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Poster PO-26
HOW TO OPERATE LNG TERMINALS WITH FLEXIBILITY / SAFETY
DESPITE THE DIVERSIFICATION OF UNLOADED LNG QUALITIES
COMMENT EXPLOITER DES TERMINAUX METHANIERS AVEC
FLEXIBILITE / SECURITE MALGRE LA DIVERSIFICATION DES
QUALITES DE GNL DECHARGES
Olivier Gorieu, Research Engineer
Dominik Uznanski, Senior Research Engineer
Research and Development Division
Gaz de France
Cryogenic Studies Section - 361, avenue du président Wilson
93211 Saint-Denis La Plaine Cedex – France
olivier.gorieu@gazdefrance.com
www.gazdefrance.com
Pascal DuPont, Associate Professor
Laboratoire Thermocinétique, UMR CNRS
Université de Nantes 6607, BP50609
44306 Nantes Cedex – France
pascal.dupont@polytech.univ-nantes.fr
www.univ-nantes.fr
ABSTRACT
GAZ de FRANCE has completed a small-scale experimental study on the behavior of
stratifications in industrial LNG storage tanks, including specific investigations on the
thick interface separating the two layers. The development of a predictive model has
revealed phenomena of particular interest in the management of industrial LNG storage
tanks. In this paper, the main R&D results on LNG stratification behavior will be
presented and their application to on-site management of LNG stratifications highlighted.
RESUME
GAZ de FRANCE a réalisé une étude expérimentale sur le comportement des
stratifications dans les réservoirs de GNL à grande échelle, en étudiant en particulier
l’interface épaisse séparant les deux couches. Le développement d'un modèle de
prédiction fait en effet apparaître des phénomènes particulièrement intéressants pour la
gestion des stockages industriels de GNL. Ce document présente les principaux résultats
de R&D sur le comportement de stratifications de GNL et l'application sur site d'un tel
modèle de prédiction.
INTRODUCTION
Since the energy market has been deregulated, end-users can select from various
sources of energy. Energy suppliers are facing fierce competition, while at the same time
human and environmental safety regulations are becoming more stringent. As a result of
above, the achievable margins are under pressure if not reduced. Nevertheless, in spite of
PO-26.1
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Poster PO-26
that, the Liquefied Natural Gas (LNG) market has proven its existence, is stronger than
ever, is growing and increasing its market share.
Consequently, the LNG industry today is faced with an ever-increasing diversification
of LNG supplies, caused by the progressive deregulation of gas markets worldwide and
by the increase of the number of liquefaction plants. Even if manufacturers have been
able to reduce the actual cost per cubic meter storage capacity, by using larger LNG
carriers and larger tanks, operating costs have to be controlled despite the diversification
of LNG grades.
Thus, in order to maintain current operating costs and safety levels of LNG storage
sites, import terminals are often faced with the need to handle different grades of LNG in
the same tank. This can impose heavy constraints on the operation of the terminal, such
as production of high boil-off rates during tank filling operations. This can also increase
the likelihood of creating stratifications, which, if left to evolve unmitigated, can lead up
to the unwanted rollover event.
It is within this context and as a response to the need for increased flexibility in
managing economically and safely different qualities of LNG in a same tank, that Gaz de
France, in association with the University of Nantes, carried out a three year
comprehensive research program between 1996 and 1998 on LNG stratification behavior.
In this paper, the main R&D results on LNG stratification behavior will be presented and
their application to on-site management of LNG stratifications highlighted.
LNG BEHAVIOR IN STORAGE TANKS
Liquefied Natural Gas is stored in heat-insulated tanks at a temperature of
approximately 110 K and under a pressure slightly higher than atmospheric pressure.
LNG storage tanks undergo a gas evaporation caused by heat conduction through the tank
walls. The gas produced by evaporation is generally recovered by boil-off gas (BOG)
compressors.
Heat leaks through the insulated tank walls generate major circulation movements by
natural convection within the liquid (Figure 1-a). The heat transferred to the stored liquid
is dissipated primarily via surface evaporation. The gas evaporation in a LNG storage
tank under steady conditions is a localized phenomenon occurring without any boiling,
through a surface boundary layer. The bulk is slightly overheated above the thermodynamic equilibrium point that is reached only at the liquid/vapor interface.
