Abstract:
A method of removing hydrogen sulfide from a hydrocarbon stream such as natural gas or refinery off gas, including those from catalytic crackers, hydrocrackers, hydrotreaters, chemical plant processes, etc. Sulfur dioxide from an external source is directly introduced into the hydrocarbon stream to promote a Claus reaction to remove the hydrogen sulfide by converting it into elemental sulfur and water. The hydrocarbons in the stream are unaffected by the process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    1. Field of Invention This invention relates to a method of removing H 2 S from hydrocarbon stream such as natural gas or refinery off gas, including those from catalytic crackers, hydrocrackers, hydrotreaters, chemical plant processes, etc. Specifically, the invention directly injects SO 2 from an external source into the hydrocarbon stream to promote a Claus reaction to remove the H 2 S.  
           [0004]    2. Related Art The Claus reaction is well known in the art of removing H 2 S from a hydrocarbon stream. The Claus reaction formula for removing H 2 S by converting it into elemental sulfur is: 
           2H 2 S +SO 2 →1.5S 2 +2H 2 O 
           [0005]    The sulfur dioxide (SO 2 ) serves as a “promoter” to convert the hydrogen sulfide (H 2 S) into elemental gaseous sulfur (S 2 ) and steam (H 2 O). The reaction can occur with or without a catalyst.  
           [0006]    In the prior art, the hydrocarbon stream is first passed through an amine treater to remove the H 2 S. The hydrocarbon stream passes up through a tray tower while liquid amine (typically methyl amine, di-ethyl amine, methyl-di-ethyl amine or DGA) flows down across the trays. The amine captures the H 2 S, and this “rich” amine then passes through a stripper, where the H 2 S is stripped off. The H 2 S then undergoes the Claus reaction in a sulfur recovery unit (SRU) using SO 2  that has been formed from the oxidation of a portion of the stripped H 2 S. The prior art oxidation of H 2 S to form SO 2 takes place at very high temperature (typically between 2,000° F and 3,000° F) and at low pressure (typically between 0.9 and 1.2 atmospheres). At these high temperatures, any residual hydrocarbons may crack, forming coke. This coke clogs up the system, and reduces the efficiency of the process. Further, there is typically a downstream post-oxidation residue of about 3% H 2 S that must undergo additional treatment after the sulfur recovery unit. This residue is typically treated in the tail gas treating unit for conversion into SO 2 or H 2 S for use in the above described Claus reaction.  
           [0007]    The prior art typically requires low pressures for a variety of reasons, most of which are due to the structure of the oxidation and reaction equipment used. This requires the hydrocarbon stream, which is typically at high pressure, to be reduced in pressure prior to processing. This low pressure results in low partial pressure of the reactants (partial pressure being a function of the reactant concentration and total pressure), making the gas phase Claus reaction less efficient compared to that efficiency found in a high pressure environment.  
           [0008]    Further, the Claus reaction is reversible. That is, if sulfur is not pulled off after being formed, the reaction can be reversed to reform H 2 S and/or SO 2 .  
           [0009]    Thus, the prior art requires the hydrocarbon stream to be internally manipulated by reducing its pressure, stripping off H 2 S and oxidizing part of the H 2 S to form SO 2 , which oxidation step often requires burning part of the valuable hydrocarbon itself.  
         BRIEF SUMMARY OF THE INVENTION  
         [0010]    The present invention eliminates most of the process and problems described above. Since hydrocarbons do not react with H 2 S or SO 2 , by injecting the promoter SO 2  directly into the hydrocarbon (HC) stream, the Claus reaction occurs while the hydrocarbons in the hydrocarbon stream remain inert to the reaction. Since the present method does not require the oxidation of H 2 S (as is common in the prior art), the method can take place under high pressure, typically between 5 and 30 ATM. Oxidation units (burners) in the industry typically operate at low pressure. By not having to oxidize the H 2 S, the method can operate at high pressure since no oxidation (burning) is required of the H 2 S. However, the method can operate at either high or low pressure, typically between 1 and 30 ATM. As there is no oxidation, there is minimal or no carbon cracking to form coke, soot, etc.  
