Overview

NeqSim provides integrated tools for analysing sulfur behaviour in gas value chains — from wellhead to sales gas. This covers:

These capabilities are relevant for both offshore (subsea pipelines, subsea processing) and onshore (gas processing trains, Claus sulfur recovery units) operations.

Key Reactions

Reaction Description NeqSim Tool
2 H₂S + O₂ → 2 H₂O + ¼ S₈ Claus reaction (direct oxidation) GibbsReactor
2 H₂S + 3 O₂ → 2 SO₂ + 2 H₂O Full oxidation of H₂S GibbsReactor
3 H₂S + SO₂ → 4 S + 2 H₂O Claus tail gas reaction GibbsReactor
S₈(gas) → S₈(solid) Desublimation / precipitation TPSolidflash
Fe + H₂S → FeS + H₂ Iron sulfide corrosion SulfurDepositionAnalyser
FeS(wall) + O₂ → Fe oxide + S⁰ Oxidation of historical wall scale IronSulfideOxidationSource

Components and Tools

1. GibbsReactor — Chemical Equilibrium

The GibbsReactor finds equilibrium compositions by minimising total Gibbs free energy subject to element balance constraints:

\[G = \sum_i n_i \left( G_i^0 + RT \ln \frac{f_i}{f_i^0} \right)\]

The solver uses Newton–Raphson iteration with Lagrange multipliers for element conservation (C, H, O, N, S, Ar). Species thermodynamic data (standard Gibbs energy of formation, heat capacity polynomials) are loaded from GibbsReactDatabase.csv and DatabaseGibbsFreeEnergyCoeff.csv.

Key configuration:

Method Description Default
setEnergyMode("isothermal") Isothermal or adiabatic operation ISOTHERMAL
setMaxIterations(5000) Maximum Newton–Raphson iterations 5000
setConvergenceTolerance(1e-4) Convergence criterion 1e-6
setDampingComposition(0.01) Damping factor for stability 0.01
setComponentAsInert(name) Mark component as non-reactive —

Java example:

SystemInterface fluid = new SystemSrkEos(273.15 + 400.0, 1.5);
fluid.addComponent("H2S", 2.0);
fluid.addComponent("oxygen", 1.0);
fluid.addComponent("water", 0.0);
fluid.addComponent("S8", 0.0);
fluid.addComponent("SO2", 0.0);
fluid.setMixingRule(2);

Stream feed = new Stream("feed", fluid);
feed.run();

GibbsReactor reactor = new GibbsReactor("Claus", feed);
reactor.setEnergyMode("isothermal");
reactor.setMaxIterations(5000);
reactor.setDampingComposition(0.01);
reactor.run();
// Outlet stream contains equilibrium composition

2. TPSolidflash — Sulfur Solubility and Precipitation

NeqSim models solid S₈ using a solid fugacity approach. The equilibrium condition is:

\[f_{\text{S8}}^{\text{solid}}(T, P) = f_{\text{S8}}^{\text{gas}}(T, P, y)\]

The solid fugacity uses a sublimation pressure correlation with Poynting correction for pressure. When gas cools below the sulfur dew point, S₈ desublimes as a solid phase.

Java example:

SystemInterface gas = new SystemSrkEos(273.15 + 50.0, 150.0);
gas.addComponent("methane", 0.90);
gas.addComponent("H2S", 0.05);
gas.addComponent("S8", 1e-8);
gas.setMixingRule(2);
gas.setMultiPhaseCheck(true);
gas.setSolidPhaseCheck("S8");

ThermodynamicOperations ops = new ThermodynamicOperations(gas);
ops.TPSolidflash();

boolean solidPresent = gas.hasPhaseType("solid");

TPSolidflash() preserves the components selected with setSolidPhaseCheck(String). In particular, selecting "S8" does not enable precipitation of water or TBP pseudo-components. Calling TPSolidflash() without first enabling solid checking retains the default of checking all components.

The current SolidFlash1 solver supports one pure solid phase together with the fluid phases. If more than one selected component is predicted to precipitate, it throws UnsupportedOperationException before adding a solid phase; simultaneous independent solids are not supported by this solver. Selecting only S8 therefore models sulfur precipitation with other solids excluded. For mixed-solid studies, do not interpret a single selected-solid calculation as full equilibrium.

