Pinching Out Sulfur; February 2006; Scientific American Magazine; by JR Minkel; 3 Page(s)
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.