Modeling the Chemistry of Carbon Dioxide - Rich Phases with Impurities

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Dense Phase CO2 Corrosion: Modeling the Chemistry of CO2 – Rich Phases with Impurities


  • - Model the solubility of So in CO2 - rich phases in order to predict whether solid So can drop out in transmission lines
    - Predict whether So can undergo reactions in the presence of water

Chemical subsystems to be modeled

  • - Pure So
    Volatility of pure So provides a baseline for its solubility in gas phase
    - So – H2O
    Solubility of So in H2O is important if an aqueous phase forms
    Model development requires considering the So – H2O system because the reference state for liquid-phase species is infinite dilution in water
    - So – CO2
    Solubility of So in CO2
    - Redox reactions of sulfur in aqueous media
    Will be important for future modeling of reactions of SOx and NOx

Polymeric species of sulfur

  • - Numerous sulfur species up to S20 have been detected
    - In the gas phase, eight multimers (So1 through So8) have been assumed in the model
    Thermochemical data are well established for So1 through So8
    So8 is dominant at normal and moderate conditions
    Lower multimers become prevalent at higher temperatures
    - In the pure liquid and solution phase, So8 predominates
    - In the CO2 phase, solvated So-CO2 species may appear

Vapor pressure of So

  • Solid-vapor equilibrium transitions into liquid-vapor equilibrium at the triple point
  • Calculations are supported by a large body of generally consistent data

Melting point of So

  • Pressure significantly affects the melting point

Solubility of So in water

  • Transition between solid-liquid equilibria and liquid-liquid equilibria at the melting point of sulfur
  • Solubility is a strong function of temperature

Solubility of So in CO2: Preliminary results

  • Two segments of solubility curves at each T corresponding to gas (or gas-like) and liquid (or liquid-like) CO2
  • Transition is sharp for subcritical CO2 and gradual for supercritical CO2
  • Fair amount of scattering in the data but the trends are clear
  • In scCO2, changes due to SVE-SLE transitions are obscured by uncertainty in data

Modeling sulfur redox

  • Provides a starting point for modeling reactions involving sulfur
  • Redox states
    - Oxidation states range from S2- through S8+
    - Practically important states:
    S2- (sulfides, hydrogen sulfide)
    S2+ (thiosulfate)
    S4+ (sulfite)
    S6+ (sulfate)
    Thiosulfates and sulfites are usually metastable as evidenced by E-pH diagrams
  • Experimental data are often kinetically constrained
    - Metastable species may be identified in addition to stable ones

Sulfur redox: Preliminary results for disproportionation

  • Disproportionation reactions proceed more readily at high T
  • This is not a direct comparison because measurements are kinetically constrained and calculations predict equilibria
  • Therefore, only semi-quantitative agreement can be expected
  • The properties of thiosulfate species (S2+) need to be reexamined

Author: Ron Springer (OLI), Andre Anderko (OLI), Jim Berthold (OLI, editor)