Characterization Of Heavy Oil Fractions Using Intrinsic Viscosity Measurement And Molecular Dynamics Simulation

For studying the diffusion and hindrance effect of heavy oil molecules on catalytic upgrading process, it is necessary to understand the transport and equilibrium properties of heavy oil molecules in the pore of catalyst. However, these properties depend on the molecular size and shape of heavy oil fractions. Thus, a thorough knowledge of molecular size and shape is foundational to develop catalyst and catalytic upgrading process. The molecular size and shape of heavy oil fractions also influence their elution characteristics in gel permeation chromatography, hence understanding molecular size and shape is necessary for the accurate interpretation of the information obtained from this chromatographic process. Moreover, for fully understanding the physicochemical properties and reaction behavior of such complex mixtures, it's necessary to characterize the chemical structures of these mixtures and study the relationship between the structure and property.In this paper, two methods, viscosity experiment and molecular simulation were used to characterize the molecular size and shape of heavy oil fractions. Since intrinsic viscosity reflects the hydraulics volume of molecule, intrinsic viscosity is measured to characterize the molecular shape and size of heavy oil fractions. The viscosity curve for solution of heavy oil fractions was determined using iso-volume dilute method, and then intrinsic viscosity is obtained according to the law that the viscosity curve follows. Based on the average molecular structures of heavy oil fractions, the density and molecular size in vacuum and solution were calculated by molecular dynamics simulation. Hence, it is accomplished to understand the essential characteristic of heavy oil fractions at the molecular level.Tahe atmosphere residue (THAR) and Suizhong 36-1 atmosphere residue (SZAR) were separated into eight fractions by Liquid-Solid Adsorption Chromatography, namely saturate, monocyclic aromatic, dicyclic aromatic, polycyclic aromatic, light resin, middle resin, heavy resin and asphaltene. Under the condition of different temperature and solvent, the flow time for solution of above heavy oil fractions was measured by Ubbelohde viscometry at the high concentration region and low concentration region. The flow time of pure solvent was also determined. The curve of flow time versus concentration was plotted and two different situations were obtained. At the whole concentration region, heavy oil fractions, including saturate, monocyclic aromatic, dicyclic aromatic, polycyclic aromatic and light resin, their curve of flow time versus concentration were consecutive and the extrapolation time of curve at zero concentration was quite consistent with the flow time of solvent. Therefore, the above five fractions were not absorbed on the tube wall of capillary. However, the curve of middle resin, heavy resin and asphaltene was not consecutive at the whole concentration region. The extrapolation time at high concentration region was obviously higher than the flow time of solvent, and both two times were very approximate at low concentration region. Thus, the above three fractions were absorbed on the tube wall of capillary at high concentration region and desorbed at low concentration region. The absorbability of middle resin, heavy resin and asphaltene is closely related to their polarity. The bigger the polarity is, the stronger the adsorptive capacity is. Whether absorbed or not, for solution of heavy oil fractions at high concentration region, the extrapolation time is used in the calculation of relative viscosity instead of the solvent flow time and then the curve of relative viscosity versus concentration was plotted. The curve accords with the Einstein law, and the slope of the curve is just the value of the intrinsic viscosity.A novel calculation method of the intrinsic viscosity of heavy oil fractions was proposed, including single-point method and multi-point method. Multi-point method is used to obtain the intrinsic viscosity of heavy oil fractions. According to the intrinsic viscosity and average relative molecular weight, the sphere equivalent hydrodynamic diameter of heavy oil fractions in three kinds of organic solvent was determined at different temperature. Under the experimental condition, the sphere equivalent hydrodynamic diameter of THAR and SZAR fractions was estimated to be 1.10-3.87 nm and 1.03-3.92 nm respectively, in which molecular size of asphaltene is much larger than that of other fractions. For the same fraction, with temperature increasing, the molecular size decreases. Compared to the pyridine and nitrobenzene solutions, the molecular size in toluene solution is higher. For the same heavy oil, the molecular size of heavy oil fractions increases as the average relative molecular weight increase. For the different types of heavy oil, the variety rule of the molecular size with average relative molecular weight is not comparable, and the molecular size is more affected by the spatial configuration of heavy oil molecule. The sphere equivalent hydrodynamic diameter in toluene solution at 40℃is used to characterize the size of heavy oil fractions, and the size is 1.20-3.84 nm and 1.12-3.90 nm for THAR and SZAR eight-components, respectively.With 1H nuclear magnetic resonance (1H NMR) and X-ray diffraction (XRD) determinations, elemental analysis and average relative molecular weight measurement, the average molecular formulas and the number ofα,β,γand aromatic hydrogen atoms of polycyclic aromatics, heavy resins and asphaltene molecules were calculated. Heteroatoms, such as S, N and O, are considered in the construction of average molecular structure. Two important structural parameters were proposed, including the number of alkyl chain substituents to aromatic rings and the number of total rings with heteroatom. Ultimately, the average molecular structures of polycyclic aromatics, heavy resins and asphaltene molecules were constructed. The number ofα,β,γand aromatic hydrogen atoms of the constructed average molecular structures agrees well with that derived from the experimental measurement. The average molecular structures of saturates were also constructed, based on the assumption that each naphthenic ring connects an alkyl side chain.On the basis of average molecular structures, the density and molecular size of saturates, polycyclic aromatics, heavy resins and asphaltene molecules were simulated by molecular dynamics simulation. The density of above fractions obtained by molecular dynamics simulation compared well with the experimental value, and the relative errors were less than 4%. Due to the swelling behavior of solvent, the alkyl side chains of heavy oil molecule in solution were much more stretched. Thus, the molecular size of heavy oil molecule in solution was larger than that in vacuum. For THAR fractions, compared with the disk-shaped molecules, such as heavy resin and asphaltene molecule, the gyration radius of line-shaped polycyclic aromatic molecule is higher. However, for SZAR fractions, disk-shaped heavy resin and asphaltene molecule are larger than line-shaped polycyclic aromatic molecule at the linear scale. Hence, the gyration radius of heavy resin and asphaltene molecule are higher. There is no necessary link between the gyration radius and average relative molecular weight, and the gyration radius is more reliant on the spatial configuration of heavy oil molecule. The gyration radius in toluene solution at 40℃is used to characterize the size of heavy oil fractions. For the above THAR and SZAR fractions, the gyration radius is 0.66-1.01nm and 0.55-1.04nm respectively.