Oil has always been the main energy source for obtaining fuel; despite the recent economic crisis, the growth of its demand has not stopped. The approach of the peak production of crude oil and the high instability in the Middle East have increased the interest in the feed-to-liquid processes, used to obtain synthetic fuel from sources such as natural gas, coal and biomass. This is the context of the Low Temperature Fischer-Tropsch Synthesis, which is used to convert the syngas into a hydrocarbon mixture mainly consisting of linear paraffins, known as syncrude. The latter, once refined, allows to obtain a high quality diesel fuel, characterized by a high cetane number and the absence of sulfur and aromatic compounds. This work aims to study the cobalt Fischer-Tropsch catalysts, supported on alumina (γ-Al2O3). The tests were conducted in a fixed bed reactor, FBR-PoliMi, inside the laboratories of the Energy Department of the Politecnico di Milano. The catalysts were prepared in laboratory using the method IWI (Incipient Wetness Impregnation) and it was observed the effect of both the addition of an organic compound (diethylene glycol) to the impregnating solution, and of the precursor salt variation (nitrate or cobalt acetate). The catalyst containing glycol came out on top in terms of activity and productivity; the use of cobalt acetate as a precursor has instead resulted in a loss of performance. Later it was analyzed how the addition of a promoter (platinum) would change the performance, both in the case of catalysts without glycol, and catalysts containing this organic compound in the impregnating solution. The sample in which platinum was added at the last impregnation step showed a very similar behavior to that of the non-promoted catalyst; the catalysts, in which both platinum and glycol were added, resulted to be less active than the unpromoted sample. The tests were carried out maintaining constant both the pressure (25 bar) and the space velocity (6410 Ncm3/gcat /h), while the effects of change of temperature (in the range 200-220°C) and syngas molar ratio H2/CO (1.7-2) were investigated. It is observed an increase of the conversion and the selectivity to the light hydrocarbons as the temperature increases; these effects were also achieved in the case of use of a syngas richer in hydrogen. Finally, we developed a kinetic model to study the performance of the sample prepared with glycol, as it provided the best performance; furthermore this catalyst, which does not require the presence of platinum, is more economical and thus, potentially, more interesting for the industrial sector. The kinetic laws that have been employed for modeling are both purely empirical and mechanistic; all regressions have come to convergence, but some of the results were not taken into account due to excessive correlation between the adaptive parameters or because they had no physical sense. In conclusion, it was determined that a mechanistic law of Sarup-Wojciechowski-type is the best to describe the behavior of the catalysts impregnated with glycol.
Il petrolio ha sempre costituito la principale fonte energetica per l’ottenimento di carburanti; nonostante la recente crisi economica, la crescita del suo fabbisogno non si è arrestata. L’avvicinarsi del picco di produzione del greggio e l’elevata instabilità che caratterizza il Medio Oriente hanno accresciuto l’interesse nei confronti dei processi feed-to-liquid, impiegati per ottenere combustibile sintetico a partire da fonti come gas naturale, carbone e biomassa. In questo contesto si colloca la sintesi di Fischer-Tropsch di bassa temperatura, la quale viene utilizzata per convertire il gas di sintesi in una miscela idrocarburica costituita principalmente di paraffine lineari, detta syncrude. Quest’ultimo, una volta raffinato, permette di ricavare un gasolio di alta qualità, contraddistinto da un elevato numero di cetano e dall’assenza di composti solforati e di aromatici. Il presente lavoro di tesi mira allo studio di catalizzatori per Fischer-Tropsch al cobalto supportati su allumina (γ-Al2O3). Le prove sono state condotte nel reattore a letto fisso FBR-PoliMi, operante presso i laboratori del Dipartimento di Energia del Politecnico di Milano. Sono stati testati catalizzatori preparati in laboratorio mediante il metodo IWI (Incipient Wetness Impregnation) e si è osservato sia l’effetto dell’aggiunta di un composto organico (glicole dietilenico) alla soluzione impregnante, sia quello della variazione del sale precursore contenente il cobalto (nitrato o acetato di cobalto). Il catalizzatore contenente glicole è risultato il migliore in termini di attività e produttività; l’impiego di acetato di cobalto come precursore ha invece determinato un peggioramento delle prestazioni. In seguito si è analizzato come l’aggiunta di un promotore (platino) modificasse le prestazioni, nel caso sia di catalizzatori non impregnati con glicole, sia di catalizzatori la cui