Why you need to dry natural gas for fueling applications

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Why you need to dry natural gas for fueling
applications
Saturday , 26 September, 2009
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• ISO 154032000(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 114392000(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 gas
• Water 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 gas
• This 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 gas 1 Atm. 14.7 PSIA 0 Bar(g) 7 Lbs.
H2O/MMSCF 112 mg/m3
1 F3 of gas 245 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 gas 1 Atm. 14.7 PSIA 7 Lbs. H2O/MMSCF

• Compressing gases
• Water per F3 increases
• Temperature increases
• Expanding gases
• Temperature decreases
• Moisture reaches
• dewpoint and turns into
• liquid!

1 F3 of gas 245 Atm. 3600 PSIG 1715 Lbs.
H2O/MMSCF
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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 liquidtoo late)
• HP gas containing moisture is traveling to the
  dispenser, enters vehicle cylinder and expands
  once againWATER!

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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 B5