Optimization of lng regasification

5/26/2018 Optimization of lng regasification

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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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& Chemical Engineering, 34(10), 16061617.

3. Chun, I., Lee, S., & Meeting, A. S. (2008). Optimized Vaporization ConditionsProcess with Unfavorable Design, 6175.

4. Chun, I. S., & Kim, C. H. (2006). Practical optimization of LNG terminalprocess. InAIChE Annual Meeting, Conference Proceedings .

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6. Deshpande, K. B., Zimmerman, W. B., Tennant, M. T., Webster, M. B., &Lukaszewski, M. W. (2011). Optimization methods for the real-time inverse

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Engineering Journal, 170(1), 4452.

7. Egashira, S. (2013). LNG Vaporizer for LNG Re-gasification Terminal, (32), 6469.

8. http://www.cleanandaffordable.ca/#clean9. http://thomaspmbarnett.com/globlogization/2012/7/20/chart-of-the-day-

economists-listing-of-top-15-natural-gas-re.html

10.Kim, H., Kim, I. H., & Yoon, E. S. (2010). Multi-objective Design of CalorificValue Adjustment Process using Process Simulators. Industrial & Engineering

Chemistry Research,49(6), 28412848.

11.Li, Y., Chen, X., & Chein, M.-H. (2012). Flexible and cost-effective optimizationof BOG (boil-off gas) recondensation process at LNG receiving terminals.

Chemical Engineering Research and Design, 90(10), 15001505.

12.Lim, G., & Park, L. (2005). LNG Terminal Operator s Design Feedbacks andTechnical Challenges.

13.Lovelady, J., Wong, J., & Minton, B. (2008). LNG Pump applications withvariable speed motor controls at LNG regasification terminals. In ACS National

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14.Miana, M., Hoyo, R. del, Rodriglvarez, V., Valds, J. R., & Llorens, R. (2010).Calculation models for prediction of Liquefied Natural Gas (LNG) ageing during

ship transportation.Applied Energy , 87(5), 16871700.

15.Munawar, S. A., Bhushan, M., Gudi, R. D., & Belliappa, A. M. (2003). CyclicScheduling of Continuous Multiproduct Plants in a Hybrid, (10), 58615882.

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& Engineering Chemistry Research, 44(11), 40014021.

17.zelkan, E. C., DAmbrosio, A., & Teng, S. G. (2008). Optimizing liquefiednatural gas terminal design for effective supply-chain operations. International

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18.Pardevised k, C., Lee, C.-J., Lim, Y., Lee, S., & Han, C. (2010). Optimization ofrecirculation operating in liquefied natural gas receiving terminal.Journal of the

Taiwan Institute of Chemical Engineers, 41(4), 482491.

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Optimization of lng regasification