Molecular Simulation of N-Octacosane – Water Gtl Mixtures in Nanopores at Elevated Temperature and Pressure

Water - wax mixtures are products of the Gas-To-Liquids (GTL) process through the Fischer-Tropsch (FT) route, a polymerization reaction converting syngas (H₂ and CO)¹ into liquid hydrocarbons and, to a smaller extent, oxygenates (e.g. alcohols). Transport properties of these mixtures, particularly inside catalyst nanopores, is a topic of significant interest for the petrochemical industry. The choice of material for catalyst support is of paramount importance as its surface structure and pore size,² is essential in ensuring FT reactor’s activity and stability, given that these features control metal catalyst dispersion, hydrocarbon selectivity reactant and product diffusion.³ Typical catalyst carriers are alumina, silica, titania, carbon, etc.⁴ In this study, we chose titanium dioxide as it is commonly used in the literature and its structure is well defined.⁵ It is generally assumed that inside the catalyst nanopores and under reaction conditions, the overall mole fraction of water inside the nanopores varies from less than 0.01 up to as high as 0.20, corresponding to the top and the bottom, respectively, of the FT fixed-bed reactor. This assumption may not hold especially in the outlet of the FT reactor, where water concentration is significantly higher.⁶ Furthermore, excess water inside the nanopore could lead to sintering of catalytic nanoparticles,⁷ shortening the catalyst’s lifetime and increasing GTL plant’s operational costs. It is thus important to understand: a) the water–wax vapor–liquid equilibrium (VLE) at FTS reactor operation conditions, as it determines the maximum allowable amount of water in the FT wax, b) the phase behavior of water–wax mixture inside the nanopores, by incorporating wall effects and c) the role of oxygenates (e.g., alcohols and acids < 10 wt % in wax)¹ on the phase behavior of water–wax inside nanopores and their interactions with the catalyst wall.

The present study focuses on simulating the phase behavior of the n-octacosane (n-C₂₈) – water mixture inside TiO₂ nanopores. Molecular Dynamics (MD) simulations with realistic molecular models^8-11 were employed, in order to account for the fluid–fluid and fluid–pore interactions, pore shape and size. Our simulations reveal the importance of confinement especially on the excess water–wax mixture transport properties, as water molecules tend to organize into two discrete layers on the TiO₂ surface.

Keywords: Gas-to-Liquids, water/n-octacosane mixture, Molecular dynamics

References

1. Wood, D. A.; Nwaoha, C.; Towler, B. F., J. Nat. Gas Sci. Eng. 2012, 9, 196-208.

2. Khodakov, A. Y.; Chu, W.; Fongarland, P., Chem. Rev. 2007, 107, 1692-1744.

3. Jahangiri, H.; Bennett, J.; Mahjoubi, P.; Wilson, K.; Gu, S., Catal. Sci. Technol. 2014, 4, 2210-2229.

4. Oh, J.-H.; Bae, J. W.; Park, S.-J.; Khanna, P. K.; Jun, K.-W., Catal. Lett. 2009, 130, 403-409.

5. Jongsomjit, B.; Sakdamnuson, C.; Praserthdam, P., Mater. Chem. Phys. 2005, 89, 395-401.

6. Panahi, M.; Rafiee, A.; Skogestad, S.; Hillestad, M., Ind. Eng. Chem. Res. 2012, 51, 425-433.

7. Qin, Q.; Ramkrishna, D., Ind. Eng. Chem. Res. 2004, 43, 2912-2921.

8. Martin, M. G.; Siepmann, J. I., J. Phys. Chem. B 1998, 102, 2569-2577.

9. Utesch, T.; Daminelli, G.; Mroginski, M. A., Langmuir 2011, 27, 13144-53.

10. Bandura, A. V.; Sykes, D. G.; Kubicki, J. D., Abstr. Pap. Am. Chem. S. 2002, 223, U603-U603.

11. Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P., J. Phys. Chem. 1987, 91, 6269-6271.