reliable, continuous operation. Based on the selected oxy- gen supply source, an intermediate mode of operation should be established, which will also serve as the normal operating mode. This approach allows the ASU to run at its minimum load, providing slight OE, enabling rapid ramp-up to full capacity and a smooth transition to oxygen-enriched operation if one train becomes unavailable. Furthermore, the SRU must ensure a stable oxygen supply and incorpo- rate robust safety measures, including interlocks that auto- matically revert the SRU to air-only operation whenever oxygen flow falls below the required level. Typically, the tail gas treatment (TGT) uses a formulated or modified tertiary amine solvent in the TGT absorber. The same solvent should be used for the AGE absorption sec- tion, leading to an overall higher demand of the solvent and a larger ‘common’ solvent regenerator. Some distinctive design and safety considerations are mandated as high-level OE is implemented in the SRU. All oxygen lines must be cleaned and built for oxygen duty. Leak detection, ventilation, and strict maintenance rules are, as expected, essential, as in any conventional plant. Using the hybrid system The plant is designed for two extreme operating conditions: the air-only mode, where both trains operate to meet the total sulphur demand, and the high-level OE mode, where a single sulphur train operates to meet the total sulphur load. However, as a ‘normal’ operating mode, it is expected that both the trains operate with a lower OE level such that the sulphur plant is ‘ready’ to shift to either of the two modes to manage any plant emergency or any planned maintenance conditions. In air mode, two SRU trains run in parallel. Each processes half the load. The AGE unit enriches the feed. The ASU idles at minimum load. Operators switch mode by reducing load on one train, increasing oxygen to the other, and adjusting the AGE unit accordingly. In oxygen mode, one train manages the entire load. The second train is idle or under maintenance. The ASU in the case study was never shut down. It ran at the minimum rate to stay active and poised. This resulted in a small enrichment of air in air mode. This kept the system stable and made transitions smooth. Case study: Hybrid SRU The facility in this case study processes natural gas and sends around 35.6 MMscfd of acid gas to the SRU system. The total sulphur load is 600 MTPD. The feed contains 44% H₂S, 52% CO₂, and trace benzene, toluene, ethylbenzene, and xylenes (BTEX). There is no ammonia. Plant configuration The AGE unit treats the full acid gas flow. Enriched gas goes to a Claus section built as two identical trains. Each train can run on air or enriched oxygen. The TGT unit and incinerator are single-train units sized for full load. No spare AGE unit, TGT unit, or incinerator was provided because their reliability is significantly higher than that of the Claus section. Figure 2 provides a high-level diagram of the over- all sulphur plant.
Age o - gas
Lean amine
Acid gas enrichment
Rich amine
Lean amine
Enriched gas
Regenerator
Rich amine
Thermal oxidiser
Acid gas
SRU
TGTU
Tailgas
Liquid sulphur
Degassing
Oxygen
AGE prepares the feed for oxygen. OE helps extract full value from the enriched feed. When combined, the SRU can do much more work with fewer trains and with much smaller equipment sizes. In practical terms, the sulphur plant can be optimised to operate under normal conditions on air with two trains at half load. When one train needs to be taken off, the other train switches to high-level OE and carries the entire load. This is the core idea of the hybrid or optimised SRU. Figure 1 provides an explanation of the hybrid and opti- mised SRU design. The main aspects are further explained in the following sections. What needs to be designed differently Even though the main aspects of design remain the same as a conventional SRU, a few aspects need special atten- tion when this optimised hybrid approach is implemented. This section lists a few major ones that demand such spe- cial attention, including: • Reaction furnace and burner design. • Sizing of SRU trains. • Oxygen supply. The furnace must handle higher flame temperatures as it processes the enriched feed gas in an oxygen-enriched atmosphere. Burners should be designed to be able to manage high-level OE with the flexibility to operate in two modes: air and oxygen. Materials, refractory, and burner internals must be rated for the worst-case operating sce- nario of the enriched service. Each train must be capable of managing the full sulphur load under oxygen-enriched conditions while also operat- ing reliably at part load in air mode. The unit design rep- resents a careful balance between these two extremes: air-only operation and oxygen-enriched operation. As the case study will demonstrate, the unit requires slightly larger equipment to accommodate air mode yet remains within acceptable limits for oxygen-enriched mode. It is recommended to source oxygen from a dedicated air separation unit (ASU) or stored liquid oxygen to ensure • More solvent of the same type. • Materials and safety in design. Figure 1 Simplified block flow diagram of a hybrid opti - mised SRU
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PTQ Q4 2026
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