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Natural Gas Engineering Students

Graduation Project Part 2

The Proposed content of the LNG Project

Liquefaction of Natural Gas

By

Eng: Mohammed Saad

2011

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1. Our project

1. Good awareness of the LNG technology including, metering and conditioning, treatment, liquefaction, storage and shipping to know the different LNG process and selection factors.

2. Studying the applied technology in SEGAS LNG plant to know every suitable procedure for every unit and its parts with the aiding of PFD & PI&D and DCS screen shots for the plant units.

3. Operation Practices, start up and shutdown for the SEGAS units and overall plant.

4. Using HYSYS Simulation for process in order to enhance the understanding of the process,

5. Conclusions, recommendations for future work.

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1. ACKNOWLEDGEMENT

We would like to thank sincerely to our guide whose able guidance gave us the direction of study and was eager enough to quench or instable thirst of answering to the smallest queries. We shall cherish his help and guidance for a long time to come.

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2. PREFACE

Liquefied natural gas takes up about 1/600 of the volume of natural gas

volume at the atmospheric conditions, the most suitable method for gas

exporting for long distance and with large amounts than CNG and pipe lines.

A refrigeration system removes thermal energy from a low-temperature

region and transfers heat to a high-temperature region.

The 1st law of thermodynamics tells us that heat flow occurs from a hot source to a cooler sink; therefore, energy in the form of work must be added to the process to get heat to flow from a low temperature region to a hot temperature region. We study the different type of refrigeration cycles and COP to obtain the minimum temperature of natural gas to be liquefied. We will know what are the procedures must be done after design and

selection of this technology and to give awareness with the SEGAS plant process

to know every suitable procedure for every unit and its parts with the aiding of

PFF, P&ID [reading drawing skills and early interaction with the practical field],

DCS [Distributed control system] screen shots for the plant units and using

Hysys simulation for process in order to enhance the understanding of the

process.

We will attempt to relate the operation procedure to technical reasoning and

justifying the learned theoretical concepts (refrigeration, heat transfer...) not

only in one field but in different engineering branches (chemical, mechanical,

and electrical …..) that will help in achieving our target.

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Contents

1. Our project objective

2. ACKNOWLEDGEMENT

3. Preface

4. Introduction

5. Refrigeration cycle

5.1. Refrigeration

5.2. Properties of Pure substance

5.3. Ton of refrigeration (TOR)

5.4. Carnot cycle

5.5. Standard vapor compression refrigeration system (VCRS)

5.5.1. Assumptions for Ideal VCRC

5.6. Cascade cycle

5.6.1. Advantages and Disadvantages

5.6.2. Assumptions

5.6.3. Applications of cascade systems

5.6.4. Optimum cascade temperature

5.7. Mixed Refrigerants

6. Natural gas treatment

7. LNG transportation

7.1. Ships

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7.2. Storage

7.2.1. Full containment Tanks

7.2.2. Comparison of full containment and In-Ground tanks

7.3. Regasification

7.3.1. Best available commercial Technologies

7.3.2. Waste Heat Recovery and engine cooling Technology

7.3.3. Selecting vaporization Method

8. Case study

8.1. SEAGAS

8.2. Qatar mega Train

9. Conclusions and recommendations for future work

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4. Introduction

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5. Refrigeration cycle

5.1. Refrigeration

Refrigeration systems are common in the natural gas processing industry and processes related to the petroleum refining, petrochemical, and chemical industries. Several applications for refrigeration include NGL recovery, LPG recovery, hydrocarbon dew point control, reflux condensation for light hydrocarbon fractionators, and LNG plants.

Selection of a refrigerant is generally based upon temperature requirements, availability, economics, and previous experience. For instance, in a natural gas processing plant, ethane and propane may be at hand; whereas in an olefins plant, ethylene and propylene are readily available. Propane or propylene may not be suitable in an ammonia plant because of the risk of contamination, while ammonia may very well serve the purpose. Halocarbons have been used extensively because of their nonflammable characteristics.