           [0011]    The modified Claus reaction of the present inventive method therefore is: 
           HC+2H 2 S+SO 2 →1.5S 2 +2H 2 O +HC 
           [0012]    This process of removing H 2 S from the HC stream can occur in any standard environment. Preferably, the inventive process occurs in essentially two steps for optimal efficiency. In Step  1 , the modified Clause reaction occurs on a first catalyst bed, which typically contains micro-porous pellets that adsorb the elemental liquid sulfur, and allow the water and hydrocarbon to pass through. Eventually, the first catalyst bed becomes saturated (clogged) and must be flushed out. This flushing occurs in Step  2 , which takes heated hydrocarbons and water from a second catalyst bed to flush out the first catalyst bed. The flushed sulfur is allowed to cool to a liquid and be pulled off, while the hydrocarbon and water pass on for further treatment. In the preferred embodiment, the liquid sulfur is trapped in a sulfur trap as described in the Smith U.S. Pat. No. 5,498,270, issued Mar. 12, 1996. By opening and closing appropriate valves, the same process is used to flush out the second catalyst bed when it becomes saturated.  
           [0013]    Alternatively, the process may occur in any standard process environment, including but not limited to a straight through Claus catalytic reactor system.  
           [0014]    Accordingly, the objectives of this invention are to provide, inter alia, a new and improved method of removing H 2 S from a hydrocarbon stream that:  
           [0015]    can be performed at high pressure;  
           [0016]    does not require high temperature, and thus does not oxidize any components in the hydrocarbon stream;  
           [0017]    does not involve oxidation of H 2 S to form SO 2 ;  
           [0018]    requires less equipment than that of prior art;  
           [0019]    minimizes the potential for a reverse Claus reaction; and  
           [0020]    is cost effective.  
           [0021]    These objectives are addressed by the inventive method. Other objects of the invention will become apparent from time to time throughout the specification hereinafter disclosed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 depicts the first step of removing sulfur from a hydrocarbon stream using a first catalytic bed.  
         [0023]    [0023]FIG. 2 depicts the second step of flushing sulfur from the first catalytic bed.  
         [0024]    [0024]FIG. 3 depicts the combined system having multiple catalytic beds and their preferred connective piping and mechanical equipment.  
         [0025]    [0025]FIG. 4 depicts the inventive process used in a straight through Claus catalytic reactor system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    The present invention is described as system  10 , depicted in a preferred embodiment in FIG. 3. System  10  comprises the steps depicted in block form in FIGS.  1  and FIG. 2. Note that the essential feature of system  10  is that SO 2 from an external source is directly introduced into the HC stream to promote the equation: 
         HC+2H 2 S+SO 2 →1.5S 2 +2H 2 O+HC 
         [0027]    The H 2 S is thus removed from the HC stream in the form of elemental sulfur without manipulating the HC itself or stripping off the H 2 S to form SO 2 by oxidation. In the best mode, the amount of SO 2  introduced into the HC stream (containing H 2 S) is slightly less than a 1:2 ratio of SO 2 :H 2 S. This ensures complete use of the SO 2  to prevent SO 2  in the downstream, which can be problematic (such as the formation of SO 3 and/or H 2 SO 4 ). However, the H 2 S:SO 2  ratio can be adjusted according to the needs of the system and operator, depending on what residue of H 2 S or SO 2 can be tolerated or are desired downstream.  
         [0028]    As depicted in FIG. 1, in the preferred embodiment the first step of system  10  is the removal of H 2 S from inlet hydrocarbon stream  21  using the promoter SO 2 in a Claus reaction. Inlet hydrocarbon stream  21  may be refinery off gas, petrochemical or chemical plant off gas, natural gas or any other hydrocarbon stream in which H 2 S is to be removed. Hydrocarbon stream  21  comprises hydrocarbon(s) (HC) and H 2 S, with the H 2 S typically of a concentration between 3% and 20%, depending on the source of inlet hydrocarbon stream  21  and its make-up.  
