SCOT Process Fundamentals

1. Process Overview and Purpose

The SCOT (Shell Claus Off-gas Treating) process is the most widely used tail gas treating technology in the sulfur recovery industry. Developed by Shell in the 1970s, it treats the tail gas from a Claus sulfur recovery unit (SRU) to achieve overall sulfur recovery efficiencies of 99.8–99.9%, far exceeding what a Claus unit alone can deliver (typically 95–97% for a two-stage unit or 97–98% for a three-stage unit).

The fundamental purpose of the SCOT process is to convert all remaining sulfur species in the Claus tail gas—including SO₂, COS, CS₂, and elemental sulfur vapor—back to H₂S, then selectively absorb the H₂S in an amine contactor. The rich amine is regenerated and the acid gas is recycled back to the Claus unit front end, creating a closed-loop system for sulfur recovery.

Four Main Process Steps

1. Reducing Gas Generation: An inline burner or reducing gas generator combusts fuel gas with a substoichiometric air supply to produce a hot gas stream rich in H₂ and CO. This reducing gas is mixed with the Claus tail gas before entering the reactor.
2. Hydrogenation Reactor: The combined stream passes over a cobalt-molybdenum (CoMo) catalyst bed, where all sulfur species are converted to H₂S through hydrogenation and hydrolysis reactions.
3. Quench and Cooling: The hot reactor effluent is cooled from approximately 650°F down to 100–130°F using a waste heat boiler and/or direct contact quench tower. Water vapor condenses and is removed.
4. Amine Absorption: The cooled gas enters a selective amine absorber (typically MDEA) that removes H₂S while allowing CO₂ to pass through. The treated gas is routed to a thermal oxidizer (incinerator) before discharge to atmosphere.

Tail Gas Treating Technology Comparison

The SCOT process remains the industry standard for facilities that must meet stringent environmental regulations requiring ≥99.9% sulfur recovery or ≤250 ppmv SO₂ in stack emissions. Its high capital cost is justified at larger plants (typically >50 long tons per day of sulfur production) where regulatory compliance demands the highest recovery efficiency.

2. Hydrogenation Reactor

The hydrogenation reactor is the heart of the SCOT process. Its function is to convert all sulfur compounds remaining in the Claus tail gas into a single species—hydrogen sulfide—so that selective amine absorption can achieve efficient removal.

Reducing Gas Generator

Upstream of the reactor, an inline burner operates under substoichiometric (fuel-rich) conditions to produce a gas stream containing 2–5 vol% H₂ and 1–3 vol% CO. This reducing gas provides the hydrogen and carbon monoxide needed for the catalytic hydrogenation and hydrolysis reactions. The burner typically fires natural gas or fuel gas with approximately 60–70% of the theoretical air requirement.

Key Reactions

The following reactions occur over the catalyst bed:

Additionally, elemental sulfur vapor reacts with hydrogen:

Catalyst Types

The most common catalyst is cobalt-molybdenum on alumina support (CoMo/Al₂O₃), which provides excellent activity for both hydrogenation and hydrolysis at operating temperatures. Titania-based catalysts are sometimes used as an alternative, particularly when higher COS conversion is required.

• CoMo/Al₂O₃: Industry standard; good activity for SO₂, COS, and CS₂ conversion; typical bed life of 3–5 years
• Titania (TiO₂): Higher COS hydrolysis activity; used in some specialized applications; more expensive

Operating Conditions

Conversion Efficiency

Under proper operating conditions with fresh catalyst, the hydrogenation reactor achieves the following conversion rates:

• SO₂ conversion: >99.9%
• COS conversion: >99.5%
• CS₂ conversion: >99%
• Elemental sulfur conversion: >99.9%

It is critical to maintain the H₂/CO ratio above 2:1 at the reactor inlet. Insufficient hydrogen leads to incomplete SO₂ conversion, causing sulfur species to break through to the amine absorber. Operators should monitor the reactor outlet for SO₂ breakthrough as an early indicator of catalyst deactivation or inadequate reducing gas supply.

3. Quench and Cooling

After the hydrogenation reactor, the gas stream exits at approximately 550–650°F and must be cooled to 100–130°F before entering the amine absorber. The cooling system also condenses and removes the large quantity of water vapor generated in the reactor.

Cooling Approaches

Two primary cooling configurations are used in SCOT units:

Waste Heat Boiler: The hot reactor effluent passes through a shell-and-tube waste heat boiler that generates low-pressure steam (typically 50–150 psig). This approach recovers thermal energy and improves the overall plant energy efficiency. The gas is cooled to approximately 300–350°F in the waste heat boiler, then further cooled in a downstream quench tower or trim cooler.

Direct Contact Quench Tower: The hot gas is cooled by direct contact with circulating water in a packed or trayed column. Water is continuously circulated through an external cooler (air-cooled or water-cooled). This approach is simpler and less expensive but does not recover energy as steam.

