Field demonstration trials of a novel method for removing hydrogen sulfide (H2S) from natural gas were recently completed in the Permian basin in Texas. The trials demonstrated the ability of the technology, known as ECOTREAT™, to remove H2S from shale gas, and the developing company, Merichem Technologies (Houston, Tex.; www.merichem.com), is now deploying the technology commercially.
ECOTREAT uses a proprietary water-soluble scavenger that reacts with H2S by forming covalent bonds. Each molecule of the organic compound can react with several molecules of H2S and does not affect the product gas. “ECOTREAT reacts with H2S to form bonds that cannot be reversed with low-grade heat or pH changes,” explains Jeff Gomach, an engineer with Merichem. “However, the scavenger material can be regenerated at ambient conditions by exposing it to oxygen in the air.”
Exposure to oxygen oxidizes the bonded sulfide and releases thiosulfate anions (which are both highly soluble in water and acidic), as well as the regenerated scavenger. “We’ve achieved upwards of 10,000 cycles in the laboratory without degradation of the material,” Gomach says. Although details of the scavenger molecule are proprietary, Merichem notes that the non-amine organic compound is considered safe and is free of metals and highly toxic components.
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An interesting aspect of the ECOTREAT technology is that it utilizes produced water from natural-gas extraction as part of the process. Produced water has high (>3.5%) salinity and contains cations such as sodium, calcium and potassium. “The general cationic strength offers an excellent buffer for the thiosulfate ions generated by the oxidation reaction,” Gomach says. Only relatively small amounts of produced water are required, Gomach continues. The produced water absorbs at least 5% of its weight in sulfur, then the water is diluted back into the bulk produced-water stream, he explains.
Especially useful for ECOTREAT are the carbonate salts. The thiosulfate produced in the scavenger-regeneration step causes an increase in pH, which is buffered by the salts in the produced water. Because of its solubility, the thiosulfate displaces the carbonate dissolved in the water to form calcium thiosulfate, Gomach says.
This avoids the problem of calcium carbonate precipitation, which occurs when you bubble air through water, while also avoiding downstream solids-handling. “Most produced water can absorb lots of hydrogen sulfide and reduce scaling at the same time,” Gomach notes.
In the field trials, tests involved feed gases with between 11,000 and 15,000 ppm H 2 S by volume, and the ECOTREAT system demonstrated greater than 99% removal of hydrogen sulfide, Merichem says.