Abstract:
A vessel is mounted external to a liquid sulfur storage pit for degassing liquid sulfur at atmospheric pressure. Liquid sulfur is re-circulated from the pit to a static mixing device extending from a head space to the liquid sulfur which provides intimate contact of the liquid sulfur as it flows downwardly and sweep air flowing through the head space above the pit. Further, the static mixing device prevents free fall of liquid sulfur and the hazards of static electricity associated therewith. Use of a heat traced gas outlet induces flow of sweep gas from the system, obviating the need for a steam eductor or blower.

Description:
FIELD OF THE INVENTION 
     This embodiment of the invention relates method and apparatus for removing hydrogen sulfide gases from liquid sulfur, and more particularly to methods for renewing liquid surfaces for evolving of gasses dissolved therein. 
     BACKGROUND OF THE INVENTION 
     An important process for removing hazardous hydrogen sulfide (H 2 S) from various waste gases, including gases produced during the refining of petroleum products, is known as the Claus process. It involves the following net reaction:
 
H 2 S+½O 2 →H 2 O+S   (1)
 
     Sulfur produced by the Claus process contains high levels of H 2 S (typically, 250 to 300 ppmw) which exists as both dissolved H 2 S and as H 2 S x  hydrogen polysulfides bound in liquid sulfur. The dissolved H 2 S separates from the sulfur readily; however, the H 2 S bound in the hydrogen polysulfides must be first decompose back into H 2 S and elemental sulfur.
 
H 2 S x →H 2 S+S x-1  
 
     This reaction is slow and accounts for the difficulty in degassing sulfur. The residence of time of liquid sulfur in conventional degassing processes can be several days. 
     The motivation to degas sulfur arrives from the toxicity, explosiveness, and corrosive nature of H 2 S. H 2 S is lethal at 600 ppmv, and is explosive at roughly 3.5% volume in air. Both of these conditions are of concern, especially during loading and unloading operations. The head space in a tank or tank truck can easily exceed the toxicity and explosive limit if the sulfur is not degassed. 
     Conventionally degassing takes place after the Claus process. Sulfur from the Claus process flows into a pit. Over time, as the sulfur cools somewhat and is agitated, H 2 S x  compounds decompose and form dissolved H 2 S and elemental sulfur. Desorbed H 2 S collects in the head or vapor space in the pit or vessel above the sulfur. 
     Despite the fact that both H 2 S and sulfur are flammable in air, the conventional industry practice is to use an air sweep of the sulfur pit vapor space to maintain the H 2 S level to well below the Lower Explosive Limit (LEL) of H 2 S. The Lower Explosive Limit for H 2 S is 3.85% at a storage temperature of 330° F. It is common industry practice to have sufficient sweep air to maintain a H 2 S concentration of less than 1% in the vapor space above the sulfur pit and thereby achieve a margin of safety. 
     Sweep air is typically drawn from the head space by a blower or a steam eductor. Such equipment is subject to fouling by crystalline sulfur. 
     Further, it is a disadvantage of modern, commercial degasification processes that they require large, complex and accordingly, expensive equipment. For example, in one process, known as the Shell process, degassing takes place in a storage tank or sulfur pit equipped with stripping columns, where liquid sulfur is vigorously agitated by bubbling air there through at atmospheric pressure. The stripping columns are open at the tops and bottoms to allow the sulfur to circulate at a rate of few hundred times per hour. The bubbling air, together with an additional flow of air, is then used as a low pressure sweep gas to displace the gases produced by the degasification process. The low pressure gases so produced are then fed to an incinerator where the H 2 S is oxidized to SO 2  and released to the atmosphere. Depending on the design, a liquid or gaseous catalyst, such as ammonia, ammonium thiosulfate, urea, morpholine, or an alkanol amine may be added for accelerating the decomposition of the polysulfide to H 2 S and elemental sulfur. 
     In an alternative process, known as the D&#39;GAASS process, degassing takes place in a vessel under pressure of at least 40 psig to 75 psig. Compressed air and high pressure sulfur are pumped to this pressure vessel. The pressure vessel contains a static mixing device which provides intimate contact between the two streams. The thus degassed liquid sulfur is discharged from the vessel and the air containing the H 2 S is discharged to an incinerator. 
