Modelling CO2 and H2S Solubility in Aqueous Solution of MDEA
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Raw natural gas contains acid gases like CO2 and H2S. These gases are important to remove for different reasons. CO2 leads to reduction in the heating value of a gas, reducing its selling value, and both CO2 and H2S may lead to corrosion in equipment. In addition to this, H2S is a hazardous gas. For removal of acid gases in natural gas, one of the most applied technologies is chemical absorption, where a solvent is reacting with the sour gas, removing it from the inlet gas. Several commercial solvents exist to be used in acid gas removal, but when H2S is to be removed MDEA is preferred. In this thesis the VLE of the absorption of CO2 and H2S using MDEA, the system containing H2O, CO2, H2S and MDEA, is modelled. The objective is to develop an updated thermodynamic model for calculating the equilibrium between these four species. Also the subsystems H2O-MDEA, H2O-CO2-MDEA and H2O-H2S-MDEA are modelled. The PARFINDER code is used in the modelling. It applies the eNRTL model to describe the non-idealities in the liquid phase, while the gas phase is modelled using the Peng-Robinson EoS. The adjustable parameters in the eNRTL model is fitted to experimental data. A literaure review is performed to find experimental data, and in the optimization procedure, the particle swarm optimization is used. The results show that the model predicts the solubility of both CO2 and H2S in aqueous MDEA well. For the binary system H2O-MDEA the total pressure, freezing point depression and excess enthalpy were modelled. The deviations were below 10% for all parameters. For the two ternary systems, H2O-CO2-MDEA and H2O-H2S-MDEA, the parameters total pressure, partial pressure of acid gas, speciation and heat of absorption were modelled. It is found that both partial pressures and total pressure are predicted well by the model. Experimental data for speciation distribution was only found for the H2O-CO2-MDEA system, and here the experimental data is very well represented by the model. The heat of absorption predictions are fairly good for some conditions, while the deviation is larger at other conditions. For the quaternary system only data for partial pressure of CO2 and H2S was available, and only these parameters were modelled. For this system more data is necessary to validate the model. The model is considered to be good as an initial model for simulation and design purposes, with some points of improvement.