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OPTIMIZATION OF LNG REGASIFICATION
TERMINAL PROCESSES
M. TECH. SEMINAR REPORT
By
Shashwat Omar
(Roll Number 133020005)
Under the guidance of
Prof. Ravindra D. Gudi
DEPARTMENT OF CHEMICAL ENGINEERING
INDIAN INSTITUTE OF TECHNOLOGY
BOMBAY400 076
APRIL, 2014
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Acceptance Certificate
Department of Chemical Engineering
Indian Institute of Technology Bombay
The seminar report entitled Optimization of LNG Regasification Terminal
Processes submitted by Mr. Shashwat Omar (Roll No. 133020005) has been
corrected to my satisfaction and can be accepted for being evaluated.
Date: 25-04-2014 Signature
(Prof. Ravindra D. Gudi)
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ACKNOWLEDGEMENT
First of all, I would like to thank the almighty who gave me the strength to complete
this report successfully.
I would like to convey my heartfelt thanks to the Indian Institute of Technology
Bombay for providing me the opportunity to work on this project and for allowing me
the access to copious amount of invaluable literature.
I would like to convey my sincere gratitude to my guide Prof. Ravindra D. Gudi, for
his invaluable guidance, supervision and encouragement throughout the course of my
study.
I thank all my friends and colleagues for their help which contributed to the
completion of this report.
At last but not the least, I want to thank my parents for being with me and giving me
inspiration when I needed it the most.
Place: Mumbai Shashwat Omar
Date: 25-04-2014
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TABLE OF CONTENTS
1 Introduction...............................................................................................52 Literature Survey......................................................................................93 Problem Formulation and Work Done so Far......................................234 List of Figures...........................................................................................285 References.................................................................................................29
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CHAPTER 1
INTRODUCTION
From the tsunami in Japan and Thailand to polar vortex in USA and Canada, the
effects of global warming are worsening day by day. The main cause of global
warming is greenhouse gases such as water vapour, Carbon dioxide (CO2) and
methane (CH4). The concentration of Carbon dioxide and methane in surrounding air
has increased 148 % and 36 % respectively in the past 250 years due to industrial
revolution.
The industrial revolution is caused by big power looms, railways, factories and new
technologies. Fossil Fuels like petroleum and Coal fulfilled are the major part of the
energy requirement of this industrial revolution. But in doing so, they have increased
the concentration of greenhouse gases in the surroundings. With the increasing
population, the energy requirements are also increasing and knowing the harmful
effects of fossil fuels, we have to go for clean fuels.
Natural Gas is one of the clean fuels. It can be found in deep underground rock
formation or with and proximity to petroleum. It consists of methane (80-90 %) some
higher alkanes and impurities like carbon dioxide, hydrogen sulphide and nitrogen.
Figure:1 Emission of combustion by-products from fossil fuels
(http://www.cleanandaffordable.ca)
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Figure 1 shows that, after burning, it generates only small amount of carbon dioxide,
nitrogen oxide and almost zero emission of sulphur dioxide. So, the use of natural gas
is getting popular all over the world as a clean environment friendly energy source.
In figure 2, we can see that the countries with most natural gas reserves are Russia,
Qatar, Iran, USA, Venezuela and Australia. The most populated countries in the world
like India, Brazil, China, Indonesia and Pakistan do not have enough natural gas
production to fulfil their energy needs. So, it is necessary for these countries to import
natural gas. But, gas can be transported only via pipeline and it is very costly to lay
pipeline from one country to another and various legal and natural constraints are also
involved in it.
Figure: 2 Country wise Natural Gas Reserves in trillion cubic meters
(http://thomaspmbarnett.com 2012)
LNG (Liquified Natural Gas) is the solution for this problem. LNG is natural gas
liquefied at atmospheric pressure when cooled to -162oC. One cubic metre of Natural
Gas is equal to the 600 cubic metre of LNG. So, for ease of transportation, large
volume of Natural Gas can be converted into small volume of LNG. LNG is
colourless, odourless, non-toxic and non-corrosive. LNG is relatively costly to
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produce and it should be stored in a cryogenic tank because of its very low
temperature.
