Application: The sour water ammonia to ammonium thiosulfate (SWAATS) process diverts sour water stripper gas (SWSG) from the Claus to convert the ammonia (NH3 ) and hydrogen sulfide (H2S) to ammonium thiosulfate (ATS) solution, thus freeing up Claus capacity and improving operability. SWAATS also can treat Claus tail gas to low emissions at low cost. For plants producing small amounts of sulfur, SWAATS can be designed to capture all of the refinery H2S. The cost of supplemental ammonia (0.54 ton NH3 / ton sulfur) is justified by having only one unit that can process both SWSG and amine-acid gas (AAG) with low emission rates.
High chemical selectivity of the process enables it to economically remove H2S or SO2 from gas mixtures including carbon dioxide. The process has been recognized by EPA as best available control technology (BACT) for control of sulfur oxide (SO2) emissions.
Description: Ammonia and a stoichiometric amount of H2S are selectively absorbed from SWSG. The reject H2S and Claus tail gas, if desired, are selectively oxidized to convert all sulfur species to SO2, which is then scrubbed from the combustion gas in a low pressure drop system to yield a vent gas with very low SOx concentration and no reduced sulfur requiring incineration:
6NH3 + 4SO2 + 2H2S + H2O 3(NH4)2S2O3(ATS)
SWAATS may be controlled to import or export H2S to balance on the NH3 available in the SWSG, or supplemental NH3 may be added to capture additional H2S.
The chemistry and internal circulation rates provide for robust operation, resistant to upsets in the feedstreams. They also prevent or reverse sulfur precipitation by converting any elemental sulfur in the Claus tail gas to thiosulfate. Control is based on simple inline analyzers (pH, combustion gas O2) and requires little attention.
Economics: Each ton of SWSG sulfur diverted from Claus to SWAATS frees up Claus capacity for about three tons of AAG sulfur. Removing NH3 from the Claus feed also greatly improves Claus operability and extends the service life of the catalyst. With SWAATS, the SWSG ammonia is an asset; the value of the process increases as higher severity hydrotreating for ultra-low-sulfur diesel production increases denitrification to over 90%, greatly increasing ammonia production. CAPEX for SWAATS to process SWSG and Claus tail gas is about a quarter of the CAPEX for construction of an equivalent amount of new Claus and tail-gas treating capacity. SWAATS OPEX is typically negative due to credit for export of MP steam, substantially lower than the OPEX for oxygen enrichment or for amine-based tail gas treatment. SWAATS consumes no external chemicals or reducing gas. Licensor removes the ATS produced.
Licensor: ThioSolv, LLC.