soluzione impregnante conteneva anche tale composto. Il campione in cui il platino è stato deposto assieme al cobalto nell’ultima impregnazione ha mostrato un comportamento molto simile a quello del catalizzatore non promosso; i catalizzatori impregnati sia con platino che con glicole sono risultati meno attivi del campione con il solo composto organico. I test sono stati eseguiti mantenendo costanti la pressione (25 bar) e la velocità spaziale (6410 Ncm3/gcat/h), mentre si sono indagati gli effetti di variazione della temperatura (in un range 200-220°C) e del rapporto molare H2/CO del gas di sintesi alimentato (1,7-2). Si è osservato un aumento della conversione e della selettività alle specie leggere al crescere della temperatura; tali effetti sono stati conseguiti anche nel caso di utilizzo di un gas di sintesi più ricco di idrogeno. Infine si è sviluppato un modello cinetico per studiare le perfomance del campione impregnato con glicole, in quanto ha fornito le migliori prestazioni; inoltre questo catalizzatore, che non richiede la presenza di platino, risulta più economico e quindi potenzialmente più interessante per il settore industriale. Le leggi cinetiche che sono state impiegate per la modellazione sono di origine sia puramente empirica che meccanicistica; tutte le regressioni sono arrivate a convergenza, ma i risultati di alcune non sono stati presi in considerazione a causa dell’eccessiva correlazione tra parte dei parametri adattivi o perché privi di senso fisico. In conclusione, si è stabilito che una legge meccanicistica del tipo Sarup-Wojciechowski è la migliore per descrivere il comportamento del catalizzatore impregnato con glicole dietilenico.
Analisi di reattività di catalizzatori Co/Gamma-Al2O3 attivi nella sintesi di Fischer-Tropsch
PERFETTI, SAMUELE;ZANCHI, GABRIELE
2015/2016
Abstract
Oil has always been the main energy source for obtaining fuel; despite the recent economic crisis, the growth of its demand has not stopped. The approach of the peak production of crude oil and the high instability in the Middle East have increased the interest in the feed-to-liquid processes, used to obtain synthetic fuel from sources such as natural gas, coal and biomass. This is the context of the Low Temperature Fischer-Tropsch Synthesis, which is used to convert the syngas into a hydrocarbon mixture mainly consisting of linear paraffins, known as syncrude. The latter, once refined, allows to obtain a high quality diesel fuel, characterized by a high cetane number and the absence of sulfur and aromatic compounds. This work aims to study the cobalt Fischer-Tropsch catalysts, supported on alumina (γ-Al2O3). The tests were conducted in a fixed bed reactor, FBR-PoliMi, inside the laboratories of the Energy Department of the Politecnico di Milano. The catalysts were prepared in laboratory using the method IWI (Incipient Wetness Impregnation) and it was observed the effect of both the addition of an organic compound (diethylene glycol) to the impregnating solution, and of the precursor salt variation (nitrate or cobalt acetate). The catalyst containing glycol came out on top in terms of activity and productivity; the use of cobalt acetate as a precursor has instead resulted in a loss of performance. Later it was analyzed how the addition of a promoter (platinum) would change the performance, both in the case of catalysts without glycol, and catalysts containing this organic compound in the impregnating solution. The sample in which platinum was added at the last impregnation step showed a very similar behavior to that of the non-promoted catalyst; the catalysts, in which both platinum and glycol were added, resulted to be less active than the unpromoted sample. The tests were carried out maintaining constant both the pressure (25 bar) and the space velocity (6410 Ncm3/gcat /h), while the effects of change of temperature (in the range 200-220°C) and syngas molar ratio H2/CO (1.7-2) were investigated. It is observed an increase of the conversion and the selectivity to the light hydrocarbons as the temperature increases; these effects were also achieved in the case of use of a syngas richer in hydrogen. Finally, we developed a kinetic model to study the performance of the sample prepared with glycol, as it provided the best performance; furthermore this catalyst, which does not require the presence of platinum, is more economical and thus, potentially, more interesting for the industrial sector. The kinetic laws that have been employed for modeling are both purely empirical and mechanistic; all regressions have come to convergence, but some of the results were not taken into account due to excessive correlation between the adaptive parameters or because they had no physical sense. In conclusion, it was determined that a mechanistic law of Sarup-Wojciechowski-type is the best to describe the behavior of the catalysts impregnated with glycol.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/122481