Abstract:
A process for removing sulfur compounds, particularly hydrogen sulfide, from a waste gas wherein sulfur dioxide is introduced into the process gas at multiple process locations. Quantities of sulfur dioxide are introduced into the process gas stream at one or more locations preceding catalytic reaction. The process of the present invention may be practiced for Claus processes involving initial thermal reactions, and may be practiced without necessity of preliminary thermal reaction. In an embodiment of the invention, one of the injection locations of sulfur dioxide is the thermal reactor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/706,594 titled “Sulfur Recovery Process” filed in the United States Patent and Trademark Office on Sep. 27, 2012. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to processes for sulfur recovery and more specifically to processes for removing sulfur compounds, including hydrogen sulfide and sulfur dioxide, from refinery and like waste gas 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, referred to herein from time to time as waste gas or offgas streams. 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 catalytic reaction is widely known in the field and commonly used in sulfur recovery units. The currently practiced modified Claus process consists of a thermal stage and a catalytic stage. In the thermal stage, a waste gas containing hydrogen sulfide is injected into a thermal reactor where hydrogen sulfide is partially oxidized with air at high temperatures to form a quantity of sulfur dioxide. The thermal reaction further serves to oxidize ammonia. Combustion gases are cooled in a waste heat boiler in which a portion of the hydrogen sulfide reacts with sulfur dioxide to form water and elemental sulfur. The elemental sulfur is condensed and removed. 
     In the Clause process catalytic stage, remaining gases are transmitted to a series of reactors to further react hydrogen sulfide and sulfur dioxide to remove elemental sulfur. Typically the reactors utilize a catalyst, such as aluminum oxide, titanium oxide or bauxite. Typical Claus process catalytic stages include heating the process gas, reaction in the catalytic reactor, condensation of the elemental sulfur, and further transmission of the remaining process gases. One to four reactor stages are typically practiced; however, a typical process involves two or three reactors. Tail gas treatment processes further treat remaining process gas after the last reactor. 
     The Claus process involves reaction of sulfur dioxide (SO 2 ) and hydrogen sulfide (H 2 S) react to produce elemental sulfur (S 2 ) and steam/water (H 2 0). The reaction formula is:
 
2H 2 S+SO 2 →1.5S 2 +2H 2 O
 
     Stoichiometric balance of two moles of hydrogen sulfide to one mole of sulfur dioxide in the process is historically difficult to achieve due to, among other things, variations in composition of off-gas streams. 
     The present invention provides a process to obtain stoichiometric balance by introducing determined flow quantities of sulfur dioxide into the process stream at determined locations during a multiple stage reactor process, analyzing hydrogen sulfide content at one or more locations, and adjusting input flows of sulfur dioxide into the process stream at one or more locations. Advantages of the present invention include reduced total mass flow during the process and consequently reduce pressure drop, thereby providing improved process efficiency and increased capacity of existing Claus process facilities. In an embodiment of the present invention wherein sulfur dioxide is introduced into the thermal reactor, the injected sulfur dioxide reduces combustion air required in the thermal reactor, thereby reducing mass flow of, among other things, water and nitrogen, and correspondingly reduces pressure drop and increases efficiency of a given sulfur recovery unit. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention comprises an improvement to a Claus process for removing sulfur compounds, particularly hydrogen sulfide, from a waste gas wherein sulfur dioxide is introduced into the process gas at multiple process locations. In particular, quantities of sulfur dioxide are introduced into the process gas stream at one or more locations preceding catalytic reaction. The process of the present invention may be practiced for Claus processes involving initial thermal reactions, and may be practiced without necessity of preliminary thermal reaction. In an embodiment of the invention, one of the injection locations of sulfur dioxide is the thermal reactor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram 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 the steps of the process of the present invention. 
