimated to be 6.9 m2/g based on both grain geometry and on the depletion of 234U/238U in the sediment via α-recoil loss. Various rate laws for plagioclase dissolution are considered in the modeling, including those based on (1) a linear transition state theory (TST) model, (2) a nonlinear dependence on the undersaturation of the pore water with respect to plagioclase, and (3) the effect of inhibition by dissolved aluminum. The major element and isotopic methods predict similar dissolution rate constants if additional lowering of the pore water 234U/238U activity ratio is attributed to isotopic exchange via recrystallization of marine calcite, which makes up about 10–20% of the Site 984 sediment. The calculated dissolution rate for plagioclase corresponds to a rate constant that is about 102 to 105 times smaller than the laboratory-measured value, with the value depending primarily on the deviation from equilibrium. The Reactive Transport simulations demonstrate that the degree of undersaturation of the pore fluid with respect to plagioclase depends strongly on the rate of authigenic clay precipitation and the solubility of the clay minerals. The observed discrepancy is greatest for the linear TST model (105), less substantial with the Al-inhibition formulation (103), and decreases further if the clay minerals precipitate more slowly or as highly soluble precursor minerals (102). However, even several orders of magnitude variation in either the clay solubility or clay precipitation rates cannot completely account for the entire discrepancy while still matching pore water aluminum and silica data, indicating that the mineral dissolution rate conundrum must be attributed in large part to the gradual loss of Reactive sites on silicate surfaces with time. The results imply that methods of mineral surface characterization that provide direct measurements of the bulk surface reactivity are necessary to accurately predict natural dissolution rates.
-
Reactive Transport modeling an essential tool and a new research approach for the earth sciences
Earth and Planetary Science Letters, 2005Co-Authors: Carl I Steefel, Donald J. Depaolo, Peter C. LichtnerAbstract:Abstract Reactive Transport modeling is an essential tool for the analysis of coupled physical, chemical, and biological processes in Earth systems, and has additional potential to better integrate the results from focused fundamental research on Earth materials. Appropriately designed models can describe the interactions of competing processes at a range of spatial and time scales, and hence are critical for connecting the advancing capabilities for materials characterization at the atomic scale with the macroscopic behavior of complex Earth systems. Reactive Transport modeling has had a significant impact on the treatment of contaminant retardation in the subsurface, the description of elemental and nutrient fluxes between major Earth reservoirs, and in the treatment of deep Earth processes such as metamorphism and magma Transport. Active topics of research include the development of pore scale and hybrid, or multiple continua, models to capture the scale dependence of coupled Reactive Transport processes. Frontier research questions, that are only now being addressed, include the effects of chemical microenvironments, coupled thermal–mechanical–chemical processes, controls on mineral–fluid reaction rates in natural media, and scaling of Reactive Transport processes from the microscopic to pore to field scale.
Li Chen - One of the best experts on this subject based on the ideXlab platform.
-
pore scale study of pore ionomer interfacial Reactive Transport processes in proton exchange membrane fuel cell catalyst layer
Chemical Engineering Journal, 2020Co-Authors: Li Chen, Qinjun Kang, Ruiyuan Zhang, Wen-quan TaoAbstract:Abstract Understanding interactions between constituent distributions and Reactive Transport processes in catalyst layer (CL) of proton exchange membrane fuel cell is crucial for improving cell performance and reducing cell cost. In this study, high-resolution porous structures of cathode CL are reconstructed, where all the constituents in CLs are resolved. A pore-scale model based on the lattice Boltzmann method is developed for oxygen diffusion in pores and ionomer, as well as electrochemical reaction at the Pt surfaces. Particularly the model considers the pore-ionomer interfacial Transport processes with distinct characteristics of sharp concentration drop, large diffusivity ratio and interfacial dissolution reaction. After validated by interfacial Transport processes with analytical solutions, the pore-scale model is applied to Reactive Transport processes inside complex CL nanoscale structures. Pore-scale results reveal that pore-ionomer interfacial Transport processes generate extremely high local Transport resistance, significantly reducing the total reaction rate. As volume fraction of carbon increases, the value of the optimum ionomer content generating the best cell performance decreases, while the value of the optimum ionomer content resulting in the lowest performance loss under low Pt loading reduces. The two values generally are different. The pore-scale model helps to understand Reactive Transport processes and to optimize the CL structures.
