Yang Haiyang; Wang Chunlu; Ren Qiang; Wang Lixin; Yan Xuemin

(1. College of Chemistry & Enνironmental Engineering, Yangtze Uniνersity, Jingzhou 434023;2. SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

Abstract: Molecular simulations were carried out to investigate the self-diffusion coefficient of asphaltene in asphaltenexylene systems, used as heavy oil models. The self-diffusion behavior of asphaltene in the asphaltene-xylene equilibrium system was mainly affected by the interaction between asphaltene molecules, with stronger interactions corresponding to a slower diffusion of asphaltene. The interactions between asphaltene molecules mainly included π-π interactions between aromatic rings and hydrogen bonds between strongly electronegative heteroatoms. These results are expected to provide theoretical guidance for reducing the viscosity of heavy oil.

Key words: heavy oil; asphaltenes; viscous mechanism; molecular simulation; molecular dynamics

1 Introduction

The rapid development of the world economy is driving a continuous increase in the demand for oil. Petroleum will remain one of the main energy sources for a long time;however, its production systems are facing very serious challenges. Conventional oil resources cannot meet the demand to support global economic development.Although heavy oil is gradually being developed as an unconventional oil resource, it exhibits high viscosity,large specific gravity, and considerable flow resistance.The viscosity of heavy oil increases exponentially with increasing concentration of asphaltenes; the latter compounds cause major problems in heavy oil recovery,transportation, and refining, because they form dense flocculations or sediments[1], thereby obstructing oil pipelines[2-3]. Low-temperature oxidation is a promising,low-cost, effective, and environmentally friendly technology for heavy oil upgrading. Therefore, in-depth analyses of the influence of oxygen-containing functional groups on the self-diffusion coefficient of asphaltene in heavy oil are urgently needed.

The asphaltene fraction obtained from Tahe (China)heavy oil was selected as a reference to establish a molecular model for asphaltene. Xylene was selected as the molecular model of the continuous phase in heavy oil.

Molecular simulations were used to investigate the selfdiffusion coefficient of asphaltene in asphaltene-xylene systems.

2 Simulation Methods

2.1 Development of molecular model

The heptane-insoluble fraction obtained from Tahe heavy oil exposed to air was used as a reference to establish the molecular model of asphaltene. The elemental composition was analyzed using a fully automatic VARIO ELⅡ instrument. A Bruker Ascend 700M nuclear magnetic resonance (NMR) spectrometer was used to analyze the NMR spectra of asphaltenes. The elemental composition and NMR spectra were used to obtain the average molecular structural parameters of asphaltene.

2.2 Molecular dynamics (MD) simulation of asphaltene-xylene system

MD simulations were carried out with the Gromacs software[4]. The simulation parameters were selected from the Optimized Potentials for Liquid Simulations-All Atoms (OPLS-AA) force field[5-6]. Molecular structures were optimized with the ORCA program[7-8]. Restrained electrostatic potential (RESP) charges were computed using the Multiwfn program[9]. Topology files were generated through the tppmktop tool[10], and periodic models were built using the Packmol software[11].

The MD simulations involved the following steps: i) the periodic models were optimized by energy minimization;ii) MD simulations at constant-pressure and constanttemperature (NPT) were carried out at 373.15 K and 0.1 MPa for 2 ns; iii) MD simulations in the canonical ensemble (NVT) were carried out at 373.15 K for 20 ns to obtain an asphaltene-xylene equilibrium system;iv) the MD simulations with NVT at 373.15 K were continued for 2 ns. The fourth step was repeated five times to calculate the mean square displacement (MSD)of asphaltene. An appropriate fitting range of the MSDνs.simulation time curve was then selected to calculate the self-diffusion coefficient of asphaltene.

3 Results and Discussion

3.1 Development of asphaltene molecular model

The elemental composition of Tahe asphaltene is shown in Table 1. The H/C atomic ratio of the sample had a low value of 0.91, indicating a higher content of aromatic carbons in the asphaltene molecules. According to the literatures[12-14], the average molecular weight of asphaltene molecules analyzed by mass spectrometry ranges between 600 and 1000 g/mol. Based on the measured elemental composition and H/C atomic ratio, the average molecular formula of asphaltene was C61H55NSO3.

Table 1 Elemental composition of Tahe asphaltene

Table 2 Analysis of NMR spectra

The13C and1H NMR spectra of Tahe asphaltene are shown in Figure 1. The distribution of C and H obtained from the analysis of the NMR spectra is shown in Table 2.Table 2shows the molar ratios of hydrogen atoms in different positions and those of aromatic and saturated carbons. The molar ratios of hydrogen atoms directly attached to the aromatic ring of the asphaltene molecule/α-hydrogens/methylene and methine hydrogens other than in the α-position to the aromatic ring/terminal methyl hydrogens other than in the α-position on aliphatic chains were 0.21:0.22:0.44:0.13. The molar ratio of aromatic/saturated carbon atoms was 0.64:0.36. Previous studies[15-18]reported that, in asphaltene molecules, heteroatoms such as S, N, and O exist in the form of thiophene sulfur, pyridine or pyrrole nitrogen, and carboxyl or hydroxyl groups,respectively. Based on the molecular structural parameters and NMR spectra of asphaltene samples discussed above, we obtained the average molecular model of Tahe asphaltene shown in Figure 2.

