Sulfur recovery refers to the conversion of hydrogen sulfide (H2S) to elemental sulfur. Hydrogen sulfide is a byproduct of processing natural gas and refining high-sulfur crude oils. The most common conversion method used is the Claus process. Approximately 90 to 95 percent of recovered sulfur is produced by the Claus process. The Claus process typically recovers 95 to 97 percent of the hydrogen sulfide feedstream.
Over 5.9 million megagrams (Mg) (6.5 million tons) of sulfur were recovered in 1989, representing about 63 percent of the total elemental sulfur market in the U. S. The remainder was mined or imported. The average production rate of a sulfur recovery plant in the U. S.
Claus Sulfur recovery unit
The process consists of multistage catalytic oxidation of hydrogen sulfide according to
the following overall reaction:
2H2S + O2 ® 2S + 2H2O (1)
Each catalytic stage consists of a gas reheater, a catalyst chamber, and a condenser.
The Claus process involves burning one-third of the H2S with air in a reactor furnace to form
sulfur dioxide (SO2) according to the following reaction:
2H2S + 3O2 ® 2SO2 + 2H2O + heat (2)
The furnace normally operates at combustion chamber temperatures ranging from 980 to 1540°C (1800 to 2800°F) with pressures rarely higher than 70 kilopascals (kPa) (10 pounds per square inch absolute). Before entering a sulfur condenser, hot gas from the combustion chamber is quenched in a waste heat boiler that generates high to medium pressure steam. About 80 percent of the heat released could be recovered as useful energy. Liquid sulfur from the condenser runs through a seal leg into a covered pit from which it is pumped to trucks or railcars for shipment to end users. Approximately 65 to 70 percent of the sulfur is recovered. The cooled gases exiting the condenser are then sent to the catalyst beds. The remaining uncombusted two-thirds of the hydrogen sulfide undergoes Claus reaction (reacts with SO2) to form elemental sulfur as follows:
2H2S + SO2 ¨3S + 2H2O + heat (3)
The catalytic reactors operate at lower temperatures, ranging from 200 to 315°C (400 to 600°F).
Alumina or bauxite is sometimes used as a catalyst. Because this reaction represents an equilibrium chemical reaction, it is not possible for a Claus plant to convert all the incoming sulfur compounds to elemental sulfur. Therefore, 2 or more stages are used in series to recover the sulfur. Each catalytic stage can recover half to two-thirds of the incoming sulfur. The number of catalytic stages depends upon the level of conversion desired. It is estimated that 95 to 97 percent overall recovery can be achieved depending on the number of catalytic reaction stages and the type of reheating method used.
If the sulfur recovery unit is located in a natural gas processing plant, the type of reheat employed is typically either auxiliary burners or heat exchangers, with steam reheat being used occasionally. If the sulfur recovery unit is located in a crude oil refinery, the typical reheat scheme uses 3536 to 4223 kPa (500 to 600 pounds per square inch guage [psig]) steam for reheating purposes. Most plants are now built with 2 catalytic stages, although some air quality jurisdictions require 3. From the condenser of the final catalytic stage, the process stream passes to some form of tailgas treatment process. The tailgas, containing H2S, SO2, sulfur vapor, and traces of other sulfur compounds formed in the combustion section, escapes with the inert gases from the tail end of the plant. Thus, it is frequently necessary to follow the Claus unit with a tailgas cleanup unit to achieve higher recovery. In addition to the oxidation of H2S to SO2 and the reaction of SO2 with H2S in the reaction furnace, many other side reactions can and do occur in the furnace. Several of these possible side reactions are:
CO2 + H2S ® COS + H2O (4)
COS + H2S ® CS2 + H2O (5)
2COS ® CO2 + CS2 (6)
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