Abstract:
Embodiments include a method and apparatus for producing synthesis gas in a catalytic partial oxidation reactor by adding hydrogen to the reactor feed stream.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 10/299,193, filed Nov. 19, 2002, now is abandoned. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a process for the preparation of synthesis gas, i.e., a mixture of carbon monoxide and hydrogen, from a hydrocarbon feedstock. Particularly, the present invention relates to adding hydrogen to the feed stream of a (CPOX) partial oxidation reactor, preferably a catalytic partial oxidation reactor. 
     BACKGROUND OF THE INVENTION 
     Large quantities of natural gas are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, a significant amount of natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane, the main component of natural gas, as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to higher hydrocarbons or hydrocarbon liquids is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into the higher hydrocarbons via a process such as Fischer-Tropsch synthesis or alcohols via alcohol synthesis. An acceptable example of Fischer-Tropsch synthesis is disclosed in U.S. Pat. No. 6,333,294 to Chao et al., incorporated herein by reference. 
     Current industrial use of methane or natural gas as a chemical feedstock proceeds by the initial conversion of the feedstock to carbon monoxide and hydrogen by either steam reforming (the most widespread process), dry reforming, autothermal reforming, partial oxidation or catalytic partial oxidation. Examples of these processes are disclosed in Gunardson, Harold, INDUSTRIAL GASES IN PETROCHEMICAL PROCESSING 41-80 (1998), incorporated herein by reference. Steam reforming, dry reforming, and catalytic partial oxidation proceed according to the following reactions respectively: 
                        
 
     As is shown schematically in  FIG. 1 , in catalytic partial oxidation, the hydrocarbon feedstock is mixed with an oxygen source, such as air, oxygen-enriched air, or oxygen, and introduced to a catalyst as syngas feed stream  10  in the CPOX reactor  1  at elevated temperature and pressure. When the feedstock comprises primarily methane, the stoichiometric H 2 :CO molar ratio of the product of a pure CPOX reaction is 2:1. However, due to secondary reactions, the observed H 2 :CO molar ratio in the syngas product stream  20  is generally less than the stoichiometric ratio. The downstream conversion of the syngas to higher hydrocarbons (e.g., fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes) via the Fischer-Tropsch or other synthesis reaction may require an H 2 :CO molar ratio greater than that observed in the syngas product stream  20 . Thus, it is generally necessary to provide an additional amount of hydrogen  40  to the CPOX syngas product stream before introduction into the Fischer-Tropsch reactor  2 . Examples of acceptable methods of producing and separating hydrogen are described in Gunardson at 41-110, incorporated herein by reference. 
     After the syngas is reacted into higher hydrocarbons (if reactor  2  is a Fischer-Tropsch reactor) or alcohols (if reactor  2  is an alcohol synthesis reactor) the gas effluent  50  is separated from the liquid product stream  30 . The liquid product stream goes on for further processing (not shown). If reactor  2  is a Fischer-Tropsch reactor, the gas effluent  50  generally comprises methane, unreacted H 2 , unreacted CO and other impurities such as CO 2  and other light hydrocarbons. If reactor  2  is an alcohol synthesis reactor, gas effluent generally comprises CO and H 2 . 
     It would generally be desirable to eliminate or reduce the amount of supplemental hydrogen added in supplemental hydrogen stream  40 . Additionally, it would be desirable to recycle a Fischer-Tropsch effluent stream rich in methane to a CPOX reactor without the need to separate hydrogen from the effluent stream. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     In preferred embodiments of the present invention, there are shown methods and apparatus for conducting a partial oxidation reaction in a reactor having a feed stream comprising a hydrogen-containing gas that has been preferably been recycled from the FT or other area within the GTL plant, an oxygen-containing gas, and a gaseous hydrocarbon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic embodiment of a conventional CPOX/Fischer-Tropsch reaction scheme. 
         FIG. 2  is a first schematic embodiment of a CPOX/Fischer-Tropsch reaction scheme in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a preferred embodiment, shown in  FIG. 2 , a CPOX feed stream  110  comprising a hydrocarbon, preferably natural gas or methane, and an oxygen-containing gas, preferably substantially pure oxygen, oxygen-enriched air, or air, most preferably substantially pure oxygen, is mixed with the gas effluent stream  140  from the downstream Fischer-Tropsch process and fed into CPOX reactor  101  where it is subject to partial oxidation, preferably catalytic partial oxidation. Examples of acceptable catalytic partial oxidation reactions are disclosed in STRUCTURED CATALYSTS AND REACTORS 179-208 (Andrzej Cybulski &amp; Jacob A. Moulijn eds. 1998), incorporated herein by reference, U.S. Pat. No. 6,402,989 to Gaffney, incorporated herein by reference, and Gunardson at 41-80, or any other catalytic partial oxidation as is known in the art. It has been thought in the past by those of skill in the art that it is unadvisable to add a hydrogen rich stream to a CPOX feed. The hydrogen would typically react exothermically with oxygen to form water. This would be counter to the preferred selective partial oxidation to CO and H 2 . Secondly, it could be indeed dangerous to include hydrogen in the feed stream of a catalytic partial oxidation reaction such as in the present case. It is the conventional wisdom that due to the conditions in the reactor (e.g., high temperatures and pressures) and the presence of substantially pure oxygen that the addition of hydrogen could create a dangerous situation. Applicants have discovered this not to be the case. Applicants have discovered that instead of reacting violently, hydrogen introduced into the catalytic partial oxidation reactor passes through substantially unchanged. 
