Revamping sulphur recovery units with high-level oxygen enrichment
Two case studies demonstrate how high-level oxygen enrichment can address two distinct issues that existing SRUs are likely to face in the future of refining
Debopam Chaudhuri, Theresa Flood, Denny Li, and Jyoti Bist Fluor
A ll crude oil contains some sulphur compounds. These compounds range from the simplest form, namely hydrogen sulphide (H₂S) found in natural gas, through simple mercaptans compounds (R-S-H) to very complex molecules. These sulphur compounds pass through to the various distillate products in various degrees of concentration. If allowed to remain in the distillates, they have adverse effects on the environment, are corrosive to equipment, and deactivate high-value catalysts of down- stream processes. Therefore, sulphur treatment and recov- ery become inevitable. The sulphur compounds present in the crude get dis- placed from the hydrocarbon phase as H₂S. This H₂S is either captured by an amine solvent in the amine treatment units or dissolved in the process condensate. The amine cir- culates in the refinery in a closed circuit, capturing the H₂S from the hydrocarbon phase in various amine treaters. This ‘rich’ amine (rich in H₂S) is then regenerated in the amine regeneration unit (ARU) to liberate the H₂S gas and regen - erate lean amine (lean in H₂S) to be circulated back to the various amine treaters. Similarly, the sour water generated from the various pro - cess units in the refinery is treated in sour water stripping unit(s) (SWSU) to liberate the H₂S and other gases. The combined stream of the H₂S-rich gases from the ARU and SWSU is then sent to the sulphur recovery unit (SRU). Figure 1 demonstrates how sulphur travels through the various process units and hydrocarbon in a refinery, start - ing from the crude oil until recovered as elemental sulphur. The H₂S in acid gas streams is converted to elemental sulphur utilising the modified Claus process. The typical feed gas streams to an SRU, originating from the amine regeneration and the SWSUs, contain varying amounts of H₂S as the sulphur source. The process involves burn - ing the acid feed gas with a sub-stoichiometric amount of air, typically just enough to combust approximately a third of the H₂S to SO₂. The SO₂ formed then reacts with the unconverted H₂S to produce elemental sulphur. The main reactions involved are:
Reaction 1 is highly exothermic, while Reaction 2 is endothermic, with a net effect of exothermicity for the net conversion described by Reaction 3. Another reaction of importance and of special interest to this discussion is the destruction of ammonia in the presence of air:
2 NH₃ + O₂ → 3 /₂ N₂ + 3 H₂O
The Claus reaction furnace needs to be hot enough to ensure near-complete destruction of ammonia to nitrogen, with the target typically 1,260°C (2,300°F) for a standard Claus. Traditional sulphur plants employing the modified Claus process utilise air as the source of oxygen in the thermal reaction furnace. The major drawback of using air as the oxy - gen source is the large amount of nitrogen that comes with the oxygen supply. The nitrogen from air adds to the hydrau- lic load of the unit, thus ‘eating up’ capacity. The nitrogen also adds thermal inertia, lowering furnace temperatures and increasing the duties of sulphur condensers and reheaters. Oxygen enrichment has been implemented in many sul - phur plants to debottleneck the process and reclaim SRU capacity. Oxygen enrichment is the process where part or all of the oxygen needed for the modified Claus reac - tion is replaced by pure oxygen. Conventionally, oxygen
Light ends treatment
Amine regen. unit
Light & mid distillate treatment
Sulphur recovery unit
Crude distillation
Heavies treatment
Sour water stripper unit
Sulphur
Sulphur in oil Sulphur in amine
Sulphur in water Sulphur in gas
(1) (2) (3)
H₂S + 3 /₂ O2 → SO2+H2O + Heat 2 H₂S + SO₂ ↔ 3 S + 2 H₂O – Heat 3 H₂S + 3/₂ O₂ → 3 S + 3 H₂O + Heat
Figure 1 Simplified sulphur map of refinery
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PTQ Q1 2025
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