Abstract:
A fuel system for a propulsion system includes a fuel deoxygenating device and a catalytic module containing catalytic materials. The fuel deoxygenating device removes dissolved oxygen from the fuel to prevent formation of insoluble materials that can potentially foul the catalyst and block desirable catalytic reactions that increase the usable cooling capacity of an endothermic fuel.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. application Ser. No. 10/805,786 filed on Mar. 22, 2004. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention generally relates to a cooling system for a high-speed propulsion system, and specifically to a cooling system including a fuel deoxygenator and a catalyst for increasing the heat sink capability of a hydrocarbon fuel capable of undergoing endothermic reaction. 
         [0003]    It is common practice to use fuel as a cooling medium for various systems onboard an aircraft. Higher engine operating temperatures increase cycle efficiency and reduce fuel consumption. The engine operating temperature is limited by the usable cooling capacity of the fuel. The cooling capacity of fuel can be increased by endothermic decomposition of the fuel into combustible products that may have improved ignition and burning characteristics. 
         [0004]    Catalysts are known that promote decomposition of endothermic fuels into combustible products with lower molecular weights than the original fuel after absorbing a heat of reaction. However, thermo-oxidative reactions caused by dissolved oxygen within the fuel can cause formation of coke that foul the catalyst and prevent the preferred catalytic reactions. 
         [0005]    At temperatures between approximately 250° F. to 800° F. dissolved oxygen within the fuel reacts to form coke precursors that initiate and propagate reactions that lead to coke deposit formation. At temperatures above approximately 800° F. the mechanism for formation of coke deposits is controlled by thermal cracking (pyrolysis) reactions where chemical bonds are broken forming coke. Reducing the amount of oxygen dissolved within the fuel decreases the rate of coke deposition at relatively lower temperatures and increases the usable cooling capacity of the fuel. 
         [0006]    It is known how to remove dissolved oxygen within fuel with de-oxygenation devices. U.S. Pat. No. 6,315,815, and U.S. patent application Ser. No. 10/407,004 assigned to Applicant, disclose devices for removing dissolved oxygen using a gas-permeable membrane within the fuel system. As fuel passes along the permeable membrane, oxygen molecules in the fuel diffuse out of the fuel across the gas-permeable membrane. Removal of dissolved oxygen from the fuel only provides limited increases in usable cooling capacity. Increasing performance demands require further increases in the usable cooling capacity of aircraft fuels. 
         [0007]    Accordingly, it is desirable to develop an endothermic fuel system that suppresses formation of coke deposits to prevent interference with desirable catalytic reactions at increased engine operating temperatures. 
       SUMMARY OF INVENTION 
       [0008]    This invention is a fuel delivery system including a fuel deoxygenating device for removing dissolved oxygen from fuel to prevent formation of insoluble materials that potentially block desirable catalytic reactions thereby increasing the usable cooling capacity of an endothermic fuel. 
         [0009]    The fuel delivery system includes the fuel deoxygenator for removing dissolved oxygen from the fuel before entering a catalytic device. The catalytic device initiates endothermic decomposition of the fuel into favorable combustible products with increased usable heat absorption capacity. 
         [0010]    The fuel-deoxygenating device includes a permeable membrane supported by a porous substrate. An oxygen partial pressure differential created across the permeable membrane drives diffusion of oxygen from the fuel and across the permeable membrane. The dissolved oxygen is then exhausted away from the fuel. Removal of dissolved oxygen from the fuel substantially reduces the formation of insoluble materials or coke that is known to form at temperatures above about 250° F. 
         [0011]    Prevention of coke formation prevents possible fouling of the catalytic device that could prevent initiation of endothermic decomposition. Endothermic decomposition occurs at temperatures well above the temperatures that cause coke formation caused by dissolved oxygen. The fuel-deoxygenating device prevents dissolved oxygen within the fuel from forming coke deposits that foul and prevent the desired endothermic decomposition of the fuel. Further, the fuel-deoxygenating device provides for substantial increases in usable cooling capacity of the fuel allowing higher engine operating temperatures. 
