Gas treatment

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Content – Energy distribution

 


 
In a gas treatment plant the raw natural gas is processed to meet the trading specifications of pipeline and gas distribution companies. As a part of the purification process various components are produced such as NGLs (prophane, ethane, butane etc.).

 The main elemnts of the purification process are:

Acid gas removal – Acid gases to be removed are typically; carbon dioxide (CO2) and hydrogen sulphide (H2S). Methods used to remove the gases are:

  • Adsorption where the gas is led through a solvent (typically amin) that dissolves the sour gases and leaves the sweet gas (the product). The rich solvent is thereafter regenerated such that the gases may be realeased and disposed.
  • Cryogenic removal uses a turbo expander, which allows gases to be removed when the gas expands and cools below the dew point for the gas to be removed. Cryogenic removal is most often used when the content of carbon dioxide is high, typically around 50%.
  • Membrane based removal is based on membranes that allow the acid gases to pass through but not the hydrocarbons.
  • Sulfur removal Unit. The “Claus process” is used to recover elental sulphur. This is a two stage process where H2S combust with air or oxygen and releases SO2 and elemental sulphur during cooling. During the second stage H2S reacts with SO2 and a catalyst such as aluminium oxide or titanium oxide to produce water and more elemental sulphur. The Claus process typically recover 95 – 97% of the sulphur.
  • Tail gas treatment unit – To recover the rest, 99,9% (250 ppm) of the sulphur a tail gas treatment unit is used. The sulfur compounds that are not recovered in the Claus process consist of unreacted H2S and SO2, uncondensed sulfur vapor, and small amounts of carbonyl sulfide (COS), and carbon disulfide (CS2). Many facilities now utilize a catalytic hydrogenation/hydrolysis reactor to convert sulfur compounds in the tail gas to H2S, followed by a selective H2S recovery unit that recycles most of the hydrogen sulfide to the front of the Claus process unit. This process utilizes a reducing gas generator where fuel gas is burned substoichiometrically to produce hydrogen (H2) and carbon monoxide (CO) for the hydrogenation reactions as well as heat. The tail gas is mixed with the combustion gases in the reducing gas generator and then is fed to the hydrogenation/hydrolysis reactor. In the reactor almost all of the sulfur compounds are catalyctically converted to H2

Dehydration – Dehydration is either by glycol-based scrubbers or based on pressure swing adsorption (PSA). Membranes are also used.

Mercury removal – Mercury removal is based on molecular sieves, a substance with a large surface area due to many small pores, such as activated carbon. The surface of the material allows certain molecules to bind by surface tension. The molecules can later be extracted and the sieve material regenerated by heating, pressure and/or purging with a carrier gas.

Nitrogen rejection – Nitrogen is removed by cryogenic distillation or by absorption with lean oil or another special solvent. Cryogenic removal permits production of helium, if present, as a valuable byproduct.

NGL recovery and treatment – Any remaining NGLs are recovered from the gas stream in most modern plants by a cryogenic turbo expander-based process followed by a fractionating process. This process leads the cooled NGLs though distillation columns called de-ethanizer, de-propanizer and de-butanizer, to extract ethane, propane and butane respectively and leave a residual stream of pentane and higher hydrocarbons.

The final step is to remove mercaptans (smelly organic gases, e.g., CH3SH) if present, in a sweetening process based on molecular sieves adsorption or catalytic oxidization such as Merox mercaptan oxidization or Sulfrex, where the main difference is the type of catalyst.

Distribution – Distribution of the treated gas from the treatment plant may by pipeline, ship, train and truck.

LNG (Liquified natural gas)
LNG is a product made to facilitate storage and transportation of primarily methane. Opposite to CNG (Compressed natural gas), the methane is liqified by reducucing the temperature to around – 162oC. The liqifaction is done through pateneted processes normally consisting of 1 -3 stages. The enrgy requirement to liquefy the gas in a modern efficient plant is around 6% of the total energy content of the gas.

Distribution – Distribution of LNG is by ship or by truck. Storage at the terminals and transportation is done in cryogenic tanks at atmospheric pressure or slightly above. The tanks are insulated, but will not keep LNG cold enough to avoid evaporation. Heat leakage will heat and boil off the LNG. Therefore LNG is stored as a boiling cryogen, which means that the liquid is stored at its boiling point for its storage pressure, around -162oC. As the vapor boils off, heat of vaporization is absorbed from and cools the remaining liquid. With efficient insulation, only a relatively small amount of boil-off is necessary to maintain temperature. Boil- off gas from land-based LNG storage tanks is compressed and fed to natural gas pipeline networks. On LNG carriers, the boil-off gas can be used for fuel. LNG is normally regasified to ambient temperature on a demand basis and then injected into the gas pipeline system.