(342av) Molecular Dynamic Investigations of Thermodynamic and Transport Properties of Trinidad and Tobago Asphaltenic Heavy Oils

Trinidad and Tobago heavy oils contain asphaltenes which can precipitate unpredictably and cause process disruptions such as coking during pyrolysis, pipe blockage and fouling of equipment and unwanted stabilization of oil-water emulsion¹. The mechanisms underlying the above are not well understood and therefore cannot be taken into account at the design stage and can impede plant utilization. Asphaltenes comprise many species of molecules with high carbon to hydrogen ratios than normal crude oil: they contain aromatic cores, hetero-atoms and aliphatic chains. Their characterization present challenges and yet such information process response to process conditions is crucial to process assurance.

Crude oil in which these asphaltene are contained are complex mixtures making it important to know how the asphaltene molecules interact with the solvent-mixture.

This research, aims to use Molecular Dynamics (MD) to shed light on asphaltene-asphalene, asphaltene-solvent and asphaltene-modifier interactions. Such interactions can be expected to influence asphaltene properties in Trinidadian-heavy oils under transportation, and processing conditions. This technique can also be used to optimize the addition of modifiers and to guide the manipulation of operational conditions which will mitigate the nuisance and risk they pose.

Figure 1: Types of Asphaltene Molecules

The objectives are to do the following:

1. Validate the use of GROMACS, a Molecular Dynamics software to calculate the trajectories of molecules and to use them to predict important transport properties of asphaltenes (Diffusion Coefficient & Viscosity), and predict thermodynamic properties of asphalt solution such as density and asphaltene molecular association.
2. Track the interactions which lead to the formation of clusters of asphaltenes.
3. Study the influence of the composition of solvent on clustering
4. Complete the design of an asphaltene molecule whose physical properties match those of asphaltenes encountered in heavy oils in Trinidad and Tobago.

METHOD:

The atomistic MD is used to represent bonded atoms in molecules by balls held together by springs. Newtonian mechanics is used to calculate molecular trajectories under Potential Fields using the GROMACS software. The motion of the molecules is determined by the application of Newton’s laws in potential field described by the Optimized Potential for Liquid Simulation All Atoms (OPLS_AA).

The OPLS_AA (eqn. (1)), with appropriately chosen parameters has been used to successfully model hydrocarbon liquids like Trinidad’s heavy oils². It comprises potentials due to: covalent bonds, Lennard-Jones forces, electrostatic forces due to partial charges on polar groups.

The Particle-mesh Ewald (PME) algorithm is used to calculate electrostatic interactions. The periodic boundary condition (PBC) and the Maxwell-Boltzmann distribution of the initial velocities are adopted. We use the Nose–Hoover thermostat and the Parrinello–Rahman barostat to control temperature and pressure in constant volume, constant temperature (NVT) simulations.

The system

The simulation box of size 7nm x 7nm x 7nm is filled with a chosen number of molecules of model asphaltene (see structure C in Figure 1) and remainder being the appropriate solvent or solvent mixture (e.g. heptane, toluene etc.) and additives such that the solute is 7wt/wt%.

Simulation Steps

The steps in the simulation are shown in the flowchart above. Asphaltene molecules are inserted in simulation box and solvent added to create a configured system. A check is carried out to see if there is net a charge and ions added to neutralize the mixture. The energy of the initial configuration in the system is minimized using steepest descent method. Next, equilibrate to the desired temperature and pressure by running the NVT ensemble (constant number of molecules, volume, and temperature) for 100ps. Then the simulation is run at constant number of molecules, temperature and pressure (NPT ensemble) for 100ps. The next stage is called Production MD.

Production Molecular Dynamics

A simulation time step of 2.0 fs was used

MD simulation lasted 40ns

Cut off for non-bonded interaction was fixed at 1.0nm

Berendsen thermostat and the Parrilello-Rahman barostat setting were used with time step of 2fs.

Radial Distribution Function (RDF) was used to analyse cluster formation.

The Results and Discussion

Diffusion Coefficient

Figure 2: Einstein’s Plot: Mean square Displacement between Molecules vs Time

The diffusion coefficient of asphaltene in heptane were estimated from the trajectories of the molecules using Einstein’s law³ in plotting the Mean Square Displacement (MSD) between molecules as a function of time as shown in the above Figure. The predicted values are comparable to measured values.

