Effect of Asphaltene Polarity on Wax Precipitation and Deposition Characteristics of Waxy Oils

Abstract

Asphaltenes are natural pour point depressants, and the effect of asphaltene polarity on the low-temperature rheology of waxy oils has been well studied. In this paper, the influence of asphaltene polarity on the wax precipitation and deposition characteristics of synthetic waxy oils was investigated through a differential scanning calorimeter (DSC), a rheometer, a polarizing microscope, and a Couette wax deposition experimental device. It was found that the asphaltenes with lower polarity have higher H/C ratio and can disperse into smaller associating particles in the synthetic waxy oils, which is beneficial for the cocrystallization interaction between asphaltenes and wax molecules, and also provide more nucleating sites for wax molecules. With the decrease of asphaltene polarity, the wax crystals of synthetic waxy oils are gradually changed from long needle-like to small needle-like or nearly spherical, which is not conducive to the overlap and insertion of wax crystals, thus greatly improving the rheological properties of synthetic waxy oils. The addition of asphaltenes with lower polarity can significantly reduce the wax deposition rate but increase the aging rate; that is, a thinner and harder wax deposit tends to form with the decrease of asphaltene polarity. Meanwhile, the formed wax deposits become heterogeneous along the radial direction after the asphaltenes addition; the wax contents of the wax deposits increase gradually from the bottom layer to the surface layer. On the one hand, with the decrease of asphaltene polarity, the low-temperature structural strength of synthetic waxy oils is continuously decreased. Therefore, the initial wax deposit layers require higher solid wax contents to resist shearing of pipe flow. On the other hand, the lower viscosity and smaller aspect ratio of wax crystals are also beneficial for the diffusion of wax molecules in the bulk oil and wax deposit, which increases the aging rate of wax deposits. Therefore, with the decrease of asphaltene polarity, the difference of wax content between the surface and bottom wax deposits in the formed heterogeneous wax deposit structure becomes larger and larger.