5.2. Properties of Pure substance

A pure substance is one whose chemical composition does not change during thermodynamic processes. Water and refrigerants are examples of pure substances. These days emphasis is on the use mixture of refrigerants. The properties of mixtures also require understanding of the properties of pure substances Water is a substance of prime importance in refrigeration and air-conditioning. It exists in three states namely; solid ice, liquid water and water vapor and undergo transformation from one state to another. Steam and hot water are used for heating of buildings while chilled water is used for cooling of buildings. Hence, an understanding of its properties is essential for air conditioning calculations. Substances, which absorb heat from other substances or space, are called refrigerants. These substances also exist in three states. These also undergo transformations usually from liquid to vapor and vice-versa during heat absorption and rejection respectively. Hence, it is important to understand their properties also. If a liquid (pure substance) is heated at constant pressure, the temperature at which it boils is called saturation temperature. This temperature will remain constant during heating until all the liquid boils off. At this temperature, the liquid and the associated vapor at same temperature are in equilibrium and are called saturated liquid and vapor respectively. The saturation temperature of a pure substance is a function of pressure only. At atmospheric pressure, the saturation temperature is called normal boiling point. Similarly, if the vapour of a pure substance is cooled at constant pressure, the temperature at which the

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condensation starts is called dew point temperature. For a pure substance, dew point and boiling point are same at a given pressure. Similarly, when a solid is heated at constant, it melts at a definite temperature called melting point. Similarly cooling of a liquid causes freezing at the freezing point. The melting point and freezing point are same at same pressure for a pure substance and the solid and liquid are in equilibrium at this temperature. For all pure substances there is a temperature at which all the three phases exist in equilibrium. This is called triple point The liquid-vapor phase diagram of pure substance is conveniently shown in temperature entropy diagram or pressure-enthalpy diagram or p-v diagram. Sometimes, three dimensional p-v-t diagrams are also drawn to show the phase transformation. In most of the refrigeration applications except dry ice manufacture, we encounter liquid and vapor phases only. Thermodynamic properties of various pure substances are available in the form of charts and tables.

Fig. 5.1 P-h diagram for a pure substance.

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The next figures show the different charts of the refrigerants and the other

refrigeration medium

.

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5.3. Ton of refrigeration (TOR)

The cooling capacity of older refrigeration units is often indicated in "tons of refrigeration" (TOR). A ton of refrigeration represents the heat energy absorbed when a ton (2000lbs.) of ice melts during a 24-hour day. The ice assumed to be solid as 32 degrees F. (0 degrees C.) initially and becomes water at 32 degrees F. (0 degrees C.). The energy absorbed by the ice is the latent heat of ice times the total weight. Today, refrigeration units are often rated in Btu/hr or KW instead of tons. The Btu equivalent of one ton of refrigeration is easy to calculate. Multiply the weight of one ton of ice (2000lbs.) by the latent heat of fusion (melting) of ice (144 Btu/lb.). Then divide by 24 hours to obtain Btu/hr. TOR = 2000 X 144/24. TOR = 288,000 Btu/24 hours. TOR = 12,000 Btu/hr. TOR = 3.51 kW.

5.4. Carnot cycle

The Carnot cycle can serve as the initial model of the ideal refrigeration cycle. Operates as a reversed heat engine cycle, transfers a quantity of heat, QL, from a cold source at temperature, TL.

𝐐𝐥 = 𝐓𝐥 𝐒𝟑 − 𝐒𝟐

QH = TH(S4 − 𝑺𝟏)

(5.1)

(5.2)

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Fig. 5.2 T-s diagram for Carnot cycle.

𝐖𝐢𝐧 = 𝐐𝐧𝐞𝐭 = 𝐐𝚮 − 𝐐𝐋

= 𝐓𝐇 − 𝐓𝐋 (𝐒𝟑 − 𝐒𝟐)

The coefficient of performance (COP) is given by:

𝐂𝐎𝐏 =benefit

cost

Where the benefit for a refrigeration process is the cooling load given as QL. This is the net benefit, i.e. heat is removed from the cold space. For a heat pump, the benefit is the heat added to the hot space, i.e. QH. From equation (5.1),(5.2) and (5.4) we get:

COPrefrig =𝐐𝐋

𝐖𝐢𝐧=

𝐓𝐋

𝐓𝐇−𝐓𝐋

COPheat pump =𝐐𝐇

𝐖𝐢𝐧=

𝐓𝐇

𝐓𝐇−𝐓𝐋

Note: The “1” accounts for the sensible heat addition in going from TL to TH.

T

S

1

2

3

4

Win

QL

QH

TH

TL

(5.3)

(5.4)

(5.5)

(5.5)

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5.5. Standard vapor compression refrigeration system (VCRS)

Vapor-compression refrigeration systems are the most common refrigeration

systems in use today. Figure 5.3 shows the different components of the vapor

compression refrigeration system. The cycle consists of four thermodynamically

processes as follows:

Process 1-2: Isentropic compression of saturated vapour in compressor

(S=const.)

