Recovery efficiency, optimum sulfur

Paskall (25) has recently reviewed the various modifications to the Claus process that result in optimum sulfur recovery efficiency. Overall plant conversion efficiencies in the range of 97% were considered to be the upper limit at the beginning of the 1970 s (26). While this is a very respectable conversion efficiency for an industrial process the unrecovered 3% in a 2,000 tonne/d sulfur plant represents 60 tonnes/d of sulfur lost, mainly to atmosphere as 120 tonnes/d of SO2. Modifications to the four stage Claus converter train however, can raise overall conversions to over 98.5% thus halving the sulfur loss to the plant tail gas. This either reduces environmental impact or the load on tail gas desulfurization units that will be discussed later. 

The hydrolysis reaction however, requires higher temperatures (650°F) than the optimum for the Claus redox. Nonetheless it has been clearly demonstrated in plant studies during the 1970 s that running the first converter bed at this higher COS hydrolysis temperature and sacrificing some Claus redox efficiency that can be recovered in later beds does result in an improved overall sulfur recovery efficiency especially in four stage Claus converter systems. 

---