Page 179 - Handbook Of Multiphase Flow Assurance

Paraffin wax 175

Waxy gels may form in the pipeline when the normal paraffin content of the components
heavier than C 18 is usually greater than 3 wt%. Oils with lower wax content are less likely to
form gels.
One method to characterize gelling tendency is pour point measurement. A stock tank oil
sample is chilled to a set temperature in a flask, which is then tilted 90°. No fluid movement
for at least 5 s represents the condition when the pour point temperature was reached, and
the liquid has gelled.
The term gel is used qualitatively only to represent a formation of interconnected matrix of
solid crystals made of normal paraffins.
While illustrative, the use of the term “gel,” for example in (Singh et al., 2001), to de-
scribe the initial wax deposit on a cold surface can be misleading. The term “gel” in wax
terminology should be used for near-uniform transformation of a waxy hydrocarbon liq-
uid into a solid-like material when it is not flowing and its temperature falls below the
pour point temperature. In the initial deposit the wax crystals interconnect and form a
lattice which has a certain mechanical strength. Just as in the regular deposit, the strength
depends on the shear stress exerted on the deposit by the fluid flowing past the deposit.
Higher flow velocity results in a more solid wax deposit, able to withstand shear of the
faster flow. A proper characteristic for initial wax deposit should be not the aspect ratio of
a crystal but a mechanical strength measurement of a deposit. While it is a technical im-
possibility at present to measure the aspect ratio of an individual micron-sized wax crystal
inside a pipeline, one can measure the force it takes to remove the deposit off a pipe wall
using a scraper.
Conversely, stronger flow results in weaker waxy gels. For example, one method to pre-
vent gelling of a waxy crude at temperatures below its pour point, is to maintain flow circu-
lation, however slow. The flow will keep disrupting the networks forming between the few
crystals, and as none of the crystals can deposit and take hold on the pipe wall due to the lack
of temperature differential driving force, no deposition or gelling would occur. Such method
has been used on a gravity-based offshore platform oil offloading system in the Northern
Atlantic.
Waxy gels are characterized by their strength, or by the yield stress required to break the
interconnected matrix of wax crystals in the gel. The strength of a gel can be measured in a
stress-controlled rheometer or in a model pipe. In a rheometer the yield stress is a direct mea-
surement. In a model pipe, the pressure required to restart the flow of a gelled oil or the gel
break pressure is measured, which then is translated into the yield stress

[ ]
]
[
[
Differential Pressure Pa] =× YieldstressPa]× PipeLengthm [ ]/Pipe Diameter m
4
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_
_
Studies have reported that the scale-up of the gel break pressure to larger diameter pipes
using the above equation provides conservative estimates. Several multi-year industrial re-
search Joint Industry Projects have developed improved correlations for gel strength predic-
tion using small size laboratory equipment such as 0.25 in. test loop. However, it would be
easier to measure gel strength more accurately in larger laboratory equipment, such as a test
loop of at least 2 in. diameter, with appropriate insulation layers and appropriate thermal
history of the oil cooldown. Laboratories aiming to improve gel break pressure correlations