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theses

In this work we model aggregation of heavy polyaromatic hydrocarbons in solution of aliphatic and aromatic hydrocarbons, effectively mimicking crude oil using dissipative particle dynamics. It has both fundamental and methodological aspects. First of all, this is the first (as far as we know) attempt to model solutions of geometrically complex molecules. Polyaromatic molecules are geometrically complex compared to, for example, common surfactants modelled by DPD due to presence of polyaromatic cores that form flat sheets in thickness of only one carbon. The anisotropy of molecules translates into anisotropic structures of the aggregates where the cores “stack” on the top of each other, therefore, computationally efficient DPD simulations should use beads of different effective diameters. The first part of the thesis describes the experience of building models of solutions of polyaromatic compounds using differently sized beads. We generally follow the “top-down” approach: the parameters are chosen to provide the best match to common thermodynamic properties of reference bulk solutions of hydrocarbons: molar volumes, activity coefficients, and solubilities. Bonded parameters are chosen from the geometrical considerations and atomistic simulation results. Having developed the DPD forcefield, we compose models of characteristic asphaltenes of different molecular mass and geometry and model their aggregation. The results show that the behavior of polyaromatic systems cannot be described with a single characteristic asphaltene model. The presence of archipelago and big asphaltenes considerably increases the size of the aggregates and makes the shape much more complex; we could follow the birth of fractalic structures typical during the asphaltene precipitation process. At the same time, the toluene insoluble fractions only weakly influences by the presence of smaller asphaltenes. The presence of smaller polyaromatic compounds with higher hydrogen to carbon ratio indeed substantially increase the dispersity of the system hindering asphaltene aggregation.

ETD_7677

M.S.

Includes bibliographical references

by Tianying Ma

doi:10.7282/T3445PS6

ETD graduate

The author owns the copyright to this work.