         FIG. 4  is a diagram of an alternative embodiment of the process of the present invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the preferred embodiment of the waste treatment process  10  of the present invention is depicted. Waste treatment process  10  of the present invention includes mixers  38 ,  48 ,  58 , heat exchangers  42 ,  54 ,  66 , reactors  46 ,  60 ,  74 , condensers  50 ,  64 ,  76 , sulfur dioxide splitter  14 , analyzer  78  and controller  52  collectively operable to provide controlled quantities of sulfur dioxide in relation to hydrogen sulfide contained in a waste gas stream containing hydrogen sulfide. Sulfur dioxide is introduced into the stream at multiple locations. 
     Referring to  FIG. 1 , hydrogen sulfide and other compounds comprise waste gas from a refinery or other industrial process and are contained in a waste gas line  18 . Waste gas line  18  typically contains various quantities of hydrogen sulfide, water, oxygen, nitrogen, carbon dioxide, sulfur dioxide, carbon monoxide and relatively small amounts of other hydrocarbon compounds. Compositions vary depending on the process, variations in the input stream and variations in the application of the process. 
     A sulfur dioxide line  12  containing sulfur dioxide is provided. A first quantity of sulfur dioxide is injected through sulfur dioxide line  12  into hydrogen sulfide waste line  18  and mixed at mixer  38 . The mixture of sulfur dioxide and waste gas containing hydrogen sulfide and sulfur dioxide downstream of mixer  38  is referred to herein as process gas. Process gas may describe various relative mixtures of sulfur dioxide and waste gas as process  10  progresses. 
     In an exemplary embodiment, sulfur dioxide is provided from a sulfur dioxide generator (not shown) or other source via sulfur dioxide line  12  to a sulfur dioxide splitter  14 . Splitter  14  is operable to allow flow of sulfur dioxide in various quantities therefrom. In an exemplary embodiment, splitter is a manifold connected to multiple output lines. 
     Flow quantity of sulfur dioxide delivered to mixer  38  may be adjusted by controller  52  in accordance with predetermined parameters or through operator input. Control of flow to mixer  38  may be accomplished by valves  22  or by valves (not shown) incorporated in mixer  38 . 
     In an exemplary embodiment the ideal amount of the first quantity of sulfur dioxide is an amount needed to provide a stoichiometric mix of sulfur dioxide and hydrogen sulfide, namely 2 moles of hydrogen sulfide to 1 mole of sulfur dioxide, to accomplish a Claus reaction, namely:
 
2H 2 S+SO 2 →1.5S 2 +2H 2 O
 
     Process gas is subsequently transmitted through process gas line  26  to heat exchanger  42  where the process gas is heated (if required) to a temperature at a determined level above dew point of sulfur vapor. As the dew point of sulfur is typically in a range of 120-150° C. (248-302° F.), the required temperature is typically a determined level above 150° C. (302° F.). Heating process gas to a determined level above the dew point of sulfur is known and commercially practiced. In the event waste gas contained in waste gas line  18  is at sufficient temperature to provide a process gas temperature at or above the determined temperature, heating of process gas at heat exchanger  42  is not required. 
     Process gas is then transmitted by line  26  to a first reactor  46  where a catalyst, such as aluminum oxide, titanium oxide or bauxite, is utilized to catalyze reaction of hydrogen sulfide and sulfur dioxide in the process gas to produce elemental sulfur and water. Reactor  46  is a conventional, commercially practiced Claus catalytic reactor. 
     Process gas is then transmitted by line  26  to a first condenser  50 . Elemental sulfur is condensed in first condenser  50 . Condensed sulfur is transmitted through line  82  to outlet container  100 . 
     Process gas remaining after condensation of sulfur from condenser  50  is transmitted through process line  26  to second mixer  48 . A second quantity of sulfur dioxide is transmitted from splitter  14  through sulfur dioxide line  12  and injected into process gas line  26  at second mixer  48 . Sulfur dioxide flow is controlled by controller  52  in combination with valve  22 . In an exemplary embodiment the ideal amount of the second quantity of sulfur dioxide is an amount needed to provide a stoichiometric mix of sulfur dioxide and hydrogen sulfide, namely 2 moles of hydrogen sulfide to 1 mole of sulfur dioxide, to accomplish a Claus reaction. Due to waste gas mix variations and flow variations, precise amounts are not readily determinable by calculation. 
