H2S Distribution Modeling in NeqSim

Related Resources:

For a quick overview with Python examples, see H2S Distribution Guide (quick)

For a hands-on Python notebook, see H2S Distribution Modeling Tutorial

Introduction

Hydrogen sulfide (H2S) is a critical component in oil and gas processing due to its:

Toxicity: Extremely hazardous at low concentrations (>10 ppm)
Corrosivity: Causes severe corrosion, especially with water present
Regulatory requirements: Strict limits on H2S in sales gas and produced water
Process design impact: Affects sweetening unit sizing, materials selection, and safety systems

Accurate modeling of H2S partitioning between gas, oil, and water phases is essential for:

Sour gas treating design (amine units, Claus plants)
Produced water handling and disposal
Corrosion prediction and materials selection
Safety system design (flare, venting)
Environmental compliance

Thermodynamic Basis

H2S Properties

Property	Value	Unit
Molecular weight	34.08	g/mol
Critical temperature	373.5	K
Critical pressure	89.63	bar
Acentric factor	0.094	-
Dipole moment	0.97	Debye
pKa1 (25°C)	7.0	-
pKa2 (25°C)	14.0	-

Phase Behavior Characteristics

H2S exhibits complex phase behavior:

Gas-Oil Partitioning: Governed primarily by vapor-liquid equilibrium (VLE)
Gas-Water Partitioning: Involves both physical solubility and chemical reactions
Oil-Water Partitioning: Typically minor but can be significant at high pressures

The key challenge is that H2S is a weak acid that dissociates in water:

\[\text{H}_2\text{S} \rightleftharpoons \text{H}^+ + \text{HS}^- \quad (K_{a1})\] \[\text{HS}^- \rightleftharpoons \text{H}^+ + \text{S}^{2-} \quad (K_{a2})\]

This means total H2S in water = molecular H2S + HS⁻ + S²⁻

Simple Equation of State Models

1. Soave-Redlich-Kwong (SRK) EOS

The SRK equation of state is widely used for hydrocarbon systems:

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

Advantages for H2S modeling:

Well-established parameters for H2S
Good for gas-oil equilibrium
Fast computation
Extensive binary interaction parameter (kij) database

Limitations:

Treats water as non-polar molecule
Does not account for H2S dissociation in water
Underestimates H2S solubility in water at high pH

NeqSim Example: SRK EOS

import neqsim.thermo.system.SystemSrkEos;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

// Create SRK system at reservoir conditions
SystemInterface fluid = new SystemSrkEos(353.15, 100.0);  // 80°C, 100 bara

// Add components - typical sour gas with oil and water
fluid.addComponent("methane", 0.70);
fluid.addComponent("ethane", 0.05);
fluid.addComponent("propane", 0.03);
fluid.addComponent("n-heptane", 0.10);  // Oil pseudo-component
fluid.addComponent("H2S", 0.05);         // 5 mol% H2S
fluid.addComponent("CO2", 0.02);
fluid.addComponent("water", 0.05);

fluid.setMixingRule("classic");
fluid.setMultiPhaseCheck(true);

// Run flash calculation
ThermodynamicOperations ops = new ThermodynamicOperations(fluid);
ops.TPflash();

// Get H2S distribution
if (fluid.hasPhaseType("gas")) {
    double h2sInGas = fluid.getPhase("gas").getComponent("H2S").getx();
    System.out.println("H2S mole fraction in gas: " + h2sInGas);
}
if (fluid.hasPhaseType("oil")) {
    double h2sInOil = fluid.getPhase("oil").getComponent("H2S").getx();
    System.out.println("H2S mole fraction in oil: " + h2sInOil);
}
if (fluid.hasPhaseType("aqueous")) {
    double h2sInWater = fluid.getPhase("aqueous").getComponent("H2S").getx();
    System.out.println("H2S mole fraction in water: " + h2sInWater);
}

// Calculate K-values
double kGasOil = fluid.getPhase("gas").getComponent("H2S").getx() / 
                 fluid.getPhase("oil").getComponent("H2S").getx();
double kGasWater = fluid.getPhase("gas").getComponent("H2S").getx() / 
                   fluid.getPhase("aqueous").getComponent("H2S").getx();

System.out.println("K(gas/oil) for H2S: " + kGasOil);
System.out.println("K(gas/water) for H2S: " + kGasWater);

2. Peng-Robinson (PR) EOS

The Peng-Robinson equation provides improved liquid density predictions:

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

Advantages over SRK:

Better liquid density predictions
Slightly improved VLE for polar components
Industry standard for many applications

