Hydrocarbon Processing - December 2020 - GP-20

SPECIAL FOCUS: PLANT DESIGN AND PROCESS CONTROL

liquefaction temperature can be reached.
The mixed refrigerant composition is varied throughout this process with ethane
and propane and, finally, with nitrogen to
reach the temperature required for storage. The process described can take more
than 10 hr to move from precooler temperatures to liquefaction temperatures.
Common challenges during startup.
A number of challenges can occur during
plant startup and from a variety of causes.
At the front end of the plant, liquids may
collect in the legs of piping during shutdown and get pushed downstream all at
once upon startup. Slugs of this nature
can cause a number of issues, such as
foaming, level fluctuations and other instabilities. FIG. 3 shows examples of contaminants present in the feed gas to LNG
liquefaction facilities including pipeline
corrosion inhibitors, glycols and heavy
hydrocarbons. Several other contaminant
types not shown here can also be present;
the variety and impact of feed contami-

nants will be the subject of a later article.
Fluctuations are another issue that often occurs when compressors and other
equipment are started and may make process stabilization difficult. Acid gas removal performance often does not meet specifications upon startup due to improper
temperature control of the lean amine or
feed gas to the contactor, which can cause
liquid hydrocarbon condensation.
Several challenges frequently occur
in the liquefaction section of the plant,
which is much more sensitive to process
variabilities. Any previously mentioned
issues that occur at the front end of the
plant will affect the liquefaction process in
some way. If the issue is severe, it can require a complete restart of the cooldown
process, resulting in a day or more of lost
production. The case study presented in
the following section details challenges
in achieving stable operations at the front
end of an LNG production facility to
properly start up the liquefaction stage.
If foaming occurs in the amine unit,

FIG. 2. An LNG production facility. An LNG storage tank can be seen on the left.

FIG. 3. Example of some contaminants commonly seen in LNG feed gas streams.

20

NOVEMBER/DECEMBER 2020 | GasProcessingNews.com

then feed gas flow to the liquefaction
process may be disrupted or stopped entirely. While conventional gas plants can
often endure some foaming events without necessarily affecting CO2 levels, LNG
facilities often cannot because of more
stringent specifications. Small foaming
events that might slightly affect treating
performance can still cause the treated gas
to miss specification, meaning it cannot be
sent to liquefaction. Gas flow to liquefaction may also be affected if the dehydration or mercury removal stages cannot remove contaminants to the required levels.
Any scenario causing flow disruption
to liquefaction can result in delayed or
discontinued cooling of the MCHE and
mixed refrigerant systems. Depending
on how long flow is disrupted, the entire
cooldown process may need to be restarted if temperatures cannot be maintained
in the interim. This chain of events can
result in a cascade of delays; therefore, effective troubleshooting of the front end of
the process must take place to ensure stability before restarting. This is critical for
the liquefaction stage to start up without
disturbance.
One of the main challenges with LNG
production startup is maintaining the
consistent and stable operating parameters required to move from startup conditions to production. Control loops and
other systems in place for correcting flow
or temperature are often programmed
too aggressively or conservatively to adequately respond to fluctuation, or they are
manually adjusted in an improper manner. A common issue with improperly
tuned control loops is a cascade of fluctuations that results in process instability
and inefficient cooling of the liquefaction
stage. Several steps in the cooldown process require precise and subtle modification to process setpoints, and significant
cascading deficiencies can occur if one
parameter is even slightly misadjusted.
For example, if the mixed refrigerant composition is not adjusted properly then the
MCHE can cool too quickly, causing materials thermal stress; or the temperature
profile can become imbalanced, resulting
in overcompensation from other control
loops and exacerbated imbalances.
A common issue with cold restarts during commissioning is the presence of residual contaminants in the liquefaction stage.
Any dust, moisture or other contaminant
in piping or vessels that is not removed


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Hydrocarbon Processing - December 2020

Table of Contents for the Digital Edition of Hydrocarbon Processing - December 2020