     Process gas is then transmitted by line  26  to second heat exchanger  54  and heated to a temperature to a determined level above the dew point of sulfur. Process gas is then transmitted by line  26  to a second Claus reactor  60  where a catalyst, such as aluminum oxide, titanium oxide, or bauxite, is utilized to catalyze reaction of hydrogen sulfide and sulfur dioxide in the process gas. 
     Process gas is then transmitted by line  26  to second condenser  64 . Elemental sulfur is condensed in second condenser  64 . Condensed sulfur is transmitted through line  82  to outlet container  100 . 
     Process gas remaining after sulfur condensation in condenser  64  is transmitted through process line  26  to third mixer  58 . If needed, a third quantity of sulfur dioxide is transmitted from splitter  14  through sulfur dioxide line  12  and injected into process gas line  26  downstream of second condenser  64  at second mixer  48 . Controller  52  in combination with valve  22  or other control mechanism inputs a determined amount, if any, of sulfur dioxide. 
     Process gas is then transmitted by line  26  through third heat exchanger  66  and third Claus reactor  74 . Sulfur dioxide flow is controlled by controller  52  in combination with valve  22 . In an exemplary embodiment the amount of the second quantity of sulfur dioxide is an amount required to provide transmitted to provide a stoichiometric mix of sulfur dioxide and hydrogen sulfide, namely 2 moles hydrogen sulfide to 1 mole of sulfur dioxide, to accomplish a Claus reaction. 
     Process gas is then transmitted by line  26  to third heat exchanger  66  and heated to a temperature in a determined level above the dew point of sulfur. Process gas is then transmitted by line  26  to a third Claus reactor  74  where a catalyst, such as aluminum oxide, titanium oxide or bauxite, is utilized to catalyze reaction of hydrogen sulfide and sulfur dioxide in the process gas to produce elemental sulfur and water. 
     Process gas is then transmitted by line  26  to third condenser  76 . Elemental sulfur is condensed in third condenser  76 . Condensed sulfur is transmitted through line  82  to outlet container  100 . 
     Remaining process gas that has not been condensed, referred to as tail gas, is transmitted through tail gas line  80  for further processing. 
     As indicated hereinabove, controller  52  adjusts flow of sulfur dioxide delivered from sulfur dioxide splitter  14  to mixer  38 , second mixer  48  and third mixer  58 . Control devices such as controller  52  are known in the art and may incorporate various processors, input sources and output as needed to receive and analyze process analyzer data, store data, provide control commands and provide output, including output displays as are known in the art. 
     Analyzer  78  is utilized to determine the quantities of hydrogen sulfide and sulfur dioxide in tail gas line  80 . Analyzer  78  is electronically connected to controller  52 . Measured quantities of sulfur dioxide and hydrogen sulfide are communicated to controller  52 . Controller  52  is then operated to control flow of sulfur dioxide to mixer  38  mixer  38 , second mixer  48  and third mixer  58  pursuant to parameters previously input to controller  52  or as determined by an operator. 
     As the process  10  of the present invention comprises a continuous operation, flow rates of sulfur dioxide to mixer  38 , second mixer  48  and third mixer  58  may be continually adjusted to provide optimum process operation responsive to controller  52 . Upon stabilized flow of adjusted volumes of sulfur dioxide further analysis and flow adjustment of sulfur dioxide may be made to achieve optimum levels of hydrogen sulfide in tail gas transmitted by tail gas line  80 . 
