Technology

Axens Sulfrex™ is an extractive sweetening process designed to remove sulfur compounds—primarily mercaptans (RSH), hydrogen sulfide (H₂S) and carbonyl sulfide (COS)—from LPG streams. It combines caustic extraction with catalytic oxidation to convert mercaptans into disulfides, yielding a sweetened LPG product that meets stringent sulfur specifications while minimizing equipment and operating costs.

Process Summary

Sulfrex™ sweetens LPG/NGL through a two-step process:

1. Caustic Extraction: H₂S is neutralized; mercaptans are absorbed as mercaptides in NaOH (12-15 wt %)
   
   H₂S + 2 NaOH → Na₂S + 2 H₂O
   RSH + NaOH → RSNa + H₂O
    
2. Catalytic Regeneration: Mercaptides oxidized to disulfides (RSSR) by sulfonated cobalt phthalocyanine catalyst at 38-55°C, regenerating NaOH
   
   2 RSNa + ½ O₂ + H₂O → RSSR + 2 NaOH
    
3. Phase Separation: Disulfides separated in gravity settler; regenerated caustic recycled
   
   Overall reaction: 
   4 RSH + O₂ → RSSR + 2 H₂O

The process is applicable to:

• Olefinic LPG: From FCC, coker, and visbreaking units
• Non-olefinic LPG: From saturated distillation columns and hydrotreating
• Commercial LPG: Meeting specifications for alkylation feedstock and petrochemical applications

Process key features:

• Single-loop caustic balance
• Ambient-temperature operation
• Compact equipment
• >99% mercaptan conversion.

Detailed Process Description

1. Extraction Section

• Feed: LPG/NGL stream at 20-45°C, near atmospheric pressure
• Caustic: 12-15 wt % NaOH (can operate up to 25-40 wt %)
• Column: Counter-current trayed extraction column
• Temperature Control: Above 20°C to avoid caustic entrainment; below 40°C for optimal mercaptan extraction efficiency

2. Oxidation/Regeneration Section

• Catalyst: Sulfonated cobalt phthalocyanine (CoSPc) dissolved in caustic loop at 100 wt ppm
• Conditions: 38-55°C, atmospheric pressure
• Equipment: Air injection system → packed-bed oxidizer → gravity settler
• Mechanism: Mercaptides oxidized via ternary complex formation involving CoSPc, mercaptide anion, and molecular oxygen

3. Temperature Optimization

Based on South Pars Gas Complex operational data:

• Extraction: Optimal temperature 36-37°C for propane, 34-35°C for butane
• Oxidation: Optimal temperature 40-41°C for maximum mercaptide conversion (98.5%)
• Upper Limit: Maximum 55°C to prevent catalyst decomposition and stress corrosion

Process Performance

Economic Efficiency

• Low OPEX: Regenerable caustic reduces chemical costs; ambient-temperature operation eliminates heating utilities
• Compact CAPEX: Single-loop design and atmospheric pressure minimize equipment sizes
• Byproduct Value: Disulfides (RSSR) can be sold or processed further
• Mild Conditions: Ambient temperature and pressure reduce infrastructure requirements

Commercial Experience

Axens comments that "85 Sulfrex™ and Sweetn’G™ units are under license" (as of Jul 2025) but does not provide a breakdown by technology. This figure supersedes earlier literature (e.g., 2011 estimates of “over 40 Sulfrex units”).

Major Projects:

• South Pars Gas Complex (Iran, 2003): Phases 9-10 implementation with optimized extraction temperatures of 36-37°C and oxidation at 40-41°C
• PRefChem (South Korea, 2015): Unsaturated LPG extractive sweetening unit upstream of polymer-grade propylene recovery
• Petroperú Talara (Peru, 2025): 8,000 bpd LPG sweetening unit as part of refinery modernization
• Pertamina Balikpapan (Indonesia, 2020): Sulfrex LPG sulfur removal unit

References

1. Afshar, Ali S. and Sayed Reza Hashemi. Jul, 29, 2011. Role and Effect of Temperature on LPG Sweetening Process. Int. J. Chem. Mat. Biomol. Sci. ISSN 2415-6620
   Vol. 5, No 7, 2011.
2. Axens SULFREX® Process: LPG, Light Naphtha Treating (Sweetening) Process. Naver Blog, 2018.
3. NGL & Condensates sweetening - Axens
4. LPG Sweetening & Olefins Recovery - Axens
5. Jul 16, 2015 - Axens PR - Axens to supply technologies for S-OIL’s residue upgrading capacity expansion project - Axens
6. Nov 9, 2009 - Axens Awarded Modernization Contract for Talara Refinery (Peru) - europétrole
7. Oct 6, 2016 - PR - Pertamina Licenses Axens Technologies for its Balikpapan and Cilacap Refineries in Indonesia - Axens