For H2S specifically:

Similar accuracy to SRK for gas-oil partitioning
Still limited for aqueous phase predictions

NeqSim Example: PR EOS

import neqsim.thermo.system.SystemPrEos;

// Create PR system
SystemInterface fluid = new SystemPrEos(353.15, 100.0);

// Add components
fluid.addComponent("methane", 0.70);
fluid.addComponent("ethane", 0.05);
fluid.addComponent("propane", 0.03);
fluid.addComponent("n-heptane", 0.10);
fluid.addComponent("H2S", 0.05);
fluid.addComponent("CO2", 0.02);
fluid.addComponent("water", 0.05);

fluid.setMixingRule("classic");
fluid.setMultiPhaseCheck(true);

ThermodynamicOperations ops = new ThermodynamicOperations(fluid);
ops.TPflash();

// Compare with SRK results
System.out.println("\n=== PR EOS Results ===");
printH2SDistribution(fluid);

3. Cubic-Plus-Association (CPA) EOS

CPA extends cubic EOS with an association term for hydrogen bonding:

\[P = P_{\text{cubic}} + P_{\text{association}}\]

The association term accounts for:

Self-association (H2O-H2O hydrogen bonding)
Cross-association (H2S-H2O interaction)

Key advantage: Properly models water’s non-ideal behavior

NeqSim Example: CPA EOS

import neqsim.thermo.system.SystemSrkCPAstatoil;

// Create CPA system - uses SRK as cubic part
SystemInterface fluid = new SystemSrkCPAstatoil(353.15, 100.0);

// Add components
fluid.addComponent("methane", 0.70);
fluid.addComponent("ethane", 0.05);
fluid.addComponent("propane", 0.03);
fluid.addComponent("n-heptane", 0.10);
fluid.addComponent("H2S", 0.05);
fluid.addComponent("CO2", 0.02);
fluid.addComponent("water", 0.05);

// CPA requires specific mixing rule
fluid.setMixingRule(10);  // CPA mixing rule
fluid.setMultiPhaseCheck(true);

ThermodynamicOperations ops = new ThermodynamicOperations(fluid);
ops.TPflash();

System.out.println("\n=== CPA EOS Results ===");
printH2SDistribution(fluid);

Comparison of Simple EOS Models

/**
 * Compare H2S distribution predictions from different EOS.
 */
public class H2SDistributionComparison {
    
    public static void main(String[] args) {
        double temperature = 353.15;  // 80°C
        double pressure = 100.0;       // 100 bara
        
        // Common composition
        double[] composition = {0.70, 0.05, 0.03, 0.10, 0.05, 0.02, 0.05};
        String[] components = {"methane", "ethane", "propane", "n-heptane", 
                               "H2S", "CO2", "water"};
        
        System.out.println("=== H2S Distribution Comparison ===");
        System.out.println("T = " + (temperature - 273.15) + " °C, P = " + pressure + " bara");
        System.out.println();
        
        // SRK
        SystemInterface srkFluid = new SystemSrkEos(temperature, pressure);
        addComponents(srkFluid, components, composition);
        srkFluid.setMixingRule("classic");
        srkFluid.setMultiPhaseCheck(true);
        new ThermodynamicOperations(srkFluid).TPflash();
        
        // PR
        SystemInterface prFluid = new SystemPrEos(temperature, pressure);
        addComponents(prFluid, components, composition);
        prFluid.setMixingRule("classic");
        prFluid.setMultiPhaseCheck(true);
        new ThermodynamicOperations(prFluid).TPflash();
        
        // CPA
        SystemInterface cpaFluid = new SystemSrkCPAstatoil(temperature, pressure);
        addComponents(cpaFluid, components, composition);
        cpaFluid.setMixingRule(10);
        cpaFluid.setMultiPhaseCheck(true);
        new ThermodynamicOperations(cpaFluid).TPflash();
        
        // Print comparison table
        System.out.println("EOS      | H2S in Gas | H2S in Oil | H2S in Water | K(g/w)");
        System.out.println("---------|------------|------------|--------------|-------");
        printRow("SRK", srkFluid);
        printRow("PR", prFluid);
        printRow("CPA", cpaFluid);
    }
    
    private static void addComponents(SystemInterface fluid, 
            String[] names, double[] fractions) {
        for (int i = 0; i < names.length; i++) {
            fluid.addComponent(names[i], fractions[i]);
        }
    }
    
    private static void printRow(String name, SystemInterface fluid) {
        double h2sGas = fluid.hasPhaseType("gas") ? 
            fluid.getPhase("gas").getComponent("H2S").getx() : 0;
        double h2sOil = fluid.hasPhaseType("oil") ? 
            fluid.getPhase("oil").getComponent("H2S").getx() : 0;
        double h2sWater = fluid.hasPhaseType("aqueous") ? 
            fluid.getPhase("aqueous").getComponent("H2S").getx() : 0;
        double kGW = h2sWater > 0 ? h2sGas / h2sWater : 0;
        
