Abstract:
A novel system and method for degassing H 2 S and H 2 S x  from liquid sulfur (sulphur) is disclosed. The system includes a degassing vessel with a plurality of cells. The cells include a sparging gas mat with a perforated surface at the bottom of the cell to allow the release of air bubbles (or sparging gas) into the cells. A catalyst may be used during the process. As a result, hydrogen sulfide and hydrogen polysulfide are efficiently and effectively removed from the liquid sulfur.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of and is a division of non-provisional U.S. patent application Ser. No. 13/598,516, filed Aug. 29, 2012 which the application is hereby incorporated by reference for all purposes in its entirety. 
     
    
     BACKGROUND OF INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to an improved process to remove hydrogen sulfide and hydrogen polysulfide from liquid sulfur (sulphur). Hydrogen sulfide is removed by mass transfer to air bubbles generated by passing air through a perforated plate. A volatile catalyst is used to rapidly decompose hydrogen polysulfide to hydrogen sulfide. 
         [0004]    2. Description of the Related Art 
         [0005]    The Claus process is extensively used to produce liquid sulfur from sour oil and gas or other gaseous hydrogen sulfide. It is known in the industry that produced sulfur contains two hydrogen sulfide species, namely, physically dissolved hydrogen sulfide, H 2 S, and hydrogen polysulfide, H 2 S x , which is the reaction product of dissolved H 2 S with liquid sulfur. Liquid sulfur produced in processing plants using the Claus process may contain upwards of 500 parts per million by weight (ppmw, where H 2 S x  is reported as H 2 S equivalent). 
         [0006]    The prior H 2 S-sulfur system involves two coupled reversible reactions, namely the physical dissolution of H 2 S in liquid sulfur, which decreases with temperature, (represented by reversible reactions (1 below), where H 2 S (g)  denotes H 2 S in the gas phase and H 2 S (d)  denotes H 2 S dissolved in liquid sulfur) and the existence of a further reversible reaction between dissolved H 2 S and liquid sulfur (2 below), which increases with temperature. 
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         [0007]    After production in Claus plants, dissolved H 2 S spontaneously degasses into the headspace of tanks and/or vessels found throughout the handling, storage and transportation chain (e.g., pits, tanks, railcars, truck tankers, and the like). Over time, the concentration in the gas phase may reach toxic or explosive levels. The lower explosive limit in air is approximately 4% by volume; the lethal concentration is approximately 600 parts per million by volume (ppmv); In addition, nuisance odor may result from fugitive emissions in concentrations lower than 0.001 ppmv. A need to degas sulfur is apparent in view of the potential accumulation hazardous levels of H 2 S in the handling chain. The degassing benchmark generally adopted by the industry (typically to avoid reaching explosive levels during transportation) is 10 parts per million by weight. 
         [0008]    The principal considerations with respect to industrial degassers involve the rates at which dissolved H 2 S is transferred from the liquid phase into the gas phase (reaction −1) and the decomposition of H 2 S x  (reaction −2). The degassing rate of dissolved H 2 S may be maximized by (a) producing a large surface for gas-liquid mass transfer and (b) reducing the boundary layer next to the interface through which the dissolved H 2 S diffuses to reach gas-liquid surface. Large surface area may be created by generating a myriad of fine sulfur droplets, generating a myriad of bubbles of stripping gas, or packing. In methods using bubbles, the preferred stripping gas in most instances is air because it is cheaper than inert gases or steam, plus air has the additional benefit that a portion of the H 2 S and H 2 S x  is consumed by reacting with oxygen (reaction products are sulfur and water). Agitation or circulation is almost always part of the process since this enhances the rate of diffusion of dissolved H 2 S through the liquid boundary layer surrounding the bubbles. 
         [0009]    Whereas dissolved H 2 S evolves directly to a gas phase, H 2 S x  generally does not. The process whereby H 2 S x  is removed is typically via a first decomposition to dissolved H 2 S (reaction −2), followed by mass transfer of dissolved H 2 S degasses across the gas-liquid boundary (reaction −1). The decomposition reaction tends to be very slow, such that H 2 S x  persists as a source of H 2 S gas for a long time. 
