Sylvain Mauran

Gas flow through highly porous graphite matrices

Highly porous graphite matrices with 20–200 kg m−3 of bulk densities have been characterized using helium pycnometer, mercury porosimeter, and fluid flow methods, i.e. both gas permeation and diffusion measurements. The gas permeability is ranging from 10−12 to 10−15 m2 and decreases as the bulk density of the graphite matrix increases. According to the Carman–Kozeny correlation, the evolution of the gas permeability with respect to the bulk density is very well correlate with, on the one hand, the increase of the tortuosity, and on the other hand, the decrease of the porosity as well as the pore diameter with the bulk density of the graphite matrix.

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.

Textural characterisation of graphite matrices using electrochemical methods

The present work describes a novel approach for the textural characterisation of a carbonaceous porous medium. Due to the electronic conduction properties of expanded natural graphite (ENG), electrochemical determination was proposed and the use of cyclic voltammetry enables determination of the surface area and porous volume of ENG. First, experiments were performed without the electrochemical electron transfer reaction. Then, the volume of the entire transformation of potassium hexacyanoferrate into the pores of the material was deduced by applying the rules of thin-layer electrochemistry. For the specific surface area and the porosity, the results are satisfying, but in comparison with classical methods, the electrochemical procedure does not allow the determination of the pore size distribution of the material. It is possible to evaluate the mean pore diameter and to assume that the double-layer capacitance may depend on the kind of graphite surface (basal or edge plane).

Potentialities of expanded natural graphite as a new transducer for NAD+-dependent dehydrogenase amperometric biosensors

The present work deals with the use of the porous texture of expanded natural graphite (ENG) as transducer in order to design electrochemical biosensors. The sensing element is a NAD+-dependent dehydrogenase. An electrochemical pretreatment of the ENG is favorable because it allows on one hand generating functional surface groups that may act as mediators for NADH oxidation and, on the other hand, eliminating enzyme-toxic compounds. The electrocatalytic oxidation of NADH on the pretreated material leads to the formation of enzymatically active NAD+. However, some persistent problems, mainly related to enzyme instability, still hamper the development of the biosensors.