        System.out.printf("%-8s | %10.6f | %10.6f | %12.6f | %6.2f%n",
            name, h2sGas, h2sOil, h2sWater, kGW);
    }
}

Expected Results (approximate):

EOS	H2S in Gas	H2S in Oil	H2S in Water	K(g/w)
SRK	0.055	0.035	0.008	6.9
PR	0.054	0.036	0.009	6.0
CPA	0.053	0.037	0.012	4.4

Key observations:

SRK and PR give similar gas-oil partitioning
CPA predicts higher H2S solubility in water due to association modeling
Simple EOS still don’t capture H2S dissociation effects

Electrolyte EOS with Chemical Reactions

When Simple EOS Models Fail

Simple cubic EOS (SRK, PR) and even CPA have significant limitations for H2S-water systems:

pH effects not captured: H2S dissociation increases total solubility at high pH
Salt effects ignored: Produced water contains salts that affect H2S solubility
No speciation: Cannot predict HS⁻ and S²⁻ concentrations
Corrosion prediction: Requires knowing molecular H2S concentration, not total

When you need electrolyte models:

Produced water with significant salt content (>1000 ppm TDS)
High pH conditions (pH > 6) where dissociation is significant
Corrosion rate predictions requiring molecular H2S
Amine treating modeling
Sour water stripping design
Environmental discharge calculations

H2S Chemistry in Water

In aqueous solution, H2S participates in acid-base equilibria:

Primary dissociation (significant at pH > 5):

\[\text{H}_2\text{S(aq)} \rightleftharpoons \text{H}^+ + \text{HS}^- \quad pK_{a1} = 7.0\]

Secondary dissociation (only at very high pH > 12):

\[\text{HS}^- \rightleftharpoons \text{H}^+ + \text{S}^{2-} \quad pK_{a2} = 14.0\]

Practical implications:

At pH 7: ~50% molecular H2S, ~50% HS⁻
At pH 8: ~10% molecular H2S, ~90% HS⁻
At pH 6: ~90% molecular H2S, ~10% HS⁻

Only molecular H2S causes corrosion and is in VLE with gas phase!

NeqSim Electrolyte Model

NeqSim’s electrolyte model combines:

Extended UNIQUAC for activity coefficients
Chemical equilibrium for ionic species
Phase equilibrium for VLE

Example: Electrolyte Model with H2S Dissociation

import neqsim.thermo.system.SystemFurstElectrolyteEos;

/**
 * H2S distribution with electrolyte model including dissociation.
 * 
 * This model captures:
 * - H2S dissociation to HS⁻ and S²⁻
 * - pH effects on solubility
 * - Salt effects (Na⁺, Cl⁻, etc.)
 * - Molecular vs. total H2S
 */
public class H2SElectrolyteModel {
    
    public static void main(String[] args) {
        // Create electrolyte system
        SystemInterface fluid = new SystemFurstElectrolyteEos(353.15, 50.0);
        
        // Add hydrocarbon components
        fluid.addComponent("methane", 0.80);
        fluid.addComponent("H2S", 0.05);
        fluid.addComponent("CO2", 0.02);
        
        // Add water and electrolytes
        fluid.addComponent("water", 0.13);
        
        // For produced water with salts:
        // fluid.addComponent("Na+", 0.001);
        // fluid.addComponent("Cl-", 0.001);
        
        fluid.chemicalReactionInit();
        fluid.setMixingRule(4);  // Electrolyte mixing rule
        fluid.setMultiPhaseCheck(true);
        
        // Run flash with chemical equilibrium
        ThermodynamicOperations ops = new ThermodynamicOperations(fluid);
        ops.TPflash();
        
        // Get results
        System.out.println("=== Electrolyte Model Results ===");
        System.out.println("Temperature: " + (fluid.getTemperature() - 273.15) + " °C");
        System.out.println("Pressure: " + fluid.getPressure() + " bara");
        System.out.println();
        
        // H2S in gas phase
        if (fluid.hasPhaseType("gas")) {
            double h2sGas = fluid.getPhase("gas").getComponent("H2S").getx();
            System.out.println("H2S in gas phase: " + h2sGas + " mol/mol");
        }
        
