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How PDO will meet its environmental mandate at Yibal Khuff
The combination of an ultra-high sulphur recovery efficiency specification of 99.9% and an ultra-low SO2 emission regulation of 35 mg/Nm3 would be very challenging for any project. This article shows how, by adopting carefully integrated and optimised sulphur recovery and tail-gas-treating technologies, PDO has pushed the technological boundaries to comply with Omani regulations.
PDO is developing new, highly sour gas reservoirs in Oman. The Yibal Khuff integrated development is a brownfield project involving stacked sour gas and oil reservoirs lying below producing sweet reservoirs.
The gas produced will contain about 3% H2S, about 5% CO2 and contaminants including RSH, carbonyl sulphide (COS) and nitrogen, and must be processed to meet an export gas total sulphur specification of <5 ppmv.
The sulphur recovery efficiency (the proportion of sulphur that is recovered from the feed gas) requirement of 99.9% is in line with the regulations in most countries.
However, the SO2 emission requirement of 35 mg/Nm3 is much lower than the World Bank guidelines for SRUs, which stipulate less than 150 mg/Nm3.
The combination of these two regulatory requirements made the design of the gas treatment and sulphur recovery facilities extremely challenging.
It was clear that best-in-class solutions would be necessary for the project to comply with the Omani standards.
Cost would also be an issue, as achieving an ultra-high sulphur recovery efficiency typically sees costs increase dramatically.
A robust integrated solution
The project is using an integrated gas processing approach based on Shell Global Solutions’ portfolio of gas processing and sulphur recovery technologies. Seven process units have been interlinked to develop a robust solution. As shown in Figure 2, the gas treating line-up has several process blocks:
- an AGRU for deep removal of H2S, CO2, RSH and COS;
- molecular sieves to dry the treated gas and remove traces of RSH;
- an aqueous methyl diethanolamine (MDEA) process to enrich the acid gas to improve the operating conditions of the SRU;
- an SRU for converting sulphur components to elemental sulphur;
- a sulphur degasser to reduce the H2S level to <10 ppmv; and
- a CANSOLV unit to quench the flue gas from the incinerator and remove SO2 and sulphur trioxide (SO3) to meet the flue gas specification from the stack (SO2 <35 mg/Nm3).
Technology line-up deployed at Yibal Khuff.
The AGRU removes most of the H2S, CO2, RSH, COS and other sulphur species from the sour gas that is routed to it using Sulfinol™-X technology. This is an enhanced treating solvent containing MDEA, sulfolane and a secondary amine (piperazine), which acts as an accelerator for CO2 and COS removal. In addition to improved sulphur species removal, Sulfinol-X offers considerably less solvent degradation and residue build-up compared with other Sulfinol processes, and can help to reduce the energy consumption of the process compared with other amine solvents.
A molecular sieve unit downstream of the AGRU dehydrates the treated gas, which also contains small amounts of RSH, COS and H2S. The molecular sieve unit has three large vessels containing a regenerative zeolite molecular sieve and an alumina adsorbent (beds) in parallel. At any time, two of the parallel beds are in adsorption mode, while the other bed is in regeneration mode. Downstream of the adsorbers, the treated,dry natural gas flows to a hydrocarbon dew-pointing unit, which is outside the scope of the licensed package, before being compressed for export to sales.
The remaining acid gas stream from the AGRU is routed to the acid gas enrichment unit (AGEU), which enriches the acid gas to the SRU to enable stable operation of the SRU. Aqueous MDEA is contacted countercurrently with the feed to selectively remove H2S over CO2 from the feed gas to the AGEU, thereby producing an H2S-rich acid gas stream. The treated gas from the AGEU contains low concentrations of acid components, including RSH, which are then routed to the incinerator unit.
