Precipitated wax content and yield stress of model wax-oil mixtures determined by arrest of flow during cooling at fixed stress

Abstract

To estimate yield stress and other rheological properties relevant to modeling of wax deposition in pipelines where flow is continuous, but stress varies across the pipe radius, model wax-oil mixtures are cooled at multiple cooling rates under constant shear stress until a temperature is reached, at which flow is arrested by gelation due to wax crystal formation. From these data, combined with a measurement of temperature-dependence of precipitated wax concentration by differential scanning calorimetry, an apparent yield stress σy, below which flow is arrested at each temperature, is related to the concentration of precipitated wax Cp and the cooling rate. Results are reported for multiple concentrations in oil of two independent wax mixtures: a many-component commercial wax mimicking the composition of field oil, and a simpler six-alkane mixture. These transient rheological data are fit to a pseudo “Herschel–Bulkley” constitutive equation from which it is found that the yield stresses obtained during flow under cooling are generally an order of magnitude, or more, lower than the yield stresses obtained in the previous work in flow at a comparable constant temperature after cooling in the absence of flow. We also find a strong decrease in the arrest temperature with a decreasing cooling rate, with no convergence even at the lowest cooling rate of 0.0625 °C min−1, indicating that under slower cooling, flow continues even under low stresses. The cooling-rate-dependent yield stress obtained in our study under constant stress provides a challenge to the recent models of gelation under flow stress and is of relevance to wax deposition in pipelines.

Interplay between wall slip and shear banding in a thixotropic yield stress fluid

“…38,39 Note that, for these gels, temperature (and more specifically, the subcooling with respect to T wa ) is the main driving force in the crystallization process, not shear. 38–41 From previous experiments, it is known that crystallization terminates relatively quickly once a temperature below T wa is reached. 33 Therefore, changes in the microstructure at the final working temperature only depend on the rearrangements of the already crystallized paraffin particles.…”

Modeling multicomponent wax deposition and aging on cold surfaces with application to pipeline fouling

“…In the previous model 5 (Equation (12) in the article of Mahir et al), the material balance equation (our Equation ( 1)) has an…”

“…While this assumption works well for the set of experimental data to which the model was compared, this solid fraction threshold will likely depend on flow characteristics, such as the wall shear stress, and the rheological properties of the gel, such as the dependence of its yield stress on volume fraction of wax precipitate. [12][13][14] In addition, the work of Hoteit et al 8 includes a significant contribution of thermally driven mass diffusion, while Merino-Garcia et al 15 dismiss its contribution as negligible for wax diffusion, and it is left out of the modeling of Veiga et al 9 Consequently, there is a need to improve the accuracy and reliability of wax deposition models, as well as measurements of critical model parameters, especially the various transport coefficients.…”