Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to processes for sulfur recovery and more specifically to processes for removing sulfur compounds, including hydrogen sulfide and sulfur dioxide, from process streams. 
     2. Description of the Related Art 
     Processing of hydrocarbon-containing fuels such as gasoline and diesel fuel results in gases containing sulfur compounds, including hydrogen sulfide (H 2 S), and hydrocarbon compounds, including ammonia (NH 3 ). Governmental regulations limit plant emissions of sulfur-bearing gases. Refineries commonly include sulfur reduction units to decrease emissions of sulfur compounds. 
     The use of a Claus reaction to recover sulfur from process off-gases is widely known in the field. Sulfur dioxide (SO 2 ) and hydrogen sulfide react to produce elemental sulfur (S 2 ) and steam (H 2 O). The reaction formula is:
 
2H 2 S+SO 2 →1.5S 2 +2H 2 O
 
The reaction can occur with or without a catalyst. As long as two moles of hydrogen sulfide are available for each mole of sulfur dioxide in appropriate concentrations with appropriate heat and pressure, elemental sulfur and water will result.
 
     A typical prior art arrangement of a Claus process with a SCOT tail gas process is outlined in  FIG. 5 . The prior art generally teaches high temperature reaction of air, through air line  16 , off-gas streams containing hydrogen sulfide, through hydrogen sulfide line  18 , and off-gas streams containing ammonia, through ammonia line  12 , in a thermal reactor  14  to oxidize a portion of the hydrogen sulfide in the process streams to create sulfur dioxide and to react the sulfur dioxide with hydrogen sulfide into elemental sulfur and water, thus eliminating a substantial portion of the hydrogen sulfide and sulfur dioxide in the streams. The quantity of air to the thermal reactor is controlled to provide a stoichiometric balance of hydrogen sulfide and sulfur dioxide in the process stream. A balanced stoichiometric ratio is difficult to maintain due to variations in composition of process streams. 
     Commonly, the process stream containing significant hydrogen sulfide is divided prior to introduction to the reactor chamber, with approximately 30-70% of the feed directed to the front end  22  of the thermal reactor chamber  14  through line  18  and the remaining 30-70% directed to the back end  20  of the reactor chamber  14  through line  18   a . The mixture of air and off-stream gases is thermally reacted near the front end  22  of the reactor chamber  14  and moves to the back end  20  of the reactor chamber  14  where the second hydrogen sulfide stream is received. 
     It is desirable to maintain a temperature at or near 1,316° C. (2,400° F.) in the thermal reactor  14  to crack process gas hydrocarbon compounds such as ammonia. Providing a determined quantity of hydrogen sulfide at the front end  22  of the reactor chamber  14  assists in maintaining desired temperature in the thermal reactor  14 . An excess of hydrogen sulfide tends to lower the temperature in the reactor chamber  14  due to increased mass flow. In prior art thermal reactors, the temperature at the back end  20  of the reactor  14  may be less than the desired temperature because of added off-gases containing hydrogen sulfide. 
     After the thermal reactor  14  the resulting gas stream is processed through a series of Claus reactors  46 ,  60  and  72 , condensers  34 ,  50 ,  64  and  76  and reheaters  42 ,  54  and  68 . The Claus reactors  46 ,  60  and  72  typically have aluminum oxide or bauxite catalyst beds. The Claus reaction produces elemental sulfur, which is recovered as liquid sulfur and water vapor. Traditional Claus systems remove greater than 95% of the sulfur from the process stream. Tail gas processes are used to remove remaining quantities of sulfur compounds to obtain an overall recovery of up to 99.9%. 
     Various processes are taught for treating tail gas to remove the remaining hydrogen sulfide and sulfur dioxide. The Shell Claus Off-gas Treating process, often referred to as the SCOT process, reacts hydrogen with remaining sulfur dioxide to generate hydrogen sulfide which is in turn absorbed in an amine compound. A typical SCOT process includes a pre-heater  82  for heating the tail gas, a hydrogenation reactor  84 , a quench tower  86  to remove water from the tail gas, an amine tower  88  for reaction of the amine solution with the tail gas, a regenerator  96  to strip the hydrogen sulfide for transmission back to the Claus reactor, and an incinerator  92  for burning off treated tail gas. The SCOT process is effective in further reducing sulfur dioxide emissions. However, the SCOT involves substantial capital and operating expense. The hydrogenation reactor  84 , required to react hydrogen with hydrogen sulfide and sulfur dioxide, is expensive because of high initial capital cost and operating costs. 
