Removing sulfur is a stinky proposition for oil refineries. The U.S. and Europe are tightening limits on the sulfur content of gasoline at the same time the crude oil coming out of the ground is becoming increasingly "sour," or sulfurous. Desulfurization technology "has pretty much been wrung out," says Thomas Wellborn, principal consultant of Denver-based Hydrocarbon Exploration and Development. "We need new, innovative technologies." A few young companies with unconventional methods may soon answer that call.
Refineries separate crude oil by boiling point, which is related to density. Most desirable are the lighter (less dense) fractions, which include gasoline and diesel. Heavier fractions contain more sulfur, and too much renders the petroleum useless. Decades ago oil refineries adopted a process called hydrodesulfurization (HDS) to strip sulfur atoms from oil molecules. Sulfurous fractions are mixed with hydrogen and a cobalt-molybdenum catalyst, yielding hydrogen sulfide. Providing hydrogen for the process is expensive, and as oils get more sour, higher pressures and more stable catalysts are needed to break the sulfur bonds. Sourer oils also tend to be heavier, which requires further refining and brings along nitrogen and heavy metals, which foul the catalyst.
Alternative technologies floated in recent years include sulfur-eating bacteria and sulfur-oxidizing reagents, and some experts see room for better-designed catalysts, too. These methods tend to operate on the distilled fractions, but pretreatment of the crude oil itself may be an attractive option. "The better the selectivity upstream, the less need for energy- and capital-intensive separation processing downstream," says Charles Russomanno, a technology transfer manager at the U.S. Department of Energy.
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One pretreatment option may be ultrasound. When blasted with ultrasonic waves, liquids can undergo a process called acoustic cavitation, in which bubbles form and violently implode. SulphCo is developing small, modular ultrasound desulfurization units based on this effect. The company, located in Sparks, Nev., claims its process can both snap loose sulfur atoms and lighten the crude, resulting in 30 to 50 percent less sulfur and about one third more diesel and kerosene. "If SulphCo's process works, it's an elegant solution to part of our problem," Wellborn says.
Refineries would have to integrate such units into their process, combining pretreatment and post-treatment. "It gets into pretty complex equations to balance the two together," says Abe Albert, a refining specialist for Hart Downstream Energy Consulting in St. Louis. On the plus side, SulphCo president Peter Gunnerman says, the treatment would make downstream desulfurization more efficient, and the capital investment would be only 5 percent that of HDS. SulphCo completed a small demonstration unit in South Korea last September.
Another technique is designed to remove all the sulfur from very heavy oils in one shot. Trans Ionics, based in the Woodlands, Tex., is focusing on so-called tar sand, or bitumen, an especially heavy and sour oil of which the Western Hemisphere holds 65 percent of the world's reserves, primarily in Canada and Venezuela. To deal with such heavy oils, refineries would typically thermally "crack" the oil (that is, cook it) and then treat the liquid products. Trans Ionics intends to extract sulfur with elemental sodium, which would then be recycled in a novel sodium sulfide battery. Funded by DOE grants, Trans Ionics has filed for patents on various components, including a novel thin-film electrolyte, says company president Robert Schucker, who expects commercialization by 2012.
The world's refineries were originally tailored to a much less stringently regulated world, notes Douglas Rundell, a BP refining technology project leader in Naperville, Ill. The challenge to the myriad proposals for new methods is to prove they can reliably augment that infrastructure. Refiners are open to a well-argued case, Rundell says: "If somebody comes along and shows that process 'x' works and the economics are compelling, people will go with it."
