Sulfur Recovery

Claus Process

Hit H₂S:SO₂ = 2:1, manage furnace temperature, and stack catalytic stages to reach 99%+ sulfur recovery.

Launch calculator Tail gas options

Stoichiometry

2:1 H₂S:SO₂

Air flow trims to hold the ratio through all stages.

Recovery

96–98%

Three catalytic stages; add TGTU for 99.5%+.

Temperature

1,800–2,500°F

Keep furnace hot enough for NH₃ destruction and good SO₂ formation.

Contents

Use this guide to:

  • Set air demand and H₂S:SO₂ control strategy.
  • Estimate stage recovery and need for TGTU.
  • Capture furnace/catalyst temperature targets.

1. Reaction Chemistry

The Claus process converts hydrogen sulfide (H₂S) to elemental sulfur through a series of thermal and catalytic reactions. It is the dominant technology for sulfur recovery worldwide.

Overall Reaction

Net Claus reaction: 2 H₂S + O₂ → 2 S + 2 H₂O ΔH = -222 kJ/mol This occurs in two steps: Step 1 (Thermal): H₂S + 1.5 O₂ → SO₂ + H₂O Step 2 (Catalytic): 2 H₂S + SO₂ ⇌ 3 S + 2 H₂O

Thermal Stage Reactions

In the reaction furnace (1,800-2,500°F), multiple reactions occur:

Primary combustion: H₂S + 1.5 O₂ → SO₂ + H₂O (exothermic) Partial combustion: 2 H₂S + O₂ → 2 S + 2 H₂O (exothermic) Thermal Claus: 2 H₂S + SO₂ ⇌ 3 S + 2 H₂O (equilibrium limited) Side reactions: CH₄ + 2 O₂ → CO₂ + 2 H₂O (hydrocarbon combustion) 2 H₂S → 2 H₂ + S₂ (H₂S dissociation)

Stoichiometry

Parameter Value Notes
H₂S:SO₂ ratio 2:1 molar Critical for efficiency
Air requirement 2.5 mol O₂/mol H₂S burned To produce SO₂
Sulfur yield 1.5 mol S/mol H₂S Theoretical maximum
Sulfur per H₂S 0.94 lb S/lb H₂S Mass basis
Key control: The H₂S:SO₂ ratio of 2:1 is critical. Deviation reduces sulfur recovery. Air flow to the furnace is controlled to maintain this ratio in the tail gas.

Air trim

Analyzer control

Use ratio control on tail gas H₂S/SO₂ to hold 2:1.

COS/CS₂

Hydrolysis

Catalyst beds hydrolyze COS/CS₂; maintain proper reheat.

NH₃ destruction

>1,950°F

Keep furnace hot to crack ammonia and avoid fouling.

2. Process Description

A typical Claus unit consists of a thermal stage followed by two or three catalytic stages with sulfur condensers between stages.

Process Stages

Stage Temperature Function S Recovery
Reaction furnace 1,800-2,500°F Combustion, thermal Claus 60-70%
Waste heat boiler 600-1,000°F Heat recovery, steam generation —
Condenser 1 300-350°F Sulfur condensation —
Catalyst bed 1 450-620°F Catalytic Claus reaction +20-25%
Condenser 2 300-350°F Sulfur condensation —
Catalyst bed 2 400-480°F Catalytic Claus reaction +5-7%
Catalyst bed 3 380-430°F Final conversion +2-3%

Key Equipment

Feed Gas Considerations

H₂S Content Classification Process Modification
>50% Rich Standard straight-through
25-50% Medium May need acid gas enrichment
15-25% Lean Split-flow or oxygen enrichment
<15% Very lean Special processes required
01

Thermal stage. Burn ~1/3 H₂S to SO₂; keep furnace hot for NH₃ destruction.

02

Condense & reheat. Drop sulfur, reheat to catalyst inlet temp to avoid sulfur freezing.

03

Catalytic beds. Sequential beds drive conversion; lower temps per stage, trim SO₂ ratio.

3. Recovery Efficiency

Claus unit efficiency depends on the number of catalytic stages, operating conditions, and feed gas composition.

Stage-wise Recovery

Cumulative recovery: 2 catalyst stages: 94-96% 3 catalyst stages: 96-98% With tail gas treatment: 99.5-99.9% Per-stage conversion: Each catalytic stage converts 65-75% of remaining H₂S + SO₂ Equilibrium limitation: The Claus reaction is equilibrium-limited. Lower temperature favors sulfur formation but slower kinetics.

