OCTOBER
2016
LNG
INDUSTRY
49
the performance of a structured packing was compared
with three random packings and two types of trays from
two internals suppliers in the context of a large LNG facility
in Australia.
Tight designs leave little room for error and, unless a
thorough sensitivity study is carried out, the likelihood of a
failed design is high. As an example,
2
consider the case of
treating a raw gas at 16 barg containing 20% CO
2
down to
50 ppm using a 33 wt% methyldiethanolamine (MDEA)
with 7 wt% piperazine solvent (an arbitrary, non-optimised
composition at this juncture). Figure 1 shows the treating
response to changes in solvent circulation rate (reported as
a percentage of design flow), and Figure 2 shows contactor
temperature profiles.
It only takes a 1% decrease in solvent rate from 89% to
88% of design flow to cause a simulated 1000s-fold
increase in CO
2
content from less than 1 ppmv to
2000 ppmv. At 89% of flow, the temperature profile shows
a normal bulge a short distance from the bottom of the
column (packed to 10 m depth with IMTP-25 random
packing). At 88.9% of design flow, this column is operating
on the edge of a steep cliff; at 88.8% it has fallen over the
edge. There is extraordinary sensitivity because of the fast
reaction rate of CO
2
with piperazine. CO
2
reacts so quickly
that at adequate solvent rates, most of the CO
2
is removed
from gas in the first 2 m of packing. The next 2 m or 3 m
polish the gas, and, above the midpoint, the rest of the
column does nothing at all. Breakthrough depends on the
solvent capacity needed to remove the CO
2
from the gas
and, provided there is enough packing in the first place, the
breakthrough solvent rate depends relatively weakly on
the amount of packing in the column.
This column was over designed. A 10 m deep bed is
more than twice what is needed for this particular packing,
as would have been shown by a sensitivity study. Indeed,
IMTP-50, or even IMTP-70, would have been adequate, and
considerably less expensive. A sensitivity study would also
show that the design solvent flow has an adequate safety
margin to permit reliable operation. But only mass transfer
rate-based simulation is capable of providing such
information reliably.
Such sharp performance sensitivity is common in LNG
absorbers using promoted MDEA solvents. This example is
actually a safe design, but without knowing beforehand
just how the internals will perform with this solvent
composition, it could just as easily have been an unsafe
design, or even a design that would not work at all. Mass
transfer rate-based simulation will allow tighter designs,
but of course, every design must have sufficient margin for
contingencies, although optimally no more than necessary.
The right tools allow one to achieve the right design while
knowing exactly how much margin there really is.
Solvent contamination
In reality and at best, a solvent is truly clean only on the
day it is offloaded from the tanker truck into the solvent
storage tank. After that, it is contaminated with either of
the following:
Reactive components that entered with the gas or
products of their reaction with the amine (heat stable
salts).
Additives (methanol, glycols) injected into the gas at
the wellhead to prevent hydrate formation in pipelines.
Contaminants in the makeup water (Na
+
, Ca
2+
, Mg
2+
,
etc.).
Amine degradation products (e.g. diethanolamine,
methylmonoethanolamine).
In LNG production, unless the gas contains sulfur
compounds or oxygen is present, heat stable salts and
reactive-amine degradation products of MDEA (such as
DEA and MMEA) are not usually present to an appreciable
extent. In any case, with an MDEA-based solvent promoted
with a highly reactive amine, such as piperazine, these
contaminants are only mildly detrimental to treating even if
they are present. However, this is not the case with glycols.
Monoethylene glycol (MEG) and diethylene glycol
(DEG) are frequently injected into wellhead gas to inhibit
hydrate formation prior to natural gas entering a pipeline.
For transportation from offshore wells to land-based
process plants, it may be more effective to dehydrate the
gas offshore using triethylene glycol (TEG). However, a
trace of TEG will inevitably be carried over with the dried
gas. Glycols have low volatility and can build to
10 or 15 wt% in the amine treating solution.
3
In effect, any
glycol build-up replaces an equal volume of high capacity
amine solvent with low CO
2
capacity glycol. The solvent
flowrate may remain the same, but it now has a much
lower acid gas capacity. It is hard to design for this, but
ProTreat
®
simulation is capable of accounting for this
effect, and can be a valuable tool in diagnosing and
quantifying this situation.
The hydrate inhibitor, methanol, is another potential
contaminant. By treating methanol as a component whose
absorption and stripping is controlled by its mass transfer
rate, ProTreat allows the residual methanol content of the
treated gas to be accurately determined. This may be
important if the gas is destined to be used as cracker
feedstock to produce ethylene and propylene, because
methanol is a serious cracking catalyst poison.
Although not strictly a contaminant, the solvent
activator concentration can change over time, because it is
considerably more volatile than the MDEA it activates.
Thus, the promoter concentration will fall with
time-on-stream, and this recommends careful solvent
monitoring. One effective way to prevent this problem
from occurring is to use a short bed (perhaps only
500 – 1000 mm depth) of random or structured packing in
the top of the absorber and run makeup water to the top of
this wash section. Similarly, feeding rich amine two or
three trays below the reflux return tray (top tray) in the
stripper will also capture almost all of the amine leaving in
the vapour from the rich solvent feed tray. Both practices
should be standard in almost any amine plant to prevent
virtually all vaporisation losses.
Liquid and vapour
distribution
Uniform liquid distribution (and periodic redistribution) is
particularly important in packed columns. By comparison,
trays tend to take care of themselves, although multi-pass
trays require some care. What is sometimes overlooked
is the need to pay careful attention to gas distribution
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