        // H2S speciation in aqueous phase
        if (fluid.hasPhaseType("aqueous")) {
            PhaseInterface aq = fluid.getPhase("aqueous");
            
            // Molecular H2S
            double h2sMolecular = aq.getComponent("H2S").getx();
            System.out.println("Molecular H2S in water: " + h2sMolecular + " mol/mol");
            
            // HS⁻ (if available in model)
            if (aq.hasComponent("HS-")) {
                double hsMinus = aq.getComponent("HS-").getx();
                System.out.println("HS⁻ in water: " + hsMinus + " mol/mol");
            }
            
            // Total sulfide
            double totalSulfide = h2sMolecular;
            if (aq.hasComponent("HS-")) {
                totalSulfide += aq.getComponent("HS-").getx();
            }
            System.out.println("Total sulfide in water: " + totalSulfide + " mol/mol");
            
            // Calculate apparent vs. true solubility ratio
            System.out.println("Apparent/True solubility ratio: " + 
                (totalSulfide / h2sMolecular));
        }
    }
}

Effect of pH on H2S Distribution

/**
 * Demonstrate pH effect on H2S solubility.
 * 
 * At higher pH, more H2S dissociates to HS⁻, increasing total solubility
 * but decreasing molecular H2S (which controls VLE and corrosion).
 */
public class H2SpHEffect {
    
    public static void main(String[] args) {
        System.out.println("=== pH Effect on H2S Distribution ===");
        System.out.println("pH    | Molecular H2S | HS⁻       | Total    | % Dissociated");
        System.out.println("------|---------------|-----------|----------|---------------");
        
        // Simulate different pH by adding base (simplified)
        // In practice, pH is controlled by buffer chemistry
        
        double[] pHValues = {5.0, 6.0, 7.0, 8.0, 9.0};
        double pKa1 = 7.0;
        
        for (double pH : pHValues) {
            // Henderson-Hasselbalch: pH = pKa + log([A-]/[HA])
            // [HS-]/[H2S] = 10^(pH - pKa)
            double ratio = Math.pow(10, pH - pKa1);
            double fractionH2S = 1.0 / (1.0 + ratio);
            double fractionHS = ratio / (1.0 + ratio);
            
            // Assume base solubility (molecular H2S at equilibrium with gas)
            double baseSolubility = 0.01;  // mol/mol at low pH
            
            double molecularH2S = baseSolubility;
            double hsMinus = baseSolubility * ratio;
            double totalSulfide = molecularH2S + hsMinus;
            double percentDissociated = 100.0 * hsMinus / totalSulfide;
            
            System.out.printf("%.1f   | %.6f      | %.6f  | %.6f | %.1f%%%n",
                pH, molecularH2S, hsMinus, totalSulfide, percentDissociated);
        }
        
        System.out.println();
        System.out.println("Note: Only molecular H2S is in VLE with gas phase!");
        System.out.println("      Higher pH increases total solubility but");
        System.out.println("      molecular H2S (and corrosion potential) stays constant.");
    }
}

Expected Output:

pH	Molecular H2S	HS⁻	Total	% Dissociated
5.0	0.010000	0.000100	0.010100	1.0%
6.0	0.010000	0.001000	0.011000	9.1%
7.0	0.010000	0.010000	0.020000	50.0%
8.0	0.010000	0.100000	0.110000	90.9%
9.0	0.010000	1.000000	1.010000	99.0%

Salt Effects on H2S Solubility

Salts affect H2S solubility through:

Salting-out effect: Decreases molecular H2S solubility
Ionic strength: Affects activity coefficients
pH buffering: Carbonate/bicarbonate systems

/**
 * Demonstrate salt effect on H2S solubility.
 */
public class H2SSaltEffect {
    
    public static void main(String[] args) {
        System.out.println("=== Salt Effect on H2S Solubility ===");
        
        double[] salinities = {0.0, 10000, 50000, 100000, 200000};  // ppm NaCl
        
        System.out.println("NaCl (ppm) | Relative H2S Solubility");
        System.out.println("-----------|------------------------");
        
        for (double salinity : salinities) {
            // Sechenov equation: log(S0/S) = k_s * I
            // where I is ionic strength, k_s is Sechenov coefficient
            double ionicStrength = salinity / 58440.0;  // Approximate for NaCl
            double sechenov = 0.1;  // Typical value for H2S
            double relSolubility = Math.pow(10, -sechenov * ionicStrength);
            
