The Claus process is the most significant gas desulfurizing process, recovering elemental sulfur from gaseous hydrogen sulfide. First patented in 1883 by the chemist Carl Friedrich Claus, the Claus process has become the industry standard. The multi-step Claus process recovers sulfur from the gaseous hydrogen sulfide found in raw natural gas and from the by-product gases containing hydrogen sulfide derived from refining crude oil and other industrial processes. The by-product gases mainly originate from physical and chemical gas treatment units (Selexol, Rectisol, Purisol and amine scrubbers) in refineries, natural gas processing plants and gasification or synthesis gas plants. These by-product gases may also contain hydrogen cyanide, hydrocarbons, sulfur dioxide or ammonia. Gases with an H2S content of over 25% are suitable for the recovery of sulfur in straight-through Claus plants while alternate configurations such as a split-flow set up or feed and air preheating can be used to process leaner feeds. Hydrogen sulfide produced, for example, in the hydro-desulfurization of refinery naphthas and other petroleum oils, is converted to sulfur in Claus plants. The reaction proceeds in two steps: 2 H2S +3 O2 → 2 SO2 + 2 H2O 4 H2S +2 SO2 → 3 S2 + 4 H2O The vast majority of the 64,000,000 tonnes of sulfur produced worldwide in 2005 was byproduct sulfur from refineries and other hydrocarbon processing plants. Sulfur is used for manufacturing sulfuric acid, medicine, cosmetics, fertilizers and rubber products. Elemental sulfur is used as fertilizer and pesticide. History The process was invented by Carl Friedrich Claus, a German chemist working in England. A British patent was issued to him in 1883. The process was later significantly modified by IG Farben. Claus was born in Kassel in the German State of Hesse in 1827, and studied chemistry in Marburg before he emigrated to England in 1852. He died in London in 1900. Process description A schematic process flow diagram of a basic 2+1-reactor (converter) SuperClaus unit is shown below: The Claus technology can be divided into two process steps, thermal and catalytic. Thermal step In the thermal step, hydrogen sulfide-laden gas reacts in a substoichiometric combustion at temperatures above 850 °C such that elemental sulfur precipitates in the downstream process gas cooler. The H2S content and the concentration of other combustible components (hydrocarbons or ammonia) determine the location where the feed gas is burned. Claus gases (acid gas) with no further combustible contents apart from