Hydrocarbon Processing - September 2021 - 48

Refining Technology
aqueous phase and that are removed as
spent caustic. Prewash effluent kerosene
should be clear and bright with a TAN less
than 0.01-mg KOH/g feed and less than
15-ppmv free water. The kerosene is then
injected with air that dissolves into the hydrocarbon,
providing oxygen for the mercaptan
oxidation reaction in the proprietary
mercaptan oxidation process reactor.
The reactor contains a fixed bed that consists
of a granulated activated carbon that
is impregnated with a proprietary catalyst.
Higher-strength caustic (typically 10°Bé)
is used to provide the required alkaline
environment for mercaptan oxidation.
This caustic is collected in a downstream
caustic settler. Any sodium naphthenates
that escape in the kerosene effluent of the
prewash will precipitate out of solution
in the highly alkaline environment of the
reactor, forming surfactants that foul the
catalyst pores of the mercaptan oxidation
process catalyst bed, leading to loss
of catalyst activity. These surfactants also
result in increased consumption of water,
salt and clay, resulting in days of production
losses due to unscheduled cleaning
and post-treatment media replacement.
In the extreme, the precipitated sodium
naphthenates will increase reactor pressure
drop to the point that it crushes the
activated carbon base and necessitates a
full catalyst replacement.
Mercaptan removal3
reacting the mercaptan (RSH) with caustic
(NaOH) to form sodium mercaptide
(NaSR) and water. The NaSR product
is then oxidized in the presence of the
dissolved oxygen, solid-bed catalyst and
caustic to produce disulfide oil (RSSR)
that is then absorbed back into the hyCoalescer
Electrostatic
coalescer
prewash
Stacked
reactor
drocarbon
phase. The basic reactions are
shown in Eqs. 1 and 2:
RSH + NaOH } NaSR + H2
O r
O (1)
NaSR + ¼ O2 + ½ H2
½ RSSR + NaOH
(2)
In the kerosene sweetening process,
the disulfide oil is soluble in the kerosene,
and it stays with the final product. Therefore,
the sulfur content is not reduced, but
the mercaptans and odor are reduced to
less than maximum specifications.
ECP. The removal of the spent caustic
from the kerosene is typically achieved
using an ECP, with target specification of
effluents being less than 1 ppmw sodium.
The electrostatic coalescer operates by
the basic principle that an electric field enhances
the merging or coalescence of the
small drops in an emulsion to form larger
drops that separate faster than the smaller
sizes. The mechanism for enhanced
coalescence has been attributed to the
creation of an induced dipole (charged
droplet) by the electric field leading to
dipole-dipole attraction for oppositely
charged droplets in alternating current
(AC) systems.4
For direct current (DC)
is achieved by
systems, the mechanism for enhanced
coalescence is accounted for by increased
electrophoretic motion due to the electric
field, whereby charged drops are attracted
to oppositely charged electrodes.5
After the larger coalesced drops are
produced, the governing settling force is
Stokes' law. For the electrostatic charge to
build up on the drops, the continuous fluid
(hydrocarbon) should have a low conductivity
and dielectric constant. If there
are upsets where slugs of the aqueous
Water
wash
Salt
filter
Clay
filter
phase enter the electrical grid, the increase
in conductivity and dielectric constant of
the mixed-phase fluid can cause short circuits
between the anode and cathode, resulting
in electrode burnout, particularly
if the electrodes are not insulated.
The vast majority of commercially
operating electrostatic separators are designed
with uninsulated electrodes, with
operating conditions ranging from 12,000
V-35,000 V and 0 milliamperes (mA)-5
mA. Typically, electrostatic coalescers are
designed for feedstocks containing between
0.5% and 10% inlet basic sediment
and water and may have difficulty separating
fine droplets in the micron size range.
The advantage is that it is a proven technology
with thousands of units deployed
globally, and it can tolerate high levels of
aqueous phase and solid contaminants.
The disadvantages are that it may not be
able to process feeds with high TANs,
caustic strength or caustic utilization that
create stable emulsions. Difficult emulsions
can be broken with the use of highefficiency
liquid-liquid cartridge coalescers
that have a droplet-to-fiber adsorption
and agglomeration mechanism that is not
present with the ECP technology. In addition,
the high-efficiency liquid-liquid cartridge
coalescers use a pre-filter stage that
effectively removes precipitates and other
solids from the kerosene stream, providing
a cleaner kerosene effluent than the
ECP. The ECP requires continuous phase
(i.e., a hydrocarbon phase) to have a low
electric permittivity. In a design with noninsulated
electrodes, there is a potential
risk for short-circuit and electrode damage
during slug conditions.
It has become apparent at several reDrain
pot
FIG.
1. Mercaptan oxidationa fixed-bed sweetening process.
48 SEPTEMBER 2021 | HydrocarbonProcessing.com
fineries (operating with particular crudes
as part of their CDU feed slates) that conventional
electrostatic coalescers were
ineffective at completely separating the
aqueous caustic when processing kerosene
containing high TAN or corrosion
inhibitors that result in tight emulsions.
Other refiners have interest in minimizing
operating and disposal costs by operating
with higher caustic utilization to
minimize caustic usage and with higher
caustic strengths to reduce spent caustic
volume-regimes where conventional
ECPs experience loss of performance. A
new approach using high-efficiency, fluoropolymer-cartridge
liquid-liquid coalescers
has been successfully pilot tested for
this application at five global refinery sites.
Kerosene feed
>3˚ Bé caustic
Spent caustic
Air
Caustic (continuous)
Spent caustic
Water
Jet fuel product
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