The process of natural gas exploration to LNG regasification is called LNG supply
chain. It is divided into four parts natural gas production, Liquefaction, LNG
transportation and LNG storage and regasification. First of all natural gas is produced
from gas field wells, and then it is transported to liquefaction plants. These plants are
built at marine terminals so the LNG can be loaded onto specially made tankers for
transporting it overseas. After delivering of LNG to importing terminals, the LNG is
stored into storage tanks, regasified and then sent into pipeline systems for delivery to
downstream customers. Next, we look at all the four processes one by one -
Figure: 3 LNG Supply Chain (Deybach 2012)
Production: - Natural gas is extracted from oil wells or deep rock formations.This gas has impurities like carbon dioxide, nitrogen and hydrogen sulphide
which should be removed before liquefaction otherwise they will freeze and
damage the equipment in which phase change is taking place besides it is
necessary to meet the demand of importer.
Liquefaction: - In this process, natural gas is converted to LNG via cooling itdown to -162
o
C at atmospheric pressure. The volume of natural gas shrinks600 times in the phase change process that means the energy content of 1
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cubic metre of LNG at -162oC is same as 1 cubic metre of natural gas
atmospheric pressure and ambient temperature. So, it becomes easy to
transport natural gas from one place to another because of less volume.
LNG Transportation: - LNG is transported in specially designed ships withdouble hulls so as to minimize the risk of potential leakage. There are three
types of LNG carriers as per the storage tank design membrane tank, IHI
prismatic and spherical tanks. Some Cargos use Boil -off Gas (BOG) generated
from the tanks as a fuel to run ships. A cargo can take 2 to 8 days to reach its
destination. So, it is necessary to calculate the BOG volume consumed by
cargo to determine actual LNG volume that can be unloaded.
Storage and Regasification:- At the importing terminal, LNG is unloaded fromcarrier and stored in big cryogenic storage tanks at atmospheric pressure and -
162oC. The quantity (100,000 m
3to 160,000 m
3) and number of storage tanks
in a terminal depend on the frequency of cargos visiting to terminal and the
demand of downstream customers. The regasification process consists of
gradually converting LNG to natural gas at 0oC and 80-100 bar pressure. The
mechanisms to convert LNG into natural gas depend on the geographical
nature of terminal and composition of the LNG imported. Sometimes,
treatment of natural gas can also be done to meet the caloric value demand by
changing the composition of propane, butane and nitrogen components before
sending it to customers.
LNG production, transportation and converting it to natural gas are a costly and
energy-intensive process. This thesis will focus on optimization of LNG storage
and regasification operation. However, optimization of the complete LNG supply
chain is beyond the scope of this thesis. For upcoming and existing LNG
regasification plants, integration of new technologies with blend of optimization
will help to minimize costs and maximize profit margins and this thesis will work
in this direction.
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operations. We can divide the whole plant into individual equipment and operations
and then will optimize them separately. First of the lot comes unloading and
recirculation operation. For most of the LNG regasification terminals, storage tanks
are located 4-5 km. far from the LNG unloading platform. The LNG transporting
pipelines must be insulated to keep LNG at liquid state. A running LNG regasification
plant is operated in two modes: first, when unloading is happening from a LNG
carrier ship and second, when there is no ship at the LNG unloading platform. In the
second situation, some LNG is pumped from storage tanks and circulated to
unloading transportation lines and then comes back to tanks. This is necessary to keep
transportation lines cool before LNG unloading. In the LNG regasification terminals,
a constant rate of recirculation is always maintained. It determines the operating cost
of the LNG pump which is needed to maintain the flow rate into transportation lines.
However, depending on the LNG send-out flow rate, the handling method and costs
of BOG to flow into the storage tanks are varied. So, the recirculation flow rate
should be decided after taking BOG handling cost into account. Park et al. (2010)
devised an optimal recirculation method to reduce total operating costs by adjusting
the recirculation flow rate dependent on demand of downstream customers. During
unloading process, flashed BOG from the pipeline flows into the storage tank. Due to
increasing amount of BOG, pressure in the storage tank rises. BOG compressor is
used to maintain pressure in storage tank. BOG from compressor meets the LNG
transported to the recondenser to meet demands. This BOG inflow rate determines the
operating cost of the BOG compressor to handle the BOG in the storage tank.
