THERMODYNAMICS OF ALKANOLAMINE WATER SOLUTIONS FROM FREEZING-POINT MEASUREMENTS

The water freezing point has been measured in several amine-water systems. The water activity coefficient derived from these measurements by regression with the nonrandom two-liquid (NRTL) model shows strong interactions in the MDEA-water binary system near 0-degrees-C. The apparent solution nonideality sequence in the amine-water system at low temperature (about 0-degrees-C) and dilute amine solution (up to 13 mol %) is methyldiethanolamine (MDEA) almost-equal-to diglycolamine (DGA) > dimethylmonoethanolamine (DMMEA) > triethanolamine (TEA) > diethanolamine (DEA) > monoethanolamine (MEA). The combined regression of freezing point depression data and total vapor pressure data predicts a significant temperature dependence of the solution properties. At 120-degrees-C, the expected solution nonideality sequence is DEA > DGA almost-equal-to MEA > MDEA. The parameters for activity coefficients from regression of these data should be useful for predicting acid gas solubility in these systems.