7/27/2019 FINAL-LNG 24.03.13

1/13

LIQUIFIED NATURAL GAS

Cooling natural gas to about -260F at normal pressure results in the condensation of the gas into

liquid form, known as Liquefied Natural Gas (LNG). LNG can be very useful, particularly for the

transportation of natural gas, since LNG takes up about one six hundredth the volume of gaseous

natural gas. While LNG is reasonably costly to produce, advances in technology are reducing the

costs associated with the liquefaction and regasifications of LNG. Because it is easy to transport,LNG can serve to make economical those stranded natural gas deposits for which the construction

of pipelines is uneconomical.

LNG, when vaporized to gaseous form, will only burn in concentrations of between 5 and 15

percent mixed with air. In addition, LNG, or any vapor associated with LNG, will not explode in an

unconfined environment. Thus, in the unlikely event of an LNG spill, the natural gas has little

chance of igniting an explosion.

Liquefaction also has the advantage of removing oxygen, carbon dioxide, sulfur, and water from the

natural gas, resulting in LNG that is almost pure methane.

LNG is typically transported by specialized tanker with insulated walls, and is kept in liquid formby auto refrigeration, a process in which the LNG is kept at its boiling point, so that any heat

additions are countered by the energy lost from LNG vapor that is vented out of storage and used to

power the vessel.

LNG takes up much less space than gaseous natural gas, allowing it to be shipped much more

efficiently.

Introduction

It is simply an alternative method to transport CH4 from producer to consumer. LNG is a

safe fuel.

CH4 is cooled to -2600F to convert to liquid having volume 600 times less than the gas. It isodorless and colorless.

It is stored and moved at cold T and low P.

Transported by ship to long distances, off-loaded, re-gasified by evaporation, and

transported by NG pipelines to consumers.

LNG chain consists of discrete sections upstream (exploration), mid-stream

(liquefaction, shipping, and re-gasification), and downstream (distribution and use).

Each of the above, being heavily capital intensive, is not owned by a single party. Therefore,

complex agreements are necessary.

LNG systems

When stored in storage tanks, LNG cannot explode or burn because no oxygen is present.

When LNG is released into the open air and turns into a gas, it can burn because of the

oxygen present in the air. Because of the evaporation and dispersion properties mentioned

earlier, the gas is only flammable within certain limits. It is flammable only within a limited

range of mixing ratios. It is flammable only when the mixing ratio of the gas to air is

between 5% and 15%. At ratios below 5% and above 15% the mixture is therefore not

flammable.

7/27/2019 FINAL-LNG 24.03.13

2/13

LNG Tankers are equipped with a full range of safety equipment such as state-of-the-art

radar and positioning systems that continuously keep the crew informed of other shipping

movements and potential hazards on board. A number of emergency systems and beacons

will automatically transmit signals should the ship experience difficulties. One of the safety

features of the loading system is a very comprehensive suite of instruments that

automatically shuts down the system as soon as certain preset parameters are exceeded. The

ships are also fitted with gas and fire detection systems.

When stored in storage tanks, LNG cannot explode or burn because no oxygen is present.

When LNG is released into the open air and turns into a gas, it can burn because of the

oxygen present in the air. Because of the evaporation and dispersion properties mentioned

earlier, the gas is only flammable within certain limits. It is flammable only within a limited

range of mixing ratios. It is flammable only when the mixing ratio of the gas to air is

between 5% and 15%. At ratios below 5% and above 15% the mixture is therefore not

flammable.

LNG Tankers are equipped with a full range of safety equipment such as state-of-the-art

radar and positioning systems that continuously keep the crew informed of other shipping

movements and potential hazards on board. A number of emergency systems and beacons

will automatically transmit signals should the ship experience difficulties. One of the safety

features of the loading system is a very comprehensive suite of instruments that

automatically shuts down the system as soon as certain preset parameters are exceeded. The

ships are also fitted with gas and fire detection systems.

Commercial viability of LNG by shipping versus pipeline transport

The gas market is more than 2000 km from the field.

The gas fields contain at least 3 to 5 tcf of recoverable gas.

Gas production costs are less than $1/MMBtu, delivered to the liquefaction plant.

The gas contains minimal other impurities (CO2, S etc.)

A marine port where a liquefaction plant could be built is relatively close to the field.

The political situation in the country supports large scale long-term investments.

The market price in the importing country is sufficiently high to support the entire chain

and provide a competitive return to the gas exporting company and the host country.

A pipeline alternative would require crossing uninvolved third-party countries and thus the

buyer is concerned about security of supply.

Heavier HCs, LPG, and condensate are removed and sold by upstream resource holders.LNG is sold at long-term contracted prices and the others at the market prices.

Units used in LNG trade

Produced gas is measured in volume cubic meters or cubic feet.

