natural gas liquefaction processes comparison comparaison entre

Poster PO-39
NATURAL GAS LIQUEFACTION PROCESSES COMPARISON
COMPARAISON ENTRE PROCEDES DE LIQUEFACTION
DE GAZ NATUREL
Pierre-Yves Martin
Jérôme Pigourier
Axens (France)
www.axens.fr
Béatrice Fischer
IFP (France)
ABSTRACT
This paper presents Axens efforts to compare LiquefinTM with the competing process,
specially the reputed most efficient ones: C3/MR, C3/MR followed by nitrogen cycle, dual
mixed refrigerant process with spiral wound exchangers.
To compare properly one process to another, it must be done with the same gas, with the
same site conditions, with the same gas turbines and with the same cooling medium
temperature (air or water). That done, to compare processes like for like is still not that easy.
For instance, it seems fair to take the same efficiencies for the compressors, however, axial
and centrifugal compressors do have different efficiencies. Similarly, equal basis leads to have
the same temperature approach for the air-cooler (or water coolers), however between mixed
refrigerant and propane, the heat exchange area will be much lower for mixed refrigerant if the
approach is kept identical. The end flash vapour quantity has also a big influence on the
process efficiency, but each process has a different fuel gas consumption. Those added factors
may lead to wide differences. Axens has calculated the effect of all those parameters on
efficiency.
The equipment characteristics play also an important role in the comparison: the
limitations of axial compressors, of centrifugal compressors (Mach number) and possibly
spiral-wound exchangers maximum size do have to be taken into account. Even the gas
turbines or alternative drivers chosen can be well adapted to one process, but not to the other.
Another important parameter is the LPG recovery: a large LPG recovery will decrease the
efficiency of any process, but not to the same extent. For Liquefin, the efficiency decrease is
not very big.
This paper shows the detailed results of those comparison studies, and the effect of several
parameters on Liquefin efficiency.
RESUME
Cet article présente le travail d’Axens pour comparer son procédé LiquefinTM aux autres
procédés avec lesquels il est en compétition, et spécialement ceux réputés les plus efficaces :
C3/MR, C3/MR suivi d’un cycle azote, DMR avec échangeur bobiné.
Pour comparer sérieusement un procédé avec un autre, on doit considérer le même gaz, les
mêmes conditions ambiantes, les même turbines à gaz , et le même moyen de refroidissement
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Poster PO-39
(air ou eau). Ceci fixé, il n’est toujours pas si aisé d’avoir une comparaison honnête. Par
exemple, il semble normal de prendre la même efficacité pour les compresseurs, cependant les
compresseurs axiaux et centrifuges n’ont pas la même efficacité. De même, on aurait tendance
à prendre la même approche thermique entre la température côté procédé et la température de
l’air ambiant (ou de l’eau de mer), cependant si on fait cela, la surface d’échange sera
beaucoup plus petite avec un mélange réfrigérant qu’avec du propane. La quantité vaporisée
dans le flash final est très importante en terme d’efficacité du schéma, cependant chaque
procédé a une consommation de fuel gaz différente. Tous ces facteurs combinés peuvent
conduire à des différences importantes. Axens a calculé l’effet de chacun de ces paramètres
sur l’efficacité.
Les caractéristiques des équipements jouent également un rôle important dans la
comparaison : les limitations des compresseurs axiaux et centrifuges (le nombre de Mach),
ainsi que la taille maximum possible pour un échangeur bobiné doivent être prises en compte.
Même les turbines à gaz ou les autres moyens d’entraînement des compresseurs peuvent être
bien adaptés à un procédé, mais pas à l’autre.
Un autre paramètre important est la récupération de GPL : une récupération poussée de
GPL va faire chuter l’efficacité de tous les procédés, mais pas dans la même mesure. Liquefin
a une baisse d’efficacité qui n’est pas trop importante dans ce cas.
L’article présente les résultats détaillés de ces études de comparaison, et l’effet de
différents paramètres sur l’efficacité de Liquefin.
INTRODUCTION
For many years, there was absolutely no problem to choose the process of a new
liquefaction plant: C3/MR was the only choice. The same process was implemented again and
again, with small improvements, sometimes bigger gas turbines, and anyway bigger capacities
along the years.
However, this process is now reaching the technology limits: maximum mach number on
the propane compressor, spiral wound exchanger becoming enormous. So now many new
processes are appearing: APCI has launched the APX process (C3/MR/N2 cycles), SHELL a
DMR process, LINDE a process with three mixed refrigerant cycles, and IFP/Axens another
DMR with plate-fin heat exchangers. The old cascade process has come back in Trinidad, with
a new concept.
Nowadays, the new projects consider capacities of 5, 6, sometimes 8 MTPA, whereas the
biggest unit in operation is below 4 MTPA. Deciding of the process to be used for a given
project is now much more difficult, and many factors must be considered to make a proper
comparison.
OLD AND NEW PROCESSES
All the natural gas liquefaction baseload plants built during the last twenty years or so are
C3/MR units, to the exception of Trinidad. The C3/MR process is well known (see figure 1):
the MR and natural gas are pre-cooled with propane, at 3 or now 4 levels of pressure. The
mixed refrigerant is only partially condensed, and separated before entering the large spiralwound exchanger.