The enriched acid gas from the AGEU is fed to two 65% SRUs, which are a two-stage modified Claus design. The aim of the SRUs is to:
- convert sulphur-containing components into liquid elemental sulphur at a temperature of 150°C so that it is suitable for degassing; and
- convert hydrocarbons in the feed to CO2 and water.
These aims have to be met with minimum pressure drop through the unit to maintain a sufficiently low back pressure at the incinerator. This is necessary to enable other process gas to freely flow from the downstream CANSOLV regenerator overhead to the incinerator.
The SRU removes approximately 95% of the sulphur in the feed gas as elemental sulphur. The sulphur product is brought to a specification of <10-ppm H2S in the sulphur degassing unit: a common above-ground degassing vessel for both SRUs. The degasser has stripping air and sweep air supplied by dedicated air blowers. The liquid elemental sulphur from the degasser vessel is then pumped to the sulphur storage tank.
The residual sulphur compounds not recovered in the SRU are incinerated to convert them to SO2. This occurs in a thermal incinerator operating at a sufficiently high temperature to ensure the destruction of the remaining aromatic hydrocarbon compounds, ethylbenzene and xylene, and to completely oxidise the CO to CO2. Destruction of the aromatic compounds governs the operating temperature of the incinerator.
The following streams are routed to the incinerator:
- tail gas from the SRUs;
- flash gas from the AGRU;
- treated gas from the AGEU, which contains most of the CO2
- and aromatic hydrocarbons; and
- vent gas from the liquid sulphur degasser.
To meet the sulphur recovery efficiency and SO2 emission regulations, the CANSOLV SO2 scrubbing process treats the tail gas from the incinerator. Before entering the CANSOLV unit, the incinerator tail gas must be scrubbed to remove dust and reduce SO3 and sulphuric acid mist to a tolerable level. This pre‑scrubbing prevents excessive degradation of the absorbent used in the CANSOLV SO2 scrubbing system and serves to cool the flue gas to an acceptable temperature for the absorption process.
The CANSOLV unit is a four-stage scrubber system comprising:
- a hot gas jet scrubber quench;
- a sub-cooling packed tower;
- a particulate separator (venturi); and
- a high-pressure system to enhance acid mist removal.
The conditioned tail gas leaves the pre-scrubber system at a maximum temperature of 37°C and containing low levels of contaminants, as per the requirements of the downstream CANSOLV process. The gas exiting the pre-scrubbing system is discharged from the droplet separator and ducted to the bottom of the CANSOLV SO2 scrubbing system absorber. This has two distinct sections:a lower section for SO2 absorption and an upper section for caustic polishing.
The process line-up of the CANSOLV unit is similar to any conventional regenerable amine unit. The key difference is that the CANSOLV unit has an online absorbent (ion exchange) purification unit to treat a slipstream of the amine solvent. The ion exchange technology removes non-regenerable heat stable salts from the solvent. The unit is sized to operate for eight cycles a day under normal operating conditions.
Setting new standards
By working with Shell Global Solutions and taking an integrated approach to the design of the sulphur recovery process line-up, PDO has managed the interfaces between the different units and optimised the configuration to enhance energy efficiency and thereby deliver maximum value. The design that the two organisations have developed through close technical collaboration is set to achieve ultra-high sulphur recovery levels and adhere to some of the most stringent emissions standards that the industry has seen. PDO is pushing technological boundaries and, as a result, can set new standards.
Senior Process Engineer, Petroleum Development Oman
|Owner||Petroleum Development Oman (owned by the Government of Oman, 60%; RoyalDutch Shell, 34%; Total, 4%; and Partex, 2%)|
|Solution type||Brownfield project|
|Gas processing and sulphur recovery facilities to be installed||AGRU with Shell Sulfinol-X solvent, an AGEU, CANSOLV technology and a Shell sulphur degassing unit|
|Feed||Sour gas containing 3% H2S, 5% CO2, RSH, COS and nitrogen|
|Product slate||Export gas with <5-ppmv sulphur|
Sulfinol is a Shell trademark