Wax crystallization and aggregation in a model crude oil

Abstract

The high-molecular-weight paraffinic (‘wax’) fraction separates from crude oils at low temperatures, a process that can lead to a sol–gel transition when the mass of wax solids exceeds 1–2%. Attractive interactions between the micron-size wax solids suspended in the non-polar medium have been suggested to be responsible for gel formation. The present study reports an optically transparent model oil system, based on a mixture of linear and branched paraffins. Rheological measurements and optical microscopy show that the model system reproduces essential features of crude oil gels. Small-angle light scattering studies conducted at temperatures intermediate between the cloud point (58 °C) and sol–gel transition (39 °C) show that phase separation and wax solid aggregation are rapid processes, leading to the formation of dynamically arrested structures well above the sol–gel transition determined rheologically. Analysis of gravity settling effects has provided a rough estimate for the yield stress of the wax particle network formed (greater than 0.7 Pa at 45 °C and 0.07 Pa at 55 °C). Clusters formed by the aggregated wax solids possess a fractal dimension of about 1.8, consistent with diffusion-limited cluster–cluster aggregation.

Structure of waxy crude oil emulsion gels

“…In the case of waxy crude oil gels, the existence of a scaling law region has been interpreted considering the inner structure of gel as characterized by a fractal order. This has been observed directly by means of light scattering measurements for a mixture of paraffins which mimics the principal characteristics of a waxy crude oil, but has a simpler composition [14]. The theoretical analysis of the framework used for describe the internal structure of waxy crude oil gels is reported in [8] but for sake of completeness we report here some of the main findings.…”

On the Influence of Micro- and Macro-Cristalline Paraffins on the Physical and Rheological Properties of Crude Oil and Organic Solvents

“…According to R. Visintin et al [5] there is a body of data that supports that this gelation process results from the creation of "… a network of wax crystals that present a strong affinity…". Furthermore, "…the fast flocculation of wax crystallites, as soon as they have been formed in solution…", is also invoked by E. Vignatti et al [6] to explain this gelation process. Despite the research efforts on this matter, waxy crudes gelation mechanisms remain contentious in many aspects.…”

Wax Segregation in Oils: A Multiscale Problem

“…An additional information we got from experiment and from literature is that the mechanism of gelification -and hence its influence on diffusivityis strongly dependent on the nature of wax since the geometry of crystallites plays an important role ( [64]). …”