THERMODYNAMIC PROPERTIES MODELING OF AQUEOUS CARBONATE ELECTROLYTE SYSTEM FOR CO2 SEPARATION FROM NATURAL GAS

Hot potassium carbonate (HPC) electrolyte solution is used in gas processing and fertilizer plant to chemically absorb CO2 and H2S gases. The HPC solvent usually contains K2CO3, KHCO3, and H2O, beside small quantities of the diethanolamine (DEA) activator and V2O5 corrosion inhibitor. The solution solubility is controlled by the concentrations of carbonate, bicarbonate and CO2 in the mixture. The problem in this study is the saturation of the potassium carbonate and potassium bicarbonate into a solid crystal state at certain conditions during the process. Consequently, the phenomena lead to accumulation of solid particles inside the units, mainly the pipelines and heat exchangers. The crystallization problem typically leads to reduction of the heat transfer rate, stripper unit temperature, and the overall process efficiency. In order to remove the solid accumulations, the process has to be shut down which lead to further production loss. The electrolyte nonrandom two liquids (ELECNRTL) model is selected for HPC thermodynamic and physical properties calculation using ASPEN PLUS simulator. The ELECNRTL model was conducted on the basis of the relationship between the solutes ion species and solvent molecules. In this study, the effective thermodynamic factors are investigated to determine the critical condition of the electrolyte crystallization in HPC solution. Furthermore, it was desired to develop these characteristics within the industrial process conditions of pressure, temperature and concentration. The observation of solution solubility detects saturation points at temperatures higher than solution boiling point for 30 wt% K2CO3 standard solution. The stable temperature simulated in this study was at temperature range between 287.15 K and 362.15 K with the error of ±4 K, respectively based on the given literature data of carbonate system. For carbonate/bicarbonate mixture system, increasing of the operation pressure from 1 bar to 2 bar increase the mixture solution boiling temperature with ΔTmean= 18 K. This gives a wider range of solvent stability in liquid phase and was also affected on the solvent transport thermodynamics. Furthermore, for binary systems of carbonate, it was found that the possibilities of solution crystallization may happen at temperatures lower than 313.15 K, pressure 1 bar for concentrations higher than 3 mole K2CO3/Kg H2O.