This paper begins by reviewing the Claus/ modified-Claus processes which have been the workhorse for sulphur recovery in gas processing plants and refineries. Then, the paper discusses the modifications and additions made to the process in order to improve the sulphur recovery from the 92- 98% range all the way to the 99.8+% range. Schematics for these improved processes are included in the paper. In closing, the development status is provided for two processes targeting the 99.5% recovery level at a much lower capital cost than current technology allows. by using other additions or adaptations to


ensure sufficient temperature in the main This paper deals mainly with the burner to maintain a stable flame (such as the additions that have been made to what is addition of acid gas and air preheat and the use referred to as the modified-Claus process to of a split-flow line-up, where up to 60% of the increase sulphur recovery over the past years: acid gas feed is bypassed directly to the first Claus stage). · Subdewpoint processes The modified-Claus process (often · Selective oxidation processes called an Sulphur Recovery Unit or SRU) has · Tail gas clean-up processes been in use in the gas processing and refinery industries for over 50 years now.

THE CLAUS PROCESS Between two and four Claus stages The Claus process involves the reaction of have been used, depending upon the sulphur oxygen with hydrogen sulphide (H2S) to make recovery requirements or the downstream sulphur and water. The process was developed processing units (if any). in London by chemist Carl Freidrich Claus and Sulphur recoveries range from 90- patented in 1883. The process is very 95% for a two-stage unit to 96-98% for a fourexothermic in nature and is limited to the 5-6 stage unit. volume % H2S content range in the acid gas SUBDEWPOINT PROCESSES feed in order to keep the reactor outlet temperature to less than the 340 to 370 oC The subdewpoint processes increase (650 to 700 oF) range (the practical limit for recovery by operating one or more Claus carbon steel). converters at below the sulphur dewpoint in order to maximize sulphur recovery. The THE MODIFIED-CLAUS PROCESS sulphur will be absorbed into the catalyst in In 1938, I.G. Farbenindustrie A.G. in this low temperature operation and necessitates Germany made an important modification the intermittent regeneration of each bed in (addition) that allowed for the handling of acid order to remove the sulphur, typically once per gases containing more than 6 volume % H2S. day per bed.

The modification involved adding a thermal In subdewpoint mode, the acid gases stage ahead of the Claus reactor(s). This from the upstream sulphur condenser are sent thermal stage typically includes a main burner, directly to the s

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