Influence of water vapor pressure on the induction period during Li2SO4*H2O single crystals dehydration

International audienceThe dehydration of Li2SO4*H2O single crystals at 80 °C has been studied by means of both isothermal thermogravimetry at 2.6, 3.6 and 4.6 hPa of water vapor and environmental scanning electron microscope. Thermogravimetric experiments allowed the determination of induction periods. Distributions of these isoconversion induction periods for a large number of single crystals showed that increasing the water vapor pressure produced a longer induction period. Moreover, the shape of the distributions of the induction periods over a large number of single crystals changed from one mode at 2.6 hPa to two modes at 3.6 and 4.6 hPa. Using an environmental scanning electron microscope, this result could be attributed to differences in nucleation rates at edges and faces of the single crystals. Highlights ► We study nucleation process during thermal decomposition of solid. ► 2 nucleation modes are considered as nuclei appear on crystal edges and faces. ► Nucleation on the edges is faster than on the faces. ► When the water vapor pressure is increased, nucleation on faces is delayed. ► Nucleation on edges withdraws when water vapor pressure is increased

From the drawbacks of the Arrhenius-f(α) rate equation towards a more general formalism and new models for the kinetic analysis of solid-gas reactions

International audienceSince many years the kinetic models used for interpreting the kinetic curves α(t) relative to the chemical transformations of solids such as thermal decomposition, reduction, oxidation, etc., rely on very restrictive assumptions to which corresponds the following equation: dalpha/dt=Aexp(-E/RT)f(alpha) (a) where A is called the "pre-exponential term", E is the "apparent activation energy", and f(α) is a mathematical function which depends of the kinetic model. This article first presents a critical analysis of Eq. (a) by detailing the conditions in which it is rigorously correct. A more general equation is then proposed on the basis of assumptions related to the nucleation and growth processes of the new phase: dalpha/dt=Phi((T, Pi)Sm(t, ...) (b) Sm(t, ...) being a function of α only in very particular cases of instantaneous nucleation or growth, and ϕ being related to the rate-determining step and varying only with thermodynamic variables (temperature, partial pressures Pi, ...).The advantages of Eq. (b) are of two types: firstly, the variables temperature and partial pressure of gases may not be separated in the expression of ϕ (no Arrhenius dependence with temperature); secondly, in gas-solid systems, when the nucleation process takes place at the surface of the solid and along the course of the transformation (nucleation and growth processes are simultaneous), the rate cannot be expressed by means of a function of α. Moreover, it is shown that new kinetic models can be obtained considering that the rate-determining step of growth may be located at the surface of the particles, and also the direction of development of the product phase may be outwards, instead of inward as generally considered. In order to simulate kinetic curves and to compare to the experimental ones, a free access software tool has been developed: CIN3. Examples of simulation and optimization are shown, illustrating the determination of constants related to nucleation and/or growth kinetics. Highlights : Discussion of the most relevant drawbacks linked to the rate equation dα/dt = A exp (− (E/RT))f(α). * Proposal of a more general equation dα/dt = ϕ(T, Pi)Sm(t, ...), with ϕ in mol m−2 s−1 and Sm in m2 mol−1. * New growth models and surface nucleation-anisotropic or isotropic growth models.* Description of CIN3 software for simulation and fitting of isothermal kinetic data

De l'analyse critique des modèles cinétiques usuels des réactions solide-gaz vers ClN3, un logiciel de simulation et d'interprétation des expériences