The temperature of the liquid bulk undergoes very little change under constant
pressure, and any warm-up is due exclusively to the aging effect (preferential evaporation
of the most volatile species, such as Nitrogen and Methane). The LNG density also
undergoes very little change under constant pressure. Depending on the evaporation rate
of nitrogen and methane, the density may decrease slightly and then increase.
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Poster PO-26
LNG Density (kg/m3)
458
35
459
460
461
462
463
1'
1
LNG height (m)
3
465
466
35
T : Whessoe LTD
D : Whessoe LTD
30
2
464
30
25
25
20
20
15
15
10
10
5
5
0
-161.0
-160.5
-160.0
-159.5
-159.0
-158.5
-158.0
-157.5
0
-157.0
LNG Temperature (°C)
(a)
(b)
Figure 1 : (a) - Natural convection in a homogeneous storage under steady conditions and
(b) -Vertical density profile (red, upper scale) and temperature profile (blue, lower scale)
monitored by a LTD traveling gage after a filling operation in a large scale storage tank.
More and more, the reduction of operating costs demands a maximization of the
number of carrier unloadings, so that the terminal is close to its design capacity. As a
consequence, there is less flexibility in managing different qualities of LNG, and quite
often tanks are completed with a non-negligible initial height, and LNG stratifications can
occur.
For example (Figure 1-b), for an initial height of 10 meters in a 120 000 m3 storage
tank, using a bottom filling device, the stored LNG (Nigeria, 459 kg/m3) cannot mix
completely with the unloaded LNG (Algeria, 462.7 kg/m3) despite the relatively small
density difference (3.7 kg/m3, 0,8 %) and the large unloading LNG flow rate (about 10
000 m3/h). As a result, a traveling LTD gage highlights that one LNG stratification is
created in the tank characterized by two homogenous layers separated by a buffer zone,
called the thick interface layer. Mainly, the upper layer is made up of the stored LNG
whereas the lower layer is made up of the injected LNG.
If initially an LNG stratification is stable, meaning that the heavy LNG is located in
the lower part of the storage and the light LNG in the upper part, many authors
highlighted that the heat conduction through the tank walls will gradually destabilize the
stratification up to the rollover event.
Because of the LNG warm-up in the first phase, the LNG of the bottom layer is
superheated with respect to conditions in the vapor space at rollover. Thus, the release of
superheat is a phenomenon accompanied by a transient high rate of vapor production that
can be 10 to 30 times greater than the tank’s normal gas boil-off rate, thus giving rise to a
hazard, due to the potentially harmful overpressures the tank can experience. Taking into
account the heat and mass transfers within the buffer zone, it is possible to introduce two
opposite phenomenon that effect the evolution of the stratification up to the rollover
event :
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- If the heat transfer through the buffer zone between the lower and the upper layers
is overestimated, the temperature increase in the lower layer is underestimated. As a
consequence, the density equalization and the rollover appear later.
- If the mass transfer through the buffer zone is overestimated, the heavy element
concentration equalization is also overestimated. As a consequence, the density
equalization and the rollover appear sooner.
The main objective of the research program was to develop a comprehensive model
describing the heat and mass transfer, and thus the dynamics and progressive erosion of
the thick interface separating the two layers of a stratification. The model development
was guided by dedicated stratification trials conducted in an 80-liter cylindrical glass tank
(Figure 2) and by numerical CFD simulations carried out with the ESTET software. This
experimental and numerical database, along with existing operational data, was used to
validate a theoretical model predicting the evolution of an LNG stratification.
SMALL-SCALE EXPERIMENTAL STUDIES USING FREON MIXTURES
In order to study the behavior of the intermediate layer of a stratification, a three part
approach was used. Firstly, a new experimental setup was developed to generate
stratifications using solutions of Freon-11 and Freon-113, characterized by a linear
density gradient within the intermediate layer. With a side and/or bottom wall heating,
this stratification was followed until rollover took place.
3
1
1
4
2
5
6
3
2
(a)
(b)
Figure 2 : The general overview of the experimental setup (a) and the test tank (b)
Because of the difficulties of using cryogenic liquids, Freon-11 solvent was used with
Freon-113 solute as the fluid stimulants to generate stratifications. Freon liquids are ideal
fluids for stratification studies, since the thermo-physical properties of the Freon solutions
are very close to LNG. Also, with a saturation temperature of around 23,6 °C, this type of
Freon solution is at equilibrium with its vapor at ambient conditions (stored LNG is
usually at equilibrium with its vapor under normal operating conditions).
