In view of the increasing interest in compact Methane Steam Reforming reactors for a distributed hydrogen economy, it is required to develop new catalytic reactors with intensified heat supply, which is one of the limiting factors for the down-sizing of the current process technology. The adoption of highly conductive metallic foams, made catalytically active by the deposition of a catalysts layer or by packing the cavities with pelletized catalyst, may be considered as a compromise between the need for activity, specific surface area and effective heat and mass transfer. In literature, conductive structured reactors adopting washcoated metallic foams have been proved to be a good alternative to conventional random packed bed systems, thus allowing to enhance the performance of this non-adiabatic endothermic process. However, washcoated structured catalysts may be affected by some limitations such as poor catalyst inventory, washcoat adhesion and a lower surface area with respect to small catalyst pellets. The choice of the foam material and morphology has strong effects on the behaviour of the process; in this work, we demonstrated the difference in terms of performances and heat transfer coefficients of a FeCrAlY foam with respect to copper foams. Innovative catalytic systems (i.e. packed foams) that provide advantages in terms of both catalytic activity and heat transfer in a lab-scale Steam Reforming process have been conceived and tested. In this work great attention was dedicated to copper open-cell foams, which were prepared in two different layouts (i.e. washcoated foams and packed foams) in order to compare their behavior to packed beds. Copper-based systems showed better performances in term of conversion at moderate and higher oven temperatures and lower thermal gradients inside the reactor thanks to the optimized heat transfer. An advantage was provided by packed copper foams: indeed, higher conversions were reached thanks to the reduction of internal mass transfer limitations and, at the same time, an enhanced heat transfer coefficient was obtained. Based on benefits provided by these conductive metallic structures, further investigations were performed on the possible production via 3D-printing replica technique of metallic Periodic Open Cellular Structures (POCS). These substrates are characterized by high porosity and specific surface area, which can be optimized by modification of POCS geometry using Computational Fluid Dynamics (CFD) simulations with the aim to improve heat transfer properties, thus further enhancing the process productivity.
L’interesse crescente nei confronti di reattori compatti per il processo di Steam Reforming di Metano ha portato allo sviluppo di nuovi reattori catalitici caratterizzati da un migliore controllo termico, aspetto che risulta limitante nella tecnologia industriale del processo in questione. L’utilizzo di schiume metalliche con un’elevata conduttività, rese cataliticamente attive mediante deposizione di uno strato di catalizzatore o mediante impaccamento delle cavità interne con pellet, può essere considerato un buon compromesso tra le richieste di elevata attività catalitica, di una grande area superficiale specifica e di un buon trasferimento sia di calore sia di materia. In letteratura, reattori strutturati conduttivi basati sull’utilizzo di schiume metalliche rivestite con washcoat sono stati confermati come valida alternativa al tradizionale packed bed, poiché consentono un miglioramento delle prestazioni del processo endotermico non adiabatico considerato. Tuttavia, questo tipo di catalizzatori strutturati presenta numerosi svantaggi, quali basso catalyst inventory, difficile adesione del washcoat e minore area superficiale rispetto a quella di pellet catalitici. La scelta del materiale e della morfologia del substrato ha una notevole influenza sul processo; in questo lavoro, abbiamo dimostrato la differenza in termini di performance e coefficienti di scambio termico introdotta dall’utilizzo di schiume in FeCrAlY e in rame. Sono stati studiati sperimentalmente sistemi catalitici innovativi (i.e. schiume impaccate) che portano vantaggi sia in termini di attività catalitica che di trasferimento di calore in un processo di Steam Reforming su scala di laboratorio. In questo lavoro, una particolare attenzione è stata dedicata alle schiume in rame, preparate in due configurazioni differenti (i.e. schiume con washcoat e schiume impaccate) al fine di confrontare il loro comportamento con quello del packed bed. I sistemi basati sull’utilizzo di strutture in rame hanno mostrato migliori prestazioni in termini di conversione a moderata e alta temperatura del forno e minori gradienti di temperatura all’interno del reattore, grazie all’ottimizzazione del processo di scambio termico. L’introduzione delle schiume in rame impaccate ha apportato dei vantaggi: infatti, è stato possibile raggiungere conversioni più elevate grazie alla riduzione delle limitazioni diffusive interne e, al contempo, e ottenere un maggiore coefficiente di scambio termico. Sulla base dei vantaggi ottenuti da queste strutture metalliche conduttive, è stato effettuato un ulteriore studio sulla produzione di Periodic Open Cellular Structures (POCS) mediante tecnica di 3D-printing replica. Questi particolari substrati sono caratterizzati da elevata porosità e area superficiale specifica, che possono essere ottimizzate mediante modifiche della geometria dei POCS partendo da simulazioni di Fluidodinamica Computazionale (CFD), allo scopo di migliorare le proprietà di scambio termico e quindi aumentare la produttività del processo.
Experimental and numerical investigation of conductive metallic structured catalysts for the intensification of methane steam reforming
TRIPI, MARIA CELESTE;BISACCIA, ALESSANDRA
2018/2019
Abstract
In view of the increasing interest in compact Methane Steam Reforming reactors for a distributed hydrogen economy, it is required to develop new catalytic reactors with intensified heat supply, which is one of the limiting factors for the down-sizing of the current process technology. The adoption of highly conductive metallic foams, made catalytically active by the deposition of a catalysts layer or by packing the cavities with pelletized catalyst, may be considered as a compromise between the need for activity, specific surface area and effective heat and mass transfer. In literature, conductive structured reactors adopting washcoated metallic foams have been proved to be a good alternative to conventional random packed bed systems, thus allowing to enhance the performance of this non-adiabatic endothermic process. However, washcoated structured catalysts may be affected by some limitations such as poor catalyst inventory, washcoat adhesion and a lower surface area with respect to small catalyst pellets. The choice of the foam material and morphology has strong effects on the behaviour of the process; in this work, we demonstrated the difference in terms of performances and heat transfer coefficients of a FeCrAlY foam with respect to copper foams. Innovative catalytic systems (i.e. packed foams) that provide advantages in terms of both catalytic activity and heat transfer in a lab-scale Steam Reforming process have been conceived and tested. In this work great attention was dedicated to copper open-cell foams, which were prepared in two different layouts (i.e. washcoated foams and packed foams) in order to compare their behavior to packed beds. Copper-based systems showed better performances in term of conversion at moderate and higher oven temperatures and lower thermal gradients inside the reactor thanks to the optimized heat transfer. An advantage was provided by packed copper foams: indeed, higher conversions were reached thanks to the reduction of internal mass transfer limitations and, at the same time, an enhanced heat transfer coefficient was obtained. Based on benefits provided by these conductive metallic structures, further investigations were performed on the possible production via 3D-printing replica technique of metallic Periodic Open Cellular Structures (POCS). These substrates are characterized by high porosity and specific surface area, which can be optimized by modification of POCS geometry using Computational Fluid Dynamics (CFD) simulations with the aim to improve heat transfer properties, thus further enhancing the process productivity.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/151220