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Oxidative dehydrogenation of ethane in short contact time reactors

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Chapter 1 Introduction 8 1.3 Understanding the oxidative dehydrogenation of ethane in short contact time reactors Initially, the process was claimed to occur by a pure heterogeneous reaction path, based on the experimental observation of large differences in performance among different noble metals (Torniainen et al., 1994). Huff and Schmidt (1996) proposed a heterogeneous model to describe ethane dehydrogenation on a Pt catalyst in 23 steps. Ethylene formation was assumed to occur on the surface following ethane dehydrogenation to ethyl and further dehydrogenation to ethylene. CO x and H 2 O formation was accounted for through the decomposition of ethylene to C and H and following oxidation of these species. All the chemistry was assumed to occur on the catalyst surface, and the gas phase reactions played no role. The validity of the results of this simulation may be diminished by the consideration that at the high temperature predicted in the reactor (~1000ºC) significant homogeneous reactions may occur. The experiments and the simulations carried out by Beretta et al. (2000, 2001a and b) suggested that at low temperature on a Pt catalyst only CO x and H 2 O are formed. At higher temperature, the ethylene yield observed at the exit of the Pt reactor can be entirely attributed to the gas phase reactions. The same authors claimed that the catalyst could be detrimental to the process, reducing the maximum attainable ethylene yield due to the formation of oxidized species (Beretta et al., 2001b). Even with homogeneous reactions, the catalyst is extremely important in igniting and sustaining the gas phase reactions (Mulla et al., 2001) and making the process feasible at short contact time. Henning and Schmidt (2002) performed a thorough investigation of the role of the homogeneous reactions in the ODH process. Downstream sampling of a Pt catalyst showed that most of the ethylene is formed in the gas phase. The catalyst is effective in oxidizing a part of ethane to CO x and H 2 O, developing the heat necessary to sustain the gas phase reactions. At the exit of the catalyst, a fraction of O 2 (∼70%) and a smaller fraction of ethane (∼40%) were consumed, while ethylene was mainly formed downstream through fast exothermic reactions sped up by the residual oxygen content of the stream, which also produces more heat. The addition of H 2 to the mixture reduced the amount of ethane consumed and ethylene formed at the exit of the monolith. Once the O 2 was consumed, slower endothermic reactions took place, leading to the formation of more dehydrogenation products. In these experiments, the back heat shield was removed, therefore significantly increasing the

Anteprima della Tesi di Francesco Donsì

Anteprima della tesi: Oxidative dehydrogenation of ethane in short contact time reactors, Pagina 8

Tesi di Dottorato

Dipartimento: Ingegneria Chimica

Autore: Francesco Donsì Contatta »

Composta da 194 pagine.


Questa tesi ha raggiunto 1055 click dal 04/11/2004.


Consultata integralmente 5 volte.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.