Thermodynamic aspects of the capture of acid gas from natural gas

Among fossil fuels, natural gas is the cleanest, in terms of CO2 emission, burn efficiency and amount of air pollutant. Methane is the prevailing element of natural gas; therefore, there are also a variety of impurities. In fact, it contains usually considerable amounts of acid gases (CO2, H2S) which can lead to corrosion in equipments and pipelines if water is present. Mercaptans are known as toxic molecules with undesirable odor, and fuel combustion of mercaptan molecules can produce SO2 which is undesirable chemical, they can cause environmental issues. Acid gases and mercaptans are needed to be removed from natural gas until acceptable standard. The treated natural gas contains as maximum as 2% of CO2, 2–4 ppm of H2S and 5–30 ppm of total mercaptans. Chemical absorption with alkanolamines [3] (such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA)) is the most well-established method to separate acid gas from natural gas. Acid gases react with alkanolamines in the absorber to form electrolyte species, mercaptans and hydrocarbons do not react with alkanolamines molecules, and they are physically absorbed by aqueous alkanolamine solution. Then the loaded solution can be regenerated by heating in the stripper.Thermodynamic model is of high importance for the conception of the process, as it is linked directly to the accurate determination of the Vapor-Liquid Equilibrium and energy balances. Reliable thermodynamic models can allow designers not only to confirm their regulatory limits, but also to minimize the loss of valuable hydrocarbons components.In this work a thermodynamic model has been developed to describe:• Alkane (methane, ethane, propane, n-butane, n-pentane, n-hexane), aromatic (ethylbenzene, benzene, toluene) and mercaptans (MM,EM) in aqueous alkanolamine solution• Acid gases (CO2,H2S) solubilities in aqueous alkanolamine solutions, and other crucial properties like: electrolyte concentration, vapor phase composition(mostly water contant)• The phase diagram for multi-component system containing CO2-H2S-alkanolamine-water-hydrocabon-mercaptan.The parameters of the model were determined with the experimental data available in the literature and the new measured data.