Coarse-grid CFD Simulation Of Methanol-to-olefins Fluidized Bed Reactors

The methanol-to-olefin(MTO)as a novel process technology performs a significant function in the C1 chemical industry.The meso-scale(sub-grid)heterogeneous flow structures such as clusters,streamers and bubbles have a key effect on gas-solid flow and reaction behavior in the fluidized bed reactor(FBR)that was used for gas-solid catalytic process.Coarse-grid simulation employing traditional two-fluid model(TFM)neglects the influences of meso-scale structures on the reactor performance,thereby resulting in over-predicted interphase momentum exchange coefficient and gas–solid heat/mass transfer coefficient.Correspondingly,these unrealistic reactor characteristic predictions are not reliable for reactor design and scaling-up.Therefore,understanding these meso-scale structures comprehensively and accurately is the key to reactor optimum design and scaling-up.To address this challenging issue,a multi-scale sub-filtered CFD reactor model was developed to comprehensively describe the gas-solid flow behavior and explore optimization for a large-scale MTO FBR.First of all,a sub-filtered CFD model for the cold-model gas-solid flow in a large-scale MTO FBR was constructed.In comparison with the simulation by classic models,coarse-grid simulation through sub-filtered CFD model predicts more accurate flow hydrodynamics such as relatively stable bed expansion height,classic radial core-annulus structure and axial S-shaped profiles.Second,based on the above cold-flow model,a reactor model for a MTO reaction system was first constructed by incorporating a filtered drag model,a filtered gas-solid heat transfer model and a MTO lumped-kinetic model to probe large-scale reactor behavior and explore optimization.More realistic reactor performance is yielded.Through coarse-grid CFD simulation,accurate predictions to the coke deposition content in affecting the olefin products(ethylene and propylene)selectivity distributions were first reported.Results suggest the optimum catalyst residence time(about 33 min)and average coke content(about 6.74%)of this MTO system,thereby increasing the economic efficiency of MTO industrial production.Finally,given that the huge investment in industrial MTO catalyst is required,whereas due to the existence of the van der Waals force,static electricity and other relevant forces between solid particles,the solid particles will unavoidably agglomerate.At the same time,because of the collision consumption,catalyst particles also show a certain degree of breakage.Thus,studying the effect of particle polydispersity on flow and reaction behavior of methanol to olefins fluidized bed reactors is desired.On the basis of the above contributions,a reactor model has been first developed with the objective to fundamentally comprehend the qualitative influence of the solid polydispersity on the features of a MTO FBR.Results show that the volume-average solid particle diameter becomes smaller with the flow time.Actually,this behavior can be due to the dynamic evolution between the aggregation and breakage process,in which the breakage process seems to be stronger.Comprehending the above principles in a MTO fluidized bed is helpful to guide the reactor optimum design and scaling-up theoretically.