Hydrocarbon Processing - February 2022 - GP-13

SPECIAL FOCUS: TREATING SOLUTIONS
* Consider the substitution of amine
in the AGRU. The O2
reacts with
amines to form bicine and heat
stable salts (HSS). Mitigation
would occur by removing amine,
using a portable reclaiming facility
in the AGRU. The reclamation
can be achieved with a mobile or
permanent unit. When properly
implemented, ion exchange
technology has proven successful
in removing HSS and bicine.
* In this process (FIG. 3), the lean
amine solution is pumped into
the ion exchange unit. The ion
exchange resin removes the
HSS, and the purified amine
solution is returned to the amine
circuit. After several minutes
of operation, the system rinses
the amine, using demineralized
water from the resin, and a
regeneration sequence takes place.
* To avoid possible operational
problems in the dehydration unit,
some dehydration unit licensors
recommend staging the heating
procedure to minimize the time in
which the O2
FIG. 3. Ion exchange filtration.
in cases where the helium concentration is
low, it is not economically feasible to extract
it out of the natural gas stream.
would be exposed to
the high regeneration temperature.
The idea is that, since most of the
regeneration step occurs at midtemperatures
(around 340°F), the
regeneration cycle is maintained
at the same temperature. The final
step is to heat the oxygen to high
regeneration temperatures (445°F).
A combination of these two operational
modes could handle up to 50 ppmv of O2
content in the composition of the feed gas.
Should the O2
content exceed 50 ppmv,
the only feasible permanent solution is the
installation of a closed-loop regeneration
system in the dehydration unit. This solution
falls within the realm of licensed designs.
In this manner, there is no impact to
the dehydration unit, and the regeneration
operation would be under normal conditions
(i.e., no high O2
present). These
types of installations would increase the
regeneration needed for each dehydration
bed; however, if the standby time is large
enough, the overall dehydration-regeneration
sequence would not be affected.
Helium recovery. Should the natural gas
be rich in helium, the helium can be recovered
in an integrated helium-nitrogen
rejection unit in the LNG plant. However,
How can the extraction of low-concentration
helium be economical in
LNG plants? Boil-off gas (BOG) from
LNG storage tanks is usually used as fuel
gas for gas turbines in liquefaction plants
(or for thermal oxidizers in plants that have
electric motors as drivers). Besides methane,
BOG also contains nitrogen, hydrogen,
helium and some other heavier hydrocarbons.
Depending on the case, a helium
extraction unit can be installed in the BOG
stream upstream of the BOG compressor;
however, the purity requirements to export
helium (> 95%) might require installing a
hydrogen removal unit downstream of the
helium extraction unit.
At present, there are several solutions
to extract helium from natural gas. These
solutions include membrane separation,
pressure swing adsorption (PSA) and
cryogenic processes. The latter seem to
be the most economical solutions for this
type of plant.
BOG components have different boiling
temperatures; therefore, processes
based on condensation and/or distillation
seem to be adequate for this purpose.
Generally, and from the perspective
of producing helium with the required
purity,
high condensation pressures
would be required to achieve such purity.
In distillation processes, higher pressures
lead to higher power consumption due to
higher condensation duties.
A combination of both condensation
and distillation is the most adequate solution
to optimize the feed stream temperature
and pressure of the cryogenic distillation
column. The possible integration
of a device to cool down the helium to
condense the BOG to a lower temperature
would provide favorable stripping
through the condensation separation
process at a relatively lower pressure. The
low operating pressure would decrease
the condenser duty and, thus, the power
consumption of the system.
Takeaways. Even small amounts of
heavy hydrocarbons in lean gas can be of
critical concern for stable and efficient
operations. While conventional technologies
for lean natural gas processing are
often infeasible due to the low amount of
heavy components, the new adsorbent
technologya
is one of the most feasible
solutions for lean natural gas processing.
The presence of O2
in natural gas is
problematic above a certain content;
however, before considering any O2
removal
installation or change in operational
procedures, it is imperative to confirm
the existence of this gas within the
natural gas stream.
While the extraction of helium also
adds complexity to LNG process facilities,
the payback period of such installations
is very short. The overall profit of
the LNG plant will be increased due to
additional revenues generated from exporting
extracted helium. Taking this into
account, helium is a prize that can be well
worth the extra effort and investment
to obtain. GP
NOTE
a BASF's Durasorb Cryo-HRU technology
b BASF's OASE technology
MADANMOHAN PATEL is a
Registered Professional Engineer
in Texas and Louisiana, with more
than 17 yr of chemical and process
engineering experience in the LNG,
petrochemical, chemical, oil and
gas, and power industries. His
experience is in supporting plant operational needs
and in providing process expertise to troubleshoot
and mitigate different plant issues. Mr. Patel earned
an MS degree in chemical engineering from Texas
A&M University.
Gas Processing & LNG | JANUARY/FEBRUARY 2022 13
Hydrocarbon Processing - February 2022

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