(150a) Investigating the Effect of Asphaltenes on Wax Deposition Using a Taylor-Couette Cold Finger Device

Wax deposition brings severe risks to the safe operation of waxy crude pipelines. In previous wax deposition studies, the role of heavy components in the crude such as asphaltenes was almost neglected. The impact of asphaltenes on wax deposition was systematically investigated utilizing a Taylor-Couette cold finger equipped with a rotary outer cylinder and a coaxial stationary inner cylinder. This apparatus could generate precisely controlled flow fields compared with traditional cold fingers with stirring. The experiments were carried out by using model oils with asphaltene concentrations ranging from 0wt% to 0.4wt%. The mass, thermal characteristics, composition and rheological properties of the deposits were obtained through weighting, DSC, HTGC and rheometer, respectively. The content, molecular weight and dipole moment of asphaltenes separated from deposits with n-heptanes were then measured or calculated. The results indicate the following three points. (a) the addition of asphaltenes significantly decreases the deposit mass but increases the wax content (see Figu re 1 a ) and critical carbon number (CCN) of the deposit. In addition, asphaltenes suppress the increment of the shorter n-paraffin fractions in the deposit and inversely accelerate the increment of longer n-paraffin fractions. Conclusively, the presence of asphaltenes at experimental concentrations facilitates the aging process of the deposit. (b) the asphaltene content in the deposits constantly increases with deposition time elapsing. As shown in Fi gure 1b , the asphaltenes are enriched approximately 3-flod in the 1-hour deposit, 5-flod in the 12-hour deposit and over 6-flod in the 24-hour deposit compared to the oil. Moreover, both the number average molecular weight (Mn) and the weight average molecular weight (Mw) of asphaltenes separated from the deposits increase while the polydispersity index (PDI) decreases in contrast with the oil. Apparent dipole moment calculations indicate that asphaltenes extracted from the deposits are more polar than those from bulk solutions. This implies that the asphaltene components with high polarity or molecular weight are prior to entering into the deposit. Based on (a) and (b), we generalize the conception of wax deposit aging via incorporating in the enrichment of asphaltenes beyond the diffusion of paraffin molecules greater or less than CCN. (c) adding high concentration asphaltenes (more than 0.2wt% for our experiments) would contribute to the shear stripping of deposits because asphaltenes could remarkably weaken the structural strength of deposits. Taking model oil with 0.2wt% asphaltenes for example, the properties of the deposits on the cold finger prior to and subsequent to shear stripping are contrasted. The results demonstrate that deposit mass, gelation temperature and yield stress of deposits prior to stripping significantly decrease from a macro perspective, and individual carbon numbers and asphaltene content also decrease from a micro perspective compared with deposit subsequent to stripping. This study provides a fundamental understanding of the role of asphaltenes on wax deposition and a basis for modeling wax deposition of waxy crude pipelines.

This work is financially supported by the National Natural Science Foundation of China (No.51534007 and No.51134006).

(a)

（b）

Figure 1. Deposit wax content and asphaltene content variation with time for model oils with different asphaltene concentrations (a) wax content (b) asphaltene content