Comprehensive Thermodynamic Model for Aqueous Nitric Acid and Sodium Nitrate Solution with Electrolyte NRTL Equation

Comprehensive Thermodynamic Model for Aqueous Nitric Acid and Sodium Nitrate Solution with Electrolyte NRTL Model

Maximilian B. Gorensek ^a

Daniel P. Lambert ^a

Meng Wang ^b

Chau-Chyun Chen ^b

^a Savannah River National Laboratory, Aiken, SC 29808

^b Texas Tech University, Lubbock, TX 79409

Abstract

Nitric acid is widely used as a solvent in reprocessing of spent nuclear fuel. It is typically neutralized with sodium hydroxide, leading to significant amounts of sodium nitrate in nuclear wastes. To support heat and mass balance calculations and process simulation for nuclear waste treatment, a comprehensive thermodynamic model is developed for the nitric-acid-sodium nitrate-water ternary system and its subsystems. Based on the symmetric electrolyte NRTL (eNRTL) activity coefficient model, the present work takes into account complete dissociation of sodium nitrate and partial dissociation of nitric acid in aqueous solution. With a maximum of three temperature coefficients for each eNRTL binary interaction parameter, the model provides an accurate and thermodynamically consistent representation for phase equilibrium properties such as vapor pressure, boiling point, dew point and salt solubility; calorimetric properties such as enthalpy and heat capacity; and speciation properties such as molecular nitric acid content. The model is validated with data covering temperatures up to 379.15 K and nitric acid concentrations up to pure acid for the nitric acid-water binary, temperatures up to 473.15 K and sodium nitrate concentrations up to saturation for the sodium nitrate-water binary, and sodium nitrate concentrations up to 0.21 mole fraction and nitric acid concentrations up to 0.3 mole fraction for the nitric acid-sodium nitrate-water ternary system. The model is being extended to cover additional ionic and organic species present in liquid waste at the Savannah River Site.