H2S Distribution Between Gas, Oil, and Water Phases

Related Resources:

Overview

Hydrogen sulfide (H2S) distribution between phases is critical for:

This guide covers modeling approaches in NeqSim from simple to advanced.

Physical Properties of H2S

Property Value
Molecular weight 34.08 g/mol
Critical temperature 373.2 K (100.05°C)
Critical pressure 89.37 bar
Acentric factor 0.0942
Dipole moment 0.97 D

Modeling Approaches

1. Simple Cubic Equations of State (SRK, PR)

The Soave-Redlich-Kwong and Peng-Robinson equations treat H2S as a non-associating, non-electrolyte component.

SRK Equation:

\[P = \frac{RT}{V-b} - \frac{a(T)}{V(V+b)}\]

When to use:

Limitations:

Python Example (SRK):

from neqsim import jneqsim

SystemSrkEos = jneqsim.thermo.system.SystemSrkEos
ThermodynamicOperations = jneqsim.thermodynamicoperations.ThermodynamicOperations

# Create system at 55°C, 50 bara
fluid = SystemSrkEos(273.15 + 55.0, 50.0)

# Add components
fluid.addComponent("H2S", 0.5)
fluid.addComponent("methane", 70.0)
fluid.addComponent("ethane", 5.0)
fluid.addComponent("water", 10.0)

fluid.setMixingRule("classic")
fluid.setMultiPhaseCheck(True)

# Flash calculation
ops = ThermodynamicOperations(fluid)
ops.TPflash()

# Get H2S distribution
for i in range(fluid.getNumberOfPhases()):
    phase = fluid.getPhase(i)
    h2s_x = phase.getComponent("H2S").getx()
    print(f"{phase.getType()}: H2S = {h2s_x:.6e}")

2. CPA Equation of State (Cubic Plus Association)

CPA combines a cubic EoS with an association term:

\[A^{CPA} = A^{SRK} + A^{assoc}\]

The association term accounts for hydrogen bonding between molecules.

When to use:

Advantages over SRK/PR:

Python Example (CPA):

from neqsim import jneqsim

SystemSrkCPAstatoil = jneqsim.thermo.system.SystemSrkCPAstatoil
ThermodynamicOperations = jneqsim.thermodynamicoperations.ThermodynamicOperations

fluid = SystemSrkCPAstatoil(273.15 + 55.0, 50.0)

fluid.addComponent("H2S", 0.5)
fluid.addComponent("CO2", 0.3)
fluid.addComponent("methane", 70.0)
fluid.addComponent("water", 10.0)

fluid.setMixingRule("classic")
fluid.setMultiPhaseCheck(True)

ops = ThermodynamicOperations(fluid)
ops.TPflash()

3. Electrolyte-CPA with Chemical Reactions

This is the most rigorous approach for H2S in aqueous systems.

H2S Dissociation Chemistry:

\[\text{H}_2\text{S}_{(aq)} \rightleftharpoons \text{H}^+ + \text{HS}^- \quad (pK_{a1} \approx 7.0)\] \[\text{HS}^- \rightleftharpoons \text{H}^+ + \text{S}^{2-} \quad (pK_{a2} \approx 14)\]

Water auto-ionization:

\[\text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^-\]

When to use:

Python Example (Electrolyte-CPA):

from neqsim import jneqsim

SystemElectrolyteCPAstatoil = jneqsim.thermo.system.SystemElectrolyteCPAstatoil
ThermodynamicOperations = jneqsim.thermodynamicoperations.ThermodynamicOperations

fluid = SystemElectrolyteCPAstatoil(273.15 + 55.0, 50.0)

# Add hydrocarbon components
fluid.addComponent("H2S", 0.5)
fluid.addComponent("CO2", 0.3)
fluid.addComponent("methane", 70.0)
fluid.addComponent("water", 10.0)

# Add electrolytes (formation water salinity)
fluid.addComponent("Na+", 0.5)
fluid.addComponent("Cl-", 0.5)

# Initialize chemical reactions
# This enables H2S dissociation, CO2 hydration, water auto-ionization
fluid.chemicalReactionInit()
fluid.setMixingRule("classic")
fluid.setMultiPhaseCheck(True)

ops = ThermodynamicOperations(fluid)
ops.TPflash()

# The aqueous phase now contains:
# - Molecular H2S
# - HS- (bisulfide ion)
# - S2- (sulfide ion)
# - OH-, H3O+ (for pH)
# - HCO3-, CO3-- (from CO2)

pH Effects on H2S Distribution

The dominant sulfur species depends strongly on pH:

pH Range Dominant Species H2S Vapor Pressure
< 5 H2S (molecular) Maximum
5-7 H2S + HS- mix Moderate
7-9 HS- dominant Low
> 9 HS- and S2- Very low
> 12 S2- dominant Negligible

Critical insight: Simple EoS models assume 100% molecular H2S in the aqueous phase. At high pH (alkaline), this overestimates H2S partial pressure significantly!

Salting-Out Effects

Dissolved salts reduce gas solubility in water. The Electrolyte-CPA model captures this through:

\[\ln \gamma_i^{salt} = k_s \cdot I\]

where $k_s$ is the salting coefficient and $I$ is the ionic strength.

Typical impact:

Only Electrolyte models capture salting-out! Simple EoS models ignore this effect entirely.

Model Selection Guide

Application Model Justification
Quick screening SRK Fast, adequate for estimates
Gas pipeline transport SRK/PR Water phase minor concern
Offshore separator sizing CPA Good 3-phase equilibrium
Gas processing plant CPA Balance of speed and accuracy
Produced water treatment Electrolyte-CPA pH and chemistry essential
Sour water stripper Electrolyte-CPA H2S speciation critical
Amine treating Electrolyte-CPA Reaction chemistry required
Corrosion assessment Electrolyte-CPA Need pH, ionic species
High salinity systems Electrolyte-CPA Salting-out effects
Sulfur recovery unit Electrolyte-CPA Complex chemistry

Computational Considerations

Model Relative Speed Memory Convergence
SRK 1× (fastest) Low Robust
PR 1× Low Robust
CPA 2-3× Medium Good
Electrolyte-CPA 5-10× High May need tuning

Tips for Electrolyte-CPA:

Common Pitfalls

  1. Ignoring water phase chemistry: Simple EoS gives wrong H2S in water at high pH

  2. Neglecting salinity: Formation water salting-out can change H2S partitioning significantly

  3. Temperature effects: H2S solubility is retrograde above ~80°C; verify model captures this

  4. Mixing rules: Use appropriate interaction parameters (kij) for H2S-water

  5. Phase identification: Ensure proper aqueous phase identification in three-phase systems

References

  1. Kontogeorgis, G.M., Voutsas, E.C., Yakoumis, I.V., Tassios, D.P. (1996). “An Equation of State for Associating Fluids”. Ind. Eng. Chem. Res., 35, 4310-4318.

  2. Carroll, J.J. (2020). “Acid Gas Injection and Carbon Dioxide Sequestration”. Wiley-Scrivener.

  3. Springer, R.D., Wang, Z., Anderko, A., Wang, P., Felmy, A.R. (2012). “A Thermodynamic Model for Predicting Mineral Reactivity in Supercritical Carbon Dioxide”. Chem. Geol., 322-323, 30-45.

  4. Haghighi, H., Chapoy, A., Burgess, R., Tohidi, B. (2009). “Experimental and thermodynamic modelling of systems containing water and ethylene glycol”. Fluid Phase Equilibria, 276, 24-30.