The Claus Sulfur Recovery Process

Sulfur recovery refers to the conversion of hydrogen sulfide (H2S) to elemental sulfur. Hydrogen sulfide is a by-product of processing natural gas and refining high-sulfur crude oils. There are many sulfur recovery technologies available for different applications, however the most common conversion method used is the Claus process. Approximately 90 to 95 percent of recovered sulfur is produced by the Claus process. The Claus process typically recovers 95 to 98 percent of the hydrogen sulfide feedstream. Only the Claus process will be described here, however please contact SRE for information related to any sulfur recovery process.



  • ‘Sour Gas’ is commonly present in natural gas and oil formations.

  • At low concentrations, it smells like rotten eggs

  • EXTREMELY dangerous as it is heavier than air, impairs our ability to smell/detect with increasing concentration, and exposure to >500 ppm can be fatal!

  • With respect to safety and product specifications, it must be removed from natural gas and petroleum products.

Thus, how do we safely rid ourselves of H2S and make another potentially valuable product?


The Sulfur Recovery Unit (SRU) is based on the modified Claus Process, which is a two-step reaction scheme overall. A portion of the total H2S is burned in the Reaction Furnace to form SO2. Then, the H2S and SO2 react, at an optimal 2:1 ratio, to form elemental Sulfur (Sx) across the Claus Reactors. After each catalytic stage, liquid sulfur is recovered in the Claus Condensers. The remaining unreacted H2S and SO2 then proceed to the next stage, where the equilibrium-limited Claus reaction continues in the presence of Claus catalyst.


In general, gas processing plants and oil refineries are required to recover between 95 and 99.99 percent of the total sulfur introduced to the SRU. A conventional 2-stage SRU, with two Claus Reactors, is typically expected to recover 96+ percent. For a 3-stage SRU, the general expectation is 98+ percent. If necessary, a Tail Gas Cleanup Unit (TGCU) is required to recover the remainder.

Emissions regulations are only becoming more stringent each and every year! That’s where SRE can help out. Our specialized testing services ensure that the operation of your SRU is optimized for long-term performance and reliability purposes!