     Even with the hydrogen in gas effluent stream  140  or if the H 2 :CO ratio in gas effluent stream  140  is lower than that needed to conduct the Fischer-Tropsch reactions, the catalytic partial oxidation product stream  120  may have an H 2 :CO ratio which is lower than that needed to conduct the Fischer-Tropsch reactions in Fischer-Tropsch reactor  102 . If that is the case, optional supplemental hydrogen stream  150  may be added to product stream  120  before it is fed into the Fischer-Tropsch reactor. It is contemplated that in many instances, the amount of supplemental hydrogen  150  needed will be greatly reduced, or even preferably eliminated, from that needed in the conventional system of FIG.  1 . Thus, greatly reducing or eliminating the cost associated with providing that supplemental hydrogen. Additionally, even if the need for supplemental hydrogen is not reduced (e.g., because the H 2 :CO ratios in the recycle stream is too low), the ability to recycle the methane from the FT effluent without the use of expensive separation techniques is extremely valuable. 
     In preferred embodiments the hydrogen is provided by a recycle stream from the Fischer-Tropsch reactor (e.g., gas effluent stream  140 ). A recycle stream could also come from other areas of the GTL plan such as product upgrading or anywhere hydrogen is produced or even from bottled or trucked-in hydrogen. Product upgrading can comprise, but is not limited to, the following processes, hydrocracking, hydroprocessing, hydrotreating, and the like, exemplary examples of which are know to one of ordinary skill in the art. The hydrogen can also come from a plant which dehydrogenates alkanes to produce olefins and hydrogen (e.g., ethane to ethylene), examples of which are well known in the art. In addition to hydrogen, CO, methane, light hydrocarbons, CO 2 , and/or inerts such as, by way of example only, N 2  and Ar, could also be present. Applicants expect that the presence of carbon monoxide and carbon dioxide in the recycle stream will not have a substantial effect on the overall efficiency of the partial oxidation reactor. For example, Applicants believe that the introduction of carbon monoxide and carbon dioxide into the partial oxidation feed will have a slight negative effect (about a 2-4% decrease in conversion), however, it is expected that any negative effect will be more than outweighed by the elimination or substantial reduction in the need for the supplemental hydrogen, the need to separate the methane in the FT effluent, or the wasted methane in the FT effluent. 
     A primary benefit to this invention is to reduce the complexity of FT tail gas handling without destroying the unconverted H 2  and CO. The separation of H 2 , inerts such as N 2  and Ar, CO and methane is difficult. Optimally, one would want to recycle H 2  and CO to the FT, purge the N 2  and Ar inerts and recycle methane to the syngas step. Complexity, cost and loss product to thermodynamics preclude this. The fact that the CPOX can handle the described recycle without destroying H 2  and CO is unique and very advantageous. 
     EXAMPLES 
     Experiments were conducted on a CPOX catalyst system with a feed gas pre-heat of 300° C. Tests reactions were run with the following feed compositions using bottled hydrogen and achieved the listed approximate H 2 :CO molar ratios in the product stream (Table 1). In Run No. 1, the total feed stream flow rate was 3.5 SLPM. In all other runs, the total feed stream flow rate was 5.0 SLPM. 
                                                           TABLE 1                               Products                   Bottled   Stream H 2 :CO       Run No.   CH 4  mol %   O 2  mol %   H 2  mol %   molar ratio                                1   64   36   0   ≈1.97       2   64   36   0   ≈2.02       3   62   36   4   ≈2.09       4   60   33   7   ≈2.14       5   58   32   10   ≈2.19       6   56   31   13   ≈2.25                    
As can be easily determined from Table 1, the addition of bottled hydrogen to the partial oxidation feed stream is shown to increase the H 2 :CO molar ratio in the partial oxidation product stream.
 
     The term “catalyst system” as used herein means any acceptable system for catalyzing the desired reaction in the reaction zone. By way of example only, a CPOX catalyst usually includes a support and a catalyst. The support may be, for example, particulates, pills, beads, granules, pellets, rings, monoliths, ceramic honeycomb structures, wire gauze, or any other suitable supports in any acceptable manufactured shape such as those listed herein. The CPOX catalyst may be selected from the group consisting of nickel, samarium, rhodium, cobalt, platinum, rhodium-samarium, platinum-rhodium Ni—MgO, Group VIII metals, nickel-based catalysts, combinations thereof, or any other catalysts mentioned herein or as is well known in the art. The above-exemplified examples of supports and catalysts are only examples. There are a plethora of catalysts systems known in the art which would be acceptable and are contemplated to fall within the scope of the present invention, such as those disclosed in STRUCTURED CATALYSTS AND REACTORS 179-208, 599-615 (Andrzej Cybulski and Jacob A. Moulijn eds. 1998), incorporated herein by reference. 
     A stream is directly recycled, if it is sent from its source to its destination without being subjected to any processing which changes its composition. Conversely, a stream is indirectly recycled if it is subjected to composition changing processing after leaving its source but before arriving at its destination. 
     Should the disclosure of any of the patents and publications that are incorporated herein by reference conflict with the present specification to the extent that it might render a term unclear, the present specification shall take precedence. 
     Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The embodiments herein are to be construed as illustrative, and not as constraining the disclosure in any way whatsoever.