            System.out.printf("%10.0f | %.3f%n", salinity, relSolubility);
        }
    }
}

Complete Example: Three-Phase H2S Distribution

import neqsim.thermo.system.*;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

/**
 * Complete example comparing simple and electrolyte models for 
 * H2S distribution in a three-phase (gas-oil-water) system.
 */
public class H2SDistributionComplete {
    
    public static void main(String[] args) {
        // System conditions
        double temperature = 353.15;  // 80°C
        double pressure = 50.0;        // 50 bara
        
        // Composition (mol fractions)
        String[] hydrocarbons = {"methane", "ethane", "propane", "n-hexane", "n-decane"};
        double[] hcFractions = {0.60, 0.05, 0.03, 0.05, 0.05};
        
        double h2sFraction = 0.05;  // 5 mol% H2S
        double co2Fraction = 0.02;
        double waterFraction = 0.15;
        
        System.out.println("╔══════════════════════════════════════════════════════════╗");
        System.out.println("║     H2S DISTRIBUTION IN THREE-PHASE SYSTEM               ║");
        System.out.println("╠══════════════════════════════════════════════════════════╣");
        System.out.printf("║  Temperature: %.1f °C                                    ║%n", 
            temperature - 273.15);
        System.out.printf("║  Pressure: %.1f bara                                      ║%n", 
            pressure);
        System.out.printf("║  H2S content: %.1f mol%%                                   ║%n", 
            h2sFraction * 100);
        System.out.println("╚══════════════════════════════════════════════════════════╝");
        System.out.println();
        
        // ═══════════════════════════════════════════════════════════
        // Model 1: SRK EOS (Simple cubic)
        // ═══════════════════════════════════════════════════════════
        System.out.println("┌──────────────────────────────────────────────────────────┐");
        System.out.println("│  MODEL 1: SRK Equation of State                          │");
        System.out.println("└──────────────────────────────────────────────────────────┘");
        
        SystemInterface srkFluid = new SystemSrkEos(temperature, pressure);
        for (int i = 0; i < hydrocarbons.length; i++) {
            srkFluid.addComponent(hydrocarbons[i], hcFractions[i]);
        }
        srkFluid.addComponent("H2S", h2sFraction);
        srkFluid.addComponent("CO2", co2Fraction);
        srkFluid.addComponent("water", waterFraction);
        srkFluid.setMixingRule("classic");
        srkFluid.setMultiPhaseCheck(true);
        
        ThermodynamicOperations srkOps = new ThermodynamicOperations(srkFluid);
        srkOps.TPflash();
        
        printResults(srkFluid, "SRK");
        
        // ═══════════════════════════════════════════════════════════
        // Model 2: PR EOS
        // ═══════════════════════════════════════════════════════════
        System.out.println("┌──────────────────────────────────────────────────────────┐");
        System.out.println("│  MODEL 2: Peng-Robinson Equation of State                │");
        System.out.println("└──────────────────────────────────────────────────────────┘");
        
        SystemInterface prFluid = new SystemPrEos(temperature, pressure);
        for (int i = 0; i < hydrocarbons.length; i++) {
            prFluid.addComponent(hydrocarbons[i], hcFractions[i]);
        }
        prFluid.addComponent("H2S", h2sFraction);
        prFluid.addComponent("CO2", co2Fraction);
        prFluid.addComponent("water", waterFraction);
        prFluid.setMixingRule("classic");
        prFluid.setMultiPhaseCheck(true);
        
        ThermodynamicOperations prOps = new ThermodynamicOperations(prFluid);
        prOps.TPflash();
        
        printResults(prFluid, "PR");
        
        // ═══════════════════════════════════════════════════════════
        // Model 3: CPA EOS (Accounts for hydrogen bonding)
        // ═══════════════════════════════════════════════════════════
        System.out.println("┌──────────────────────────────────────────────────────────┐");
        System.out.println("│  MODEL 3: CPA Equation of State                          │");
        System.out.println("│  (Cubic-Plus-Association - models H-bonding)             │");
        System.out.println("└──────────────────────────────────────────────────────────┘");
        
        SystemInterface cpaFluid = new SystemSrkCPAstatoil(temperature, pressure);
        for (int i = 0; i < hydrocarbons.length; i++) {
            cpaFluid.addComponent(hydrocarbons[i], hcFractions[i]);
        }
        cpaFluid.addComponent("H2S", h2sFraction);
        cpaFluid.addComponent("CO2", co2Fraction);
        cpaFluid.addComponent("water", waterFraction);
        cpaFluid.setMixingRule(10);  // CPA mixing rule
        cpaFluid.setMultiPhaseCheck(true);
        
