LNG Book

Acid gas concentrations in natural gas

  • Although many natural gases are free of objectionable amounts of H2S and CO2, substantial quantities of these impurities are found in both gas reserves and production in the United States.
  • In a survey of U.S. gas resources, Meyer(2000) defined subquality gas as that containing CO2≥2%, N2≥4%, or H2S≥4 ppmv.
  • These criteria were selected because gases that contain these amounts of impurities generally require upgrading or blending. Using these criteria, Meyer (2000) estimated that 41% of proven gas reserves are subquality and 34% of 1996 gas production in the lower 48 states was subquality.
  • It is obvious that removal of H2S and CO2is a major concern in gas processing.

Purification levels

  • The inlet conditions at a gas processing plant are generally temperatures near ambient and pressures in the range of 300 to 1,000 psi (20 to 70 bar), so the partial pressures of the entering acid gases can be quite high.
  • If the gas is to be purified to a level suitable for transportation in a pipeline and used as residential or industrial fuel, then the H2S concentration must be reduced to 0.25 gr/100 scf (6 mg/m3), and the CO2 concentration must be reduced to a maximum of 3 to 4 mol%.
  • However, if the gas is to be processed for NGL recovery or nitrogen rejection in a cryogenic turbo expander process, CO2 may have to be removed to prevent formation of solids.
  • If the gas is being fed to an LNG liquefaction facility, then the maximum CO2 level is about 50 ppmv (Klinkenbijl et al., 1999) because of potential solids formation.

Purification processes

  • Four scenarios are possible for acid gas removal from natural gas:

1.CO2 removal from a gas that contains no H2S

2.H2S removal from a gas that contains no CO2

3.Simultaneous removal of both CO2 and H2S

4.Selective removal of H2S from a gas that contains both CO2 and H2S

  • Because the concentrations of CO2 and H2S in the raw gas to be processed and the allowable acid gas levels in the final product vary substantially, no single process is markedly superior in all circumstances, and, consequently, many processes are presently in use.

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