Figure 1 13C and 1H NMR spectra of asphaltene obtained from Tahe heavy oil

Figure 2 Molecular model of Tahe asphaltene

3.2 Asphaltene-xylene equilibrium system

We built a periodic system consisting of 44 asphaltene molecules and 1508p-xylene molecules (44AS-1508XYL), with an asphaltene mass concentration of 19.5%. After energy minimization followed by MD simulations in the NPT and NVT ensembles, we obtained the stable equilibrium system shown in Figure 3.

Asphaltene molecules mainly exhibited paralleldisplaced stacking[19-20], along with a small amount of T-shaped stacking. Some hydrogen bonds were present between asphaltene molecules. As shown in Figure 4,these bonds mainly connected two carboxyl groups,a carboxyl and a hydroxyl group, or two hydroxyl groups. Two asphaltene molecules were found to be connected through the following configurations: (a)parallel-displaced stacking; (b) T-shaped stacking; (c)two hydrogen bonds between two carboxyl groups; (d)one hydrogen bond between a carboxyl and a hydroxyl group; (e) one hydrogen bond between the carbonyl oxygen of an asphaltene molecule and a hydrogen atom of the carboxyl group of another asphaltene molecule;(f) partial π-π stacking with one hydrogen bond between two hydroxyl groups.

Figure 3 Asphaltene-xylene equilibrium system (44AS-1508XYL). Asphaltene and p-xylene molecules are shown in red and grey, respectively

For non-periodic systems, the interaction energy between two molecules is equal to the sum of the energies of the two molecules alone minus the energy of the two molecules interacting with each other[19], as shown in Equation (1):

whereE1and E2are the respective energies of the two isolated molecules andEtotalis the total energy of the two molecules stacked together.

The interaction energies in Figure 4 were calculated according to Equation 1, and the results are shown in Table 3.

Table 3 Interaction energies between asphaltene molecules

As shown in Table 3, the largest interaction energy among the six possible configurations was found for two asphaltene molecules in parallel-displaced stacking; in this arrangement, the system would exhibit the lowest energy and the highest stability. This indicates that the asphaltene molecules tended to stack in parallel-displaced configuration. Because xylene can penetrate the interlayer of asphaltene laminates to disperse asphaltene molecules,not all asphaltene molecules were stacked together in parallel-displaced orientation.

3.3 Simulations of self-diffusion in asphaltene-xylene system

In order to investigate the effect of carboxyl and hydroxyl groups on the self-diffusion coefficient of asphaltenes,we built two molecular models of decarboxylated and dehydroxylated asphaltene, respectively, as shown in Figure 5.

Using the Packmol software, 10 pristine, 10 decarboxylated, and 10 dehydroxylated asphaltene molecules were randomly arranged together with 848p-xylene molecules within three periodic systems. MD simulations were performed with the OPLS-AA force

field using the Gromacs software. The periodic models of the stable systems are shown in Figure 6.

Figure 4 Types of connection between asphaltene molecules. Grey, white, red, blue, and yellow spheres represent carbon,hydrogen, oxygen, nitrogen, and sulfur atoms, respectively

Figure 5 Molecular models of decarboxylated and dehydroxylated asphaltene

Figure 6 Asphaltene-xylene equilibrium systems (10AS-848XYL)

The number of hydrogen bonds were calculated using the gmx hbond program implemented in the Gromacs software. The numbers of hydrogen bonds between asphaltene molecules in the pristine asphaltene-xylene,decarboxylated asphaltene-xylene, and dehydroxylated asphaltene-xylene equilibrium systems were 13, 0, and 10, respectively. No hydrogen bonds were found in the decarboxylated asphaltene-xylene equilibrium system,which indicated that hydrogen bonds mainly originated from carboxyl groups. Carboxyl groups would increase the interaction energies between asphaltene molecules,due to the easy formation of hydrogen bonds.

After continuing the MD simulations of asphaltene-xylene equilibrium systems in the NVT ensemble, we calculated the MSD and self-diffusion coefficient of the asphaltene molecules. The obtained MSDνs. simulation time curves are shown in Figure 7.

The 500-1500 ps time window in Figure 7 was selected as the interval for calculating the diffusion coefficient of the asphaltene molecules. These sections of the curves were approximately linear and were located in the middle of the plots, thus eliminating interferences at the beginning and end of each simulation; the slope was calculated every 50 ps. Each MD simulation in the NVT ensemble was repeated five times to calculate the MSD and slope values. The statistically calculated slope data are shown in Figure 8, where each point represents one calculated MSDsimulation time slope value. The mean values of the MSD slopes for the pristine asphaltene-xylene, decarboxylated asphaltene-xylene, and dehydroxylated asphaltene-xylene equilibrium systems were 0.00037, 0.00077, and 0.00066 nm/ps, respectively. The corresponding self-diffusion coefficients, calculated as one-sixth of the slope value[21],were 0.062, 0.13, and 0.11 m/s, respectively.

