Hydrogen sulfide (H2S) is a smelly, corrosive, highly toxic gas. Besides its other bad habits, it also deactivates industrial catalysts. H2S is commonly found in natural gas and is also made at oil refineries, especially if the crude oil contains a lot of sulfur compounds.
Because H2S is such an obnoxious substance, it is converted to non-toxic and useful elemental sulfur at most locations that produce it. The process of choice is the Claus Sulfur Recovery process.
First the H2S is separated from the host gas stream using amine extraction. Then it is fed to the Claus unit, where it is converted in two steps:
A catalyst is needed in the second step to help the components react with reasonable speed. Unfortunately the reaction does not go to completion even with the best catalyst. For this reason two or three stages are used, with sulfur being removed between the stages. Engineers know how different factors like concentration, contact time and reaction temperature influence the reaction, and these are set to give the best conversions.
The reaction is as follows: 2H2S + SO2 ==> 3S + 2H2O
Inevitably a small amount of H2S remains in the tail gas. This residual quantity, together with other trace sulfur compounds, is usually dealt with in a tail gas unit. The latter can give overall sulfur recoveries of about 99.8%, which is very impressive indeed.
Sulfur Production by the Claus Process
Over the years many improvements have been made to the Claus process. Recent developments include:
More improvements can be expected. Here are some possibilities.
CS2 destruction. Carbon disulfide (CS2) is a side product made in the furnace. Laboratory work has shown that special catalysts operating in the furnace can destroy the CS2 before it gets into the catalytic section. A commercially available catalyst like this might be developed for use in a Claus plant.
Catalyst Temperature Policy. The conversion of H2S goes faster at higher temperatures, but a more favorable equilibrium is obtained at lower temperatures. It isn't obvious whether higher or lower temperatures are needed in the third converter. Kinetic modelling may supply the answer, thereby improving conversion or reducing catalyst replacement cost.
Some H2S-containing gas is unsuitable for treatment by amine extraction because of high CO2 levels. These streams often lend themselves to processing by so-called liquid redox processes such as SulFerox® or ARI-LO-CAT®. Instead of air, these processes use a liquid solution containing oxidized iron.
Several novel processes are being developed to make hydrogen as well as sulfur from H2S. These are sometimes called H2S splitting processes. Hydrogen is a valuable gas that is needed in oil processing and for the manufacture of ammonia and methanol.
For imaginative solutions to chemical process issues, contact
Ed Luinstra, Ph.D.
Chemical Process Consultant
Calgary, Alberta, Canada
With 25 years experience in oil and gas research.
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SUPERCLAUS is a trademark of STORK Comprimo B.V. and Gastec N.V. (Netherlands).
ARI-LO-CAT is a registered trademark of Wheelabrator Clean Air Systems, Inc. SulFerox is a
registered service mark of Shell Oil Company.