Abstract:
A process of producing degassed liquid sulfur using process gas containing H2S to agitate the liquid sulfur being degassed while in contact with a degassing catalyst. Process gas is less costly and less complicated and quickly accomplishes substantial degassing rendering the liquid sulfur much safer in storage and transportation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/837,927 filed Jun. 21, 2013, entitled “APPARATUS FOR IN-SITU PRODUCTION OF LOW DISSOLVED HYDROGEN SULPHIDE, DEGASSED, SULFUR FROM CLAUS SULFUR RECOVERY,” and to U.S. Provisional Application Ser. No. 61/837,944 filed Jun. 21, 2013, entitled “APPARATUS FOR IN-SITU PRODUCTION OF LOW DISSOLVED HYDROGEN SULPHIDE, DEGASSED, SULFUR FROM CLAUS SULFUR RECOVERY,” and to U.S. Provisional Application Ser. No. 61/837,950 filed Jun. 21, 2013, entitled “PROCESS FOR IN-SITU PRODUCTION OF LOW DISSOLVED HYDROGEN SULPHIDE, DEGASSED, SULFUR FROM CLAUS SULFUR RECOVERY,” and to U.S. Provisional Application Ser. No. 61/837,958 filed Jun. 21, 2013, entitled “PROCESS FOR IN-SITU PRODUCTION OF LOW DISSOLVED HYDROGEN SULPHIDE, DEGASSED, SULFUR FROM CLAUS SULFUR RECOVERY,” and to U.S. Provisional Application Ser. No. 62/010,766 filed Jun. 11, 2014, entitled “PROCESS FOR DEGASSING CONDENSED SULFUR FROM A CLAUS SULFUR RECOVERY SYSTEM”, all five of which are incorporated herein by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None. 
     FIELD OF THE INVENTION 
     This invention relates to the recovery of degassed sulfur from a Claus sulfur recovery plant and especially to substantially reducing the H 2 S content of liquid sulfur for the safe storage and transportation of liquid sulfur. 
     BACKGROUND OF THE INVENTION 
     The Claus process is a gas desulfurizing process for recovering elemental sulfur from gaseous hydrogen sulfide. It was first developed in the 1880&#39;s and has become an industry standard for refineries, chemical plants and natural gas processing plants. As petroleum and natural gas is tending to contain ever increasing amounts of sulfur compounds while fuel regulations are tending to mandate less allowable sulfur in fuel, Claus processes become increasingly important. 
     A Claus plant, which is a multi-step process within a larger industrial plant is arranged to recover sulfur from gaseous hydrogen sulfide. Typically, elemental sulfur is produced by a thermal step and several catalytic steps. Elemental sulfur is separated from the Claus plant as a liquid at one or more condensers. 
     While current sulfur condensers have proven satisfactory for condensing sulfur, there is a need for improvement in the quality of the sulfur condensed. The problem is that the condensed sulfur includes dissolved H 2 S. Over an extended time, the H 2 S will eventually disassociate from the liquid sulfur and accumulate as a toxic and flammable gas in vapor spaces at the top of the storage or transport vessels. Since an unsafe condition is possible until the sulfur is fully degassed of dissolved H 2 S, precautionary steps are required prior to opening a sulfur vessel and while transferring liquid sulfur from one vessel to another. 
     It has been found that it is the nature of a liquid sulfur produced in a sulfur condenser process that reactant hydrogen sulfide (H 2 S) is incorporated into the sulfur as simple dissolved H 2 S and also as chemically bound with sulfur in the form what is sometimes called a sulfane or polysulfane. Sulfane is H 2 S x , (with x&gt;1). H 2 S x  will convert back to H 2 S and elemental sulfur in time through an equilibrium reaction which may be accelerated with a catalyst. This is a known problem and most efforts to remove H 2 S from the elemental sulfur include bubbling various gases such as air and preferably inert gases such as nitrogen and carbon dioxide through the liquid sulfur while in a catalyst bed. This degassing process, while necessary, takes time and adds to the expense of capturing sulfur from refineries, gas plants and chemical plants that deal with sulfur. 
     Thorough degassing is imperative as capturing and disposing of H 2 S that is emanating from liquid sulfur storage is another issue. If the elemental sulfur is not adequately degassed, H 2 S emanating from liquid sulfur storage may become a fugitive emission in an area that is closely monitored for environmental compliance. In some instances, up to half of the reported emissions from a Claus sulfur recovery plant and Claus Tail Gas Cleanup unit can come from H 2 S emanating from liquid sulfur in storage. Without degassing operations or adequate capture and disposal technology, these additional emissions may limit the sulfur processing capability of the Claus/TGU (Tail Gas Unit) unit. 
