Thermal vs Catalytic Claus Process Fundamentals

1. Claus Process Overview

The Claus process is the dominant industrial method for converting hydrogen sulfide (H₂S) from acid gas streams into elemental sulfur. Developed by Carl Friedrich Claus in 1883 and subsequently modified into the modern two-stage configuration, the process remains the cornerstone of sulfur recovery in gas processing, refining, and petrochemical operations worldwide. Every sour gas processing facility that produces a concentrated acid gas stream from amine treating or physical solvent regeneration relies on some variant of the Claus process for sulfur disposal and recovery.

The Claus process operates in two distinct stages: a high-temperature thermal stage (reaction furnace) followed by multiple low-temperature catalytic stages (converter beds). The overall chemistry converts H₂S to elemental sulfur and water through a controlled partial oxidation and catalytic reaction sequence.

Overall Claus Reaction Chemistry

The modified Claus process involves two sequential reactions. In the thermal stage, one-third of the incoming H₂S is combusted with a controlled amount of air to produce SO₂:

In both the thermal and catalytic stages, the remaining H₂S reacts with the generated SO₂ in the Claus reaction:

The combined overall reaction is:

The Claus reaction is equilibrium-limited, which is why multiple catalytic stages with intermediate sulfur condensation are required. Removing the sulfur product between stages shifts the equilibrium forward, driving conversion higher. This is the fundamental thermodynamic principle underlying the multi-stage Claus design.

Recovery Efficiency by Configuration

Feed Gas Requirements

The composition of the acid gas feed has a profound effect on Claus unit design and operation. The primary feed quality considerations are:

• H₂S concentration: The Claus reaction furnace requires a minimum acid gas H₂S concentration of approximately 50 mol% for a stable flame without supplemental fuel. Below 50% H₂S, acid gas enrichment, combustion air preheat, or fuel gas co-firing may be needed. Lean acid gas feeds (15–50% H₂S) require split-flow or other modified configurations
• Hydrocarbon content: Hydrocarbons in the acid gas (from amine co-absorption or solvent carryover) burn in the reaction furnace and increase soot and COS/CS₂ formation. Total hydrocarbon content should generally be below 2–4 mol% to avoid excessive carbon deposition on catalyst
• Ammonia (NH₃): Sour water stripper acid gas may contain 15–40 mol% ammonia. Ammonia must be completely destroyed in the reaction furnace to prevent ammonium salt plugging of downstream equipment. This requires higher furnace temperatures (2300–2800°F) and longer residence times
• CO₂ content: Carbon dioxide is inert in the Claus process but acts as a diluent, reducing flame temperature and requiring larger equipment. High CO₂ feeds (>40 mol%) may require oxygen enrichment or acid gas enrichment

2. Thermal Stage (Reaction Furnace)

The thermal stage is the heart of the Claus unit, where the initial combustion and high-temperature Claus reactions occur in a refractory-lined reaction furnace. This stage accomplishes several critical functions simultaneously: partial combustion of H₂S to establish the required 2:1 H₂S-to-SO₂ ratio, thermal destruction of contaminants (ammonia, hydrocarbons, BTEX), conversion of 60–70% of the total sulfur in a straight-through configuration, and generation of high-pressure steam through the downstream waste heat boiler.

Combustion and Ratio Control

The reaction furnace operates by burning approximately one-third of the incoming H₂S with a precisely controlled amount of air. The combustion air flow is regulated to produce a 2:1 molar ratio of H₂S to SO₂ in the furnace effluent, which is the stoichiometric requirement of the Claus reaction. This ratio is the single most critical operating parameter in the entire Claus process.

Deviations from the 2:1 ratio directly reduce sulfur recovery. An excess of H₂S (ratio > 2.0) results in unreacted H₂S passing through to the tail gas. An excess of SO₂ (ratio < 2.0) leaves unreacted SO₂ in the tail gas. Both conditions reduce overall conversion and increase emissions. Modern Claus units use tail gas analyzers (UV or photometric H₂S/SO₂ analyzers) to continuously trim combustion air flow and maintain the ratio within ±0.05 of the 2.0 target.