With this new experimental setup, the evolution of stratifications characterized by a
thick interface was studied. By applying lateral and bottom heating, the mean evolution
of the temperature, the concentration and the density in each layer was recorded. The
results give interesting indications as to the progressive merging of the convective
intrusions and the entrainment of the interface by the upper and lower layers.
PO-26.4
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Poster PO-26
Initial
2530 s
3620 s
5510 s
7360 s
9190 s
11110 s
12670 s
14480 s
16220 s
z (cm)
30
20
10
40
Initial
2530 s
3620 s
5510 s
7360 s
9190 s
11110 s
12670 s
14480 s
16220 s
30
z (cm)
40
20
10
0
0
24.5
25
25.5
26
T (°C)
26.5
1486
(a)
1488
1490
3
ρ (kg/m )
1492
1494
(b)
Figure 3 : Time evolution of vertical profiles of temperature (a) and density (b)
Secondly, using ESTET 3.2 CFD software, numerical simulations were conducted in
order to reproduce the evolution of several stratifications in liquid Freon. This software,
developed by EDF, was adapted in order to take into account LNG stratifications. The
simulation of three theoretical stratifications demonstrated the feasibility of this
numerical approach. Each run required more than twenty hours CPU time to simulate the
evolution of a 25 cm high and 25 cm large (radius) stratified liquid up to rollover.
Figure 4 : ESTET numerical simulations of convective intrusions
Qualitatively, convective intrusions and entrainment have been reproduced, as shown
in Figure 4, which shows the temperature field and vertical temperature profile obtained
after 800 seconds of evolution. Thirdly, using a theoretical approach, it was possible to
develop a new comprehensive model of a thick interface that has been compared and
validated with the experimental results using solutions of Freon-11 and Freon-113.
MEDIUM-SCALE EXPERIMENTAL STUDIES USING MIXTURE OF LNG
The Research Division of Gaz de France has developed and is the owner of a software
called LNG MASTERTM, the purpose of which is to perform LNG tank management and
to predict the behavior of LNG inside industrial storage tanks [ 3 ]. LNG MASTER is a
Windows-based commercially available software with an on-line ”help” facility that has
been designed to be “user-friendly” for use by operational engineers. LNG MASTER
helps to optimize the handling of LNG storages in terms of safety and cost reduction. Its
main functionalities are : the prediction of LNG aging, the prediction of LNG tank filling
operations, the prediction of stratification evolutions and the rollover phenomenon, the
PO-26.5
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Poster PO-26
coupling of boil-off rate with the operating pressure and its variations, the taking into
account of tank and site gas recovery and safety devices (compressors, flare, safety
valves, rupture disk), the specification of operating scenarios for the pressure, the
unloading of methane carriers, the site emission and LNG transfer between tanks.
6.8
8
Initial
TRIAL : measured
vertical density profil
(21/04/1989 - 18h00)
6.4
6
Roll-over
Upper layer
Thick Interface
LNG MASTER :
Initial vertical density
fil
5.6
Upper layer
Height (m)
Height (m)
6.0
4
Lower layer
Thick Interface
5.2
Industrial data
LNG Master 4.0
2
LNG Master 4.0 (Interface Lower limit)
4.8
4.4
455
Lower layer
457
459
461
463
465
LNG Master 4.0 (Interface Upper limit)
0
21/04/1989
22/04/1989
23/04/1989
LNG Density (kg/m3)
24/04/1989
25/04/1989
Date
(a)
(b)
Figure 5 : (a) - Analysis of a vertical density profile to determine the upper and lower limits
of the thick interface between the lower and the upper layers, and (b) - evolution of the thick
interface (thin lines), up to the rollover, and the total height of LNG in the tank (thick line)
As LNG MASTER is able to simulate the time evolution of an LNG stratification up
to the rollover taking into account a simple law of heat and mass transfer through a thin
interface, it was interesting to implement such a new comprehensive model in the core of
the software in order to simulate the time evolution of a LNG stratification taking into
account the thick interface. And, to validate this new comprehensive model with LNG
stratifications, LNG MASTER simulations were compared with experimental databases,
constituted during Gaz de France's experimental campaign conducted between 1987 and
1989 in a 500 m3 LNG tank [ 4 ]. As an example of this validation, the results of the LNG
MASTER simulations were compared with the stratification evolution of April 1989. In
this tank, at the cryogenic studies section of Gaz de France (Nantes), a stratification had
been built by a top-filling of a light LNG (Algeria, 456.6 kg/m3, -159.5 °C) onto a heavy
LNG (Algeria, 463.3 kg/m3, -158.9 °C), characterized by a medium density difference
(6.7 kg/m3, 1,4 %). Using an LTD traveling gage, manufactured by Whessoe, it was
possible to measure the initial properties of the LNG stratification, namely the density
(Figure 5-a) and the temperature. In order to simulate the time-evolution of the
stratification with LNG MASTER, the user have to specify the initial conditions, which
are :
-
the properties of the lower and the upper layer, namely the thickness, the LNG
temperature, the LNG composition, and the LNG density being calculated using
the Kloseck McKinley (KMK) method [ 1 ],
-
the properties of the thick layer interface namely the thickness.
