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Oxidative dehydrogenation of ethane in short contact time reactors

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Chapter 1 Introduction 4 ODH. These catalysts are characterized by basic sites and comprise ions or metal oxides of the group IA and IIA (Cavani and Trifirò, 1995). In our research group we deeply investigated both types of catalysts, and in particular the second one. From experimentation on basic oxides (promoted and non-promoted MgO, Sm 2 O 3 and La 2 O 3 ) used in the ODH reaction, as reported in a recent work conducted in our laboratories (Ciambelli et al., 2000; Ruoppolo, 2000), it was found that for temperatures above 700°C the selectivity of the process towards ethylene formation seems to be substantially independent on the composition of the catalyst, and is determined by gas-phase reactions. Thus, none of the catalysts investigated for oxidative dehydrogenation of ethane in conventional reactors seemed to be significant in large-scale applications, due to the limitations in yield and selectivity occurring by decreasing the dilution and increasing the ethane to oxygen ratio in the passage from lab scale to industrial scale (Cavani and Trifirò, 1998). 1.2.2 ODH in short contact time reactors In the second approach the development of new catalytic systems was joined to novel reactor configurations, namely short contact time reactors. The typical contact times in this kind of reactors are of the order of the milliseconds, and can be considered short with respect to the order of the residence times of comparable industrial processes (for instance in steam cracking the residence time is ~1 s). The major advantage of the use of short contact time reactors is that the residence time may be closely tuned on the characteristic time of the reactions to the desired products, while the reactions to undesired but thermodynamically favored products do not significantly occur due to the short time scale. In conclusion in a short contact time reactor high yields to non- equilibrium products may be achieved. In addition, since a short contact time reactor is operated at high space velocity, it works under quasi-adiabatic conditions, due to the high rate of heat production (high flow rate) and the low heat dispersion (little reactor dimensions). The particular temperature profile (with temperature gradients up to 10 6 K/cm) developing in such kind of reactors may also be significant in determining product distribution. In comparison with fixed beds, a number of issues may be overcome in short contact time reactors. At high flow rates, the radial and axial dispersion is lower (high number of Peclet), hence the radial temperature gradient is reduced and a uniform temperature may be

Anteprima della Tesi di Francesco Donsì

Anteprima della tesi: Oxidative dehydrogenation of ethane in short contact time reactors, Pagina 4

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.