Theoretical Modeling of Tertiary Structure of Paraffin Inhibitors

Abstract

Summary Theoretical molecular-mechanical modeling of conventional paraffin inhibitors was performed to determine a correlation between the tertiary structure of paraffin inhibitor polymers and their melting and glass transition temperatures, crystallinity, viscosity, and performance. The results allowed implementation of targeted syntheses of new advanced paraffin inhibitors. The physical properties and performance of these new paraffin inhibitors were successfully predicted by the molecular-mechanical modeling. Introduction n-Paraffins are the major component of waxy solid deposits from crude oil1–3. The preventive methods of paraffin deposition involve the use of chemical additives such as wax crystal modifiers also known as paraffin inhibitors, to distort or modify the undesirable crystal structure. These compounds, which are usually polymeric in nature, are able to interact with the wax crystals thus improving the flow characteristics and reducing the cohesive strength of a paraffin deposit, hence inhibiting wax deposition.4–7 The mechanism of the inhibition process is well described in the literature8–9. Commercial wax inhibitors are typically synthesized to be branched and/or to contain some bulky, preferably polar, functional groups that interfere with the n-paraffin nucleation and induce crystal imperfections. Crystallization of paraffin wax from crude oil in the presence of such polymer inhibitors results in the formation of smaller crystals that are less prone to further agglomeration. It has been theorized that paraffin inhibitor performance is based on the primary structure of the polymer chain including the effects of different functional groups and the length of the main/side chains. This extended abstract comprises describes structure of polymer wax inhibitors with experimental data on their physical properties and relates that information to their inhibiting performance. Also, it reveals a correlation between structures and properties of conventional polymers, using this knowledge to target synthesis of premium paraffin inhibitors. Results and Discussion It is important to comprehend how the steric structure of polymer inhibitors and conformation of their hydrocarbon chains correlate with the ability to interact and inhibit n-paraffins of crude. n-Paraffins are flexible hydrocarbon molecules and, hence, they have a tendency to align together upon cooling and precipitate from the crude oil as "stable" agglomerates of wax solids. The interaction of n-paraffins with hydrocarbon chains of paraffin inhibitors operates by the same mechanism, but the position of polar segments or bulky functional groups of the additive plays a key role in its capability to modify the wax crystal morphology. These effects are necessary to inhibit the agglomeration and control the n-paraffins in the oil. The experimental challenge is in determining the exact structure of polymer molecules. It is practically impossible to discern the specific structure of actual wax inhibitors because of the complex and diverse multi-component nature of the polymer products. Therefore, theoretical modeling of basic polymer structures is a more feasible approach. Three main types of conventional paraffin inhibitors were chosen for this study: ethylene-vinyl acetate (EVA) polymers, alpha-olefin-maleic anhydride (OMAC) polymers, and ester type of paraffin inhibitors such as polyacrylate and polymethacrylate. The most simplified and typical units of each polymer were taken in consideration for theoretical modeling by molecular mechanics calculation. The PC Spartan Plus program was used for the theoretical calculation of these paraffin inhibitors. Standard bond distances and bond angles were used to describe the molecular structure of the polymers. Molecular mechanics models were used to describe the energy of deviation of the polymer chain from the idealized geometry. The followed assumptions were made: the molecular geometry should correspond to the configuration with minimum energy of the distortions in bond distances, bond angles, and dihedral angles, together with contributions from "non-bonding" (van der Waals and Coulombic) interactions10.