Efficiency Calculation

Sulfur recovery efficiency: η = (S_produced / S_inlet) × 100% Where: S_produced = Elemental sulfur recovered (lb/hr) S_inlet = Sulfur in feed as H₂S (lb/hr) S_inlet calculation: S_inlet = Q × y_H₂S × (32/34) × ρ Where: Q = Acid gas flow (scfh) y_H₂S = H₂S mole fraction 32/34 = MW ratio (S/H₂S)

Example: Recovery Calculation

Given: 10 MMSCFD acid gas, 85% H₂S, 2-stage Claus (95% recovery)

H₂S flow = 10 × 0.85 = 8.5 MMSCFD H₂S mass = 8.5 × 10⁶ / 379.5 × 34 = 761,500 lb/day Sulfur in feed = 761,500 × (32/34) = 716,700 lb/day Sulfur recovered = 716,700 × 0.95 = 680,900 lb/day = 340 long tons/day (LTD)

Factors Affecting Recovery

Factor Effect Mitigation
H₂S:SO₂ ratio drift Reduces conversion Ratio analyzer control
Catalyst deactivation Lower conversion Regular replacement
COS/CS₂ formation Sulfur loss Hydrolysis catalyst
Sulfur carryover Catalyst fouling Proper condenser operation
Ammonia in feed Catalyst plugging High furnace temperature

4. Design Parameters

Claus unit design must ensure adequate residence time, proper temperatures, and correct stoichiometry.

Reaction Furnace Design

Parameter Typical Range Notes
Residence time 0.5-2.0 seconds Longer for NH₃ destruction
Temperature 1,800-2,500°F Higher for lean gas
Thermal recovery 60-70% Depends on feed H₂S
Heat release ~250 BTU/scf H₂S For furnace sizing

Catalyst Bed Design

Space velocity (GHSV): GHSV = Q_gas / V_catalyst (hr⁻¹) Typical values: First bed: 800-1,200 hr⁻¹ Second bed: 600-1,000 hr⁻¹ Third bed: 400-800 hr⁻¹ Bed depth: Typical: 3-5 feet Pressure drop: 2-5 psi per bed

Catalyst Types

Type Application Operating Temp
Activated alumina Standard Claus 400-650°F
Titania (TiO₂) COS/CS₂ hydrolysis 450-600°F
Claus + hydrolysis First bed (combined) 500-620°F
Selectox Direct oxidation 350-450°F

Air Demand Calculation

Theoretical air requirement: Air = H₂S × (1/3) × (100/21) = 1.59 × H₂S (molar) For 1/3 of H₂S burned to SO₂: O₂ = (1/3) × 1.5 × H₂S = 0.5 × H₂S Actual air (with excess): Air_actual = Air_theoretical × (1 + excess) Typical excess: 0-5%

⚠ Temperature limits: Furnace temperature below 1,700°F risks incomplete combustion and ammonia breakthrough. Above 2,800°F risks refractory damage and NOₓ formation.

Residence

0.5–2 s

Furnace residence time target for conversion and NH₃ destruction.

Bed temps

450→380°F

Lower each catalytic stage to favor equilibrium without sulfur freezing.

Air excess

0–5%

Trim air to ratio; limit excess to avoid SO₂ breakthrough.

5. Tail Gas Treatment

Environmental regulations often require tail gas treatment to achieve overall sulfur recovery of 99.5% or higher.

Tail Gas Composition

Component Typical Range Notes
H₂S 0.5-1.5% Unreacted
SO₂ 0.25-0.75% Should be ~½ of H₂S
S vapor 0.1-0.5% Uncondensed
COS + CS₂ 0.01-0.1% Side products
CO₂ 2-10% From feed gas
H₂O 25-35% Reaction product
N₂ 50-65% From combustion air

Treatment Technologies

Environmental Limits

Regulation Requirement Typical Standard
EPA NSPS Subpart J >20 LTD capacity 99.8% recovery or 250 ppm SO₂
EPA NSPS Subpart J 2-20 LTD 99% recovery
State/local Varies May be more stringent

References

Select

Pick TGTU type. SCOT for high recovery, SUPERCLAUS for simpler oxidation, redox for small streams.

Control

Hold ratio. Tail gas analyzer feedback keeps H₂S:SO₂ near 2:1 upstream.

Verify

Check emissions. Confirm stack SO₂ meets NSPS/state limits; adjust recovery or TGTU severity.

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