     In another alternative process, known as the SNEA process, degassing takes place by repeated circulation and spraying the liquid sulfur into the sulfur pit. Release of dissolved H 2 S is achieved by spraying liquid sulfur through jets at a specific velocity. Ammonia, injected at the suction of the recirculation pump, is typically used as a catalyst. After the H 2 S gas is released, it is removed by a sweep gas and fed to an incinerator. 
     Both the stripping columns used by the Shell process and the circulation/spraying equipment used in the SNEA process are costly and require a large amount of space. The D&#39;GAASS process ignores the requirement to have sweep air in the sulfur pit vapor space or safe operation of the sulfur pit. 
     Accordingly, there has existed a definite need for a degasification process that, not only effectively reduces the H 2 S content of liquid sulfur but, is simple, requires a minimum of space and is inexpensive and results in safe conditions. 
     SUMMARY OF THE INVENTION 
     Now of the invention, there has been found a simple, effective and relatively inexpensive process employing apparatus for degassing liquid sulfur at low pressures including atmospheric pressure. Liquid sulfur is intimately mixed with oxidizing gas for maximal evolution of dissolved hydrogen sulfide without associated risks of the prior art. As earlier described, due to excellent insulating properties of molten sulfur, static electricity discharge can build up where free fall of sulfur is allowed. Several incidents have been reported where static electricity buildup was believed to have initiated a sulfur fire or explosion. 
     Accordingly, in one embodiment, degassing of liquid sulfur is achieved through agitation of the sulfur as it falls along a static mixing device such as one or more link-chains. Further, the static mixing device is preferably electrically grounded in which free fall is minimized, avoided or otherwise neutralized. 
     An upper end of one or more chains is suspended in the head space above the sulfur and extend to below the surface of the liquid sulfur residing in a pit or sump. A stream of liquid sulfur containing hydrogen polysulfides and H 2 S is introduced onto the upper end of the chains for flow downwards on the chains clinging thereto. As the liquid sulfur flows over the chain, the liquid is agitated and the vapor-liquid degassing surface is continually renewed for evolution of dissolved H 2 S. The liquid sulfur flows from the chain onto the vapor-liquid interface of the liquid sulfur in the pit. The circulated sulfur remains on the surface where there is continued contact with the sweep air to complete the dissolution of the H 2 S. The liquid sulfur is continuously re-circulated from the pit to the chain by a pump. Sweep air is introduced over the pit and along the sulfur flowing over the chains so that evolved H 2 S is collected. 
     In another embodiment, heat tracing of the sweep air outlet, such as through steam tracing, heats the sweep gas and induces flow of sweep gas from the head space by convection, thereby obviating the need for an eductor and avoiding blockages and complications due to crystalline sulfur deposition. 
     In some embodiments for even greater removal of H 2 S, the recirculation of liquid sulfur to the chain flows through a section of pipe that contains a solid catalyst for promoting the oxidation of the hydrogen polysulfides to H 2 S and elemental sulfur. Also in some embodiments, a liquid or gaseous catalyst for promoting the decomposition of hydrogen polysulfides into H 2 S is introduced at the suction of the re-circulating pump. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a degassing system implementing one embodiment of the invention and demonstrating co-current sulfur and sweep gas flow; 
         FIG. 2  is a schematic representation of a degassing system implementing another embodiment of the invention for demonstrating counter-current sulfur and sweep gas flow; and 
         FIG. 3  is a perspective drawing showing one alternate helical embodiment of a static mixing device for use in some embodiments of the process of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , a degassing apparatus  10  of one embodiment of the invention comprises a main vessel or pit having a sump or pit  11  for receiving liquid sulfur  12 . The materials of construction for the pit  11  and related equipment are chosen to be essentially inert to oxidizing gases, liquid sulfur, and the like and typically comprise carbon steel. The pit  11  has a roof closure  13  and a vertically-oriented degassing vessel  14 . The pit  11  has a liquid sulfur inlet  8 . 
     An influent stream of liquid sulfur  12  rich in hydrogen polysulfides and H 2 S, such as that produced by a Claus processing unit, flows through inlet  8  and into the pit  11 . The liquid sulfur accumulates in the pit  11  and forms a liquid surface  15 . A vapor head space  16  is formed above the liquid surface  15  which is in communication with the degassing vessel  14 . 
     Rich liquid sulfur  12  is re-circulated by a loading or transfer pump  20  through piping  21  and is introduced into an upper portion  22  of the degassing vessel  14  through a liquid sulfur inlet  23  for return to the pit  11 . 