Furnace Temperature

The reaction furnace operates at extremely high temperatures, with the actual temperature depending on the acid gas composition:

The minimum furnace temperature of approximately 1800°F is necessary for adequate reaction kinetics and contaminant destruction. Below this temperature, the combustion reactions become sluggish, COS and CS₂ hydrolysis is incomplete, and ammonia destruction is inadequate. Plants processing ammonia-bearing acid gas from sour water strippers must maintain furnace temperatures above 2300°F to achieve 99.9%+ ammonia destruction and prevent downstream ammonium salt plugging.

Residence Time

The reaction furnace must provide adequate gas residence time for the combustion and high-temperature Claus reactions to approach equilibrium. Typical residence time requirements are:

• Standard acid gas (no NH₃): 0.5–1.5 seconds at flame temperature
• NH₃-bearing acid gas: 1.0–2.0 seconds minimum, often with a dedicated ammonia destruction zone upstream of the main furnace chamber
• High-hydrocarbon feed: 1.0–1.5 seconds to ensure complete hydrocarbon combustion and minimize soot formation

Residence time is calculated based on the volumetric gas flow at furnace conditions (temperature and pressure) divided by the furnace internal volume. The furnace is typically a horizontal cylindrical vessel with length-to-diameter ratios of 2.5:1 to 4:1.

Burner Design

The reaction furnace burner is a specialized piece of equipment designed for acid gas combustion service. Unlike conventional fuel-fired burners, the Claus burner must handle a low-calorific-value fuel (acid gas), maintain stable combustion over a wide turndown range (typically 3:1 to 5:1), and achieve thorough mixing of acid gas with combustion air to establish the correct H₂S/SO₂ ratio. Key burner design features include:

• Air/gas mixing: Intensive mixing at the burner throat ensures uniform combustion and prevents localized oxygen-rich or fuel-rich zones that would impair ratio control and increase side-product formation
• Flame stability: Swirl vanes, bluff-body stabilizers, or pilot flames maintain a stable flame across the operating range. Flame failure detection (UV scanners) is a safety requirement per API 556
• Multiple nozzles: Plants processing both amine acid gas and sour water stripper gas may use separate burner nozzles or a two-zone furnace to optimize ammonia destruction while maintaining proper H₂S/SO₂ ratio
• Oxygen enrichment capability: For lean acid gas applications, the burner may include provisions for oxygen injection to increase flame temperature and process capacity

Refractory Lining

The reaction furnace is lined with multiple layers of high-temperature refractory to protect the steel shell from the extreme operating temperatures. A typical refractory system consists of:

• Hot face layer: High-alumina (85–95% Al₂O₃) castable or firebrick capable of withstanding continuous service at 2800°F+ and resistant to sulfidation attack. Typical thickness: 4.5–6 inches
• Insulating layer: Lightweight insulating castable or insulating firebrick that reduces heat flux to the steel shell. Typical thickness: 4–6 inches
• Shell temperature: The refractory system is designed to maintain the steel shell temperature below 450–550°F to prevent sulfide corrosion and maintain shell integrity

Refractory maintenance is one of the major cost drivers for Claus unit turnarounds. Hot face refractory life is typically 5–10 years depending on operating severity, thermal cycling, and the quality of the original installation. Refractory failure modes include spalling from thermal shock during startups and shutdowns, sulfidation attack from reducing atmospheres, and mechanical erosion from high-velocity combustion products.

Side Reactions: COS and CS₂ Formation

In addition to the desired combustion and Claus reactions, several side reactions occur in the reaction furnace that produce carbonyl sulfide (COS) and carbon disulfide (CS₂):

COS and CS₂ represent sulfur that is not readily recovered in the standard alumina catalyst beds. At typical catalytic converter temperatures, COS hydrolysis is relatively slow over alumina catalyst. Titania (TiO₂) catalyst in the first converter bed provides superior COS and CS₂ hydrolysis, converting these species back to H₂S and CO₂ for subsequent Claus conversion. In well-designed units with proper catalyst selection, COS and CS₂ typically contribute less than 0.5% to total sulfur losses.