     In an alternative embodiment depicted in  FIG. 2 , analyzers  78  are provided to determine hydrogen sulfide and sulfur dioxide quantities in process line  26  at other locations. Each analyzer is electronically connected to controller  52  (electronic connections are depicted by lines on  FIG. 2 ) to transmit measurements obtained. An analyzer  78  is provided at waste gas line  18  upstream of mixer  38  to determine quantities of hydrogen sulfide and sulfur dioxide in waste gas. Such determination allows determination of flow rate of sulfur dioxide to mixer  38  to achieve desired balance of sulfur dioxide to hydrogen sulfide at mixer  38 . An analyzer  78  is provided downstream of sulfur condenser  50  and upstream of mixer  48  to determine hydrogen sulfide and sulfur dioxide in process line  26  upstream of mixer  48 . Such placement allows determination of flow rate of sulfur dioxide to mixer  48  to achieve desired stoichiometric balance of sulfur dioxide to hydrogen sulfide at mixer  48 . An analyzer  78  is provided downstream of sulfur condenser  64  and upstream of mixer  58  to allow measurement of hydrogen sulfide and sulfur dioxide in process line  26  upstream of mixer  58  and allow determination of flow rate of sulfur dioxide to mixer  58  to achieve desired stoichiometric balance of sulfur dioxide to hydrogen sulfide at mixer  58 . Accordingly, the alternative embodiment of  FIG. 2  allows control of fluid flow to allow controlled mixing of hydrogen sulfide and sulfur dioxide for a Claus reaction at multiple locations based on determined quantities of hydrogen sulfide and sulfur dioxide in the process gas as analyzed at multiple locations. 
     In a third exemplary embodiment, an analyzer  78  is used to analyze hydrogen sulfide content of tail gas  80  and at least one other analyzer  78  is provided at one or more, but not all locations described in second exemplary embodiments thereby allowing selective determination of content of sulfur dioxide and hydrogen sulfide in the waste gas or process gas and adjustment of sulfur dioxide flow responsive thereto. In such embodiment, while precise hydrogen sulfide content of process gas and sulfur dioxide is not known upstream of all mixers, the multiple data points provide information useful to determine desired flow rate of sulfur dioxide to mixer  38 , mixer  48  and mixer  58  to obtain optimal hydrogen sulfide elimination by process  10 . 
     A fourth exemplary embodiment of the present invention comprises addition of a fourth mixer, heat exchanger, reactor and condenser to provide a fourth stage of Claus reaction. This embodiment is not depicted as it is repetitive of the second and third stages described herein. 
     A fifth exemplary embodiment of the present invention comprises deletion of a Claus reaction stage, thereby defining a two-stage process. In such embodiment, mixer  48 , heat exchanger  54 , reactor  60  and condenser  64  would not be included in the process. 
     In all exemplary embodiments wherein reference to stoichiometric balances of sulfur dioxide with hydrogen sulfide are referenced, it is noted that balances and flow rates other than exact stoichiometric quantities may be desirable at specific mixers  38 ,  48 , and  58  and that other than exact stoichiometric quantities may be required for operational reasons. Accordingly, the teachings of the process  10  are not limited to stoichiometric quantities. 
     Referring to  FIG. 3 , a method  200  of the present embodiment comprises: 
     A first mixing step  202  of mixing a sulfur dioxide stream with an waste stream, said waste stream containing hydrogen sulfide, the mixed stream referred to herein as a process stream; 
     A first heating step  204  of heating the process stream to a determined temperature. 
     A first reacting step  206  of reacting said hydrogen sulfide with said sulfur dioxide in the presence of a catalyst; 
     A first condensing step  208  of condensing sulfur from said process stream; 
     A second mixing step  210  of mixing a sulfur dioxide stream with said process stream; 
     A second heating step  212  of heating said process stream to a determined temperature. 
     A second reacting step  214  of reacting said hydrogen sulfide with said sulfur dioxide in the presence of a catalyst; 
     A second condensing step  216  of condensing sulfur from said process stream; 
     A third mixing step  218  of mixing a sulfur dioxide stream with said process stream; 
     A third heating step  220  of heating said process stream to a determined temperature. 
