Influence of Measuring Geometry on Rheomalaxis of Macrocrystalline Wax–Oil Gels: Alteration of Breakage Mechanism from Adhesive to Cohesive

Abstract

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 non-affine 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. In order to elucidate the functional role of mechanically fixating geometries during gel breakage processes, 3 model solutions were prepared containing 5 wt%, 7.5 wt%, and 10 wt% macro-crystalline 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 solidfraction 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 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 simultaneously ensuring cohesive breakage of high solid-fraction wax-gels that otherwise rupture in adhesive mode.

New Method for Gelation Temperature Measurement without Disturbing the Crystal Network

“…However, this method suffers due to the rheomalaxis and shear-thinning nature of the gel, as the destruction of the sample during shearing affects the gelation process. If the imposed shear causes irreversible changes in the gel structural state, then the gel is referred to as rheomalaxis, whereas the shear-thinning effect leads to reversible changes in the gel structure . DSC is commonly used to measure the glass-transition temperature of polymers, where the phase transition happens at a sharp temperature.…”

“…If the imposed shear causes irreversible changes in the gel structural state, then the gel is referred to as rheomalaxis, whereas the shear-thinning effect leads to reversible changes in the gel structure. 52 DSC is commonly used to measure the glass-transition temperature of polymers, where the phase transition happens at a sharp temperature. However, in the case of crude oil, the transition from the liquid to the gel phase occurs over a range of temperatures.…”

Advances and Future Challenges of Wax Removal in Pipeline Pigging Operations on Crude Oil Transportation Systems

“…It was reported that there exists a threshold solid wax fraction delineating the transition of wax yielding from cohesive breakage at low solid wax fraction to adhesive breakage at large solid wax fraction . To incorporate this transition, the following correlation may be more appropriate for calculating the wax layer yield stresswhere is the threshold solid wax content.…”

Wax Deposition in Crude Oil Transport Lines and Wax Estimation Methods

“…CPM provides information about the wax precipitation as wax appearance can be noticed with high resolution in small-size dimensions up to 0.5 μm. The volume of the sample stored for testing is very small, and CPM is sensitive to film thickness of sample which is dependent on the concentration of the sample [57][58][59][60][61][62][63][64].…”