Figure 7 MSD vs. simulation time curves of asphaltenexylene equilibrium systems

Figure 8 Scatter plots of MSD slopes of asphaltene-xylene equilibrium systems

The self-diffusion behavior of asphaltene molecules is mainly affected by molecular interactions which are divided into van der Waals and Coulomb interactions. The former interactions are mainly short-range interactions.

And the corresponding energies are mostly reflected inEVDW-SRterm. The long-range component is represented in the form of dispersion correction, i.e.,EDisper.-corr.. The latter interactions are electrostatic interactions. Using the particle-mesh Ewald (PME) method, the Coulomb interactions are expressed as the sum of short-range Coulomb energies at a cutoff distance (ECoulomb-SR) and long-range Coulomb energies calculated in reciprocal space (ECoul.-recip.). The trajectories of the asphaltene and xylene components were extracted separately from the appropriate MD trajectories in the NVT ensemble.Two single reruns (gmx rerun) of the MD simulations were carried out to calculate the respective energies of the asphaltene and xylene molecules. The molecular interaction energies of the asphaltene-xylene equilibrium systems are shown in Table 4.

Table 4 Molecular interaction energies of asphaltenexylene equilibrium systems (kJ/mol)

The interaction energy between the asphaltene and xylene components was calculated by subtracting the molecular interaction energies of each component from the total molecular interaction energy of the system[22], as shown in Equation (2):

whereEAandEBare the molecular interaction energies of the asphaltene and xylene components, respectively,whileESYSis the total molecular interaction energy of the asphaltene-xylene equilibrium system.

As shown in Table 4, the molecular interaction energies of the asphaltene component of the pristine asphaltenexylene, decarboxylated asphaltene-xylene, and dehydroxylated asphaltene-xylene equilibrium systems were 6045.49, 4247.88, and 5507.67 kJ/mol, respectively.The interactions between asphaltene molecules mainly originated from π-π interactions between aromatic rings and hydrogen bonds between strongly electronegative heteroatom groups. As the molecular models of pristine,decarboxylated, and dehydroxylated asphaltene contained the same number of aromatic rings, their π-π interaction energies were similar. The difference in the molecular structures of pristine, decarboxylated, and dehydroxylated asphaltene molecular models was that decarboxylated asphaltene contained one less carboxyl group and dehydroxylated asphaltene had one less hydroxyl group than the pristine molecule. Therefore, the differences in the interaction energies of asphaltene components in these three systems were mainly due to differences in hydrogen bonds.The interaction energies between the asphaltene and xylene components of the pristine asphaltene-xylene,decarboxylated asphaltene-xylene, and dehydroxylated asphaltene-xylene equilibrium systems calculated according to Equation (2) were 3585.25, 3695.14, and 3733.42 kJ/mol, respectively. The molecular interactions between asphaltene and xylene components mainly originated from π-π interactions between aromatic rings.The pristine asphaltene-xylene equilibrium system exhibited a lower interaction energy between asphaltene and xylene than the other systems, which could be because the asphaltene molecules in this system were more tightly aggregated. The reduced contact between the asphaltene and xylene molecules resulted in a lower interaction energy between the two components.

The comparison of the molecular interaction energies and the asphaltene self-diffusion coefficients of the three systems showed that the asphaltene self-diffusion coefficient was inversely proportional to the molecular interaction energy between asphaltene molecules. The viscosity of heavy oil is mainly affected by the asphaltene components[23]; asphaltene diffuses slowly in heavy oil,resulting in high viscosity[24]. This means that the viscosity of heavy oil is mainly affected by the interaction energy between asphaltene molecules: the larger the interaction energy, the higher the viscosity. In order to reduce the viscosity of heavy oil, it is necessary to weaken the molecular interactions involving asphaltene, including the π-π interaction between aromatic rings and the hydrogen bonds between strongly electronegative heteroatom groups.

4 Conclusion

Several models of asphaltene-xylene systems were built to simulate heavy oil. The arrangement of asphaltene molecules in the asphaltene-xylene equilibrium system was dominated by parallel-displaced stacking, with a small amount of T-shaped stacking configurations.Some hydrogen bonds were present between asphaltene molecules. These bonds mainly existed between two carboxyl groups, a carboxyl groups and a hydroxyl group,and two hydroxyl groups; they generally originated from the carboxyl groups of asphaltene molecules. Larger interaction energies of asphaltene molecules corresponded to smaller self-diffusion coefficients of asphaltene. These results may provide important theoretical guidance for reducing the viscosity of heavy oil.

Acknowledgement:This work is financially supported by the National Natural Science Foundation of China (Grant No.41872168).