     U.S. Pat. No. 5,021,232 issued to Hise et al. on Jun. 4, 1991 discloses a process for the cleanup of sulfur-containing constituents in a gaseous stream such as a tail gas from a sulfur recovery unit. A Claus reaction is used to convert sulfur-containing compounds to elemental sulfur in the presence of a stoichiometric excess of hydrogen sulfide. The elemental sulfur is separated from the tail gas and the sulfurous compounds remaining in the tail gas are separated by crystallization for recycle through the Claus process. Carbon dioxide is the crystallization material. Sulfur-containing compounds are at least partially excluded from a solid (frozen) phase of the carbon dioxide for recycling through the Claus process. 
     U.S. Pat. No. 5,741,469 issued to Bhore et al. on Apr. 21, 1998 discloses a process that may be used to treat Claus plant tail-gas utilizing solid oxides to remove sulfur oxides from gas streams. 
     U.S. Pat. App. No. US 2003/0082096, invented by Lynn and published on May 1, 2003 discloses treating sulfur dioxide-rich gas by combusting it with a substoichiometric amount of oxygen to produce a combustion gas with water vapor and sulfur vapor. The combustion gas is cooled to form water containing suspended solid sulfur and polythionic acids. 
     U.S. Pat. No. 6,610,264 issued to Buchanan et al. on Aug. 26, 2003 discloses a process and system for removing sulfur from tail-gas emitted from a Claus sulfur recovery process. The tail-gas is first oxidized so as to convert sulfur therein to sulfur oxides. Oxidized tail-gas is directed into an absorber where a solid absorbent absorbs substantially all the sulfur oxides thereon. After allowing sufficient time for a desired amount of sulfur oxides to be absorbed, absorption is ceased. Next, the solid absorbent containing the absorbed sulfur oxides is contacted with a reducing gas so as to release an off-gas containing hydrogen sulfide and sulfur dioxide. Upon releasing sulfur from the solid absorbent, the solid absorbent is regenerated and redirected into the absorber. Sulfur in the off-gas emitted by regeneratio is concentrated to an extent sufficient for use within a Claus sulfur recovery process for conversion to elemental sulfur. 
     SUMMARY OF THE INVENTION 
     Objects of the present invention include providing a process for treating process off-gases that:
         effectively removes sulfur compounds from the gases;   handles variations and operational fluctuations in the process stream composition without upset;   eliminates the need for a hydrogenation reactor and the hydrogen supply required for such a reactor; and   reduces or eliminates the need for amine towers.       

     Other objects of the present invention will become evident throughout the reading of this document. 
     The present invention comprises a method of treating an off-gas stream from a refining process to remove sulfur compounds, including hydrogen sulfide. In the present invention, a portion of the off-gas stream containing hydrogen sulfide is injected at the front end of the thermal reactor and in at least one other location downstream of the thermal reactor. A ratio of hydrogen sulfide to sulfur dioxide at the outlet of the thermal reactor is less than the stoichiometric requirement. The ratio is adjusted downstream of the thermal reactor so that a ratio of hydrogen sulfide to sulfur dioxide is maintained substantially in excess of the stoichiometric requirement for a Claus reaction through the Claus reactors. Substantially complete reaction of all sulfur dioxide, whether initially present in process gas or generated in the thermal reactor, occurs concurrently with transmission of the process gas through the Claus reactors, such that the tail gas contains virtually no sulfur dioxide. 
     The tail gas, containing hydrogen sulfide but virtually no sulfur dioxide, is treated by a process including removal of water, heating the tail gas, introducing sulfur dioxide into the tail gas in a stoichiometricly balanced quantity, processing the tail gas in a Claus reactor, recovering elemental sulfur and sub-cooling the remaining tail gas to the sulfur dewpoint. 
     In an alternative embodiment, the tail gas may be treated by a sub-dewpoint reactor intermediate the tail gas Claus reactor and the sub-cooler. 
     In a second alternative embodiment, the tail gas is treated as in a SCOT tail gas treatment process. However, the elimination of sulfur dioxide from the tail gas eliminates the need for a prior art hydrogenation unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of the preferred embodiment of the process of the present invention. 
