NMR Studies of Emulsions in Porous Media and Applications to Fischer-Tropsch Synthesis
As global reserves of conventional fossil fuels are dwindling, Fischer-Tropsch (FT) is a promising and greener method to convert natural gas into liquid fuels. However, it is highly complex and remains thus far poorly understood. The work presented in this thesis employs nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) to gain insight into and elucidate the multiphase behaviour and transport processes of Fischer-Tropsch Synthesis (FTS) reactants and products. In order to achieve this, the world’s first operando NMR compatible reactor was designed, commissioned and built to ultimately optimise the conversion of Gas-to-Liquids (GTL) through the FT process. A particular focus of this work lies on the application of a pulsed-field gradient (PFG) NMR emulsion detection technique to characterise hydrocarbon-in-water emulsions and study their droplet size distribution (DSD) inside porous media under operando conditions ($\geq$180 $\degree$ C and $\geq$36 bar). The initial study into droplet formation was done on $\theta$-Al2O3 pellets by relaxometry and PFG NMR measurements at ambient conditions, where dodecane and water were model compounds for FT products. The relaxation results indicated that water was the surface wetting phase, while dodecane was isolated from the pore surface. Spontaneous dodecane-in-water droplets formed, suggested by the restrictive diffusion behaviour for dodecane in the binary mixture. This trend was confirmed in Q-silica beads as well as in Ru-based FT catalyst pellets. Subsequently, the influence of (i) temperature, (ii) pore size, (iii) surfactant and (iv) surface modification on emulsion formation was investigated. Little impact of temperature and pore size was found for the ranges studied. However, a longer alcohol chain length lead to a small increase in droplet size. Furthermore, if the catalyst surface became hydrophobic with stearic acid, spontaneous emulsion formation did no longer occur. Studying FTS $\textit{in situ}$ can be very complex due to its high P and T. This thesis contains the first ever reported operando study of FT under real operating conditions. This work resulted in spatially-resolved MRI measurements of the diffusion and spatial mapping of product formation inside catalysts. These results can be used to validate numerical reactor simulations and optimise catalyst development. Finally, the phase behaviour of wax and water was studied post-FTS $\textit{in situ}$ in 1~wt$\%$~Ru/TiO2 catalysts at both 180 and 220~$\degree$ C. Emulsion formation was detected by slowly increasing the water partial pressure between 0.3~-~0.95. At $\textit{P}/\textit{P}\textsubscript{0}~\leq$~0.5, both water and wax diffused freely. However, at $\textit{P}/\textit{P}\textsubscript{0}~\geq$~0.8, capillary condensation of water inside the catalyst pores was reported. This resulted in the formation of a water-rich, surface-wetting layer, located in between the pore surface and the wax layer. The wax species were displaced and became isolated from the pore space, which was confirmed by the significant increase in both $\textit{T}\textsubscript{1}$ and $\textit{D}$ values. For $\textit{P}/\textit{P}\textsubscript{0}~\geq$~0.9, wax-in-water emulsions spontaneously formed. This was concluded from the sudden drop in the diffusion behaviour for wax.
Gas-to-Liquids, Fischer-Tropsch, Emulsions, Porous Media, Nuclear Magnetic Resonance, Magnetic Resonance Imaging, Pulsed-Field Gradient, Diffusion, Oil and Gas
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