État stationnaire (économie)

thumb|350px|Les ressources naturelles traversent l'économie et finissent comme des déchets et de la pollution. Une économie stationnaire ou état stationnaire est une économie dont le stock de capital physique et la taille de la population sont constants et qui ne croît pas avec le temps. Normalement, ce terme fait référence à l'économie nationale d'un pays donné, mais il peut également s'appliquer au système économique d'une ville, d'une région ou du monde entier. Au , l'économiste classique Adam Smith a développé le concept d'état stationnaire d'une économie. Il pensait que toute économie nationale finirait par se stabiliser dans un état stationnaire final. Depuis les années 1970, le concept d'économie stationnaire est principalement associé aux travaux de l'économiste écologique Herman Daly. Le concept d'état stable de Daly implique l'analyse écologique des flux de ressources naturelles dans l'ensemble de l'économie et diffère donc du concept classique d'état stationnaire. L'interv

Analysis of Fluid-Fluid Reaction Systems with Reactions in Both Phases

Adrien Calabria

Developing mathematical models for chemical reaction systems is essential for analysis, development, design, optimization and control of chemical, or biochemical industrial processes. Nowadays, more and more companies across the world have to deal with innovative concepts such as the Sustainable Chemistry (or "Green Chemistry"), which particularly require good knowledge of chemical reactions systems. Well-defined chemical operations involved in a process lead to its efficiency, energy saving and an improvement of the product quality. Furthermore, plants adopting this new philosophy minimize their byproduct production and pollutant formation. This master thesis works on developing a methodology for analysis and kinetic modeling of chemical reaction systems. In this present study, the case where chemical reactions take place in both phases of a heterogeneous biphasic fluid-fluid (F-F) reactor under isothermal conditions is considered. In order to model homogeneous or heterogeneous (F-F) chemical systems, a new approach called “Extent-based Incremental Identification” has recently been developed by the Laboratoire d’Automatique (EPFL). In contrast to the commonly used simultaneous approach, this extent-based incremental approach can compute parameters corresponding to reaction and mass transfer rates individually for each reaction and mass transfer laws, and also does not require prior postulation or knowledge of rate laws. This particular case is an extension of the dissertation done by Nirav Bhatt [1], which considers a heterogeneous chemical reaction system with reactions in one of the phases with steady state mass-transfer between the phases, and work done by Michael Amrhein [2], who developed a linear transformation that computed the extents of reaction from the numbers of moles in homogeneous reaction systems, with inlet and outlet streams. [1] Nirav Bhatt, EPFL Diss. n° 5028 (2011) [2] Amrhein et al., AIChE J. 56 (2010), 2873-2886

2013

Multi-physics modeling of VVERs with high fidelity high-resolution codes

Marianna Papadionysiou

The calculations performed for the design and operation of a Nuclear Power Plant (NPP) are a key factor for their safety analyses. The standard for the computational analysis of NPPs is the so called conventional approach, which relies on coarse mesh diffusion for the neutronic solver and 1D channels for the T/H solver. The recent evolution of computing clusters allows the use of a novel approach, with codes performing first-principle based multi-physics simulations with high-resolution of the calculated parameters. Due to their computational cost, these methods can only be used as an audit tool of the conventional approach. In recent years, the nuclear industry has moved towards the establishment of Best Estimate Plus Uncertainty safety limits. The novel approach is a step forward to that direction. At the same time, VVER technology is expanding, with new reactors being built worldwide. However, there is currently no high-resolution tool available for VVER steady state and cycle analysis. There is a clear need for more refined simulation tools capable of handling hexagonal geometries.The goal of this PhD is the development of a novel computational scheme based on the 3D sub-pin neutron transport code nTRACER-FAST (nTF), coupled to the sub-channel code COBRA-TF (CTF) for VVER full core steady state and cycle analysis. The coupling of a neutronic code with sub-pin resolution to a sub-channel T/H solver, as well as the use of CTF for full core VVER sub-channel analysis are one of the main novelties of this work, to the extent of the writer's knowledge. In this thesis, nTF is verified & validated for VVER 3D core standalone neutronic calculations against data published in international benchmarks. The internal sub-pin coupling of nTF to CTF for hexagonal geometries is fully developed on the course of this study. A double domain decomposition scheme that allows both codes to be executed in parallel is designed. The multi-physics core solver is verified for steady state simulations. nTF/CTF is also compared with nTF using a simplified 1D T/H solver, proving that to achieve accurate predictions, computational tools of different fidelity should not be mixed. Finally, nTF/CTF is developed for 3D full core cycle analysis. The coupled code system is validated with experimental data, achieving the target accuracy set for industrial use. The capability of the solver for sub-pin predictions throughout the depletion cycle is demonstrated.A conventional computational route for VVER analysis, built with CASMO5-VVER as a lattice code for the nodal neutronic solver PARCS, is also established. The use of PARCS with CASMO5-VVER is another novelty of the present work. CASMO5-VVER/PARCS is verified and validated for VVER standalone neutronic and multi-physics steady state simulations and cycle analysis. The modeling options that optimize the use of the code system and the generation of CASMO5-VVER cross-sections for PARCS are described. The comparison of the novel and conventional computational routes demonstrates the accuracy enhancement enabled by the high-resolution core solver, during steady state and depletion calculations. The work performed during this thesis also showed that the nTF/CTF computational requirements are manageable for VVER cycle analysis, when a state-of-the-art computing cluster is used. This illustrates that such high fidelity, high-resolution computational scheme can be used as an audit tool for the conventional route.