National audienceDepuis de nombreuses années, les modèles analytiques utilisés pour l'interprétation des courbes cinétiques alpha(t) relatives aux transformations chimiques de solides (isotherme, isobare) (décompositions thermiques, réduction d'oxydes par un gaz, etc ...) reposent sur des hypothèses très restrictives conduisant à une équation de vitesse (1) où A est appelé le « terme pré-exponentiel », E est l'énergie d'activation apparente et f(alpha) est une fonction analytique qui dépend du modèle cinétique. Cet article présente d'abord une analyse critique de l'utilisation abusive de cette équation en précisant les conditions dans lesquelles elle est valable. Une nouvelle formulation plus générale de la vitesse est ensuite explicitée sur la base d'hypothèses relatives aux processus de germination et de croissance de la nouvelle phasedécrite par l'équation(2) E(t) pouvant être une fonction analytique de la variable 'alpha" dans des cas très particuliers de germination ou croissance instantanée, et la variable 'phi" étant une fonction des contraintes thermodynamiques (pression, température, ...). Dans le cas général où les processus de germination et croissance sont concomitants, deux familles de modèles sont envisagées selon que la croissance est isotrope ou anisotrope. En particulier, les modèles de germination-croissance sont basés sur l'apparition des germes en surface des grains, conformément à la réalité physique, contrairement aux lois d'Avrami (germination en volume, volume infini) qui peuvent conduire après approximation à des équations de vitesse de type (1). L'équation (2) offre la possibilité de calculer une quarantaine de modèles cinétiques différents, incluant ceux relatifs à l'équation (1). Afin de visualiser l'allure des courbes cinétiques calculées et de les comparer aux courbes expérimentales, un outil de simulation a été développé : CIN3 (IDDN N° FR001130014.000.SP.2009.000.30625). Des simulations basées sur des réactions étudiées au laboratoire sont présentées afin d'illustrer les potentialités du logiciel CIN3

Kinetic modeling of low temperature oxidation of copper nanoparticles by O2

International audienceThe mechanism and kinetics of copper nanoparticles oxidation at low temperature were investigated using thermogravimetry (TGA), differential scanning calorimetry (DSC), X-Ray diffraction (XRD) and transmission electron microscopy (TEM). Isothermal and isobaric studies of the oxidation reaction were carried out at various temperatures. It was found that working under an oxygen partial pressure of 1kPa in the temperature range 125 -145°C leads to reaction where nucleation of the oxide phase is in competition with its growth. The study of the dependency of the growth rate on the oxygen partial pressure under 10 kPa has shown the adsorption of oxygen at the surface of the oxide to be the rate-determining step. A mechanism and a kinetic model have been established to interpret the experimental curves

Impact of atmospheric water vapor on the thermal decomposition of calcium hydroxide: a universal kinetic approach to a physico-geometrical consecutive reaction in solid–gas systems under different partial pressures of product gas

International audienceThermal decomposition of Ca(OH)2 under atmospheric water vapor exhibits special features, including an induction period (IP) and a subsequent sigmoidal mass-loss behavior under isothermal conditions. Atmospheric water vapor reduces the reaction rate at a specific temperature and causes a systematic shift of the mass-loss curve, which was recorded at a specific heating rate, to higher temperatures as the water vapor pressure, p(H2O), increases. The challenge in this study was to universally describe the kinetics of thermal decomposition under various p(H2O) conditions by introducing an accommodation function in the fundamental kinetic equation. The accommodation function in the multiplied form of two p(H2O) components with a variable exponent in each component was derived on the basis of the classical nucleation and interface reaction theories. The universal kinetic approach was realized by applying the accommodation function to formal kinetic analyses of the Arrhenius plot for the IP and the Friedman plot for the mass-loss process. Furthermore, the overall reaction process under isothermal conditions was analyzed kinetically on the basis of the physico-geometrical consecutive reaction model, which was composed of an IP, a surface reaction (SR), and a phase boundary-controlled reaction (PBR). Subsequently, the kinetic parameters for each physico-geometrical reaction step were determined by the modified Arrhenius plot with the accommodation function. The impact of the atmospheric water vapor on the kinetics of thermal decomposition was characterized in connection with physico-geometrical reaction mechanisms through the interpretation of the kinetic parameters and these variation behavior patterns as the overall reaction advanced

