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

Abstract

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 (To) and wax interface (Ti) 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, To 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˙(t), accounting for the wall shear stress, deposit radius and θ, which was taken to be an adjustable parameter to describe the sloughing intensity.