-
pore scale study of Reactive Transport processes in catalyst layer agglomerates of proton exchange membrane fuel cells
Electrochimica Acta, 2019Co-Authors: Li Chen, Qinjun Kang, Wen-quan TaoAbstract:Abstract Porous structures of agglomerates in cathode catalyst layers (CLs) of proton exchange membrane fuel cells are reconstructed, in which all the four phases are resolved including Platinum, carbon, ionomer and pore. A pore-scale Reactive Transport model based on the lattice Boltzmann method is developed, in which oxygen dissolution reaction at pore-ionomer interface, oxygen diffusion inside ionomer, and electrochemical reaction at ionomer-Pt interface are considered. Emphasis is put on structural parameters, especially Pt/C mass ratio, on the Reactive Transport process and the volumetric reaction rate (or current density). Pore-scale results show that while under high Pt loading oxygen is depleted quite close to the surface of the spherical agglomerate, it has to penetrate deep into the porous agglomerate before it is completely consumed under low Pt loading which is not captured by classical agglomerate model based on homogeneous mixture assumption. Pore-scale results also found that effects of Transport inside the agglomerate decreases as reaction rate, porosity or ionomer thickness increases. Finally, local Transport resistance inside the agglomerate is evaluated, and it increases as the agglomerate size increases or the dissolution reaction rate decreases.
-
pore scale study of multiphase Reactive Transport in fibrous electrodes of vanadium redox flow batteries
Electrochimica Acta, 2017Co-Authors: Li Chen, Wen-quan Tao, Piotr Zelenay, Rangachary Mukundan, Qinjun KangAbstract:Abstract The electrode of a vanadium redox flow battery generally is a carbon fibre-based porous medium, in which important physicochemical processes occur. In this work, pore-scale simulations are performed to study complex multiphase flow and Reactive Transport in the electrode by using the lattice Boltzmann method (LBM). Four hundred fibrous electrodes with different fibre diameters and porosities are reconstructed. Both the permeability and diffusivity of the reconstructed electrodes are predicted and compared with empirical relationships in the literature. Reactive surface area of the electrodes is also evaluated and it is found that existing empirical relationship overestimates the Reactive surface under lower porosities. Further, a pore-scale electrochemical reaction model is developed to study the effects of fibre diameter and porosity on electrolyte flow, VII/VIII Transport, and electrochemical reaction at the electrolyte-fibre surface. Finally, evolution of bubble cluster generated by the side reaction is studied by adopting a LB multiphase flow model. Effects of porosity, fibre diameter, gas saturation and solid surface wettability on average bubble diameter and reduction of Reactive surface area due to coverage of bubbles on solid surface are investigated in detail. It is found that gas coverage ratio is always lower than that adopted in the continuum model in the literature. The current pore-scale studies successfully reveal the complex multiphase flow and Reactive Transport processes in the electrode, and the simulation results can be further upscaled to improve the accuracy of the current continuum-scale models.
-
pore scale simulation of multicomponent multiphase Reactive Transport with dissolution and precipitation
International Journal of Heat and Mass Transfer, 2015Co-Authors: Qinjun Kang, Li Chen, Bruce A Robinson, Qing Tang, Wen-quan TaoAbstract:Abstract Multicomponent multiphase Reactive Transport processes with dissolution–precipitation are widely encountered in energy and environment systems. A pore-scale two-phase multi-mixture model based on the lattice Boltzmann method (LBM) is developed for such complex Transport processes, where each phase is considered as a mixture of miscible components in it. The liquid–gas fluid flow with large density ratio is simulated using the multicomponent multiphase pseudo-potential LB model; the Transport of certain solute in the corresponding solvent is solved using the mass Transport LB model; and the dynamic evolutions of the liquid–solid interface due to dissolution–precipitation are captured by an interface tracking scheme. The model developed can predict coupled multiple physicochemical processes including multiphase flow, multicomponent mass Transport, homogeneous reactions in the bulk fluid and heterogeneous dissolution–precipitation reactions at the fluid–solid interface, and dynamic evolution of the solid matrix geometries at the pore-scale. The model is then applied to a physicochemical system encountered in shale gas/oil industry involving multiphase flow, multicomponent Reactive Transport and dissolution–precipitation, with several reactions whose rates can be several orders of magnitude different at a given temperature. The pore-scale phenomena and complex interaction between different sub-processes are investigated and discussed in detail.
