Thermodynamic basis of heat-stable salt precipitation during industrial MDEA reclaiming

Heat-stable salt (HSS) accumulation in methyldiethanolamine (MDEA) solvents progressively impairs absorption performance, accelerates corrosion, and complicates solvent regeneration in industrial gas sweetening systems. In this study, an industrially contaminated MDEA solution from the Shurtan gas sweetening unit was subjected to vacuum distillation to examine precipitation behavior during reclaiming. Water removal induced solvent enrichment and visible salt precipitation, indicating a reclaiming-driven phase instability. To interpret this behavior, industrial sample observations were integrated with binary MDEA-H 2 O phase analysis and surrogate divalent electrolyte modeling. The binary solvent matrix remained thermodynamically stable over the investigated composition range, whereas electrolyte-containing systems exhibited marked contraction of the liquid stability region under water-depleted conditions. The calculated ionic strength increased from approximately 2.15 to 5.3 mol L −1 during solvent enrichment, and a critical instability window of about 3.5-4.0 mol L −1 was identified for the transition from stable liquid behavior to metastable and precipitation-prone regimes. These results indicate that precipitation during MDEA reclaiming is governed by the combined effects of ionic-strength amplification, dielectric suppression, and activity-driven crystallization. The study therefore provides a quantitative thermodynamic basis for defining solvent-enrichment limits during regeneration and for evaluating the potential of controlled crystallization as an HSS withdrawal strategy.