For inventory checks, sum each component’s getNumberOfMolesInPhase() over the active phases and compare it with getNumberOfmoles() for that component. Phase mole fractions and phase fractions must each sum to one.

Reusing separated gas in a recompression train

For a component-selected sulfur calculation, enable setSolidPhaseCheck("S8") and run TPflash(). After the flash, phaseToSystem("gas") creates an independent fluid containing only the gas-phase component inventory. Liquid and solid inventories remain in the parent system and must be accounted for as separate outlets or deposits.

The extracted fluid preserves the EOS, mixing rule, multiphase option and solid selection. It can be passed directly to a Stream, compressor, mixer or cooler; there is no need to reconstruct a fresh fluid to avoid carrying over the parent inventory. Solid checking remains enabled so later cooling can precipitate more sulfur. Disable it explicitly on the extracted fluid only when the downstream model is intended to omit solid equilibrium.

The fluid-phase solve remains subject to the material-balance and equilibrium checks when solid checking is enabled but no solid is present. In particular, a stalled three-fluid-phase trial may recover to a balanced two-fluid-phase state before the selected solid is checked. An active solid phase is excluded from that fluid-only recovery.

When checking conservation, compare each component’s overall molar inventory with the sum over active phases before and after each operation. For the H2S/S8 system, count sulfur atoms as n(H2S) + 8 * n(S8) and include every liquid drain and removed solid. Phase extraction transfers existing material; it does not represent an additional chemical reaction.

3. SulfurDepositionAnalyser — Integrated Unit Operation

The SulfurDepositionAnalyser is a process equipment unit that combines all sulfur analysis capabilities in a single run() call:

  1. Chemical equilibrium — Gibbs reactor for H₂S + O₂ reactions
  2. Sulfur solubility — TP-solid flash for S₈ in gas
  3. Temperature sweep — Scans temperature range to find deposition onset
  4. Corrosion assessment — FeS risk, NACE sour severity, SO₂ corrosion

Java example:

SystemInterface gas = new SystemSrkEos(273.15 + 80.0, 150.0);
gas.addComponent("methane", 0.82);
gas.addComponent("ethane", 0.05);
gas.addComponent("propane", 0.02);
gas.addComponent("CO2", 0.03);
gas.addComponent("H2S", 0.05);
gas.addComponent("nitrogen", 0.02);
gas.addComponent("water", 0.005);
gas.addComponent("oxygen", 0.00005);
gas.addComponent("S8", 1e-8);
gas.addComponent("SO2", 0.0);
gas.setMixingRule(2);

Stream feed = new Stream("feed", gas);
feed.setFlowRate(100000, "kg/hr");
feed.run();

SulfurDepositionAnalyser analyser =
    new SulfurDepositionAnalyser("Sulfur Analyser", feed);
analyser.setTemperatureSweepRange(0, 200, 5);  // 0-200 C in 5 C steps
analyser.run();

// Results
double onsetT = analyser.getSulfurDepositionOnsetTemperature();   // °C
double solubility = analyser.getSulfurSolubilityMgSm3();          // mg/Sm³
boolean corrosionRisk = analyser.hasCorrosionRisk();
String jsonReport = analyser.getResultsAsJson();

Key result accessors:

Method Returns
getSulfurSolubilityInGas() S₈ mole fraction in gas at inlet
getSulfurSolubilityMgSm3() S₈ concentration in mg/Sm³
getSulfurDepositionOnsetTemperature() Onset temperature (°C) or NaN
isSolidSulfurPresent() Whether solid S₈ exists at inlet
hasCorrosionRisk() Overall corrosion risk flag
getEquilibriumComposition() Map of component → mole fraction
getTemperatureSweepResults() List of T-sweep data points
getCorrosionAssessment() Map of corrosion assessment data
getResultsAsJson() Comprehensive JSON report

4. Historical FeS Wall Inventory

SulfurDepositionAnalyser assesses the current fluid and uses FeS indicators for corrosion risk, but it does not retain kilograms of corrosion product from an earlier operating period. Use IronSulfideWallInventory with IronSulfideOxidationSource when the proposed route is historical water/seawater or FeCO3 scale → FeS → later O2 ingress → elemental sulfur.