-
pore scale modeling of multiphase Reactive Transport with phase transitions and dissolution precipitation processes in closed systems
Physical Review E, 2013Co-Authors: Qinjun Kang, Li Chen, Bruce A Robinson, Wen-quan TaoAbstract:A pore-scale model based on the lattice Boltzmann (LB) method is developed for multiphase Reactive Transport with phase transitions and dissolution-precipitation processes. The model combines the single-component multiphase Shan-Chen LB model [X. Shan and H. Chen, Phys. Rev. E 47, 1815 (1993)], the mass Transport LB model [S. P. Sullivan et al., Chem. Eng. Sci. 60, 3405 (2005)], and the dissolution-precipitation model [Q. Kang et al., J. Geophys. Res. 111, B05203 (2006)]. Care is taken to handle information on computational nodes undergoing solid-liquid or liquid-vapor phase changes to guarantee mass and momentum conservation. A general LB concentration boundary condition is proposed that can handle various concentration boundaries including Reactive and moving boundaries with complex geometries. The pore-scale model can capture coupled nonlinear multiple physicochemical processes including multiphase flow with phase separations, mass Transport, chemical reactions, dissolution-precipitation processes, and dynamic evolution of the pore geometries. The model is validated using several multiphase flow and Reactive Transport problems and then used to study the thermal migration of a brine inclusion in a salt crystal. Multiphase Reactive Transport phenomena with phase transitions between liquid-vapor phases and dissolution-precipitation processes of the salt in the closed inclusion are simulated and the effects of the initial inclusion size and temperature gradient on the thermal migration are investigated.
C T Kelley - One of the best experts on this subject based on the ideXlab platform.
-
convergence of iterative split operator approaches for approximating nonlinear Reactive Transport problems
Advances in Water Resources, 2003Co-Authors: Joseph F Kanney, Cass T Miller, C T KelleyAbstract:Numerical solutions to nonlinear Reactive solute Transport problems are often computed using split-operator (SO) approaches, which separate the Transport and reaction processes. This uncoupling introduces an additional source of numerical error, known as the splitting error. The iterative split-operator (ISO) algorithm removes the splitting error through iteration. Although the ISO algorithm is often used, there has been very little analysis of its convergence behavior. This work uses theoretical analysis and numerical experiments to investigate the convergence rate of the iterative split-operator approach for solving nonlinear Reactive Transport problems.
Qinjun Kang - One of the best experts on this subject based on the ideXlab platform.
-
pore scale study of pore ionomer interfacial Reactive Transport processes in proton exchange membrane fuel cell catalyst layer
Chemical Engineering Journal, 2020Co-Authors: Li Chen, Qinjun Kang, Ruiyuan Zhang, Wen-quan TaoAbstract:Abstract Understanding interactions between constituent distributions and Reactive Transport processes in catalyst layer (CL) of proton exchange membrane fuel cell is crucial for improving cell performance and reducing cell cost. In this study, high-resolution porous structures of cathode CL are reconstructed, where all the constituents in CLs are resolved. A pore-scale model based on the lattice Boltzmann method is developed for oxygen diffusion in pores and ionomer, as well as electrochemical reaction at the Pt surfaces. Particularly the model considers the pore-ionomer interfacial Transport processes with distinct characteristics of sharp concentration drop, large diffusivity ratio and interfacial dissolution reaction. After validated by interfacial Transport processes with analytical solutions, the pore-scale model is applied to Reactive Transport processes inside complex CL nanoscale structures. Pore-scale results reveal that pore-ionomer interfacial Transport processes generate extremely high local Transport resistance, significantly reducing the total reaction rate. As volume fraction of carbon increases, the value of the optimum ionomer content generating the best cell performance decreases, while the value of the optimum ionomer content resulting in the lowest performance loss under low Pt loading reduces. The two values generally are different. The pore-scale model helps to understand Reactive Transport processes and to optimize the CL structures.
-
pore scale study of Reactive Transport processes in catalyst layer agglomerates of proton exchange membrane fuel cells
Electrochimica Acta, 2019Co-Authors: Li Chen, Qinjun Kang, Wen-quan TaoAbstract:Abstract Porous structures of agglomerates in cathode catalyst layers (CLs) of proton exchange membrane fuel cells are reconstructed, in which all the four phases are resolved including Platinum, carbon, ionomer and pore. A pore-scale Reactive Transport model based on the lattice Boltzmann method is developed, in which oxygen dissolution reaction at pore-ionomer interface, oxygen diffusion inside ionomer, and electrochemical reaction at ionomer-Pt interface are considered. Emphasis is put on structural parameters, especially Pt/C mass ratio, on the Reactive Transport process and the volumetric reaction rate (or current density). Pore-scale results show that while under high Pt loading oxygen is depleted quite close to the surface of the spherical agglomerate, it has to penetrate deep into the porous agglomerate before it is completely consumed under low Pt loading which is not captured by classical agglomerate model based on homogeneous mixture assumption. Pore-scale results also found that effects of Transport inside the agglomerate decreases as reaction rate, porosity or ionomer thickness increases. Finally, local Transport resistance inside the agglomerate is evaluated, and it increases as the agglomerate size increases or the dissolution reaction rate decreases.