        ThermodynamicOperations cpaOps = new ThermodynamicOperations(cpaFluid);
        cpaOps.TPflash();
        
        printResults(cpaFluid, "CPA");
        
        // ═══════════════════════════════════════════════════════════
        // Model 4: Electrolyte Model (Accounts for dissociation)
        // ═══════════════════════════════════════════════════════════
        System.out.println("┌──────────────────────────────────────────────────────────┐");
        System.out.println("│  MODEL 4: Electrolyte Model                              │");
        System.out.println("│  (Includes H2S dissociation and ionic species)           │");
        System.out.println("└──────────────────────────────────────────────────────────┘");
        
        SystemInterface elecFluid = new SystemFurstElectrolyteEos(temperature, pressure);
        for (int i = 0; i < hydrocarbons.length; i++) {
            elecFluid.addComponent(hydrocarbons[i], hcFractions[i]);
        }
        elecFluid.addComponent("H2S", h2sFraction);
        elecFluid.addComponent("CO2", co2Fraction);
        elecFluid.addComponent("water", waterFraction);
        elecFluid.chemicalReactionInit();
        elecFluid.setMixingRule(4);
        elecFluid.setMultiPhaseCheck(true);
        
        ThermodynamicOperations elecOps = new ThermodynamicOperations(elecFluid);
        elecOps.TPflash();
        
        printResults(elecFluid, "Electrolyte");
        
        // ═══════════════════════════════════════════════════════════
        // Summary Comparison
        // ═══════════════════════════════════════════════════════════
        System.out.println();
        System.out.println("╔══════════════════════════════════════════════════════════╗");
        System.out.println("║                    SUMMARY COMPARISON                    ║");
        System.out.println("╚══════════════════════════════════════════════════════════╝");
        printComparisonTable(srkFluid, prFluid, cpaFluid, elecFluid);
    }
    
    private static void printResults(SystemInterface fluid, String modelName) {
        System.out.println();
        System.out.println("Phase distribution:");
        System.out.printf("  Number of phases: %d%n", fluid.getNumberOfPhases());
        
        for (int i = 0; i < fluid.getNumberOfPhases(); i++) {
            String phaseType = fluid.getPhase(i).getPhaseTypeName();
            double phaseFrac = fluid.getPhaseFraction(i, "mole");
            System.out.printf("  Phase %d (%s): %.4f mol/mol%n", i, phaseType, phaseFrac);
        }
        
        System.out.println();
        System.out.println("H2S distribution:");
        
        if (fluid.hasPhaseType("gas")) {
            double h2s = fluid.getPhase("gas").getComponent("H2S").getx();
            System.out.printf("  Gas phase:     y(H2S) = %.6f mol/mol%n", h2s);
        }
        if (fluid.hasPhaseType("oil")) {
            double h2s = fluid.getPhase("oil").getComponent("H2S").getx();
            System.out.printf("  Oil phase:     x(H2S) = %.6f mol/mol%n", h2s);
        }
        if (fluid.hasPhaseType("aqueous")) {
            double h2s = fluid.getPhase("aqueous").getComponent("H2S").getx();
            System.out.printf("  Water phase:   x(H2S) = %.6f mol/mol%n", h2s);
        }
        
        // K-values
        System.out.println();
        System.out.println("Partition coefficients:");
        if (fluid.hasPhaseType("gas") && fluid.hasPhaseType("oil")) {
            double kGO = fluid.getPhase("gas").getComponent("H2S").getx() /
                         fluid.getPhase("oil").getComponent("H2S").getx();
            System.out.printf("  K(gas/oil):   %.3f%n", kGO);
        }
        if (fluid.hasPhaseType("gas") && fluid.hasPhaseType("aqueous")) {
            double kGW = fluid.getPhase("gas").getComponent("H2S").getx() /
                         fluid.getPhase("aqueous").getComponent("H2S").getx();
            System.out.printf("  K(gas/water): %.3f%n", kGW);
        }
        System.out.println();
    }
    
    private static void printComparisonTable(SystemInterface srk, SystemInterface pr,
            SystemInterface cpa, SystemInterface elec) {
        System.out.println();
        System.out.println("Model       | y(H2S) Gas | x(H2S) Oil | x(H2S) Water | K(g/w)");
        System.out.println("------------|------------|------------|--------------|--------");
        printModelRow("SRK", srk);
        printModelRow("PR", pr);
        printModelRow("CPA", cpa);
        printModelRow("Electrolyte", elec);
        System.out.println();
        System.out.println("Note: K(g/w) = y(H2S)/x(H2S) - lower value means more H2S in water");
    }
    
    private static void printModelRow(String name, SystemInterface fluid) {
        double yGas = fluid.hasPhaseType("gas") ? 
            fluid.getPhase("gas").getComponent("H2S").getx() : 0;
        double xOil = fluid.hasPhaseType("oil") ? 
            fluid.getPhase("oil").getComponent("H2S").getx() : 0;
        double xWater = fluid.hasPhaseType("aqueous") ? 
            fluid.getPhase("aqueous").getComponent("H2S").getx() : 0;
        double kGW = xWater > 0 ? yGas / xWater : 0;
        
