Page 243 - Pipeline Pigging Technology

Pipeline Pigging Technology

PARAFFIN TREATMENT

Paraffin treating compounds are used for three main reasons:

(1) to reduce the viscosity of an oil as it cools while traversing a pipeline,
so that if flow in the line is stopped and it cools to ambient
temperature, flow can be re-started within the burst strength of the
pipe;
(2) to minimize paraffin deposition on the walls of the pipe; and
(3) to minimize plugging of instrumentation and metering equipment.

High-viscosity oil is difficult to pump, and can cause a major problem if a
line is shut down and cools off. Deposit formation reduces the effective
diameter of the line with an increase in pressure drop and a corresponding
reduction in line capacity.
Two types of paraffin treating compounds are used in pipelines: crystal
modifiers and dispersants. Crystal modifiers function by distorting the growth
and shape of paraffin crystals. The result is that when a waxy oil cools below
its cloud point, the paraffin precipitates as small, rounded, particles rather
than acicular (needle-like) crystals. Needle-shaped crystals can interlock and
form gels, greatly increasing the viscosity of the oil. Crystal modifiers change
the paraffin crystal shape and surface energy, making it less likely to attach to
the walls of the pipe, and to other wax crystals. Also, the crystal size remains
so small that the crystals are less prone to sedimentation and agglomeration.
For this reason, crystal modifiers are known as pour-point depressants or flow
improvers.
Dispersants are surfactant compounds which alter the surface energy of
paraffin crystals, making them less attractive to each other. Dispersants
function by changing the interfacial energy between the paraffin crystal and
the solvent oil, which also make the crystals less likely to deposit on solid
surfaces such as pipe walls. This leaves them dispersed in the oil solvent in a
non-agglomerated form. Both crystal modification and dispersion cause a
reduction in the rate of paraffin fouling on the walls of pipes. Typical use rates
for both paraffin compounds are in the range of 100 to 200 parts per million.
Crystal modifiers must be continuously added at a temperature above the
"cloud point" of the oil to be effective. The cloud point of the oil is that
temperature at which the oil becomes "cloudy" due to precipitation of
paraffin crystals, and as such represents the solubility limit of paraffin in the
oil. It is not the same as the "pour point" of the oil, which is the temperature
at which the oil no longer pours out of a beaker under standard conditions.
Oil below the pour point is still pumpable.

224