PO-26.6
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Poster PO-26
Taking into account the general behavior of the LNG and the specific behavior of the
interface, LNG MASTER simulates the time-evolution of the stratification up to the
rollover (Figure 5-b). During the first phase, the thick interface controls the heat and mass
transfers between the lower and upper layers. Then, during the second phase, affected by
the transfers mainly from the lower to the upper layer, the thick interface moves down
while its thickness is decreasing with the decreasing density difference between the lower
and upper layers. Close to the rollover, the interface is reduced to a thin interface, as a
result of which heat and mass transfers are increasing, the BOG exiting tank is increasing.
During the rollover, the lower and upper layers mix to create a homogenous layer,
consequently the interface disappears.
-157.5
-158.5
-159
461
459
457
-159.5
-160
21/04/1989
Industrial data (z=2m)
Industrial data (z=5.5m)
Industrial data (z=6.5m)
LNG Master 4.0 (Upper)
LNG Master 4.0 (Lower)
463
Density (kg/m3)
Temperature (°C)
-158
465
Industrial data (z=2m)
Industrial data (z=5.5m)
Industrial data (z=6.5m)
LNG Master 4.0 (Lower)
LNG Master 4.0 (Upper)
455
21/04/1989
22/04/1989
23/04/1989
24/04/1989
22/04/1989
23/04/1989
25/04/1989
24/04/1989
25/04/1989
Date
Date
(a)
(b)
Figure 6 : Evolution of the temperature (a) and density (b) in the lower (bleu line) and the
upper (bleu line) layers compared with the LNG temperatures measured at three positions
or densities extracted from vertical density profiles : z=2 m (lower layer), z=5.5 m (interface,
at the beginning) and z=6.5 m (upper layer)
During the first phase, the heat leaks contribute to warm up the temperature (Figure 6)
in the lower layer and consequently the density decreases (Figure 6), and the BOG exiting
the tank is reduced to 10 m3(n)/h, in agreement with the experimental data. Then, during
the second phase, the heat and mass transfers increase the slopes of the temperature and
the density curves, in agreement with the experimental data. Close to the rollover, LNG
MASTER predicts very well the evolution of the temperature both in the lower and upper
layers and the occurrence of the mixing process.
BOG exiting the 500 m3 tank (m3(n)/h)
300
Industrial data
LNG Master 4.0
250
200
150
100
50
0
21/04/1989
22/04/1989
23/04/1989
24/04/1989
25/04/1989
Date
(a)
(b)
Figure 7 : (a) - Evolution of the BOG exiting the 500 m3 tank, compared with the BOG flow
rate measured by a gas flow meter, (b) - LNG density and temperature profiles (blue)
highlight a stratification in the tank partially full of LNG when the unloading begins
PO-26.7
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LNG MASTER is able to predict the rollover onset time within 8 % in this case (+ 6
hours for a 72 hours simulation), and the BOG exiting tank is slightly underestimated by
LNG MASTER (165 m3(n)/h against 200 m3(n)/h measured by the flow meter, for this
example) (Figure 7-a).
LARGE-SCALE OBSERVATIONS USING MIXTURE OF LNG
On LNG terminals, receiving various grades of LNG, stratifications can occur in
storage tanks. Mainly, stratifications occur when the LNG cargo is injected at the bottom
of the tank under a lighter heel LNG (Figure 7-b). Even if stratifications can decrease
electrical power consumption for BOG compression by limiting BOG production during
filling operations, operators need to ensure that the stratifications are compatible with the
terminal’s operating conditions.
