Modeling And Optimization Of Dehydrogenation-regeneration-selectivity Hydrogen Combustion For Propane Dehydrogenation To Propene

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.