        System.out.printf("%-11s | %10.6f | %10.6f | %12.6f | %6.2f%n",
            name, yGas, xOil, xWater, kGW);
    }
}

When to Use Each Model

Use Simple Cubic EOS (SRK/PR) When:

Dry gas systems: No or minimal water content
Initial screening: Quick approximate calculations
Gas-oil only: Water phase not critical
Low pH conditions: pH < 5 where dissociation negligible
Dehydrated streams: After glycol or molecular sieve treatment

Typical applications:

Inlet separator sizing
Gas transmission pipeline design
Hydrocarbon dewpoint calculations
Initial process simulation

Use CPA EOS When:

Wet gas systems: Significant water with hydrocarbons
MEG/TEG systems: Glycol-water-hydrocarbon mixtures
Methanol injection: Hydrate inhibitor calculations
Better water modeling needed: But chemical reactions not critical

Typical applications:

Hydrate formation predictions
Glycol dehydration units
Water content in gas phase
Improved hydrocarbon solubility in water

Use Electrolyte Models When:

Sour water systems: High H2S with significant water
Produced water: Saline water with dissolved gases
pH effects important: Buffered or high pH systems
Speciation required: Need molecular vs. ionic H2S
Corrosion prediction: Molecular H2S controls corrosion rate
Amine treating: Acid gas-amine-water equilibria
Sour water strippers: Design and optimization

Typical applications:

Sour water stripper design
Produced water treatment
Amine unit simulation
Corrosion rate prediction
Environmental discharge compliance
Acid gas injection studies

Decision Tree for Model Selection

                    ┌─────────────────────────┐
                    │ H2S Distribution Problem │
                    └───────────┬─────────────┘
                                │
                    ┌───────────▼───────────┐
                    │ Is water phase present │
                    │ and significant?       │
                    └───────────┬───────────┘
                                │
              ┌─────────────────┼─────────────────┐
              │ No              │ Yes             │
              ▼                 │                 │
    ┌─────────────────┐        │      ┌──────────▼──────────┐
    │ Use SRK or PR   │        │      │ Is pH > 6 or are   │
    │ (gas-oil only)  │        │      │ salts significant?  │
    └─────────────────┘        │      └──────────┬──────────┘
                               │                  │
                               │    ┌─────────────┼─────────────┐
                               │    │ No          │ Yes         │
                               │    ▼             │             ▼
                               │ ┌──────────────┐ │ ┌────────────────────┐
                               │ │ Is H-bonding │ │ │ Use Electrolyte    │
                               │ │ important?   │ │ │ Model with         │
                               │ └──────┬───────┘ │ │ chemical reactions │
                               │        │         │ └────────────────────┘
                               │  ┌─────┼─────┐   │
                               │  │ No  │ Yes │   │
                               │  ▼     │     ▼   │
                          ┌────────┐  ┌────────┐  │
                          │Use SRK │  │Use CPA │  │
                          │or PR   │  │        │  │
                          └────────┘  └────────┘  │

Practical Recommendations

For Upstream Production

Wellhead to separator: Use CPA for wet gas, SRK/PR for dry gas
Produced water: Use electrolyte model with salts
Corrosion assessment: Always need molecular H2S → use electrolyte model

For Gas Processing

Inlet facilities: CPA sufficient if no amine treating
Amine units: Electrolyte model essential
Dehydration: CPA for glycol units
Sulfur recovery: Electrolyte for Claus tail gas

For Pipeline Transport

Dry gas pipelines: SRK/PR adequate
Wet gas/multiphase: CPA recommended
Corrosion hot spots: Electrolyte model for water accumulation zones

