Gas Treat General Overview
Gas treating is the process employed to remove acidic components (primary H2S and CO2) and other impurities from hydrocarbon streams. That is to meet the set specifications and demand. There are several methods to remove the acidic components from the natural sour gas and these can be broadly categorized as chemical reaction or absorption.
Chemical reaction processes remove H2S and/ or CO2 from the gas stream by chemical reactions that maybe reversible or irreversible. In reversible reactions such as alkanolamine processes the solvent could be regenerated. Because the water content of the solution minimizes heavy hydrocarbon absorption, these processes are well suited for gases rich in heavier hydrocarbons. Some amines can be used to selectively remove H2S in the presence of CO2 such as the MDEA amine.
Chemistry—The overall equilibrium reactions applicable for H2S and CO2 and primary and secondary amines are shown below with a primary amine. A qualitative estimation of the velocity of the reaction is given.
For hydrogen sulfide removal
RNH2 + H2S —— RNH3+ + HS-
RNH2 + HS- —— RNH3+ + S—
The overall reactions between H2S and amines are simple since H2S reacts directly and rapidly with all amines to form the disulfide and sulfide.
For hydrogen sulfide removal
2RNH2 + CO2 —— RNH3+ + RNHCOO-
RNH2 + CO2+ H2O —— RNH3+ + HCO3-
RNH2 + HCO3 —— RNH3+ + CO3–
The majority of sour gas is treated with a regenerative solvent (normally amine) to separate the acid gases from the hydrocarbon components. After separating any condensed liquids in the gas in the inlet separator, the sour gas is flowed into the bottom of a contactor or absorber tower. This vessel contains from 20 to 24 trays, or packing giving equivalent contact stages. Lean solution containing sweetening solvent in water is pumped into the tower near the top. As the liquid flows down from tray to tray, it is in intimate contact with the sour gas as the gas bubbles upwards through the liquid on each tray. When the gas reaches the top, virtually all of the H2S and, depending on the solvent, all of the CO2 will have been removed from the gas stream. The gas is now sweet, and meets the specifications for H2S and CO2 content.
The rich solution leaves the contactor at the bottom, and is flowed through a pressure letdown valve, allowing the pressure drop to about 70 psig. Upon reduction of the pressure, the rich solution is flowed into a flash drum, where most dissolved hydrocarbon gas and some acid gas flash off. The solution then flows through a heat exchanger if available picking up heat from the hot regenerated lean solution stream. The rich stream then flows into the stripper, where the regeneration of the solvent occurs at a pressure of about 15 psig and at the boiling temperature of the solution. Heat is applied from an external source, such as a steam reboiler. The librated acid gases and any dissolved hydrocarbon gases leave the stripper at the top, together with some solvent and water vapors. This stream is flowed through a condenser, usually an air cooler, to condensate the solvent and water vapors. The liquid and gas mixture is flowed into a separator, referred to normally as a reflux drum, where the acid gases are separated from the condensed liquids. The liquids are pumped back into the top of the stripper as reflux. The gas stream, consisting mainly of H2S and CO2, is piped to a sulfur recovery unit or to a flare stack in some cases if the total sulfur rate in the gas stream is meeting MEPA requirement such as the case with Haradh gas plant.
The regenerated solution is flowed from the reboiler or the bottom of the stripper through the rich/lean solution heat exchanger if available through a cooler to adjust the temperature to the appropriate treating temperature in the contactor. The stream is then pumped with a high pressure pump back into the top of the contactor, to continue the sweetening of the sour gas. Most amine systems incorporate means of filtering in the solution. This is a accomplished by flowing a portion of the lean amine solution through a particle filter and sometimes through a carbon filter as well. The purpose is to maintain a high degree of solution cleanliness in order to avoid solution foaming. Some solvent systems also have a means of removing degradation products, by having an additional reboiler (reclaimer) for this purpose.