         [0029]    Inlet hydrocarbon stream  21  enters system  10  via main inlet line  12 . In the preferred embodiment, if inlet hydrocarbon stream  21  is less than 260° F, it is heated in heat exchanger  40  to a temperature between 260° F and 300° F to prevent sulfur in inlet hydrocarbon stream  21  from solidifying. SO 2 is then introduced into main inlet line  12  to combine with hydrocarbon stream  21  form process stream  15   a,  which comprises HC, H 2 S and SO 2 . The introduced SO 2 is produced from an external source, typically a nearby SO 2 generating unit that uses any SO 2 producing method known in the art of chemical and petrochemical processing.  
         [0030]    Process stream  15   a  continues through first inlet line  22  into first catalytic reactor  20 , which comprises first catalyst bed  23 . On first catalyst bed  23 , the H 2 S in process stream  15   a  reacts with the SO 2 , following the equation of the exothermic Claus reaction: 
         [0031]    [0031]         HC   +     2        H   2        S     +     SO   2            →                catalyst                         1.5        S   2       +     2        H   2        O     +   HC                             
         [0032]    It is significant that the HC does not react and is not structurally affected by first catalyst bed  23 .  
         [0033]    The catalyst in first catalyst bed  23  typically comprises catalyst beads, which comprise alumina, activated charcoal, or aluminum carbonate (AL 2 CO 3 ). The catalyst beads are typically ⅛″ to ⅜″ diameter beads having a high porosity for absorbing and/or adsorbing condensed elemental sulfur.  
         [0034]    Because the Claus reaction is exothermic, in the preferred embodiment first catalyst bed  23  is cooled, typically using cooling coils, to promote the reaction, which is more efficient at temperatures just above the freezing point of sulfur (248° F). Further, cooling catalyst bed  23  promotes the condensation of the sulfur to a liquid. The preferred range of temperature of first catalyst bed  23  is between 250° F and 280° F.  
         [0035]    Desulfured process stream  34   a,  comprising HC, H 2 O, S 2 vapor and trace amounts of H 2 S (typically 100 to 1000 ppm) leaves first catalytic reactor  20 , and enters vaporous sulfur recovery unit  50   a,  where S 2 vapor is removed using any device and/or method for capturing sulfur vapor known in the art, including absorbing dry beds or liquid processing systems.  
         [0036]    Leaving vaporous sulfur recovery unit  50  is devapored process stream  36   a  , which comprises HC, H 2 O and trace amounts of H 2 S. Devapored process stream  36   a  may optionally then proceed to further treatment unit  70 , depending on regulatory or process requirements. For example, further treatment unit  70  may perform dehydration to remove the H 2 O, or further treatment unit  70  may remove or lower the concentration of trace amounts of H 2 S, using any process for lowering H 2 S concentrations known either in the prior art (including an amine unit) or taught by the inventive system  10 .  
         [0037]    When first catalyst bed  23  becomes saturated with liquid S 2 , the S 2 is flushed out as depicted in FIG. 2. Inlet HC stream  21  mixes with SO 2 , typically after inlet HC stream  21  is heated in heat exchanger  40 . By closing valve  80  and opening valve  82  shown in FIG. 3, the mixture of inlet HC stream  21  and SO 2 passes through second inlet line  32  as process stream  15   b  (comprising HC, H 2 S and SO 2 ) into second catalytic reactor  30 , where it reacts in a Clause reaction on second catalytic bed  31 . Typically, second catalytic reactor  30  and second catalytic bed  31  comprise the same structure, temperature control and catalytic beads described above for first catalytic reactor  20  and first catalytic bed  23 . Liquid sulfur remains in and/or on second catalytic bed  31 , and desulfured process stream  34   b,  comprising HC, H 2 O, a small amount of vapor sulfur and a trace amount of H 2 S, leaves second catalytic reactor  30  and enters vaporous sulfur recovery unit  50   b.  Typically vaporous sulfur recovery unit  50   b  is structurally and functionally equivalent to vaporous sulfur recovery unit  50   a.  