Oxidative dehydrogenation of ethane in short contact time reactors
Francesco Donsì - Università degli Studi di Napoli - [2002]
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  • Abstract
    Oxidative dehydrogenation of ethane at short contact times in the last years has been proven to be effective in achieving high yields and selectivities to ethylene. Nevertheless, a number of issues remains unresolved that we discuss in Chapter 1.
    In Chapter 2 we try to provide insights into the role of the catalyst. To do this, we propose a novel non-noble metal catalyst alternative to Pt, which until now is the only reliable catalyst for performance and lifetime. LaMnO3-based catalysts are cheaper than Pt and more thermally stable, and above all perform better than the noble metal. From the experimental results in short contact time reactors we have seen higher ethane conversion and ethylene selectivity on LaMnO3 under a wide range of conditions.
    The role of the catalyst was further investigated by comparison with a blank reactor. Both experimental results and numerical simulations showed that the catalyst is important not only as ignitor to sustain gas phase reaction and to limit their extent to a well defined length, but also to produce the heat to drive the homogeneous dehydrogenation reactions by combustion to CO2 rather than to CO, thus sacrificing less ethane.
    In Chapter 3 we addressed the investigation of different catalysts, alternative to noble metals. We studied mixed catalysts, where Pt is substituted into the perovskite structure, and Pt/CeO2. Experimental results showed good performance of substituted perovskite for the ODH process, as high as on Pt/Sn, but without the highly volatile Sn. Experimental results also suggested that a catalyst suitable for this process is required to oxidize C2H6 under fuel-rich conditions preferentially to CO2 and H2O rather than to syngas. Thus we interpreted the experimental results in terms of C and H oxidation and built a surface mechanism based on these considerations.
    In Chapter 4 the catalyst supports were object of our investigation. We explained the experimentally observed differences among ceramic supports commonly used in short contact time reactors, with the estimation of gas dispersion and in terms of specific geometric surface and pore size. Also the effect of washcoat was critically evaluated.
    In Chapter 5 we investigated the effect of the main operating parameters of the process, such as C2H6/O2, dilution, preheat temperature and flow rate. Particular attention was devoted to fuel addition. As alternative to H2, we proposed CO addition, which on a suitable catalyst active in CO oxidation, such as LaMnO3, yields to significantly improvements in ethane conversion (~10%) and ethylene selectivity (~10%). Also the transient behavior of the system was followed and coke formation investigated.
    Finally, in Chapter 6 we used a mathematical model to provide deeper understanding of the process. With a purely homogeneous model we studied ethylene formation at short contact times, together with the formation of undesired PAH, and the effect of O2. Oxygen presence is important to boost the kinetics of ethylene formation and to provide the heat to drive an autothermal process.
    With a hetero-homogeneous 2-D model, we tried to assess the concurrent phenomena of gas-phase reactions, mass transfer and surface reactions occurring in the system. We proposed a three-zone model, where the first zone is dominated by heterogeneous reactions producing heat to increase gas temperature, in the second zone, still rich in oxygen, hetero-homogeneous reactions occur, and most of ethylene is formed, and in the third one, when homogeneous reactions are faster than mass transfer rate because of temperature, purely homogeneous reactions occur. We also studied the hetero-homogeneous model parametrically with the surface reaction rate and the inlet temperature.
    In chapter 7 we conclude with a summary of the results.
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