Hydrocarbon and Fixed Gas Solubility in Amine Treating Solvents: A Generalized Model

Hydrocarbons are sparingly soluble in aqueous solvents and to a first approximation their solubility can be described by the traditional Henry’s Law, which linearly relates the mole fraction in water to gas-phase partial pressure. The presence of amines in the aqueous phase increases hydrocarbon solubility over that in water, an effect referred to as “salting-in”. Commercial amine treating solutions can contain as much as 25 mol% amine, so it is important to take account of salting-in. The presence of ionic species, on the other hand, has the opposite effect and hydrocarbon solubility is reduced—this is termed “salting-out”. The treating solution may be highly loaded with acid gases and therefore it may have high concentrations of numerous ions. The result is significantly reduced hydrocarbon solubility. The once simple ideal-gas, ideal-solution Henry’s Law is further complicated by the non-ideal behavior of gases at the high pressures common in gas treating. This paper provides a generalized approach to representing the equilibrium solubility of hydrocarbons in acid-gas-loaded, high-strength, commercial, amine treating solvents. The methodology is valid for system pressures up to values where the hydrocarbon solubility is so high it begins to compete with the acid gases for a place in the solution and, therefore, it starts to displace acid gas from the solvent. The validity of the generalized model is shown by excellent agreement with GPA, GTI and other data for all generic amines in commercial use over a wide range in concentration and acid-gas loadings. The model correlates all the available data and provides the basis for very reliable hydrocarbon solubility predictions. Finally, using the model in an LNG sweetening application, the implications of salting in and salting out effects are demonstrated in the context of transient operations and foaming in commercial plants. Hydrocarbon Solubility in Amine Treating Solvents: A Generalized Model. Laurence Reid Gas Conditioning Conference, Norman, Oklahoma, February, 2013