Study On The Average Molecular Structure Of Different Asphaltenes

With the deterioroation of crude oil,researchers have paid more and more attention to the technology of converting heavy oil into light fractions.Heavy oil is composed of a variety of moleculars.The most complex structure of heavy oil fractions are asphaltenes.Compared with other heavy oil fractions,asphaltene has the highest boiling point,the largest average molecular mass,the lowest hydrocarbon ratio.It also contains large amounts of oxygen,nitrogen,sulfur and heavy metal heteroatom.Therefore,complex structure of asphaltene has become the difficulty of heavy oil converting into light fractions.In this paper,experimental analysis methods and molecular simulation methods are utilized to study the average molecular structure of asphaltenes from Tahe atmospheric residue and Suizhong 36-1 atmospheric residue aiming to provide the foundation data for heavy oil processing.¹H-NMR,elemental analysis and XRD analysis are used to investigate the asphaltene representative fragments.The average molecular structure parameters of asphaltene are calculated by improved B-L method,which are used to construct the initial average molecular structure.The density of the initial average molecular structure is simulated by simulation technology to verify its rationality.Then the reasonable average molecular structure configuration are established,which can help researchers to understand the asphaltene molecular structure.The most possible average molecular structures are studied by changing the average length of the side chain,the length of the bridge chain between two structural units,the connecting position of bridge chain and the connectivity style.The results show that the twist energy and van der waals energy have the maximum influence on total energy of the most stable molecules conformations of asphaltene.The most possible side chain length of Tahe atmospheric residue asphaltene is C5-C9 alkyl chain.When the alkyl side chain is shorter or longer,the average molecular structure is not stable.The most possible side chain length of Suizhong 36-1atmospheric residue asphaltene is C7-C11.The most possible bridge chain length of asphaltene is C6-C9 with low potential energy.The sulfur bridge bond is much more posibble to connect two asphaltene molecular conformation than the same length of alkyl chain.In addition,the chain bridge connecting position tend to connect in the position that is off long side chain.The solubility parameter of asphaltene is reasearched to find the solubility differences between Tahe and Suizhong 36-1 asphaltenes from the angle of solubility parameter.When the solvent is toluene completely,the solubility parameter of Tahe asphaltene is 10.54MPa⁰.5.5 and the solubility parameter of Suizhong 36-1 asphaltene is 10.90MPa^0.5.The solubility parameter of Tahe asphaltene in toluene solution obtained by molecular simulation is 10.66 MPa^0.5,which is slightly larger than the experimental value.The solubility parameter of Suizhong asphaltene in toluene solution obtained by molecular simulation is 11.21 MPa^0.5,which is larger than the experimental value.The results show that the average molecular structure of the asphaltenes are reasonable.