Contents
Hydrocarbon Processing - December 2020 - Intro
Hydrocarbon Processing - December 2020 - Cover1
Hydrocarbon Processing - December 2020 - Cover2
Hydrocarbon Processing - December 2020 - Contents
Hydrocarbon Processing - December 2020 - 4
Hydrocarbon Processing - December 2020 - 5
Hydrocarbon Processing - December 2020 - 6
Hydrocarbon Processing - December 2020 - 7
Hydrocarbon Processing - December 2020 - 8
Hydrocarbon Processing - December 2020 - 9
Hydrocarbon Processing - December 2020 - 10
Hydrocarbon Processing - December 2020 - 11
Hydrocarbon Processing - December 2020 - 12
Hydrocarbon Processing - December 2020 - 13
Hydrocarbon Processing - December 2020 - 14
Hydrocarbon Processing - December 2020 - 15
Hydrocarbon Processing - December 2020 - 16
Hydrocarbon Processing - December 2020 - 17
Hydrocarbon Processing - December 2020 - 18
Hydrocarbon Processing - December 2020 - 19
Hydrocarbon Processing - December 2020 - 20
Hydrocarbon Processing - December 2020 - 21
Hydrocarbon Processing - December 2020 - 22
Hydrocarbon Processing - December 2020 - 23
Hydrocarbon Processing - December 2020 - 24
Hydrocarbon Processing - December 2020 - 25
Hydrocarbon Processing - December 2020 - 26
Hydrocarbon Processing - December 2020 - 27
Hydrocarbon Processing - December 2020 - 28
Hydrocarbon Processing - December 2020 - 29
Hydrocarbon Processing - December 2020 - 30
Hydrocarbon Processing - December 2020 - 31
Hydrocarbon Processing - December 2020 - 32
Hydrocarbon Processing - December 2020 - 33
Hydrocarbon Processing - December 2020 - 34
Hydrocarbon Processing - December 2020 - 35
Hydrocarbon Processing - December 2020 - 36
Hydrocarbon Processing - December 2020 - 37
Hydrocarbon Processing - December 2020 - 38
Hydrocarbon Processing - December 2020 - 39
Hydrocarbon Processing - December 2020 - 40
Hydrocarbon Processing - December 2020 - 41
Hydrocarbon Processing - December 2020 - 42
Hydrocarbon Processing - December 2020 - 43
Hydrocarbon Processing - December 2020 - 44
Hydrocarbon Processing - December 2020 - 45
Hydrocarbon Processing - December 2020 - 46
Hydrocarbon Processing - December 2020 - 47
Hydrocarbon Processing - December 2020 - 48
Hydrocarbon Processing - December 2020 - 49
Hydrocarbon Processing - December 2020 - 50
Hydrocarbon Processing - December 2020 - 51
Hydrocarbon Processing - December 2020 - 52
Hydrocarbon Processing - December 2020 - 53
Hydrocarbon Processing - December 2020 - 54
Hydrocarbon Processing - December 2020 - 55
Hydrocarbon Processing - December 2020 - 56
Hydrocarbon Processing - December 2020 - 57
Hydrocarbon Processing - December 2020 - 58
Hydrocarbon Processing - December 2020 - 59
Hydrocarbon Processing - December 2020 - 60
Hydrocarbon Processing - December 2020 - 61
Hydrocarbon Processing - December 2020 - 62
Hydrocarbon Processing - December 2020 - 63
Hydrocarbon Processing - December 2020 - 64
Hydrocarbon Processing - December 2020 - 65
Hydrocarbon Processing - December 2020 - 66
Hydrocarbon Processing - December 2020 - 67
Hydrocarbon Processing - December 2020 - 68
Hydrocarbon Processing - December 2020 - 69
Hydrocarbon Processing - December 2020 - 70
Hydrocarbon Processing - December 2020 - 71
Hydrocarbon Processing - December 2020 - 72
Hydrocarbon Processing - December 2020 - 73
Hydrocarbon Processing - December 2020 - 74
Hydrocarbon Processing - December 2020 - 75
Hydrocarbon Processing - December 2020 - 76
Hydrocarbon Processing - December 2020 - 77
Hydrocarbon Processing - December 2020 - 78
Hydrocarbon Processing - December 2020 - 79
Hydrocarbon Processing - December 2020 - 80
Hydrocarbon Processing - December 2020 - 80A
Hydrocarbon Processing - December 2020 - 80B
Hydrocarbon Processing - December 2020 - 81
Hydrocarbon Processing - December 2020 - 82
Hydrocarbon Processing - December 2020 - Cover3
Hydrocarbon Processing - December 2020 - Cover4
Hydrocarbon Processing - December 2020 - GP-1
Hydrocarbon Processing - December 2020 - GP-2
Hydrocarbon Processing - December 2020 - GP-3
Hydrocarbon Processing - December 2020 - GP-4
Hydrocarbon Processing - December 2020 - GP-5
Hydrocarbon Processing - December 2020 - GP-6
Hydrocarbon Processing - December 2020 - GP-7
Hydrocarbon Processing - December 2020 - GP-8
Hydrocarbon Processing - December 2020 - GP-9
Hydrocarbon Processing - December 2020 - GP-10
Hydrocarbon Processing - December 2020 - GP-11
Hydrocarbon Processing - December 2020 - GP-12
Hydrocarbon Processing - December 2020 - GP-13
Hydrocarbon Processing - December 2020 - GP-14
Hydrocarbon Processing - December 2020 - GP-15
Hydrocarbon Processing - December 2020 - GP-16
Hydrocarbon Processing - December 2020 - GP-17
Hydrocarbon Processing - December 2020 - GP-18
Hydrocarbon Processing - December 2020 - GP-19
Hydrocarbon Processing - December 2020 - GP-20
Hydrocarbon Processing - December 2020 - GP-21
Hydrocarbon Processing - December 2020 - GP-22
Hydrocarbon Processing - December 2020 - GP-23
Hydrocarbon Processing - December 2020 - GP-24
Hydrocarbon Processing - December 2020 - GP-25
Hydrocarbon Processing - December 2020 - GP-26
Hydrocarbon Processing - December 2020 - GP-27
Hydrocarbon Processing - December 2020 - GP-28
Hydrocarbon Processing - December 2020 - GP-29
Hydrocarbon Processing - December 2020 - GP-30
Hydrocarbon Processing - December 2020 - GP-31
Hydrocarbon Processing - December 2020 - GP-32
Hydrocarbon Processing - December 2020 - GP-33
Hydrocarbon Processing - December 2020 - GP-34
Hydrocarbon Processing - December 2020 - GP-35
Hydrocarbon Processing - December 2020 - GP-36
Hydrocarbon Processing - December 2020 - GP-37
Hydrocarbon Processing - December 2020 - GP-38
Hydrocarbon Processing - December 2020 - GP-39
Hydrocarbon Processing - December 2020 - GP-40
Hydrocarbon Processing - December 2020 - GP-41
Hydrocarbon Processing - December 2020 - GP-42
Hydrocarbon Processing - December 2020 - GP-43
Hydrocarbon Processing - December 2020 - GP-44
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