Wax deposition using a cold rotating finger: An empirical and theoretical assessment in thermally driven and sloughing regimes

Time-dependent wax deposition was studied using a newly-built Cold Rotating Finger (CRF). Wax deposition rates and pseudo-steady state deposit mass values were shown to depend on the CRF rotational speed. Relative to 0 rpm, the wax deposit mass decreased with increasing rotational speed, reducing by 54% at 100 rpm and 82% at 700 rpm. At low rotational speeds, the reduced wax deposit correlated with changes in the bulk oil (T-o) and wax interface (T-i) temperatures as a function of increasing CRF rotational speed, with the behavior described by a diffusive model which accounted for the CRF fluid motion. The model described the temperature profile in the boundary layer by considering the heat transfer coefficient as a function of the CRF rotational speed, with heat transfer governing wax deposition. Mass transfer was described by Fick's law assuming a linear solubility with temperature and constant diffusivity determined from experimental data at CRF = 100 rpm. The change in heat transfer governed the mass deposited, with deposition at low rotational speeds described by a thermally-driven process. At higher rotational speeds, T-o was independent of CRF rpm, although the wax deposit mass continued to decrease. Visual assessment of the CRF revealed sloughing at rotational speeds >= 400 rpm. For high CRF rotational speeds, the molecular diffusion model could not accurately describe the wax deposit mass and was modified to include a sloughing term, (S) over dot (t), accounting for the wall shear stress, deposit radius and theta, which was taken to be an adjustable parameter to describe the sloughing intensity.

Automated summary

Time-dependent wax deposition was investigated using a Cold Rotating Finger (CRF), revealing that deposition rates and mass values were influenced by the rotational speed. A diffusive model considering fluid motion described the reduced wax deposit at low rotational speeds, while a modified molecular diffusion model was needed for high speeds due to observed sloughing phenomena.