EPFL2022

Operando XAS insight in the redox dynamics of VOx species and reducible supports involved in oxidative dehydrogenation of alcohols

Anna Zabilska

Supported vanadia (VOx) belongs to some of the most versatile selective oxidation and reduction catalysts applied for many reactions in the chemical industry and pollution control. Among them, the oxidative dehydrogenation (ODH) of alcohols is particularly attractive since it is one of the main industrial methods of formaldehyde production and a possible route for the transformation of renewable bioethanol to value-added products, such as acetaldehyde and acetic acid. Despite many spectroscopic methods used to investigate the mechanism of this reaction, the details about the redox involvement of VOx species and promoting supports (CeO2 and TiO2) remain debated. In this study, we used an element-specific operando X-ray absorption spectroscopy (XAS) to probe the redox transformations of active sites in vanadia-titania and vanadia-ceria catalysts during ethanol and methanol ODH. By carefully designing the steady-state and transient operando XAS experiments and by applying advanced chemometric methods, we could discriminate specifically and often quantitatively, which role plays each redox couple during the alcohol ODH cycle and uncover hidden structure activity-relationships.Chapter 1 introduces the relevant literature and summarizes the goals and the scope of the thesis.In Chapter 2, we investigate the redox dynamics of active sites in a bilayered VOx/TiOx/SiO2 catalyst (an analog of a widely used VOx/TiO2 catalyst with comparable activity) during ODH of ethanol. Operando time-resolved V and Ti K-edge XAS coupled with a transient experimental strategy quantitatively showed that the formation of acetaldehyde over VOx/TiOx/SiO2 catalyst is kinetically coupled to the formation of a V4+ intermediate, while the formation of V3+ is delayed and its rate is 10-70 times lower. The only very weak redox activity of the Ti4+/Ti3+ couple was detected, which suggests that TiOx species promote the redox activity of VOx indirectly, either via the fast electron transport between VOx species or by changing the electronic structure of VOx species.In Chapter 3, we report on possible difficulties, which can occur during in situ XAS studies of supported vanadia (VOx) catalysts at synchrotron beamlines related to X-ray-induced photochemical reduction of vanadium. We rigorously tested this phenomenon and suggest approaches, which can help to identify and mitigate vanadium photoreduction interfering chemical speciation.In Chapter 4, we focused on the structure-activity relationships in VOx/CeO2 system. We prepared a series of VOx/CeO2 catalysts using high-surface-area CeO2 Using time-resolved operando quick XAS at V K- and Ce L3-edges, we demonstrated that during oxygen cut-off experiments vanadium and cerium change their oxidation states in concert with each other and with the production of acetaldehyde. It suggests that both reducible vanadium and cerium are parts of the same active sites. The maximum VOx/CeO2 activity per mass of catalyst observed at ca. 3 V/nm2 coincides with the maximal amount of cerium atoms, which reversibly change their oxidation state during oxygen cut-off experiments.In Chapter 5, we prove that the mechanistic findings described in Chapter 4 for ethanol ODH over VOx/CeO2 catalysts are also valid for methanol ODH. Moreover, we confirm that the VOx surface density is the main descriptor of the activity for VOx/CeO2 catalysts in alcohol ODH.

EPFL2022

Multi-physics modeling of VVERs with high fidelity high-resolution codes

Marianna Papadionysiou

EPFL2022

Analysis of Fluid-Fluid Reaction Systems with Reactions in Both Phases

Adrien Calabria

2013

Operando XAS insight in the redox dynamics of VOx species and reducible supports involved in oxidative dehydrogenation of alcohols

Anna Zabilska

EPFL2022