(155d) Measuring and Modelling Wax Deposit Kinetics in Thermally-Driven and Sloughing Regimes

Paraffin wax, a
major component of heavy crude oil, solidify when subjected to temperatures
below its cloud point, causing freezing fouling problem on wall of sub-sea
pipelines.

background:white">To assess wax deposition, kinetic (0-30 minutes) and
dynamic (0-750 s^-1) experiments were performed on a novel laboratory-scale
device termed “the Cold Rotating Finger” (figure a). The CRF features a cooled
rotating shaft where deposition takes place under well-defined heat
and mass transfer characteristics.

A one-dimensional mathematical
model based on molecular diffusion was derived and used as a framework to
analyse the experimental results obtained with the CRF. It assumes a constant wax
diffusivity and solubility and the heat transfer coefficient which vary with
shear rate is the main driving force of wax deposition on the wall of the CRF.
The model assumes that the system evolves in two stages, linear and asymptotic
(see figure b).

The kinetic results
show an initial rapid deposition of wax (with ~85% of the total mass deposited occurring within 5 min) followed
by a slower growth rate that tends to be asymptotic after a relatively short
time (15 min).

The dynamic study shows however, a
difference in wax deposition behaviour with increasing shear rate. The deposited
mass decreases with increasing shear rate (a decrease of 47% for 150 s^-1
and 70% for 750 s^-1). Compositional analysis of the deposited wax
layer also revealed a shift toward heavier alkane chains at higher shear rate.

Comparison of the mathematical
model with experimental measurements shows satisfactory agreement at low shear
rate (0-150 s^-1) where the wax build-up is thermally-driven as shown
in figure b. The model, coupled with temperature monitoring in the CRF, shows a
significant difference in heat transfer with shear rate (the bulk oil temperature
shows a difference of 4^oC between 0 and 150s^-1). However,
when increasing the shear rate up to 750 s^-1, the temperature in the
system tends to an asymptotic value (a difference of 4.5^oC between 0
and 750s^-1) and the mathematical model over predicts the final mass of
the wax deposit. Analyses of the experimental data at 750s^-1 show
that sloughing (i.e., mechanical removal of
wax induced by high wall shear stress) becomes significant and contribute to reduce
wax deposition. The sloughing effect, proved experimentally, is now being
incorporated in the model to predict wax build-up accurately under high shear
rate regime.

(Words: 382)

Figure (a): The Cold Rotating Finger set-up; Figure (b): Experimental and
theoretical mass deposit growth at 0, 50 & 150 s^-1.