The Rule And Mechanism Of The Formation Of Mesophase During Thermal Condensation Process Of Aromatic Enriched Fraction Of Heavy Oil

The petroleum residues are polydisperse.The difference between the conversion process of the fractions of petroleum residue can destroy the lamellar orientation and the fluidity of the reaction system.Therefore,it is important to characterize the conversion behavior of different fractions during the whole process in order to modify the properties of carbonaceous mesophase,and provide the basis for the industrialization of preparation process.Firstly,the transition of the aromatics in earlier stages of the thermal conversion and its effect on subsequent carbonization behavior were analyzed.It was found that more monocyclic and dicyclic aromatics was converted to polycyclic aromatics in a comparatively shorter time between 450 and 500 ℃.The higher polycyclic aromatics content of the oil accelerated the formation of the anisotropic phase,thus the excessive carbonization of the system was prevented and the lamellar orientation of the pyrolysis product was modified.The modified oil obtained by thermal pretreatment was converted to a carbonaceous mesophase with a large domain texture and good lamellar orientation(Lc = 21.94 (?)),as well as a low softening point(273 ℃)in a short carbonization time,which is attributed to the higher polycyclic aromatics content(which accelerated the formation of an anisotropic phase)and fewer long side chains(which controlled an appropriate free radical concentration)of the thermal pretreated oil.Secondly,the parent pitch was extracted by solvent to modify the molecular weight distribution of pitch.The fractions with different molecular weight distribution were identified to determine the carbonization behavior of the fractions and detect the effect of the fractions on the carbonization behavior of the whole system.The pitch residue with less low molecular weight fractions(280–350)converted to a fully developed carbonaceous mesophase with low condensation extent and low softening point in a short carbonization time.The fractions with molecular weight between 350 and 450 can help the system contain a good viscosity to control the free radical reaction which can enhance the proportion of components with a high degree of order in the carbonaceous mesophase.Subsequently,to concentrate pre-mesogens(molecules can convert to a mesophase quickly)from petroleum pitch and prevent system from suffering from excessive carbonization,the parent pitch was extracted by various solvents with different solubility parameters and the corresponding pitch residues were carbonized.It was found that the dispersive solubility parameter has a major effect on the FT-IR structural indices(i.e.,the indices calculated from Fourier transform infrared spectroscopy)of the pitch residue;thus,four pitch residues enriched in molecules characterized by different condensation degrees and numbers of alkyl chains were successfully obtained and their carbonization behaviors were identified to determine the carbonization behavior of pre-mesogens.The pitch residue obtained by hexamethylene extraction formed a large domain texture with good lamellar orientation(Lc = 24.45 (?))and low softening point(SP = 257 ℃)in a short carbonization time,which is attributed to the removal of molecules with a high alkyl carbon atom percentage(which accelerates the formation of an anisotropic phase)and the enrichment of specific molecules.The concentration of molecules with a low condensation degree,fewer alkyl chains,and a higher condensation reactivity in A1 HEX accelerated the formation of the anisotropic phase and provided alkyl free radicals during thermal treatment,which hindered crosslinking between larger free radicals and contributed to the large domain texture with good lamellar orientation and a low softening point.In addition,because of the lower reactivity of the self-assembled polycyclic aromatic hydrocarbons(PAHs)contained in the anisotropic phase(compared to that of the isolated PAHs in the isotropic matrix),the rapid formation of a mesophase during carbonization prevented excessive condensation to some extent.Finally,the effect of hydrogenation on the composition and molecular structure of the petroleum pitch was studied.With hydrogenation,the aromaticity and condensation extent of the pitch were decreased and the number of alkyl side chain was increased.In addition,the thermal conversion behavior of the hydrogenated pitch obtained from the parent pitch with different extent of carbonization.Results show that the hydrogenated pitch HP11 obtained from the parent pitch P11(which converted from the petroleum residue after 11 h heat treatment)has high short alkyl side chain content(which provided alkyl free radicals during thermal treatment,which hindered crosslinking between larger free radicals).In addition,the high large planar macromolecules content in HP11(which attributed to the high planar macromolecules content in P11)can shorten the time required to obtain a fully developed carbonaceous mesophase(which inhibited the excessive carbonization of the molecules with a high thermal reactivity,and increased the order degree(Lc = 22.57 (?)),and the homogeneity(Order Index = 0.8340)of the pyrolysis product,as well as decreased the softening point(261 ℃)of the pyrolysis product).