Rigorous sulfur-recovery process simulation

NeqSim’s sulfur-recovery package provides a complete steady-state Claus flowsheet plus the stateful core needed for sub-dew-point bed cycling. Public thermochemistry and correlations provide the base model; vendor catalyst data or plant measurements should be used to fit the explicit calibration factors before equipment rating or guarantee work.

Six implementation layers

  1. Audited S8 and COS identity and formation properties; ideal-gas S2-S8 equilibrium, sulfur vapor pressure, and dew point.
  2. Opt-in fully coupled thermodynamic residual evaluation and catalyst-pellet effectiveness in PlugFlowReactor; its historical frozen-property and activity-only defaults are preserved.
  3. A finite-rate Claus reaction furnace and cell-discretized reactive waste-heat boiler.
  4. Mixed-allotrope condensers and finite-rate alumina, titania, or mixed catalytic beds, including COS and CS2 hydrolysis.
  5. An integrated SRU with combustion-air control, sulfur products, material-balance closure, KPIs, and a fluent builder.
  6. Tail-gas hydrogenation/absorption/recycle, incineration, oxygen enrichment, split flow, and a stateful sub-dew-point bed.

The primary package is neqsim.process.equipment.reactor.sulfurrecovery.

Process topology and convergence

flowchart TD
  A[Acid gas and oxidant] --> B[Reaction furnace]
  B --> C[Reactive WHB]
  C --> D[Thermal condenser]
  D --> E[Catalytic stages and condensers]
  E --> F[TGTU recycle and incinerator]

Every catalytic converter is followed by a sulfur condenser. The air controller converges the final Claus tail gas to the configured H2S/SO2 ratio, normally 2.0. When tail-gas treatment is enabled, absorbed H2S is relaxed to a fixed-point recycle and air demand is recalculated on the combined fresh and recycle feed.

Thermodynamic basis

SulfurThermodynamics treats sulfur vapor as an ideal reacting mixture of S2 through S8. For each species i,

\[y_i P = P^\circ \exp\left(\frac{n_i\lambda-G_i^\circ(T)}{RT}\right),\]

where n_i is its sulfur-atom count and lambda is the sulfur atomic chemical potential. A scalar root solve enforces total sulfur vapor pressure. This gives allotrope fractions and mean sulfur atoms per vapor molecule without treating all sulfur vapor as S8.

Sulfur vapor pressure is interpolated in ln(P) versus 1/T over 312.15-1308.15 K. Calls outside the reviewed range fail explicitly. The process EOS continues to carry elemental sulfur as an S8 equivalent for compatibility; sulfur equipment converts between that atom inventory and the S2-S8 vapor distribution.

Reaction models

The reduced furnace mechanism balances H2S oxidation, the thermal Claus reaction, ammonia and methane destruction, H2S dissociation, and COS/CS2 formation. Elemental Gibbs minimization is also available through ClausReactionFurnace.ModelMode. The reduced mechanism is the default because residence time, contaminants, and calibration factors remain explicit.

The reactive waste-heat boiler advances thermal Claus recombination and COS/CS2 hydrolysis through finite cells at local temperature. Catalytic converters use Arrhenius kinetics, GHSV contact time, catalyst activity, pellet effectiveness, an equilibrium approach, and catalyst-specific hydrolysis activity.

Basic use

SystemInterface fluid = new SystemSrkEos(313.15, 2.0);
fluid.addComponent("H2S", 10.0);
fluid.addComponent("CO2", 2.0);
fluid.addComponent("methane", 0.05);
fluid.addComponent("water", 1.0);
fluid.setMixingRule("classic");

Stream acidGas = new Stream("acid gas", fluid);
acidGas.run();

SulfurRecoveryUnit sru = new SulfurRecoveryProcessBuilder("SRU", acidGas)
    .configuration(SulfurRecoveryUnit.Configuration.STRAIGHT_THROUGH)
    .catalyticStages(2)
    .tailGasTreatment(true)
    .incinerator(true)
    .build();
sru.run();

SulfurRecoveryPerformance result = sru.getPerformance();
double recovery = result.getOverallSulfurRecoveryPercent();
double sulfurKgPerHour = result.getRecoveredSulfurKgPerHour();
double stackSO2KgPerHour = result.getStackSO2KgPerHour();

The builder supports STRAIGHT_THROUGH, SPLIT_FLOW, OXYGEN_ENRICHED, and SUB_DEW_POINT. The built unit exposes temperatures, catalyst types, oxidant composition, split fraction, control tolerance, recycle settings, all equipment, sulfur products, tail gas, and recycle streams.

For higher-fidelity generic PFR calculations, enable the new modes explicitly:

pfr.setThermodynamicCoupling(PlugFlowReactor.ThermodynamicCoupling.FULLY_COUPLED);
pfr.setCatalystMolecularDiffusivity(1.0e-5);
pfr.setCatalystEffectivenessEnabled(true);

Compatibility

The sulfur-recovery API is additive: no existing public class, constructor, method, or enum value is removed, so existing source and compiled callers remain API-compatible. Compatibility with previously serialized object graphs is outside this change’s scope. PlugFlowReactor retains its historical frozen-property thermodynamic coupling and activity-only catalyst behavior unless the new options are enabled explicitly.

Calculations that already use S8, sulfur(S8), or COS can produce different reaction or energy results because their incorrect CAS identities and gas-phase formation properties have been corrected. This is an intentional data-quality correction, not numerical result compatibility.

Exact numerical reproducibility is also not claimed for existing non-isothermal PlugFlowReactor cases. Its heat-capacity request now uses the supported J/molK unit instead of falling through to the fixed 30 J/(mol K) estimate, and axial-profile reaction rates are reported on their declared volumetric basis with catalyst activity included. These are defect corrections; the public API and opt-in defaults remain unchanged.

Results and validation

SulfurRecoveryPerformance reports fresh-feed and recovered sulfur, Claus and overall recovery, sulfur-atom closure, tail H2S/SO2, oxygen demand, furnace temperature, stack SO2, and air/recycle iteration counts and convergence flags.

Validate in dependency order:

  1. Component identity and reference thermochemistry.
  2. Sulfur vapor pressure, normal boiling point, dew point, and S2-S8 distribution.
  3. Reaction stoichiometry and element closure for every unit.
  4. Furnace temperature and NH3/CH4 destruction against a measured or published case.
  5. WHB outlet COS/CS2 and steam production.
  6. Per-bed Claus conversion, hydrolysis, temperature rise, and sulfur condensation.
  7. Final ratio control, total recovery, TGTU conversion/removal, and stack SO2.

Recommended acceptance criteria for a calibrated steady-state case are sulfur closure below 1e-5 relative and controlled H2S/SO2 within 0.02. Assign tolerances to each measured temperature and composition from its source uncertainty. Do not fit all kinetic multipliers to total recovery alone because that produces a non-identifiable model.

Current limits

This offers commercial-style topology, convergence, balances, and calibration hooks. Commercial rating parity still requires appropriate licensed or plant-specific kinetic and equipment data.

Public references