The CFD Modeling In Radial Flow Moving Bed Reactor And Its Application Of Catalytic Pyrolysis Reaction

Recently, radial flow moving bed reactor (RFMBR) is selected for the technology of catalytic pyrolysis of light hydrocarbons with the characteristic of long catalyst life cycle due to its high flow capacity, low pressure drop, wide variation in solids residence time, and continuous regeneration of catalyst particles. However, the inhomogeneous distributions of flow, heat transfer and species yield/conversion can be found in axial and radial directions in RFMBR due to complex structure and catalytic pyrolysis reaction. Therefore, it is important to establish a complete gas-solid two phase flow reaction model to analyze the flow and catalytic pyrolysis of light hydrocarbons behaviour in RFMBR.In this study, a complete3D gas-solid two phase flow CFD model is developed and used to simulate the flow fields in annular RFMBRs with different flow configurations at cold model firstly. The results show that the simulated data using the two-phase CFD model are in better agreement with the experimental data than the single-phase CFD model. The inhomogeneous flow distributions always exist in all the four type RFMBRs, and the flow is more uniform in RFMBRs with CF flow configuration. In addition, the bed voidage has a great effect on the flow fields. The normalized radial pressure drop decreases with the increase of the bed voidage, which leads to more inhomogeneous distributions of flow fields. Furthermore, a higher gas velocity plays the most important role to generate the cavity. The size of the cavity increases with the increase of gas velocity and it also appears a "generation-growth-collapse" process with the flow proceeding.Secondly, based on the cold model above, a thermal CFD model coupled with six lump reaction kinetic model is established and used to simulate the gas-solid translation and catalytic pyrolysis behaviour in RFMBR with CP-z flow configuration at thermal model. Then, using this thermal model, the effects of some key operation parameters and reactor structures are also evaluated and optimized numerically. Based on these simulations, there exists a good heat transfer performance between gas and solid phase in the catalyst bed, while the temperature profiles and the species yield/conversion distributions are inhomogeneous along the axial direction of CP-z reactor. Moreover, the results indicate that product yields are more sensitive to the reaction temperature than to the dilution rate and the reaction residence time. A initial catalyst temperature of923.15K, a width of catalyst bed in range of0.119m-0.139m and a dilution rate of3are preferable for a higher low carbon olefins yields in this simulation. Besides, for the Z-type centripetal flow RFMBR, the annular tube using an inverted cone structure is positive to improve the uniformity of flow distribution and the low carbon olefins yields.Finally, the study of the flow behavior of gas-solid two phase flow fluid in rectangular radial flow moving bed reactor is conducted as well. The results show that the cavity phenomenon appears on the top of catalyst bed as increasing the superficial gas velocity. And the cavity size increased with the increase of the superficial gas velocity. In addition, bed voidage and solid-seal height are the key factors to affect the formation of cavity phenomenon. Increasing the bed voidage appropriately and keep a high solid-seal height both can effectively eliminate the cavity phenomenon, which ensure the operating flexibility and stability of rectangular RFMBR. Besides, for the RFMBR, using a trapezoidal structure instead of rectangular structure is also useful to inhibit the formation of cavity phenomenon.