(801b) Thermodynamic Modeling of Carbon Dioxide Solubility in Aqueous Morpholine Solutions

Thermodynamic Modeling of Carbon Dioxide Solubility in Aqueous Morpholine Solutions

Naser S. Matin, Joseph E. Remias, James K. Neathery and Kunlei Liu*

Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, United States

Abstract

Employing the experimental CO₂ solubility in aqueous solutions of morpholine (6.14, and 8.85 m), a rigorous thermodynamic modeling is introduced. Both total pressure and a gas chromatography method were applied to determine CO₂ partial pressure over the solutions under study. In addition to solubility data the experimental speciation data was also included in system thermodynamic modeling. The Electrolyte-NRTL equations along with the needed thermodynamic equations were used to model the experimental CO₂ solubility and morpholine speciation data. Using this regression, the molecule-molecule and molecule-electrolyte binary parameters necessary for thermodynamic modeling of the system were determined. Using the CO₂ solubility data, the CO₂ heat of absorption in the solutions was also calculated. The regressed parameters from the experimental data yielded good agreement between the experimental and model values. A temperature dependent function for the morpholine carbamate stability constant was introduced, which displays a decreasing trend for carbamate formation with increasing temperature. The calculated morpholine carbamate stability constant is in acceptable agreement with literature. Considering the cyclic and non-cyclic secondary alkanolamines, morpholine shows a stronger carbamate bond as evidenced by its higher carbamate equilibrium constant compared to that of a linear secondary amine (DEA). This is partially reflected in the higher rate for CO₂ absorption in aqueous solutions of morpholine compared to the linear amine. The heat of absorption for morpholine was determined as a function of temperature and CO₂ solution loading. The temperature dependent Henry’s law constant and heat of dissolution (physical solubility) for CO₂ in pure morpholine were also introduced.  The determined CO₂equilibrium properties are in good agreement with most of the experimental data.

Keywords: Henry’s constant; carbon dioxide; morpholine; vapor liquid equilibrium; modeling,