Study On Formation Mechanism And Orderly Coking Performance Of Mesophase During The Stepwise Carbonization Of Aromatic-rich Heavy Oil

Limited by technical barriers of foreign technology,needle coke produced by domestic companies is mainly mid-and low-end products with serious homogeneity.At present,the core technology for producing high-quality needle coke is still lacking in domestic market.Moreover,at present,the reaction mechanism of carbonization into coke needs to be further understood,which leads to the difficulty in improving the preparation of high-quality needle coke.Needle coke is a typical mesophase-based carbon material,and its carbonaceous mesophase structure is the key factor that affects its specific performance and final application.In view of this,aromatic-rich heavy oil was used as the raw material,and the evolution of the mesophase structure during coking and subsequent calcination were served as main line of this thesis to reveal the key control factors of the orderly coke-forming process and provide theoretical guidance for developing new strategies and exploring new applications of needle coke.In this study,the aromatic-rich heavy oil was selected as feedstock.the conventional carbonization process featured by constant temperature and pressure was firstly utilized to prepare needle coke,and then reaction conditions were optimized.Under this process condition,the formation of the mesophase in the early stage of the carbonization and its internal driving force were explored;and the main control factors of the mesophase orientation was analyzed by investigating the changes in the system viscosity,gas evolution rate and polarized light texture in the later stage of the conventional carbonization reaction.The research results show that the formation process of the mesophase structure presents the characteristics of multi-stage self-assembly.The relative molecular weight distribution o and the molecular configuration of aromatic hydrocarbons are the crucial factors for regulating this construction process.Intermolecular interactions such as van der Waals forces andπ-πconjugation between fused-ring aromatic molecules are the main driving force for inducing the association between aromatic hydrocarbons;Microdomain then appears via aggregate of the formed lamellar stacks in the system under the solvophobic force and the cohesive force between stacks;and fractal characteristics could be found for these microdomains;under the interaction of surface tension,cohesion and external forces,the rearrangement of lamellar stacks inside the microdomain always occurs to varying degrees during the gradual coalescence,resulting in different optical texture observed by polarized-light microscope.The study also found that the gas escape rate in the solidification stage of conventional carbonization is often insufficient to induce the uniaxial orientation of the formed mesophase optical structure during the conventional carbonization process.Based on the difference in the chemical reaction environment required for mesophase development and the subsequent uniaxial orientation,a stepwise carbonization process was proposed and used to prepare needle coke.And from the level of"component"and"molecule",the coupling law of the mesophase development in the early stage and the mesophase solidification in the late stage was discussed.It is found that the reaction conditions including 440℃,2MPa and 8h during the early stage would ensure fully formation and development of mesophase as well as produce good eutectic effect origined from reasonable distribution between the sub-components.Thus,not only obtains a broad domain mesophase texture with good melt deformability,but also can providing sufficient gas evolution via cleavage of side chain or light component volatilization under subsequent high temperature and low pressure conditions.Subsequent utilization of high-temperature and low-pressure solidifed conditions（selecting 470℃ and 0.5MPa）is conducive to promoting the removal and escape of alkyl side chains in the mesophase molecules and achieving appropriate regulation of the viscosity growth of the system,resulting into“overlap”between adequate gasflow and suitable viscosity in the time interval.So improvement of this coupling between above two factors could produce a premium coke rich in needle-domain mesophase.On this basis,the chemical changes and multi-scale structural evolution of mesophase in the green coke derved from mesophase pitch with different optical textures were systematically investigated during the calcination process,and its physical and chemical properties were further correlated.The results indicates that the volatilization of light components,the cleavage of residual alkyl side-chains and the dehydro-condensation reaction of aromatic molecules mainly occurs at the calcined temperature lower than 800℃,accompanied by the transition from"meta-stable crystalline phase"to stable"polycrystalline phase".Meanwhile,the laminar size continues to increase and the defects within the lamellas gradually decrease,while the stacking height of the lamellas temporarily decreases.During the calcination temperature range of 800-1200℃,dehydro-condensation is turned into the predominant reactions,along with a certain amount of scission of carbon-nitrogen bonds.At this time,the elimination of interstitial defects such as isolated molecules between the stacks and the continue increase of graphene size contributes to the rapid growth of graphene stacking height,and the order of the lamellar stacking is gradually improved.Whreas the carbon-sulfur bond fracture reaction also begins to increase significantly,and a certain amount of defect structure is generated,resulting in an increase in the average layer spacing.When the calcined temperature is promoted to 1200-1600℃,the incessant dehydro-condensation reaction could significantly improved the order of the stacked structure of the mesogen,and the wrinkled degree of lamellar lattice fringe was significantly reduced,although the carbon-sulfur bond cleavage reaction began to increase significantly.However,during the calcination process,the interaction between the lamellae stacks is continuously enhanced by the local rearrangement,but this arrangement could not change the overall orientation of stacks.Therefore,macro-scale optical texture reflected by polarized light microscopy and lamella morphology characterized by SEM remains basically unchanged during the calcinatio.Compared with broad-domain and mosaic mesophase cokes,needle coke has the lamellar lattice fringes with smaller wrinkled degree,a smaller interlayer spacing as well as more compact and orderly arrangement of lamellae stacks,thereby greatly enhancing theπ-πinteraction between the lamellaes.So it has a higher true density and better charge conduction performance.Besides,the lamellaes stacks and microcracks induced by contract of these stacks were oriented along the direction of needle-like texture,thus needle coke possesses good axial thermal conductivity and the capacity to accommodate the volumn expansion in the vertical direction of axis.Therefore,the thermal expansion coefficient of needle coke is lower.In view of the fact that the multi-scale structural characteristics of mesophasecould be flexibly controlled by optical texture of green coke and calcination temperature,this study also used the calcined coke as the anode material of the potassium ion battery,and then investigated the performance and mechanism of potassium storage.The experimental results show that,compared to the mosaic optical structure,the needle-like optical texture helps the calcined coke to readily form a more ordered carbon layer lattice at 1200℃,providing a richer and diverse active sites for K⁺intercalation,and then exhibits a specific capacity of 308 m A·g-1 at a current density of 0.1C.In additions,the formed short-range-ordered turbostratic carbon not only has good structural elasticity,but also makes those intercalation compounds that require a higher degree of order(especially KC₂₄ and KC₃₆)are difficult to form.The large interlayer spacing makes it easy to form KC₈ compounds,which in turn facilitates the rapid migration of K⁺and reduces volume expansion,thereby showing excellent rate performance and cyclic charge-discharge stability.