From Components to Phase-Dependent Dynamics of Diffusivity in Wax Solutions Subjected to Fluid–Solid Phase Transition: Insights from Pulsed Field Gradient NMR

The evolution of solvent and solute diffusivity during fluid–solid phase transition was studied in model wax in n-dodecane solutions in a wide concentration range. Studied systems were characterized using viscosity measurements to provide supplementary information related to wax precipitation onset, while diffusion coefficients of n-dodecane and paraffin molecules were quantified using Pulsed Field Gradient (PFG) NMR. It was revealed that above the wax appearance temperature (WAT), the Hayduk–Minhas equation adequately predicts the solute and solvent diffusivity. At lower temperatures (below the WAT), three distinct diffusive components appear, which no longer originate from individual molecular components but correspond to a liquid phase differing in terms of association to the wax crystal network. These diffusion components were concluded to contain dodecane and the residual dissolved wax; the major components among them correspond to fluid, which relatively freely diffuses between the wax microcrystals and experiences the hindrance due to the wax gel network, and the minor components correspond to the fluid closely associated with the wax crystals. Unlike at high temperatures, the Hayduk–Minhas equation was found to be unable to predict adequately the diffusivity below the WAT. Using Singh’s approach, the aspect ratio of wax crystals was calculated for different temperatures and concentrations and its complex nonlinear behavior was observed. It turned out that none of the models available differentiate the fluids with respect to the wax crystal network that leaves out of modeling the diffusion components with reduced mobility. The results indicate that the intuitive paradigm of component-dependent dynamics of solvent and solute diffusivity should be changed to phase-dependent dynamics once the system turns into wax gel since the diffusion of separate components becomes the diffusion of separate phases. This understanding shows a new route to improving the wax deposition modeling, which will facilitate an increase of effectiveness of the remedial strategies in the petroleum industry.