-
pore scale study of multiphase Reactive Transport in fibrous electrodes of vanadium redox flow batteries
Electrochimica Acta, 2017Co-Authors: Li Chen, Wen-quan Tao, Piotr Zelenay, Rangachary Mukundan, Qinjun KangAbstract:Abstract The electrode of a vanadium redox flow battery generally is a carbon fibre-based porous medium, in which important physicochemical processes occur. In this work, pore-scale simulations are performed to study complex multiphase flow and Reactive Transport in the electrode by using the lattice Boltzmann method (LBM). Four hundred fibrous electrodes with different fibre diameters and porosities are reconstructed. Both the permeability and diffusivity of the reconstructed electrodes are predicted and compared with empirical relationships in the literature. Reactive surface area of the electrodes is also evaluated and it is found that existing empirical relationship overestimates the Reactive surface under lower porosities. Further, a pore-scale electrochemical reaction model is developed to study the effects of fibre diameter and porosity on electrolyte flow, VII/VIII Transport, and electrochemical reaction at the electrolyte-fibre surface. Finally, evolution of bubble cluster generated by the side reaction is studied by adopting a LB multiphase flow model. Effects of porosity, fibre diameter, gas saturation and solid surface wettability on average bubble diameter and reduction of Reactive surface area due to coverage of bubbles on solid surface are investigated in detail. It is found that gas coverage ratio is always lower than that adopted in the continuum model in the literature. The current pore-scale studies successfully reveal the complex multiphase flow and Reactive Transport processes in the electrode, and the simulation results can be further upscaled to improve the accuracy of the current continuum-scale models.
-
pore scale simulation of multicomponent multiphase Reactive Transport with dissolution and precipitation
International Journal of Heat and Mass Transfer, 2015Co-Authors: Qinjun Kang, Li Chen, Bruce A Robinson, Qing Tang, Wen-quan TaoAbstract:Abstract Multicomponent multiphase Reactive Transport processes with dissolution–precipitation are widely encountered in energy and environment systems. A pore-scale two-phase multi-mixture model based on the lattice Boltzmann method (LBM) is developed for such complex Transport processes, where each phase is considered as a mixture of miscible components in it. The liquid–gas fluid flow with large density ratio is simulated using the multicomponent multiphase pseudo-potential LB model; the Transport of certain solute in the corresponding solvent is solved using the mass Transport LB model; and the dynamic evolutions of the liquid–solid interface due to dissolution–precipitation are captured by an interface tracking scheme. The model developed can predict coupled multiple physicochemical processes including multiphase flow, multicomponent mass Transport, homogeneous reactions in the bulk fluid and heterogeneous dissolution–precipitation reactions at the fluid–solid interface, and dynamic evolution of the solid matrix geometries at the pore-scale. The model is then applied to a physicochemical system encountered in shale gas/oil industry involving multiphase flow, multicomponent Reactive Transport and dissolution–precipitation, with several reactions whose rates can be several orders of magnitude different at a given temperature. The pore-scale phenomena and complex interaction between different sub-processes are investigated and discussed in detail.
-
lattice boltzmann based approaches for pore scale Reactive Transport
Reviews in Mineralogy & Geochemistry, 2015Co-Authors: Hongkyu Yoon, Qinjun Kang, Albert J ValocchiAbstract:Important geoscience and environmental applications such as geologic carbon storage, environmental remediation, and unconventional oil and gas recovery are best understood in the context of Reactive flow and multicomponent Transport in the subsurface environment. The coupling of chemical and microbiological reactions with hydrological and mechanical processes can lead to complex behaviors across an enormous range of spatial and temporal scales. These coupled responses are also strongly influenced by the heterogeneity and anisotropy of the geologic formations. Reactive Transport processes can change the pore morphology at the pore scale, thereby leading to nonlinear interactions with advective and diffusive Transport, which can strongly influence larger-scale properties such as permeability and dispersion. Therefore, one of the greatest research challenges is to improve our ability to predict these processes across scales (DOE 2007). The development of pore-scale experimental and modeling methods to study Reactive processes involving mineral precipitation and dissolution, and biofilm dynamics allows more fundamental investigation of physical behavior so that more accurate and robust upscaled constitutive models can be developed for the continuum scale. A pore-scale model provides fundamental mechanistic explanations of how biogeochemical processes and pore-scale interfacial reactions alter flow paths by pore plugging (and dissolving) under different geochemical compositions and pore configurations. For example, dissolved CO2 during geological CO2 storage may react with minerals in fractured rocks, confined aquifers, or faults, resulting in cementation (and/or dissolution) and altering hydrodynamics of Reactive flow. This can be observed in a natural analogue where primary porosity in sandstone is cemented by carbonate precipitates, affecting dissolved CO2 flow paths at the Little Garde Wash Fault, Utah (e.g., Fig. 1a–b). Several other examples demonstrating macroscopic characteristics of calcium carbonate (CaCO3) precipitation in Figure 1 include an elongated concretion along the groundwater flow direction, CaCO3 precipitation along the vertical pathway sealed …