At the Montoir-de-Bretagne methane terminal, LNG stratifications are created
regularly due to the fact that the terminal receives various grades of LNG. But operators
need to make sure that the stratified tank’s emptying rate is high enough to withdraw the
lower layer before rollover can occur.
(a)
(b)
(c)
(d)
Figure 8 : Simulation of a LNG stratification with LNG MASTER 4.0, without the use of
submerged pumps : behavior of the LNG thick interface (a), LNG temperature (b) and
LNG density (c) in the lower and upper layer, and the BOG exiting tank (d)
PO-26.8
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Poster PO-26
Using LNG MASTER as a predictive tool, operators can characterize the LNG
stratification, and simulate its evolution and the rollover intensity (Figure 8). Without the
use of LNG submerged pumps (no send out from this tank), the LNG MASTER software
predicts the rollover occurrence in 5 days, for a maximum BOG flowrate of 20000
m3(n)/h. Thus, operators have to anticipate by withdrawing the lower with LNG
submerged pump.
(a)
(b)
Figure 9 : Simulation of a LNG stratification with LNG MASTER 4.0, with the use of
one LNG submerged pump (@ 450 m3/h) behavior of the LNG thick interface (a)
and the BOG exiting tank (b)
With the use of one LNG submerged pump (@ 450 m3/h send out rate) during 5 days,
the LNG MASTER software simulates the variation of LNG level, and predicts the
rollover event to occur in 4 days, for a maximum BOG flow rate of 8 000 m3(n)/h ((a)
(b)
Figure 9-b). By using two submerged pumps, operators are able to entirely avoid the
rollover event. This example highlights the interest of being able to calculate with a
predictive software such as LNG MASTER the adequate LNG emptying rate of a
stratified tank, in order to safely manage the stratification and thus to be able not only
to avoid or minimize the rollover event, but also cut operating costs by reducing BOG
compressor output (due to significantly reduced BOG evaporation rates induced by
the stratification).
CONCLUSION
The comprehensive stratification predictive model and the databases developed in this
project should help gas companies optimize the handling of different LNGs in their
storage tanks and thus be better prepared to operate within a context of deregulation of
gas markets and increase of LNG production sites. In order to validate this new
comprehensive model with LNG stratifications, LNG MASTER software simulations
were compared with experimental databases, constituted during Gaz de France's
experimental campaign conducted between 1987 and 1989 in a 500 m3 LNG storage tank.
On LNG terminals, receiving various grades of LNG, stratifications can occur in
storage tanks. In this view, as a predictive tool, LNG MASTER can be use to characterize
the stratifications, to simulate their evolutions and thus to confirm their compatibility
PO-26.9
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Poster PO-26
with operating constraints, namely to determine the minimum number of LNG submerged
pumps which need to be use for a safe withdrawal of the stratification’s lower layer
before the occurrence of the rollover event.
REFERENCES CITED
[ 1 ] - KLOSEY J. and McKINLEY C., Density of liquified natural gas and of the low
molecular weight hydrocabons, Proceedings of 1st International Conference on
LNG, 1968.
[ 2 ] - LANTERI-MINET P.-L., Reception of different LNG qualities in LNG receiving
terminals / Prediction tools for improving LNG storage tank management, World
Petroleum Congress (Rio, Bresil), 2002.
[ 3 ] - MARCEL O. UZNANSKI, D. and DUBOST J., Un Logiciel pour Gérer les
Réservoirs de GNL en toute Sécurité, "GNL11 Proceedings", Spare paper 1,
Session 3, 1995.
[ 4 ] - MARCEL O., Rollover trials in a 500 m3 LNG storage tank, GDF report
M.CERMAP 96I.682, N 1157, 1992.
[ 5 ] - UZNANSKI, D., Innovative Optimization Techniques for LNG Storage Tank
Management, Osaka Gas R&D Forum '99, 1999.
[ 6 ] - UZNANSKI, D., GORIEU O., AOYAGI Y. and BENITO A., Recent advances in
the optimized management of LNG storage tank filling operations, "GNL13
Proceedings", PO-16, 2001.
PO-26.10
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