Waste Heat Boiler

The waste heat boiler (WHB) is a critical piece of equipment that cools the hot furnace effluent gas from reaction furnace temperature (1800–2800°F) down to approximately 550–650°F while generating high-pressure or medium-pressure steam. The WHB serves the dual purpose of heat recovery and gas conditioning for the downstream sulfur condenser and catalytic stages.

The WHB tube sheet at the hot end operates in one of the most demanding thermal environments in the process industry. The tube sheet face exposed to the furnace effluent can see temperatures exceeding 2500°F, while the back side is cooled by boiling water at 400–600°F. This extreme temperature differential (1000–2000°F across the tube sheet thickness) creates severe thermal stresses that are the most common cause of WHB failure. Refractory ferrules or metallic shields at the tube sheet face protect the tube-to-tubesheet welds from direct exposure to the hottest gas.

3. Catalytic Stage (Converter Beds)

The catalytic stage of the Claus process consists of multiple fixed-bed catalytic reactors (converters) arranged in series, each followed by a sulfur condenser. The purpose of the catalytic stage is to continue the Claus reaction at temperatures well below those in the reaction furnace, where equilibrium conversion is higher. By removing sulfur between stages (in the condensers) and reacting at progressively lower temperatures, each successive catalytic stage drives the overall conversion closer to completion.

Claus Reaction Thermodynamics

The Claus reaction is exothermic and equilibrium-limited. Lower temperatures favor higher equilibrium conversion, which is why multiple stages operating at decreasing temperatures are used:

As the reaction proceeds and heat is released, the catalyst bed temperature rises, which reduces the equilibrium conversion. This is the fundamental trade-off in catalytic converter design: the inlet temperature must be high enough to initiate the reaction on the catalyst, but the temperature rise across the bed limits the conversion achievable in a single stage.

Catalyst Types

Two primary catalyst materials are used in Claus converters:

Activated alumina is the workhorse catalyst used in the majority of Claus converter beds worldwide. It provides excellent Claus reaction activity at relatively low cost. However, alumina has limited effectiveness for hydrolyzing COS and CS₂, particularly at the lower operating temperatures of second and third converter beds.

Titania catalyst provides significantly better COS and CS₂ hydrolysis activity compared to alumina. The hydrolysis reactions convert these species back to H₂S and CO₂:

For this reason, many operators specify titania catalyst in the first converter bed (which operates at the highest temperature and sees the highest COS/CS₂ concentrations) and alumina in the second and third beds. This combination provides cost-effective COS/CS₂ management while maintaining high overall Claus conversion.

Operating Temperatures

Each catalytic converter operates within a specific temperature window determined by the balance between reaction kinetics (which require higher temperatures), equilibrium conversion (which favors lower temperatures), and sulfur dewpoint avoidance (which sets a minimum temperature). The operating temperatures decrease from the first to the last converter:

The inlet temperature to each converter must remain above the sulfur dewpoint of the process gas (typically 250–280°F depending on sulfur vapor concentration) to prevent liquid sulfur condensation on the catalyst. Liquid sulfur on the catalyst causes deactivation by blocking active sites and, if severe, can cause permanent damage through pore blockage and structural degradation. A minimum approach of 30–50°F above the sulfur dewpoint is maintained as a safety margin.

Bed Sizing and Space Velocity

Catalytic converter beds are sized based on the gas hourly space velocity (GHSV), which is the ratio of the volumetric gas flow (at standard conditions) to the catalyst volume:

Where Q_std is the gas volumetric flow at standard conditions (scf/hr) and V_catalyst is the total catalyst volume (ft³). Typical design GHSV values for Claus converters are:

Catalyst bed depth is typically 3–5 feet, with the bed supported on a grid or screen with an inert ceramic ball support layer. The converter vessel diameter is determined by the allowable superficial gas velocity through the bed, typically 3–6 ft/s to maintain acceptable pressure drop while ensuring adequate gas distribution.