     Technology is needed to reduce costs and overcome and resolve these problems without creating new disadvantages. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     The invention more particularly relates to a process for producing liquid sulfur that is degassed of H2S. The process includes a sulfur degassing catalyst and liquid sulfur in a vessel wherein the sulfur degassing catalyst and liquid sulfur define a contact zone and condensed products are directed to the vessel from a Claus plant into the contact zone of the vessel. These condensed products include elemental sulfur, dissolved H 2 S and H 2 S x  where x≧2. The conversion of H 2 S x  is catalyzed on the surface of the sulfur degassing catalyst to form H 2 S and elemental sulfur and process gas from the Claus plant is directed at an elevated pressure into the contact zone of the vessel to agitate the sulfur degassing catalyst and liquid sulfur. The process gas also carries H 2 S that has formed on the surface of the sulfur degassing catalyst away from the sulfur degassing catalyst. The process gas includes H 2 S prior to entering the vessel. The processes gases along with H 2 S from the contact zone are exhausted for further processing in the Claus plant and liquid sulfur that is degassed of H 2 S is extracted from the contact zone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a flow diagram showing a basic and conventional Claus sulfur recovery system; 
         FIG. 2  is a flow diagram showing liquid sulfur being degassed of H 2 S; 
         FIG. 3  is a flow diagram of the vessel connected to the Claus process; and 
         FIG. 4  is a flow diagram of an alternative embodiment showing the liquid sulfur being degassed of H 2 S. 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow. 
     Referring now to  FIG. 1 , a line diagram for a conventional Claus Sulfur Recovery Plant is generally indicated by the numeral  10 . Claus Plants have been in use for more than a century at petroleum refineries to remove sulfur from gases containing H 2 S. Undertaking a brief explanation of a conventional Claus Plant, referring to Claus Sulfur Recovery Plant  10 , gas having sulfur, typically in the form of H 2 S, enters via conduit  12 . A burner  15  along with reaction furnace  18  are provided to burn and oxidize at least part of the H 2 S to elemental sulfur SO 2  and water wherein the reaction is:
 
10H 2 S+5O2→2H 2 S+SO 2 +7/2S 2 +8H 2 O.
 
     These very hot gases and vapors are cooled down in a waste heat boiler  19  and a first condenser  22  where the elemental sulfur is condensed and removed at liquid discharge conduit  25 . Cooling water is provided to both the waste heat boiler  19  and to the condenser  22 , as shown at inlet  23  to make steam, as shown at outlet  24 , for use in making electricity or elsewhere in the in the Claus Sulfur Recovery Plant  10  or in the larger industrial plant that is not shown. The remaining gases from the first condenser  22  are directed through the gas conduit  28  to reheater  30  where the gases are reheated and then delivered to a catalytic conversion to elemental sulfur for converting remaining H 2 S and SO 2  to elemental sulfur. The chemical process is generally described as follows:
 
2H 2 S+SO 2 →3S+2H 2 O.
 
     Again, the process gases are cooled in the second sulfur condenser  32  so that elemental sulfur may be condensed to a liquid and removed at the second liquid discharge conduit  35 . The gases are conventionally directed by a conduit  38  to further sulfur recovery steps including catalytic reactor  41  and  51  to recover liquid sulfur at discharge conduits  45  and  55 . 
     It should be noted that more thorough descriptions of a Claus system may be found in many other places and there are doubtless variations known in the art. This description has been presented simply to describe the improvement related to degassing the liquid sulfur acquired by most any Claus system. 
     Referring now to  FIG. 2 , a sulfur degassing vessel  60  is arranged to receive the liquid sulfur from liquid discharge conduits  25  and  35  at a lower portion of the vessel  60  or at the bottom of the vessel  60 . Inside the vessel  60  is liquid sulfur with a contained catalyst  62  held within a contact zone  65  that is generally above the lower portion of the vessel  60 . A degassed liquid sulfur discharge line  66  is arrange to remove liquid sulfur above the contact zone such that liquid sulfur entering the vessel  60  must pass completely through the contact zone  65  or at least through a substantial portion of the contact zone  65 . The catalyst  62  may take one of several forms. The first form is a plurality of high surface area alumina particles (spheres, extrudates, etc.) constrained to prevent being removed or carried away by sulfur flow from the vessel  60 . A second form is a plurality of similarly constrained high surface area alumina particles impregnated with iron oxides. A third form is one or more low surface area alumina porous ceramic foam supports coated with high surface alumina particles with or without impregnated iron oxide. 
