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Melchiori, Tommaso (2014) Study of non catalytic gas-solid reactions: development of a single particle model. [Tesi di dottorato]

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Abstract (inglese)

This thesis investigates single particle models to describe non catalytic gas-solid reactions. A comparative study was made between the traditional shrinking core model and more detailed continuous models, involving the solution of microscopic balances for the solid and gas phases inside a single porous particle. Such a study proved that in some cases the use of the shrinking core model can lead to severe errors in the prediction of conversion, and that kinetic parameters in SCM are affected by the particle size. Different diffusion models were tested for the continuous model, and the inaccuracy of the Fick law compared to multicomponent Stefan-Maxwell was evaluated, depending on the concentration of the reaction gas in the mixture. The thesis also proved that natural convection inside the particle can be neglected by changing the balance from mass to molar basis or vice versa, depending on the type of reaction considered. An equation for the local particle porosity was also included, to account for the local changes of gas diffusivity as effect of the reaction. The effect of the pore size distribution was studied, by writing the particle model as a population balance, including different diffusive resistances for different pore sizes, for the cases when Knudsen or solid state diffusion can be important. Sintering phenomena were included, by extending the grain model with an empiric equation. Simulations with simultaneous gas solid reactions were performed, also considering non uniform initial distributions of the solid phases inside the particle: sensitivity studies proved that the position of the solid reagents in the particle may have a great influence on the model results, even when intra particle diffusion is fast compared to the chemical reactions. Gas-solid models were also used to simulate real processes. In particular, thanks to collaboration with an industrial research project, a kinetic study with a CFD model was developed, applying a shrinking core model to simulate real reactors for the direct reduction of iron ores with syn gas at high temperature and pressure. Finally, thanks to the collaboration with the Technical University of Eindhoven, a continuous model was used to simulate reactions of reduction of iron-titanium oxides in chemical looping combustion processes, comparing the results with experimental data.

Abstract (italiano)

Questa tesi investiga modelli di singola particella per descrivere reazioni gas-solido non catalitiche. E’ stato fatto uno studio comparativo fra il tradizionale shrinking core model e modelli continui più dettagliati che comprendono la risoluzione dei bilanci microscopici per le fasi gas e solida dentro una singola particella porosa. Tale studio ha provato che in alcuni casi lo shrinking core model può condurre ad errori importanti nella predizione della conversione, e che i parametri cinetici nel SCM dipendono dalla dimensione della particella. Sono stati testati diversi modelli di diffusione all’interno del modello continuo, e la non accuratezza della legge di Fick rispetto alla Stefan-Maxwell multicomponente è stata valutata, a seconda della concentrazione del gas reagente nella miscela. La tesi prova anche che la convezione naturale all’interno della particella può essere trascurata cambiando i bilanci da massivi a molari o vice versa, a seconda del tipo di reazione considerata. Un’equazione che descrive la porosità locale della particella è stata inclusa al modello, per tener conto dei cambiamenti della diffusività effettiva del gas per effetto della reazione. L’effetto della distribuzione della dimensione dei pori è stato investigato, riscrivendo il modello di particella come bilancio di popolazione, includendo diverse resistenze diffusive per diverse dimensioni dei pori, per i casi in cui La diffusione di Knudsen o la diffusione in stato solido possono essere importanti. Fenomeni di sinterizzazione sono stati inclusi, estendendo il tradizionale grain model con un’equazione empirica. Sono state fatte simulazioni di reazioni gas solido con più reazioni, considerando anche distribuzioni disomogenee delle fasi solide all’interno della particella: studi di sensitività hanno dimostrato che la posizione dei reagenti solidi nella particella possono avere un effetto importante sui risultati del modello, anche nel caso in cui la diffusione all’interno della particella è veloce rispetto alle reazioni chimiche. Modelli di reazione gas-solido sono stati usati anche per simulare processi reali. In particolare, grazie alla collaborazione con un progetto di ricerca industriale, uno studio cinetico con modelli CFD è stato sviluppato, applicando lo shrinking core model per simulare reattori reali per la riduzione diretta di ossidi di ferro con gas di sintesi ad alte temperature e pressioni. Infine, grazie alla collaborazione con l’Università Tecnica di Eindhoven, un modello continuo è stato usato per simulare reazioni di riduzione di ossidi di ferro-titanio in processi di chemical looping combustion, confrontando i risultati con i dati sperimentali.

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Tipo di EPrint:Tesi di dottorato
Relatore:Canu, Paolo
Dottorato (corsi e scuole):Ciclo 26 > Scuole 26 > INGEGNERIA INDUSTRIALE > INGEGNERIA CHIMICA, DEI MATERIALI E DELLA PRODUZIONE
Data di deposito della tesi:27 Gennaio 2014
Anno di Pubblicazione:27 Gennaio 2014
Parole chiave (italiano / inglese):gas-solid reactions, single particle models, porous particles, shrinking core model, chemical looping combustion, iron oxides
Settori scientifico-disciplinari MIUR:Area 09 - Ingegneria industriale e dell'informazione > ING-IND/24 Principi di ingegneria chimica
Area 09 - Ingegneria industriale e dell'informazione > ING-IND/23 Chimica fisica applicata
Struttura di riferimento:Dipartimenti > Dipartimento di Ingegneria Industriale
Codice ID:6402
Depositato il:19 Mag 2015 17:04
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