LNG is measured in mass tons or million tons. MMT or MT.

7/27/2019 FINAL-LNG 24.03.13

3/13

LNG ship sizes are specified in cargo volume, typically thousands of cubic meters.

Re-gasified product is sold in energy units MMBtu.

1 ton LNG 48,700 ft3 NG

1 MM cft LNG 1,000,000 ft3 NG

1,000 ft3 NG 1,000 Btu

NG liquefaction

Gas received at the plant must be as close to CH4 as possible.

HCs higher than CH4 are removed by cooling to -300C and sold directly to the market or

used as refrigerant in the liquefaction cycle. Remaining gas, pure CH4, is then fed to liquefaction plant which either follows the pure

refrigerant cascade process (Phillips process) or the pre-cooled propane mixed refrigerant

(Multi-Component Refrigerant) process,promoted by Air Products, Shell.

The first LNG plants in Algeria and Alaska were based on the Phillips process using

propane, ethylene, and methane as refrigerants. Many of the later plants were developed on

the MCR process.

The evaluation of NG liquefaction processes is done on thermal efficiency. Although, both

processes do not show a major difference in efficiency values (~92%) of their main

processors, the choice is mainly governed by license fees, perceived advantages, and the

companys choice.

CASCADE PROCESS

This process uses three refrigeration circuits propane, ethylene, and methane to cool the

purified gas to the required temperature of -161.50C.

In the first circuit, compressed propane cools feed gas to the liquid temperature of propane,

-300C through a series of evaporators.

The second circuit repeats the process with ethylene, cooling the gas to the liquid

temperature of ethane, -1000C.

The third circuit drops the temperature to that of liquid methane, -1600

C. Boil-off from the final tanks is further compressed and injected back into the circuit to

increase the efficiency.

In this connection cascade system of gas cooling is worthy to be mentioned. Propane being

produced in refinery is cheap and acts as a good refrigerant and the starting material for

this system. As much low temperature as -1600C is achieved by cascade cooling. The

principle of which is: Ethylene at 19atm pressure is liquefied by propane evaporation at

7/27/2019 FINAL-LNG 24.03.13

4/13

400C. Propane is liquefied at 18atm pressure by water cooling at 50 0C. Ethylene at -1000C

can condense methane at 35atm.

Separation of ethane and ethylene mixture is done by distillation at low temperature.

Ethane-ethylene mixture is distilled at 21atm pressure in a 60 plate column, where liquid

propane acts as coolant in condenser. The temperature of the reboiler is kept around -70C,

in these columns.

7/27/2019 FINAL-LNG 24.03.13

5/13

Nine-stage Cascade Liquefaction Process

7/27/2019 FINAL-LNG 24.03.13

6/13

Improved cascade cooling to liquefy methane, has been the main feature of cryogenics in the industry.

Turbo expander technology, the new comer in this field has been acclaimed and has gained momentum.

Turbo expander liquefaction:

The underlying principle in this design is quite simple: the gas is compressed in two or three stages and

expanded in turbines; the cooled gas is permitted to exchange heat with incoming gas. Thus the inability

prevailing in Joule-Thomson cycles to recover and utilize mechanical energy has been successfully

overcome in turbo expanders. In such expander, as condensation of liquids should not take place in any

engine, the liquids are taken from heat exchangers and further fractionated.

Purification of natural gas is essential before such cycle is attempted.

Absorber gases can be directly fed into fuel system, without much purification. Removal of acid gases

and sulfur compounds will suffice if desirable methane, ethane, ethylene fractions can be separated by

low temperature distillation or by hyper sorption. Low temperature fractionation depends upon the

efficient production of such low temperatures.

7/27/2019 FINAL-LNG 24.03.13

7/13

Multi Component Refrigeration Process

(Also Called Mixed Component Refrigeration Process)

After initial developments of cascade LNG plants, the mixed refrigerant cycle was

developed to simplify the refrigeration system. This system uses a single mixed refrigerant

composed of nitrogen, methane, ethane, propane, butane and pentane. The refrigerant is

designed so that the refrigerant boiling curve nearly matches the cooling curve of the gas

being liquefied. The closeness of the match of these two curves is a direct measure of the

efficiency of the process.

To increase process efficiency, the system first pre cools the gas to -30 0C by propane in a

process similar to the first circuit of the cascade process. The next cycle uses a combination of CH4 + C2H6 + C3H8 + N2 refrigerants to bring the

temperature down to -161.50.C in a series of stages.

Liquefaction is achieved when CH4 gas is allowed to come in contact with cold metal spiral

tube HE containing liquefied refrigerant mixture that cools the gas.

The process has two major components:

1. The refrigeration system and

2. The main exchanger cold box.

The cold box is a series of aluminum plate fin exchangers which provide very close

temperature approaches between the respective process streams. The low pressure

refrigerant is compressed and condensed against air or water in a closed system. The

refrigerant is not totally condensed before being sent to the cold box. The high pressure

vapor and liquid refrigerant streams are combined and condensed in the main exchanger.

