SUPPORTING FUNCTIONAL UNITS IN LNG

PLANTS

LNG Production Plant

• The process and utility requirement design depends on site conditions, compositions and products specification.

• The natural gas entering a liquefaction facility will often contains several contaminants that must be first reduced to acceptable levels to ensure satisfactory liquefaction plant performance or to meet LNG sales specifications or both.

Composition Natural gas is a complex mixture of hydrocarbon and non-hydrocarbon constituents and exists as a gas under atmospheric conditions.

Raw natural gas typically consists primarily of methane (CH4), the shortest and lightest hydrocarbon molecule. It also contains varying amounts of:

Heavier gaseous hydrocarbons: ethane (C2H6), propane (C3H8), normal butane (n-C4H10), iso-butane (i-C4H10), pentanes and even higher molecular weight hydrocarbons. When processed and purified into finished by-products, all of these are collectively referred to NGL (Natural Gas Liquids).

Acid gases: carbon dioxide (CO2), hydrogen sulfide (H2S) and mercaptans such as methanethiol (CH3SH) and ethanethiol (C2H5SH).

Other gases: nitrogen (N2) and helium (He).

Water: water vapor and liquid water.

Liquid hydrocarbons: perhaps some natural gas condensate (also referred to as casing-head gasoline or natural gasoline) and/or crude oil.

Mercury: very small amounts of mercury primarily in elementary form, but chlorides and other species are possibly present.

Typical LNG plant specifications are:

• Hydrogen Sulphide ex acid gas treating unit < 3.5 ppmv

• Carbon Dioxide ex acid gas treating unit < 50 ppmv

• Total Sulphur (Hydrogen Sulphide +Carbonyl Sulphide + Organic Sulphur Compounds ex acid gas treating unit) < 20 mg /Nm3

• Total sulphur in fuel gas < 300 ppmv (depending use)

• SO2 emission ex incinerator < 250 mg /Nm3

• Sulphur purity > 99.9 %wt Sulphur recovery > 95 - 99.9 %

• Water in LNG ex driers < 0.5 ppmv

• Mercury in LNG < 0.01 mmg/Nm3

• Effect of Impurities Found in Natural Gas Field processing operations of natural gas, which is classified as a part of gas engineering, generally include the following:

1. Removal of water vapor, dehydration

2. Removal of acidic gases (H2S and CO2)

3. Separation of heavy hydrocarbons

Overall Process Flow Scheme

INLETFACILITY

OFFSHORE

PRODUCTIONACID GASREMOVAL

DEHYDRATIONMERCAPTANREMOVAL

NGLRECOVERY

LIQUEFACTION

REFRIGERATION(C3, MR, N2)

07

FRACTIONATIONC3, C4 TREATING

NITROGENREJECTION

SULFURRECOVERY

LNG

PROPANE

BUTANE

PLANTCONDENSATE

SULFUR

FIELDCONDENSATE

CONDENSATETREATING

TRAIN 4

TO TRAIN 5

ChilledSelexol