This research activity aims to present and analyze the results of a heat-transfer experimental campaign on open-cell metal foams, in order to prove their potential applicability as catalyst supports for strongly endo-/exo-thermic catalytic reactions in tubular reactors. Open-cell metal foams, also known as metal sponges, belong to the general class of cellular media: they appear as formed by void cells interconnected by solid metal ligaments, called struts, forming an irregular network of randomly oriented polyhedrons. Recently, open-cell metal foams have drawn the attention of the scientific community because of their promising heat and mass transfer performances that make them useful for a variety of applications, both in the chemical industry and in other industrial sectors. Innovative lab-produced open-cell metal foam samples, each with different manufacturing routes and geometrical characteristics, were tested and the results compared in this study with the ones obtained from as commercial aluminum foam taken as a benchmark in order to assess their applicability as catalyst supports for the filling of tubular reactors. In particular, aluminum metal foams produced with cheap and innovative manufacturing methods, such as sintering and dissolution from aluminum powders accomplished by the MUSP laboratories, and a special kind of regular cubic-cell foam (belonging to the general class of POCS) produced in the labs of the University of Salento were tested. Attractive results were reached when testing the same commercial metal foam sample sintered to the wall of the reactor; the positive effect of sintering was qualitatively and quantitatively investigated. Attention was also devoted to packed foams, an innovative solution given by the packing of a void open-cell foam with particles. As a hybrid solution, packed foams could combine and exploit the synergies in the heat-transfer mechanisms of a packed bed and a bare foam, resulting in a better global heat management. For the heat-transfer experimental campaign, a pre-existing test rig located in the Laboratories of Catalysis and Catalytic Processes (LCCP) of the Department of Energy of Politecnico di Milano was used. Some of the experimental results were analyzed with an existing model, used in previous works. However, the model proved to be unsuited for representing the heat-transfer mechanisms in some of the currently tested metal foams. A different quantitative approach based on the estimate of an overall heat-transfer coefficient is then presented. This work begins with a short description of the advantages of open-cell metal foams in comparison to common packed beds or monoliths and some of the possible manufacturing routes and applications, along with a brief description of the heat-transfer mechanisms inside porous media (chapter 1). In chapter 2, the experimental set ups and procedures used to acquire the experimental data are presented. Chapter 3 will present the characterization of the tested foams and packed foams while chapter 4 will focus on the experimental results, showing the positive effects of enhancing the direct contact of the foam with the tube wall by diffusion bonding of an aluminum commercial foam. In chapter 5, the lab-manufactured open-cell foams will be compared to the commercial one, and the results discussed both qualitatively and qualitatively. Chapter 6 will show the benefits of packed foams. Finally, some conclusions will be drawn.
L’obiettivo del presente lavoro è quello di presentare e analizzare i risultati di una campagna sperimentale volta a caratterizzare le prestazioni di scambio termico di schiume metalliche a celle aperte e valutare il loro potenziale utilizzo come supporto per catalizzatori all’interno di reattori tubolari a letto fisso che promuovono reazioni fortemente endo/eso-termiche. Le schiume metalliche a celle aperte, anche note come spugne, appartengono alla classe di materiali altamente porosi: appaiono come formate da celle vuote, interconnesse le une alle altre da legamenti metallici solidi a formare dei poliedri orientati in maniera casuale. Negli ultimi anni le schiume metalliche a celle aperte hanno ricevuto una crescente attenzione da parte della comunità scientifica grazie alle interessanti e promettenti proprietà di scambio termico e di massa che le rendono utilizzabili in vasti campi applicativi, in ambito chimico e nella industria di processo. Nel corso del presente lavoro, sono state testate schiume metalliche di tipo commerciale opportunamente inserite all’interno di un reattore tubolare collocato in un forno termostatico per valutare la loro risposta termica. Fissando come riferimento le prestazioni di una schiuma commerciale in alluminio, i risultati sperimentali sono stati messi a confronto con quelli ottenuti da schiume metalliche prodotte in laboratorio e quindi aventi proprietà geometriche e processi di manifattura differenti: l’obiettivo è valutare la possibilità di utilizzare tali soluzioni come supporto per catalizzatori in reattori tubolari. In particolare, sono state testate schiume in alluminio prodotte con metodi innovativi poco costosi, come la schiuma prodotta dai laboratori MUSP di Piacenza con un metodo di sintering e dissoluzione da polveri di alluminio di scarto e la schiuma a cella cubica regolare appartenente alla classe POCS (“Periodic Open-Cell Structures”), prodotta dai laboratori dell’università del Salento. Risultati interessanti sono stati ottenuti dalla stessa schiuma commerciale in alluminio utilizzata come benchmark ma sinterizzata alla parete di un reattore tubolare in alluminio: l’effetto positivo del contatto a parete è stato valutato da un punto di vista qualitativo e quantitativo. Inoltre, particolare attenzione hanno ricevuto le schiume impaccate, una soluzione innovativa ottenuta grazie all’impaccamento di una schiuma vuota con particelle sferiche, generalmente usate per i letti impaccati. Questa soluzione