         [0010]    The slow decomposition of H 2 S x  represents a main obstacle in the degassing processes. For this reason, various degassing processes make use of a catalyst to accelerate the decomposition reaction. Catalysts may be liquid or solid (generally, a bed of granules). Many different chemical types have been used, including amines. In the past, use of amines fell into disfavor by the industry because solid sulfur, subsequently solidified, was unacceptably friable which resulted in a very dusty product. 
         [0011]    Once the H 2 S is transferred to the gas phase, it may be removed from the degasser using various removers, such as fans, eductors, and the like. The effluent containing the H 2 S extracted from the liquid sulfur may be delivered to an incinerator, a tail gas treatment unit or back to front end of the Claus processing plant. 
         [0012]    Numerous degassing patents have been granted, some of which are relevant based on the method used with regards to gas-liquid contact (sparging) and the use of amine-type liquid catalysts. Whereas the degassing rate is critically dependent on the efficiency of sparging, patents that disclose a sparging gas provide little to no description of the sparging apparatus. Illustrative examples from relevant patents are identified below. 
         [0013]    U.S. Pat. No. 4,729,887 (Pendergraft) discloses a vessel which is a concrete pit with 3 cells. The middle cell contains a bed of alumina or cobalt-molybdenum impregnated alumina (solid catalyst). Air is delivered to distributor manifold provided with a plurality of perforated pipes under the catalyst bed. Air assists in circulating sulfur through the bed. 
         [0014]    U.S. Pat. No. 5,935,548 (Franklin) discloses a system where sulfur is agitated and mixed using an eductor supplied with partly degassed sulfur (which agitates/mixes liquid sulfur). Air is supplied through a pipe and discharged (a) in the vicinity of the eductor (b) into the stream of partly degassed sulfur or (c) to a “sparger” underneath the eductor(s). The sparger appears to consist of a pipe provided with openings. The diameter of the pipe or openings therein is not specified. 
         [0015]    U.S. Pat. No. 6,149,887 (Legas) discloses an apparatus consisting of various arrangements of cells and baffles. Heated gas is fed to distributors in each cell. Franklin purports to generate finely divided gas bubbles using tubes with a multiplicity of small openings. 
         [0016]    U.S. Pat. No. 6,676,918 (Wu) discloses a method to degas in Claus rundown seal pots. Compressed air is injected under pressure via a line having a small opening nozzle into the annular space of the seal pot. 
         [0017]    US patent application 2011/0182802 A1 (Garg) discloses a system that supplies compressed air to a gas diffuser located below a packing. The difusser has a predetermine shape and size and is provided with holes ¼ of openings in the packing. Use of a sintered metal diffuser may also be used. 
         [0018]    Canadian Patent No. 2,170,021 (Ellenor) discloses up to four cells equipped with an impeller/shroud assembly. Air is ingested into liquid sulfur by the impeller and the mixture then passed through a perforated shroud creating small bubbles to aerate the cell. High turbulence combined with tiny bubbles results in very fast degassing. A mixture of morpholine and cyclohexylamime is added to catalytically decompose H 2 S x . The last cell is dedicated to the removal, by degassing, of the volatile catalyst. Doing so eliminates the objection of producing friable (solid) product. 
       SUMMARY OF INVENTION 
       [0019]    According to one aspect of one or more embodiments of the present invention, the invention consists of a compact, portable, inexpensive apparatus and process that produces a high-quality product with less than 10 ppmw of H 2 S. Rapid degassing is achieved using a novel sparging system. In one embodiment, the sparging system uses a perforated plate and compartments to produce a high concentration of gas bubbles in the sulfur liquid. The high concentration of gas bubbles rising through the sulfur results in a large surface area and promotes vigorous agitation that results in the rapid removal of hydrogen sulfide. Use of a catalyst such as n amine or a mixture of amines results in the rapid removal of hydrogen polysulfide from the liquid sulfur. 
         [0020]    Other aspects of the present invention will be apparent from the following description, drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1  shows a cutaway elevation view of a degasser apparatus in accordance with one or more embodiments of the present invention. 
           [0022]      FIG. 2  shows an isometric view of a component of a sparger apparatus in accordance with one or more embodiments of the present invention. 
           [0023]      FIG. 3  shows an isometric view of a component of a sparger apparatus in accordance with one or more embodiments of the present invention. 
