Experimental Study And Numerical Simulation On Hydrodynamic Heterogeneity In Fluidized Catalyst Strippers

Catalyst stripping technology is widely used in fluidized catalytic cracking(FCC),methanol to olefins(MTO)and other chemical processes.Due to the low stripping efficiency of the baffle-free strippers,various baffles are installed in the commercial catalyst strippers and the disc-donut baffles are the most frequently encountered ones.Assumption that gas-solid distributions are axially symmetric was made in all of the previous hydrodynamic studies on catalyst strippers.The circumferential gas-solid heterogeneity was then neglected by previous researchers.However,an efficient stripping performance needs a good gas-solid contact all over the entire cross-section of a catalyst stripper.In order to comprehensively evaluate and understand the hydrodynamic heterogeneity in the catalyst strippers,a 4-channel fiber optical densitometry was used in the experiment of this study to synchronously measure the radial gas-solid distributions at four circumferential angles around the stripper column.Meanwhile,a 3-dimensional stripper model was established and Computational Particle Fluid Dynamics(CPFD)was used to investigate the hydrodynamic heterogeneity in catalyst strippers.This study comprehensively investigated the hydrodynamic heterogeneity in the traditional disc-donut stripper,which is widely applied in the commercial FCC units.It was found in experiment that the circumferential gas-solids distribution is highly heterogeneous in the disc-donut stripper and local gas bypassing was found to exist.Unlike previous simulation studies by using Two Fluids Model(TFM),CPFD model successfully predicted the observed gas bypassing and circumferential gas-solids heterogeneity in the disc-donut stripper.It is also shown that the consideration of particle size distribution(PSD)is a possible key reason leading to the circumferentially heterogeneous results in CPFD simulation.Through CPFD simulation,the distribution of catalyst"dead zone"on the disc-donut baffles was obtained for the first time.The results indicate that opening holes on disc-donut baffles have a positive effect on shrinking the volume of the catalyst"dead zones".Finally,based on the analysis of the gas-solid heterogeneity above and beneath the disc-donut baffles,the reason of the severe wear reported in the commercial strippers was proved to be the continuous impact of the particles entrained by the high-velocity bypassing gas.In view of the highly heterogeneous gas-solid distribution in the traditional disc-donut stripper,a packed stripper with louver baffles was designed based on our previous studies to further improve the hydrodynamic homogeneity and the stripping performance of the catalyst strippers.The comparative study on this packed stripper shows that it can eliminate both the circumferential gas bypassing found in the traditional disc-donut stripper and the radial gas bypassing found in the disc-donut stripper with opening holes.Compared to the baffle-free and disc-donut strippers,the packed stripper has a more uniform radial gas-solid distribution and a higher stripping efficiency.Compared to the disc-donut stripper,the packed stripper has a wider operating range of solids circulating rate.The results of CPFD simulation show that the designed louver baffle in this study can effectively redistribute and redirect the gas flow.The radial distribution of the gas velocity in the packed stripper is more uniform and close to plug flow.In contrast,the radial distribution of the gas velocity in the baffle-free and disc-donut strippers is highly heterogeneous.Ring distributor is a typical gas distributor applied in commercial strippers.Gas can only be firstly distributed to the annular area covered by the location of the ring distributor and this is very likely to be one of the causes that leads to the hydrodynamic heterogeneity in the catalyst stripper.This study investigated the gas distribution of the ring distributor and the results show that the gas is firstly distributed to the annular area near the stripper wall and then transfers to the bed center of the stripper as the gas flows up in the baffle-free stripper.Further studies show that when ring distributor combines with the disc-donut baffles with opening holes,bypassing gas will be formed at the holes of the bottom donut baffle.The bypassing gas will then gradually transfer to the bed center of the stripper.Finally new bypassing gas will be formed at the holes of the higher disc baffle.When ring distributor combines with the new designed louver baffles,the heterogeneity of the gas distribution decreases as the gas flows through every layer of the louver baffle.Eventually,a far more uniform gas-solid distribution will be achieved in the packed stripper.When operated with a certain solids circulating rate(G_s),the hydrodynamic heterogeneity of a catalyst stripper could also be influenced.The experimental results of this study show that the gas-solid distribution can be improved by increasing G_s.CPFD simulation reveals that the particle radial motion caused by the location difference between particle feeding inlet and discharging outlet is the reason of this improvement.