Caratterizzazione di membrane metalliche e ionomeriche per applicazioni energetiche

The work of this thesis has been focused on the characterization of metallic membranes for the hydrogen purification from steam reforming process and also of perfluorosulphonic acid ionomeric (PFSI) membranes suitable as electrolytes in fuel cell applications. The experimental study of metallic membranes was divided in three sections: synthesis of palladium and silver palladium coatings on porous ceramic support via electroless deposition (ELD), solubility and diffusivity analysis of hydrogen in palladium based alloys (temperature range between 200 and 400 °C up to 12 bar of pressure) and permeation experiments of pure hydrogen and mixtures containing, besides hydrogen, also nitrogen and methane at high temperatures (up to 600 °C) and pressures (up to 10 bar). Sequential deposition of palladium and silver on to porous alumina tubes by ELD technique was carried out using two different procedures: a stirred batch and a continuous flux method. Pure palladium as well as Pd-Ag membranes were produced: the Pd-Ag membranes’ composition is calculated to be close to 77% Pd and 23% Ag by weight which was the target value that correspond to the best performance of the palladium-based alloys. One of the membranes produced showed an infinite selectivity through hydrogen and relatively high permeability value and is suitable for the potential use as a hydrogen separator. The hydrogen sorption in silver palladium alloys was carried out in a gravimetric system on films produced by ELD technique. In the temperature range inspected, up to 400°C, there is still a lack in literature. The experimental data were analyzed with rigorous equations allowing to calculate the enthalpy and entropy values of the Sieverts’ constant; the results were in very good agreement with the extrapolation made with literature data obtained a lower temperature (up to 150 °C). The information obtained in this study would be directly usable in the modeling of hydrogen permeation in Pd-based systems. Pure and mixed gas permeation tests were performed on Pd-based hydrogen selective membranes at operative conditions close to steam-reforming ones. Two membranes (one produced in this work and another produced by NGK Insulators Japan) showed a virtually infinite selectivity and good permeability. Mixture data revealed the existence of non negligible resistances to hydrogen transport in the gas phase. Even if the decrease of the driving force due to polarization concentration phenomena occurs, in principle, in all membrane-based separation systems endowed with high perm-selectivity, an extensive experimental analysis lack, at the moment, in the palladium-based membrane process in literature. Moreover a new procedure has been introduced for the proper comparison of the mass transport resistance in the gas phase and in the membrane. Another object of study was the water vapor sorption and permeation in PFSI membranes with short and long side chains was also studied; moreover the permeation of gases (i.e. He, N2 and O2) in dry and humid conditions was considered. The water vapor sorption showed strong interactions between the hydrophilic groups and the water as revealed from the hysteresis in the sorption-desorption isotherms and thermo gravimetric analysis. The data obtained were used in the modeling of water vapor permeation, that was described as diffusion-reaction of water molecules, and in the humid gases permeation experiments. In the dry gas experiments the permeability and diffusivity was found to increase with temperature and with the equivalent weight (EW) of the membrane. A linear correlation was drawn between the dry gas permeability and the opposite of the equivalent weight of PFSI membranes, based on which the permeability of pure PTFE is retrieved in the limit of high EW. In the other hand O2 ,N2 and He permeability values was found to increase significantly, and in a similar fashion, with water activity. A model that considers the PFSI membrane as a composite matrix with a hydrophilic and a hydrophobic phase was considered allowing to estimate the variation of gas permeability with relative humidity on the basis of the permeability in the dry PFSI membrane and in pure liquid water.

Produzione di idrogeno "on-board" mediante deidrogenazione catalitica di kerosene

L’H2 è attualmente un elemento di elevato interesse economico, con notevoli prospettive di sviluppo delle sue applicazioni. La sua produzione industriale supera attualmente i 55 ∙ 1010 m3/anno, avendo come maggiori utilizzatori (95% circa) i processi di produzione dell’ammoniaca e quelli di raffineria (in funzione delle sempre più stringenti normative ambientali). Inoltre, sono sempre più importanti le sue applicazioni come vettore energetico, in particolare nel settore dell’autotrazione, sia dirette (termochimiche) che indirette, come alimentazione delle fuel cells per la produzione di energia elettrica. L’importanza economica degli utilizzi dell’ H2 ha portato alla costruzione di una rete per la sua distribuzione di oltre 1050 km, che collega i siti di produzione ai principali utilizzatori (in Francia, Belgio, Olanda e Germania). Attualmente l’ H2 è prodotto in impianti di larga scala (circa 1000 m3/h) da combustibili fossili, in particolare metano, attraverso i processi di steam reforming ed ossidazione parziale catalitica, mentre su scala inferiore (circa 150 m3/h) trovano applicazione anche i processi di elettrolisi dell’acqua. Oltre a quella relativa allo sviluppo di processi per la produzione di H2 da fonti rinnovabili, una tematica grande interesse è quella relativa al suo stoccaggio, con una particolare attenzione ai sistemi destinati alle applicazioni nel settore automotivo o dei trasposti in generale. In questo lavoro di tesi, svolto nell’ambito del progetto europeo “Green Air” (7FP – Transport) in collaborazione (in particolare) con EADS (D), CNRS (F), Jonhson-Matthey (UK), EFCECO (D), CESA (E) e HyGEAR (NL), è stato affrontato uno studio preliminare della reazione di deidrogenazione di miscele di idrocarburi e di differenti kerosene per utilizzo aereonautico, finalizzato allo sviluppo di nuovi catalizzatori e dei relativi processi per la produzione di H2 “on board” utilizzando il kerosene avio per ottenere, utilizzando fuel cells, l’energia elettrica necessaria a far funzionare tutta la strumentazione ed i sistemi di comando di aeroplani della serie Airbus, con evidenti vantaggi dal punto di vista ponderale e delle emissioni.