         FIG. 2  is a diagram of an alternative embodiment of the process of the present invention. 
         FIG. 3  is a diagram of an alternative embodiment of the process of the present invention incorporating a sub-dewpoint reactor. 
         FIG. 4  is a diagram an alternative embodiment of the present invention depicting a modified SCOT tail gas treatment process. 
         FIG. 5  is a diagram of a prior art process treating system with a SCOT tail gas treatment process. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the preferred embodiment of the process treatment process of the present invention is depicted. An off-gas line  18  containing hydrogen sulfide as a primary sulfur-containing compound, referred to herein as hydrogen sulfide off-gas line  18 , and an off-gas line  12  containing ammonia, referred to herein as the ammonia off-gas line  12 , each include by-products from a primary hydrocarbon process. Off-gas lines  12  and  18  each typically also contain various quantities of water, oxygen, nitrogen, carbon dioxide, sulfur dioxide, carbon monoxide and small amounts of other hydrocarbon compounds. Compositions vary depending on the process, variation of the input stream and variations in the application of the process. 
     Hydrogen sulfide off-gas line  18  and ammonia off-gas line  12  are each connected to thermal reactor  14  for transmittal of respective process gases to thermal reactor  14 . Air is supplied to thermal reactor  14  through air line  16  for reaction with process gases from hydrogen sulfide off-gas line  18  and ammonia off-gas line  12 . 
     Relative quantities of air, hydrogen sulfide off-gas line  18  gases and ammonia off-gas line  12  gases introduced into reactor chamber  14  are determined to create an excess of sulfur dioxide over the stoichiometric amount of sulfur dioxide and hydrogen sulfide required pursuant to the Claus equation. Accordingly, an excess of sulfur dioxide will be result at the outlet of reactor  14 . 
     The preferred temperature within thermal reactor  14  is at least 1,316° C. (2,400° F.) in order to oxidize or reduce various other constituents of hydrogen sulfide off-gas line  18  gases and ammonia off-gas line  12  gases. 
     In the preferred embodiment of the present invention, all hydrogen sulfide containing gas introduced into thermal reactor  14  through hydrogen sulfide off-gas line  18  is introduced at the front end  22  of thermal reactor  14 . Hydrogen sulfide is not fed into the thermal reactor  14  after the initial introduction through first hydrogen sulfide line  18 . This allows the temperature of reactor chamber  14  to remain near 1,316° C. (2,400° F.) in a greater area of the reactor chamber  14  and extends exposure time of the process gas to such temperature. Accordingly, increased oxidation or reduction of contaminants results at consistent flow rates of process gases. 
     After exiting reactor chamber  14 , the thermally-reacted process gas is fed by gas line  26  through a waste heat boiler  30  to cool the process gas and recover thermal energy from the process gas. Process gas is then fed to first condenser  34  where the process gas is cooled to allow condensation and collection of elemental sulfur. 
     A second quantity of hydrogen sulfide line  18  gas is transmitted by hydrogen sulfide gas line  18  and injected into thermally-reacted gas line  26  downstream of first condenser  34  at mixer  38 . The amount of hydrogen sulfide added is sufficient to create an excess of the amount hydrogen sulfide required for a stoichiometric balance with sulfur dioxide in the process gas line  26 . 
     As the Claus reaction is exothermic, reaction of hydrogen sulfide and sulfur dioxide is readily accomplished at a wide range of temperatures and pressures. Addition of excess hydrogen sulfide at mixer  38  accordingly enhances effective Claus reaction downstream of mixer  38  and enhances sulfur dioxide removal from the gas stream. 
     Through control devices known in the art, the quantity of hydrogen sulfide containing process gas delivered to mixer  38  through line  24  may be adjusted. Control of flow may be accomplished by valve  36  or by a control mechanism incorporated into mixer  38 . The quantity of hydrogen sulfide line  18  gas injected into thermally-reacted gas line  26  at mixer  38  may be controlled by controller  52 . 
     A sulfur dioxide analyzer  78  determines the quantity of sulfur dioxide in tail gas line  80  upstream of quench tower  84 . Such determination is input to controller  52 . If input of additional hydrogen sulfide line  18  gas is required at mixer  38 , controller  52  adjusts the amount of hydrogen sulfide line  18  gas input to mixer  38 . Analyzer  78  and controller  52  are commercially practiced control devices. 
