【作者】 羡小超；

【导师】 米镇涛；

【作者基本信息】 天津大学 ， 化学工艺， 2010， 博士

【摘要】 当现代飞行器的飞行速度达到或超过高超音速(6马赫)时,气动热导致的飞行器的热负荷变得非常严重,因此,可采用进入燃烧室前的“吸热燃料”作冷却剂对飞行器各子系统进行主动冷却。液体碳氢燃料的催化裂解反应是强吸热过程,其作为“吸热燃料”具有很大的发展潜力。在飞行器的燃料系统较高的工作温度和压力下,碳氢燃料处于超临界状态。本文以正十二烷和含多支链的异十二烷(HBID)为模型燃料,对碳氢燃料中烷烃组分的超临界催化裂解特性进行研究,建立超临界催化裂解反应动力学模型(S-C模型),并采用S-C模型进行动力学研究。以正十二烷作为直链烷烃的模型燃料,HZSM-5和USY分子筛为催化剂,研究不同压力条件下催化裂解反应的初始转化率和催化剂的失活。研究表明,与常压条件相比,超临界催化裂解反应具有以下特点:(1)反应以双分子裂解为主,产物中烷烃、芳烃和积炭增加,烯烃减少,裂解反应的表观反应速率常数降低;(2)反应流体密度增大,反应物停留时间延长,流体对积炭前驱体的呈现萃取特性。在孔径较大的USY分子筛上的超临界催化裂解反应具有明显的特征(2),而在HZSM-5分子筛上则具有明显的特征(1)。根据烷烃催化裂解的单分子和双分子反应机理,并考虑反应物与产物在催化剂活性位上的竞争吸附,建立了吸附——反应动力学模型。通过对超临界反应流体萃取积炭前驱体过程的分析,以及积炭前驱体与催化剂活性的关系,建立了超临界萃取与催化剂失活的关系;并以TOS(time on stream)失活函数来反映因生成积炭而导致的催化剂失活,其与积炭前驱体的超临界萃取项相结合构成了超临界催化剂失活函数。动力学模型与失活函数共同组成了烷烃超临界催化裂解反应的S-C模型。将S-C模型应用于正十二烷在HZSM-5分子筛上的超临界催化裂解反应,通过非线性拟合得到了模型参数,并通过一系列的统计检验证明了模型参数的显著性。然后根据400℃、420℃和450℃时得到的模型参数,计算了正十二烷在HZSM-5分子筛上超临界催化裂解反应的表观活化能为125.4kJ/mol,正十二烷在HZSM-5分子筛上的超临界吸附热为109.5kJ/mol。定义并根据S-C模型模拟计算了超临界萃取对催化剂活性保持的贡献率(CRSE)。以HBID为模型燃料,研究了多支链取代烷烃超临界催化裂解反应的特点。研究过程中首先对催化剂进行筛选。HZSM-5、USY和Al-MCM-41三种类型分子筛中,USY因其合适的孔径和酸性,被用作该反应体系的催化剂。不同压力条件下反应结果表明,超临界催化裂解反应能在一定程度上提高初始转化率,并显著提高催化剂活性的稳定性。在450℃、4.0MPa的超临界条件下,进行HBID的催化裂解的动力学实验,并采用S-C模型对实验数据进行拟合。结果表明S-C模型适用于HBID在USY分子筛上的催化裂解的反应体系,并且模型中表示竞争吸附的参数可简化掉。在400℃、4.0MPa的条件下,以HZSM-5分子筛为催化剂,对正十二烷/HBID、正十二烷/MBID(含单支链的异十二烷)双元混合燃料,以及MBID/HBID/正十二烷三元混合燃料,进行了超临界催化裂解的初步研究。结果表明:HBID和MBID均对混合物中的正十二烷组分的裂解有促进作用,且前者的促进作用更显著。这是由于HBID自身不能进入分子筛孔隙内部进行裂解反应,而MBID能发生裂解,但转化率远低于正十二烷组分,因此异十二烷与正十二烷的复配存在一个最优配比,对两种异十二烷而言最优值均在25%左右。在三元混合燃料中HBID对MBID的裂解也有一定的促进作用。更多还原

【Abstract】 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.更多还原