Sulfur Condensers

A sulfur condenser is placed after the waste heat boiler and after each catalytic converter to cool the process gas and condense elemental sulfur vapor. Removing sulfur between catalytic stages shifts the Claus reaction equilibrium forward, which is essential for achieving high overall recovery. Sulfur condensers are shell-and-tube heat exchangers with the process gas on the shell side and boiler feedwater or low-pressure steam on the tube side.

The condenser outlet temperature must be maintained above the sulfur solidification point (approximately 246°F) to prevent solid sulfur plugging. Liquid sulfur drains from each condenser through drain legs (seal legs) into the sulfur collection system. The seal legs must maintain a liquid seal to prevent process gas from short-circuiting into the sulfur pit.

Reheater Types

Before entering each catalytic converter, the process gas must be reheated from the upstream condenser outlet temperature (300–350°F) to the required converter inlet temperature (380–620°F). Several reheating methods are used in the industry:

• Hot gas bypass: A portion of the hot gas from the WHB outlet is bypassed around the condenser and mixed with the cold condenser outlet gas. This is the simplest and most common method for the first reheater. The disadvantage is that the bypassed gas carries unseparated sulfur vapor, which reduces the benefit of intermediate sulfur removal
• Inline burner (acid gas burner): A small burner fires a portion of the acid gas feed directly into the process gas stream between the condenser and converter. This method provides precise temperature control but introduces additional combustion products (SO₂, H₂O) and can upset the H₂S/SO₂ ratio if not properly controlled
• Indirect reheater (steam or hot oil): A shell-and-tube heat exchanger uses HP steam, hot oil, or another heating medium to reheat the process gas without direct contact. This method provides clean heating without affecting process chemistry and is preferred for the second and third reheaters where precise ratio control is most critical

4. Sulfur Handling and Storage

The liquid sulfur produced in the Claus condensers must be collected, degassed, stored, and formed into a solid product suitable for transportation and sale. Sulfur handling may seem straightforward compared to the process chemistry of the Claus unit itself, but it involves significant operational and safety challenges related to H₂S dissolved in liquid sulfur, sulfur solidification in piping, and the environmental impact of sulfur storage emissions.

Liquid Sulfur Collection

Liquid sulfur from each condenser flows by gravity through drain legs (seal legs) into a common sulfur collection system. The drain legs serve a dual purpose: they transport liquid sulfur out of the condensers and they maintain a liquid seal that prevents process gas from flowing backward into the sulfur pit. Key design features of the sulfur drain system include:

• Seal leg height: Sufficient liquid head to overcome the pressure differential between the process gas side and the sulfur pit (typically atmospheric). Seal legs are sized for 1.5–2.0 times the maximum expected pressure differential
• Steam jacketing: All sulfur drain legs, rundown lines, and collection headers are steam-jacketed or steam-traced to maintain the sulfur in liquid form (melting point approximately 246°F) and prevent solidification and plugging
• Sulfur pit: An underground or ground-level concrete pit that collects liquid sulfur from all condensers. The pit is heated (typically with steam coils embedded in the concrete) to maintain sulfur temperature above 260–280°F

Degassing

Freshly produced liquid sulfur from Claus condensers contains significant quantities of dissolved H₂S, typically 200–400 ppm by weight. This dissolved H₂S exists as both free dissolved gas and as chemically bound hydrogen polysulfide (H₂S_x). The dissolved H₂S presents serious safety and quality concerns:

• Safety hazard: H₂S released from liquid sulfur during storage, handling, forming, and transportation is toxic at concentrations above 10 ppm in air. Multiple fatalities have occurred in the sulfur industry from H₂S release during loading and forming operations
• Product quality: Many sulfur buyers specify maximum H₂S content below 10–30 ppm to reduce handling hazards during downstream use
• Explosive atmospheres: H₂S released in enclosed spaces above liquid sulfur can form flammable/explosive mixtures with air. Vapor space management in sulfur pits and tanks is a critical safety consideration

