Modeling the Chemistry of Carbon Dioxide - Rich Phases with Impurities

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

Objectives:

• - Model the solubility of S^o in CO₂ - rich phases in order to predict whether solid S^o can drop out in transmission lines

  - Predict whether S^o can undergo reactions in the presence of water

Chemical subsystems to be modeled

• - Pure S^o

  Volatility of pure S^o provides a baseline for its solubility in gas phase

  - S^o – H₂O

  Solubility of S^o in H₂O is important if an aqueous phase forms

  Model development requires considering the S^o – H₂O system because the reference state for liquid-phase species is infinite dilution in water

  - S^o – CO₂

  Solubility of S^o 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 (S^o₁ through S^o₈) have been assumed in the model

  Thermochemical data are well established for S^o₁ through S^o₈

  S^o₈ is dominant at normal and moderate conditions

  Lower multimers become prevalent at higher temperatures

  - In the pure liquid and solution phase, S^o₈ predominates

  - In the CO₂ phase, solvated S^o-CO₂ species may appear

Vapor pressure of S^o

• 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 S^o

• Pressure significantly affects the melting point

Solubility of S^o 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 S^o 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 S^2- through S⁸⁺

  - Practically important states:

  S²- (sulfides, hydrogen sulfide)

  S⁰

  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

Deployment plan

• Upcoming update of the OLI software

  - Version 9.3.1 is in the final stage of development

  - It will contain the recently developed sulfur chemistry

  - Members will be able to download the new version from the OLI download site

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