     Process gas is subsequently transmitted to reheater  42  where the process gas is heated. Process gas is then transmitted by line  26  to a first Claus reactor  46  where a catalyst, such as aluminum oxide (Al 2 O 3 ) or bauxite, is utilized to catalyze reaction of hydrogen sulfide and sulfur dioxide in the process gas. Condensers such as second condenser  50 , reheaters such as first reheater  42  and reactors such as first Claus reactor  46  are known in the art. 
     Claus reactor  46  is operated to achieve an outlet temperature greater than 315° C. (600° F.). The hydrogen sulfide off-gas of line  18  contains quantities of carbonyl sulfide (COS) and carbon disulfide (CS 2 ). Operating Claus reactor  46  at such temperature allows such compounds to be cracked during the reaction process. 
     Process gas is then transmitted by line  26  through second condenser  50 . 
     A third quantity of hydrogen sulfide line  18  gas is transmitted by hydrogen sulfide gas line  18  and injected into process gas line  26  downstream of second condenser  50  at mixer  48 . Control of flow may be accomplished by valve  46  or by a control mechanism incorporated into mixer  48 . Process gas is then transmitted by line  26  through second reheater  54  and second Claus reactor  60 . 
     Process gas is then transmitted by line  26  through third condenser  64 , third reheater  68 , third reactor  72  and fourth condenser  76 . Preferably a ratio of 50 lbmol of hydrogen sulfide to 1 lbmol of carbon dioxide is provided at the inlet to third reactor  72  (25:1 ratio on a stoichiometric basis. 
     Condensers such as condensers  50  and  64 , reheaters such as reheaters  42  and  68 , and reactors such as reactors  60  and  72  are known in the art. 
     The excess hydrogen sulfide feed at mixer  38  and mixer  48  introduces sufficient quantities of hydrogen sulfide through hydrogen sulfide line  18  that contact of substantially all sulfur dioxide remaining in the process gases may be made within process line  26 , thereby achieving substantial elimination of sulfur dioxide at the outlet of condenser  76 . The quantity of sulfur dioxide in the tail gas line  80  at the outlet of condenser  76  is reduced below a level that would cause corrosion or sulfur formation in the quench tower. A desired concentration is substantially below one hundred (100) parts per million. 
     OPERATION EXAMPLE 
     In an illustrative material balance calculation, a quantity of hydrogen sulfide line  18  gas, a quantity of ammonia line  12  gas and a quantity of air line  12  gas are reacted in reactor  14  creating a process line  26  gas having a concentration of sulfur dioxide of 41.9 pound moles (“lbmols”) of sulfur dioxide and of 27.5 lbmols of hydrogen sulfide at the outlet of condenser  34 . This is a 0.66:1 ratio of hydrogen sulfide to sulfur dioxide on a lbmol basis and a 0.33:1 ratio on a stoichiometric basis (as 2 lbmols of hydrogen sulfide are required to each lbmol of sulfur dioxide). 
     A second quantity of hydrogen sulfide line  18  gas is injected at mixer  38  to provide a concentration of hydrogen sulfide of 21.7 lbmols and a concentration of sulfur dioxide of 9.4 lbmols at the outlet of condenser  50 , a 2.31:1 ratio of hydrogen sulfide to sulfur dioxide on a lbmol basis and a 1.16:1 ratio on a stoichiometric basis. 
     A third quantity of hydrogen sulfide line  18  gas is subsequently injected at mixer  48  to provide a concentration of hydrogen sulfide of 14.4 lbmols and a concentration of sulfur dioxide of 0.34 lbmols at the outlet of condenser  64 , a 48:1 ratio on a lbmol basis and a 24:1 ratio on a stoichiometric basis. In a preferred embodiment, the ratio of hydrogen sulfide to sulfur dioxide downstream of reactor  60  us at least 20:1 on a lbmol basis. This is the ratio of hydrogen sulfide to sulfur dioxide at the inlet to reactor  72 . As further reaction occurs and sulfur is condensed at reactor  72 , a concentration of hydrogen sulfide of 13.7 lbmols and a concentration of sulfur dioxide of 0.003 lbmols results at the outlet of condenser  76 , a ratio of 4,566:1 on a lbmol basis and a ratio of 2,283:1 on a stoichiometric basis. In a preferred embodiment, the ratio of hydrogen sulfide to sulfur dioxide downstream of reactor  72  is at least 200:1 on a lbmol basis. 
