Abstract:
A novel hydrocarbon feedstream catalyst bed for the desulfurization of a gas or a liquid hydrocarbon feedstream and a process comprising passing a hydrocarbon feedstream over the catalyst bed is described. The bed comprises at least two catalysts having different sulfur compound affinities and/or specificities thereby improving the overall amount of sulfur compound removal. The process reduces the sulfur content in a gas hydrocarbon feedstream from up to about 300 ppm to less than about 500 ppb, and in a liquid hydrocarbon feedstream from up to about 3% to less than about 500 ppb.

Description:
BACKGROUND  
         [0001]    The present invention relates to a catalyst bed for the desulfurization of a hydrocarbon feedstream. The catalyst bed comprises at least two catalysts, each having an affinity for sulfur-containing compounds. When used in combination, the catalyst bed demonstrates significant reductions in the sulfur concentration in the feedstream. A process for reducing the concentration of sulfur compounds in a hydrocarbon feedstream to a level of less than about 500 ppb is also disclosed.  
           [0002]    Hydrocarbon feed streams, such as natural gas (NG), liquified petroleum gas (LPG) and gasoline, are used as the starting materials for several chemical processes, many of which utilize catalysts in one or more reaction steps. However, problems frequently arise during the chemical processing if the hydrocarbon feed stream also contains sulfur compounds. These sulfur compounds can poison the reaction catalysts rendering the catalyst bed ineffective. Nickel catalysts, which are generally useful for in hydrogenation reactions, are especially sensitive to sulfur poisoning on their active surfaces. Similarly, many precious metals which are used in a variety of catalysts, are sensitive to sulfur and can be easily poisoned by the presence of sulfur or sulfur-containing compounds. Poisoning of the catalysts results in longer then desired reaction times, formation of undesired side reaction products, reduction in the life expectancy of the nickel catalyst, and, in some instances, poor quality of the finished product. Thus, it is beneficial to reduce the sulfur content in the hydrocarbon feed stream before it reaches the chemical processing catalyst bed.  
           [0003]    However, hydrocarbon feed streams have different sources of origin. This means that each feed stream has unique sulfur compound contaminants and contaminants present at different concentrations. For example, Table I shows some sulfur species commonly found in natural gas, LPG and gasoline streams. Moreover, the sulfur species vary not only by type of feed stream but also by source of origin. In other words, the natural gas feed stream composition originating in Alaska can vary significantly from the natural gas feed stream composition originating in northern Russia.  
                                 TABLE 1                           Some sulfur species commonly found in hydrocarbon feedstocks            Species   Natural Gas   LPG   Gasoline               H 2 S   X   X   —       Carbonyl Sulfide (COS)   X   —   —       t-Butyl Mercaptan   X   X   —       Di-sulfides   X   X   X       Dimethyl Sulfide (DMS)   X   —   —       Tetrahydrothiophene (THT)   X   —   X       C2-C3 mercaptans   X   X   X       thiophene   —   —   X       C4+ mercaptans   —   —   X       benzothiophenes   —   —   X       subs. - benzothiophenes   —   —   X       sulfides   —   —   X                                  
 
           [0004]    A number of different catalysts that are effective for removing sulfur compounds are known in the art. For example, activated carbon has a high capacity for ethyl mercaptans, manganese oxide is effective for dimethyl sulfoxide removal, and zinc oxide can be used to remove hydrogen sulfide. Other catalysts known to be effective in desulfurization processes include carbon, copper/zinc oxides, nickel-based sorbents, nickel oxides, zeolites, molecular sieves and faujasites, among others. In addition, different methods have been used to reduce the sulfur level in feedstreams. The most commonly used procedure involves the application of a hydrogen recycle stage to convert the sulfur-containing compounds to H 2 S, and then the removal of the sulfur compounds in a separate step. This can be an arduous and time-consuming procedure. Thus, a better method for removal of sulfur-containing compounds is needed. However, because of the variations in the specific sulfur compound species and the concentration of the sulfur compounds in the feed streams, it can be difficult to find a single catalyst composition that is universally effective for the removal of essentially all sulfur compounds from gas and liquid hydrocarbon streams.  
         SUMMARY  
         [0005]    The present invention is for a novel hydrocarbon feedstream catalyst bed for the desulfurization of a gas or a liquid hydrocarbon feedstream. The bed comprises at least two catalysts having different sulfur compound affinities and/or specificities thereby improving the overall amount of sulfur compound removal. In one embodiment, the catalyst bed is configured such that the feed stream has initial contact with a first catalyst that is more selective or that has the greater affinity for the sulfur compound that is present in relatively high concentration within the feedstream. As the feedstream passes over the first catalyst, the targeted sulfur compounds are removed generating a cleaner stream for reaction with a second catalyst. Because the stream is cleaner when it reaches the second catalyst, the efficiency of the second catalyst is enhanced. In an alternative embodiment, the catalysts are mixed within the catalyst bed. As the feedstream passes over the catalyst bed, the sulfur compounds are adsorbed by the catalyst having the highest affinity for the particular sulfur compound.  
           [0006]    The present development further describes a process comprising passing a hydrocarbon feedstream over a catalyst bed comprising at least two catalysts having different sulfur compound affinities and/or specificities thereby improving the overall amount of sulfur compound removal. The process reduces the sulfur content in a gas hydrocarbon feedstream from up to about 300 ppm to less than about 500 ppb, and in a liquid hydrocarbon feedstream from up to about 3% to less than about 500 ppb. 
       