ibrida può combinare e sfruttare le sinergie di un letto impaccato e di una schiuma metallica semplice, risultando in una migliore gestione e risposta termica globale. Per lo studio delle proprietà di scambio termico, è stato utilizzato un impianto collocato nei laboratori LCCP (“Laboratori di Catalisi e Processi Catalitici”) del Dipartimento di Energia del Politecnico di Milano. Alcuni dei risultati sperimentali (quelli ottenuti con la schiuma metallica commercial in alluminio) sono stati elaborati con un modello matematico di regressione preesistente ed utilizzato in lavori recenti, basato su una modellazione pseudo-omogena della scambio termico in mezzi porosi. Tuttavia, in virtù dei risultati ottenuti, il modello è stato valutato inadatto a descrivere il fenomeno termico per le schiume metalliche a bassa densità di pori, come quelle studiate in questo lavoro. Un differente approccio quantitativo, basato sul calcolo di un coefficiente di scambio termico globale ottenuto risolvendo con un metodo alle differenze finite l’equazione dell’energia, viene quindi presentato e i risultati letti quantitativamente sfruttando anche questo parametro per i confronti. Inizialmente, nel capitolo introduttivo, vengono descritti i vantaggi delle schiume metalliche a celle aperte, rispetto ai comuni letti impaccati e ai monoliti. Inoltre, vengono mostrate le possibili applicazioni e i processi di produzione delle stesse, con una maggiore attenzione per la descrizione dei fenomeni termici e della loro modellazione in mezzi porosi. Nel secondo capitolo, viene introdotto l’impianto sperimentale e le procedure utilizzate per ottenere i risultati. Il terzo capitolo si focalizza invece sulla caratterizzazione delle schiume metalliche testate mentre il capitolo 4 presenterà alcuni risultati sperimentali di scambio termico inerenti alla schiuma commerciale in alluminio; viene quindi spiegato l’effetto positivo del contatto a parete ottenuto tramite sinterizzazione. Nel capitolo 5, le schiume metalliche prodotte in laboratorio sono confrontate con quella commerciale e i risultati discussi quantitativamente e qualitativamente. Il capitolo 6 sarà interamente centrato sui benefici di scambio termico apportati dalle schiume impaccate. Alcune conclusioni saranno a chiusura del lavoro.
Experimental study of the heat transfer performances of open cell metal foams as catalyst support
GARANCINI, LORENZO
2014/2015
Abstract
This research activity aims to present and analyze the results of a heat-transfer experimental campaign on open-cell metal foams, in order to prove their potential applicability as catalyst supports for strongly endo-/exo-thermic catalytic reactions in tubular reactors. Open-cell metal foams, also known as metal sponges, belong to the general class of cellular media: they appear as formed by void cells interconnected by solid metal ligaments, called struts, forming an irregular network of randomly oriented polyhedrons. Recently, open-cell metal foams have drawn the attention of the scientific community because of their promising heat and mass transfer performances that make them useful for a variety of applications, both in the chemical industry and in other industrial sectors. Innovative lab-produced open-cell metal foam samples, each with different manufacturing routes and geometrical characteristics, were tested and the results compared in this study with the ones obtained from as commercial aluminum foam taken as a benchmark in order to assess their applicability as catalyst supports for the filling of tubular reactors. In particular, aluminum metal foams produced with cheap and innovative manufacturing methods, such as sintering and dissolution from aluminum powders accomplished by the MUSP laboratories, and a special kind of regular cubic-cell foam (belonging to the general class of POCS) produced in the labs of the University of Salento were tested. Attractive results were reached when testing the same commercial metal foam sample sintered to the wall of the reactor; the positive effect of sintering was qualitatively and quantitatively investigated. Attention was also devoted to packed foams, an innovative solution given by the packing of a void open-cell foam with particles. As a hybrid solution, packed foams could combine and exploit the synergies in the heat-transfer mechanisms of a packed bed and a bare foam, resulting in a better global heat management. For the heat-transfer experimental campaign, a pre-existing test rig located in the Laboratories of Catalysis and Catalytic Processes (LCCP) of the Department of Energy of Politecnico di Milano was used. Some of the experimental results were analyzed with an existing model, used in previous works. However, the model proved to be unsuited for representing the heat-transfer mechanisms in some of the currently tested metal foams. A different quantitative approach based on the estimate of an overall heat-transfer coefficient is then presented. This work begins with a short description of the advantages of open-cell metal foams in comparison to common packed beds or monoliths and some of the possible manufacturing routes and applications, along with a brief description of the heat-transfer mechanisms inside porous media (chapter 1). In chapter 2, the experimental set ups and procedures used to acquire the experimental data are presented. Chapter 3 will present the characterization of the tested foams and packed foams while chapter 4 will focus on the experimental results, showing the positive effects of enhancing the direct contact of the foam with the tube wall by diffusion bonding of an aluminum commercial foam. In chapter 5, the lab-manufactured open-cell foams will be compared to the commercial one, and the results discussed both qualitatively and qualitatively. Chapter 6 will show the benefits of packed foams. Finally, some conclusions will be drawn.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/118344