Process 2-3: Isobaric heat rejection in condenser (Pc=const.)

Process 3-4: Isenthalpic expansion of saturated liquid in expansion device

(h=const.)

Process 4-1: Isobaric heat extraction in the evaporator (Pe=const.)

Figure 5.3 Components of vapor compression refrigeration system.

1

2

3

4

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By utilizing the Pressure-Enthalpy (P-H) diagram, the refrigeration cycle can be broken down into four distinct steps:

1. Expansion 2. Evaporation 3. Compression 4. Condensation

The vapor-compression refrigeration cycle can be represented by the process flow and P-H diagram shown in Fig5.4.

Figure 5.4 (P-h) diagram of vapor compression refrigeration cycle.

1 -2s A reversible, adiabatic (isentropic) compression of the refrigerant. The saturated vapour at state 1 is superheated to state 2.

Wc = h2s − h1 2s-3 an internally, reversible, constant pressure heat rejection in which the working substance is de superheated and then condensed to a saturated liquid at 3. During his process, the working substance rejects most of its energy to the condenser cooling water.

qH = h2s − h3 3-4 An irreversible throttling process in which the temperature and pressure decrease at constant enthalpy.

h3 = h4

4-1 An internally, reversible, constant pressure heat interaction in which the working fluid is evaporated to a saturated vapour at state point 1. The latent enthalpy necessary for evaporation is supplied by the refrigerated space surrounding the evaporator. The amount of heat transferred to the working fluid in the evaporator is called the refrigeration load.

qL = h1 − h4

1

2 3

4

P

h h3=h4 h1

2s

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The thermal efficiency of the cycle can be calculated as

𝜂 =𝑞𝑒𝑣𝑒𝑝

𝑤com=

ℎ1 − ℎ4

ℎ2𝑠 − ℎ1

5.5.1. Assumptions for Ideal VCRC

1. Irreversibility within the evaporator, condenser and compressor are ignored.

2. No frictional pressure drops.

3. refrigerant flows at constant pressure through the two heat exchangers

(evaporator and condenser)

4. stray heat losses to the surroundings are ignored

5. compression process is isentropic

5.6. Cascade cycle

A cascade system consists of two separate single-stage refrigeration systems: a lower system that can better maintain lower evaporating temperatures and a higher system that performs better at higher evaporating temperatures. These two systems are connected by a cascade condenser in which the condenser of the lower system becomes the evaporator of the higher system as the higher system’s evaporator takes on the heat released from the lower system’s condenser. See figures 1.7 and 1.8. It is often desirable to have a heat exchanger between the liquid refrigerant from the cascade condenser and the vapor refrigerant leaving the evaporator of the lower system. The liquid refrigerant can be sub-cooled to a lower temperature before entering the evaporator of the lower system, as shown in the next figure. Because the evaporating temperature is low, there is no danger of too high a discharge temperature after the compression process of the lower system. When a cascade system is shut down while the temperature of the ambient air is 25°C, the saturated vapor pressure of the refrigerant increases. For a lower system using HFC- 125 as the refrigerant, this saturated pressure may increase to 1440 kPa abs. For safety reasons, a relief valve at the cascade condenser connects to an expansion tank, designed to store the refrigerant from the lower system in case of shutdown. For extremely low evaporating temperatures, a multistage compression system may be used in either the lower or higher system of a cascade system.

5.6.1. Advantages and Disadvantages

The main advantage of a cascade system is that different refrigerants, equipment, and oils can be used for the higher and the lower systems. This is especially helpful when the evaporating temperature required in the lower system is less than -60°C. One disadvantage of a cascade system is the overlap of the condensing temperature of the lower system and the evaporating temperature of the higher system for heat transfer in the condenser. The overlap results in higher energy

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consumption. Also a cascade system is more complicated than a compound system. The performance of the cascade system can be measured in terms of 1 kg of refrigerant in the lower system, for the sake of convenience. If the heat transfer between the cascade condenser and the surroundings is ignored, then the heat released by the condenser of the lower system is equal to the refrigerating load on the evaporator of the higher system.

5.6.2. Assumptions

The following assumptions are made in all the examples provided in this chapter: 1. The pressure drop in all heat exchangers and phase separators is zero. 2. The ambient temperature is 300 K. 3. The minimum temperature approach between the hot and cold streams is

3 K in all cold heat exchangers. 4. The adiabatic efficiency of all compressors is 80% and that of all pumps is

90%. 5. The heat in leak from ambient is negligible.