    
    
     SUMMARY OF THE FIGURES  
       [0007]    [0007]FIG. 1 is a perspective view of a catalyst bed of a hydrocarbon feedstream desulfurization system wherein the catalyst bed is made in accordance with the present invention and the selective adsorbent section is positioned near the inlet port and the general adsorbent section is positioned near the exit port;  
         [0008]    [0008]FIG. 2 is a perspective view of a catalyst bed of a hydrocarbon feedstream desulfurization system wherein the catalyst bed is made in accordance with the present invention and the general adsorbent section is positioned near the inlet port and the selective adsorbent section is positioned near the exit port; and  
         [0009]    [0009]FIG. 3 is a perspective view of a catalyst bed of a hydrocarbon feedstream desulfurization system wherein the catalyst bed is made in accordance with the present invention and the general adsorbent is intermixed with the selective adsorbent to form the filter bed. 
     
    
     DETAILED DESCRIPTION  
       [0010]    The present invention is for a catalyst bed that is intended to be used to remove contaminants from a gas or liquid hydrocarbon feedstream. As is known in the art, some of the most pervasive contaminants in these feedstreams are sulfur-containing compounds, such as, but not limited to, hydrogen sulfide, carbonyl sulfide, sulfides, mercaptans, thiophenes, tert-butyl mercaptan, di-sulfides, dimethyl sulfide, tetrahydrothiophene, ethyl mercaptan, and benzothiophene. Many of these sulfur contaminants not only have strong odors, making it unpleasant to work around processes utilizing the feedstreams, but the sulfur is also poisonous for many catalysts that use hydrocarbon starting materials.  
         [0011]    As shown in FIG. 1, a hydrocarbon feedstream desulfurization system  10  includes a catalyst bed reactor  12  having an inlet port  14  and an exit port  16 . The catalyst bed reactor  12  houses a catalyst bed  20 . A hydrocarbon feedstream, F, enters the reactor  12  at the inlet port  14 . The hydrocarbon feedstream is in contact with the catalyst bed  20  for a predetermined residence time, determined by the dimensions of the bed  20  and the rate of flow of the feedstream. As is known in the art, the catalyst bed  20  can have a controlled temperature and pressure. The feedstream F then exits the catalyst bed  20  through the exit port  16 . As the feedstream F passes over the bed  20 , contaminants are removed from the feed stream.  
         [0012]    The hydrocarbon feedstream may be supplied as a gas or as a liquid. The typical sulfur concentration of the raw gas-phase hydrocarbon feedstream can have a sulfur concentration of up to about 300 ppm and the liquid-phase feedstream can have a sulfur concentration of up to about 3%. The process of the present invention reduces the sulfur concentration to less than about 500 ppb.  
         [0013]    Referring again to FIG. 1, in the present invention, the catalyst bed  20  comprises a first catalyst or a general adsorbent catalyst  22 , and a second catalyst or a selective adsorbent catalyst  24 , each having an affinity for sulfur-containing compounds. The first catalyst  24  is positioned near the inlet port  14  of the bed  20 . The second catalyst  22  is positioned near the exit port  16  of the bed  20 .  
         [0014]    The first or selective adsorbent catalyst  24  is preferably selected based on the material&#39;s  24  specificity for a predetermined class of chemical compounds. For example, a non-limiting list of some selective catalyst materials  24  would include copper/zinc catalysts, zinc oxide catalysts, copper/zinc/molybdenum oxide catalsyts, nickel aluminas, nickel silicas or combinations thereof As used herein, a “selective adsorbent catalyst” is a material that fails to adsorb at least one of the sulfur compounds—ethyl mercaptan, tert-butyl mercaptan, tetrahydrothiophene and dimethyl sulfide—at a temperature of about 38° C., a pressure of about 15 psig, and a feedstream space velocity of not less than about 3000 hr −1 . If desired, the relative degrees of specificity for a series of adsorbents can be graded by increasing the reaction temperature and/or decreasing the space velocity. The greater the temperature gradient between the adsorption of a first sulfur-containing compound and a second sulfur-containing compound, the greater the specificity of the selective adsorbent for the first sulfur-containing compound. Similarly, the greater the space velocity gradient between the adsorption of a first sulfur-containing compound and a second sulfur-containing compound, the greater the specificity of the selective adsorbent for the first sulfur-containing compound.  
