Multi-scale CFD Simulation Of MTO/MTP Large-scale Fluidized Bed Reactor

The methanol-to-olefins(MTO)and the methanol-to-propylene(MTP)technologies are two newly important technologies to produce ethylene or propylene.MTO/MTP provides an economical route to produce light olefins from abundant non-petroleum natural materials(e.g.,coal or natural gas).MTO/MTP is believed to link the coal or natural gas based chemical industry to the moderm petrochemical industry,and thus MTO/MTP technology is very important,expecially for China.Among many MTO techniques,DMTO is the most successfully used technology.Based on the first DMTO technology(DMTO-I),DICP is developing a MTP process(called DMTP).The DMTP process combines MTO reaction,alkylation reaction of methanol and ethylene and the catalytic cracking reaction of components above C4.This study will focus on CFD simulation of the DMTO/DMTP,and the purpose of this study is to provide reliable parameters for the next generation DMTO/DMTP process.In Chapter 1,a literature survey for the development of MTO/MTP process,the related simulation approaches and simulation studies of MTO/MTP fluidized beds is presented.DICP cooperated with IPE(Institute of Process Engineering)to study the hydrodynamic behaviors and reaction behaviors in MTO reactors by using computational fluid dynamics(CFD)simulations,but there exist some problems for simulations of large-scale MTO reactors:1)the seven-lumped kinetic model used in CFD simulation only considers the parallel reactions and neglects the olefins interconversion.The parameters of deactivation function heavily depend on the types of reactors;2)the coke content is closely related to the selectivity of the light olefins(ethylene and propylene).There is a wide distribution of coke content which could be attributed to the circulation of catalysts(the continous feed-in of fresh catalysts and the discharge of spent catalysts).However,the present TFM combined with the kinetic model could not capture the chemical differences between particles(i.e.,the particles with different coke content),and thus it is unable to accurately predict the distribution of products.This study aims to resolve these problems.In Chapter 2,an industrial MTO reactor is simulated,and comparison of a seven lumped kinetic model(called Kinetic A)and the new kinetic model(called Kinetic B)considering the further conversion between products is made.The results show that hydrodynamic behaviors can be well predicted using both models combined with EMMS-based drag model.For reaction behaviors,the predicted ratio of ethylene to propylene by using Kinetic B(1.544)is closer to the experimental value(1.008)than that by using Kinetic A(1.5675),however this improvement is very limited.The present TFM simulation may be responsible for the aforementioned predictions,because the variation of coke distribution with time and reaction is not accurately captured.So an approach of tracking two kinds of coke(the coke on fresh catalysts and the coke on patched catalysts)is designed to mainly investigate the effect of the evolution of coke content on reaction behaviors.The simulation shows that the ratio of ethylene to propylene is decreased with the increase of simulation time,and it is expected to approach the experimental value if the simulation time is long enough.This result also proves that the circulation of catalyst results in a wide distribution of coke content in the industrial DMTO reactor.Limited to current computational ability(simulation of a 100-second realistic process needs 66 days),so the required time for simulation of a process with average residence time of 4800 s is virtually formidable at present.In future,PBM(Population Balance Model)will be introduced and further combined with a reasonable initial model to more accurately reveal the influence of coke distribution on reaction behaviors.Based on the sucessfully developed DMTO process,DICP is developing the DMTP process.In DICP’s patent CN101177374,the combination of MTO reaction,alkylation reaction of methanol and ethylene,and the catalytic cracking reaction of components above C4 is proposed to maximize the selectivity of propylene.Given the similarity of DMTO and DMTP technologies,a similar simulation method for studying MTO reactor can be also used to investigate the MTP process.In chapter 3,TFM combined with EMMS/bubbling is used to simulate three industrial MTP reactors with different geometries,and the effect of various geometrical parameters(such as the position of solid inlet,and the plate for partitioning the reaction zone,etc.)on the hydrodynamic behaviors are investigated.The results show that the position of solid inlet has effect on the overall distribution of solid volume fraction in the reactor(the solid volume fraction varies from 0.34 to 0.03 for Case2 and varies from 0.41 to 0.03 for casel).The introduction of the partition plate results in an obvious difference between two reaction zones(the upper part has lower solid volume fraction within the range of 0.03-0.25 and the lower part has higher solid volume fraction within the range of 0.1-0.35).These hydrodynamic features may be combined with different reaction routes to improve the yield and selectivity of propylene in future.At last,conclusions and future work are presented.