Eni started producing oil reserves from the Aquila reservoir in the Adriatic Sea after the discovery in 1981. As primary production decreased, a decision was made to start enhanced recovery with artificial gas lift. Located in deep waters (815 meters) and 46 km off the southern coast of Italy, a floating production, storage and offloading vessel (FPSO) was needed.

As part of the production process scheme, the vessel needed to generate steam and electricity from the produced associated gas. Equipment was installed to remove hydrogen sulfide (H2S) from a combination of the oil stabilizer overhead vapors, the sour water stripper overhead vapors and, if required, a slip stream of the produced gas. The treated gas must meet an H2S specification of 100 parts per million vapor (ppmv) to provide stripping gas for the sour water stripper and meet post combustion emissions specifications from the steam boiler and turbine generator.

The anticipated sulfur removal requirement was 2.3 metric tons per day (MTPD). Eni requested a process that would be economical while minimizing environmental impact, operator attention and logistical support. Following a detailed evaluation, the liquid redox process from Merichem Company (Merichem) was selected for the Aquila Phase II Project and installed as part of the topsides on the FPSO Firenze.

After a five-year run (2013-2018), the FPSO Firenze has stopped production due to low oil production. This case study looks at the decision to use LO-CAT® H2S removal technology (a liquid reduction-oxidation process), the cost of operation, and the unit availability over its' lifetime.


As the energy industry searches for reserves in ever-deeper formations, there appears to be more sulfur with which to contend. Deep oil reservoirs in the Caspian Sea, Gulf of Mexico and offshore Brazil show significant amounts of H2S in the produced well fluids. H2S at low levels (just 100 ppmv) is a life-threatening, corrosive and flammable gas. Exploration and production of fields with significant H2S levels must be done under very strict safety precautions. Ultimately, disposal of the H2S must be designed into the production facilities.

Several H2S removal technologies are available, including non-regenerative liquid scavengers (triazine-based), non-regenerative solid-bed absorbents and the regenerative liquid reduction-oxidation (redox) process. These technologies remove sulfur from associated gas streams and do not release them to the environment. The non-regenerative technologies are often referred to as scavengers.

Process Evaluation
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