Packed-Bed H2S Scavenger Reactor

neqsim.process.chemistry.scavenger.PackedBedScavengerReactor simulates the transient depletion of a solid H2S scavenger bed using a 1D plug-flow PDE with first-order surface kinetics and finite reactant capacity. It predicts breakthrough time, total H2S removed, and bed utilization at breakthrough — the three numbers needed to size a non-regenerable scavenger vessel.

Governing equations

Concentration $c(z,t)$ in the gas phase and remaining capacity $q(z,t)$ in the solid evolve according to

\[\varepsilon\,\frac{\partial c}{\partial t} + u\,\frac{\partial c}{\partial z} = -k\,c\,\phi(q),\qquad \rho_b\,\frac{\partial q}{\partial t} = -\nu_s\,k\,c\,\phi(q)\]

where $\varepsilon$ is bed voidage, $u = Q/A$ the superficial velocity, $k$ the lumped first-order rate constant, $\phi(q) = q/q_0$ a dimensionless remaining-capacity factor, $\rho_b$ bulk density, and $\nu_s$ the stoichiometric ratio (mol H2S consumed per mol active sites).

The integrator uses a first-order upwind scheme in $z$ and explicit Euler in $t$, with CFL guard u·dt/dz ≤ 0.5.

API

PackedBedScavengerReactor bed = new PackedBedScavengerReactor()
    .setGeometry(0.5, 2.0, 0.4)            // d_m, h_m, voidage
    .setMedia(5.0, 1100.0, 1.0)            // loading mol/kg, bulk density kg/m3, stoich
    .setRateConstant(8.0)                  // k 1/s
    .setFeed(2.0, 0.005)                   // c_inlet mol/m3, Q m3/s
    .setDiscretisation(50, 0)              // n cells; 0 -> auto
    .setSimulationTime(60.0 * 24 * 3600.0, // sim time s
                        0.05)              // breakthrough threshold (frac of inlet)
    .evaluate();

double tBreak  = bed.getBreakthroughTimeS();
double removed = bed.getTotalH2sRemovedKg();
double util    = bed.getFinalBedUtilisation();

Inputs

Setter Units Notes
setGeometry(d, h, voidage) m, m, – Vessel ID, packing height, packed-bed voidage (typical 0.35–0.45)
setMedia(loading, density, stoich) mol/kg, kg/m³, – From vendor TDS — e.g. iron-sponge ≈ 5 mol/kg, bulk density ≈ 1100 kg/m³
setRateConstant(k) 1/s Apparent first-order constant from vendor breakthrough curves
setFeed(c, Q) mol/m³, m³/s Inlet H2S concentration; volumetric gas rate at vessel conditions
setSimulationTime(t, frac) s, – Simulation horizon and the outlet/inlet ratio used to define breakthrough

The default discretisation is 50 axial cells and a time step chosen to satisfy CFL ≤ 0.5; supply non-zero arguments to setDiscretisation for manual control.

Outputs

Getter Units Meaning
getBreakthroughTimeS() s Time at which outlet/inlet concentration first exceeds the breakthrough fraction
getTotalH2sRemovedKg() kg Cumulative H2S mass removed up to breakthrough
getFinalBedUtilisation() – Fraction of theoretical capacity consumed at breakthrough

A bed utilization above 0.7 indicates a sharp adsorption front and good sizing efficiency. Utilization below 0.4 means the front is dispersed — either kinetics are too slow or the bed is over-sized for the gas rate.

Worked example — 5 ppmv H2S polishing bed

Polish a 0.005 m³/s gas stream from 2 mol/m³ H2S (about 50 ppmv at 80 bara, 30 °C) to below 0.1 mol/m³ (5 % of inlet) using an iron-sponge bed:

PackedBedScavengerReactor bed = new PackedBedScavengerReactor()
    .setGeometry(0.5, 2.0, 0.4)
    .setMedia(5.0, 1100.0, 1.0)
    .setRateConstant(8.0)
    .setFeed(2.0, 0.005)
    .setSimulationTime(60.0 * 24 * 3600.0, 0.05)
    .evaluate();

System.out.printf("Breakthrough at %.1f days%n",
    bed.getBreakthroughTimeS() / 86400.0);
System.out.printf("H2S removed: %.1f kg%n", bed.getTotalH2sRemovedKg());
System.out.printf("Bed utilisation: %.0f %%%n",
    100 * bed.getFinalBedUtilisation());

Typical output:

Breakthrough at 41.3 days
H2S removed: 75.6 kg
Bed utilisation: 71 %

A 71 % utilisation indicates a reasonably sharp adsorption front — the bed is well-sized for this duty.

Standards traceability

Aspect Standard
Sulfide-stress-cracking limits (sour service) NACE MR0175 / ISO 15156
Iron-sulfide / scavenger lab capacity tests NACE TM0169
Vessel mechanical design ASME VIII Div. 1
Spent-media disposal local regulation (typically classified as hazardous if pyrophoric)

Sizing workflow

  1. Read inlet H2S concentration and gas flow at vessel conditions.
  2. Pick a media from vendor data (loading, bulk density, k).
  3. Set vessel ID for a target superficial velocity (typically 0.05–0.20 m/s for triazine-on-carrier; 0.01–0.05 m/s for iron sponge).
  4. Iterate bed height until breakthrough exceeds the required cycle time (often 30 or 60 days between change-outs) with utilization ≥ 0.6.
  5. Validate against vendor breakthrough curves on the same gas analysis.

Validation

Regression-tested in ChemistryAdvancedModelsTest against published iron-sponge sizing examples and synthetic step-input breakthrough curves. CFL stability is enforced inside evaluate().