Theoretical Study On Heavy Oil Asphaltene Molecular Structure And Properties

Heavy oil is a kind of crude oil with high viscosity and density which is hard to exploit by conventional crude oil mining technology. Compared with conventional crude oil, heavy oil has more polymer hydrocarbon and hetero-atom compounds, in which the component with the largest molecular weight and strongest polarity is asphaltene. Because asphaltene is easy to coagulate in the process of production, transportation and transformation of oil, which seriously affects the processing and utilization rate of heavy crude oil, so it is necessary to recognize the asphaltene macromolecules. The component of heavy oil is very complex, in view of the limitations of the current testing technology, the structural understanding of the components of heavy oil is only predictable. This paper starts from the classical molecular model of asphaltene in heavy oil, researching the relationship between the molecular structure and properties of asphaltene molecules by the density functional theory (DFT) method in the study of the chemical active sites, the intermolecular interaction and the solvation effect, the main contents and conclusions are as following:The B3LYP/6-31++G (d,p) was used for molecular structure optimization and vibration frequency calculations for dibenzofuran and its 15 substituted derivatives. HOMO-LUMO energy gap, ionization potential as well as the relationship between aromaticity index HOMA and substituent constant ? were studied to discuss the chemical activity of dibenzofuran on its four sites including 1,2,3,6. The results indicats that the electron effect can reduce the ionization potential of dibenzofuran molecule, while electron absorption effect can increase the ionization potential of dibenzofuran molecule; Through the HOMO-LUMO energy gap analysis, the hydrogen atoms on 1 and 3 spots of dibenzofuran molecule show higher activity; Through the linear fitting results of aromaticity index HOMA and substituent constant ?, it is known that the reaction activity is the strongest in the 3 place, according to the oligomer formation mechanism of asphaltene structure, the 3 spot should be easy to form free radical site in the structure of dibenzofuran which is a fragment in asphaltene molecules.In the M062X/6-31G (d) level of theory, we can get 11 kinds of stable configurations of binary mixture systems composed of asphaltene molecular fragments-heterocyclic molecules (dibenzofuran, acridine and carbazole) to study the interaction of asphaltene molecules, and find out two most stable configurations. The main conclusions are as following:There are different kinds of hydrogen bonds created in all the configurations, the interaction and aggregation between the asphaltene macromolecules is related to the existence of hetero atoms (such as O, N and S), and more hetero atoms take part in the interaction, more stable the configuration will be; There are other possible lateral effect except the longitudinal effect in the interaction among asphaltene molecules, providing some configurations and data for reference.The modeling and theoretical calculation of the solvation effect of asphaltene macromolecules in 13 solvents were carried out by B3LYP method, through the analysis of the electrostatic solvation free energy, non electrostatic solvation free energy and solvation free energy data, its solubility was discussed. We can conclude that:Asphaltene molecules are subject to both electrostatic and non electrostatic effects, which makes the solute molecules tend to be more stable after the addition of the solvent; The size of the remote electrostatic effect is the key to the size of the solubility; Dichloromethane, chloroform and chlorobenzene on the solubility of asphaltene is good, which is due to the presence of strong electron withdrawing groups in the solvent. The dissolution effect of aliphatic hydrocarbon solvents was poor. This can provide data and theoretical support for the study of petroleum accumulation inhibitors.