Hydrocarbon Processing - September 2021 - 49

Refining Technology
Liquid-liquid coalescer
cartridge
system. A high-efficiency liquid-liquid
coalescer was developed for the separation
of caustic from hydrocarbons using
polymeric and fluoropolymer media that
operate in a horizontal configuration and
can separate liquids with low interfacial
tensions.6
The overall system7
two stages (pre-filter and coalescer) and
is depicted in FIG. 2.
Pre-filter. A separate vessel with absolute-rated
filter cartridges is typically
used in the pre-filter stage. It is used for
the separation of solids and preconditioning
of the fluid. The filtration stage
protects the coalescer and increases its
service life. Removal of solids can also
destabilize emulsions, as solids can congregate
around the caustic droplets and
increase the barrier to coalescing.
Coalescence and separation. The
liquid-liquid cartridge coalescer system
(FIG. 2) consists of a horizontal coalescer
cartridge stage, followed by a settling zone
that relies on the differences in density for
separation of the coalesced droplets from
the primary product fluid (in this case,
consists of
kerosene). The fluid enters at the left side
of the housing after treatment by the prefilter
and flows from the inside of the coalescer
cartridges radially outward, causing
the transformation of the inlet fine
dispersion of droplets into large drops in
the outlet stream. This process is known
as " coalescing " and can be described by
the small droplets adsorbing weakly to the
coalescer medium fibers, where they are
subsequently subjected to collisions by
incoming droplets that result in mergers
and agglomerations into larger drops.
In the coalescer medium, the pore sizes
are adjusted in a gradient of increasing
sizes to allow for optimum growth of the
drops through successive adsorptions,
collisions and agglomerations. The drag
force of the continuous phase flow causes
the enlarged drops to disengage from the
fibers. The coalesced droplets then flow
in the horizontal direction (left to right)
through a settling zone. For caustic removal
from kerosene, the caustic phase
has a higher specific gravity than kerosene;
therefore, the coalesced droplets
settle downward by gravity and are collected
in a sump located at the bottom of
the housing. The purified kerosene exits
the housing from a nozzle on the topright
far end of the housing.
Coalescer performance. High-efficiency
cartridge liquid-liquid coalescer
systems can treat streams with up to 10%
inlet discontinuous phase concentrations.
They can reduce the discontinuous phase
down to ppm levels in the outlet and can
process difficult emulsions with interfacial
tensions as low as 0.5 dyne/cm. For the
aqueous phase from hydrocarbon separations,
coalescer outlets below 15 ppmv per
a specific water testingb
method (ASTM
D3240) can be achieved. The integration
FIG. 2. Horizontal liquid-liquid fiber-bed
coalescer system with pre-filter.
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Hydrocarbon Processing | SEPTEMBER 2021 49
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Hydrocarbon Processing - September 2021

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