Nathalie Mazet

Modelling of gas-solid reaction—Coupling of heat and mass transfer with chemical reaction

A general gas-solid reaction model is formulated. This work is the further development of the previous modelling work of Mazet (1988, Ph.D. Thesis, University of Perpignan) and Goetz (1991, Ph.D. Thesis, University of Perpignan) to simulate reversible gas-solid reactions that have been extensively applied to the new chemical heat pump technology developed at CNRS-IMP. In the present paper, a general coupled heat and mass transfer model has been developed and solved numerically. The global and local information yielded from this model are essential for a better knowledge of transfer mechanism concerning the significant influence of mass transfer in the pellet at low pressure and low permeability, important phenomena ignored by the previous models. Two reactive fronts in the transformation of the reactor have been found by this model, i.e. the “heat front” and the “mass front”. Parameter sensitivity study from this model has classified the domain of influence of permeability and operating pressure on the global transformation of the reactor. This model has shown that when constraint pressure Pc > 4 bars and permeability k > 10−13 m2, previous models that consider heat transfer only at the global pellet level are still valid. In any other cases, the present model should be applied. Good agreement has been found between the simulation results of this model and the available analytical theory and experimental results for extreme boundary and initial conditions, and also for those within the practical experimental range. When a reactive material is given, this model can find out the pressure range in which this particular material can function correctly and efficiently. The simulation results from this model have also been compared with the recent experimental work of Prades (1992, Ph.D. Thesis, University of Perpignan) on a new material—IMPEX developed at CNRS-IMP. The comparison has confirmed the domain of influence yielded from this model and have indicated the variation of permeability during the reaction process.

Experimental investigation of an innovative thermochemical process operating with a hydrate salt and moist air for thermal storage of solar energy: Global performance

This paper investigates an innovative open thermochemical system dedicated to high density and long term (seasonal) storage purposes. It involves a hydrate/water reactive pair and operates with moist air. This work focuses on the design of and experimentation with a large scale prototype using SrBr2/H2O as a reactive pair (400kg of hydrated salt, 105kWh of storage capacity and a reactor energy density of 203kWh/m3). Promising conclusions have been obtained regarding the feasibility and performance of such a storage process. Hydration specific powers from 0.75 to 2W/kg have been reached for a bed salt energy density of 388kWh/m3. Moreover, two important parameters that control the storage system have been identified and investigated: the equilibrium drop and the mass flow rate of moist air. Both have a strong influence on the reaction kinetics and therefore on the reactor’s thermal power.

An experimental and theoretical investigation of the transient behavior of a two-phase closed thermosyphon

This paper presents an experimental and theoretical investigation of the two-phase closed thermosyphon (TPCT) behavior in transient regimes. Experimental results show two kinds of TPCT response. We focus on regular variations of operating system variables, where a mathematical model has been developed in order to obtain an analytical expression of the system response time. The dependence of this response time according to the various parameters is linked to geometry and heat transfer laws. The model can be considered as a simple and efficient tool for designing TPCTs in both transient and steady regimes.

Comparison of a general model with a simplified approach for the transformation of solid-gas media used in chemical heat transformers

The influence of mass transfer has drawn great attention in the study of gas-solid reversible reactions. Experiments have been conducted for the IMPEX blocks (Mauran et al., 1991, patent no. 91 0303) in reaction with ammonia at low pressure when the influence of mass transfer is significant (Prades, 1992, These de doctorat, Universite de Perpignan; Lu et al., 1996b, A.I.Ch.E. J., to be submitted). A general gas-solid reaction model has been developed (Lu et al., 1996a, Chemical Engineering Science51, 3829–3845) to quantify the non-negligible influence of mass transfer on the global transformation of the reactive media (especially for the cases of less-permeable material). Two levels of coupling have been considered. Firstly, at the grain level, mass transfer and chemical kinetics are coupled, and secondly, at the pellet level, heat transfer and mass transfer are coupled. The simulation results of the general model have revealed that in the process of reaction, there exist two progressive reactive fronts in the pellet, one is the ‘heat front’ moving from the heat exchanger to the gas diffuser and the other is the ‘mass front’, moving from the gas diffuser to the heat exchanger. In this paper, a simplified approach has been made by simplifying the progressive reactive fronts shown in the general model as ‘sharp’ reactive fronts. This simplified approach does not enter into the details at the grain level and does not have the accumulative terms in the model equations. Therefore, it has significantly simplified the numerical calculation. The comparison has shown similarities between the results of these two models, regarding the global advancement and local profiles. If the physical characteristics of the reactive block are constant in the process of the reaction, the simplified model has confirmed to be basically reliable to predict the gas-solid reversible reaction and therefore will serve as an efficient tool for the optimal sizing of a chemical heat transformer. In any other cases, the general model should be used.