CO2 adsorption on calcium oxide: An atomic-scale simulation study

International audienceWe present a detailed study of CO2 adsorption on CaO, by means of atomic-scale simulations relying on Density Functional Theory. Combining ab initio thermodynamics of the CO2 gas phase and a thorough analysis of its interaction with the oxide, we build an orientation-sensitive adsorption model, which demonstrates that low coverage by the gas is expected in a wide range of working conditions, including the domain of stability of CaCO3 calcite. Investigation of the interactions between the adsorbed molecules reinforces this conclusion. Our work thus provides a strong hint that calcite nucleation should occur by a localised mechanism, discarding the possibility of collective surface transformation

Experimental study and Monte-Carlo simulation of the nucleation and growth processes during the dehydration of Li2SO4,H2O single crystals

9 pagesInternational audienceA kinetic model for the dehydration of lithium sulfate monohydrate is proposed in order to account for experimental data obtained on single crystals by thermogravimetry at 80°C under fixed water vapour pressure, and by optical microscopy. This model is based on the assumptions of Mampel's model, the nucleation takes place randomly at the surface of the solid and is followed by isotropic growth toward the centre of the crystal. Calculated rates d/dt are obtained by means of Monte-Carlo simulations and compared to the experimental ones, which leads to the determination of two kinetic constants: the areic frequency of nucleation (in number of nuclei.m-2.s-1) and the areic reactivity of growth (in mol.m-2.s-1)

Kinetic modeling of solid-gas reactions at reactor scale: A general approach

International audienceA rigorous simulation of industrial reactors in the case of solid-gas reacting systems is a complicated task due to several difficulties linked to the kinetic problem at the scale of the solid grains and to the problem of gas and heat transfers within the powder bed. Firstly it requires the knowledge of the kinetic model for the calculation of the speed of reaction in one part of the reactor and for given conditions of temperature and gas composition, and secondly it necessitates solving the material and heat balance equations for the thermohydraulic conditions settled in the reactor

Kinetic modeling of solid-gas reactions at reactor scale: A general approach

International audienceUnderstanding the industrial reactors behavior is a difficult task in the case of solid state reactions such as solid-gas reactions. Indeed the solid phase is a granular medium through which circulate gaseous reactants and products. The properties of such a medium are modified in space and time due to reactions occurring at a microscopic scale. The thermodynamic conditions are driven not only by the operating conditions but also by the heat and mass transfers in the reactor. We propose to numerically resolve the thermohydraulic equations combined with kinetic laws which describe the heterogeneous reactions. The major advantage of this approach is due to the large variety of kinetic models of grains transformation (~40) compared to the usual approach, especially in the case of surface nucleation and growth processes which need to quantitatively describe the grain conversion kinetics at a microscopic scale due to nucleation frequency and growth rate laws obtained in separate isothermal and isobaric experiments. The heat and mass transfers terms entering in the balance equations at a macroscopic scale depend on the kinetics evaluated at the microscopic scale. These equations give the temperature and partial pressure in the reactor, which in turn influence the microscopic kinetic behavior

CIN4: a software tool for simulation of heterogeneous reactions at a reactor scale based on a micro-meso-macro coupling

National audienceUnderstanding the industrial reactors behavior is a difficult task in the case of solid state reactions such as solid-gas reactions. Indeed the solid phase is a granular medium through which circulate gaseous reactants and products. The properties of such a medium are modified in space and time due to reactions occurring at a microscopic scale. The thermodynamic conditions are driven not only by the operating conditions but also by the heat and mass transfers in the reactor. CIN4, a multiphysic software resulting from the collaboration between ASTEK and EMSE, offers the resolution of the thermohydraulic equations combined with kinetic laws which describe the heterogeneous reactions. The heat and mass transfers terms entering in the balance equations at a macroscopic scale depend on the kinetics evaluated at the microscopic scale. These equations give the temperature and partial pressure in the reactor, which in turn influence the microscopic kinetic behavior