     A third reacting step  222  of reacting said hydrogen sulfide with said sulfur dioxide in the presence of a catalyst; 
     A third condensing step  224  of condensing sulfur from said process stream; 
     An analyzing step  226  of analyzing the amount of hydrogen sulfide in said process stream after said third condensing step; and 
     A control step  228  of adjusting flow of sulfur dioxide to the process stream at any of said first mixing step  202 , said second mixing step  210 , and said third mixing step  218 . 
     In an application wherein the temperature of said process gas exceeds the dew point of sulfur vapor downstream of said first mixing step  202 , first heating step  204  may be omitted. 
     In an alternative embodiment of the method, analyzing step  226  further comprises a step of analyzing the amount of hydrogen sulfide and sulfur dioxide in said waste gas prior to said first mixing step and in said tail gas. 
     In an alternative embodiment of the method analyzing step  226  comprises a step of analyzing the amount of hydrogen sulfide and sulfur dioxide in said waste gas prior to said first mixing step, in said tail gas and in said process gas prior to said second mixing step. 
     In an alternative embodiment of the method analyzing step  226  comprises a step of analyzing the amount of hydrogen sulfide and sulfur dioxide in said waste gas prior to said first mixing step, in said tail gas, in said process gas prior to said second mixing step, and in said process gas prior to said third mixing step. 
     Exemplary Simulation 
     In an exemplary simulation utilizing commercially practiced simulation methods and software developed for and used in the industry, a simulation produced the following exemplary process with calculated results. Quantities herein are generally rounded to whole numbers. 
     A first quantity of sulfur dioxide is injected by sulfur dioxide line  12  into hydrogen sulfide waste line  18  at mixer  38 . In the simulation, 39.5 kilogram moles (87 pound moles) per hour of sulfur dioxide are transmitted from splitter  14  to mixer  38 . At mixer  38 , the sulfur dioxide is mixed with a waste gas mixture containing, among other things, 132 kilogram moles (291 pound moles) per hour of hydrogen sulfide. 
     Process gas is heated at heat exchanger  42  to 448° C. (839° F.). Process gas is then transmitted by line  26  to a first Claus reactor  46  where an aluminum oxide based catalyst is utilized to catalyze reaction of hydrogen sulfide and sulfur dioxide. Process gas is transmitted to condenser  50  wherein sulfur is condensed at a temperature of approximately 177° C. (350° F.). Elemental sulfur is condensed at condenser  50  with 15 kilogram moles (33 pound moles) per hour of elemental sulfur transmitted to outlet container  100 . 
     Process gas from condenser  50  is transmitted to mixer  48  containing, among other things, 55.8 kilogram moles (123 pound moles) per hour of hydrogen sulfide and 1.4 kilogram moles (3 pound moles) per hour sulfur dioxide. From splitter  14 , 26.3 kilogram moles (58 pound moles) per hour of sulfur dioxide are transmitted to mixer  48 . Mixed process gas is then transmitted by line  26  to second heat exchanger  54  and then to second Claus reactor  60  at a reactor temperature of 193° C. (380° F.) to reactor  60 . At second reactor  60 , aluminum oxide is utilized to catalyze reaction of hydrogen sulfide and sulfur dioxide. Process gas is then transmitted from reactor  60  to second condenser  64 . At second condenser  64 , 10.9 kilogram moles (24 pound moles) per hour of elemental sulfur is condensed at a temperature of 135° C. (275° F.) and transmitted to outlet container  100 . 
