One such modification is all-catalytic Selectox process of UOP (McIntyre and Lyddon, 1997). A special catalyst bed replaces gas burner in a conventional Claus plant. The Selectox catalyst occupies the top few inches of the first bed, where it promotes the selective oxidation of H2S to SO2. The remainder of the bed is filled with Claus catalyst where the Claus reaction occurs to about 80% completion. The highly exothermic nature of these reactions requires that the feed gas be monitored for the concentration of H2S to avoid overheating. The Selectox bed is then followed by two conventional Claus beds. There are two versions for the Selectox process: the “Straight through Selectox” which can be used for H2S concentration no more than 5 mol%; and the “Recycle Selectox”,which is normally used for H2S between 5 to 40 mol%. These processes are reliable, and the sulfur recovery rate could reach around 96% (McIntyre and Lyddon, 1997). In our study, the acid gas stream of the base case contains 4.05 mol% H2S, and so the former design is suitable.