MODOP process

Claus plant tail gas is normally quite dilute, and more concentrated H2S streams can be made dilute by recycling product gas. When direct oxidation is used in conjunction with a conventional Claus plant it is necessary to assure that essentially all sulfur in the tail gas is in the form of H2S. This can be accomplished by operating the Claus system with slightly less than the stoichiometric amount of air or by providing a separate hydrogenation/hydrolysis step to convert sulfur compounds to H2S before direct oxidation. Commercial direct oxidation processes include the Superclaus process offered by Comprimo, the Selectox process offered by Unocal/Parsons, Mobil s MODOP process, and the previously discussed Catasulf process. 

Although the selective oxidation reaction (8-1) is not reversible, the products of reaction 8-1 (S and H2O) can react by the reverse of reaction 8-3 to reduce overall sulfur recovery. Therefore, in the MODOP process, water is removed from the gas stream in the quench tower to improve the conversion of hydrogen sulfide to sulfur. Conversion can be further increased by removal of additional water in a glycol dehydration tower after the quenching... 

Figure 8-27. Schematic flow diagram of the MODOP process applied to Claus plant tailgas treatment (Kettner and Llermann, 1988). Reprinted with permission from (XI Gas Journal, Jan. 11,1988, copyright PennWeliPidrHshing Co.

The MODOP process may be used to directly treat low hydrogen sulHde content amine regenerator off gas. In this application, the hydrogenation and quench equipment are generally not required due to the absence of non-H2S sulfur species and the low water content of the feed gas. 

Modop [Mobil Oil Direct Oxidation Process] A process for removing residual sulfur-containing gases from the tail gas from the Claus process. The catalyst is titanium dioxide pelletized with calcium sulfate. Developed in the 1980s by Rhone-Poulenc, Procatalyse, and Mobil Oil. Three plants were operating in Germany in 1995 and one in the United States. European Patents 60,742 78,690. 

When treating Claus unit tail gas, the process is capable of overall sulfur recoveries of up to 99.6% The number of reactors included is dependent on the feed gas concentration and the required sulfur recovery. Catalyst selectivity is maximized at a MODOP reactor outlet temperatures of 250°-270°C (482°-518 F). As the oxidation reaction is highly exothermic, additional stages must be employed to limit the reactor outlet temperature to below 320°C (608°F) at high concentrations of H2S in the feed gas. 

Kettner, R.. Luebcke, T., and Stemfels, E. A., 1988, Experience with the MODOP Tail Gas Treating Process, paper presented at 38th Canadian Chemical Engineering Conference, Edmonton, Canada, October 4. 

Mobil, 1994, Modop in Gas Processes 94, Hydro. Process., April, p. 94. 

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