Figure 5.5.Refrigeration Cascade system layout.

1

2 3

4

5

6 7

8

Condenser (Tch)

Ch

CL

Evaporator h(Teh)

Condenser (TcL)

Evaporator (TeL)

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Figure 5.6. (p-h) diagram for cascade refrigeration system.

5.6.3. Applications of cascade systems

1. Liquefaction of natural gas and petroleum vapours. 2. Liquefaction of industrial gases 3. Manufacturing of dry ice 4. Deep freezing etc. 5. Medical applications

5.6.4. Optimum cascade temperature:

For a two-stage cascade system working on Carnot cycle, the optimum cascade temperature at which the COP will be maximum, Tcc,opt is given by:

𝑇𝐶𝐶𝑜𝑝𝑡 = 𝑇𝑒 𝑇𝑐

Where Te and Tc are the evaporator temperature of low temperature cascade and condenser temperature of high temperature cascade, respectively. Refrigeration effect

𝑄°𝑒𝑙 = 𝑚°𝑙 ℎ5 − ℎ8 Total Power

𝑃𝑐𝑡 = 𝑃𝑐𝑙 + 𝑃𝑐ℎ

Pch = mh(h2 − h1)

Pcl = ml h6 − h5

Heat balance of the cascade condenser

𝑚ℎ ℎ1 − ℎ4 = 𝑚𝑙 ℎ5 − ℎ8

p

h

1

2 3

4

5

6 7

8

Pch

Peh

PcL

Pel

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And the C.O.P

COP =Qel

Pct

In a refrigeration cycle, energy is transferred from lower to higher temperature levels economically by using water or ambient air as the ultimate heat sink. If ethane is used as a refrigerant, the warmest temperature level to condense ethane is its critical temperature of about 90°F. This temperature requires unusually high compression ratios — making an ethane compressor for such service complicated and uneconomical. Also in order to condense ethane at 90°F, a heat sink at 85°F or lower is necessary. This condensing temperature is a difficult cooling water requirement in many locations. Thus a refrigerant such as propane is cascaded with ethane to transfer the energy from the ethane system to cooling water or air. One example of cascade cycle is ConocoPhillips currently has at least two trains in operation: Atlantic LNG, and Egyptian LNG. More trains are being constructed since this process is expanding to compete with the APCI. It shares about 5% of the world’s LNG production and it has been in operation for more than 30 years. The process uses a three stage pure component refrigerant cascade of propane, ethylene, and methane .The pretreated natural gas enters the first cycle or cooling stage which uses propane as a refrigerant. This stage cools the natural gas to about -35oC and it also cools the other two refrigerants to the same temperature. Propane is chosen as the first stage refrigerant because it is available in large quantities worldwide and it is one of the cheapest refrigerants. The natural gas then enters the second cooling stage which uses ethylene as the refrigerant and this stage cools the natural gas to about -95oC. At this stage the natural gas is converted to a liquid phase (LNG) but the natural gas needs to be further sub cooled so the fuel gas produced would not exceed 5% when the LNG stream is flashed. Ethylene is used as the second stage refrigerant because it condenses methane at a pressure above atmospheric and it could be also condensed by propane. After methane has been condensed by ethylene, it is sent to the third stage where it sub cools the natural gas to about - 155oC then it is expanded through a valve which drops down the LNG temperature to about - 160oC. Methane is sent back to the first cooling stage and the LNG stream is flashed into about 95% LNG (which is sent to storage tanks) and 5% fuel gas used as the liquefaction process fuel. Methane is used as the sub cooling stage refrigerant because it could sub cools up to -155 oC and it is available in the natural gas stream so it is available at all times and at lower costs.

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Figure 5.7 ConocoPhillips simple cascade schematic

We will now attempt to perform a simulation of this simple process. We first recognize that the boiling points of each of the refrigerants will limit the temperatures at the outlet of each exchanger.

Table 1.1: Refrigerants boiling points

Refrigerant Boiling Point (°c) Propane -42 Ethylene -103 Methane -161

The T-Q profile of the simple cascade is shown in the next figure:

Figure 5.8 Simple ConocoPhillips Cascade LNG Cooling Curve

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5.7. Mixed Refrigerants

The MR process uses a mixture of hydrocarbons and nitrogen as a single refrigerant. Selecting the correct composition of refrigerant to maintain proper temperatures and pressures during separation and flashing is an important part of optimization.

