Equilibrium and thermal properties of selected CO2-methane-tertiary amine systems; an experimental and modelling study

Abstract

In this work semi-differential heat of absorption of CO2 into aqueous solutions of Nmethyldiethanolamine (MDEA) and N,N-dimethylmonoethanolamine (DMMEA) has been investigated in a reaction calorimeter. The experiments were conducted at concentrations 2.0M and 4.2 M, and the temperatures 313 K, 353 K and 393 K. The resulting differential heats of absorption were plotted as a function of liquid CO2 loading. An effect of temperature on the heat of absorption was observed for MDEA. This effect was larger at low loadings and the lower amine concentration. This was also observed for DMMEA, but here the effect of temperature was larger. The heat of absorption of CO2 into aqueous DMMEA is slightly larger than for MDEA. The vapor-liquid equilibrium of the system MDEA-H2O-CO2 and DMMEA-H2O-CO2 has been investigated in different types of apparatuses to obtain data that span a wide range of pressures and CO2 loadings. In addition the vapor-liquid equilibrium of the binary system MDEA-H2O and DMMEA-H2O have been measured in an ebulliometer setup. From the data obtained from the low pressure equilibrium apparatus and the calorimeter a few conclusions can be made: • DMMEA is more reactive towards CO2 than MDEA. • MDEA is more easily stripped to low loadings than DMMEA • DMMEA has a higher cyclic capacity than MDEA • DMMEA is a smaller molecule, allowing the use of higher concentrations than MDEA • A higher concentration affects the partial pressure of CO2 above the solvent to a less degree for DMMEA than MDEA. A new high pressure vapor-liquid equilibrium apparatus was purchased and brought into use for the first time in this work. There were several problems with the apparatus, mostly related to leaking from the equilibrium cell or the surrounding valves, during the period of this work. Some conclusions can be made from the high pressure equilibrium data: • The solubility of methane is higher in MDEA than in DMMEA • Pressure seems to affect the equilibrium CO2 loading to a greater extent in MDEA solutions than in DMMEA solutions. Obtained vapor-liquid equilibrium data were used to optimize e-NRTL parameters for the binary MDEA-H2O and DMMEA-H2O systems and the ternary MDEA-H2O-CO2 and DMMEA-H2O-CO2 systems. The model performed well for both the binary systems. For the ternary MDEA-H2O-CO2 system the model performed satisfactorily but not before the VLE data from the low pressure apparatus were excluded from the optimization. At high CO2 partial pressures and 313 K the model struggled to replicate the experimental data. This was also the case for DMMEA.