ThioSolv SWAATS Process

The ThioSolv SWAATS process uses sulfur chemistry to capture the ammonia and H2S in the sour water stripper gas (SWSG)

6 NH3 + 4 SO2 + 2 H2S + H2O  3 (NH4)2S2O3
Ammonium Thiosulfate (ATS)
Equation for ATS

Effect of SWAATS on SRU capacity.

Current Claus sulfur capacity100 
Sulfur in SWSG10 
Claus efficiency0.96 
Existing tail gas scrubber? Y=1, N=01 
Claus equivalent amine acid gas sulfur capacity123 
Claus tail gas
SWSG sulfur captured in SWAATS106
Tail gas sulfur captured in SWAATS04
Total sulfur captured in SWAATS1010
Maximum amine acid gas to Claus117119
Total SRU capacity with SWAATS127129
*Capacity Calculations are available if asked for
  • Replace amine-based tail gas treating with ThioSolv scrubbing!
  • Eliminates recycle to Claus, increasing Claus capacity

Conventional tail gas treatment reduces SO2 in the tail gas to H2S, captures the H2S by amine scrubbing, and recycles it to the Claus feed. The amine also captures CO2 from the tail gas and recycles it, too, to Claus. Each ton of sulfur removed from the tail gas displaces about 1.5 tons of capacity for sulfur as H2S in amine acid gas.

  • SWAATS captures the sulfur from the Claus tail gas and removes it as ATS product.

Reduce operating cost of Claus tail gas treatment by 80%

Operating cost
Units / ton S
PowerkWh 600xxx
HeatMillion BTU none60
Solventgal none.6
Fuel gasMillion BTU nonexxx
Catalyst lifeyears 10-203-5

Claus capacity estimate

Capacity in the Claus unit is typically limited by pressure drop, which is primarily a function of gas traffic. This calculation expresses the gas traffic capacity of a unit in equivalent amine acid gas sulfur capacity.

H2S dissolves in water in the processes because ammonia renders it soluble as NH4HS, so the H2S in sour water stripper gas is accompanied by a roughly equimolar amount of ammonia. Because of the additional air required to combust the ammonia, a ton of S in the SWSG produces as much gas traffic in Claus as about 2.5 tons of S in amine acid gas.

The concentration of sulfur species in the tail gas from Claus is about constant, so reducing the gas traffic reduces the recycle of H2S and CO2 from the tail gas unit. Because the amine absorber on the tail gas absorbs so much CO2 along with the H2S, one ton of recycle H2S takes up about 1.5 tons of equivalent amine acid gas capacity.

Adding a SWAATS unit to divert one ton of SWSG sulfur from Claus adds about 3.5 tons of sulfur recovery capacity.

Eliminates the cause of 90% of the operating problems in SRU

In Claus, exactly one third of the feed sulfur must be oxidized to SO2, requiring close control of the supply of oxygen in excess of that consumed by other reactions, including combustion of hydrocarbon and ammonia. Fluctuations in the rate of ammonia in the feed have a disproportionate effect upon the oxygen demand.

Destruction of ammonia requires proper distribution of sulfur between the first chamber in the thermal reactor and the second. Too little H2S in the first chamber allows high temperatures that produce SO3, which deposits in the Claus catalyst. Too much H2S to the first chamber prevents the oxidizing atmosphere required for complete destruction of the ammonia. The residual ammonia then deposits in the catalyst. Low residual oxidizing potential also deposits coke in the reactors. The life of the Claus catalyst is shortened whenever the air rate is either above or below optimum.

SO3 formed in the thermal reactor has a high dew point. In the downstream system, flaws in insulation allow heat loss that can condense sulfuric acid, which is aggressively corrosive to the equipment.

With no ammonia in the feed, the distribution of H2S between chambers is not critical and may be operated with some margin of comfort.

  • Reduce emissions of SO2
  • Revamp existing TGT equipment, use TGT amine capacity for other services
  • Reduce H2S hazards