Numerical Simulation Of Gas-Solid Flow For Propylene Polymerization In Multizone Fluidized Bed Reactor

Recently a novel gas-phase technology for polyolefin production is under development by Basell Inc., Italy. The innovative technology uses a multi-zone circulating reactor (MZCR), which is similar to circulating fluidized bed reactor (CFBR), to produce olefin polymers with onion ring structure. This has greatly improved the phase distribution, the homogeneity and properties of production. Typical core-annular radial flow structure and heterogeneous axial one which mostly affect the performance of production exist in the riser of CFBR. The dynamic behaviour of the riser is complex. This work has developed numerical simulation methods to investigate fluid and solid behaviour using computational fluid dynamics (CFD) method. Based on the dynamics behaviours of fluid catalytic cracking (FCC) particles and air system, it has considered the multiphase gas-solid flow characters of propylene polymerization in the MZCR.One of the key points for the two fluid model is to establish the constitutive correlations for the relationship between the solid stress and the strain. The kinetic theory of granular flow (KTGF) is a good choice to close the constitutive equations by describing the granular temperature, solid viscosity and pressure. Particle-particle restitution coefficient is critical to the parameters in the KTGT and the heterogeneous flow structure in the riser as well. Another key point is how to describe the interphase drag coefficient, which represents the intensity of momentum transfer between the gas phase and solid phase. This work has used the Gidaspow drag model. It takes into consideration the influence of solid volume fraction on the drag force that is suitable for the gas - solid multiphase system.To verify the validity of the model and various conditions adopted in this simulation, gas and particle flow profiles are obtained for the time-averaged radial solid volume fraction, axial solid volume fraction and axial bed pressure drop. Furthermore, the typical core-annular flow structure is observed. The granulartemperature which is the scale of the fluctuant energy of single particle can be predicted by the simulation. The granular temperature is related to the solid volume fraction. When the solid volume fraction is between 0.01 and 0.05, the granular temperature is in the transition region from dispersed values to well-regulated ones. The Fast Flourier Transform (FFT) method has been sued to analyze the power spectrum density of the pressure fluctuations in the different regions of the riser. The main frequencies of the pressure fluctuation are different between single particle and clusters.Moreover, based on the CFD model for FCC system, the gas and solid flow behaviours of another system in the riser are simulated. The solid and gas phases are composed of polypropylene (PP) particles and air, respectively. Flow profiles are obtained for the volume fraction and pressure for each of the two phases at different operating conditions. The computational values agree reasonably well with the experimental ones. The former show that the solid volume fraction at different regions of the riser decrease and the overall flow structure become more uniform when increasing the superficial gas velocity or decreasing the solid mass flux. The value at the annular region is more significantly affected than the one at the core. Density of PP particles is so small that they are easily carried by the upward gas flow, which leads to a small difference in velocity between the gas and solid. This means that the inlet conditions have significant effects on the flow behaviour of the under region. Moreover, the flow structure tends to become uniform more easily. Finally, the Discrete Particle Model is adopted to study the resident time distribution of solid particles. It has an obvious tail.