Abstract:
A method to reduce the lightoff temperature of a catalyst. In the method, the working fluid (the fuel and oxidant to be reacted in the presence of the catalyst) is supplemented with a starter fluid at least until the lightoff temperature of the catalyst is reached. Preferably, the amount of starter fuel added is sufficiently small such that a reaction of the starter fuel and oxidant in the presence of the catalyst does not have a sufficient heat release to raise the temperature of the catalyst to the lightoff temperature.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is generally directed to a method for the operation of a catalytic reactor, and, more specifically, to a method for reducing the lightoff temperature of a catalyst within the catalytic reactor.  
         BACKGROUND OF THE INVENTION  
         [0002]    Catalytic reactors are well known devices for supporting many chemical reactions, such as exothermic oxidative reactions. However, in order for a catalytic reactor to support an exothermic oxidative reaction at a desired rate, a catalyst, which is in a catalyst bed, in the reactor needs to reach threshold temperature, commonly referred to as lightoff temperature. Upon reaching the lightoff temperature, the chemical reaction proceeds at the desired rate generally at a temperature substantially higher than the lightoff temperature.  
           [0003]    Several methods for raising the temperature of the catalyst to the lightoff temperature are known in the art. Common methods of preheating the catalyst to the lightoff temperature include; passing an auxiliary fluid, such as air, that has been heated through the catalytic bed; resistive heating of a substrate on which the catalyst is positioned, e.g. passing an electrical current through the substrate; or preheating with a homogenous flame. The working fluid, e.g., fuel and oxidant in an oxidation reactor, may also be pre-heated to raise the temperature of the catalyst to the lightoff temperature. All these methods require auxiliary systems such as heaters, burners, or electric power sources adding to the cost and complexity of the system.  
           [0004]    Efforts to lower the lightoff temperature of a catalyst to minimize the rise in temperature required to lightoff the catalyst have been concentrated on engineering various aspects of the catalyst. One such engineering effort has focused on catalyst structure. For example where the catalyst is positioned on a substrate, high area washcoats have been used in conjunction with the catalyst to increase catalyst surface area and dispersion. While achieving reduced catalyst lightoff temperatures, this technique tends to reduce the durability of the catalyst.  
           [0005]    Other methods that do not lowered lightoff temperature can be used to lightoff a catalyst. In one such method, a starter fuel/oxidant mixture is used to raise the temperature of the catalyst to the lightoff temperature. After the lightoff temperature is achieved, the starter fuel/oxidant mixture is gradually replaced with an operational fuel/oxidant mixture. This substitution strategy requires storage of multiple fuel types, storage of significant amounts of a starter fuel, and precise flow control during the switching process to avoid catalyst or catalytic reactor damage.  
           [0006]    Based on the foregoing it is the general object of the present invention to overcome or improve upon the problems and drawbacks of the prior art.  
         SUMMARY OF THE INVENTION  
         [0007]    A catalyst supports an exothermic reaction between constituents of a working fluid. Based on the working fluid, the catalyst has a baseline lightoff temperature it must achieve before the exothermic reaction proceeds at a desired rate. In the method of the present invention, the working fluid is supplemented with a starter fuel prior to associating the working fluid with the catalyst. The working fluid in association with the starter fluid in the presence of the catalyst defines a supplemented lightoff temperature that is less than the baseline lightoff temperature. As a result, the temperature rise required to lightoff the catalyst is reduced. As an option, the start fuel may be discontinued after the supplemented lightoff temperature is reached.  
           [0008]    The precise starter fuel is application dependent. For example, each working fluid, which has fuel and oxidant components, interacts differently with a catalyst, or catalysts, within a catalytic reactor. As a result, the selection of a starter fuel depends on the interaction characteristics of the working fluid, starter fuel and the catalyst(s).  
           [0009]    The amount of starter fuel added to the working fluid need not be significant. Preferably, the amount added is not sufficient in and of itself to interact with the catalyst(s) within the catalytic reactor to raise the catalyst(s) to a temperature at or above the lightoff temperature. It, however, is preferred that the starter fuel be more reactive than the fuel, or fuels, within the working fluid as to the catalyst, or catalysts, within the catalytic reactor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1. is a graph illustrating the relationship between lightoff temperature and operational temperature of a catalyst.  
         [0011]    [0011]FIG. 2 depicts a cross-sectional view of a catalytic reactor in which the method disclosed herein might be employed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    In exothermic oxidative reactors, a catalyst therein lights off when the catalyst temperature rises dramatically from an initial catalyst temperature to an operational temperature. FIG. 1 shows this relationship. The auxiliary heating profile, denoted by reference number  10 , shows the temperature of the catalyst as a function of the heat applied thereto. As depicted in FIG. 1, the catalyst is heated from, for example, ambient conditions to a lightoff temperature, say T 1 . Upon reaching the lightoff temperature T 1 , the catalyst temperature jumps nearly instantaneously from the lightoff temperature to a considerably higher operational temperature designated A.  
