Evaluation of Chilled Methanol Processes for Acid Gas Removal

Abstract

As the main goal achieved with this master thesis, a plant design was constructed for an acid gas removal process with methanol operating at low temperatures. First, a bibliographical research was made in terms of sour gas treatment; with special focus of physical absorption processes involving methanol as the solvent to achieve separation; such as Rectisol and Ifpexsol. The literature research was extended to thermodynamic data; compiling equilibrium values for binary systems between methanol and carbon dioxide (CO2), hydrogen sulphide (H2¬S) and methane (CH4); respectively. The simulator Pro II with Provision was selected as the computational tool to achieve thermodynamic calculations for the gas stream to be treated. The thermodynamic Equation of State (EOS) utilized to model the properties of the system was a simulator built in modified version of the Soave-Redlich-Kwong-Panagiotopuolos-Reid EOS. Comparisons between the researched equilibrium values and the simulated data were done; corroborating that the model was strong enough to perform calculations for components related with acid gas removal. A natural gas stream rich in Carbon Dioxide (CO2), Nitrogen (N2) and heavy-hydrocarbons was selected from Statoil s Snøhvit gas treatment processing in order to be subject of acid gas removal. The plant design for the sour gas treatment was developed in three individual stages that were later integrated: heavy-hydrocarbons removal, absorption with methanol and solvent regeneration. The design proposed was effective into removing the CO2 present in the natural gas stream down to a value of 40 ppmv.Finally, a brief pinch analysis was sketched; thus identifying the actual possibility of heat integrating the system with an LNG processing unit. In conclusion, simple simulation and thermodynamic tools can conduct to efficient designs for integral acid gas removal plants.