Acid gas treatment upgrade for Qatargas
Upgrading acid gas treatment at the world’s largest LNG facility safeguards the plant’s design capacity and its ability to process increasingly sour feeds.
Gauthier PERDU, Laurent NORMAND and Geraldine LABORIE, Prosernat
Omar AlHatou, Qatargas
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Qatargas is the world’s largest liquefied natural gas (LNG) company. It annually produces and supplies the globe with 42 million tonnes of LNG from its four ventures (Qatargas 1-4). A joint venture of Qatar Petroleum, ExxonMobil, Total, Mitsui and Marubeni, the company has headquarters in Doha, Qatar and maintains its upstream assets in Ras Laffan. Natural gas is supplied to Qatargas’s LNG trains from the massive North Field, by far the world’s largest non-associated gas field. Together with RasGas, the company achieves LNG production of 77 million t/y.
The company was originally formed to operate three LNG trains with a design capacity of 2 million t/y each. Then, Qatargas 1 was upgraded in 2005 through a debottlenecking project, which resulted in a new capacity of 10 million t/y.
Plateau Maintenance Project
The recent completion of the Plateau Maintenance Project (PMP) ensures that the production capacity of Qatargas 1 is maintained at 10 million t/y of LNG for many years to come. This has been accomplished by drilling additional offshore wells, modifying associated offshore facilities, modifying the existing Qatargas 1 Trains 1, 2 and 3, and adding onshore facilities to accommodate expected increases of hydrogen sulphide (H2S) and carbon dioxide (CO2) contents in the feed gas.
With these new gas sources, the H2S and CO2 content would be above the maximum levels that can be handled by the existing facilities to keep production of LNG at maximum flow. Additionally, the content of H2S and CO2 will not be stable over time, but is expected to rise according to the wells in production. Contents can even be adjustable, but they will definitively stay above the H2S and CO2 levels the original plant was designed for.
The existing gas pretreatment units of Qatargas 1 LNG plant consist of three parallel Sulphinol D units, designed by Shell and capable of handling a gas with 0.9% H2S and 2.4% CO2. The acid gas produced by the Sulphinol plant is sent to an existing 634 t/d sulphur production plant. The Sulphinol units accommodate the presence of mercaptans in the feed gas, with the support of polishing units installed downstream.
The additional quantities of H2S are treated in new onshore facilities composed of a new acid gas removal unit (AGRU) and a new sulphur recovery unit (SRU), as part of the project. The new plant is based on a single train concept and is capable of treating over 1.1 billion cubic feet per day (bcfd) of offshore feed gas, located upstream the existing LNG trains. The basic principle is to remove the extra amounts of H2S and CO2 from sourer feed gas, down to levels acceptable to the existing downstream facilities. The acid gas from this new AGRU is sent directly to a new dedicated conversion unit in order to produce sulphur and meet emission standards. Numerous new or upgraded compression and utility units were also included as the onshore project had a major brownfield component, with the scope of work conducted within the existing operating facilities and equipment.
After Chiyoda and Technip had carried out early conceptual studies in 2004 and 2005, followed by the feasibility study, pre-front end engineering design (pre-FEED) and FEED of the facilities from 2006 to 2008, the Technip-Chiyoda joint venture (TCJV) was awarded the engineering procurement and construction (EPC) contract by Qatargas for the onshore work of the project in 2010. Technology selection for the licensors of a new AGRU and sulphur plant was carried out in 2006. Based on its preflash low BTX process, within the AdvAmine technology portfolio, Prosernat was initially appointed to supply the design of the new selective AGRU, while the sulphur plant is based on a technology supplied by another company.
The initial phase had considered two parallel 600 million scfd AGRU + SRU/TGTU trains within a separate plot area dedicated to AGRUs, another dedicated to SRU/TGTUs, and a third for solvent storage tanks. For that phase, there was already a constraint in available space within the existing LNG trains, congested after the 2005 debottlenecking phase. The initial scheme had resulted in stretching the distance between the AGRU and the SRU of each train.
The project eventually went to a fast track review FEED in 2008. Qatargas decided that more space constraints could be solved by a single train, with additional and significant benefits on project costs. Qatargas therefore considered a single train of 1.1 bcfd with integrated AGRU + SRU + TGTU, installed within a single plot area. Optimisation has also been achieved thanks to a patented design proposed by Prosernat, in which all the amine units and sections of the plant (low BTX AGRU + acid gas enrichment + amine section of the TGTU + common regeneration) are fully integrated. The available area allocated for the greenfield part of the project was a single 365 x 150 m zone, already occupied by approximately 100 x 150 m of existing instrumentation and HVAC facilities. One of the most impressive achievements of the project has been the installation of one 1.1 bcfd gas treatment plant plus 880 t/d sulphur, tail gas and incinerator units within 40 000 m2. The storage tank area was left at the initial location.
The project was completed in 2014 and the plants went into operation in September 2014. The feed flow of gas was quickly and successfully raised to the design capacity. The new plant satisfactorily passed performance checks and reception was at the beginning of 2015.
The PMP project also hit records in term of HSE management since the project team, along with their contractors, worked over 42 million man-hours in a very difficult operating environment, achieving world class safety performance, including an exceptional total recordable incident rate (TRIR) of 0.23 for every 200 000 hours worked. More importantly for Qatargas, the project met the objectives of the company in term of its safety ‘incident and injury free’ culture, particularly in the application of the STOP safety observation programme, where a peak workforce of 6500 completed over 665000 observation cards, which helped identify and mitigate safety risks.
Design of the new gas treatment unit
The selected process scheme for the new AGRU is the preflash low BTX (see Figure 1). It includes a low pressure preflash of rich MDEA solvent in a dedicated column installed upstream the MDEA regenerator, with routing of the recovered gas to the TGT absorber. The possibility to recycle a part of the acid gas from the regenerator overhead to the preflash column for an enrichment step was also implemented for leaner H2S feed gas cases (containing 1 mol% H2S) and for the turndown case. The proposed scheme is based on AdvAmine technology. It gives additional flexibility in terms of gas composition range and also efficient operating adjustment. Acid gas feeds the SRU, followed by a hydrogenation and quench section for tail gas. The tail gas from the TGT absorber goes to an incinerator, installed close to the new units.
The plant also takes advantage of the integration of a high pressure absorber and tail gas absorber. Semi-lean solvent from the TGT absorber is recycled to the high pressure absorber after mixing with fresh lean solvent from regeneration.
The scope of Prosernat as AdvAmine process licensor covered the design of the complete amine unit, including the AGRU’s high pressure absorber, the preflash column, the TGT absorber and the solvent regenerator. Due to the highly integrated amine, SRU process configuration, heat and material balances were prepared by iterative calculations between Prosernat and the SRU licensor.
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