Design Considerations

• Water condensation: The Claus tail gas contains significant water vapor (typically 30–35 vol%), and additional water is produced by the hydrogenation reactions. Most of this moisture condenses during cooling and must be removed before the amine absorber.
• Sulfur dewpoint: Upstream of the reactor, the gas temperature must be maintained above the sulfur dewpoint (approximately 250–300°F depending on sulfur vapor concentration) to prevent liquid sulfur from condensing on the catalyst and deactivating it.
• Sour water management: The condensed water contains dissolved H₂S and must be routed to a sour water stripper for treatment before disposal or reuse.
• Acid gas knock-out drum: A separator vessel downstream of the quench section removes entrained liquid droplets from the gas before it enters the amine absorber. Proper mist elimination is essential to prevent amine foaming and degradation.

Quench System Performance

4. Amine Absorber Design

The amine absorber is the final treatment step in the SCOT process. It selectively removes H₂S from the cooled tail gas using a chemical solvent, producing a treated gas clean enough for discharge to the thermal oxidizer.

Selective Absorption with MDEA

Methyldiethanolamine (MDEA) is the preferred solvent for SCOT absorbers because of its high selectivity for H₂S over CO₂. This selectivity is critical: the Claus tail gas contains substantial CO₂ (typically 20–30 vol%), and if the amine co-absorbs CO₂, the recycled acid gas to the Claus unit becomes diluted with CO₂, reducing Claus burner temperature and sulfur recovery efficiency.

MDEA achieves selectivity because H₂S reacts with the amine via a fast proton-transfer mechanism, while CO₂ absorption requires a slower carbamate formation reaction. By limiting the contact time (using fewer trays or shorter packing height), CO₂ absorption is minimized while H₂S removal remains high.

Design Parameters

• Number of trays: 15–25 (valve or sieve trays); packed columns use 15–25 ft of structured packing
• Lean amine loading: 0.005–0.015 mol H₂S per mol amine (very lean for tail gas service)
• Rich amine loading: Varies with gas composition; typically 0.10–0.25 mol/mol
• Amine concentration: 40–50 wt% MDEA in water
• Operating pressure: Near atmospheric (0–5 psig); low pressure limits absorption driving force

Amine Circulation Rate

The required amine circulation rate is determined by the H₂S mass balance:

Where ΔLoading is the difference between rich and lean amine loadings, and the solution density and molecular weight are determined by the amine concentration.

Amine Performance Comparison

The rich amine from the SCOT absorber is typically regenerated in the main amine regeneration system shared with the upstream acid gas removal unit (AGRU). The regenerated acid gas is recycled to the Claus SRU front end, completing the sulfur recovery loop.

5. Environmental Compliance

The primary driver for installing a SCOT tail gas treating unit is to meet increasingly stringent environmental regulations governing sulfur dioxide emissions from sulfur recovery operations. After the SCOT absorber, the treated tail gas is routed to a thermal oxidizer (incinerator) that converts any remaining H₂S to SO₂ before discharge to atmosphere through a stack.

Regulatory Requirements

Stack Emissions and Thermal Oxidizer

The treated gas leaving the SCOT absorber typically contains 10–100 ppmv H₂S. This gas is routed to a thermal oxidizer (incinerator) operating at 1,200–1,400°F, where all remaining H₂S is oxidized to SO₂. The resulting stack gas SO₂ concentration is typically 100–250 ppmv, well within regulatory limits when the SCOT unit is operating properly.

The thermal oxidizer also destroys any residual COS, CS₂, and mercaptans, converting them to SO₂ and CO₂. Proper combustion chamber design ensures complete oxidation with a minimum residence time of 0.5–1.0 seconds at temperature.

Continuous Emissions Monitoring (CEMS)

Regulatory agencies require continuous emissions monitoring systems (CEMS) on the thermal oxidizer stack to verify ongoing compliance. Key monitored parameters include:

• SO₂ concentration: Primary compliance parameter; measured by UV fluorescence or pulsed fluorescence analyzers
• Stack gas flow rate: Used to calculate total mass emissions (lb/hr or tonnes/day)
• O₂ concentration: Ensures proper incinerator operation and verifies excess air
• Stack temperature: Confirms adequate combustion temperature for complete oxidation

Facilities are also required to report total sulfur recovery efficiency on a rolling average basis (typically 12-hour or 24-hour rolling average). Exceedances must be reported to regulatory agencies and may trigger enforcement actions. Proper SCOT unit operation, including maintaining adequate reducing gas supply, catalyst activity, and amine circulation, is essential for sustained environmental compliance.

References

1. GPSA, Chapter 22 — Sulfur Recovery
2. API Publication 955 — Sulfur Recovery Operations
3. EPA 40 CFR Part 60, Subpart Ja — Standards of Performance for Sulfur Recovery Units
4. Shell Claus Off-gas Treating (SCOT) Process Technical Manual