Selectox process

A derivative of the Claus process is the Recycle Selectox process, developed by Parsons and Unocal and Hcensed through UOP. Once-Thm Selectox is suitable for very lean acid gas streams (1—5 mol % hydrogen sulfide), which cannot be effectively processed in a Claus unit. As shown in Figure 9, the process is similar to a standard Claus plant, except that the thermal combustor and waste heat boiler have been replaced with a catalytic reactor. The Selectox catalyst promotes the selective oxidation of hydrogen sulfide to sulfur dioxide, ie, hydrocarbons in the feed are not oxidized. These plants typically employ two Claus catalytic stages downstream of the Selectox reactor, to achieve an overall sulfur recovery of 90—95%. 

For feeds greater than 5 mol % hydrogen sulfide, but less than 40 mol %, the Recycle Selectox process can be used. In this variation of the Selectox process, a portion of the process gas leaving the condenser downstream of the Selectox reactor is recycled so as to limit the oudet gas temperature from the reactor to >205 C. 

Another variation of the Selectox process can be used with the Beavon process in tail gas treating. The hydrogenated Claus tail gas stream is sent to a Selectox reactor. Overall recoveries of up to 98.5% are possible. Use of Beavon/Selectox, however, typically costs more than use of Superclaus. 

The new Selectox process can achieve ninety-five percent conversion of hydrogen sulfide even though the initial H2S concentration is below the level previously feasible for Claus processing. The effects of various minor components of Claus feed are discussed. 

Figure 4. Two-stage Selectox process with recycle.

Since the Selectox process is capable of well over 90 percent sulfur recovery, the authors recommend a careful cost-benefit study to see if Claus tail gas processing should be required with such plants. It might well be found that—say—95 percent capture of sulfur in the coal conversion plant is acceptable. 

Removal of hydrogen sulfide om the hydrogenation reactor off-gas in the presence of carbon dioxide is accomplished by use of a selective amine such as Exxon s Flexsorb SE Plus or Union Carbide s UCARSOL HS-103 (see Chapters 2 and 3). Regeneration of the selective amine produces a hydrogen sulfilde-rich gas which is sent to a sulfur recovery unit. In the Parsons FGC process, the Recycle Selectox process (see Chapter 8) is used for sulfur recovery. 

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. 

The principal advantage of the Selectox process, if used in conjunction with the BSR process, is that substantial capital cost savings can be realized by replacing a Stretford system with a Selectox reactor. In addition, problems with liquid effluents are eliminated. The advantage of the other two versions of the process is that they are suitable for the treatment... 

Figure 8-2S. BSR/Selectox process for Claus plant emission control. (Beavonetal., 1980f...

Further variations of the Selectox process, including packaged units and three-stage units with recycle, have been reported by Hass et al. (1981). 

Introduction. A process similar to the Selectox process, the Mobil Oil Direct Oxidation Process (MOIX)P), has been developed by Mobil Oil AG of Celle, Germany, the German... 

Figure 8-26, Two-stage Selectox process with recycle. (Beavon etal., 19801...

Delaney, D. D., Bertram, R. V., and Gowdy, H. W., 1990, The Selectox Process Commercial Experience, paper presented at the AIChE 1990 Summer National Meeting, San Diego, CA, August 19-22. 

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