           [0024]      FIG. 4  shows an exploded view of the uniformly spaced perforations of the sparging gas mat in accordance with one or more embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Specific embodiments of the present invention will now be described in detail with reference to the accompanying figures. Like elements in the various figures are denoted by like reference numerals for consistency. Further, in the following detailed description of embodiments of the present invention, numerous specific details are set forth in order to provide a more thorough understanding of the present invention. In other instances, well-known features have not been described in detail to avoid obscuring the description of embodiments of the present invention. 
         [0026]      FIG. 1  shows a cutaway elevation view of a degasser apparatus in accordance with one or more embodiments of the present invention. The degasser apparatus consists of a vessel  1 . The shape of the vessel  1  shown in the figure is a box like shape. One skilled in the art will recognize that the shape of the vessel does not have to be a box shaped, rather what is important is that the vessel is sized to suit the production rate. In one embodiment of the present invention, the vessel  1  is box-like, measuring 2.1×6.4×2.5 meters (W×L×H), which is large enough to degas sulfur at 90 tonnes per hour. Higher or lower throughputs are contemplated. That is, degassing throughputs is scalable. Higher throughputs can be accommodated by using more than one degasser in parallel. A scaled-down (smaller) version can be used for throughputs significantly lower than 90 TPH to optimize efficiency and minimize cost in construction, footprint, energy consumption and the like. It is contemplated that the throughputs of the degassing process may be reduced by having cells with reduced volumes, such as smaller footprints. 
         [0027]    In one embodiment, the outer walls of vessel  1  are constructed of dimple plates consisting of channels for the passage of steam and condensate. Steam is used to keep the contents in the vessel  1  above the melting point of sulfur. The preferred liquid sulfur temperature range is 125° C. to 155° C. The temperature of the sulfur may be measured (instruments not shown) and controlled by a PCL system. The vessel  1  can also be insulated. 
         [0028]    In one embodiment of the present invention, the vessel  1  is provided with partitions  2 ,  3 ,  4  and  5  that divide the vessel  1  into four degassing cells  6 ,  7 ,  8 ,  9  and one pump cell  10 . 
         [0029]    The partition  2 ,  3 , and  4  divide the vessel  1  into a plurality of degassing cells or tanks, each being approximately the same size. The preferred embodiment includes 4 degassing cells. These partitions extend above the normal sulfur level. Also, in the preferred embodiment, the height of partitions  2 ,  3 , and  4  is 1.6 meter tall. The height of partition 5 determines operating sulfur level in cell  9 , so it is typically lower than the other cells (generally up to the level of the standpipes (as discussed below)). In plan view, the cells are 2.1 m wide×1.5 m long×2.5 m high. In one embodiment, the liquid sulfur levels 52 (i.e., height) of each cell is somewhat lower from cell to cell as the liquid sulfur traverses the degassing cells  6 ,  7 ,  8 ,  9 . 
         [0030]    A sulfur line  11  (generally, steam-jacketed) admits liquid sulfur LS (having elevated levels of H 2 S and H 2 S x ) to the cell  6 . In the preferred embodiment, the liquid sulfur LS is admitted continuously at an approximately constant rate. The sulfur flows from the cell  6  into the degassing cell  7  through a standpipe  12 . Likewise, the liquid sulfur from the cell  7  flows to the degassing cell  8  through a standpipe  13  and from the degassing cell  8  to the degassing cell  9  through a standpipe  14 . The liquid sulfur is degassed while in resident in the cells  6  to  9 . The cell  9  is further dedicated to the removal of catalyst (as discussed below). A degassed sulfur from the cell  9  flows over a partition  5  into a pump cell  10 . 
         [0031]    A sulfur pump supplying the liquid sulfur via line  11  may or may not be necessary (and is not shown). In one embodiment, the standpipes  12 ,  13  and  14  are identical in size and shape. The preferred diameter of the standpipes is 0.10 to 0.30 meter, and further preferred from 0.15 to 0.25 meter. The preferred top to bottom length of the standpipe is 0.3 to 2.0 meter (the height can be variable to get the desired cell liquid volume dictated by desired residence time). The standpipes are supported by bulkhead flanges (not shown) through the partitions. It is also contemplated in a second embodiment of the invention that standpipes not be used to transfer the liquid sulfur from one cell to another. In the second embodiment, sulfur flows from cell to cell through openings (or perforations or slots) in the partitions  2 ,  3  and  4 . The openings may be rectangular perforations/slots or circular holes located close to the floor so that flow entering a cell tends to swept into the column of rising (as disclosed below) gas bubbles. 