Viscosity

The TCAF routine in GROMACS⁴ was used to estimate time constants in the decay of the transverse-current autocorrelation functions of the fluctuations in velocity gradients in the liquid and the viscosity over timeframes inferred from them. In Fig. 3 the viscosity is plotted against k, a function of the decay constant. It shows viscosity converges to values which agree with experimental data.

Figure 3: Plot of Viscosity vs Parameter k

Cluster Analysis

Molecules separated by 3.5 â„« or less are considered associated and thus are part of a cluster. Fig. 4 below, shows the distance apart of two molecules in events, as a function of time. In vacuum, all the events recorded are below the red line and indicate all molecules are clustered, as expected. However, in the adjacent figure, the molecules tracked have not clustered in the presence of solvents molecules.

Figure 4: The distance between asphaltene molecules one and two as a function of time.

The picture frames of asphaltene molecules in various configurations in vacuum annealing experiment were recorded: to begin with all molecules are apart, nearly all the molecules have clustered during a simulation run for 5 ns of the molecules in vacuum. The molecules are stack laterally indicating interaction of π electrons above and below the plane of the molecules.

The picture frame in Fig. 5A show the molecules in heptane after 10 ns of production NVT simulation: some molecules are unclustered and others are clustered in pairs and in trimers. Some molecules are attached horizontally but displaced indicating π bonding between the aromatic cores; others are attached edgewise to the aromatic core. Edgewise attachments are not observed in MD runs in vacuo but may have been made possible due to solvation by heptane. The arrangements of the molecules in the clusters are expected to have implications for cluster growth, agglomeration, flocculation and other phase transitions.

Asphaltene in Heptane Asphaltene in “Crude” Dispersed Asphaltene in toluene

Figure 5A Figure 5B Figure 5C

Figure 5C shows the simulation box containing toluene and asphaltene molecules. Again some toluene molecules have been edited out for clarity. Unlike the simulation of asphaltene in heptane the formation of clusters of asphaltene molecules does not occur. We propose that the mechanism for reducing the probability of cluster formation (stacking) is steric in nature. The toluene molecules orient themselves transversely to the asphaltene molecules thereby making it unlikely that the asphaltene molecules are able to stack. This seems to explain why toluene is an effective solvent for asphaltenes.

A histograms of cluster sizes are recorded. The largest cluster contains 5 molecules however the relative occurrence is lowest compared to the monomer.

Figure 6 shows the probability of an asphaltene molecule belonging to clusters of varying sizes including the monomer after the simulation time of 100ns. The maximum cluster size contained 5 molecules. The proportion of molecules belonging to a cluster reduced from 36% for the monomer to 12.5% for the cluster with 5 molecules.

Figure 6

The Visual Molecular Dynamics software can be used to view the trajectories of the MD simulation over 40ns of 12 molecules of asphaltene in heptane (7wt/wt%). Clustering events are visible.

SUMMARY AND CONCLUSION

The use of GROMACS for atomistic MD simulation of asphaltene in vacuo and in heptane, toluene and “crude” are validated. The transport and thermodynamic properties can be evaluated from the trajectories to yield values in agreement with published experimental data. The phenomena of molecular aggregation and clustering in heptane is observed. Two types of stacking are observed – flat stacking and edgewise association with a cluster. In heptane only a minority of molecules remain unassociated. These results confirm that MD simulation can be used as a tool for investigating risks to flow assurance posed by asphaltenes and their risk mitigation.

FUTURE WORK

1. Complete the design an Asphaltic molecule to mimic better the properties of the cocktail of Trinidad and Tobago Asphaltenes.
2. Simulate more accurately the composition and processing conditions of Trinidad heavy oils to predict the inclusion of designer additives which can modify the properties of these oils favourably.
3. Parallize the computing facilities in the laboratory to improve computing power at minimal cost.

REFERENCES

1. M. Spiecker, KL Gawrya, PK Kilpatrick, J. Colloid & Interface Sci. 267, (2003) 178-193
2. Adrian Lutchman, F. Addo-Yobo. UTT Research Symposium September 2018
3. Kubo (1966), Report Progr. Phys 29, 255-259
4. Bruce J. Palmer. Rev. E49, 359
5. Gromacs User Manual version 5.0.4, gromacs.org