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Trinidad plant is built with a cascade process (propane/ethylene/methane), with also
several levels of pressure on each cycle . The process (see figure 2) is arranged so as to have
the same power on the three cycles. Another interesting feature is to install parallel lines of
compression, with Frame 5 - variable speed gas turbine - so as to have a high availability and
easier operation: no compressor trip will shut down completely the unit, and the restart of the
compressor can be done without loss of refrigerant.
Propane cycle
CW
MR cycle
CW
CW
LNG
Feed Gas
Figure 1: C3/MR process simplified scheme
CW
C1
CW
C2
CW
2 Frame 5
2 Frame 5
C3 2 Frame 5
PFHE
PFHE
PFHE
Feed
gas
LNG
Figure 2: Cascade process
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APCI APX process [1]
This process (see figure 3) is a three cycle process: propane, mixed refrigerant, nitrogen.
The exchangers used are kettles for the propane, spiral wound for the mixed refrigerant,
another spiral wound and plate-fin exchanger for the nitrogen cycle. Compared to the C3/MR
process, the new third cycle allows to decrease the propane and MR flow-rates, so as to
achieve with existing equipment much higher capacities (7 – 8 MTPA).
Propane cycle
MR cycle
N2 Cycle
LNG
CW
CW
CW
CW
Feed Gas
Figure 3: APX process simplified scheme
LNG
Natural
gas
First mixed
refrigerant
cycle
Figure 4: Shell DMR process simplified scheme
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SHELL DMR Process [2]
This process (see figure 4) is a dual mixed refrigerant process, with different power on the
two cycles, and with two spiral-wound exchangers. Having mixed refrigerant on the first cycle
allows to have a smaller condenser, and also to remove the propane compressor bottleneck:
For propane compressors, the compressor size, thus the capacity of the unit is limited by the
mach number at the tip of the blades. Using a mixed refrigerant, with a lower molecular
weight, allows to push further this limit as the mach number is lower with this gas. (see figure
7)
LINDE process [3]
Liquefaction
MR cycle
Sub-cooling
MR cycle
Pre-cooling
MR cycle
Natural
gas
Plate-fin
Exchangers
Spiral-wound
heat-exchanger
Spiral-wound
heat-exchanger
LNG
Figure 5: LINDE process simplified scheme
This process is a three cycle process, like the cascade process, but with mixed refrigerant
on all cycles (see figure 5). Compared to the cascade, the efficiency is better, as mixed
refrigerants allow to have a closer approach. However, the power is not the same on all three
cycles, unlike the new cascade. Plate-fin exchangers are used on the first cycle, and spiralwound exchangers on the two colder cycles.
IFP/Axens Liquefin Process [4] [5]
This process (see figure 6) is a dual mixed refrigerant process, with the same power on
both mixed refrigerant cycles. Plate-fin heat exchanger are used for the whole exchange line.
As for all processes with mixed refrigerant on the first cycle, the main condenser is smaller
(see figure 11) and the compressor of the first cycle has a lower mach number (see figure 7).
The lower amount of mixed refrigerant on the cold cycle allows to reach much higher LNG
capacities with the existing axial compressors.
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Poster PO-39
CW
CW
Feed gas
Heavy mixed
refrigerant
compression line
Scrubber
Hot Oil
CW
Light mixed
refrigerant
compression line
Main exchange line
LNG
Figure 6 : Simplified Liquefin process scheme
All these processes were developed to overcome the technology limits reached by the
C3/MR process. The main limitations being the propane compressor as explained already , but
also to some extend the axial compressor and the spiral-wound exchanger.
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
1.25
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
Tip relative mach number (inlet)
Peripheral Mach number (Mu)
Biggest propane compressors
0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16
Flow Coefficient (φ1)
0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16
Flow Coefficient (φ1)
Liquefin MR1 compressors (4.8MTPA)
Figure 7 : Mach number vs flow coefficient for the
first stage compressor (propane or MR)
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To require proven compressors imposes to fit the light refrigerant flow-rate and pressure
ratio to one existing axial compressor. This is another constraint making difficult to increase
very much the LNG production with a proven process.
In case the process could not accommodate an axial compressor, there will be an
efficiency penalty : one consider usually a polytropic efficiency of 86% for the axial
compressors instead of 82% for the centrifugal compressors. This axial compressor will be
used on the coolest stage of the refrigeration. On LIQUEFIN, the simulations give a difference
of about 1.5 % on the LNG production. The use of axial compressor is also beneficial for
operation: the inlet vanes possible angle variation will be very useful for control.
The size of the spiral wound exchanger, already huge, cannot be increased forever. So for
very high capacities, it would be necessary to increase the LMDT of this exchanger to stay
within a feasible size, but with an efficiency penalty. Another possibility would be to have 2
spiral-wound exchangers in parallel, but it would increase the cost and the delivery time.
SIMULATION PARAMETERS
To compare one process with another, it is necessary to be very careful about several
parameters, which can change hugely the result: end flash quantity, compressor efficiencies,
condenser temperature approach, LPG recovery.
