Insights on natural gas purification: Simultaneous absorption of CO2 and H2S using membrane contactors

Abstract

Natural gas plays a critical role in today's energy supply. However, before it can be utilized as a fuel for chemical processing or energy generation, it must undergo extensive treatment to remove all contaminants such as CO2 and H2S. This paper presents the simultaneous absorption of CO2 and H2S from natural gas using aqueous potassium carbonate solutions as the absorbent solvent while utilizing hollow fiber membrane contactors. The reaction mechanisms of CO2 and H2S absorption into aqueous carbonate solutions are considered complicated due to the several species involved in the reaction scheme that undergo reversible reactions. A comprehensive 2D mathematical model has been developed for the simultaneous absorption of CO2 and H2S while considering reversible and equilibrium reactions rates. The model results showed that lower concentrations of carbonate solutions resulted in higher absorption rates, while the presence of high bicarbonate concentrations in the solution was the main incentive for the enhanced reversible reactions and thus, lower absorption rates were achieved. A complete removal of CO2 and H2S in a gas mixture containing 5% CO2, 5% H2S, and 90% CH4 was possible by using two membrane modules in series while using fresh 1M carbonate solution in each module. The first module removes all H2S in the system and small amounts of CO2, while the second module removes the remaining CO2 coming out from the first module. This scheme is favorable than using only one membrane module, because the majority of bicarbonate ions that favors the reversible reaction, are generated from the consumption of H2S with carbonate solution. Therefore in the second module, CO2 can be consumed significantly due to the absence of bicarbonate ions.