     The catalyst  62  converts H 2 S x  to H 2 S and elemental sulfur. Reducing the content of H 2 S x  at this stage of the production of liquid sulfur has been found to substantially reduce the tendency of stored liquid sulfur to slowly yield H 2 S gas. The productivity of the catalyst  62  is enhanced by agitation, especially by gas. In the present invention, as distinguished from prior known systems, the gas used to stir the liquid in the contact zone  65  is process gas from the Claus process delivered through conduit  28 A of H 2 S containing gas. A metering valve may be arranged so that a small amount of process gas may be injected near the bottom of the vessel in a sparger  63  to agitate the liquid and the catalyst  62  to carry produced H 2 S back to the Claus Sulfur Recovery Plant  10 . The process gas may alternatively be taken off conduit  38  as shown by dotted line conduit  28 B. 
     Once the gas has passed through the vessel  60 , it exits at exit conduit  68  and rejoins the Claus process downstream of the source of the process gas at conduit  28 . In the preferred arrangement, the process gas rejoins the Claus process at conduit  48  via conduit  48 A. It should be noted that the process gas may optionally be arranged to rejoin the Claus process at conduit  38  as shown by dotted line  38 A. However, there is a more significant pressure drop between conduits  28  and  48  to allow for more vigorous stirring of the catalyst  62  by the process gas from sparger  63 . For even more vigorous stirring, the process gas may optionally be arranged to rejoin the Claus process further along the system such as at conduit  51 A. This arrangement is shown by dotted line  48 B and may be preferred if the source of the process gas used in vessel  60  comes from conduit  38  via conduit  28 B. While the process gas from the vessel  60  will not have been subjected to all of the successive treatments in the catalytic reactors  31 ,  41  and  51 , it may have some vaporous elemental sulfur that could be condensed in condenser  52  and may be subjected to further sulfur removal treatment in a tail gas unit, which are conventional in Claus plants. 
     One additional side reaction occurring in the contact zone  65  worth mentioning is additional conversion of H 2 S to elemental sulfur. The process gas includes some SO 2  and may reaction on the surface of the catalyst with H 2 S that may be condensed in the liquid sulfur, emanating from the liquid sulfur by the decomposition of H 2 S x , or contained in the process gas. This reaction is the same chemical reaction occurring in the Claus process and is generally described as: 2H 2 S+SO 2             3/xS x 2H 2 O. Having additional active catalyst for this chemical reaction to occur yields more liquid sulfur separated from the industrial process and less sulfur compounds in process gas.
     It is noted that it has long been recognized that the catalytic process occurring in contact zone  65  is an equilibrium reaction and therefore, gases that have been used for agitating the catalyst always exclude H 2 S. This is more expensive than simply using a side stream of process gas. And, the process gas includes enough H 2 S to warrant further sulfur recovery steps so that H 2 S recovered in the vessel  60  is simply and efficiently disposed. As compared to an arrangement using air or nitrogen, the air and nitrogen will have acquired small amounts of H 2 S and elemental sulfur vapor that must be handled. These gases usually must be directed to a part of the sulfur recovery unit where combustion can convert the elemental sulfur to SO 2  to avoid plugging the vent line from the process either contributing additional SO 2  emissions or requiring recycling with some motive fluid such as air or steam. This adds additional costs to operating a Claus process. 
     The process gas, as noted above generally includes H2S. Process gases in line  28  may contain about 4% to about 9% by volume H 2 S and typically about 8% by volume H 2 S. Process gases in line  38  typically comprise less H 2 S, but certainly have sufficient pressure to agitate the catalyst  62  and return to the Claus process  10 . Process gases in line  38  may have between 2% to 5% H 2 S by volume and typically about 4% by volume H 2 S. Process gases in line  48  still retain sufficient pressure to be used to agitate the catalyst  62  and has a lower H 2 S content being about 0.5% H 2 S to about 3% H 2 S by volume and typically about 1% H 2 S to about 2% H 2 S by volume. 
     The full Claus process with the vessel  60  is generally shown in  FIG. 3  showing the side stream of process gas being taken from line  28  and being carried through the vessel  60  and back into the Claus process at line  38 . 
     The vessel  60  may optionally be arranged to receive liquid sulfur discharged through drains  45  and  55  for degassing. In the preferred arrangement, the liquid sulfur discharge lines  45  and  55  are combined with the degassed liquid sulfur in line  66 . It has been found that such small streams of liquid sulfur really do not contain much H 2 S x  that needs degassing. Most of the liquid sulfur is gathered from the first two condensers  22  and  32 . 
     In an alternative embodiment shown in  FIG. 4 , the liquid sulfur may be received at the top of the vessel  60  and liquid elemental sulfur having the sulfanes removed may be withdrawn at the bottom of the vessel  60 . In this embodiment, the sulfur is travelling counter to the flow of the process gas through the reaction zone  65 . In  FIG. 2 , the catalyst zone  65  is shown as being liquid continuous, whereas, in the alternative embodiment shown in  FIG. 4 , the catalyst zone may be gas continuous with the liquid sulfur trickling down through the contact zone  65 . 
     Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.