Hydrocarbon Processing - September 2021 - 47

Special Focus Refining Technology
T. H. WINES, Pall Corp., Port Washington, New York;
A. GORIN, Pall Corp., DeLand, Florida; and J. RIOS and
J. TRUCKO, Honeywell UOP, Des Plaines, Illinois
Improve kerosene mercaptan sweetening with
fluoropolymer-cartridge liquid-liquid coalescers
Many refineries are experiencing
new problems related to the formation
of stable emulsions and poor separation
due to the increased use of lower-cost opportunity
crude oils that have a high total
acid number (TAN), higher gravities and
more sulfur content. Various naphthenic
acids, pipeline flow enhancers and injected
corrosion inhibitors will distill off
in the kerosene cut from the crude distillation
unit (CDU). The kerosene is often
treated with caustic (sodium hydroxide or
ammonia) and a fixed-bed catalyst, using
a proprietary mercaptan oxidation processa
for
mercaptan sweetening. Sodium
naphthenates will form after caustic treatment,
and may foul the porous structure
of the mercaptan oxidation process catalyst
bed and lead to loss of production due
to unscheduled cleaning operations.
Originally, a conventional batch prewash
with gravity settling was sufficient
treatment of naphthenic acids from lowacid-number
kerosene (less than 0.02-mg
KOH/g feed) and for the removal of the
spent caustic from the effluent kerosene.
However, the increasingly complex origin
of crude blends and their associated additives-including
drilling chemicals, pipeline
slip enhancers and corrosion inhibitors-can
result in emulsive kerosene in
these simple low-TAN applications. When
the acid number exceeds 0.02-mg KOH/g
feed, the two phases become more emulsified
due to sodium naphthenate formation.
Historically, an electrostatic coalescer
prewash (ECP) has been utilized
to resolve the two phases. Over the past
decade, electrostatic coalescers at some
sites have been found to be ineffective at
completely separating the aqueous caustic
at several refineries (and with particular
crude oils) when processing kerosene containing
a high TAN, corrosion inhibitors
or other contaminants that result in tight
kerosene/aqueous phase emulsions.
A new approach using high-efficiency,
fluoropolymer cartridge liquid-liquid coalescers
has been successfully pilot tested
for this separation at five global refinery
sites. Results from the pilot tests are presented
that demonstrate the effectiveness
of the high-efficiency cartridge coalescers
for breaking these tight emulsions-thus
reducing the caustic levels in the kerosene
and providing clear and bright kerosene
effluents. As a result of these improvements,
spent caustic volumes can be reduced
by operating with caustic strength
up to 10°Bé and more fully utilized caustic
than other options.
Potential benefits
of the
cartridge
liquid-liquid coalescer approach are presented
based on fouling issues in the
downstream fixed-bed reactors and on increases
in media life in the post-treatment
section. Fouling issues can require the
kerosene mercaptans sweetening process
to be bypassed for several days, while the
catalyst bed structure is hot-water washed
to restore catalyst activity, resulting in
margin losses associated with the inability
to upgrade the kerosene to finished jet
fuel while the reactor bed is being cleaned.
A description of the first integrated, commercial
pre-filter/cartridge coalescer
modular unit that is ready to start up this
year is provided. Immediately deployable
commercial-scale rental separation equipment
options are also provided.
Mercaptan oxidation process. Crude
oil feeds to refineries have been trending
toward heavier gravities, with increased
corrosiveness, sour (sulfur) content and
a higher TAN as lower-cost " opportunity "
options. To take advantage of the favorable
economics associated with these opportunity
crudes, many refineries have
experienced new problems in various unit
operations, including the slop tank area,
electrostatic desalting, tower overhead
corrosion, heat exchanger fouling and excessive
contaminants in the plant wastewater
treatment.1
Many of these issues are
related to the formation of stable emulsions
and resultant poor separation due
to either the absence of separation equipment
or the use of ineffective physical or
physiochemical separation equipment.
The kerosene cut from the CDU is also
an area of particular concern, as natural
contaminants in high-TAN crude oil (e.g.,
naphthenic acids) and intentionally injected
(or added) corrosion inhibitors have
boiling points in this distillate cut range
(302°F-482°F). These feed contaminants
are known to form stable surfactant-based
emulsions in refinery applications.
After the kerosene cut leaves the CDU,
the refinery will process the stream by
either steam or reboiler stripping to remove
undesirable light hydrocarbons, as
required to meet cut point specifications.
If the kerosene has been steam stripped,
then water will precipitate out of solution
as the stream cools past its dewpoint.
The kerosene is then treated with a dilute
caustic solution (typically, sodium
hydroxide), followed by a fixed-bed catalysis,
using the proprietary mercaptan
oxidation process. A schematic2
of the
mercaptan oxidation fixed-bed kerosene
process is shown in FIG. 1.
Initially, a low-concentration caustic
prewash (typically, limited to 3°Bé-5°Bé
for previous technologies) is used to react
with naphthenic acids that form sodium
salts that have high solubility in the
Hydrocarbon Processing | SEPTEMBER 2021 47
Hydrocarbon Processing - September 2021

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