The condensed stream is flashed across a J-T valve and this low pressure refrigerant

provides the refrigeration for both the feed gas and the high pressure refrigerant. Removal

of pentane and heavier hydrocarbons from the feed gas is accomplished by bringing the

partially condensed gas out of the cold box and separating the liquid at an intermediate

temperature. The liquid removed is then further processed to produce a specification C5+

product. Light products from this separation are returned to the liquefaction system.

Mixed Refrigerant Liquefaction Process

7/27/2019 FINAL-LNG 24.03.13

8/13

7/27/2019 FINAL-LNG 24.03.13

9/13

PreCooled Mixed Refrigerant Process

The propane PreCooled mixed refrigerant process (Fig. 16-27) was developed from a combination of the

cascade and mixed refrigerant processes. In this process, the initial cooling of the feed gas is

accomplished by using a multistage propanerefrigeration system. The gas is cooled with this system to

around 40C at which point the gas is processed in a scrub column to remove the heavy hydrocarbons.

The gas is thencondensed in a two step mixed refrigerant process. The chillingof the gas is accomplished

in a single, large, spiral-wound heat exchanger. This exchanger allows extremely close temperatureapproaches between the refrigerant and the gas to be achieved.

The mixed refrigerant in this process is a lighter mixture composed of nitrogen, methane, ethane and

propane with a molecular mass around 25. The mixed refrigerant after recompression is partially cooledwith air or water and then further cooled in the propane refrigeration system. The partially condensed

refrigerant from the propane chilling is separated and the high pressure vapor and liquid streams sentseparately to the main exchanger. The liquid is flashed and provides the initial chilling of the gas. The

high pressure vapor is condensed in the main exchanger and provides the low level, final liquefaction of

the gas. As in the other processes, the LNG leaves the exchanger sub cooled and is flashed for fuel

recovery and pumped to storage.

7/27/2019 FINAL-LNG 24.03.13

10/13

FIG. 16-27

Propane PreCooled Mixed Refrigerant Process

LNG PROJECTS AND TRANSPORTATION

These are the most expensive plants because 8 to 10% of gas delivered to the plant is used tofuel the refrigeration process; overall operating costs are high, although maintenance and

labor costs are low.

The actual LNG production for sale often exceeds the designed capacities because ofdebottleneckingincreases efficiencies. The extra volumes can be sold as short term contacts

or spot sales.

LNG is usually transported by specially designed refrigerated ships which operate at lowerpressure, keeps the boil-off to a minimum, and employs suitable insulation.

The boil-off produced depends on thickness of insulation, surface area and the surface ofthe tank, ambient temperature, and distance to market.

On a typical voyage, 0.1 to 0.25% of the cargo converts to gaseous phase daily. In a typical20 day return voyage from the LNG plant to the customer, the total loss, net of voyage +

loading / unloading + boil-off, is 2 to 6% of the total volume.

Regular steel cannot be used to make ship tanks. A special alloy of steel with Ni and Al isused.

7/27/2019 FINAL-LNG 24.03.13

11/13

Only a small number of shipyards (mostly in Japan, France, and Korea) have the capabilityand experience to build LNG tankers with either spherical or rectangular membrane tanks,

using steel and PVC or polystyrene insulation.

LNG ship size is expressed in m3 of maximum LNG volume. A mid-size 138,000 m3 is able totransport 65,115 tons of LNG. Shipping costs are usually on daily charter rates. It takes

around 3 yrs to build an LNG tanker.

Most plants have their own dedicated fleet of LNG ships, operating a virtual pipeline. Asa ship is being loaded, a sister ship may be discharging its cargo, and the remaining

members of the fleet are either en route to the buyers re-gas facility or on the way back to

the LNG plant to pick up new cargo.

LNG regasifications

The main components of this train are offloading berths and port facilities, LNG storage

tanks, vaporizers to convert liquid to gas, and pipeline link to the local gas grid.

LNG storage tanks, vaporizers typically account for 25 to 35% of capital costs. Safety

features, unloading berths, and general construction absorb the remaining 40% costs.

Vaporizers warm LNG from -161.5 to 50C. These are relatively simple units in which LNG is

pumped through tubular or paneled HE, allowing the T to rise. Contact with seawater in warmer climates or heated water in colder climates keeps the HE

warm. Large volumes of seawater are kept flowing through the system to avoid ice buildup

on panels.

If water must be heated the cost increases as 1.5-3% of throughput energy goes to fuel the

water heater system. Warm water may be used from adjoining power stations.

Since LNG is immediately converted to CH4 and rises above the area, locating a regasing

facility is difficult.

