Design Variations of the CLAUS Process

Claus units have to be tailoted to die hydrogen sulfide content of the acid gas and to its ammonia and hydrocarbon contents. Hence, numerous process variations have evolved. 

The flow sheet of a Standard Claus unit is shown in Fig. 5.10. Essentially, such a unit consists of the combustion chamber section with a waste heat boiler and two reactors. Such plants are normally used for sour gases containing more than some 45 vol. % of H2S. The sour gas is fed to the combustion chamber together with air or, if required, with fuel gas it is already in this combustion chamber that as much as 70% of the hydrogen sulfide are reacted to elemental sulfur. The gas mixture leaving the combustion chamber is cooled down in a waste heat boiler to the inlet temperature required for the first catalytic stage. The temperature is controlled by what has been termed a hot gas bypass. 

If acid gases containing between approximately 20-50 vol.% of H2S are to be treated, the Standard Process has to be modified as addition of the stoichiometric air rate would get the concentration of combustible components in the combustion chamber outside the ignition range. Consequently, only a slip stream of the sour gas is fed to the burners in the combustion chamber together with the full air rate, while the rest of the sour gas is fed either directly to the combustion chamber downstream of the burners or immediately upstream of the first catalytic stage. 

Whenever the acid gas contains less than 20 vol. % of H2S, the process has to use either oxygen-eiuiched air or even pure oxygen. The operating principle is the same as for air, but the burners are different from those in a Standard unit. The use of oxygen also requires more stringent monitoring and safety facilities. 

The above concentration limits refer to cold sour gases and cold combustion air. If the gases are preheated, the limits will be shifted downward. The limit for using the Claus process is an H2S concentration around 5 %. Leaner gases can be treated only if the reaction is supported by burning additional sulfur. Nor can gases be treated in Claus units if they contain major proportions of hydrocarbons or ammonia. Such gases can only be incinerated and the SO2 then absorbed or reduced by suitable processes (Sect. 5.3.2). 

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