This process is used to treat gas streams containing high concentrations of H2S. The chemistry of the units involves partial oxidation of hydrogen sulfide to sulfur dioxide and the catalyticaily promoted reaction of H2S and SO2 to produce elemental sulfur. The reactions are staged and are as follows:
Figure 7-7 shows a simplified process flow diagram of the Claus® process. The first stage of the process converts H2S to sulfur dioxide and
to sulfur by burning the acid-gas stream with air in the reaction furnace, This stage provides SO2 for the next catalytic phase of the reaction. Multiple catalytic stages are provided to achieve a more complete conversion of the H2S. Condensors are provided after each stage to condense the sulfur vapor and separate it from the main stream. Conversion efficiencies of 94-95% can be attained with two catalytic stages, while up to 97% conversion can be attained with three catalytic stages. The effluent gas is incinerated or sent to another treating unit for “tail-gas treating” before it is exhausted to atmosphere.
There are many processes used in tail-gas treating. The Sulfreen® and the Cold Bed Absorption® (CBA) processes use two parallel reactors in a cycle, where one reactor operates below the sulfur dew point to absorb the sulfur while the second is regenerated with heat to recover molten sulfur. Even though sulfur recoveries with the additional reactors are normally 99-99.5% of the inlet stream to the Claus unit, incineration of the outlet gas may still be required.
The SCOTT® process uses an arnine to remove the H2S. The acid gas off the amine still is recycled back to the Claus plant. Other types of processes oxidize the sulfur compounds to SO2 and then convert the SO2 to a secondary product such as ammonium thiosulfate, a fertilizer. These plants can remove more than 99.5% of the sulfur in the inlet stream to the Claus plant and may eliminate the need for incineration. Costs of achieving this removal are high.