Asia continues to lead in providing regasing facility. There are 2 in India.

LNG projects have traditionally been the domain large energy companies because of high

capital costs, long term time-horizon,, and often reliance on a single gas supplier and

discreet number of customers.

Process Cost range ($/ MMBtu)

Gas production (upstream) 0.5 0.75

Gas processing and liquefaction 1.3 1.8

Shipping (1000-8000 km) 0.4 1.0

Delivered LNG cost 2.20 3.55

LNG storage and regasification 1 1.5

Total LNG cost 3.2 5.05

LNG storage tanks

http://en.wikipedia.org/wiki/LNG_storage_tankhttp://en.wikipedia.org/wiki/LNG_storage_tank

7/27/2019 FINAL-LNG 24.03.13

12/13

Modern LNG storage tanks are typically full containment type, which has a prestressed concrete

outer wall and a high-nickel steel inner tank, with extremely efficient insulation between the walls.

Large tanks are low aspect ratio (height to width) and cylindrical in design with a domed steel or

concrete roof. Storage pressure in these tanks is very low, less than 10kPa (1.45 psig). Sometimes

more expensive underground tanks are used for storage. Smaller quantities (say 700 m (190,000

US gallons and less), may be stored in 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 reliquefied and returned to storage.

1. Single Containment Tank

Consists of a primary container and generally an outer shell. Only the primary container is

required to meet the low temperature ductility requirement for storage of the product. The outer

shell is primarily for retention and protection of the insulation, and for containing the gas pressure.

In the event of leakage from the primary container the outer tank is not designed to contain the

refrigerated liquid. An aboveground single containment tank is surrounded by a bund wall or dike

to contain any leakage.

2. Double Containment Tank

Both the inner self-supporting primary container and the secondary container are capable ofindependently containing the refrigerated liquid The secondary container is located at a distance of

6 meters or less, from the primary container The secondary container is intended to contain any

leakage of the refrigerated liquid from the primary container. However, it is not intended to contain

any vapor resulting from such a leakage.

3. Full Containment

Both the self-supporting primary container and the secondary container are capable of

independently containing the refrigerated liquid. The outer tank wall is 1 to 2 meters distant from

the inner tank. Normally the inner tank contains the refrigerated liquid. The outer tank is capable

both of containing the refrigerated liquid and of controlled venting of the vapor resulting from

product leakage after a credible event The roof is supported by the outer tank.

How pure is LNG?

http://en.wikipedia.org/wiki/LNG_storage_tankhttp://en.wikipedia.org/wiki/Prestressed_concretehttp://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Natural_gashttp://en.wikipedia.org/wiki/LNG_storage_tankhttp://en.wikipedia.org/wiki/Prestressed_concretehttp://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Natural_gas

7/27/2019 FINAL-LNG 24.03.13

13/13

LNG is a very pure form of Natural Gas and is not carcinogenic or toxic. For the Natural

Gas to be liquefied all impurities are removed such as:

Sulphur, carbon dioxide, and mercury which are corrosive to LNG equipment

Water, which could freeze and cause equipment blockage

Heavier hydrocarbons which could also freeze like water

The removal of these containment makes LNG, when re-gasified in an import terminal, avery clean and reliable natural gas source for cooling, heating and power.

As a fuel, LNG is much cleaner than gasoline or diesel, reducing particle emissions to near

zero and carbon dioxide (CO2) emissions by 70%. When burned for power generation, the

results are even more dramatic - sulphur dioxide (SO2) emissions are virtually eliminated

and CO2 emissions are reduced by 40%. LNG has the least environmental impact of all

fossil fuels.

What is the typical specification of Re-gasified LNG?

The Gross Heating Value in a gaseous state is in the range of 1050 BTUs per Standard

Cubic Foot (approximately equal to 9,340 kilo calories per Standard Cubic Meter) to 1170

BTUs per standard Cubic Foot (approximately equal to 10,420 kilo calories per Standard

Cubic Meter).

Component Specifications

Methane (C1) Not less than 85 Mol %

Ethane (C2) Not more than 9.2 Mol %

Propane (C3) Not more than 3.00 Mol %

Butanes (C4) and heavier Not more than 2.00 Mol %

Pentanes (C5) and heavier Not more than 0.25 Mol %

Nitrogen (N2) Not more than 1.25 Mol %

Oxygen (O2) Not more than 0.5 Mol %

Total Non Hydrocarbons Not more than 2.0 Mol %

Total Sulphur including H2S Not more than 10 ppm by weight expected H2S content not

more than 4ppm by volume.

Impurities Gas shall be reasonably free from dust (max size 5 microns), gum forming

constituents and other deleterious solid and/or liquid matter, which will cause damage to or

interfere with the Operations of Gas Transporters Facilities.

Water content not more than 112 Kg/MMSCM