Therefore, recirculation flow rate creates a trade-off between the cost of BOG
compressor and operating cost of the recirculation flow pump. If the necessary
recirculation flow rate is high, the pump operating cost increases, but the compressor
operating cost decreases due to the reduction of BOG inflow and vice-versa.
After unloading, LNG is stored in cryogenic storage tanks. These are of different
kinds based on the natural climate and necessity. The most popular one is full
containment tank. LNG is characterized by its density. Less dense LNG tends to have
lower chain hydrocarbon content and less calorific value and denser LNG tends to
have higher chain hydrocarbon content and high calorific value. During the unloading
operation, terminal operators have to face three different cases. In the first case,
incoming LNG is lighter than the LNG stored in the tank to be filled. A complete mix
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of two LNG quantities is ensured by tank bottom filling operation with a limited BOG
generation and stratification risk is eliminated which can potentially lead to a rollover
event. For the second case, incoming LNG is heavier than the stored in storage tank.
By tank top filling operation, stratification and risk of rollover is avoided but it
usually results in excessive BOG production and tank pressure increases as shown in
figure 5.
Figure: 5Total Boil-off gas flowrate and pressure evolutions during tank top filling
in a 120,000 m3LNG tank (Zellouf and Portannier 2011)
The last situation is tank bottom filling with a heavy LNG under a light stored LNG
heel producing less BOG but leading to stratification which will need to be managed
in order to avoid rollover. Figure 6 shows how a stratification profile looks like inside
a tank. Rollover phenomenon is the overturning of two LNG stratified layers.
Figure: 6Characterization of LNG stratification (Zellouf and Portannier 2011
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Rollover occurs when the two LNG layers densities equalizes mainly further to the
LNG expansion in the lower layer due to the heat ingress. It is accompanied by a
sudden rise in BOG which is due to the sudden release of the energy accumulated in
the lower density layer through time.
Zellouf and Portanni er (2011)showed that by optimizing the operating pressure in
the storage tank, the total BOG rate generated during filling can be reduced by about
half. This can be done in three easy steps. First, pre-cooling of the tank heel before
unloading occurs by lowering the operating pressure. Secondly, before unloading, the
operating pressure is increased above the atmospheric pressure to sub-cool the LNG
and this pressure is maintained throughout the unloading process. Third, once tank is
filled, the pressure is then lowered progressively to the atmospheric value.
Figure: 7Pressure optimization of BOG rate during tank filling of heavy LNG
under/over light heel LNG at a filling rate of 10,000 m3/h, obtained using the GDF
SUEZ LNG MASTER software(Zellouf and Portannier 2011)
Figure 7 shows that by operating pressure optimization, the total BOG rate generated
during filling can be decreased by about 50 %. This highlights the advantages gained
from this procedure, not only in terms of the costs saving by reducing compressor
input, but also in terms of avoiding the use of safety equipments like flares.
Deshpande et al. (2011)presented a lumped parameter model in order to predict time
for the rollover phenomenon and to investigate its sensitivity to heat and mass transfer
coefficients. The originality about this work is its ability to estimate heat and mass
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transfer coefficients from the real time data using an inverse methodology. The real
time data of Level, Temperature and density from storage tanks is assimilated in order
to estimate heat and mass transfer coefficients from the densities of the stratified
layers. Then, the optimized heat and mass transfer coefficients are used in prediction
of time of rollover as shown in figure 8.
Figure: 8procedure used to infer HTC and MTC from the real time LTD data
profile (Deshpande et al. 2011)
BOG generation is uncalled for but inevitable in a LNG regasification terminal. BOG
generation happens in LNG storage tanks and all LNG transporting lines due to some
heat ingress. Querol et al. (2010)proposed number of methods to handle BOG so as
to minimize the operating cost.
Figure: 9Diagram of the unloading procedure of a LNG regasification terminal(Querol et al. 2010)
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As seen in figure 9, during the unloading operation, heat is absorbed by the carrier
(QC), tank (QT), unloading arms (QA) and LNG transporting lines (QL). For a normal
LNG regasification terminal with BOG generation at its worst case and normal case,
these methods are discussed to handle BOG
Flaring: This is used when there is no other option to handle BOG and safetyof equipment and plant is necessary. No additional investment is needed for it
because it is installed compulsorily in each terminal.