         [0013]    In FIG. 1, it has been assumed that the catalyst temperature results directly from the auxiliary heat applied to the catalyst. This assumption is based on the further assumption that if the working fluid is flowing over the catalyst before the catalyst is at the lightoff temperature, the exothermic reactions occurring in the presence of the catalyst are minimal. Thus for all practical purposes, these reactions do not significantly increase the temperature of the catalyst. In the event the exothermic reactions are more than minimal, the catalyst temperature before lightoff will be equal to the sum of the catalyst temperature resulting form the auxiliary heat applied thereto and the temperature rise resulting from the exothermic reactions. Therefore, the auxiliary heating profile  10  will have to be adjusted accordingly.  
         [0014]    In the present method, the working fluid is supplemented with a starter fluid. As a result, the lightoff temperature of the catalyst is reduced. Referring to FIG. 1, the baseline lightoff temperature, i.e., the catalyst temperature that is required to lightoff the catalyst if only the working fluid is considered, denoted T 1  is reduced to a supplemented lightoff temperature denoted T 2 . As a result of the addition of the starter fluid, the auxiliary heating required for the catalyst is similarly reduced.  
         [0015]    As the operational temperature of the catalyst is a function of lightoff temperature of the catalyst, the operational temperature of the catalyst is reduced from point A to point B when the lightoff temperature is reduced from the baseline lightoff temperature T 1  to the supplemented lightoff temperature T 2 . After lightoff, the temperature increase of the catalyst above the lightoff temperature results from the exothermic reactions between the components of the working fluid. Thus, the operational temperature of the catalyst after lightoff is approximately equal to the sum of lightoff temperature and the temperature rise due to the exothermic reactions. Thus, a reduction in catalyst lightoff temperature equates to a similar reduction in catalyst operational temperature. As a result, points A and B are on a line designated 12 that is generally parallel to the auxiliary heating profile  10 .  
         [0016]    The amount of starter fuel required is minimal as compared to the amount of fuel in the working fluid. Preferably, the heat release of the starter fuel when reacted with oxidant in the working fluid in the presence of the catalyst is sufficiently low that it is incapable in and of itself of raising the initial catalyst temperature of the catalyst to the baseline lightoff temperature T 1 . While the method reduces the lightoff temperature, it is still anticipated that some auxiliary heating of the catalyst may still be required. This auxiliary heating could be by conventional methods known to those skilled in the art.  
         [0017]    The method of the present invention is shown in conjunction with a fixed geometric catalytic reactor suitable for performing the method. As shown in FIG. 2, the catalytic reactor, generally denoted by the reference number  50 , is comprised of housing  52 . The housing  52  defines a chamber  54 , an entrance  56 , and an exit  58 . A plurality of conduits  60 , each having a first opening  62 , a second opening  64 , and an exterior surface  66 , penetrate the housing  52 . The penetration is such that a portion of each conduit  60  is positioned within the chamber  54  with the first opening  62  outside the chamber  54  and the second opening  64  inside the chamber. A catalyst  68  is positioned on a portion of an exterior surface  66  of at least one conduit  60  within the chamber  54  between the first opening  62  and the second opening  64 .  
         [0018]    In this fixed geometry catalytic reactor, a mixture, working fluid, with or without starter fuel,  70  enters through the entrance  56  and flows toward the exit  58 . Additional air  72  flows into the entrances  62  of the conduits  60  passing through the conduits and exiting through the conduit exits  64 . The mixture  70  contacts the additional oxidant  72  as the additional oxidant exits into the chamber  54 . Mixing of the mixture  70  and additional oxidant  72  begins almost immediately.  
       EXAMPLE 1  
       [0019]    The method was employed with a catalytic reactor similar to that depicted in FIG. 2 using methane and oxygen, as a constituent of air, as the working fluid. The catalytic reactor had a platinum catalyst supported on a La-stabilized γ-alumina washcoat. The average flow rate of the working fluid with starter fluid, if present, was about 50 ft/sec. The working fluid with starter fluid, if present, had an initial temperature of about 200° C. Auxiliary heating of the catalyst was accomplished by preheating the working fluid. The tests were conducted at a pressure of 1 atm. The results are as follows:  
         [0020]    Starter Fuel—Propane (LPG)  
         [0021]    Fuel/Air Equivalence Ratio* —3.0 (working fluid plus starter fuel)  
         [0022]    *the actual fuel/oxidant ratio divided by the stoichiometric fuel/oxidant ratio based on a complete combustion reaction.  
                                                       vol. % to fuel within   0   2.1   4.4   10   40       working fluid       Lightoff Temp.   Before - 386   306   287   &lt;200   &lt;200       Degrees C.   After - 393                  
 
