Hydrocarbon Processing - February 2021 - GP-13

SPECIAL FOCUS: TREATING SOLUTIONS
be reached in the extraction products by installation of a newly
designed disulfide scrubber to drastically reduce the disulfide
present in the regenerated caustic. The disulfide scrubberd
virtually eliminates reentry of S into the product streams by
counter-currently contacting the caustic stream with a hydrocarbon stream, reducing the total product S by more than 95%
over a design without it.
The bottoms of the debutanizer can be processed through
a naphtha sweetening process.e This process treats C5-C12+ to
less than 5 wppm mercaptan S with minimum alkalinity requirements, which can then be used to dilute bitumen when
creating synthetic crudes.
Another option after sweetening is fractionating the naphtha to produce a low-S C5 and C6 material.f Fractionating out
the light C5 and C6 material concentrates the heavy disulfides
created during the sweetening process in the bottoms product.

Even though the typical impurities are present, atypical
impurities can affect the design of a complex. These atypical
impurities include oxygen, methanol, glycol, corrosion inhibitors, naphthenic acids and unusual mercaptan distributions.
Various corrosion inhibitors and slip enhancers will initiate
and stabilize emulsions between hydrocarbons and caustic.
In addition, methanol can come from many upstream sources
(e.g., dehydrators) and form emulsions with caustic. Excessive
methanol quantities should be removed with a water wash prior to amine or caustic treating. Additionally, shared regeneration sections, discussed later in this article, transfer methanol
from one segregated stream to another.
Glycol and other oxygenates also must be considered.
These impurities can cause acid corrosion in sweetening
units. Two options exist to minimize the occurrence of acid
corrosion.c The first option is to construct the existing reactors from 316L stainless steel. The second option is to apply
special epoxy to the carbon steel and concrete surfaces. This
creates an epoxy layer to protect the carbon steel of the reactor. However, the epoxy will need to be replaced at every turnaround of the unit.

Methods for disposal of waste caustic and spent air.
Caustic prewashing is necessary to completely remove the residual H2S and CO2 from the feed hydrocarbon. It is used upstream of the extraction process so that the feed is free of H2S
and CO2, which creates an S-laden caustic stream that must be
disposed of.
During the extraction process, mercaptan transfers from oil
to the aqueous caustic phase. The generated sodium mercaptide remains in the caustic, which is then separated from the hydrocarbon phase. The sodium mercaptide is then catalytically
oxidized with injected air at almost ambient conditions to produce disulfide, which separates from the caustic. This regeneration process creates an S-laden spent air stream, caustic stream
and disulfide oil stream that must be disposed of (FIG. 3). The
naphtha sweetening process also makes use of caustic and will
produce a waste caustic stream that must be disposed of.
After the extraction and/or sweetening processes have
been completed, several methods exist to dispose of the effluent waste caustic, disulfide oil and spent air from the treating
units. Each of these effluent streams have different disposal

Integration of caustic treating with the complex. Different options exist to integrate caustic treating units into a grassroots or existing plant. One configuration may treat the entire
LPG stream with caustic extraction for use as power plant fuel,
low-S motor fuels, or sales to other processors.
One example, shown in FIG. 2, places a C3 extraction process unit just after the depropanizer, thereby producing low-S
propane required by a dehydrogenation process to produce
propylene. Butanes from the debutanizer are treated in the
C4 extraction process unit, which creates a low-S C4 cut for
further fractionation by the deisobutanizer to generate lowS isobutane and n-butane. The caustic is rich in mercaptides
from both the C3 and C4 extraction systems and are sent to a
common regeneration system to reduce capital cost.
Ultra-low-S specifications of less than 2 wppm total S can

nViro
MET

Low sulfur C2CO2
H2S
RSH
C2
NGL

Deethanizer
C3+
COS
CO2
H2S
RSH

Amine absorber

Caustic
regeneration

Gas extraction

Depropanizer
C4+
H2S
CO2
RSH

Clean water

DSO, spent air, spent caustic
Low sulfur C3

C3
H2S
CO2
COS
RSH

Clean air

Propane to dehydrogenation

DSO, spent air, spent caustic
C3 extraction

Caustic regeneration

C4+
H2S
CO2
RSH

C4 extraction

Low
sulfur C4

i-Butane
Deisobutanizer

Debutanizer
C5+
H2S
RSH

n-Butane
Naphtha
sweetening

Sweetened C5

Naphtha

Spent caustic

FIG. 2. An example integration of caustic treatment into an existing plant.
Gas Processing & LNG | JANUARY/FEBRUARY 2021 13

Hydrocarbon Processing - February 2021

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