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Figure 2  Alternatives for natural gas sweetening.


            11.4  BATCH PROCESSES


            In this case, H 2 S is basically removed and the presence of CO 2 does not
            affect the processes. Usually, batch processes are used for low-sulfur-content
            feeds.


            11.4.1  Iron Sponge
            Iron sponge fixed-bed chemical absorption is the most widely used batch
            process. This process is applied to sour gases with low H 2 S concentrations
            (300 ppm) operating at low to moderate pressures (50–500 psig). Carbon
            dioxide is not removed by this treatment. The inlet gas is fed at the top of
            the fixed-bed reactor filled with hydrated iron oxide and wood chips. The
            basic reaction is the formation of ferric sulfide when H 2 S reacts with ferric
            oxide:
                 2Fe 2 O 3 þ 6H 2 S ! 2Fe 2 S 3 þ 6H 2 O                  ð1Þ
            The reaction requires an alkalinity pH level 8–10 with controlled injection
            of water. The bed is regenerated by controlled oxidation as
                 2Fe 2 S 3 þ 3O 2 ! 2Fe 2 O 3 þ 6S                        ð2Þ
            Some of the sulfur produced might cake in the bed and oxygen should be
            introduced slowly to oxide this sulfur, Arnold and Stewart [2]:

                 S 2 þ 2O 2 ! 2SO 2                                       ð3Þ






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