Fluidization Attrition And Fluidized Bed Process Optimization Of Mo/HZSM-5 Microspheres For Methane Dehydro-aromatization

As a major component of primary energy such as natural gas,coalbed methane,oil field gas,shale gas and natural gas hydrate,the efficient and clean utilization of methane has attracted much attention.In the absence of oxygen,methane can be catalysed and directly converted into liquid or solid aromatics with high extra value and hydrogen required for coal chemical industry and petrochemical industry,and which has short process and good atomic economy.After nearly thirty years of continuous exploration as to methane dehydro-aromatization（MDA）,some achievements have been made in catalyst,reaction process and equipment,and the thermodynamics,kinetics and catalytic mechanism of the reaction have been deeply understood,and the long-term stable operation in the small-scale test system has been achieved,but there is no pilot test and industrial practice at present.Limited by thermodynamics,the MDA need to be carried out at a higher temperature to obtain the product yield with production significance.And it needs to take measures of system pressurization which is not conducive to MDA in kinetics,or increase the feed rate under normal pressure,in order to improve the output,reduce the cost and cut the reactor size.The above process methods will cause significant catalyst attrition,like the FCC process in refining industry,it may bring some negative effects to future scale-up test,pilot scale and even industrial production,such as blocking pipelines,polluting products and reducing catalytic performance.And some fine particles produced by catalyst attrition were also collected in the gas-phase outlet pipeline of our earlier related tests.The existing catalysts with better activity are hydrogenated ZSM-5 zeolite molecular sieves with Mo as the main supporting metal.In actual use,they need to be shaped into regular particles of hundred micron level,and the fluidized bed process with better heat transfer effect or circulating fluidized bed process attached continuous regeneration of catalyst needs to be adopted.However,the catalyst will be inactivated rapidly in the process due to competitive carbon deposition reaction and deep dehydrogenation reaction caused by steric hindrance,and the change in type and size of reactor may affect the catalytic performance and suitable process parameters.In order to further promote the MDA technology to pilot scale and industrialization,based on our previous research,focusing on the problems of mass loss and unstable aromatization performance of the catalyst,this thesis mainly carried out the following four aspects of related research.（i）Fluidization attrition of Mo/HZSM-5 catalystThe catalyst loss is due to its attrition under process conditions.In order to understand the real attrition of catalyst under high temperature,long-time rapid movement and MDA reaction,the circulating fluidized bed was taken as the simulation object in Section 2.3 of Chapter 2,and the fluidization attrition evaluation device was made on the basis of existing50 mm fluidized bed.In Chapter 3,the effects of system temperature,running time and process factors（activation,aromatization and regeneration）on the fluidization attrition of Mo/HZSM-5 microspheres were studied.The results show that thermal stress,mechanical stress and chemical stress together result in surface abrasion and/or body fragmentation of the catalyst.The higher the system temperature and the longer the running time,the more serious the catalyst attrition.Surface abrasion mainly occurs below 200℃,the attrition is nearly stable between 200℃and 600℃,and the multistage crushing is dominant above600℃.The coke caused by process factors increases the size and attrition resistance of catalyst particles,and the treatment depth（activation→aromatization→regeneration）has a relatively negative effect.H₂regeneration can reduce the particle size of catalyst with coke,but can not make it return to the original state.It is speculated that Mo₂C and/or MoO_xC_y,graphite precursor and/or carbon particles,and oligomeric polycyclic aromatics are the three types of carbon deposits on process treated Mo/HZSM-5.（ii）Relationship between particle size and performance of Mo/HZSM-5 catalystIf the variation in active component and carrier structure of catalyst is ignored,the most direct manifestation of catalyst attrition is the change of particle size（distribution）.In order to understand the effect of particle size on aromatization performance,the MDA properties of Mo/HZSM-5 microspheres with different sizes and amorphous broken Mo/HZSM-5 particles were systematically studied in Chapter 4.The results show that there is no regular particle size effect in the fixed bed,and the catalysts at the two poles of the particle size range show poor MDA effect.In the fluidized bed,the methane conversion of larger particles is better and more stable,and the stable value of benzene selectivity is not affected by the change of particle size,and the catalyst of 200-280μm has the best MDA performance.The fine catalyst particles produced by fluidization attrition increase the contact surface with methane and reduce the diffusion resistance,which can improve the initial performance of Mo/HZSM-5.But the fines also promote the coke formation and increase the mass loss of catalyst,which eventually led to the rapid deactivation.（iii）Process optimization of Mo/HZSM-5 for MDA in fluidized bed（system）The optimal process parameters of single size fluidized bed often can not adapt to the scale-up and type change of reactor,resulting in the instability of MDA performance.In order to understand the influence of the size and type of fluidized bed on the process optimization of Mo/HZSM-5 for MDA,in Chapter 5,the combination of 8/15/50 mm fluidized bed optimization and 35/50 mm fluidized bed continuous reaction-regeneration system verification was used to study some suitable MDA process parameters.The results show that the suitable regeneration H₂temperature and flow rate are respectively 850℃and 0.5 L·min^-1.The suitable reaction space velocity（RSV）is 4000 m L·g^-1·h^-1,and the suitable particle size of Mo/HZSM-5 is 177.7-194.0μm.Before and after activation,the suitable heating mediums are N₂or H₂and H₂,respectively.The suitable catalyst circulation rate of the system is closely related to the size of reactor and regenerator,and the system has a limited self-regulation characteristic.High space velocity will enlarge the heterogeneous fluidization of the system and make some"dead space"catalyst inactivated continuously without H₂regeneration.Appropriate increase of catalyst hold-up in reaction zone,prolongation of regeneration time and reduction of regeneration times are beneficial to the stable formation of aromatics.（iv）Reactor scale up effect of Mo/HZSM-5 for MDAThe increase of reactor size often leads to the change of catalytic effect.In order to understand the influence of fluidized bed（system）scale-up on Mo/HZSM-5 for MDA,in Chapter 6,based on the existing experimental scale,the scale-up effect of Mo/HZSM-5catalyzing MDA in three sizes of fluidized bed and two sizes of fluidized bed continuous reaction-regeneration system was studied.The results show that the amplification effect is affected by RSV.At lower RSV(≤4000 m L·g^-1·h^-1),when the inner diameter of the fluidized bed（system）increases（8 mm→50 mm and 35 mm→50 mm）,the methane conversion,benzene selectivity and formation rate decrease,while naphthalene selectivity and formation rate increase.At higher RSV(8400～11000 m L·g^-1·h^-1),the methane conversion decreases slightly with the fluidized bed scale-up（8 mm→15 mm→50 mm）,but the selectivity and formation rate of benzene and naphthalene increase.It is speculated that higher RSV makes Mo/HZSM-5 contact with CH₄more fully,reduces the diffusion resistance of aromatics,and improves heat transfer,resulting in smoother deactivation.In order to promote the further scale-up of MDA technology,this paper focuses on the attrition cause,possible mechanism and catalysis loss of Mo/HZSM-5 microsphere catalyst in fluidized bed（system）,and studies the process parameters optimization,validation and amplification effect aiming at the instability of MDA performance in small-scale test,so as to provide theoretical and technical support for the future reactor design and large-scale practice of this technology.