     Process gas from condenser containing, among other things, 0.54 kilogram moles (1.2 pound moles) per hour of hydrogen sulfide, 0.28 kilogram moles (0.61 pound moles) per hour sulfur dioxide and 8.6 kilogram moles (19 pound moles) per hour of sulfur is 64 is transmitted to mixer  58 . No sulfur dioxide is transmitted from splitter  12  to mixer  58 . (Note that sulfur dioxide transmission from splitter  14  to mixer  58  may be done if needed to obtain needed molar balance). Process gas is then transmitted by line  26  to third heat exchanger  66  and then to third Claus reactor  74  at a reactor temperature of 135° C. (275° F.) at third reactor  74 . At third reactor  74 , aluminum oxide is utilized to catalyze reaction of hydrogen sulfide and sulfur dioxide. Process gas is transmitted from reactor  74  to third condenser  76 . At third condenser  76 , 0.4 kilogram moles (8 pound moles) per hour of elemental sulfur are condensed and transmitted to outlet container  100 . Process gas containing, among other things, 2.9 kilogram moles (6.5 pound moles) per hour of sulfur and 0.2 kilogram moles (0.5 pound moles) per hour of hydrogen sulfide, is transmitted by tail gas line  80  from condenser  76  to mixer  103 . 
     Tail gas is transmitted from third mixer  103  to incinerator  106 . While tail gas may be further processed if desired, the simulation indicates incineration. 
     In the simulation an input flow of 132 kilogram moles (291 pound moles) per hour of hydrogen sulfide is treated to accomplish an outflow of 0.2 kilogram moles (0.5 pound moles) per hour of hydrogen sulfide under the process of the present invention. 
     Referring to  FIG. 4 , a process  300  of an alternative embodiment of the present invention comprises injection of sulfur dioxide into thermal reactor  90 . In the embodiment of  FIG. 4 , a waste gas is introduced into reactor  90  by line  92 . A flow of waste gas is directed through line  86  to the burner chamber  94  where it is combusted with air. Air is injected into burner chamber  94  by air line  96 . The combusted waste gas flows from burner chamber  94  to thermal reactor  90 . A second flow of waste gas is injected into thermal reactor  90  by waste gas line  92 . Within chamber  94  and thermal reactor  90 , among other things, hydrogen sulfide reacts with air to produce sulfur dioxide, water and nitrogen, and sulfur dioxide reacts with hydrogen sulfide to produce elemental sulfur and water. 
     In the embodiment of  FIG. 4 , sulfur dioxide is introduced into thermal reactor  90  by way of sulfur dioxide line  12 . Sulfur dioxide is introduced into thermal reactor  90  to provide a quantity of sulfur dioxide for reaction with hydrogen sulfide existing in the waste gas and thereby reduce required quantity of combustion air. Reduced combustion air in the thermal reactor results in less mass of both nitrogen and water in the waste gas and process gas and decreases pressure drop during the sulfur removal process  10 . 
     From reactor  90  waste gas is transmitted to waste gas boiler  98  for cooling of the waste gas and heat recovery. The heat recovery process is not depicted as waste heat recovery is commonly practiced and known in the industry. Waste gas is then transmitted by waste gas line to condenser  88 . Elemental sulfur is condensed from waste gas at condenser  88 . Waste gas is then transmitted through waste gas line  18  to mixer  38  for processing according to the embodiments previously described herein. 
     Controller  52  and valves  22  are operable to control flow of waste gas through lines  92  and  86 , of air through line  96  and of sulfur dioxide through line  12 . Flow rates of waste gas through lines  92  to reactor  90 , of waste gas through line  86  to burner chamber  94 , of air through line  96  and of sulfur dioxide through line  12  may be determined by predetermined parameters. As indicated in previously-described embodiments, controller  52  may comprise a single controller or multiple controllers. As indicated in previously-described embodiments, multiple analyzers may be used. The process  300  of  FIG. 4  is practiced after condenser  88  as described in previously-described embodiments. 
     While the preferred embodiments of the invention have been described and illustrated, modifications thereof can be made by one skilled in the art without departing from the teachings of the invention. Descriptions of embodiments are exemplary and not limiting. The extent and scope of the invention is set forth in the appended claims and is intended to extend to equivalents thereof. The claims are incorporated into the specification. Disclosure of existing patents, publications and known art are incorporated herein to the extent required to provide reference details and understanding of the disclosure herein set forth.