Leaving vaporous sulfur recovery unit  50   b  is devapored process stream  36   b,  comprising HC, H 2 O and trace amounts of H 2 S. Devapored process stream  36   b  passes through and is heated to a preferred temperature between 380° F and 420° F in heat exchanger  41 , which is preferably a shell tube cross heat exchanger for recovering downstream heat from desulfured wash stream  26   a  and/or devapored wash stream  38   a  described below. Heated devapored process stream  36   b  then passes through first flush line  35  as inlet flush stream  43   a  (the heated stream of HC, H 2 O and trace amounts of H 2 S). Inlet flush stream  43   a  passes across the catalyst beds of first catalyst bed  20 , flushing out the liquid S 2 . Sulfur rich wash stream  24   a  coming from first catalyst bed  20  comprises HC, H 2 O, liquid S 2  and trace amounts of H 2 S (typically 100 to 1000 ppm). Sulfur rich wash stream  24   a  enters liquid sulfur recovery unit  60   a,  which is any liquid sulfur recovery device known in the art of chemical and petrochemical processing, including the sulfur trap described by Smith in U.S. Pat. No. 5,498,270, issued Mar. 12, 1996. The liquid sulfur is eventually removed from liquid sulfur recovery unit  60   a.  Leaving liquid sulfur recovery unit  60   a  is desulfured wash stream  26   a,  which comprises HC, H 2 O, small amounts of S 2 vapor, and trace amounts of H 2 S. In the preferred embodiment, desulfured wash stream  26   a  passes through vaporous sulfur recovery unit  50   a  to remove the small amounts of S 2 vapor. Devapored wash stream  38   a,  comprising HC, H 2 O and trace amounts of H 2 S, leaves vaporous sulfur recovery unit  50   a  for further processing, if necessary, in further treatment unit  70 , as described above for devapored process stream  36   a  .  
         [0038]    When first catalytic reactor  20  has been flushed of liquid S 2 , appropriate valves are opened and closed to allow first catalyst bed  23  to start removing H 2 S from process stream  15   a  as described above. The desulfured process stream  34   a  from first catalytic reactor  20  can then be used to flush out second catalyst bed  31  in an analogous manner to the process described above and depicted in FIG. 2 for first catalyst bed  23 .  
         [0039]    Valves  80 - 95  can be opened and closed in a wide range of permutations. One such setting may allow one or both desulfured process streams  34   a  and  34   b  to be released downstream, either through or around further treatment unit  70 . While it is essential that liquid sulfur recovery unit  60  remove the flushed liquid sulfur from either catalytic reactor, vaporous sulfur recovery units  50  may be circumvented.  
         [0040]    Further, while FIG. 3 shows only a first catalytic reactor  20  and a second catalytic reactor  30  with associated piping, heaters, and vapor and liquid sulfur traps, it is understood that more than two catalytic reactors may be utilized in the same interacting manner described above for just two reactors.  
         [0041]    Alternatively, system  10  may be used in any process environment capable of introducing into the hydrocarbon stream a desired amount of SO 2 from an external source to remove H 2 S from that hydrocarbon stream without manipulating the hydrocarbon itself. For example, system  10  may be used in a straight through Claus catalytic reactor system, as depicted in FIG. 4. HC stream  21  is heated in heat exchanger  40  and then passes through first pass through catalytic reactor  120 . Unlike first catalytic reactor  20  described above, first pass through catalytic reactor  120  does not absorb the elemental sulfur produced in the Claus reaction, but allows the sulfur to pass through to a first liquid sulfur recovery unit  60   a,  where the sulfur is pulled out of process stream  15   a  leaving desulfured process stream  134   a.  Desulfured process stream  134   a  typically still has some residual H 2 S, so desulfured process stream  134   a  is reheated in heat exchanger  41 , additional SO 2  from an external source is introduced to form process stream  15   b, which reacts in second pass through catalytic reactor  130 . Sulfur is pulled off in liquid sulfur recovery unit  60   b,  and desulfured process stream  134   b  continues to further treatment unit  70 , which may be a dehydrator, amine unit or additional Claus catalytic reactors as described herein.  
         [0042]    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.