A Sweet Solution to Sour Gas

“It’s a way to improve the safety and purity of the gas, but at the same time be more energy efficient.”

Along with affordable and reliable energy, natural gas can harbor an extremely toxic threat—hydrogen sulfide—which must be removed before the gas can be used safely. In a surprising twist, PNNL researchers in search of carbon dioxide capture solutions have discovered a way to remove hydrogen sulfide from natural gas using a water-free solvent that promises to be much more effective and efficient than current techniques.

“We hope that this is a new ‘sweetening’ solvent, so to speak, where we’re able to be more energy efficient and more selective at removing hydrogen sulfide,” said Dave Heldebrant, who is the principal investigator for the project. “It’s a way to improve the safety and purity of the gas, but at the same time be more energy efficient.”

Why it matters

Natural gas contains varying levels of hydrogen sulfide, so-called “sour gas” for its poisonous properties, that can be deadly even at low concentrations and corrosive to pipelines and equipment.  This research delivers a new hybrid chemical/physical solvent for removing hydrogen sulfide that is three times more selective than the industry standard solvent and is much more energy efficient—saving dollars, conserving energy, and potentially opening up natural gas resources worldwide.

Methods

Initially, researchers were investigating methods to remove CO₂ from natural gas using a water-free chemical solvent–significant because chemical-based CO₂ removal from natural gas currently requires millions of pounds of water-based amine solvents per hour that must be boiled to release the CO₂. That requires a lot of energy. Alternative methods include forcing the CO₂ to dissolve into a “physical” solvent using the high pressure of the natural gas as the driving force.

“Processing chemical solvents in water on an industrial scale is extremely energy intensive, and physical solvents generally have lower selectivity,” Heldebrant said. “Our thought was: Can we make this process better by first getting rid of the water and somehow combining the two processes?”

Researchers began with a base and alcohol combination that traditionally does not react with CO₂ at atmospheric pressure, but when they added higher pressures of CO₂, the blend chemically reacted with the CO₂ to make a liquid carrier. Researchers then reduced the pressure and observed the chemically reacted CO₂ being released from the solvent without heat. This first-of-a-kind hybrid approach combining chemical selectivity with pressure-based CO₂ removal delivered a major breakthrough:  demonstrating that CO₂ could indeed be removed from natural gas without water.

Encouraged by the results with CO₂, researchers moved on to other naturally occurring compounds found in natural gas, including hydrogen sulfide. That’s when they really hit the sweet spot, discovering that the water-free solvent was even more effective at hydrogen sulfide capture.

“Nobody had ever seen a chemical solvent react with hydrogen sulfide in the absence of water and still remain a liquid. It had never been published,” Heldebrant said. “We set out and proved that the reaction was occurring and the produced liquid further absorbed more hydrogen sulfide, bringing the total capacity up to 43 percent by weight. You’re getting the same capture that you would have in the water-based solution, but now you don’t need the water.”

See Also

Natural Gas Saves Water and Reduces Drought Vulnerability, Even When Factoring in Water Lost to Hydraulic Fracturing

Analysis under real-world natural gas processing conditions has confirmed the initial results and indicates that the hybrid solvent has excellent potential for hydrogen sulfide removal, absorbing it 10 times more selectively than CO₂ and having a much higher selectivity for hydrogen sulfide than the industry standard solvent. What’s more, this unique hybrid solvent favors hydrogen sulfide removal, while all other solvents favor CO₂.

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