Figure 5.9 Refrigerant vs. LNG cooling curve

One of the examples of MR is APCI. This process accounts for a very significant proportion of the world's base load LNG production capacity. Train capacities of up to 4.7 million tpy were built or are under construction. It's illustrated in Figure 4 as part of an overall LNG plant flow scheme. There are tow main refrigeration cycles. The precooling cycle uses a pure component, propane. The liquefaction and sub-cooling cycle uses a mixed refrigerant (MR) made up of nitrogen, methane, ethane and propane. The precooling cycle uses propane at three or four pressure levels and can cool the process gas down to -40° C. it's also used to cool and partially liquefy the MR. The cooling is achieved in kettle-type exchangers with propane refrigerant boiling and evaporating in a pool on the shell side, and with the process streams flowing in immersed tube passes. A centrifugal compressor wih side streams recovers the evaporated C3 streams and compresses the vapour to 15-25 Bara to be condensed against water or air and recycled to the propane kettles. In the MR cycles, the partially liquefied refrigerant is separated into vapour and liquid streams that are used to liquefy and sub-cool the process streams from typically -35°C to between -150°C-160°C. This is carried out in a proprietary spiral wound exchanger, the main cryogenic heat exchanger (MCHE). The MCHE consists of two or three tube bundles arranged in a vertical shell, with the process gas and refrigeration entering the tubes at the bottom which then flow upward under pressure.

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The process gas passes through all the bundles to emerge liquefied at the top. The liquid MR stream is extracted after the warm or middle bundle and is flashed across a joule Thomson valve or hydraulic expander onto the shell side. It flows down wards and evaporates, providing the bulk of cooling for the lower bundles. The vapour MR streams passes to the top(cold bundle) and is liquefied and sub-cooled, and is flashed across a JT valve into the shell side over the top of the cold bundle . It flows downwards to provide the cooling duty for the top bundle and, after mixing with liquid MR, part of the duty for the lower bundles. The overall vaporized MR streams from the bottom of the MCHE is recovered and compressed by the MR compressor to 45-48 Bara. It's cooled and partially liquefied first by water or air and then by the propane refrigerant, and recycled to the MCHE. In earlier plants all stages of the MR compression were normally centrifugal; however, in some recent plants axial compressors have been used for the LP stage and centrifugal for the HP stage.

Figure 5.9 APCI propane precooled mixed refrigerant process.

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6. LNG transportation

6.1. Ships

Typical ships are shown in the next two figures 45&46. The concept of

membranes is based on the idea that the forces exerted by the LNG cargo are

transmitted by a metallic membrane to the ship’s inner hull (these are double

hull ships). The Norwegian company Moss Rosenberg introduced the concept of

spherical tanks. The capacity of these ships is between 100,000 to 140,000 m3.

The ships are powered with a medium speed 4-stroke diesel engine, which is capable of providing fuel efficiency and operates on burning low pressure gas. It consumes 0.15% of the cargo through what vaporizes of the LNG and that adds up to consuming 2 % of ads up to consuming 2 % of the LNG produces over the life of the project.

Figure 6.1 Schematic and photo of LNG carrier with GAZ Transport Membrane

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Figure 6.2 Schematic and photo of LNG carrier with Moss Rosenberg self

supporting tanks

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6.2. Storage

All field erected LNG storage tanks have a primary and a secondary

containment system. The primary container is for normal operation and the

secondary containment is for the highly unlikely event of a leak in the primary

container. World wide a variety of storage tank types have been developed and

constructed over the years. Those that have been successful can be categorized

into the following types:

1. Single containment types have a cylindrical metal primary tank and an

earthen dike wall secondary containment. Single containment tanks were

the first type developed and are now used mainly in remote locations

2. Double containment types have a cylindrical metal primary tank and an

independent metal or reinforced concrete, open top secondary

containment outer tank. This type was developed for small sites; however

few have been built because the full containment type, below, was soon

developed.

3. Full containment type tanks have a cylindrical metal inner primary tank

and metal or pre-stressed concrete outer secondary containment tank

structurally independent but combined into one structure. Today full

containment tanks are the most common type used.

4. Full containment membrane type has a cylindrical thin metal membrane

primary container structurally supported by an outer pre-stressed concrete

cylindrical tank. The outer concrete tank also serves as the secondary leak

containment. Applications of membrane tanks have been far less than the

other types of tanks except in Japan and Korea.

5. Even though all of the above listed structures can be built in-ground, only

membrane tanks, type 4, have been regularly built below grade. The outer

wall of an in-ground tank is not pre-stressed. The outer wall is held in

compression by soil pressure which in turn also supports the LNG’s

hydrostatic load.

