1.  Field of the Invention
The invention relates generally to a sulfur recovery process and more particularly to an improved Claus plant tail gas cleanup process. 
2.  Setting of the Invention
The conventional catalytic Claus reaction, EQU 2H.sub.2 S+SO.sub.2 .revreaction.3S+2H.sub.2 O,
for sulfur recovery from sulfur-containing gas streams is widely practiced and accounts for approximately one-half of total sulfur production.  However, the effluent tail gas from the final of a series of reactors in a Claus plant contains varying quantities of different compounds, including sulfur, water, sulfur vapor, sulfur dioxide, hydrogen sulfide, carbonyl sulfide and carbon disulfide.  Normally, this effluent tail gas stream is burned with air to convert all sulfur compounds present in the stream to sulfur dioxide before discharge into the atmosphere. 
Ever stricter environmental standards on SO.sub.2 emissions require reduction, however, in the total quantity of sulfur compounds present in a Claus unit tail gas.  As the Claus reaction is equilibrium controlled, it is difficult to further improve by additional Claus conversion the percent conversion to sulfur to meet these stricter standards.  Thus, efforts have been directed to methods of treating Claus tail gases to remove sulfur compounds other than additional Claus conversion.  One example is U.S. Pat.  No.  3,798,316 entitled "Purification of Claus Plant Tail Gas by Water Washing" issued to D. K. Beavon on Mar. 19, 1974.  In this method, the Claus tail gas is treated with an aqueous absorbent solution to reduce sulfur compounds in the tail gas to acceptable levels.  A similar process is described in U.S. Pat.  No.  3,752,877 issued to D. K. Beavon on Aug. 14, 1973, wherein a Claus tail gas is reduced to convert all sulfur compounds to H.sub.2 S, the H.sub.2 S-rich stream is water quenched to condense water, and the H.sub.2 S is then extracted by an absorbent solution such as the alkaline solution used in the Stretford process.  Another extraction process is disclosed in U.S. Pat.  No.  4,113,849 issued to G. R. Atwood on Sept. 12, 1978, entitled "Recovering Sulfur from Gas Streams Containing Hydrogen Sulfide".  This method converts all sulfur compounds in a Claus tail gas to sulfur dioxide before cooling and contacting with an SO.sub.2 -absorbing solution.  While sulfur recovery from Claus tail gas by methods other than additional Claus conversion is quite high, on the order of 99.9% recovery in combination with the Claus reactors, these methods suffer from operating problems such as high chemical makeup costs, chemical waste disposal, and either increased plant emissions or the necessity for plant shutdown when a process upset occurs. 
Other processes have attempted to promote Claus conversion by operation of a Claus catalytic reactor at lower temperatures.  An example, known as the Cold Bed Adsorption Process, is described in U.S. Pat.  No.  3,702,884, entitled "Method for Reducing Sulfur Compounds in Sulfur Plant Effluent" issued to E. B. Hunt, Jr. et al.,  on Nov. 14, 1972.  This method removes up to 99% of the sulfur compounds present in the original acid gas by catalytic conversion of H.sub.2 S and SO.sub.2 in the tail gas to sulfur which is adsorbed on the catalyst bed at a temperature of 250 to about 350.degree.  F. A further variation of the Cold Bed Adsorption Process is U.S. Pat.  No.  3,749,762, issued to N. R. Montgomery on July 31, 1973, and entitled "Process for Producing Sulfur from Sour Gas".  U.S. Pat.  No.  2,785,056, entitled "Process for the Production of Sulfur from Hydrogen Sulfide and Sulfur Dioxide or Oxygen-Containing Gas", issued to W. Thumm, et al.,  on Mar. 12, 1957, illustrates a similar process for using a series of Claus reactors operating above and below the sulfur dew point.  However, both of these processes, while yielding sulfur recovery of up to 99%, suffer in that they may fail to meet more stringent environmental protection standards or to provide recoveries in excess of 99%. 
An additional Claus process variation is disclosed in British Pat.  No.  717,483, issued to W. M. Catchpole, et al.,  on Oct. 27, 1954, entitled "Process for Recovery of Sulfur from Hydrogen Sulfide".  This process discloses operation of two reactors, the first operating at a temperature of 300.degree.  C. and the second of which is operated at a temperature of 100.degree.-150.degree.  C., preferably 150.degree.  C., to deposit sulfur on the catalyst. 
"BSR/Selectox I Sulfur Recovery Process for Claus Tail Gas Treating", by D. K. Beavon and R. H. Hass, presented at the Fifth Canadian Symposium on Catalysis, Oct. 26-27, 1977, discloses a process for removal of sulfur from a Claus tail gas by the direct oxidation process.  In this process, a Claus tail gas stream is hydrogenated, then cooled and water quenched to remove water.  The resulting stream is combined with a stoichiometric amount of air and fed to a reactor at a temperature of about 325.degree.  F. for oxidation of the H.sub.2 S present in the tail gas stream to sulfur over a proprietary Selectox-32 catalyst.  The process achieves overall recoveries of up to 98%. 
It is becoming increasingly evident, however, that the permissible sulfur emission levels will mandate sulfur recoveries on the order of 99.9% and, in any event, methods of achieving recoveries in excess of 99% are highly desirable.  Thus, a method for achieving high sulfur removal without the problems of the known absorbent solutions is required.