1883

Claus Process

Carl Friedrich Claus (1827–1900)

Natural gas, as it comes out of the ground, is a messy substance. One of its most significant drawbacks is that it almost always contains hydrogen sulfide, which is very toxic and bears its highly objectionable rotten-egg smell. The same problem shows up as crude oil is refined—the crude deposits, characterized as “sour,” have many sulfur-containing compounds. These are stripped out in a process that produces still more hydrogen sulfide, which has to be removed as well. This is done by the Claus process, invented and patented in 1883 by German-English chemist Carl Friedrich Claus. It was originally intended to recover sulfur from calcium sulfide, a waste product from manufacturing plants producing soda ash, but it was adapted to work for other sulfur-containing compounds as well. Refined in the 1930s, it is still in use worldwide.

That’s because the chemistry involved is so direct that it’s hard to improve on. First, hydrogen sulfide is burned to produce sulfur dioxide, which reacts with more unburned hydrogen sulfide to produce elemental sulfur and water. This takes place under both heating and catalytic conditions to maximize the removal of sulfur, which comes off as a hot gas and is then condensed to liquid sulfur so it can be pumped into tanks for storage. Any unreacted hydrogen sulfide gas that’s dissolved in the liquid sulfur is usually removed by degassing and sent back to undergo the Claus process again. As the sulfur cools to a solid, it may be stored in huge piles—they look like bright yellow mountains—since a surplus of sulfur is produced by the Claus process in some oil-producing regions. Some of the sulfur produced this way is used in sulfuric acid manufacturing, in the rubber industry, as a fertilizer, and as a general chemical feedstock. And some of it just sits around. As long as the world needs to tap high-sulfur oil reserves, no one will ever get very rich selling it.

Mountains of sulfur, extracted from natural gas and waiting to be put to use.