The tail gas treating unit converts the small amount of sulfur compounds (< 5%), which were not converted in the sulfur recovery unit (SRU), into hydrogen sulfide (H2S) and recycles it back to the SRU for additional processing. Sketches of the process are shown below. The SRU tail gas is heated and sent to the catalytic reactor where essentially all of the sulfur compounds are converted into H2S. The gas from the catalytic reactor is cooled in the waste heat exchanger and the quench tower. Excess water is removed in the cooling process and is sent to the sour water stripper.
The cooled gas is the sent to the absorber column, where amine removes the H2S and some of the CO2 in the gas stream. The remaining gas (vent gas) is sent to the thermal oxidizing unit. The rich amine from the absorber is heated in the lean/rich exchanger and fed to the regenerator column. Steam, generated in the reboiler, heats the amine and removes the hydrogen sulfide (H2S) and carbon dioxide (CO2) from the amine. The lean amine from the stripper is cooled, in the lean/rich exchanger and the lean solvent cooler and returned to the absorber. The H2S and CO2 removed from the amine is cooled (and water removed) in the overhead condenser and recycled to the sulfur recovery unit for additional processing into sulfur.
Design optimization can evaluate cost based on TIC or NPV for:
- Air cooling versus cooling water exchangers
- Shell & tube versus plate & frame feed-bottoms exchanger
- Regeneration trays versus reboiler heat input