Accurate prediction of physical properties for the Gas-To-Liquid process based on molecular simulation methods

The FT route is a polymerization reaction used extensively in the GTL process, which involves the transformation of synthesis gas (syngas, H2 and CO) to liquid hydrocarbons. This process is heavily utilized in the production... [ view full abstract ]

The FT route is a polymerization reaction used extensively in the GTL process, which involves the transformation of synthesis gas (syngas, H₂ and CO) to liquid hydrocarbons. This process is heavily utilized in the production of clean, high quality transportation fuel, which lacks aromatic hydrocarbons and its usage generates lower emissions (lower NO_x formation). The main FT reaction products, namely water, wax and small amounts of oxygenates (e.g. alcohols and carboxylic acids, < 10 wt %), form a mixture through which the dissolved reactants diffuse, reach the catalytic nanoparticles and react. The transport properties of these mixtures, particularly inside catalyst nanopores, is a topic of significant interest for the petrochemical industry. Liquid hydrocarbon production is controlled by both the choice of the metal catalyst and its support material. Key factors for FT catalyst design is the selection of the active materials, usually involving transition metals Fe, Co, Ni and Ru. AAmong them, Co has been established as the best choice in terms of its performance in industrial FTS applications. The choice of catalyst support material is also an important factor, as its structural features are essential in ensuring FT reactor’s activity and stability. Typical catalyst carriers include titanium dioxide (TiO₂), silicon carbide (SiC), silicon dioxide (SiO₂) and aluminium oxide (Al₂O₃).TiO₂ is commonly used as such, however, SiC –particularly its beta polymorph (β-SiC or 3C-SiC)– has also emerged as an attractive support in FT synthesis in the past years.

Unfortunately, Co FTS catalysts deactivate over time. Contributing factors to deactivation have been extensively reviewed in the literature. Among them, excess water reaction conditions are of particular interest, since – under these conditions – sintering of catalytic nanoparticles is observed, which is responsible for FT reactor’s reduced lifetimes and increased operational costs. The degree of catalyst deactivation increases with increasing water partial pressure caused by high water loads. Furthermore, oxygenates bear significant impact on FT refining catalysis, hydrocracking as well as catalyst deactivation. We have previously demonstrated by means of atomistic molecular simulations that inside TiO₂ nanopores and under the aforementioned conditions, water and wax form two separate phases, with water organizing into two discrete layers on the TiO₂ surface. The present study focuses on simulating the phase behavior of the n-octacosane (n-C₂₈) – high water content mixtures inside both TiO₂ and SiC nanopores, with explicit consideration of oxygenates. Molecular Dynamics (MD) simulations are employed in order to elucidate the fluid–fluid and fluid–pore interactions, as well as the importance of confinement on the wax – water mixture transport properties. Our study highlights: a) the phase behavior of water–wax mixture inside hydrophilic (TiO₂) and hydrophobic (SiC) nanopores and b) the role of oxygenates on the phase behavior of water–wax and their interactions with the catalyst wall.