Shear-Induced Change in Morphology of Wax Crystals and Flow Properties of Waxy Crudes Modified with the Pour-Point Depressant

摘要

Flow properties of waxy crude oils treated with the pour-point depressant (PPD) are affected by shear during pipeline transportation, which is called the shear history effect. In this study, the influence of shear on both the fractal dimension characterizing the wax crystal morphology and flow properties of eight waxy crudes treated with the PPD was examined. Further, the characteristic groups of waxes, resins, and asphaltenes contained in the crudes and the PPD were analyzed with Fourier transform infrared spectroscopy. A correlation was developed, with the fractal dimension of wax crystals being the dependent variable and both the entropy generation because of viscous flow during the shear and die parameters of oil composition being independent variables; furthermore, developed were correlations with flow properties of the gel point, viscosity, and yield stress being the dependent variables and both the decrease of the fractal dimension of wax crystals after being sheared and the parameters of oil composition being independent variables. The results suggest that shear at temperatures below the wax appearance temperature (WAT) could destroy the interactions because of the weak van der Waals forces between waxes, between waxes and resins, between waxes and asphaltenes, and between waxes and the PPD. As a result, die wax aggregates are broken, and the fractal dimension of wax crystals decreases. The new surfaces of broken wax aggregates provide new wax crystallization sites, which lead wax crystals to intergrow. Intergrowing wax crystals result in the decline in effectiveness of the PPD. The decrease in the fractal dimension of wax crystals and the decline in the effectiveness of the PPD have positive correlations with the average carbon number of waxes, the concentration of precipitated wax, and the wax content; however, they have negative correlations with the contents of resins and asphaltenes. This may help in making a new probe into the rheology- microstructure relationship.