Hydrocarbon Processing - February 2022 - GP-11

SPECIAL FOCUS: TREATING SOLUTIONS
The best ways to deal with heavy hydrocarbons,
oxygen and helium in LNG plant feed gas
M. PATEL, Cheniere Energy, Corpus Christi, Texas
Natural gas is one of the world's most
important sources of energy. Where natural
gas pipelines are infeasible or do not exist,
LNG is a way to move natural gas from
producing regions to demand markets.
Several recent LNG projects are based on
pipeline gas (lean feed gas), which contains
higher methane, with low natural
gas liquid (C2
-C5) and lower C5
hydrocarbon content than typical conventional
natural gas. It is very challenging to
remove a small amount of impurities to
ensure a stable and efficient operation.
Where does heavy hydrocarbon
come from? Natural gas is available from
conventional natural gas reservoirs and
unconventional gas, such as shale gas,
tight gas and coalbed methane. There are
two primary sources for feed gas to an
LNG plant: dedicated natural gas reservoirs
and pipeline gas from mixed sources.
Hydrocarbon streams produced at the
wellhead are composed of gas, liquid hydrocarbons
and, sometimes, free water.
The liquids from the gas phase are separated
by passing the well stream through
an oil-gas or oil-gas-water separator.
For pipeline gas, the removal of C2
+
heavy hydrocarbon components from
natural gas is required to avoid the unsafe
formation of a liquid phase during
transportation. Therefore, pipeline gas is
usually lean. However, a small amount of
aromatics-such as benzene, toluene and
xylene (BTX)-and C5
+ often remain in
the gas. The typical composition of natural
gas is detailed in TABLE 1.
What are the possible effects of trace
amounts of heavy hydrocarbons in
the lean natural gas feed of an LNG
plant? The presence of heavy hydrocarbons
in natural gas can result in a freezing
out of the gas at liquefaction temperature,
+ heavy
causing a significant adverse performance
impact. Even trace concentrations of
heavy hydrocarbons and aromatics can
cause precipitation of solids (freezing)
and fouling of the main liquefaction heat
exchangers. When the freeze-out of heavy
hydrocarbons occurs inside the exchange
cores, pressure drop increases across the
cores. This leads to flow maldistribution
in the cores, which results in thermal
stresses and core leaks.
When the core pressure drop reaches
a high limit, the LNG train has to be
shut down to defrost the exchangers
and remove the heavy hydrocarbon deposits.
Defrosting results in production
loss and increases process risks such as
equipment failure.
Hydrocarbons can also create operational
and performance problems in the
amine unit or the reinjection system.
What can be done to mitigate the
effects of heavy hydrocarbons? This
trace of heavy components must be removed
prior to liquefaction. A few conventional
separation schemes are available
that use a scrub column and a liquids
extraction unit that is integrated with the
liquefaction process. These conventional
technologies are often not feasible due
to the low amount of heavy components.
In addition, front-end natural gas liquids
recovery processes may be feasible; however,
an increase in capital and operational
expenditures is associated with these
configurations for the removal of only a
small content of aromatics and heavy hydrocarbons.
A
proprietary adsorption technologya
is one of the most feasible solutions
for lean gas natural gas processing. The
adsorption technology is specifically designed
for the removal of aromatic hydrocarbons
(BTX) and heavy hydrocarbons
from lean gas feeds in LNG pre-treatments
(FIG. 1). This advanced technology
combines both unit functionalities into
one system by utilizing a multi-material
approach to achieve heavy hydrocarbon
and water removal to the required cryogenic
specifications. This approach provides
a 30% increase in capacity for BTX
components, allowing for the effective removal
of BTX to less than 1 ppm without
compromising unit efficiency.
Natural gas extracted from conventional
natural gas and/or oil fields does
not generally contain oxygen (O2
). However,
it is common to find certain high
O2
TABLE 1. Typical composition of natural gas
Rich-feed gas composition, mol%
Methane
Ethane
Propane
i-Butane
n-Butane
i-Pentane
n-Pentane
+
C6
85-90
3-10
1-5
0.5-2
0.5-2
0.5-2
0.5-2
0.5-2
concentrations in U.S. natural gas
pipelines (greater than 10 ppmv) in some
Lean-feed gas composition, mol%
96-98
1-3
0.1-0.4
0.005-0.01
0.005-0.01
0.005-0.01
0.005-0.01
0.005-0.01
Gas Processing & LNG | JANUARY/FEBRUARY 2022 11
Hydrocarbon Processing - February 2022

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