The source is explicitly mass-transfer and inventory limited, produces low/base/high S0 scenarios when kinetics are uncalibrated, and writes S0 to the outlet as S8. See the Iron-Sulfide Wall Source Guide.

Application Scenarios

Offshore Subsea Pipeline

A sour gas pipeline from wellhead (120 °C, 200 bar) to shore (4 °C, 80 bar) experiences continuous cooling. Sulfur solubility decreases with temperature, leading to S₈ precipitation in mid-pipeline sections. Use the temperature sweep to identify the critical pipeline segment where solid deposition begins.

Risk factors:

Mitigation:

Onshore Gas Processing

In gas processing plants, the highest sulfur deposition risk locations are:

  1. JT valve / turboexpander outlet — Joule–Thomson cooling causes the largest temperature drop (can reach −25 °C)
  2. Cold separator internals — Condensed phases collect solid sulfur
  3. Heat exchanger tubes — Gradual cooling zones
  4. Gas metering stations — Pressure reduction

Use the analyser at each process point to map risk levels.

Claus Sulfur Recovery Unit (SRU)

The GibbsReactor directly simulates the Claus process equilibrium:

The O₂/H₂S ratio is critical — stoichiometric ratio of 0.5 maximises S₈ yield; excess O₂ produces SO₂ instead.

Corrosion Assessment

The analyser evaluates corrosion risk based on industry standards:

NACE MR0175 Sour Severity

H₂S Partial Pressure Classification
< 0.3 kPa Non-sour
0.3 – 1.0 kPa Mild sour
1.0 – 10 kPa Moderate sour
> 10 kPa Severe sour

FeS Formation

Iron sulfide (FeS) forms whenever H₂S contacts carbon steel in the presence of water:

SO₂ / H₂SO₄ Corrosion

When SO₂ is present (from H₂S oxidation) and water condenses:

Database Entries

Sulfur species thermodynamic data is stored in:

File Species Data
GibbsReactDatabase.csv H₂S, S, S₂, S₈, SO₂, SO₃, H₂SO₄, FeS, Fe₂O₃, FeS₂ Element counts, Cp, ΔHf, ΔGf, ΔSf
DatabaseGibbsFreeEnergyCoeff.csv S₈, SO₂, SO₃, H₂S, H₂SO₄ Polynomial coefficients for G(T), H(T)
COMP.csv S₈, SO₂ Component properties for EOS
COMPSALT.csv FeS Salt equilibrium data

Demo Notebook

A comprehensive Jupyter notebook demonstrating all capabilities is available at:

examples/sulfurtask/SulfurDepositionAnalysis.ipynb

The notebook covers:

  1. Gas composition setup with H₂S and S₈
  2. Solid phase configuration and TP-solid flash
  3. Sulfur solubility maps (T-P grids)
  4. Sulfur saturation envelope / deposition zone
  5. Gibbs reactor for Claus reactions
  6. Temperature sweep of equilibrium products
  7. O₂/H₂S ratio sensitivity
  8. SulfurDepositionAnalyser with corrosion assessment
  9. Offshore pipeline sulfur dropout simulation
  10. Onshore gas processing risk mapping
  11. H₂S concentration sensitivity analysis
  12. JSON reporting

Source Code

File Description
neqsim.process.equipment.reactor.SulfurDepositionAnalyser Integrated analyser unit operation
neqsim.process.equipment.reactor.IronSulfideWallInventory Stateful wall-scale inventory and exposure history
neqsim.process.equipment.reactor.IronSulfideOxidationSource Wall formation/oxidation source with S8 outlet
neqsim.process.equipment.reactor.GibbsReactor Gibbs energy minimisation reactor
neqsim.process.equipment.reactor.GibbsReactorCO2 Specialised CO₂/acid gas reactor
neqsim.thermodynamicoperations.flashops.SolidFlash1 TP-solid flash implementation