【作者】 张新平；

【导师】 袁渭康； 周兴贵；

【作者基本信息】 华东理工大学 ， 过程系统工程， 2010， 博士

【摘要】 近年来丙烯生产能力的增长远远低于丙烯需求的增长,丙烷脱氢是增产丙烯的一条重要技术路径。丙烷脱氢氧化制丙烯是指在丙烷脱氢后,对反应过程中产生的氢进行选择性氧化的过程,与传统的催化脱氢相比,不但能提高平衡转化率,还能够实现反应体系的自热或放热操作。但脱氢与氢选择性氧化的耦合增加了过程的复杂性。本研究针对丙烷脱氢氧化制丙烯技术,对脱氢、氢选择性氧化和再生反应过程开展模拟和优化研究工作,目的是要解决反应器优化设计与放大及反应-再生和脱氢-氢选择性氧化组合工艺优化的关键工程问题,为丙烷脱氢氧化新技术的开发提供理论依据。本文大致可分为四个部分：一是脱氢过程的模型化与模拟研究,在Cr2O3/Al2O3催化剂和Pt-Sn/Al2O3催化剂脱氢动力学的基础上,分别建立了催化剂失活时的轴向和径向一维固定床绝热脱氢反应器动态模型。模拟显示脱氢催化剂的活性下降迅速。虽然等温操作时在反应初期C3H8的转化率较绝热操作要高,但因其反应器整体温度更高,催化剂的活性下降更快,导致C3H8的转化率和C3H6的选择性也下降更快。二是催化剂再生过程的模型化与模拟研究,应用内扩散效率因子修正的均匀模型,在研究了内外扩散影响计算方法的基础上,建立了综合考虑内外扩散影响的轴向与径向绝热固定床一维非均相烧焦动态模型。模拟显示,对于有催化烧焦作用的Cr2O3/Al2O3催化剂,由于外扩散的影响,催化剂颗粒温度与气相主体温度有显著的差别并造成催化剂颗粒多态的存在,降低氧浓度或提高气体流速能够消除多态现象的发生；当烧焦再生过程受外扩散控制时,烧焦首先在反应器入口处完成,外扩散影响减弱时,则首先完成烧焦再生的位置向后移动直至反应器出口；对于烧焦速率很慢的Pt-Sn催化剂,气相主体温度和催化剂颗粒温度相差很小,床层各处结焦量基本上以同一速率下降；不管是Cr2O3/AlO3催化剂还是Pt-Sn催化剂,实行反转烧焦操作并没有明显的优势。反应器出口压力相同时,脱氢过程径向反应器丙烯的选择性和收率都高于轴向反应器。但再生过程轴向反应器烧焦速度要快于径向反应器。采用径向反应器常压烧焦是不利于整个丙烷-脱氢再生周期的,要想使径向反应器有竞争力,必须提高再生过程反应器的入口压力。三是脱氢单元和再生单元的动态优化以及在此基础上的反应-再生周期优化。针对再生过程优化目标-再生时间不确定的特殊性,提出了两种动态优化方法。两种优化方法的结果很接近,但第二种优化方法能大大减少优化执行的时间；合理的优化方案是在氧浓度上限优化反应器入口温度,它不但得到了优化问题的最优解,而且也降低了优化问题的复杂性,节省了优化时间；通过反应-完全再生和反应-不完全再生两种操作模式的分层优化,得到了最优残余焦水平,最佳脱氢反应时间和反应/再生过程操作参数三个层次的优化值。结果表明反应-不完全再生模式是最佳的操作模式,最优的再生后残余焦含量为0.00071 g coke/g cat,最优的脱氢反应时间为12分钟,对应的烧焦再生时间为9.27分钟。四是氢选择性氧化过程的模型化与模拟及脱氢-氢选择性氧化耦合过程的模拟与优化。建立了氢选择性氧化反应器数学模型并实现了脱氢-氢选择性氧化耦合过程的模拟和优化。模拟显示耦合级数越大,总转化率、选择性和收率随时间下降越快。稳态优化显示将氢全部烧掉是最优的操作模式。而动态优化显示当采用多级氢选择性氧化操作时,在反应后期氢选择性氧化反应器入口氧氢比有所降低。通过这样层层递进的研究,不但对脱氢、再生和氢选择性氧化过程有了深刻的认识,而且还获得了过程的最优操作参数和最优的反应-再生-氢选择性氧化系统操作模式。研究的结果将对反应器的设计和过程的开发起到重要的指导作用。更多还原

