Mancino, Gabriella (2014) Advances in short contact time catalytic partial oxidation systems for syngas and olefins production. [Tesi di dottorato]


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Item Type: Tesi di dottorato
Resource language: English
Title: Advances in short contact time catalytic partial oxidation systems for syngas and olefins production
Date: 30 March 2014
Number of Pages: 129
Institution: Università degli Studi di Napoli Federico II
Department: Ingegneria Chimica, dei Materiali e della Produzione Industriale
Scuola di dottorato: Ingegneria industriale
Dottorato: Ingegneria chimica
Ciclo di dottorato: 26
Coordinatore del Corso di dottorato:
Date: 30 March 2014
Number of Pages: 129
Keywords: partial oxidation; sulphur poisoning; structured catalysts
Settori scientifico-disciplinari del MIUR: Area 09 - Ingegneria industriale e dell'informazione > ING-IND/27 - Chimica industriale e tecnologica
Aree tematiche (7° programma Quadro): ENERGIA > Produzione di combustibile rinnovabile
Date Deposited: 10 Apr 2014 20:04
Last Modified: 28 Jan 2015 09:50

Collection description

The catalytic partial oxidation (CPO) of natural gas over noble metal catalysts is an attractive way to obtain syngas (CO and H2) which can be employed for downstream processes to produce synthetic fuels. The preliminary conversion of methane to syngas is also interesting in the context of advanced combustion systems with reduced NOx emissions. Several catalysts have been studied for CPO of methane, but Rh-based systems have established as the best performing catalysts in terms of both activity and selectivity. Sulphur poisoning is a tricky issue for industrial processes, leading to catalysts loss of activity. Recently, the presence of sulphur containing compounds naturally occurring in natural gas or added as odorants necessary for safety reasons, was recognized as a serious drawback for the costly Rh catalysts. Indeed, sulphur adversely affects the catalytic performance during catalytic partial oxidation adsorbing onto Rh active sites, causing a (reversible) suppression of their steam reforming activity, with an associated risk of catalyst overheating during CPO autothermal operation. Therefore the development of catalysts that are intrinsically sulphur tolerant and are not readily poisoned by the amounts of sulphur commonly found in natural gas is desirable. On the basis of recent works that have shown that metal phosphide catalysts have promising hydrodesulphurization (HDS) properties due to a higher sulphur tolerance than the correspondent metal based catalysts, in this work, a novel structured Rh catalyst doped with phosphorous and supported on alumina, has been prepared and investigated during the CPO of methane under self-sustained conditions at short contact time in the presence of sulphur. Results were compared with the reference undoped Rh/γAl2O3, showing a significant enhancement of the specific steam reforming reaction rate of P-doped catalyst, and a higher sulphur tolerance. Both the findings were correlated to the presence of phosphorous, whose interaction with Rh improved the metal dispersion on the support and inhibited the strong sulphur adsorption lowering the resulting surface S coverage. Since natural gas may comprise significant amounts of ethane beside methane, the CPO of ethane is also object of study, in the view of a direct use of natural gas as feed at the on-site gas field without separating respective components. The importance of the role of the gas phase chemistry, which was found negligible in the case of CH4 CPO over Rh and Pt catalysts, increases for higher alkanes following the progressive lower stability of the C-H bond, as in the case of CPO of ethane or propane, when large quantities of ethylene can be formed with Pt based catalysts (but not with Rh). However, the literature data available in the field of sulphur poisoning during CPO of higher hydrocarbon is so far scarce and somehow contradictory. Therefore, in the second part of this PhD thesis, the effect of sulphur addition to the feed during the CPO of ethane for syngas production (C2H6/O2=1) has been investigated over more conventional Rh and Pt catalysts supported on a γ-alumina washcoat anchored to honeycomb monoliths. The findings obtained confirmed the previous results on CPO of methane on Rh: sulphur addition resulted in a rapid and reversible poisoning, depending on its concentration. However the adverse impact of sulphur is much larger on Rh than on Pt. Due to the more complex chemistry of ethane, two main effects related to the presence of sulphur during its CPO have been identified: i) the strong inhibition of the hydrogenolysis of ethane to methane occurring on Rh but not on Pt; ii) the progressive inhibition of steam reforming of both the reactant (ethane) and one of the products, ethylene, whose formation in turn increases after the introduction of sulphur. The latter result was also observed for Pt catalyst. Since ethylene is thought to be mainly formed by the homogeneous oxidative dehydrogenation of ethane, this result suggested the possibility to take advantage of selective sulphur poisoning in order to maximize the formation of ethylene. The oxidative catalytic conversion of light alkanes (in particular ethane) to olefins is an attractive solution to obtain ethylene and propylene, the most important building blocks for the polymers industry. The CPO of ethane to ethylene (stoichiometric feed ratio C2H6/O2=2) is characterized by a complicated chemistry which involves both heterogeneous and homogeneous reaction paths, whose contribution and synergy may be strongly alterated by S-poisoning. In the final part of this PhD work, S-poisoning of catalytic steam reforming of C2H6 and C2H4 has been investigated as a strategy to increase the process selectivity and yield to ethylene during the CPO of ethane to ethylene (C2H6/O2=2) over Rh and Pt catalysts. In other words, the possibility to use sulphur as an intentional selective poison of undesired heterogeneous reactions to boost the production of ethylene has been exploited. At the same time, taking advantage of the selective poisoning of sulphur on catalytic reforming paths, an attempt has been done to shed light on the complex interaction of hetero-homogeneous chemistry available in a C2H6 CPO reactor, to understand the contribution of heterogeneous reforming reactions to the overall performance of the reactor.


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