Degassing methods reduce the dissolved H₂S content to acceptable levels (typically < 10 ppm):

Sulfur Forming

Degassed liquid sulfur must be solidified (formed) into a product suitable for storage, handling, and transportation. The primary forming methods are:

• Prilling: Liquid sulfur is sprayed from the top of a prilling tower, and droplets solidify as they fall through an ascending air stream. Produces uniform spherical pellets (1–4 mm diameter). High capacity (up to 3000+ t/d per tower) but requires significant plot space and air emissions management
• Pastillation (rotoforming): Liquid sulfur is deposited as small drops onto a moving steel belt cooled from below with water. Produces uniform hemispherical pastilles. Lower emissions than prilling, moderate capacity (500–1500 t/d per unit), increasingly popular for new installations
• Flaking: Liquid sulfur is applied as a thin film to a water-cooled rotating drum, and the solidified film is scraped off as flakes. Produces irregular flat flakes. Lower capital cost but generates more dust than prilling or pastillation
• Block pouring: Liquid sulfur is poured into large open-air blocks and allowed to solidify over days to weeks. Lowest capital cost but requires large storage area, creates dust during reclamation, and H₂S emissions are a concern during the solidification period

Environmental Considerations

Sulfur storage and handling operations generate several environmental concerns that must be managed:

• H₂S emissions from sulfur storage: Even degassed sulfur continues to release small amounts of H₂S. Enclosed or covered sulfur storage with vent gas collection and treatment (thermal oxidation or scrubbing) may be required to meet ambient air quality standards
• SO₂ from sulfur fires: Molten sulfur is combustible, and sulfur fires produce large quantities of SO₂. Prevention measures include proper grounding to prevent static discharge, inert gas blanketing of enclosed spaces, and temperature control to prevent auto-ignition (auto-ignition temperature approximately 450°F for liquid sulfur)
• Sulfur dust: Solid sulfur handling generates dust that is both a respiratory hazard and a combustion/explosion risk. Dust collection and suppression systems are required at forming, conveying, and loading operations
• Stormwater runoff: Sulfur storage pads exposed to rainfall produce acidic runoff (from dissolved SO₂ and sulfuric acid formation) that must be collected and neutralized before discharge

5. Tail Gas Treatment

The tail gas leaving the final sulfur condenser of a Claus unit still contains 1–5% of the original sulfur feed in the form of H₂S, SO₂, COS, CS₂, and sulfur vapor. While a well-designed three-stage Claus unit achieves 97–99% sulfur recovery, increasingly stringent environmental regulations—particularly EPA NSPS Subpart Ja for petroleum refineries and state-level regulations for gas processing plants—often require overall sulfur recovery of 99.5% or higher. Tail gas treatment (TGT) processes bridge the gap between Claus-only recovery and the required regulatory limits.

SCOT Process (Shell Claus Off-gas Treating)

The SCOT process is the most widely used tail gas treating technology worldwide. It achieves overall sulfur recovery of 99.8–99.9+% by converting all residual sulfur species in the Claus tail gas back to H₂S and then selectively absorbing the H₂S for recycle to the Claus unit. The SCOT process consists of three main sections:

• Hydrogenation reactor (reducing reactor): The tail gas is heated to 500–600°F and passed over a cobalt-molybdenum (CoMo) catalyst with added hydrogen (H₂) or a mixture of H₂ and CO. All sulfur species (SO₂, COS, CS₂, elemental sulfur vapor) are converted to H₂S through hydrogenation and hydrolysis reactions:
  SO₂ + 3 H₂ → H₂S + 2 H₂O
  COS + H₂O → H₂S + CO₂
  CS₂ + 2 H₂O → 2 H₂S + CO₂
  S_n + n H₂ → n H₂S
• Quench tower (direct contact cooler): The hot reactor effluent (approximately 500–600°F) is cooled to 95–130°F by direct contact with circulating water. This step condenses water vapor and cools the gas for efficient amine absorption. Excess water from the quench loop is routed to the sour water stripper
• Amine absorber: The cooled gas (now containing H₂S as the only significant sulfur compound) is contacted with a selective amine solvent (typically MDEA or a proprietary selective formulation) that preferentially absorbs H₂S while rejecting CO₂. The treated gas (containing < 10–250 ppmv H₂S depending on design) exits the absorber top and is routed to the tail gas incinerator. The rich amine is regenerated in a stripper column, and the regenerated acid gas (concentrated H₂S) is recycled to the Claus reaction furnace

Alternative Tail Gas Processes

While SCOT is the industry standard, several alternative tail gas treating processes have been developed for specific applications:

Sub-Dewpoint Processes (CBA, MCRC)

Sub-dewpoint processes operate the final Claus catalytic stage below the sulfur dewpoint, deliberately allowing sulfur to condense on the catalyst. This shifts the equilibrium strongly toward higher conversion by continuously removing the sulfur product. The trade-off is that the sulfur-laden catalyst must be periodically regenerated by heating to re-vaporize the deposited sulfur. Multiple reactors operate in a cyclic sequence: while one or two beds are in reaction mode (sub-dewpoint), another bed is being regenerated with hot gas.

CBA and MCRC processes can boost Claus recovery from 97% to 99–99.5% without requiring the hydrogenation reactor and amine system of the SCOT process. They are most attractive as a retrofit option for existing Claus units that need moderate improvement in recovery without the cost and complexity of a full SCOT unit.

Tail Gas Incinerator

Regardless of whether a tail gas treating process is installed, all Claus sulfur recovery units include a tail gas incinerator. The incinerator thermally oxidizes all remaining sulfur species (H₂S, COS, CS₂, sulfur vapor, and any residual mercaptans) to SO₂ before discharge through the stack. The incinerator operates at 1000–1400°F with natural gas or fuel gas as supplemental fuel, and must achieve complete conversion of H₂S to SO₂ (destruction efficiency > 99.9%).

The incinerator stack is the single point source of SO₂ emissions from the sulfur recovery complex. SO₂ emissions are directly proportional to the unrecovered sulfur from the Claus/TGT system. Stack continuous emission monitoring systems (CEMS) for SO₂ are typically required by regulatory permits.

Stack Emissions and Monitoring

The tail gas incinerator stack must comply with applicable emission limits, which vary by jurisdiction and facility type:

Recent Regulatory Trends

Regulatory requirements for sulfur recovery have become progressively more stringent over the past several decades, and this trend continues. Key developments include:

• Higher recovery requirements: Many jurisdictions now require 99.9%+ overall sulfur recovery for new facilities, effectively mandating tail gas treatment for any Claus unit of significant size
• Continuous monitoring: Stack CEMS for SO₂ and total reduced sulfur (TRS) are increasingly required in place of periodic stack testing
• Fugitive emissions: Regulations are expanding beyond stack emissions to include fugitive emissions from sulfur pits, seal legs, and sulfur handling equipment. Enclosed sulfur storage with vent gas treatment may be required
• Startup/shutdown emissions: Some regulations now limit emissions during SRU startup and shutdown events, which historically were exempt. This drives the adoption of startup procedures that minimize sulfur compound releases
• Best Available Control Technology (BACT): For facilities subject to Prevention of Significant Deterioration (PSD) review, BACT for sulfur recovery units is increasingly defined as Claus + SCOT (or equivalent) achieving 99.9%+ recovery

References

1. GPSA, Chapter 22 — Sulfur Recovery
2. API Standard 560 — Fired Heaters for General Refinery Service
3. API Standard 556 — Instrumentation, Control, and Protective Systems for Gas Fired Heaters
4. EPA NSPS 40 CFR 60, Subpart Ja — Standards of Performance for Petroleum Refineries
5. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1