End Flash Quantity. The quantity of end flash usually corresponds to the plant fuel gas
consumption (minus some margin). If it is possible to increase this quantity (fuel gas export to
other plants, recycle, etc), the cold end temperature of the main exchange line will increase,
and the efficiency of the plant, thus the quantity of LNG produced will increase.
LNG Production vs End Flash Quantity
106
LNG production
105
104
103
102
101
100
100
110
120
130
140
End flash quantity
Figure 8 : Effect on LNG production of end flash quantity
If no end flash is wanted for any reason, this is a large decrease of LNG production (but a
simplification of the scheme: removing of the end flash compressor). In many cases however,
the quantity of fuel gas cannot be decreased below a certain quantity because of the nitrogen
content of the feed gas. The figure 8 shows the production variation with the quantity of end
flash. If the end flash can be increased by 40%, the LNG production will be increased by 5%
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with the same power on the refrigeration compressors (but of course more power on the fuel
gas compressors). This also must be checked carefully for process comparison.
Compressor Efficiency. Depending upon the compressor efficiency considered, the LNG
production can vary a lot: the LNG production is increased by nearly 10% when the polytropic
efficiency is changed from 79 to 85% (see figure 9). This has to be checked carefully when
making comparisons.
LNG Production vs Compressor Polytropic Efficiency
105
104
LNG Production
103
102
101
100
99
98
97
96
95
79
80
81
82
83
84
85
Compressors Polytropic Efficiency
Figure 9 : Effect on LNG production of compressor efficiency
Temperature Approach on The Main Condenser
Whatever the process, a large condenser on the first refrigerant cycle must evacuate the
heat produced by the refrigeration compressors. As the outlet of this condenser is at bubble
point, to modify the outlet temperature of this condenser will change the discharge pressure of
the corresponding compressor, so the power of this compressor, and thus the overall
efficiency. The temperature approach of the other coolers will also have an impact on the
power, but less important than this one.
We have plotted for a Liquefin case the capacity versus the temperature approach of the
condenser (see figure 10). The closer the temperature approach, the larger the LNG
production. However, in air-cooling case, the size of this condenser can be a problem, as it
governs more or less the size of the plant area, so a part of the cost.
The size of the condenser will depend upon whether the refrigerant of the first cycle is a
pure component (propane as in the C3/MR and cascade), or a mixed refrigerant (Liquefin and
Linde process). With a pure component, the condensation is done at a fixed temperature (the
dew point temperature is the same as the bubble point temperature), whereas with a mixed
refrigerant, the temperature varies linearly between the dew point temperature and the bubble
point temperature. (see figure 11). Either the condenser will be much smaller with a mixed
refrigerant in the first cycle, or inversely with the same condenser size, the LNG production
will be increased.
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Plant capacity vs MR1 Condenser Outlet temperature
103.0%
Plant Capacity (%)
102.5%
102.0%
101.5%
101.0%
100.5%
100.0%
39
40
41
42
43
44
45
°C
Figure 10: Effect on LNG production of condenser temperature approach
Inlet
temperature
60
MR Condensation
55
outlet
temperature
Temperature °C
50
Propane
45
40
35
Condensation
Larger LMDT (+35%)
CONDENSER 35% Smaller
30
Air or Water
25
20
0
50
100
150
200
250
DUTY MW
300
350
Figure 11: Effect of using propane or mixed refrigerant on the size of the condenser
LPG Recovery. To recover LPG from the gas can help sometimes to make the project
economically sound. However, this recovery will increase the power for LNG liquefaction,
and not with the same amount for all processes, nor for all gas compositions. We have
simulated for LIQUEFIN the effect on efficiency of different C3 recovery ratios with a very
lean gas (1.2% C3 only).
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LNG production vs LPG recovery
100
LNG production (%)
99
98
97
96
95
94
93
92
91
30
40
50
60
70
80
90
C3 recovery (% of C3 in feed)
Figure 12 : LNG production decrease with increased LPG recovery
CONCLUSION
To solve the capacity increase problem, many new processes have proposed solutions. The
comparison of these processes for a specific case must be done with care, taking into account
all parameters including site conditions, end flash quantity, compressor efficiency, temperature
approach and LPG recovery. The equipment availability and risk factor must also be taken
into account. In this respect, the flexibility of Liquefin could make it the right choice in many
cases.
REFERENCES CITED
1. M.J. Roberts, J.C. Bronfenbrenner, Yu-Nan Liu, J.M. Petrowski - Large Capacity Single
Train AP-X Hybrid LNG Process - Gastech 2002, Qatar
2. R. Nibbelke, S. Kauffman, B. Pek - Liquefaction Process Comparison of C3MR and
DMR for Tropical Conditions - GPA 81st annual convention, 2002
3. H. Bauer - A Novel Concept for Large LNG Baseload Plants - AICHE Spring National
Meeting, 2001
4. M. Khakoo , B.Fischer, J.C.Raillard - The Next Generation of LNG plants - LNG13,
Seoul, Korea, 2001
5. B.Fischer - A New Process To Reduce LNG Cost - AICHE Spring National Meeting,
2002
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