         [0023]    Starter Fuel—Hydrogen  
         [0024]    Fuel/Air Equivalence Ratio—3.0 (working fluid only)  
                                                               vol. % to fuel   0   3.8   4.6   5.4   6.2   6.9   7.7       within working       fluid       Lightoff Temp.   Before - 406   374   345   327   303   276   &lt;200       Degrees C.   After -- 396                  
 
         [0025]    Starter Fuel—Hydrogen Sulfide  
         [0026]    Fuel/Air Equivalence Ratio —3.0 (working fluid only)  
                                                               vol. % to fuel within                           working fluid   0   0.001   0.003   0.01                           Lightoff Temp.   Before - 412   344   310   318           Degrees C.   After - 383                      
 
         [0027]    The before and after temperatures provided under the condition of zero starter fuel indicate the minor hysteresis effects associated with the operation of catalytic reactors. Based on the above data, it is clear that the lightoff temperature for the catalyst was reduced in all cases with a minor addition of a starter fuel to the working fluid.  
       EXAMPLE 2  
       [0028]    Same conditions as Example 1, except a rhodium catalyst was used instead of a platinum catalyst. The results are as follows:  
         [0029]    Starter Fuel—Propane (LPG)  
         [0030]    Fuel/Air Equivalence Ratio—3.0 (working fluid plus starter fuel)  
                                               vol. % to fuel within working                       fluid   0   4.4   10   40                   Lightoff Temp.   Before - 415   408   396   387       Degrees C.   After - 420                  
 
         [0031]    Starter Fuel—Hydrogen  
         [0032]    Fuel/Air Equivalence Ratio —3.0 (working fluid only)  
                                                       vol. % to fuel within   0   3.8   11.1   15.4   18.2       working fluid       Lightoff Temp.   Before - 413   394   336   &lt;200   &lt;200       Degrees C.   After - 391                  
 
         [0033]    Starter Fuel—Hydrogen Sulfide  
         [0034]    Fuel/Air Equivalence Ratio—3.0 (working fluid only)  
                                               vol. % to fuel within working                       fluid   0   0.001   0.003   0.01                   Lightoff Temp.   Before - 402   430   455   473       Degrees C.   After - 410                  
 
         [0035]    As shown in the data above, H 2  reduced the lightoff temperature over a rhodium catalyst, but propane did not. Hydrogen sulfide, on the other hand, increased the lightoff temperature. This illustrates that the starter fuel is dependent upon the catalyst.  
         [0036]    Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the invention should not be limited to the description of the preferred versions contained herein.