Recondenser: All the regasification terminals have this equipment to handlethe total amount of generated BOG in the normal case.
BOG compressor: This equipment is installed to compress BOG as per therequirement of recondenser and LNG carrier pressure.
Most of the LNG terminals buy the electricity supplied by the state or national grid. In
the plant, LNG is converted into natural gas which is a very good fuel and it can be
used to generate electricity inside the terminal. For generation of electricity, a
cogeneration power plant should be set up. In this gas fired power plant, gas will enter
inside the equipment at very high pressure and will be burned. The burned gases will
generate heat and pressure that will be used to rotate a Gas turbine. By rotation of this
Gas turbine, electricity will be generated. The hot gases after burning can be used for
producing hot water from cold water in a heat exchanger. This cogeneration plant are
very effective in terms of energy saving and cost minimization. In addition to that,
Variable costs occurring due to the submerged combustion vaporiser could be reduced
if submerged combustion and heat recovered from cogeneration plant are integrated.
This equipment succeeds to obtain nearly 100 % efficiency in reducing electricity bill
of LNG terminal and its energy dependence on outer sources.
In the LNG storage tank the liquid temperature is approximately -162oC and the
pressure is nearly above the atmospheric pressure. Part of LNG stored in tanks is
continuously evaporated into BOG (Boil-Off Gas) because of heat transfer from
surroundings. This BOG should be removed by compressors because it is necessary to
maintain tank pressure within a safe range. However, excessive operation of
compressors spends excess energy, and even sometimes generates excess BOG. So,
proper handling of BOG is essential for the safe and efficient LNG terminal operation.
Shin et al. (2008)proposed an empirical BOR (boil off rate) model and a simplified
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dynamic tank model. Proposed algorithm generates an optimal operation schedule for
BOG compressors which minimizes the power consumption given potential failure of
one of the operating compressors.
They used following empirical equation in order to estimate the rate of BOG
generation in a LNG storage tank
(2-1)
Where,
CR= rollover coefficient ( 1)
BS= boil off rate on specification (h-1
)
L= LNG density (kg/m3)
VL= LNG volume (m3)
K1 = correction factor for the offset of the tank pressure from the LNG vapour
pressure
K2= correction factor for the LNG temperature
K3= correction factor for the ambient temperature
A range of correction of correction factors was definedbased on real situations and
also defined an equation for calculating LNG vapour pressure from a Antoine model
equation. Using all these equations, the BOG generation rate can be calculated from
the tank pressure. Under the assumption that, LNG volume is very large so the
temperature and volume rate of change will be very slow and thus can be taken as
constant. So, for a target steady state tank pressure P s, the target compressor load F0
can be calculated using the MILP optimization problem solving. A simple dynamic
model based on the ideal gas law is used for the prediction of the tank pressure change
which can be effectively applied to safety analysis for preparing against the potential
failure of one of the running compressors. Based on a case study, the performance
evaluation indicated that the energy consumption could be reduced by half as
compared with the conventional method by operating the tanks at a higher pressure
within the safe range.
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As indicated in the paper, the recondenser is the heart of a LNG regasification
terminal. Like, for a body when heart stops beating, it becomes dead. Same is true for
a LNG plant. Recondenser is the most vital equipment of LNG terminal. It converts
BOG to LNG and gives positive head to High pressure pumps. In storage tanks, LNG
is at 1 bar pressure and -1620C. It is send to recondenser at 8 bar pressure, so it
becomes subcooled. This energy difference between subcooled and boling state is
used to condense BOG into LNG inside recondenser. BOG recovery is an essential
thing and also from that perspective, this equipment is pretty valuable. Yajun et al.
(2012)proposed an optimized control theory to control pressure and level inside the
recondenser and optimum flow rate of LNG to condense BOG. In the figure 10, it is
showed that in some existing BOG terminals, pressure is controlled via recondenser
pressure controller PIC-1 and to prevent High Pressure pumps from cavitations by
regulating the pressure control valves PCV-1 and PCV-2 also used for LNG
bypassing.
