The aim of this thesis is to perform a detailed investigation of the homogeneous-heterogeneous interaction by means of reaction path analysis and to assess the adequacy of reactor modelling in the context of oxidative coupling of methane. The OCM is still under development but it is foreseen as a promising source of light hydrocarbons, mainly ethane and ethylene, exploiting natural gas as raw material. Through this computational thesis work, we want to investigate the effect of operating conditions and of thermal control by modeling the catalytic reactor for the OCM process with a one-dimensional model, which is then validated on the basis of a detailed CFD analysis. The results obtained through the one-dimensional model allow for assessing the range of operating conditions which maximizes the process yield. However, it is impossible to define unique set of operating conditions due to the significant influence of each variable on the other ones. Nevertheless, temperatures around 1100 K, high pressures and high catalytic load with sufficient gas-phase volume have positive effects on the reacting system. Moreover, the comparison between the results obtained through the one-dimensional model and the CFD allow for the validation of the one-dimensional model to describe the system in chemical regime, despite small deviations are experienced. These small deviations are investigated through an analysis of transport phenomena, which highlighted the significant influence of the coupling between the two kinetic mechanisms also at the modeling-scale. An analysis of the kinetic mechanism (RPA) is performed to complete the study and to understand in detail the main reaction paths. In addition to the assessment of dominant reactions, the key role of the gas-surface coupling is a further result of the RPA. The gas-surface coupling influences not only the conversion of methane to ethylene, but also the selectivity of ethylene with respect to other by-products. Although many issues still remain unresolved, this thesis work provides several contributions which can aid in defining the configuration of a catalytic reactor for the OCM process.
Lo scopo di questa tesi è di analizzare nel dettaglio l’interazione tra il meccanismo omogeneo ed eterogeneo, mediante l’analisi dei percorsi di reazione e la valutazione dell’adeguatezza del modello reattoristico per il processo di accoppiamento ossidativo del metano. L’OCM è un processo ancora in via sperimentale che si propone di produrre idrocarburi leggeri, principalmente etano ed etilene, utilizzando gas naturale come materia prima. Con questo lavoro di tesi investighiamo l’effetto delle condizioni operative e del controllo termico, simulando il reattore catalitico per l’OCM tramite un modello monodimensionale che viene poi validato a seguito di una dettagliata analisi CFD. Dai risultati del modello monodimensionale si individuano i range per le variabili operative che massimizzano la resa del processo, anche se una definizione assoluta risulta impossibile a seguito della forte influenza che certe variabili esercitano sulle altre. Tuttavia temperature nell’ordine di 1100 K, alte pressioni, elevati carici catalitici con sufficiente volume di fase gas, hanno un effetto positivo sul sistema. Inoltre, il confronto tra i risultati del modello monodimensionale con quelli ottenuti dalla CFD, consente di validare l’utilizzo del primo per la descrizione del sistema, nonostante piccoli discostamenti siano presenti. Queste piccole deviazioni vengono investigate attraverso una dettagliata analisi del reattore, evidenziando la fondamentale importanza dell’effetto dell’accoppiamento tra i due meccanismi cinetici anche a livello modellistico. Per completare lo studio e comprendere in dettaglio la chimica del processo, viene eseguita un’analisi del meccanismo cinetico attraverso lo studio dei principali percorsi reattivi (RPA). In aggiunta alla delineazione delle principali reazioni, desiderate e non, ancora una volta risulta evidente la marcata importanza dell’accoppiamento homo-hetero, determinante non solo per la conversione del metano a dare etilene, ma anche per la selettività dell’etilene rispetto ad altri sottoprodotti. Nonostante restino ancora aperti molti degli interrogativi sul sistema in analisi, questo lavoro di tesi fornisce nuove conoscenze importanti che possono essere usate come punto di partenza per definire la configurazione di un reattore catalitico per l’OCM.
Oxidative coupling of methane : assessment of homogeneous-heterogeneous reaction pathways and adequacy of reactor models
PECCHI, CHIARA;PACCIANI, CLAUDIA
2016/2017
Abstract
The aim of this thesis is to perform a detailed investigation of the homogeneous-heterogeneous interaction by means of reaction path analysis and to assess the adequacy of reactor modelling in the context of oxidative coupling of methane. The OCM is still under development but it is foreseen as a promising source of light hydrocarbons, mainly ethane and ethylene, exploiting natural gas as raw material. Through this computational thesis work, we want to investigate the effect of operating conditions and of thermal control by modeling the catalytic reactor for the OCM process with a one-dimensional model, which is then validated on the basis of a detailed CFD analysis. The results obtained through the one-dimensional model allow for assessing the range of operating conditions which maximizes the process yield. However, it is impossible to define unique set of operating conditions due to the significant influence of each variable on the other ones. Nevertheless, temperatures around 1100 K, high pressures and high catalytic load with sufficient gas-phase volume have positive effects on the reacting system. Moreover, the comparison between the results obtained through the one-dimensional model and the CFD allow for the validation of the one-dimensional model to describe the system in chemical regime, despite small deviations are experienced. These small deviations are investigated through an analysis of transport phenomena, which highlighted the significant influence of the coupling between the two kinetic mechanisms also at the modeling-scale. An analysis of the kinetic mechanism (RPA) is performed to complete the study and to understand in detail the main reaction paths. In addition to the assessment of dominant reactions, the key role of the gas-surface coupling is a further result of the RPA. The gas-surface coupling influences not only the conversion of methane to ethylene, but also the selectivity of ethylene with respect to other by-products. Although many issues still remain unresolved, this thesis work provides several contributions which can aid in defining the configuration of a catalytic reactor for the OCM process.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/135893