     The sulfur transfer pump  20  can include those typically used in conventional Claus plants for delivering liquid sulfur from sulfur recovery units to a liquid storage container or to a sulfur loading station for transport. Consequently, conventional sulfur transfer or loading pumps  20  are readily adaptable for use in this embodiment. 
     Degassed or lean liquid sulfur  12  is removed from the pit  11  by transfer pump  40  and through discharge piping  41 . Discharge pump  40  is typically similar to re-circulation pump  20  and is positioned for receiving and discharging degassed liquid sulfur as a product. 
     The degassing vessel  14  is fit with one or more static mixing devices  30 . The number of devices  30  is related to the re-circulation rate of sulfur  12 , the higher the flow rate, the larger the device or the greater the number of devices  30 . Liquid sulfur  12  from the liquid sulfur inlet  23  is directed onto the one or more static mixing devices  30 . 
     In one embodiment shown in  FIGS. 1 and 2 , and for simplicity, one device  30  is shown in the cross-section illustrations although device  30  can represent one or more link-chains. The static mixing device  30  promotes agitation of the stream of liquid sulfur  12  as it flows downwards; the sulfur clings onto the device  30  by the surface tension of the liquid sulfur. Other embodiments of the static mixing device  30  include rope or metal rope, pipe, or a conventional fluid mixing device such as a helical device. 
     The static mixing device  30  minimizes free fall of the liquid sulfur  12 . Free fall of liquid sulfur has been shown to create static electricity which can cause ignition of the H 2 S, sulfur vapors or other combustible gases found in the head space  16  above the liquid sulfur in the pit  12 . The static mixing device is electrically grounded  31  to earth so prevent the build-up of static electricity. 
     With reference to  FIG. 3 , a helical static mixing device  30  consists of a series of stationary, rigid elements  32  placed to similar effect as the link-chain embodiment. The helical elements form intersecting channels that split, rearrange, and recombine the component streams. Similar static mixing devices are manufactured by Koch Engineering Co., Wichita, Kans. and Chemineer Kenics, North Andover, Mass. (www.kenics.com). 
     As the liquid sulfur  12  containing hydrogen polysulfides and H 2 S flows over the mixing device  30  the liquid is agitated and a vapor-liquid degassing surface is continually renewed for evolution of dissolved H 2 S therefrom. 
     The static mixing device  30  not only efficiently agitates the liquid sulfur, but also because it has no moving parts, it adds to the simplicity and low cost of the process. 
     The devices  30  extend substantially continuously between the liquid sulfur inlet  23  and vapor-liquid interface or surface  15  of the liquid sulfur in the pit  11 . The sulfur inlet can include a reservoir having one or more discharges through which the one or more static mixing devices nozzle extend for directing the liquid sulfur onto each of the one or more devices before they extend out of the reservoir and downwardly to the pit  11 . The liquid sulfur flows along and from the static mixer  30  onto the liquid sulfur surface  15 . 
     Sweep air  33  is introduced into the apparatus through gas inlet  42  to traverse the degassing vessel and the head space  16  for extraction through outlet  44  so that evolved H 2 S in the head space  16  is removed from the pit  11 . 
     The liquid sulfur  12  traverses the pit  11 , much of which remains adjacent surface  15  where there is continued contact with the sweep air  33  to complete the dissolution of the H 2 S. Rich liquid sulfur is continuously re-circulated from the pit  11  to the mixing device  30  by pump  20 . 
     Returning to  FIG. 1 , one embodiment of a method of operation is shown. A stream of sweep air  33 , such as an oxidizing gas or air, is introduced at the inlet  42  located near the top of the degassing vessel  14 . The sweep air  33  is drawn downward in the degassing vessel  14  and into the vapor space  16  above the sulfur pit. In a co-current flow, the sweep air and liquid sulfur both progress down the degassing vessel  14 . 
     The sweep air collects H 2 S through the head space  16  and is expelled through a discharge  44  at the opposite end of the pit  11 . The sweep air  33  is expelled by the vacuum action provided by a vacuum pump such as an eductor or more preferably is induced by convective chimney effect through a heat tracing  34  of the discharge  44  such as through heat-jacketing of related piping. 
     Conventional processing would have the sweep air  33  containing H 2 S disposed in an incinerator or recycled to the suction of combustion air blowers of the sulfur recovery plant. The rate of sweep air  33  through the head space  16  removes evolved H 2 S gas so that the concentration of the H 2 S gas is below the lower explosive limit (LEL) of the H 2 S in air and more preferably to about ¼ of the LEL to provide a safety factor of 4:1. The residence time of liquid sulfur in the degassing vessel  14  is typically less than about one minute and more preferably from about one second to about 30 seconds. 