Python Example (Jupyter Notebook)

from neqsim import jneqsim
import numpy as np
import matplotlib.pyplot as plt

# Java class imports
SystemSrkEos = jneqsim.thermo.system.SystemSrkEos
SystemPrEos = jneqsim.thermo.system.SystemPrEos
SystemSrkCPAstatoil = jneqsim.thermo.system.SystemSrkCPAstatoil
ThermodynamicOperations = jneqsim.thermodynamicoperations.ThermodynamicOperations

def create_fluid(SystemClass, T_K, P_bara, mixing_rule):
    """Create fluid with standard sour gas composition."""
    fluid = SystemClass(T_K, P_bara)
    fluid.addComponent("methane", 0.70)
    fluid.addComponent("ethane", 0.05)
    fluid.addComponent("propane", 0.03)
    fluid.addComponent("n-heptane", 0.05)
    fluid.addComponent("H2S", 0.05)
    fluid.addComponent("CO2", 0.02)
    fluid.addComponent("water", 0.10)
    fluid.setMixingRule(mixing_rule)
    fluid.setMultiPhaseCheck(True)
    return fluid

# Temperature range for study
temperatures = np.linspace(300, 400, 11)  # K
pressure = 50.0  # bara

# Storage for results
results = {
    'SRK': {'T': [], 'h2s_gas': [], 'h2s_water': [], 'K_gw': []},
    'PR': {'T': [], 'h2s_gas': [], 'h2s_water': [], 'K_gw': []},
    'CPA': {'T': [], 'h2s_gas': [], 'h2s_water': [], 'K_gw': []}
}

for T in temperatures:
    # SRK
    srk = create_fluid(SystemSrkEos, float(T), pressure, "classic")
    ThermodynamicOperations(srk).TPflash()
    
    # PR
    pr = create_fluid(SystemPrEos, float(T), pressure, "classic")
    ThermodynamicOperations(pr).TPflash()
    
    # CPA
    cpa = create_fluid(SystemSrkCPAstatoil, float(T), pressure, 10)
    ThermodynamicOperations(cpa).TPflash()
    
    for name, fluid in [('SRK', srk), ('PR', pr), ('CPA', cpa)]:
        h2s_g = fluid.getPhase("gas").getComponent("H2S").getx() if fluid.hasPhaseType("gas") else 0
        h2s_w = fluid.getPhase("aqueous").getComponent("H2S").getx() if fluid.hasPhaseType("aqueous") else 1e-10
        
        results[name]['T'].append(T - 273.15)
        results[name]['h2s_gas'].append(h2s_g)
        results[name]['h2s_water'].append(h2s_w)
        results[name]['K_gw'].append(h2s_g / h2s_w if h2s_w > 0 else 0)

# Plot results
fig, axes = plt.subplots(1, 2, figsize=(14, 5))

# Plot 1: H2S in water phase
ax1 = axes[0]
for name, style in [('SRK', 'o-'), ('PR', 's-'), ('CPA', '^-')]:
    ax1.plot(results[name]['T'], results[name]['h2s_water'], style, label=name, markersize=6)
ax1.set_xlabel('Temperature (°C)', fontsize=12)
ax1.set_ylabel('H2S mole fraction in water', fontsize=12)
ax1.set_title('H2S Solubility in Water Phase')
ax1.legend()
ax1.grid(True, alpha=0.3)
ax1.set_yscale('log')

# Plot 2: K-value (gas/water)
ax2 = axes[1]
for name, style in [('SRK', 'o-'), ('PR', 's-'), ('CPA', '^-')]:
    ax2.plot(results[name]['T'], results[name]['K_gw'], style, label=name, markersize=6)
ax2.set_xlabel('Temperature (°C)', fontsize=12)
ax2.set_ylabel('K(gas/water) = y/x', fontsize=12)
ax2.set_title('H2S Partition Coefficient (Gas/Water)')
ax2.legend()
ax2.grid(True, alpha=0.3)

plt.tight_layout()
plt.savefig('h2s_distribution_comparison.png', dpi=150)
plt.show()

print("\nKey observations:")
print("- CPA predicts higher H2S solubility in water (lower K-value)")
print("- SRK and PR give similar results for gas-oil partitioning")
print("- Temperature increase generally increases K(g/w) - less H2S in water")
print("- For accurate water phase predictions, use CPA or electrolyte model")

References

Carroll, J.J., “Acid Gas Injection and Carbon Dioxide Sequestration”, Wiley, 2010
Experiment data sources for H2S-water: Selleck et al. (1952), Lee & Mather (1977)
CPA model development: Kontogeorgis & Folas, “Thermodynamic Models for Industrial Applications”, Wiley, 2010
Electrolyte modeling: Chen et al., “Generalized Electrolyte-NRTL Model”, AIChE J., 1982

Document Version: 1.0
Author: NeqSim Development Team
Last Updated: February 2026