Hydrocarbon Processing - August 2022 - 62

Gas Processing & LNG Industry Spotlight
is transferred onto a ship for its primary
journey. Once it arrives at a shoreside receiving
terminal, the LNG is transferred
to storage tanks for regasification before
entering consumption pipelines. Some
may be loaded to trucks and railcars
while still in liquid form.
By the time any of this natural gas
reaches an ultimate customer, it can
change ownership multiple times, passing
through custody transfer facilities at each
handoff point. Various loads with potentially
differing characteristics get mixed
into the process, so there is no assurance
that all loads are necessarily on-spec.
LNG also changes while in transport,
as it boils off its light components, with
these components escaping into the tank
or other enclosure. This alters the overall
composition, and depending on the
elapsed time and storage conditions, can
significantly modify the calorific value of
the load. Given the typical LNG tanker
load can have a value of $50 MM or more,
a change of even 1% is worth $500,000.
Each time LNG passes through a custody
transfer point, the new owner must
verify its composition and calorific value
characteristics to avoid paying for lost value
or for passing off-spec product into the
larger supply chain. This analysis does not
need to be as stringent as during pretreatment
processes, but basic contaminant
levels and calorific values must be verified.
The challenge is finding an analyzer
technology suited to the application.
Gas chromatography (GC). The traditional
method for analyzing natural gas
after the pretreatment stage is with GC.
Various models are available that can
FIG. 2. TDLAS analyzers, such as the
proprietary one showna
O, H2S and CO2
, use differential
spectroscopy techniques to quantify low parts
per billion (ppb) to parts per million (ppm)
levels of H2
in the outlet gas
stream of an amine treatment unit.
quantify the relevant components with
the required precision and calculate the
calorific value and Wobbe index. A typical
GC analysis takes a sample of the natural
gas, mixes it with an inert carrier gas
and pushes it through a packed column
enclosed in an oven. During the time in
the column, the gases separate and exit
individually, passing through a detector
that measures the amount of each. Some
models use multiple columns for a higher
degree of separation.
Operationally, GC tends to be complex.
First, these models depend on a
sampling system to deliver gas to the analyzer.
This requires tubing and valving
over a limited distance, so the analyzer
must be near the source. GC models often
require a shelter, although some models
can be installed in the open in forgiving
environments. Second, GC models
require a supply of consumables in the
form of carrier gas and test gases for calibration,
so there is an ongoing cost.
Third, while GC can perform this
FIG. 3. Variability with the sample vaporizer
results in erratic Btu readings from the GC.
62 AUGUST 2022 | HydrocarbonProcessing.com
analysis routinely with sufficient precision,
there is a weaker link in the analysis
chain. An LNG sample must pass
through a vaporizer to phase change the
sample into a gaseous form suitable for
GC analysis. The vaporizer stage is often
more problematic than the analyzer
itself, and in day-to-day operations, the
vaporizer may not always provide a truly
representative sample of the LNG-resulting
in an inaccurate picture of the
product's characteristics.
The task of the vaporizer is especially
tricky because of the compositional
complexity of natural gas. It must completely
vaporize the sample without performing
what is effectively a fractional
distillation action. All the components
do not vaporize simultaneously, so the
vaporizer must ensure that it does not
lose the lighter fractions or stop the process
while some of the heavier fractions
remain partially liquefied.
The vaporizer must operate within a
very narrow range that is easily disrupted
by changes in LNG flow, pressure or
temperature. This makes it especially
difficult to extract truly representative
samples while the process is undergoing
startup and shutdown actions. Vaporizer
performance can be adversely impacted
by changes in flowrate or changes in
composition of the LNG. This can result
in unstable vaporization, providing nonrepresentative
gas to the GC. Per the GIIGNL
LNG custody transfer handbook,
this data is not to be included in any CT
calculations. Typical instability is visible
as a noisy reading with random high and
low spikes (FIG. 3), even when the supply
being analyzed is very homogenous.
The overall sampling system-covering
extraction, vaporization and transport
to the GC analyzer-is highly
complex, and all elements must work
together to prevent pre-vaporization,
incomplete vaporization or sample loss.
This requires careful design, installation
and maintenance throughout all steps,
with precise temperature control. Given
the procedure's difficulties, the motivation
for simple and reliable alternative
mechanisms is understandable.
Because natural gas is burned in gaseous
form, it may seem counterintuitive
to suggest evaluating its characteristics
in liquid form, but its chemical composition
and calorific value are not dependent
on its phase. It may also seem counterintuitive
to suggest using an analyzer
at cryogenic temperatures. The solution
is Raman spectroscopy, which can be applied
to analyze natural gas whether in
gas or liquid form.
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