     Any suitable oxidizing gas can be employed as sweep gas  33 . Representative oxidizing gases include air, oxygen-enriched air, mixtures of gases containing oxygen, sulfur dioxide and sulfur dioxide-enriched gases. Air or oxygen-enriched air is preferred. 
     The pit  11 , re-circulation piping  21 , sweep air inlet and discharges  42 , 44  and the liquid sulfur influent  8  and effluent  41  are typically steam jacketed  34  which provides for the flow of steam or other suitable heating media therebetween. This enables the various streams to be heated to a temperature above the melting temperature of solid sulfur to a temperature of from about 265° F. to about 285° F. Preferably, the temperatures are maintained at about 270° F. for optimal release of H 2 S. The degassing vessel  14  is also surrounded by a carbon steel jacket for a heating media such as steam to circulate between the vessel and the jacket and avoid formation of solid sulfur onto the vessel&#39;s inner surfaces. 
     As seen in  FIG. 1 , the liquid sulfur stream and the oxidizing gas streams pass co-currently through the degassing vessel  14 . 
     In an alternative embodiment, the streams of liquid sulfur  12  and sweep gas  33  pass counter-currently as shown in  FIG. 2 , or in any arrangement therebetween including cross flow (not shown). 
     With reference to a counter-current embodiment of  FIG. 2 , the liquid sulfur stream is continuously circulated by pump  20  and down the degassing vessel  14 . Sweep air is introduced from inlet  42  and flows through the head space  16  to rise up the degassing vessel  14  against the downward flow direction of the liquid sulfur. Again, the sweep gas  33  is continuously circulated to remove H 2 S and other combustible gases from the head space  16  and the degassing vessel  14 , shown as being induced by heat tracing  34  of the discharge  44 . 
     As shown as an option in  FIG. 2 , to further enhance the degassing reaction, the liquid sulfur and oxidizing gas streams are contacted in a catalyst section  24  containing a solid catalyst bed for promoting the oxidation of hydrogen polysulfides to H 2 S and elemental sulfur. Preferred catalysts include Claus catalysts, including activated alumina. Claus catalysts are well known in the art. They are typically made of activated alumina in a suitable shape, such as spheres or pellets. Other suitable catalysts include Claus-like catalysts, such as titanium dioxide, Selectox™ (manufactured by Davisson Chemical Co.) and the like. The catalyst section  24  can comprises a pipe spool between pump  20  and the Inlet  23  to the degassing vessel  14 . The catalyst bed is kept in place in the catalyst section  24  with screens mounted between flanges located at both ends of the pipe spool. In an alternative embodiment (not shown), the catalyst bed is located inside the degassing vessel  14 , below the sulfur inlet connection  23 . 
     In some embodiments, a liquid or gaseous fluid catalyst for promoting the decomposition of hydrogen polysulfides to H 2 S is added to either or both streams. In the embodiment shown in  FIG. 1  and  FIG. 2 , the fluid catalyst is introduced through piping  21  before the liquid sulfur stream is introduced into the degassing vessel  14 . Representative fluid catalysts include ammonia, ammonium thiosulfate, morpholine, urea, alkanol amines, such as diisopropanol amine, and mixtures thereof. 
     It is another major advantage of the method of the invention that it results in the removal of substantially all of the total H 2 S from the initial liquid sulfur stream, where “total H 2 S” means the total of both hydrogen polysulfides and H 2 S by weight. Using the inventive process, the total H 2 S can be reduced to less than about 30 ppmw. 
     One understands that total H 2 S can be further reduced through adjusting operational parameters including increased re-circulation, increased effective length of static mixing and increase residence times. Total H 2 S can be reduced to less than about 10 ppmw. One may opt to employ one of the above catalyst embodiments in addition to adjusting other operational parameters. 
     Furthermore, the degassing vessel  14  operates at atmospheric pressure and is considerably smaller, simpler, and less expensive than the pressurized vessel used in the D&#39;GAASS process, than the stripping columns used in the Shell process, and the circulation/spraying equipment used in the SNEA process. 
     While the invention has been described in detail with reference to certain preferred embodiments thereof, it will be understood that modifications and variations are within the spirit and scope of that which is described and claimed.