Why you need to dry natural gas for fueling applications

Saturday , 26 September, 2009

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ISO 15403:2000(E), Paragraph 5.1

“The single most important safety requirement of compressed natural gas (CNG) fuel is very low water dew-point temperature to preclude the formation of liquid water at any time. Liquid water is the precursor to the formation of corrosive compounds through combination with components in natural gas, namely carbon dioxide and hydrogen sulfide. The combination of corrosive agents, and the pressure cycling, caused by fuel consumption and subsequent refilling of the fuel storage container, can result in crack growth in metals and ultimately damage and failure. Also, liquid water itself can be detrimental as it may cause blockages, both liquid and solid, in the fuel system.

Thus, the water dew-point of the fuel gas at the fuelling station outlet shall be sufficiently below the lowest ambient temperature in which the fuelling station and vehicles will operate.”

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ISO 11439:2000(E), Paragraph 4.5-Gas Composition

4.5.1 General: Cylinders shall be designed to tolerate being filled with natural gas meeting the specification either of dry gas or wet gas as follows. Methanol and/or glycol shall not be deliberately added to the natural gas.

4.5.2 Dry gasWater vapour shall be limited to less than 32 mg/m3 (i.e. a pressure dewpoint of - 9 °C at 200 bar)...

4.5.3 Wet gasThis is gas that has a higher water content than that of dry gas…

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• Water does not cause a problem as long as it stays in vapor form…

…it is when it condenses to form liquid water (increase in pressure, decrease temp.)

that problems start...

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The tip of the Iceberg,

the “Joule-Thomson” effect...

• Freeze-ups at dispenser nozzles

• Freeze-ups in the vehicles fuel injectors

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Relation between pressure and volume Boyle & Marriott Principle

1 F3 of gas1 Atm. = 14.7 PSIA 0 Bar(g)7 Lbs. H2O/MMSCF 112 mg/m3

1 F3 of gas245 Atm. = 3600 PSIG 248 Bar (g)1715 Lbs. H2O/MMSCF 27,470 mg/m3

And what about gas temperature??...

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This is where Joule-Thompson effect kicks into place.

1 F3 of gas1 Atm. = 14.7 PSIA7 Lbs. H2O/MMSCF

1 F3 of gas245 Atm. = 3600 PSIG1715 Lbs. H2O/MMSCF

Compressing gases:-Water per F3 increases-Temperature increases

Expanding gases:-Temperature decreases-Moisture reaches dewpoint and turns into liquid!

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Formation of hydrates

• Hydrates are a solid phase complex of water and light hydrocarbons whose formation is dependent on gas composition, pressure and temperature.

• Hydrates form when enough water vapor is present in the gas.

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Effect without dehydration system...

• Gas enters compressor, gets compressed (temperature and moisture concentration increases),

• Gas enters storages and expands (temperature decreases, moisture turns into liquid…too late)

• HP gas containing moisture is traveling to the dispenser, enters vehicle cylinder and expands once again…WATER!

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The different methods available to remove

moisture from natural gas

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• Pre-compression or post compression

– Pre-compression protects the compressor!– Media not affected by oil carryover from

compressor discharge– Media not affected by high compressor

discharge temperature– No recuperation of purge gases required

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Desiccant type dehydrators

• Various configurations available to meet operating and budget requirements

• Single tower system (need to replace desiccant)

• Single tower system with manual regeneration

• Twin tower system with manual regeneration

• Twin tower system with fully automatic regeneration

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Thank you for your time!

QUESTIONS?

Guy Couturier

Senior Applications Specialist

Xebec Adsorption Inc.

Booth: B518