Hydrocarbon Processing - February 2022 - GP-12

SPECIAL FOCUS: TREATING SOLUTIONS
the dehydration unit because of the high
temperature during the regeneration step.
At the regeneration temperature, O2
can
produce an oxidation reaction that would
generate water. If the natural gas also contains
sulfur, the O2
present in the regener).
Adding the presation
gas stream will lead to the formation
of sulfur dioxide (SO2
ence of water will lead to hydrogen sulfide
(H2
S) and sulfur. Sulfur can cause adsorbent
deactivation and corrosion of materials
in cooler parts of the regeneration
section, as well as channeling and high
pressure drop in the molecular sieves.
FIG. 1. The proprietary adsorption technologya is specifically designed for the removal of aromatic
hydrocarbons (such as BTX) and heavy hydrocarbons from lean gas feeds in LNG pre-treatments.
What can be done to mitigate the effects
of O2
? The simple answer would be
to remove O2 from the feed gas (FIG. 2). In
removal systems exist in LNG plants.
this sense, not many commercially proven
O2
O2
FIG. 2. A typical feed gas O2
removal unit.
sampling analyses (where pipeline specifications
are normally set at 10 ppmv).
Usually, the presence of O2
in natural gas
sampling measurements is due to poor
sampling procedures or to misread gas
chromatograph results (it is not uncommon
to confuse argon with O2
content, and
.
(i.e., through tank vacuum breakers and
inadequate control in vapor recovery units
of storage tanks or O2
rejection from helium
recovery units) failure or lack of tank
blanketing systems.
ing gas chromatograph results). It is also
proven that some samples show much
greater than 20 ppmv O2
most pre-treatment licensors can design
systems to handle up to 20 ppmv of O2
In some cases, the O2
when read- What can O2 in natural gas feed do
in natural gas can result
in an LNG plant? Even at low levels, the
presence of O2
in the corrosion of pipes in LNG plants,
which can cause safety hazards due to catastrophic
gas leaks. O2
can potentially imcontent
may show
values of 100 ppmv.
Helium is a very valuable gas in the
industry. It is extracted from natural gas,
which typically contains 1 vol%-8 vol%
helium. While O2
is a problem, helium
might be a blessing. The following will
analyze the challenges with O2
.
Where does that O2 come from? Natu,
which
ral gas produced in conventional wells (either
gas or oil) does not contain O2
can be found in gas lines fed by landfills,
vacuum oil recovery systems, coal mine
methane (CMM) or other low-pressure
gas collection systems-all of which can
contain several percent of O2
es of O2
. Other sourccan
be related to air ingress from
field compressors in natural gas extraction
systems (i.e., packing on low-pressure well
compressors) or through ingress from upstream
oil and/or gas production facilities
pact the acid gas removal unit (AGRU) or
the molecular sieve dehydration system.
In the AGRU, O2
can lead to a continuous
formation of contaminants (bicine)
by the degradation of the amine in methyl
diethanolamine systems.
The reaction of O2
with amines leads
to the formation of different acids (i.e.,
formic, glycolic and oxalic), in which ions
will form salts stable to heat. These salts
increase the risk of foaming and fouling in
the amine system, causing change in the
amine strength and reducing the treating
capacity of the amine system. Therefore,
the AGRU will require more actions to
maintain the amine quality.
In the dehydration unit, O2
in the regeneration
gas will result in the formation
of water due to the oxidation of the light
hydrocarbons during the regeneration
heating stage. When present in excess of
design limits, O2
can cause problems in
12 JANUARY/FEBRUARY 2022 | GasProcessingNews.com
removal systems for LNG applications
are based on catalytic reactions and
solid scavengers (although there are not
many references for the latter). A continuous
adsorption-based process-using
copper as the adsorber in at least two
columns-could be used, but a reducing
agent (hydrogen) would also be needed,
which is very costly and not available in
LNG plants. Therefore, we are left with
systems based on catalytic reactions. For
this option, the catalytic combustion of
O2
can be used to remove O2
gas. The combustion of O2
from natural
in natural gas
yields water and carbon dioxide (CO2
Produced CO2
less than 50 ppmv of CO2
).
will be removed in the
in the feed gas, there will be
in the feed to
the AGRU, which will have minimal impact
on AGRU operations.
These systems consist of a catalytic
reactor with enough catalyst to remove
the O2
and produce CO2
and water. The
water could ultimately be separated in a
separator or coalescer. However, it does
not seem to be a feasible solution in LNG
plants that might expect O2
used to remove CO2
to be present
in the feed gas.
Membrane technologies, like the ones
from natural gas, have
been considered in some projects. However,
there are no commercial membrane
materials with proven results for the removal
of O2
from the natural gas.
Operational procedures in the LNG
contamination. These include the
plant can be implemented to deal with
O2
following:
downstream amine unit. Based on 50
ppmv of O2

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Hydrocarbon Processing - February 2022

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