         [0015]    The second or general adsorbent catalyst  22  is preferably selected from a group of relatively materials which are capable of adsorbing sulfur constituents without a high degree of specificity. For example, a non-limiting list of some general adsorbent catalysts would include activated carbon, magnesium oxide, copper/manganese, silver on alumina, nickel silicates, nickel silica/magnesia/alumina, zeolites, molecular sieves, faujasites and combinations thereof, have been shown to be. As used herein, a “general adsorbent catalyst” is a material that adsorbs ethyl mercaptan, tert-butyl mercaptan, tetrahydrothiophene and dimethyl sulfide at a temperature of about 38° C., a pressure of about 15 psig, and a feedstream space velocity of not less than about 3000 hr −1 .  
         [0016]    In the embodiment of FIG. 1, when the hydrocarbon feedstream passes over the catalyst bed  20 , the hydrocarbons initially passes over the selective adsorbent  24  where the targeted sulfur-containing components are adsorbed by selective adsorbent material  24 . The remaining hydrocarbons then pass over the general adsorbent  22  where other sulfur-containing components may be retained by the adsorbent material  22 . The remaining hydrocarbons then exit the catalyst bed  20 .  
         [0017]    As shown in the embodiment of FIG. 2, a catalyst bed  120  comprises a general adsorbent catalyst  122  positioned near an inlet port  114  and a selective adsorbent catalyst  124  positioned near an exit port  116 . With the alternative relative positioning of the general catalyst  122  and the selective catalyst  124 , the hydrocarbon feedstream first passes over the general catalyst  122 , and then over the selective catalyst  124 . If the selective adsorbent catalyst  124  is highly selective, it will be relatively unaffected by the presence of other sulfur-containing compounds. However, if the selective adsorbent catalyst  124  has an affinity for sulfur-containing compounds other than its  124  target compound, this configuration risks allowing the selective catalyst  124  to function in some respects as a general adsorbent, thereby decreasing the overall efficacy of the catalyst bed  120 .  
         [0018]    [0018]FIG. 3 shows a second alternative embodiment for a catalyst bed  220 . In the bed  220 , a general adsorbent catalyst  222  is intermixed with a selective adsorbent catalyst  224  throughout the length of the catalyst bed  220 . As the hydrocarbon feedstream passes over the catalyst bed  220 , selected sulfur-containing compounds are adsorbed preferentially onto the selective catalyst  224  leaving the general catalyst  222  available to adsorb other sulfur-containing compounds. The bed  220  with the catalysts intermixed is most effective when each catalyst  222 ,  224  has an affinity for a particular class of sulfur-containing compounds. For example, if the “general” catalyst preferentially adsorbs thiophenes and the “selective” catalyst preferentially adsorbs mercaptans, both classes of sulfur compounds can be removed as the feedstream passes over the mixed bed.  
         [0019]    The embodiments of FIGS.  1 - 3  have been presented and described in terms of only two catalysts or adsorbents. However, more than one catalyst can be combined to form the “general adsorbent catalyst” and/or more than one catalyst can be combined to form the “selective adsorbent catalyst”.  
         [0020]    Further, the embodiments of FIGS.  1 - 3  have been presented and described in terms of removal of sulfur-containing compounds from a hydrocarbon feedstream. However, the selection of the catalyst materials can vary and the selection will be dependent on the particular contaminants that are to be removed from the feedstream.  
         [0021]    From a reading of the above, one with ordinary skill in the art should be able to devise variations to the inventive features. For example, the catalyst bed may vary in design and equipment from what is illustrated herein. Further, the general adsorbent catalyst and the selective adsorbent catalyst may be optimized to a particular hydrocarbon feedstream or contamination mixture. These and other variations are believed to fall within the spirit and scope of the attached claims.