Condensing LNG flow rate is maintained by the control valve FCV. The Condensing
LNG flow rate is governed by the following equation:
(2-2)
Flow rate is fixed by the ratio calculation module in FX1. Where, Q LNG (m3/h) is
volumetric flow rate of condensing LNG, (Nm3/h) is standard volume flow ofBOG; (MPa) is pressure of HP suction header: is 0.58, a parameter referringto LNG composition.
Value of QLNGwill be transformed into the setting point of FIC-1. The normal high
set point of recondenser liquid level is 60 %. Once liquid level exceeds 60 %, LIC-2
will sendout signal to LCV to send high pressure make up gas inside recondenser to
lower the liquid level. This move will increase the pressure of recondenser and can
also prevent sufficient amount of LNG not to enter recondenser. If the level still keeps
on rising, there is a interlocked level controller LT-2 which will automatically give
HH-SD a signal to shut down control valve XV-1. When the recondenser level goes
below the set point, LIC-1 could start override control to reduce BOG flow rate
entering into recondenser.
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Figure: 11Optimized control system to BOG recondenser (Yajun et al. 2012)
Pressure will be controlled by the new valve PIC-3. If pressure inside the recondenser
keeps on increasing PC-1 sends signal to PCV-1 to open and decrease the pressure.
When PCV-1 is fully open and pressure inside recondenser still keeps on increasing,
then pressure controller PIC-2 gives command to PCV-3 to vent BOG. For the
condition of decreasing pressure inside recondenser, PC-1 will send signal to PCV-1
to close and if this move is not sufficient then, PCV-4 will operate PCV-2 and make
up gas will enter recondenser and maintain the pressure. For liquid level control,
LCV-1 and LCV-2 will come into use now. When the required LNG output is more
than summation of BOG condensate and condensing LNG stream, the liquid level will
drop down. The liquid controller LIC-1 will increase the opening of valve LCV-1 andLCV-2 to maintain a desired liquid level. Conversely, in the case of liquid level rise,
the opening of LCV-1 and LCV-2 will be decreased.
The advantage of this control is LNG storage tank pressure is held constant.
Recondenser comes into normal position sooner. It is more energy efficient because
makeup is not entered into system frequently. New system can operate at a flexible
operating pressure of recondenser. BOG can be totally liquefied even for the low flow
rate of LNG.
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High pressure Pumps are used to pressurize LNG that comes out of recondenser to
100-100 bar pressure. This high pressure is required because after LNG is converted
into natural gas, it must be transported over a long distance. These pumps can be 4-5
in numbers based on the capacity of the plant. These pumps take considerable amount
of energy to run. A single stage and multi-stage cryogenic pump is shown in figure
12. Presently, most of the LNG terminals have high pressure pumps running on fixed
speed motors and produce a single flow versus head curve at that speed. These pumps
are controlled by throttling the discharge pressure with a control valve or bypassing a
portion of the flow to a secondary stream. Pumps running on a fixed speed have a
single Best Efficiency Point (BEP) at which the pump is designed to run at the rated
point. Pump flows that are throttled by reduction of the discharge pressure run off of
the BEP and therefore the efficiency is reduced. The efficiency and range of operation
for cryogenic pumps can be improved by used of variation of motor speed.
Figure: 12Multistage and Single Stage High Pressure Cryogenic Pumps(Lovelady
et al. 2008)
Lovelady et al. (2008)showed that by adjusting the speed user can accurately control
the operating characteristics of the pump over a greater range with better overall
efficiency. Efficiency of pump operation at off design flow points can be improved by
varying the pump speed to a point on the pump hydraulic curve which matches the
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BEP to the desired flow rate. This paper discussed how the variable speed operation
of cryogenic pumps resulted in improved operating costs with more effective control
of the output flow and head. In addition, the paper also discussed other design, control
and economic benefits of utilizing variable speed motors at LNG terminals.
The vaporizers are used in LNG regasification terminal to convert LNG to natural gas.
High pressure LNG at about -1600C comes into the vaporizer and it is vaporized until
a suitable temperature is achieved. This temperature is generally close to 0oC and it
can vary according to the demand of customers. There are several types of vaporizers
that can be used according to the climate, cost and easily available vaporization fluid.
