Search

Rheological measurement of wax–oil gel breakage is highly susceptible to the phenomenon of adhesive breakage, hindering instrument-scale replication of cohesive breakage processes. Adhesive breakage measurements are notoriously irreproducible due to strongly nonaffine gel deformation. Efforts to ensure mechanical fixation give rise to spatially inhomogeneous deformation fields in the measuring geometry, particularly with respect to azimuthal and radial location. To elucidate the functional role of mechanically fixating geometries during gel breakage processes, three model solutions were prepared containing 5, 7.5, and 10 wt % macrocrystalline wax in dodecane. Rheograms were acquired in controlled deformation mode at imposed shear rates in the range of 0.1–1.0 s–1 using a vane or a cone and plate geometry. Yield stress values, nominally ascribed to primary peak height, were established based on 95% confidence intervals. Yielding trends confirm that adhesive breakage is particularly pronounced in high solid-fraction gels. A solid-fraction threshold delineates cohesive breakage in low solid-fraction gels from inherent adhesive breakage in high solid-fraction gels. Mechanical fixation in a vane geometry precludes wall slippage, ensuring cohesive breakage; resultant yield stress values follow a modified power-law dependency on the total wax content, characterized by a power-law exponent of ∼1.25. Nonuniform deformation within the vane geometry confers a modest (artificial) reduction in apparent yield stress value as a consequence of azimuthal integration of the torque signal. Nonuniform deformation also confers a distinct (artificial) broadening of the breakage peak and is accompanied by the appearance of a new shoulder peak located at a deformation value of ∼5. Conversely, in the cone and plate geometry, adhesive breakage occurs inherently for high solid-fraction gels and is manifested by a substantial reduction in measured yield stress, albeit without a concomitant peak broadening. Hence, the practical utility of the cone and plate geometry is limited to low solid-fraction gels that inherently exhibit cohesive breakage behavior. Mechanical fixation afforded by the vane geometry effectively precludes wall slippage, enhancing measurement reproducibility while ensuring a cohesive breakage of high solid-fraction wax-gels that otherwise rupture in adhesive mode. Locked until 22.1.2020 due to copyright restrictions. This document is the Accepted Manuscript version of a Published Work that appeared in final form in [Energy & Fuels], copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.energyfuels.8b02725

The influence of microcrystalline wax addition upon the rheological properties of model wax-oil gels is investigated. Addition of less than 1 wt % microcrystalline wax to a model oil consisting of 5 wt % macrocrystalline wax in dodecane shows no significant impact on the WAT and gelation temperature. Beyond 1 wt % added microcrystalline wax, increases in WAT and gelation temperature are observed and are attributable to the higher crystallization temperature of the microcrystalline wax. The effect of microcrystalline wax addition upon the WAT and gelation temperature is shown to be attributed to merely overlapping compositions of macro- and microcrystalline wax. However, a substantive effect of microcrystalline wax addition is observed on the yield stress. Addition of 0.13 wt % microcrystalline wax reduces the yield stress of the waxy oil model from 238.0 to 22.5 Pa. Addition of 0.5 wt % microcrystalline wax decreases the yield stress to 5.4 Pa, which is close in value to the yield stress of neat 5 wt % microcrystalline wax gel. Microscopic images reveal two mechanisms leading to formation of weak mixed wax gels. At low to medium addition of microcrystalline wax, microcrystalline crystallites formed during cooling provide nucleation sites for subsequent precipitation of macrocrystalline wax. Macrocrystalline wax crystals formed in contact with microcrystalline crystallites are smaller in size and the growth is localized, in comparison to neat macrocrystalline wax. Modified macrocrystalline wax precipitation leads to uneven dispersion of the macrocrystalline crystals in the liquid phase. At high concentration, microcrystals form throughout the sample prior to precipitation of macrocrystalline wax. Hence, only a small and discrete space remains for macrocrystalline crystals to grow, forming small crystals. Interlocking among macrocrystals is spatially hindered by the presence of microcrystalline crystallites. In this condition, the system completely behaves as a microcrystalline gel. This investigation provides a plausible mechanistic account for the known gel weakening activity of microcrystalline wax. © American Chemical Society 2018. This is the authors accepted and refereed manuscript to the article. Locked until 23.04.2019 due to copyright restrictions.

Ice crystallization in confined spherical geometries is investigated experimentally at ambient pressure conditions. Water-in-crude (w/o) oil emulsions are formed by homogenization of an acidic North Sea crude oil with water or brine, at aqueous phase fractions ranging from 1 to 30 wt % and varying electrolyte contents. Ice-in-oil dispersions are formed from the emulsions by cooling, and they provide a justified analogue to gas hydrate formation in water-in-crude oil emulsions, due to analogous wettability conditions that govern agglomeration. Nuclear magnetic resonance (NMR) spectroscopy and digital video microscopy (DVM) imaging establish droplet size distributions (DSDs) and mean droplet diameters, and demonstrate emulsion stability with an absence of coalescence over extended time durations. Differential scanning calorimetry (DSC) establishes the crystallization temperature of the dispersed water droplets. It is demonstrated that the crystallization temperature decreases with a decreasing length scale of the dispersed water droplet phase, in accordance with theoretical knowledge. © 2017. This is the authors’ accepted and refereed manuscript to the article. Locked until 21.5.2018 due to copyright restrictions.