Hydrocarbon Processing - February 2021 - GP-12

SPECIAL FOCUS: TREATING SOLUTIONS

Most customers choose to catalytically
regenerate caustic, using air to
convert and remove the extracted
mercaptans while returning the
caustic to the extractor with minimal
sulfur content. This regeneration
method has a higher capital cost than
once-through caustic usage but
a much lower operational cost.
The other type of caustic treatment is sweetening, which
requires injection of air that dissolves into the hydrocarbon.
The oxygen from the dissolved air and the mercaptan already
present in the hydrocarbon react in the presence of the solid
bed catalyst and create disulfide that remains in the liquid hydrocarbon stream. This disulfide present in the product is less
corrosive than the mercaptan in the feed, despite no change in
the total S. Caustic is easily separated from the hydrocarbon in
the reactor vessel while spent air is separated from the hydrocarbon, either in a degassing vessel in the process or in downstream tankage, depending on the volume of spent air. Sweetening can be used to treat naphtha (C5-C12) and to produce jet
fuel (C12-C20+) hydrocarbons.
Pros and cons to fractionation prior to caustic treatment.
Treating the entire NGL stream without fractionation allows
for minimum capital investment, but minimum efficiency.
Using one caustic concentration results in poor selectivity
for mercaptan reaction or removal. In addition, contamination
of caustic from pipeline chemicals can include interfering ions,

Equilibrium constant, log scale

C1SH

C2SH

C3SH

C4SH

C5SH

Caustic concentration

FIG. 1. Mercaptan extraction curve.

12 JANUARY/FEBRUARY 2021 | GasProcessingNews.com

acids, pipeline slip and flow enhancers, and upstream processing chemicals. Treating this variety of contaminants can increase fresh caustic consumption and spent caustic disposal.
Entire-stream extraction preferentially removes light mercaptans as opposed to larger mercaptans. Heavy mercaptans remaining in the hydrocarbon would likely cause off-specification product (see FIG. 2).
Conversely, entire-stream sweetening can better convert
most of the heavy mercaptans to disulfide in the presence of
lower-strength caustic. This will cause a loss of light hydrocarbon from spent air vented from the hydrocarbon. Additionally,
this process would need to have large fixed-bed reactors to accommodate the liquid flow and required space velocity.
Fractionation and individual treatment of narrowly cut fractions maximizes treating efficiency. Using the most effective
caustic concentration for each treating objective optimizes selectivity for mercaptan removal or conversion.
Extraction designs are extremely flexible and can be optimized for mercaptan removal from lighter feeds [C1 and C2
(gas phase) and C3 to C5 (liquid phase)] containing 2 wppm
mercaptan S-20,000+ wppm mercaptan S in one or multiple
tight cut extractors. This process can be designed to achieve
minimum reentry of S, reducing the S to less than 5 wppm S in
the product, and in some cases less than 1 wppm S. Since heavy
mercaptans are not in this cut, stronger caustic can be used,
which increases the efficiency of the extractor, reduces caustic
circulation rates and reduces vessel sizes.
Heavier condensate feeds (C5-C20+) will contain heavier
mercaptans with concentrations ranging from 20 wppm S to
2,000+ wppm S. These streams are more efficiently treated via
sweetening with weaker caustic strengths to increase heavy mercaptan solubility. Since the treated hydrocarbon has low vapor
pressure, minimum loss of hydrocarbon product occurs with
the spent air vented from the hydrocarbon. Smaller-size fixedbed reactors can accommodate the liquid flow at the required
space velocity for the desired conversion. As a result, mercaptans are sweetened to disulfide to less than 5 wppm mercaptan
S for naphtha and less than 20 wppm mercaptan S for jet fuel.
Additionally, the product naphtha can be fractionated to
concentrate heavy disulfide S formed from sweetening the
mercaptans in the heavy cut, leaving a sweet, saleable light cut
of naphtha.
Typical and atypical impurities, and effect on design.
Several common impurities, such as CO2, COS, H2S and RSR,
can exist in the feed to the systems. Although it is expected
that entire-stream amine treating and fractionating of the
ethane cut would remove all the CO2 in the C3 and C4 cuts,
CO2 has been observed in these cuts at midstream facilities to
the concentration of 100 wppm-1,000 wppm. Caustic can be
used to completely remove CO2 and H2S, but it is consumed
irreversibly and would represent an operating cost in the range
of millions of dollars.
The preferred method of treatment at these high concentrations is an amine absorber to reduce the concentration to less
than 25 wppm, followed by a caustic prewash upstream of the
caustic treater. COS can be removed to less than 1 wppm by
pretreatment in a proprietary prewash design,a or during posttreatment with a modified caustic blend.b

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Hydrocarbon Processing - February 2021

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