Some of them are Open Rack vaporizer (ORV), Fired heater with Shell and Tube
vaporizer, Submerged Combustion Vaporizer (SCV), Intermediate Fluid Vaporizer,
Ambient Air Vaporizer etc. It is the most important process to select the right
vaporizer system in designing a LNG terminal as the regasifying costs contribute
major portion of an LNG terminal operation. Low operating cost with high reliability
of the regasifying system is a key parameter for a successful operation of an LNG
receiving terminal. I n-Soo Chun (2008) reviewed ORV performance and optimum
heat recovery temperature under harsh seawater temperature. The practical
optimization of vaporization with the unfavourable seawater conditions in winter isdiscussed. An economic comparison between conventional vaporization and
optimized vaporization is also discussed. As shown in figure 13, an ORV is a
vaporizer in which LNG flows inside a heat-transfer tube and exchanges heat with
seawater that flows outside the heat-transfer tube to gasify the LNG. The LNG flows
in form an inlet nozzle near the bottom and passes through an inlet manifold and a
header pipe to be sent to a set of panels which each of them consisting of a curtain-
like array of heat-transfer tubes. LNG exchanges heat with seawater that flows
downward in a flim-like manner utside the heat-transfer tubes as it flows upward
inside the heat-transfer tubes. This operation results in normal temperature gas to be
sent out from an outlet nozzle via an outlet header and manifold pipe.
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Figure: 13Schematic of an Open Rack Vaporizer (Egashira 2013)
The paper discussed that for achieving low cost vaporization and to minimize impact
on the environment, ORVs and SCVs should be combined as a backup. The seawater
temperature drops to 0oC after it is used for LNG vaporization, which is not
recommended for operational purpose especially during the winter. The following
parameters were investigated for the vaporization optimization:
Performance of ORV with the lower seawater temperature Optimum seawater operating temperature Economic combination of ORV and SCV
Economic justification of seawater heating Overall economic assessment of vaporization facility
In figure 14, it is shown that seawater requirement abruptly increases as the
temperature of inlet seawater decreases, while seawater requirement is constant after a
certain point even though its temperature increases. As the temperature of seawater
decreases below design temperature, the SCV operation and seawater heating process
need to be operated. This results an increase in operating cost. Fuel gas requirements
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for the operation of SCVs increase linearly during the winter as the recovered heat
from seawater decreases.
Figure: 14Economic Analysis of different Seawater Temperature(In-Soo Chun
2008)
The results showed that, for selection of optimum vaporization method, designconstraints with unfavourable seawater temperature have been successfully solved
with heating up seawater. This results in heat recovery increment from cold seawater.
Unless otherwise, the seawater cannot be used when its temperature is lower than the
design temperature.
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CHAPTER 3
Problem Formulation and Work Done so Far
In this thesis, we consider the problem of optimizing LNG regasification terminal
processes. The scope of the optimization that we seek spans from the upstream
operations of unloading LNG cargos at the jetty to the downstream system that sends
out natural gas to customers. Given a schedule of LNG carriers to the terminal and
demand of downstream customers, a typical LNG regasification terminal has
unloading arms, storage tanks, Low pressure pumps, BOG compressor, Recondenser,
High pressure pumps and vaporizers as its main unit operations. The key decisions for
optimization are related to recirculation operation, BOG maintenance and handling,Recondensing BOG, vaporization of LNG. After going through, several research
papers in field of optimization of LNG process, it is seen that no work has been done
to optimize the whole plant in an integrated manner. The works are done in individual
sub-systems such as optimization of storage tank BOG handling, optimization of
recirculation operation and optimization of recondenser operation. This work will
attempt an integrated look at the entire plant, based on mass & energy balances, and
will also consider minimizing external work that may be necessary to maintain phase
of the LNG at appropriate points. As such therefore, this work proposes to optimize
the entire LNG regasification plant by formulating an optimization problem having
objective function as minimization of power consumption and maximization of profit.
In the first phase of the work, we look at optimizing the wait time of LNG cargos
given a schedule of their arrival to the terminal. According to demand, constant
discharge of LNG from each storage tank is assumed. The terminal consists of two
jetties and four LNG storage tanks. A cargo can be assigned to any one of the jetty
based on the availability of space there and total storage tank volume. Once assigned
to a jetty, a cargo will unload its contents to 4 tanks assuming constant discharge flow
rate. There can be three cases for this operation. First, both the jetties have cargos.
