Comb-like Polyoctadecyl Acrylate (POA) Wax Inhibitor Triggers the Formation of Heterogeneous Waxy Oil Gel Deposits in a Cylindrical Couette Device

In real crude-oil pipelines, the formed wax deposits could be heterogeneous along the radial or axial directions. However, studies on this aspect are scarce. In this paper, the effect of the polyoctadecyl acrylate (POA) wax inhibitor on the wax deposition of 10 wt % model waxy oil was investigated with the aid of an in-house wax deposition device, differential scanning calorimetry test, high-temperature gas chromatography Fourier transform infrared analysis, and microscopic observation. The results showed that adding POA in the model waxy oil greatly modifies the morphology of precipitated wax crystals, thus greatly changing the wax deposition behavior of the oil. For oil sample 1 (without POA), the precipitated wax crystals (needle-like) more easily form porous network structures, causing the formation of a thick and porous wax deposit with relatively low WAT, wax content and critical carbon number (CCN). With the increase of POA concentration (oil samples 2-5), the precipitated wax crystals become more regular (spherical-like) and more compact, which favors the formation of thinner but more-compact wax deposits with relatively high WAT, wax content, and CCN. It was also found that adding a small amount of POA (50-200 ppm, oil samples 2-4) triggers the formation of heterogeneous wax deposits along the radial direction. When the POA concentration increases further (>400 ppm, oil sample 5), however, the formed wax deposit structure turns back into a homogeneous one. Based on the fact that the POA molecules participate in wax precipitation and concentrate in the wax deposits, it is deduced that the continuous depletion of POA in bulk oil with deposition tithe triggers the formation of the heterogeneous wax deposits. The lower the original POA concentration is, the more obvious the depletion of POA in bulk oil is, resulting in the formation of a more-explicit heterogeneous wax deposit structure. At the POA concentrations >400 ppm, however, the depletion of POA in bulk oil is so slow that it has little influence on the wax deposition process, thus turning back the wax deposit structure into a homogeneous one.