Assessing Anhydrous Tertiary Alkanolamines for Gas Purifications

Paul M. Mathias, Kash Afshar, Satish Reddy, Arnold R. Smith, Rubin J. McDougal
Louis V. Jasperson, Phillip K. Koech, Mark D. Bearden, Charles J. Freeman, David J. Heldebrant

Anhydrous tertiary alkanolamines chemically react with CO₂ and H₂S, with greater selectivity for the latter. This is in direct contrast to aqueous amine-based solvent systems with higher selectivity for CO₂ due to stronger chemical fixation than for H₂S. Anhydrous tertiary alkanolamines exhibit pressure-induced chemical fixation of CO₂ to form zwitterionic ammonium alkylcarbonate ionic liquids, while the same tertiary alkanolamines react with H₂S at atmospheric pressures to form hydrosulfide ionic liquids. This difference in capture pressure implies that anhydrous alkanolamines could be chemically selective for H₂S over CO₂ under specific conditions.  We present here the first vapor-liquid equilibrium (VLE) data of anhydrous ethyldiethanolamine (EDEA) with CH₄, C₃H₈, H₂S and CO₂ at 10 and 50 °C measured by the PTx and PTxy methods. The data are modeled in Aspen Plus® using an electrolyte NRTL model. Data trends and phenomena are discussed for each gas.  We also compare anhydrous EDEA’s simulated performance for CO₂ and H₂S separations to other solvents such as Fluor Solvent, Econosolv, and aqueous methyldiethanolamine (MDEA) for representative gas-processing applications.