Second, one jetty has cargo and one does not. Third, both the jetties do not have any
cargo at a time. The total unloading time of a cargo will depend on the amount of
LNG in it, constant unloading rate from the cargo and total volume of the storage
tanks. It is essential to minimize unloading time of a LNG cargo because the profit of
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operating a LNG terminal is based on the demand of Natural Gas from customers and
demand cannot be fulfilled without supply. Secondly, there is a maximum time limit
in which a cargo has to be unloaded otherwise there will be a huge penalty on
terminal for every minute of delay.
There are two approaches used in the literature for scheduling problems formulation:
continuous and discrete. There are uniform slots for a job in continuous approach and
variable length slots for discrete time approach. For the LNG cargo scheduling
formulation, we will adopt a continuous time approach. The benefit of using this
approach is, it has fewer time slots as compared to the discrete time formulation and
less variables. Having said that, nonlinearities in the system is also introduced and we
need to accommodate that in the solution procedure. The following figure shows the
structure of jetty and storage tanks.
Figure 15: Block diagram of LNG unloading operation
I ndex notations & constraints:
i = jetty (1,2)
j= cargos (1,.... , N)
k = Number of tanks (1,2,3,4)
m = Number of slots (1,2,3...........,M)
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, If jth cargo is assigned to ith jetty in its mth slot.
At a time, one jetty can have only one cargo. So,
(3-1)
The same constraint can be written for ship. At a time, a ship can be boarded to only
one of the jetties. So,
(3-2)
Now, after completion of unloading of a cargo another cargo should be assigned to
the jetty. For that,
=Start time of the unloading of cargo at ith jetty in its mth slot.
End time of the unloading of cargo at ith jetty in its mth slot
(3-3)
Processing time of the jth cargo at ith jetty in its mth slot.
Then,
(3-4)
(3-5)
Where, =Processing time in the unloading
All of the tanks can be filled from both of the jetties or from the single one which will
be based on the availability of cargos. Given there is no tank at its highest level, The
feed to tank coming from each jetty will be divided into four parts otherwise, that tank
will not be filled.
Volume of cargos to be taken as
Volume of tanks to be taken as
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Let the fraction of feed stream divided to each tank is
(3-6)
and (3-7)
Now mass balance equation for a single tank can be written as,
(3-8)
Additional Constraints:
(3-9)
(3-10)
Let where, kland (3-11)
(3-12)
(3-13)
Objective Function: minimization of standing time of the LNG carrier which is
(3-14)
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Where,
Tss(j) = standing time of jth cargo
Tar(j) = arrival time of jth cargo
It is essential to maintain the same level in each tank during the operation of terminal
because in the case of outlet pump failure of one or two tanks, we can send LNG out
from the rest of the tanks. Similarly, If there is a problem in upstream of the tank, we
can take LNG into rest of the tanks. So for that the objective function should be
Minimize
+
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LIST OF FIGURES
Figure No. Title Page No.
1 Emission of combustion by-products from fossil fuels 5
2 Country wise Natural Gas Reserves in trillion cubic meters 6
3 LNG Supply Chain 7
4 Dahej LNG regasification terminal of Petronet LNG Ltd. 9
5 Total Boil-off gas flowrate and pressure evolutions during tank
top filling in a 120,000 m3LNG tank
11
6 Characterization of LNG stratification 11
7 Pressure optimization of BOG rate during tank filling of heavyLNG under/over light heel LNG at a filling rate of 10,000 m
3/h,
obtained using the GDF SUEZ LNG MASTER software
12
8 procedure used to infer HTC and MTC from the real time LTD
data profile 13
9 Diagram of the unloading procedure of a LNG regasification
terminal
13
10 Existing control system to BOG recondenser 17
11 Optimized control system to BOG recondenser 18
12 Multistage and Single Stage High Pressure Cryogenic Pumps 19
13 Schematic of an Open Rack Vaporizer 21
14 Economic Analysis of different Seawater Temperature 22
15 Block diagram of LNG unloading operation 24
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