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A combined process for the recovery of sulfur from an acid gas stream. The combined process includes both a Claus unit and a unit for treating the Claus tail gas with caustic to remove hydrogen sulfide therefrom and to biologically oxidize the hydrogen sulfide using certain types of sulfur bacteria

A combined process for the recovery of sulfur from an acid gas stream. The combined process includes both a Claus unit and a unit for treating the Claus tail gas with caustic to remove hydrogen sulfide therefrom and to biologically oxidize the hydrogen sulfide using certain types of sulfur bacteria to make elemental sulfur. The combined process provides for an exceptionally low concentration of hydrogen sulfide in the finally treated sweet gas.

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That which is claimed is: 1. A sulfur recovery process, comprising: charging an acid gas stream comprising hydrogen sulfide as a feed to a sulfur recovery system operated so as to yield a first sulfur product and a Claus tail gas comprising hydrogen sulfide and less than about 1000 ppmv sulfur diox

That which is claimed is: 1. A sulfur recovery process, comprising: charging an acid gas stream comprising hydrogen sulfide as a feed to a sulfur recovery system operated so as to yield a first sulfur product and a Claus tail gas comprising hydrogen sulfide and less than about 1000 ppmv sulfur dioxide; charging said Claus tail gas to a biological gas desulfurization system comprising absorber means for contacting said Claus tail gas with a lean solvent to yield a sweet gas and a rich solvent; bioreactor means for biologically oxidizing the dissolved hydrogen sulfide contained in said rich solvent and to yield a lean solvent containing sulfur; and sulfur recovery means for recovering sulfur from a portion of said lean solvent and to yield a second sulfur product and a sulfur reduced lean solvent and returning said sulfur reduced lean solvent to said bioreactor means; and operating said biological gas desulfurization system, wherein said lean solvent comprises an alkaline solution having a pH in the range of from 7.5 to 12 and sulfur bacteria capable of biologically oxidizing the dissolved hydrogen sulfide contained in said rich solvent and which is selected from the genera of Beggiatoa, Thiothrix and Thiobacillus to yield a second sulfur product and a sweet gas comprising less than 100 ppmv hydrogen sulfide. 2. A sulfur recovery process as recited in claim 1, wherein said sulfur recovery system comprises: combustion means for partially oxidizing the hydrogen sulfide in said acid gas stream to yield a combustion gas stream; and Claus reaction means for reacting said combustion gas stream to yield said Claus tail gas and said first sulfur product. 3. A sulfur recovery process as recited in claim 2, wherein said combustion gas stream comprises hydrogen sulfide and sulfur dioxide in such amounts as to have a molar ratio of hydrogen sulfide-to-sulfur dioxide that exceeds 2:1 but is less than 6:1. 4. A sulfur recovery process as recited in claim 3, wherein further charged to said combustion means of said sulfur recovery system is an oxygen-containing gas, comprising oxygen, in such an amount that the molar ratio of oxygen-to-hydrogen sulfide charged to said combustion means is less than 0.5. 5. A sulfur recovery process, comprising: passing an acid gas stream comprising hydrogen sulfide to a combustion zone defined by combustion means and providing for partially oxidizing the hydrogen sulfide in said acid gas stream with oxygen to form sulfur dioxide thereby providing a combustion gas stream comprising sulfur dioxide and hydrogen sulfide; controlling the amount of hydrogen sulfide oxidized in said combustion zone such that less than ⅓ (one third) of the hydrogen sulfide in said acid gas stream is oxidized to sulfur dioxide; passing said combustion gas stream to a Claus reaction zone operated under Claus conversion conditions to yield a reaction gas comprising sulfur; recovering sulfur from said reaction gas to yield a Claus tail gas comprising a concentration of hydrogen sulfide; passing said Claus tail gas to an absorption zone defined by absorber means for contacting said Claus tail gas with a lean caustic solution whereby hydrogen sulfide is recovered from said Claus tail gas and from which is yielded a sweet tail gas comprising less than 100 ppmv H2S and a rich caustic solution, wherein said lean caustic solution comprises an alkaline solution having a pH in the range of from 7.5 to 12 and sulfur bacteria capable of biologically oxidizing the dissolved hydrogen sulfide contained in said rich caustic solution and which is selected from the genera of Beggiatoa, Thiothrix and Thiobacillus, and wherein said sweet tail gas comprises less than 100 ppmv hydrogen sulfide; and passing said rich caustic solution to a bioreaction zone defined by bioreactor means for the biological oxidation of the dissolved sulfide in said rich caustic solution by use of said sulfur bacteria to elemental sulfur, yielding from said bioreaction zone said lean caustic solution; passing a portion of said lean caustic solution to sulfur recovery means for recovering sulfur from a portion of said lean caustic solution and to yield a sulfur reduced lean caustic solution and sulfur; and returning said sulfur reduced lean caustic solution to said bioreaction zone. 6. A sulfur recovery process as recited in claim 5, wherein said controlling step includes controlling the amounts of sulfur dioxide and hydrogen sulfide in said combustion gas stream such that said combustion gas stream further comprises a molar ratio of hydrogen sulfide-to-sulfur dioxide exceeding 2:1 but is less than 6:1. 7. A sulfur recovery process as recited in claim 6, wherein an oxygen-containing gas, comprising oxygen, is passed with said acid gas stream to said combustion zone in such an amount relative to said acid gas stream so that the molar ratio of oxygen-to-hydrogen sulfide passed to said combustion zone is less than 0.5. 8. A sulfur recovery process as recited in claim 5, wherein said sweet tail gas comprises less than 50 ppmv H2S. 9. A sulfur recovery process, comprising: reacting an acid gas steam with oxygen under such oxidation conditions to yield a combustion gas comprising relative amounts of hydrogen sulfide and sulfur dioxide so as to have a ratio of hydrogen sulfide to sulfur dioxide exceeding 2:1 but is less than 6:1; reacting said combustion gas under Claus reaction conditions to yield a reaction gas comprising sulfur; recovering sulfur from said reaction gas to yield a Claus tail gas comprising a concentration of hydrogen sulfide and less than 1000 ppmv sulfur dioxide; contacting said Claus tail gas with a lean absorbent thereby removing from said Claus tail gas a portion of the hydrogen sulfide contained therein and to yield a sweet gas, comprising less than 100 ppmv H2S, and a rich solvent comprising dissolved hydrogen sulfide; and biologically oxidizing the dissolved hydrogen sulfide of said rich solvent to elemental sulfur by contacting said rich solvent with a sulfur bacteria capable of biologically oxidizing the dissolved hydrogen sulfide of said rich solvent and which is selected from the genera of Beggiatoa, Thiothrix and Thiobacillus under biological oxidation conditions to yield a lean solvent containing said elemental sulfur, recovering said elemental sulfur from said lean solvent by use of sulfur recovery means for recovering sulfur from said lean solvent and to yield a sulfur product and a sulfur reduced lean solvent; and returning said sulfur reduced lean solvent to the bioreactor. 10. A sulfur recovery process as recited in claim 9, wherein said sweet gas comprises less than 50 ppmv H2S. 11. A sulfur recovery process as recited in claim 9, wherein said sulfur bacteria includes those organisms of the genus Thiobacilli. 12. A sulfur recovery process as recited in claim 11, wherein said lean absorbent comprises a buffered alkaline solution.