LNG Book

Solvent absorption process

  • In solvent absorption, the two major cost factors are the solvent circulation rate, which affects both equipment size and operating costs, and the energy requirement for regenerating the solvent.
  • Table 5.3 summarizes some of the advantages and disadvantages of chemical and physical solvents.

[table id=30 /]

Amines

  • Amines are compounds formed from ammonia (NH3) by replacing one or more of the hydrogen atoms with another hydrocarbon group.
  • Replacement of a single hydrogen produces a primary amine, replacement of two hydrogen atoms produces a secondary amine, and replacement of all three of the hydrogen atoms produces a tertiary amine. Primary amines are the most reactive, followed by the secondary and tertiary amines.
  • The amines are used in water solutions in concentrations ranging fromapproximately 10 to 65 wt% amines.
  • Amines remove H2S and CO2 in a two step process:

1.The gas dissolves in the liquid (physical absorption).

2.The dissolved gas, which is a weak acid, reacts with the weakly basic amines

  • Absorption from the gas phase is governed by the partial pressure of the H2S and CO2 in the gas, whereas the reactions in the liquid phase are controlled by the reactivity of the dissolved species.

When a gas stream that contains the H2S, CO2, or both, is contacted by a primary or secondary amine solution, the acid gases react to form a soluble acid-base complex, a salt, in the treating solution.

  • The reaction between the amine and both H2S and CO2 is highly exothermic.
  • The fact that CO2 absorption occurs by two reaction mechanisms with different kinetic characteristics, significantly affects the relative absorption rates of H2S and CO2 among the different alkanolamines
  • The primary and secondary amines achieve essentially complete removal of H2S and CO2. Tertiary amines must react with CO2 via the slow hydrolysis mechanism.

Process flow diagram

Amine reclaiming

  • Amines react with CO2 and contaminants, including oxygen, to form organic acids.These acids then react with the basic amine to form heat stable salts (HSS). As theirname implies, these salts are heat stable, accumulate in the amine solution, and mustbe removed.
  • For MEA and DGA solutions, the salts are removed through the use of a reclaimer,which utilizes a semi-continuous distillation process.
  • The reclaimer is filled with lean amine, anda strong base, such as sodium carbonate or sodium hydroxide, is added to the solutionto neutralize the heat stable salts. A slipstream of 1 to 3% of the circulating amineis then continuously added to the reclaimer while the mixture is heated.
  • Water and amine vapor are taken off the top, which leaves the contaminants in the liquidn bottoms.
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