Sulfur Recovery Unit Troubleshooting Guide

A practical guide for Claus SRU operators and engineers. Focused on the symptom-to-root-cause patterns we actually see in the field, not a textbook rehash of the chemistry.

Know Your Baseline

Before troubleshooting, confirm your unit's "known good" state. For a modified Claus with two catalytic stages plus tail gas treatment, typical steady-state indicators:

Symptom-to-Cause Decision Matrix

Symptom: Low Recovery / High Tail Gas SO2

1. Check H2S:SO2 ratio. If off 2:1, the air demand (AGR) controller is misadjusted. Re-zero the tail gas analyzer first — drift here causes more false calls than any other single issue.
2. Look for BTEX breakthrough. Aromatics from upstream ADIP/Sulfinol/MDEA solvent slippage deactivate Claus catalyst. Signs: sudden bed outlet temperature rise (burning coke off), black catalyst on inspection. Remedy: upstream solvent cleanup.
3. Check ammonia in acid gas. NH3 from SWS creates ammonium salts in condensers. Must burn completely in reaction furnace (2400 degF minimum). If RF is < 2200 degF, NH3 breakthrough is likely.
4. Catalyst aging. After 3-4 years, bed 1 activity drops. Deactivation typically 3-5% per year. If bed 1 dT (temperature rise across bed) has fallen > 30% vs new, plan changeout.
5. Condenser fouling. Liquid sulfur carryover from cold condensers indicates plugging. Check delta-P across each condenser.

Symptom: Converter Bed Plugging / High dP

Usually liquid sulfur condensation on the catalyst. Root causes:

• Inlet temperature too low. Dew point of sulfur vapor at bed 1 inlet is ~400 degF at typical concentration. Inlet must be > 450 degF with margin.
• Reheater problem. Gas reheater (inline burner or HX) may be underperforming. Check reheater outlet temp.
• Catalyst swelling/crushing. Some aluminas physically degrade after extended hot/wet service. Replace with a harder alumina (CR, CRS-31, S-201) on next turnaround.
• Sulfation of catalyst. Deep bed sulfation ("aging") forms aluminum sulfate, permanent deactivation. Visible as blue-tinted catalyst on unload.

Symptom: Rich / Lean H2S:SO2 Oscillation

Your trim air controller is hunting. Before tuning, check:

• Tail gas analyzer response time (should be < 60 s). UV analyzers drift — verify against certified gas.
• Feed-forward from acid gas composition. If ABS (acid balance system) is not receiving fresh acid gas compositions, ratio control lags.
• Cascade controller tuning. Typical: PI on AGR, no derivative. Reset time > 2x transport delay.

Symptom: Burner Flame Flutter / Refractory Damage

• Low turndown. Most RF burners turn down 3:1 at best. Below that, flame becomes unstable and refractory cools below dewpoint.
• Wrong O2 target. Excess O2 in RF is a real problem: it oxidizes the SS tube sheet, increases CO2 formation, and lowers furnace temperature.
• Water droplets. Amine carryover from upstream absorber drops furnace temp rapidly. Verify amine KO drum level/coalescer.
• Refractory spalling. Cycle from shutdown to startup without a proper heat-up ramp (50 degF/hr max) destroys refractory. Use a heat-up gas burner, not the acid gas.

Symptom: Sulfur Pit Issues

• Dark sulfur. High H2S content in liquid sulfur (> 10 ppmw) — indicates insufficient degassing. Check degassing system (Shell, D'GASS, or equivalent).
• Pit fire. H2S buildup in vapor space above 3-4 vol% is explosive. Check sweep steam/air ejector flow and hatch integrity.
• Solidification in lines. Steam jacket temp too low. Target 270-290 degF on jacket (above sulfur freeze point 239 degF, below viscosity spike at 320 degF).

The "Viscosity Spike" Trap

Liquid sulfur has a viscosity minimum around 280 degF and a rapid viscosity increase above 320 degF as S8 rings polymerize. Never heat a sulfur line above 310 degF. Operators attempting to thaw a blocked line with high-pressure steam can convert liquid sulfur to a plug that will require hydroblasting out.

Tail Gas Treatment Unit (TGTU) Interactions

If you have a SCOT/TGTU downstream, its performance depends on getting everything converted to H2S upstream. Symptoms and root causes:

• High SO2 into TGTU hydrogenation reactor. Incomplete hydrogenation — check H2 supply, reactor inlet temp (target 570-620 degF), and catalyst activity (Co-Mo aging).
• Amine foaming in TGTU absorber. Solid sulfur or oxygenates from upstream carryover. Filter, and verify upstream quench water quality.
• COS/CS2 slippage. Only a titania catalyst (Axens CRS-31, Porocel TK) converts COS/CS2 hydrolytically. If using alumina alone, expect 10-20% slip.

Routine Checks

Daily: tail gas H2S/SO2, condenser outlet temps, sulfur rundown rate, unit recovery by mass balance.

Weekly: converter bed dT trend, reheater outlet temps, degassing H2S spot check, amine reclaimer sample (upstream).

Monthly: catalyst bed sample (dP and activity), TGTU hydrogenation reactor temp profile, air preheater approach.

Turnaround: full catalyst changeout on bed 1 if activity < 70% of new, condenser tube inspection, RF refractory survey.

References

• API Publication 932-B — Sulfur Recovery Operating Guide
• Brimstone Sulfur Symposium Papers (annual) — industry best references
• Kohl & Nielsen — Gas Purification, 5th ed., Chapter 8
• Paskall & Sames — Sulfur Recovery, Western Research Institute
• Goar, B. — Claus Process Operating Practices, Sulfur Experts Monograph