Technology

Name

Axens Sulfrex

Owner

/ Axens SA

Brand

Sulfrex™

Process

Gas processing

Type

Acid Gas Removal

Available

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:

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
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
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

Metric	Value
Mercaptan conversio	>99% (mercaptide to RSSR)
Total sulfur removal	84-97.7% (depending on conditions)
Operating temperature (extraction)	20-45°C
Operating temperature (oxidation)	38-55°C
Pressure	Near atmospheric
Caustic concentration	12-15 wt % (up to 25-40 wt %)

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