Selective removal of Hydrogen Sulfide from Natural Gas by use of MDEA

Abstract

There have been a lot of investigations on acid gas removal using absorption process in which different types of solvents including physical solvents, chemical solvents and hybrid solvents are used. One of these chemical solvents is a kind of tertiary amine called MDEA. The main goal of this master thesis is modeling an acid gas removal plant using MDEA. At first, a brief explanation about Natural gas chain in Norwegian continental shelf is given, then a bibliographical research was made regarding acid gas removal processes; in addition, some criteria for selection of proper acid gases removal process are discussed.Asgård B has an offshore absorption unit to remove selective acid gas using FLEXORB- a solvent which is used to remove selectively H2S. Another solvent that can be used in the absorption process to remove H2S selectively is MDEA .In this project, MDEA is replaced with FLEXORB to observe the solvent behavior from different angles in the Asgård B acid gas removal unit. For this purpose, a simulation of the process has been done in which MDEA was used as a solvent and the obtained results will be discussed.The simulation of this plant is designed based on the data collected from Statoil. Unisim software has been chosen to simulate amine based acid gas removal unit. It has also been tried to optimize the process. To do so, heat exchanger network was designed using Pinch Analysis method and the required external energy has been calculated. It helps to use heat flows of the streams in the process in order not to waste the energy.Finally, it is concluded that required recycled flow rate of MDEA is much more than FLEXORB and consequently, the required make up water for MDEA solvent will be more than required water in the process using FLEXORB. Furthermore, FLEXORB acts more selectively than MDEA in the case of H2S removal. Changing different parameters in the simulation shows that (i) inlet lean amine temperature entering the absorber does not affect the absorption process, (ii) increasing of heat duty in the reboiler of regenerator decreases acid gases in the treated gas, (iii) lean amine flow rate with an acceptable loading has a direct effect on the product quality. (iv)increase of trays in the absorber leads to remove bulk CO2.