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Chemical Process Technology

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Saturday, December 10, 2011

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 :


ComponentEstimated
Solubility
Concentration
(ppmV)
Practical
Acceptable
Concentration
(ppmV)
Remark
n-C58,9001,000
neo-C5-5
C6217150
C77050
C80.50.5
C90.10.1
C100.0000010
Cyclo-C6115100
M-Cyclo-C5575550
M-Cyclo-C6335300
Benzene1.530.5
Toluene / M-Benzene24.920
O-Xylene0.220.1
M-Xylene1.541.0
P-Xylene120100
CO24050 - 100Practical experience shown higher CO2 concentration is acceptable / practical.
H2O0.00011.0Practical 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 :

ComponentPractical
Acceptable
Concentration
(ppmV)
Remark
H2S4                                                
COS0.5
Total Sulfur      10 - 200Subject to country
Merceptants Sulfur   6 - 15Subject 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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posted by Webworm, 10:17 AM

1 Comments:

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