         [0032]    In the preferred embodiment, a sulfur pump  15  removes the degassed sulfur DS from the pump cell  10  via a line  16 . The level in the pump cell  10  is generally maintained by a control valve  47 . The sulfur pump  15  or its operation is not always required. In some cases, the degassed sulfur DS in the cell  10  may be simply gravity drained into a pit (not shown). Generally, level control in the pump cell  10  is not strictly required in this case. However, in the preferred embodiment the vessel  1  is “sealed.” This means that the exit point from the drain pipe is always below the sulfur level in the pit. The sulfur line  11  is provided with a shutoff valve or flow control valve  49 , but is not required. 
         [0033]    An air line  17  is in fluid communication with an air line  18  and a sparging gas mat  19  in the cell  6 , an air line  21  and a sparging gas mat  22  in the cell  7 , an air line  24  and a sparging gas mat  25  in the cell  8  and an air line  27  and a sparging gas mat  28  in the cell  9 . In the preferred embodiment, the sparging gas mats  19 ,  22 ,  25  and  28  are provided with perforated plates  20 ,  23 ,  26  and  29 , respectively. A blower  30  is used to provide a sparging gas  70  to the line  17 . The pressure in line  17  is controlled by a valve  31 . The preferred pressure is 1 to 10 psi (the pressure is dictated primarily by the height of the sulfur above the sparging gas mat (the level may vary, depending on process requirements)). Valves  32 ,  33 ,  34  and  35  in the lines  18 ,  21 ,  24 , and  27 , respectively, are provided to control the flow of the sparging gas to each respective sparging gas mat (as discussed below). 
         [0034]    It is not critical that every degassing cell gets the same sparging gas flow, but in the preferred embodiment, the sparging gas flow should be roughly equal. According to one or more embodiments of the present invention, a sparging gas mat for each degassing cell is shown in  FIGS. 2 and 3 . A sparging gas mat  200  consists of a perforated plate  210  that covers a substantial portion of the degassing cells&#39; footprint. In one embodiment of the present invention, the efficiency of the sparging gas mat  200  is due to (a) exposing the liquid sulfur in a cell to small sparging gas bubbles rising up the cell and (b) the upward flow of sparging gas bubbles results in the liquid sulfur being agitated and circulated in the cell(s) (see e.g., induced sulfur circulation  220  for the degassing cell  8 , in  FIG. 1 ). The degree of exposure, circulation and agitation of the sparging gas bubbles and the liquid sulfur depends on the air/sulfur ratio. When air is used as the sparging gas, the preferred air/sulfur ratio ranges from 0.008 to 0.15 m 3  air per kg sulfur and further preferred from 0.037 to 0.094 m 3  air per kg sulfur. 
         [0035]    With reference to  FIGS. 2 and 3 , in one embodiment, the sparging gas mat  200  consists of a welded frame  300  with internal partitions  310 . A cutaway view of a cell wall  230  is shown in  FIG. 2 . The partitions  310  create a multiplicity of compartments or chambers (in the preferred embodiment, the welded frame  300  has eight partitions  310  creating 8 compartments). The perforated plate  210  is attached (in the preferred embodiment, the plate is bolted) to the frame  300  and the partitions  310 . A sparging gas pipe  220  (which could be for example, the line  18  in  FIG. 1  for the cell  6 ) is attached at the center of the frame  300 . The sparging gas pipe  220  extends to the bottom of the frame  300  where openings  255  are provided for sparging gas to flow into a plurality of compartments  240   a,    240   b,  and  240   c,    240   d  (not shown in this embodiment, are the other 4 compartments of the sparging gas mat  200 ) below the perforated plate  210 . The compartments  240   a - h  assist in distributing the sparging gas evenly over the surface area of the plate  210 . 