6.2.1 Full Containment Tanks

A liquefied Modern LNG storage tanks are typically full containment type, which has a pre-stressed concrete outer wall and a high-nickel steel inner tank, with extremely efficient insulation between the walls. The common characteristic of LNG Storage tanks is the ability to store LNG at the very low temperature of -162°C. Large tanks are low aspect ratio (height to width) and cylindrical in design with a domed steel or concrete roof. LNG storage tanks can be found in ground, above ground or in LNG carriers. Storage pressure in these tanks is very low, less than 10 kPa (1.45 psig). Sometimes more expensive underground tanks are used for storage. Smaller quantities (700 m³ and less), may be stored in

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horizontal or vertical, vacuum-jacketed, pressure vessels. These tanks may be at pressures anywhere from less than 50 kPa to over 1,700 kPa (7 psig to 250 psig).

LNG must be kept cold to remain a liquid, independent of pressure. Despite efficient insulation, there will inevitably be some heat leakage into the LNG, resulting in vaporization of the LNG. This boil-off gas acts to keep the LNG cold. The boil-off gas is typically compressed and exported as natural gas, or is re-liquefied and returned to storage. Figure 47 illustrates the full containment cryogenic tank internals.

Figure 6.2 Full Containment Tank Internals.

6.2.2 Comparison of full containment and In-Ground tanks

6.2.2.1. Economics

The capital cost of constructing an in-ground LNG tank is over twice that

of a full containment tank.

In-ground tanks consume more electrical energy for increased boil-off

compression, soil and foundation heating and ground water pumping. The

extra power consumption is approximately a constant 1,500 kW load.

6.2.2.2. Design and safety

When LNG tanks are located in areas of possible aircraft impact full

containment tanks have a higher chance of impact than in-ground tanks.

Structures that are built into the ground generally have reduced

acceleration loads generated from seismic events. This is because motions

of in-ground storage system follow the seismic ground shaking and are not

amplified through the structure of the tank as is the case for an above

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ground storage system. In addition, sloshing responses of LNG tanks

resulting from seismic activity are lower for underground tanks. This

however does not mean that an in-ground tank is safer than an above

ground tank. It means that an aboveground tank is designed to higher

seismic loads than an in-ground tank. In all cases LNG tanks are designed

to maximum seismic activity for each tank type and its location.

Based on the seismic hazard studies, the Hong Kong region is an area of

low seismic activity. For example, seismic loading is not explicitly

considered for general building design in Hong Kong. Hence, the design

driver for selecting underground tank system to lower seismic loads is not

applicable and the aboveground storage tanks are an appropriate choice

for this location.

Ground water can be very problematic for in-ground LNG tanks. The

density of LNG is less than one-half that of water. If for some reason

ground water was to rise around an in-ground tank or leak into it, buoyant

forces could lift the tank or displace LNG over the tank wall. However such

an event is considered highly unlikely.

6.2.2.3. Operation and Maintenance

The soil heating cables on an in-ground tank are located such that they are

almost impossible to repair. Redundant heating cables will be installed to

lessen the possibility of failure.

Since most equipment and piping is located on the roof of an LNG tank,

access to this equipment is generally easier for in-ground tanks.

Above-ground LNG tanks do not require the operation and maintenance of

dewatering pumps.

Because much on an in-ground is covered with soil, tank inspection and

monitoring is difficult and possible problems may go unnoticed. When problems

do occur, it is much harder to repair them. For example, the in-ground tanks in

Yung-An (Taiwan) have been leaking for years, but due to the difficulty in

pinpointing the leak location and accessibility, have elected not to try to repair

the leak. Figure 6.3 shows a variety of design in installation of LNG storage tanks.

Figure 6.3 LNG Tank designs

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6.3. Regasification

Treated “Natural Gas” is odorless, colorless, nontoxic, and noncorrosive, and, when cooled to -162 C, turns into liquefied natural gas (LNG). Liquefying natural gas reduces its volume by more than 600 times, which makes it more efficient and practical to store and transport. At present, most of the worldwide produced natural gas has to be transported from its production facilities to distant consumption markets. LNG is mostly transported via special-purpose LNG tankers. When the LNG cargo reaches its destination, the liquefied natural gas is re-vaporized back into a gas, which is then linked to pipelines that transport the gas for use. This re-vaporization (or regasification) activity is significant in terms of operating costs and possible impact on the environment. Offshore facilities, also known as deepwater ports, face unique challenges in structural design and sitting. However, the opportunity to build deepwater ports for LNG importation has also opened doors for new and unique methods to bring natural gas into the United States. Newer technologies utilize engine cooling technology and waste heat recovery from generators, boilers, or a combination of heat sources to warm the LNG, thus improving the efficiency of regasifying and reducing impact on the environment.