【Abstract】 The increasing demand for propene derivatives has produced a correspondingly heavy increase in propylene demand during the last 20 years. Propane dehydrogenation is believed to have a great potential as a propene booster in the future. In contrast to conventional catalytic dehydrogenation, alternative approach for obtaining higher-than-equilibrium olefin yields and for making the overall process thermoneutral or exothermic is combination processes of catalytic dehydrogenation (DH) and selective hydrogen combustion (SHC). But the process becomes complex due to coupling of dehydrogenation and selective hydrogen combustion.In this study, simulation and optimization are carried out to the reaction process of the dehydrogenation, the selective hydrogen combustion and the regeneration of coked catalyst. The purpose is to solve reactor optimization design and scale up, the process optimization of the reaction-regeneration and the dehydrogenation-selective hydrogen combustion, so as to provide theoretical reference for new technology development of propane dehydrogenation oxidation. This paper can be divided into four parts:First, modeling and simulation of the dehydrogenation process:based on dehydrogenation kinetics of Cr2O3/Al2O3 catalyst and Pt-Sn/Al2O3 catalyst, the one-dimensional dynamic adiabatic fixed bed reactor models of axial and radial are established respectively with catalyst deactivation. Simulations show the rapid decline of catalyst activity. Although C3H8 conversion is higher at initial reaction stage under the isothermal operation than the adiabatic operation, higher overall temperature and the faster decrease of the catalyst activity result in faster decrease of the C3H8 conversion and C3H6 selectivity under the isothermal operation.Second, modeling and simulation of the catalyst regeneration process:an effectiveness factor modified coke burning-off physical model is used to establish the axial and radial heterogeneous dynamic reactor model for regeneration process by considering the pore and film diffusion influence. Simulations show that, due to the catalytic effect of Cr2O3/Al2O3 catalyst on the coke combustion, multi-steady states exist for the catalyst pellets and the catalyst temperature is sensitive to gas temperature caused by the film diffusion influence, however, at increased mass flow rate or lowered oxygen concentration, multi-steady states will not appear. Under the strong influences of film diffusion, the coke in the packed bed reactor will first be exhausted at the inlet, while if the film diffusion resistance is decreased, the position of first coke exhaustion moves toward the outlet of the reactor. For Pt-Sn catalyst, the difference between the gas temperature and the catalyst temperature are very small because of the very slow coke combustion rate, coke decline at the same rate basically at different location of the bed. Whether the Cr2O3/Al2O3 catalyst or the Pt-Sn catalyst, the reverse coke combustion operation has no obvious advantage.Keeping the same outlet pressure, the yield and selectivity of C3H6 at the radial reactor is higher, however, the regeneration time is shorter at the axial reactor. Therefore, increase of the inlet pressure of radial reactor is needed so that the average yield of the radial reactor exceeds the axial reactor in the entire reaction-regeneration cycle.Third, dynamic optimization of the dehydrogenation and the regeneration units and the optimization of reaction-regeneration cycle:two kinds of dynamic optimization methods are proposaled for the regeneration process due to the specificity of uncertain regeneration time. The results of two kinds of optimization methods are closed, but the second optimization method can significantly reduce the implementation time of the optimization. Reasonable optimization strategy is to optimize the inlet temperature at upper limit of oxygen concentration, which not only get the optimal solution but also reduces the complexity and save optimization time. Through hierarchical optimization to two operation modes of reaction-complete regeneration and reaction-incomplete regeneration, the optimization value at three levels has obtained-the optimal residual coke content, the best dehydrogenation time and the optimal operating parameters of reaction/regeneration unit. The results show that the optimal operation mode is reaction-incomplete regeneration, the optimal residual coke content is 0.00071 g coke/g cat, the best dehydrogenation time is 12 minutes and the corresponding regeneration time is 9.27 minutes.Fourth, modeling and simulation of the SHC and simulation and optimization of DH-SHC coupling process:a model of the SHC is established, simulation and optimization are realized to DH-SHC process. Simulations show that the more DH-SHC stage, the total conversion of C3H8, the selectivity and yield of C3H6 decrease faster with time. Steady-state optimization shows that the optimal operation mode is that all the hydrogen is combustion. But dynamically optimization shows that, to multi-stage DH-SHC operation, the ratio of O2 to H2 of SHC reactor need decrease at the later period of reaction.Through this research, there is more understanding to the dehydrogenation, regeneration and selective hydrogen combustion processes, further, the optimal operating parameters of these processes and the optimal operation mode of the reaction-regeneration-selective hydrogen combustion are obtained. The results are helpful to the reactor design and the process development.更多还原