New Sorption Cycles for Heat and/or Cold Production Adapted for Long Distance Heat Transmission

The analysis of the combinations of dipoles, linked by a gas transfer between an endothermic element and an exothermal element, through an exothermal physical-chemical process in thermal contact with an other endothermic process, is the basis of a new process for the transmission at long distance of heat/cold production. The yield of such cycles is identified through the values of the unused exergy, the untapped exergy and the exergy which is produced in excess by the process.Copyright

Optimisation de la diffusion du gaz dans des matériaux réactifs, siège de transferts de chaleur, de masse et d’une réaction chimique

Abstract Au cours du fonctionnement des transformateurs thermochimiques, differents phenomenes interviennent: les transferts thermiques, massiques et la reaction chimique solide⧹gaz. Ceux-ci sont analyses, modelises sous forme dequations aux derivees partielles, et resolus par la methode des elements finis. Le modele a ete valide a partir dune experimentation. Les parametres importants pour la transformation du materiau sont la pression operatoire et la permeabilite du milieu poreux utilise, appele Impex. Cet article propose une optimisation de la repartition des diffuseurs de gaz afin dameliorer le transfert de masse et daugmenter les performances du reacteur.

Thermochemical Solar Reactor: Simplified Method for the Geometrical Optimization at a Given Incident Flux

This paper aims to describe a simplified method to optimize the geometry of a solar thermochemical reactor. As a first step, this paper focuses on a purely thermal analysis. The chemical reaction is represented by a uniform heat sink inside the material. The heat transfer modes are radiation in the empty part (cavity) and conduction inside the reactive material. The aim is to find the optimal geometry of the reactor, by maximizing its efficiency, for a fixed value of the incident solar flux and of the total volume of the reactor. An analytical solution can be found thanks to some simplifying hypothesis. The influence of different operational parameters on the maximal efficiency and on the optimal shape is studied. A comparison between different reactor designs (cylindrical and cavity reactors) is shown. A 2D study, based on CFD software using a finite element method, allows for quantifying the effects of the simplifying assumptions. The constructal theory aims to optimize the internal structure of a system in order to provide easier access to its internal currents and increase the system efficiency. Thus, this study can be seen as the optimization of the elemental volume of the constructal approach. In a next step this optimization method will be used to optimize more complex reactor design, as for example, a honeycomb reactor obtained by the assembling of several cavities, in order to optimize a thermochemical reactor for hydrogen production or high temperature heat storage.

Gravitational heat pipes for discontinuous applications

Abstract Gravitational heat pipes or thermosiphons (TS) are simple and efficient heat exchangers. This paper deals with the adaptation of these exchangers to discontinuous or cyclic processes such as chemical heat pumps. Such TS must fulfil a two-mode working too: the TS working must be active/inactive according to the successive endothermal/exothermal phases of the cyclic process. Two kinds of TS blockage processes have been successfully experimented: they act either on the liquid or on the vapor phase inside the TS. A simple model demonstrates that realistic configurations of TS and reactor can easily reduce the energy and duration required by the blockage process itself on the performances of the coupling.

Gibbs Systems Dynamics: A Simple But Powerful Tool for Process Analysis, Design and Optimization

Dynamic process modeling by the mean of Equivalent Gibbs systems is described here. It allows to model a large number of processes and only requires standard engineering knowledge. This method is issued from thermodynamics of irreversible processes, initiated by I. Prigogine, but applied here to process engineering. First, an Equivalent Gibbs System (EGS) is defined for each component involved in the process. In such system, mass, energy and entropy are linked through Gibbs equation and entropy production can easily be expressed according to fluxes and their related forces. Assuming linear phenomenological laws, phenomenological coefficients can be calculated from common engineering correlations, or evaluated from technical data if available. As an example, a conventional vapor compression chiller is simulated. Three control modes are analyzed on an exergy basis: on/off control with constant or floating condensing pressure, PID control with variable compressor speed.Copyright