     In such calculation, the total lbmols of all constituents of process gas at the outlet of condenser  76  is 872.6 lbmols (including 435.6 lbmols of nitrogen and 360.3 lbmols of water vapor), providing a calculated sulfur dioxide concentration of 3.4 parts per million on a lbmol basis. 
     In operation, sulfur dioxide levels are monitored sulfur dioxide analyzer  78  determines the quantity of sulfur dioxide in tail gas line  80  upstream of quench tower  84 . Controller  52  adjusts quantities of hydrogen sulfide off-gas line  18  input to process line  26  at mixer  38  and at mixer  48  to obtain the desired concentration of sulfur dioxide at the analyzer measurement location. Such control may be exercised using defined constraints or through user input. 
     Liquid sulfur condensation utilizing Claus reactors may be accomplished using fewer or greater than the three series of reactors and condensers identified in  FIG. 1 . 
     Referring to  FIG. 2 , an alternative embodiment of the gas treatment process is shown. An additional mixer  58  is installed downstream of condenser  64  to provide an additional input source of hydrogen sulfide line  18  gas at such location. Introduction of a relatively small quantity of additional hydrogen sulfide at such location greatly impacts the ratio of hydrogen sulfide to sulfur dioxide at such location to react remaining sulfur dioxide and reduce the quantity of sulfur dioxide at the outlet of condenser  76 . A fourth hydrogen sulfide line  18  gas input location is provided at mixer  58  downstream of third condenser  64 . Control of flow may be accomplished by valve  24  or by a control mechanism incorporated into mixer  58 . A hydrogen sulfide/sulfur dioxide analyzer  28  is provided downstream of first condenser  34 . Analyzer  28  is connected to controller  52 . provides additional data regarding the composition of process line  26  gas at condenser  34 . Mixer  58  provides additional operational flexibility in adjusting hydrogen sulfide input. 
     Additional analyzers (not shown) may be utilized as desired. Placement of analyzers may be adjusted to provide information at locations deemed relevant. Mixers may be placed at alternate locations. 
     In a second alternative embodiment of the process gas treatment process, mixer  48  may eliminated. Adjustments to the amount hydrogen sulfide line  18  gas input to process gas line  26  are made at mixer  38 . 
     In a third alternative embodiment of the process gas treatment process, a separate source of hydrogen sulfide may be utilized to introduce a hydrogen sulfide feed to mixer  38  in lieu of feeding a portion of the hydrogen sulfide off-gas line  18  process gas to mixer  38 . Such alternative embodiment would be useful in instances wherein the hydrogen sulfide off-gas line  18  process gas contains significant quantities of other contaminants. 
     In a fourth alternative embodiment of the process gas treatment process, mixer  38  is placed downstream of thermal reactor  14  and upstream of condenser  34  to increase dwell time of excess hydrogen sulfide in the process gas stream. 
     Tail Gas Treatment—Sulfur Dioxide Addition 
     The process gases remaining after removal of elemental sulfur, which are now referred to as the “tail gas,” are transmitted from condenser  76  by tail gas line  80  to a quench tower  84  to remove water vapor from the tail gas. Quench towers, such as quench tower  84 , are known in the art and are commonly used for removal of water vapor from the tail gas. Water is known to inhibit Claus reactions, so it is advantageous to remove water vapor from the tail gas prior to a Claus reaction to be subsequently initiated. 
     The tail gas is then heated at heater  104  to achieve a temperature in the range of 149 to 260° C. (300 to 500° F.). A quantity of sulfur dioxide is added to the tail gas from sulfur dioxide line  106  at valve  110  such that a stoichiometric ratio of sulfur dioxide and hydrogen sulfide is obtained to achieve a Claus reaction. The amount of sulfur dioxide to be added is determined by an analyzer  118  upstream of valve  110 . Analyzer  118  monitors the quantity of hydrogen sulfide in the tail gas line  80  and through valve control devices known in the art adjusts the quantity of sulfur dioxide added to tail gas line  80 . 
     Addition of sulfur dioxide to the tail gas creates an additional Claus reaction with the hydrogen sulfide remaining in the tail gas. 