         [0036]    The sparging gas mats  200  are located at the center of each cell ( 6 ,  7 ,  8  and  9 ) and are generally positioned centrally on the bottom floor  205  of the cell ( 6 ,  7 ,  8  and  9 ). In one embodiment, the sparging gas mats  200  measure 0.07×1.3×1.3 meters (surface area), which generally covers 54% of the footprint of the cell&#39;s bottom floor  205 . The preferred area of mats  200  range from 25% to 95% of the footprint of the cell ( 6 ,  7 ,  8  and  9 ). As shown in  FIG. 1 , sparging gas (e.g., air bubbles)  400  from the sparging gas mat  200  rise to the surface where they disengage from the liquid sulfur to occupy a headspace  50 . The column of rising air bubbles  400  results in vigorous agitation and circulation  220  of the liquid sulfur. 
         [0037]    In one embodiment, perforations, such as holes  250  in the perforated plate  210  are 1.02 mm in diameter on 2.26 mm stagger  260  providing 22% open area, defined as the area of the holes relative to the area of the perforated plate.  FIG. 4  illustrates the uniformed spaced holes in the perforated plate  210 . In another preferred embodiment, the holes are 0.838 mm in diameter on 3.327 mm stagger  260 , providing 5.8% open area. A larger stagger  260  means that the perforations  250  are spaced wider apart which, in combination with smaller holes, reduces the chance that bubbles will coalesce as they rise to the surface of the liquid sulfur. This is desirable since the surface area for mass transfer (hence degassing rate) is not diminished as the bubbles rise through the sulfur column. 
         [0038]    Referring to  FIG. 1 , a sparging gas heater  36  may be provided to heat the sparging gas flowing in the line  17 . The sparging gas heater  36  may be used when the vessel  1  is used in very cold climates or environments. The sparging gas heater  36  prevents the liquid sulfur LS from freezing. A line  37  is provided to remove stripping air enriched with H 2 S plus other volatile gasses that may be found in liquid sulfur (such as COS, CS 2  and H 2 O), catalyst, water, sparging gas, containing hydrogen sulfide (H 2 S), sulfur dioxide (SO 2 ), and sulfur vapour out of the headspace  50  of the vessel  1 . The gases  60  are removed to downstream treatment (not shown) by a fan  38  which keeps the headspace  50  under slight vacuum. 
         [0039]    A catalyst pump  40  supplies a catalyst from a catalyst tank  39  and pumps it to a line  41 . The line  41  is in fluid communication with lines  42 ,  43  and  44  that terminate with a check valve (not shown). The check valves prevent sulfur from flowing up the lines. The line  42  is in fluid communication with the sulfur line  11  in order that the catalyst mixes with the liquid sulfur LS before the liquid sulfur LS enters the degassing cell  6 . The line  43  is disposed to flow the catalyst into the standpipe  12  so as to mix with the liquid sulfur in the degassing cell  7 . The line  44  is disposed to flow the catalyst into a standpipe  13  to mix with the liquid sulfur in the degassing cell  8 . Flow indicator/control valve assemblies  45 ,  46  and  47  are provided in lines  42 ,  43  and  44  to control the flow of the catalyst independently to each degassing cell. A sulfur flowmeter  48  is provided in the line  11  to control the rate at which the catalyst is supplied to the line  41 . Control systems (not shown) may be used to control the catalyst flow rates. 
         [0040]    In the preferred embodiment, the catalyst is usually distributed unequally to the cells  6 ,  7  and  8 . The majority of catalyst is injected into the line  11  and at diminishing rate into the degassing cells  7  and  8 . This process allows the catalyst to be depleted by degassing from cell to cell. 
         [0041]    The dosage rate of catalyst to each cell is adjusted according to the concentration of 
         [0042]    H 2 S x , which may be known beforehand. Concentration rates are provided to the control system (not shown). In the preferred embodiment, the total catalyst injection rate may range from 0 to 15 ppmw depending on the H 2 S x  concentration. In one embodiment, the catalyst is an aqueous mixture of morpholine and cyclohexylamime. 
         [0043]    A control system (not showed) may be used to control various components in the novel system, such as rates for the introduction of liquid sulfur and as indicated above, the catalyst into the system and flow rates for the introduction of sparging gases and the removal of effluent gases from the system. 
         [0044]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. It will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not be considered limited to what is shown and described in the specification and drawings.