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Kinetics Of Supercritical Catalytic Cracking Of Hydrocarbon Fuels

Posted on:2011-09-15Degree:DoctorType:Dissertation
Country:ChinaCandidate:X C XianFull Text:PDF
GTID:1101330338483239Subject:Chemical processes
Abstract/Summary:PDF Full Text Request
As the flight speed of aircraft reaches or exceeds the hypersonic speed (e.g. 6 Mach), aerodynamic heating results in severe heat load of the aircraft, thus it is necessary to use the fuel as the primary coolant. The fuel is used as a"heat sink"to remove waste heat from various subsystems and components of the aircraft, and then the fuel can be called as"endothermic fuel". The catalytic cracking of liquid hydrocarbon fuels is a potential endothermic process. Because that the real operating temperature and pressure on board are higher than the critical temperature and critical pressure of general liquid hydrocarbon fuels, the catalytic cracking reaction is carried out under supercritical conditions. In this work, n-dodecane and HBID (highly brached iso-dodecane) were selected as the model fuels to investigate the characteristics of paraffin catalytic cracking over zeolite catalyst under supercritical conditions. Then, a supercritical catalytic kinetic model (S-C model) was developed and applied to the catalytic cracking of paraffins.The different behaviors of initial conversion and catalyst decay of n-dodecane (a model fuel of linear alkanes) catalytic cracking over HZSM-5 and USY zeolites were investigated by changing operating pressures from 0.1MPa to 4.0MPa at supercritical temperature. The results showed that, compared to atmospheric catalytic cracking, the supercritical catalytic cracking of n-dodecane had the following characteristics. (1) Bimolecular cracking mechanism is the dominant reaction mechanism, which results in higher content of alkanes, aromatics and coke in the products, and lower content of alkenes, as well as lower apparent reaction rate constant. (2) The density of reaction fluid increases, which results in the prolongation of residence time and the extraction of coke precursors by supercritical reaction fluid. The characteristic of (2) is significant for the supercritical catalytic cracking over USY zeolite with relatively large pore size, while (1) is significant for that over HZSM-5 zeolite. Based on both monomolecular and bimolecular cracking mechanisms, anAdsorption-Reaction kinetic model was built by considering the competing adsorption between reactant and product on active sites of the catalyst. The relationship between supercritical extraction of coke precursors and the maintenance of catalyst activity was obtained by the analysis of the supercritical extraction process and the effect of coke precursor on catalyst activity. Then the catalyst decay function associating with supercritical catalytic reaction was developed by adding the supercritical extraction term to traditional TOS catalyst decay function, which was involved to account for the catalyst decay due to coke production. The combination of Adsorption-Reaction kinetic model and supercritical catalyst decay function makes up a complete kinetic model accounting for the supercritical catalytic cracking of paraffin reactant over acid zeolite catalyst (S-C model). The S-C model was applied to the supercritical catalytic cracking of n-dodecane over HZSM-5 zeolite, and the model parameters were obtained by nonlinear fitting. Then a series of statistical analysis were conducted, which verify the significance of parameters estimation. According to the model parameters obtained from 400, 420 and 450℃, the apparent activation energy of n-dodecane supercritical catalytic cracking over HZSM-5 zeolite and supercritical adsorption heat of n-dodecane on HZSM-5 zeolite were calculated, which were 125.4 and 109.5 kJ/mol, respectively. Finally, the contribution ratio of supercritical extraction to maintenance of catalyst activity (CRSE) was defined and calculated using S-C model.The characteristics of highly branched alkane catalytic cracking over acid zeolite catalyst under supercritical condition were investigated by choosing HBID as a model fuel. First, the catalytic activities of HZSM-5,Al-MCM-41 and USY zeolite were compared, and the results showed that USY zeolite was a proper catalyst with appropriate pore size and acid distribution. Therefore, USY was chosen as the catalyst in the following investigation of HBID. Results of experiments under different operating pressures showed that supercritical condition could promote the initial conversion in some extent and significantly enhance the stability of catalyst. A series of kinetic experiments were carried out for the supercritical catalytic cracking of HBID over USY zeolite at 450℃under 4.0MPa. The fitting results of S-C model were acceptable, which indicated that S-C model could also be applied to the catalytic cracking reaction system of branched alkanes over USY zeolite. Additionally, the parameter accounting for the competing adsorption could be removed for simplification.The supercritical catalytic cracking reactions over HZSM-5 zeolite at 400℃under 4.0MPa were preliminarily investigated using binary mixtures of n-dodecane/HBID, n-dodecane/MBID (mono-branched iso-dodecane) and tertiary mixture of MBID/HBID/n-dodecane. The results show that HBID and MBID both can enhance the catalytic cracking of n-dodecane component, and the effect of former one is more significant. HBID itself can not cracking over HZSM-5 zeolite, and the coversion of MBID is also lower than that of n-dodecane. Therefore, there is an optimal value of the content of the two iso-dodecanes in binary mixtures with n-dodecane, which are approximate 25%. During the tertiary mixture, the component of HBID could also enhance the conversion of both n-dodecane and MBID components.
Keywords/Search Tags:Endothermic fuel, Catalytic cracking, Supercritical, Zeolite, Catalyst decay, Kinetic model
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