6.3.1. Best Available Commercial Technologies The three sources of thermal energy typically used to warm LNG from a liquid to a gaseous state are ambient air, natural gas (heat from combustion), and seawater. The basic types of vaporization systems that utilize these sources of thermal energy include:

intermediate fluid vaporizers,

ambient air vaporizers,

open rack vaporizers,

shell and tube vaporizers,

Submerged combustion vaporizers.

Each system uses a vaporization process that passes the liquefied natural gas through pipes that are surrounded by a heating medium to transfer heat into the LNG. “Direct” heat is when the heating medium directly warms the LNG. “Indirect” heat is when the heating medium is used to warm an intermediate (or secondary) medium that transfers the heat to the LNG. Lastly, each vaporization system can be set up as an “open-loop” or “closed-loop” system. Using seawater to heat the LNG is referred to as open-loop vaporization, while using natural gas combustion is closed-loop vaporization.

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6.3.1.1 Intermediate Fluid Vaporizers, IVS

An intermediate fluid vaporizer (IFV) (figure 50) uses an intermediate heat transfer fluid to re vaporize LNG. IFV technology can be configured to operate in a closed-loop, open-loop, or combination system. The most common intermediate fluid vaporizers use propane, refrigerant, or a water/glycol mixture as an intermediate fluid. Although propane and refrigerant have low flash points that are ideal for heat transfer, the operational risks are much higher when handling these types of fluids, and these fluids are very costly. The water/glycol mixture has a high flash point, requiring a larger heat transfer area, which results in a larger system than the propane or refrigerant systems. However, the water/glycol fluid system is more cost effective and the associated operational risks are relatively low.

Figure 6.4Intermediate Fluid Vaporizer

An IFV typically uses a “shell and tube” heat exchanger, where LNG flows through the tubes with the intermediate heating medium circulating inside the shell and around the tubes. There are two stages to heating the LNG with an intermediate fluid vaporizer. First the liquefied natural gas is heated by an intermediate fluid in a heat exchanger, in which the LNG becomes a gas. The intermediate fluid flows through tubes in separate heating equipment (such as a propulsion boiler) to absorb heat. Then the vaporized natural gas is circulated through a second shell and tube heat exchanger, with seawater as the heating medium used to bring the gas to the temperature required to send it out through pipelines. The open-loop IFV technology requires seawater intake. Therefore, environmental issues include adverse effects (and assumed mortality) to marine life from entrainment in the intake, as well as exposure to seawater discharged back into the ocean at a temperature lower than the surrounding water. If an

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intermediate fluid vaporizer system operates with propane or refrigerant as the intermediate fluid, then these fluids add a potentially hazardous material to the facility operations. An IFV system that uses the water/glycol mixture is considered a safer way to operate. Lastly, depending on the combustion process used to heat the intermediate fluid, air emissions are also an environmental concern, unless the system uses waste heat recovery.

6.3.1.2. Ambient Air Vaporizers, AAV

Ambient air vaporization (AAV) technology uses ambient air as the thermal energy source to vaporize the liquefied natural gas. The LNG is distributed through a series of surface heat exchangers where the air travels down and out the bottom of the vaporizer. The air flow is controlled on the outside of the exchanger through natural buoyancy of the cooled, dense air, or by installing forced-draft air fans. This process can be set up as either a direct heat or indirect heat system. AAV technology is best suited for areas with warmer ambient temperatures. In cooler climates, a supplemental heat system would be necessary to maintain effective use during colder weather conditions. Frost forming on the vaporizer is an issue because the LNG is vaporized directly against the air (direct heat system) and the water vapor in the air condenses and freezes. Frost build-up reduces performance and heat transfer. To maintain continual operation, additional units are typically installed to provide the required throughput. The ambient air vaporization system requires a significant amount of space to prevent ambient air recirculation and to maintain the vaporizer capacity.

