Chemical & Process Technology: Contaminants and Impurities Limit in LNG

Saturday, December 10, 2011

Contaminants and Impurities Limit in LNG

Liquefied Natural gas (LNG) contains of majority Methane (CH4) which is more than 90% and other light hydrocarbon such as Ethane (C2H6), Propane (C3H8), Butane (C4H10) and Nitrogen (N2) inert gas. LNG is non-corrosive, non-toxic, non-carcinogenic, odorless and colorless. However, it is flammable and explosive and create greenhouse effect to environment.

Natural gas may contains of other contaminants such as Carbon Dioxide (CO2), Hydrogen Sulfide (H2S), Water (H2O), Mercury (Hg), Nitrogen (N2), Helium (He), Heavy Hydrocarbon, BTEX, CO2, S, etc. These component need to be extracted from natural gas to an acceptable, practical and economic level when it is liquefied and transported. The following are general reasons why these contaminants need to be extracted :

solidify / freezing results cryogenic heat exchanger blockage (contaminants such as CO2, H2O, C5+, BTEX, etc)
corrosion & cracking of cryogenic heat exchanger made of aluminum material (contaminant such as Hg)
affecting product heating value (component such as N2, C2, C3, C4, etc)
extraction highly valuable / important component (such as N2, He, etc)
product toxicity (such as H2S, mercaptans i.e. methanethiol (CH3SH) and ethanethiol (C2H5SH), etc)
stratification results roller (such as N2)

Freeze & Blockage
Component in natural gas potentially freeze and blocking cryogenic heat exchanger need to be reduced to level at or below it freezing point. The following table list out the solubility of several components in LNG :

Component	Estimated Solubility Concentration (ppmV)	Practical Acceptable Concentration (ppmV)	Remark
n-C5	8,900	1,000
neo-C5	-	5
C6	217	150
C7	70	50
C8	0.5	0.5
C9	0.1	0.1
C10	0.000001	0
Cyclo-C6	115	100
M-Cyclo-C5	575	550
M-Cyclo-C6	335	300
Benzene	1.53	0.5
Toluene / M-Benzene	24.9	20
O-Xylene	0.22	0.1
M-Xylene	1.54	1.0
P-Xylene	120	100
CO2	40	50 - 100	Practical experience shown higher CO2 concentration is acceptable / practical.
H2O	0.0001	1.0	Practical experience shown higher H2O concentration is acceptable / practical.

Corrosion & Cracking

Mercury (Hg) present in natural gas can cause corrosion & cracking of cryogenic heat exchanger made of aluminum material. Typically Hg concentration shall be limited to 0.01 microgram / Nm3.

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Stratification & Rollover

At LNG storage condition, Nitrogen will have lower dew point compare to Methane. Nitrogen in LNG tends to vaporize first compare to Methane. As Nitrogen is heavier than Methane, top layer of LNG tank would tends to rollover and generate excessive vapor which can cause overpressure and relieve BOG to atmosphere. Through experience, Nitrogen content in LNG is normally limited to maximum 1 mol%.

Toxicity
Present of H2S, COS and merceptants in LNG would results high toxicity LNG. The following list out typical level in LNG :

Component	Practical Acceptable Concentration (ppmV)	Remark
H2S	4
COS	0.5
Total Sulfur	10 - 200	Subject to country
Merceptants Sulfur	6 - 15	Subject to country

Above are typical value for quick reference. Figures may change according to LNG composition, liquefaction temperature, etc and will varies from case to case. Shall be used as reference only.

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Labels: LNG

posted by Webworm, 10:17 AM

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