LNG Book

Physical absorption

  • In the amine and alkali salt processes, the acid gases are removed in two steps:physical absorption followed by chemical reaction.
  • In processes such as Selexol or Rectisol, no chemical reaction occurs and acid gas removal depends entirely on physical absorption.
  • Advantages and disadvantages :

: Absorption processes are generally most efficient when the partial pressures of the acid gases are relatively high.

: Solvents can be chosen for selective removal of sulfur compounds, which allows CO2 to be slipped into the residue gas stream and reduce separation costs.

: Energy requirements for regeneration of the solvent are lower than in systems that involve chemical reactions.

Solvent properties

  • Selexol is a polyethylene glycol and has the general formula:

Process flow diagram

  • The plant is designed to process 26 MMscfd (0.74MMSm3/d) entering the Selexol unit at 603 psia (41.6 bar) and 32°F (0°C).
  • The lean solvent, cooled to 25°F (−3.9°C) with propane refrigerant, enters the absorber where it absorbs CO2 and some of the ethane and heavier hydrocarbons.
  • The rich solvent from the absorber is regenerated by reduction of the pressure in three flash drums, from 603 to 106 psia (41.6 to 7.3 bar) in the high-pressure drum, from 106to 16 psia (7.3 to 1.1 bar) in the medium-pressure drum, and from 16 to 3 psia (1.1to 0.21 bar) in the vacuum drum.
  • Lean Selexol from the vacuum drum is recompressed and sent to the propane chiller. The treated gas that leaves the absorber passes through a knockout drum and filter separator to remove entrained Selexol and condensed hydrocarbons.

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  • The plant pretreats the gas to reduce CO2, ethane, and heavier hydrocarbon levels before final purification in molecular sieve units and subsequent liquefaction.

Adsorption

  • Acid gases, as well as water, can be effectively removed by physical adsorption on synthetic zeolites. Applications are limited because water displaces acid gases on the adsorbent bed.
  • Molecular sieve can reduce H2S levels to the 0.25 gr/100 scf (6 mg/m3)specification. However, this reduction requires regeneration of the bed at 600°F(315°C) for extended time with the potential for COS formation if 4A molecular sieve is used
  • Chi and Lee (1973) studied the coadsorption of H2S, CO2, and H2O on a 5A molecular sieve from a natural gas mixture under a variety of conditions.
  • y is the concentration in the exit stream and yo is the concentration in the inlet to the bed. The gas that entered the bed was saturated with H2O and contained both CO2 (1.14 mol%) and H2S (0.073 mol%.).
  • The bed quickly saturated with CO2, and its breakthrough was almost instantaneous.
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