     The tail gas is then transmitted to a Claus reactor  114  having a catalyst bed of aluminum oxide or bauxite and subsequently to a condenser  120 . 
     Remaining tail gas is then transmitted to a subcooler  128  where the tail gas is further cooled preferably to a temperature of 66° C. (150° F.). At subcooler  128 , the Claus reaction continues and sulfur continues to condense. 
     Liquid sulfur is continuously collected at a sulfur trap  70  at condenser  120  and a sulfur trap  70  at subcooler  128 . 
     Tail gas remaining at the output of subcooler  128  contains less than 150 parts per million of hydrogen sulfide and sulfur dioxide and may be burned to atmosphere at burner  132 . 
     Referring to  FIG. 3 , an alternative embodiment of the tail gas treatment process provides a subdewpoint reactor system  124  for treatment of tail gas remaining after condenser  120 . The subdewpoint reactor system  124  includes at least one pair of subdewpoint reactors, reactor  124   a  and  124   b . Tail gas is transmitted to subdewpoint catalyst bed  124   a . Catalyst bed  124   a  is a catalyst bed utilizing aluminum oxide (Al 2 O 3 ) or bauxite as a catalyst for a Claus reaction. Catalyst bed  124   a  operates effectively when the temperature of the tail gas is around 260° F. Catalyst bed  124   a  and the condensing process are known in the art. As is known in the art, catalyst bed  124   a  is porous and subject to saturation as sulfur condenses from tail gas onto catalyst bed  124   a . Upon saturation of the catalyst bed  124   a , flow of tail gas is redirected to subdewpoint catalyst bed  124   b . Catalyst bed  124   b  is like catalyst bed  124   a  and likewise is subject to condensation of sulfur and saturation. 
     Upon redirection of tail gas to catalyst bed  124   b , the sulfur may be cleaned from catalyst bed  124   a . In like manner, upon saturation of catalyst bed  124   b  with sulfur, tail gas may be redirected to catalyst bed  124   a  and catalyst bed  124   b  may be cleaned. Such process of cleaning catalyst beds is known and practiced in the art. 
     In the alternative embodiment of  FIG. 3 , tail gas remaining after subdewpoint reactor system  124  is transmitted to subcooler  128  where the tail gas is further cooled preferably to a temperature of 66° C. (150° F.). Tail gas remaining at the output of subcooler  128  may be burned to atmosphere at burner  132 . 
     Modified SCOT Process. 
     Referring to  FIG. 4 , an alternative embodiment of the tail gas process comprises a modification to the Shell Claus Off-gas Treatment Process. In such alternative embodiment, the process gases remaining after condenser  76 , which are now referred to as the “tail gas,” are transmitted from condenser  76  by tail gas line  80  to a quench tower  84  to eliminate water vapor from the tail gas. Quench towers, such as quench tower  84 , are known in the art and are commonly used for removal of water vapor from the tail gas. 
     The tail gas from quench tower  84  is transmitted to an amine tower  88 . In amine tower  88 , hydrogen sulfide is absorbed by an amine solution contacting the tail gas. The remaining tail gas contains sufficiently reduced quantities of sulfur dioxide that the remaining tail gas may be burned to the atmosphere at burner  92 . 
     The amine and hydrogen sulfide solution from amine tower  88  is transmitted to an amine regenerator  96 . At amine regenerator  96 , hydrogen sulfide is stripped from the amine. Hydrogen sulfide from the amine regenerator is transmitted by hydrogen sulfide feed line  18  to thermal reactor  14 . 
     Amine towers, such as amine tower  88 , and amine regenerators, such as amine regenerator  92 , are commonly practiced in the art. 
     It is noted that the tail gas treatment of the present invention eliminates the expensive hydrogenation step of the prior art SCOT tail gas treatment process as the quantity of sulfur dioxide in the tail gas line  80  at the outlet of condenser  76  is reduced below a level that would cause corrosion or sulfur formation in the quench tower. This embodiment allows practice of the process of the present invention in plants having existing SCOT tail gas treatment facilities in place, but at reduced operation costs. 
     The foregoing disclosure and description of the invention is illustrative and explanatory thereof. Various changes in the details of the illustrated construction may be made within the scope of the appended claims without departing from the spirit of the invention. The present invention should only be limited by the following claims and their legal equivalents.

Technology Category: 4