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Oxidative dehydrogenation of ethane in short contact time reactors

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Chapter 1 Introduction 6 Huff and Schmidt, 1996; Bodke et al., 2000) and can be regulated by the appropriate choice of the active component (Schmidt and Huff, 1994), support morphology (Bodke et al., 1998), and reaction mixture composition (Huff and Schmidt, 1993). This seems to be true only at moderate temperatures, while above 800°C, it has been shown, both experimentally (Lødeng et al., 1999; Beretta et al., 2000; Beretta et al., 2001a and b; Mulla et al., 2001; Huff et al., 2000) and theoretically (Huff et al., 2000; Beretta et al., 2001b; Zerkle et al., 2000), that the kinetics of homogeneous reaction paths is not negligible, even for millisecond residence time scales. The alternative possibility to a purely heterogeneous mechanism of ethylene production is a hetero-homogeneous reaction path. As proposed in recent works (Lødeng et al., 1999; Mulla et al., 2001), the catalyst could be assumed to act mostly as an igniting agent, active for total oxidation reactions, while ethylene-forming reactions mainly occur in the gas phase. In the same way, Bodke at al. (2000) sketched a short contact time reactor as a 2 zones reactor where in the first zone oxygen depletion occurs through C 2 H 6 oxidation to CO x and H 2 O with consequent temperature increase, while in the second zone, where O 2 is absent, the endothermic dehydrogenation of ethane to ethylene and H 2 takes place. Moreover, the authors also reported that the use of hydrogen co-feeding allows a minor ethane consumption in the first zone of the reactor, with H 2 being oxidized easier than ethane over the Pt-Sn surface, and consequently C 2 H 6 dehydrogenates in the second zone with very high selectivity (>80%) and yield (about 60%) to ethylene. Detailed heterogeneous and homogeneous chemical kinetic mechanisms employed in the two-dimensional computational fluid dynamics model developed by Zerkle et al. (2000) confirmed the main scheme of Bodke et al. and evidenced that the proportion between the contribution of heterogeneous and homogenous reactions to the ethane overall consumption is strongly dependent on C 2 H 6 /O 2 feed ratio. More specifically, they reported that the formation of ethylene must be mainly attributed to heterogeneous dehydrogenation paths, especially when H 2 is added to the feed. The use of materials other than noble metals has been considered by some authors (Mulla et al., 2002; Flick and Huff, 1999; Beretta and Forzatti, 2001). Flick and Huff proposed the use of Cr 2 O 3 based foam monoliths in ethane ODH (1999), showing that they exhibit very promising performances, even better than Pt-based monoliths, but with a strongly limited catalyst life-time, likely due to the well-known deactivation processes occurring on transition metal oxides in the catalytic combustion of hydrocarbons (i.e. under operating conditions characterized by high temperatures, oxidizing atmosphere and the presence of water and CO 2 ).

Anteprima della Tesi di Francesco Donsì

Anteprima della tesi: Oxidative dehydrogenation of ethane in short contact time reactors, Pagina 6

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.