In sulfur recovery units for petroleum refining, natural gas purification and coal chemical plants, conventional two-stage and three-stage Claus processes are limited by thermodynamic equilibrium, with a maximum total sulfur recovery rate of only 95%~97%. The tail gas contains residual SO₂, COS, CS₂ and gaseous elemental sulfur, which fail to meet the current national emission standards for air pollutants in the petrochemical industry. SCOT (Shell Claus Off-gas Treating), the Shell Claus off-gas treatment process, is the most widely used hydrogenation-based deep tail gas treatment technology in the refining and chemical industry. It operates in tandem with the main Claus unit to realize deep removal and resource recovery of sulfides.
1. Process Development and Application Background
Developed by Shell Netherlands, the SCOT process was industrialized in 1973 and designed as a post-treatment process matched with Claus sulfur recovery. Following the implementation of ultra-low emission standards in China's petrochemical sector, direct discharge of tail gas from conventional Claus units leads to severe emission excess. As a result, SCOT has become a standard tail gas treatment facility for refineries, LNG terminals and coal-to-gas projects. It raises the total sulfur recovery rate of the combined unit to above 99.9%, complying with national ultra-low SO₂ emission control requirements.
2. Process Reaction Mechanism
The whole process consists of three core sections: hydrogenation reduction, solvent absorption and solvent regeneration.
Tail gas flows into the hydrogenation reactor, where reduction and hydrolysis reactions take place over Co-Mo sulfur-resistant hydrogenation catalysts at 270~310 °C under slight positive pressure. SO₂ and S₈ are hydrogenated into H₂S, while organic sulfur species COS and CS₂ are hydrolyzed into H₂S.
Cooled hydrogenated tail gas enters the absorber, where MDEA (methyldiethanolamine) selective solvent absorbs H₂S. Purified flue gas is incinerated before reaching emission standards. Rich amine solvent is sent to the regenerator for stripping, yielding high-concentration H₂S which is recycled back to the Claus furnace for sulfur production.
3. Technical Advantages
Compared with selective oxidation and liquid-phase oxidation tail gas processes, SCOT boasts outstanding operational stability:
①High conversion rate of organic sulfur, suitable for working conditions with fluctuating high and low sulfur loads;
②High selectivity of amine solvent, low co-absorption loss of CO₂ and controllable unit energy consumption;
③Hydrogenation catalysts feature strong resistance to heavy metal and sulfide poisoning, with a service life of 5 to 8 years;
④Closed-loop process without secondary hazardous waste, and by-product sulfur reaches premium industrial grade.
4. Industrial Application Value
Modified SCOT processes are universally equipped for sulfur recovery units in large domestic integrated refining and chemical complexes, balancing environmental compliance and sulfur resource recovery. This technology breaks the thermodynamic bottleneck of traditional Claus processes, reduces the operation and maintenance difficulty of tail gas treatment units, and aligns with the low-carbon and clean production goals of the petrochemical industry. It serves as a standardized and mainstream post-treatment technology in the full sulfur recovery industrial chain.