Physical and theoretical modelling of wax deposition in thermally-driven and sloughing regimes: the effect of the physical and chemical environment

This research considers the fundamental understanding of physical and theoretical modelling of wax deposition under dynamic conditions. The effect of physical environment i.e. the shear rate and the associated shear stress and heat transfer was investigated using a custom-built bench top Cold Rotating Finger (CRF) and a Quartz Crystal Microbalance (QCM) with heat transfer. The effect of chemical environment was studied by assessing wax deposition phenomena in the presence of wax inhibitors using the aforementioned techniques.

The CRF with an inner rotating cylinder is developed and characterised to provide the opportunity to study wax deposition on cold surfaces in the region of high fluid shear stress and laminar/transitional fluid flow. Time-dependent wax deposition study was performed at fixed operating temperatures and CRF rotational speed ranging from 0 to 700 rpm. The mass of the wax deposited was shown to decrease with the increase in the CRF rotational speed. At low rotational speeds (< 400 rpm), the reduction in the amount of deposited wax corresponded to a decrease in the bulk oil temperature (To) with increasing CRF rotational speed reducing therefore the overall temperature gradient driving force. The wax deposition at low rotational speeds was then described as a thermally-driven process. At higher rotational speeds (≥ 400 rpm), the change in temperature was negligible with sloughing of wax layers visually observed. The wax deposition at high rotational speeds was then described as a sloughing-driven process. A diffusive mathematical model based on molecular diffusion considering the heat transfer as governing wax deposition was elaborated assuming a linear solubility with temperature. For high CRF rotational speeds the molecular diffusion model could not accurately describe the wax deposit mass and the model was modified to include a sloughing term, described in terms of wall shear stress.

The study of formation of the first incipient wax layer on a cold surface was not possible with the CRF. Hence, a modified QCM with heat transfer across the quartz sensor was developed to enable the understanding of the crystallization of the first n-alkane wax on a cold surface as well as the effect of inhibitors on the n-alkane crystallization. The QCM was able to detect the deposition of wax crystals by analyzing the frequency shift as compared to the resistance. A large shift in the frequency (as compared to viscous frequency shift), all along with a small resistance, indicates a rigid mass load on the crystal due to the crystallization of the n-alkane on the QCM sensor.

The research considers the effect of the chemical environment on wax deposition by studying the performance of four ethylcellulose n-alkane wax inhibitors using the CRF at the static regime (0 rpm), the thermally-driven regime (100 rpm) and the sloughing regime (400 rpm). The inhibitors were characterized by 3 different degree of substitution (DS = 1, 0.6 and 0.4) and two alkyl pendant chain lengths (C22 and C18). The presence of inhibitors in a model solution of 5 wt% wax-in-dodecane/toluene (1:1) reduce the wax deposition with an inhibition that range from 10 to 35% with inhibitors efficiency depending on two main parameters: the structure of the inhibitor itself (DS and pendant alkyl chain) and the flow regime. The structure of the wax inhibitor determines its ability to interact with the n-alkane wax molecules and inhibit their deposition. A higher degree of substitution and a longer alkyl pendant chain induce a higher van der Waals interaction with wax molecules and a higher inhibitor efficiency, the inhibitor is more effective with similar carbon chain length, i.e. LMW n-alkane. A given flow regime, however, might promote the deposition of heavier or lighter molecules changing therefore the efficiency of the inhibitors as they are more effective for lighter wax.