There is no seawater intake associated with this system. However, cooling ambient, moist air (which condenses into fresh water) necessitates treatment to prevent bio fouling in the freshwater discharge piping. Discharging the treated freshwater back into the ocean could potentially have an adverse impact on the sea water. Also, depending on geographical locations (such as areas with high dew points) cooling the ambient air can generate a “fog bank.” This is essentially benign, but you must consider sitting issues. Since the AAV technology typically burns natural gas only for supplemental heating during colder months, air emissions overall are relatively low compared to the other vaporization technologies.

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Figure 6.5LNG Ambient Air vaporizer

6.3.1.3. Open Rack Vaporizers, ORV

Open rack vaporizers (ORV) use seawater as the thermal energy source in a direct heat system to vaporize the LNG (figure 51). To control algae growth within the system, sodium hypochlorite (chlorine) is injected on the intake side of the system. The treated seawater is then pumped to the top of the water box and travels down along the outer surface of the tube heat exchanger panels, while LNG flows upward through these tubes and under the open rack vaporizer and is discharged through the water outfall, while the vaporized natural gas is removed from the top header of the system. Because this technology relies on seawater as the primary heat source, it is only effective where seawater temperatures exceed approximately 63 degrees Fahrenheit. The ORV technology does not require combustion and this process poses no new ignition sources. There are several environmental issues, including seawater intake, seawater outfall, and air emissions. Open rack vaporizer technology requires large volumes of water, which could adversely affect marine life. Further, the cooled and treated seawater that is returned to the ocean could potentially affect marine life and water quality. Although the ORVs do not directly produce air pollution emissions, powering the seawater pumps does.

Figure 6.6LNG Open Rack vaporizer

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6.3.1.4. Shell and Tube Vaporizers, STV

Shell and tube vaporizers (STV) also use seawater as the thermal energy source (figure 6.7). In an open-loop STV system, LNG enters the bottom of the STV, which is mounted vertically to optimize vaporization efficiency. The liquefied natural gas passes through multiple tubes while seawater enters a shell surrounding the tubes. A closed-loop system uses an intermediate fluid (such as propane or a water/glycol mixture) to transfer heat. The intermediate fluid flows through tubes in separate heating equipment (such as a propulsion boiler) to absorb heat, then the fluid passes through the STV unit to re-gasify the LNG. Since there are two heat exchangers, this requires a large amount of space. The open-loop technology reduces air emissions, since there is no combustion. Further, these STVs are generally small. Conversely, since the open-loop system uses seawater as the thermal energy source, there are environmental issues similar to the ORV system.

Figure 6.7 Shells and Tube Vaporizer

6.3.1.5. Submerged Combustion Vaporizers, SCV

Submerged combustion vaporizers (SCV) do not use seawater for LNG vaporization (figure 6.8). Instead, the LNG is warmed by flowing through tube bundles that are submerged in a water bath, which is heated by natural gas combustion. The submerged combustion burner emits hot exhaust gas that directly heats the water bath by bubbling through the water to an exhaust stack. Since the thermal capacity of the water bath is high, it is possible to maintain a stable operation even for sudden start-ups/shutdowns and rapid load fluctuations. Thus, they provide great flexibility to quickly respond to changing demand requirements. Since the SCV has such a huge reserve heat bank, even when the

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combustion process fails, surges can be mitigated with the heat just from the water bath.

Figure 6.8 Submerged Combustion Vaporizer

During operation, SCVs consume anywhere from 1.5 to 2.0 percent of the LNG cargo to fuel the combustion burner, which is a significant operating cost. In addition, the bathwater becomes acidic as the combustion products are absorbed during the heating process. It’s necessary to add chemicals to the water bath, which results in excess combustion water that must be neutralized before being discharged. Lastly, the submerged combustion vaporizer system produces large quantities of air emissions from the flue gas. This can be reduced through exhaust gas control technology, but adds significant operating costs to the SCV system.

6.3.2 Waste Heat Recovery and Engine Cooling Technology

Deepwater ports can use re-gasification vessels (which are equipped with re vaporization systems onboard) to vaporize the LNG to natural gas. Waste heat recovery and engine cooling technologies have been incorporated as part of the re-vaporization system to improve the efficiency (and reduce the emissions) of these re-gasification vessels. Additionally, using engine cooling technology reduces the amount of seawater intake because, instead of cooling the engines solely with seawater, cooled water from the LNG vaporization process is used to cool the engines. Additionally, any cooling systems can be tied into the intermediate fluid, such as the heating, ventilating, and air conditioning systems.

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6.3.3. Selecting vaporization Method

The following table 11 illustrates the factors which are used in selecting the LNG vaporization method.

Table 1.2 Comparison of vaporization methods

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