         [0012]    Accordingly, the endothermic fuel delivery system of this invention includes a fuel-deoxygenating device that suppresses formation of coke deposits to prevent interference with desirable catalytic reactions at increased engine operating temperatures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
           [0014]      FIG. 1  is a schematic view of a propulsion system and fuel delivery system according to this invention; 
           [0015]      FIG. 2  is a schematic view of a fuel deoxygenator according to this invention; 
           [0016]      FIG. 3  is a schematic view of another deoxygenator according to this invention; 
           [0017]      FIG. 4  is a schematic view of a permeable membrane according to this invention; and 
           [0018]      FIG. 5  is a schematic view of the catalyst and a fuel deoxygenator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    Referring to  FIG. 1 , a propulsion system  10  includes a fuel delivery system  20 . The fuel delivery system  20  includes a fuel deoxygenator  22  and a catalyst module  24 . The fuel system  20  also includes a heat exchanger  26  for direct or indirect cooling of propulsion system components and other systems  28  by rejecting heat to the fuel. The propulsion system  10  is preferably a gas turbine engine, ramjet or scramjet engine for high-speed aircraft, although a worker with the benefit of this disclosure will recognize the benefits applicable to other known energy conversion devices. The other systems  28  can include cooling of bleed air or other fluids for cooling components of the propulsion system  10 . 
         [0020]    The catalyst module  24  includes a catalytic material  36  that promotes endothermic decomposition of the fuel. The catalytic material  36  can be a metal such as copper, chromium, platinum, rhodium, iridium, ruthenium, palladium, and any combination of these materials. Further, the catalytic material  36  may also be a zeolite. A worker having the benefit of this disclosure would understand the specific composition of catalyst required to break down the fuel into favorably combustible components. 
         [0021]    Zeolites are preferred because they are more reactive and produce less insoluble products than the metals. As a result of the reduced amount of insoluble products produced greater usable cooling capacity is obtainable in the endothermic fuel. The specific type of zeolite can include faujasites, chabazites, mordenites, silicalites, or any other type of zeolite known to catalytically crack fuel. 
         [0022]    Preferably, the catalytic material  36  is supported on a honeycomb structure  38  disposed within the catalytic module  24 . However, the catalytic material may be supported on granules, extrudates, monoliths, or other known catalyst support structures. The catalytic materials required reaction temperatures of between about 1000° F. and 1500° F. Lower temperatures provide for lower conversions and therefore lower usable heat sink capacity of the fuel. 
         [0023]    The catalytic module  24  is disposed adjacent heat producing components of the propulsion system  10 . Preferably, the catalytic module  24  is disposed within a housing  19  of the engine assembly  10 . The heat generated by the propulsion system  10  elevates the temperature of the catalytic module  24  to temperatures required to initiate catalytic reactions that cause the endothermic decomposition of the fuels. 
         [0024]    The temperature of the catalytic module  24  can also be elevated by the heat of the fuel itself. The fuel flowing through the catalytic module  24  is used to absorb heat from other systems. The heat absorbed will elevate the temperature of the catalytic module  24  to temperatures providing optimum operation. Further, it is within the contemplation of this invention to heat the catalytic module  24  by any means or device. 
         [0025]    Fuels have long been used as coolants on aircraft because of the capacity to absorb heat. The capacity to absorb heat without chemical reaction is known as the fuels physical heat sink. The physical heat sink is limited by formation of insoluble materials formed as a result of dissolved oxygen reacting with components of the fuel in the presence of heat. 
         [0026]    The formation of insoluble materials related to the amount of dissolved oxygen within the fuel occurs at temperatures lower than those required for the catalytic reactions. Disadvantageously, the formation of insoluble materials can create a layer of coke deposits on the catalytic material  36 . The layer of coke deposits prevents a substantial portion of fuel from contacting the catalytic material  28 , thereby preventing the desired catalytic reactions. The fuel delivery system  20  of this invention includes the fuel deoxygenator  22  that removes a substantial amount of dissolved oxygen from the fuel. The removal of dissolved oxygen delays the formation of coke deposits typically formed at temperatures below about 800° F. At increased temperatures catalytic reactions begin cracking the fuel into desired components with favorable combustion properties and greater heat absorption capabilities. 
         [0027]    Referring to  FIG. 2 , a schematic view of a fuel deoxygenator  22 ′ according to this invention is shown and includes a plurality of tubes  42  disposed within a housing  40 . The fuel  30  is flowed around the tubes  42  from an inlet  44  to an outlet  46 . Tubes  42  include a composite permeable membrane  48  that absorbs oxygen molecules dissolved within the fuel  30 . A strip gas  50  flowing through the tubes  42  creates a partial pressure differential across the composite permeable membrane  48  that draws dissolved oxygen from the fuel  30  into the tubes  42  and out with the strip gas  50 . Oxygen is then removed from the strip gas  50  and exhausted from the system  20 . The strip gas  50  may than be recycled through the fuel deoxygenator  22 ′. Deoxygenated fuel exits through the outlet  46  and into the catalyst module  24  for catalytic reaction with the now deoxygenated fuel  30 . 
         [0028]    Referring to  FIG. 3 , another embodiment of a fuel deoxygenator  22 ″ is shown and includes a series of fuel plates  52  stacked one on top of the other. The composite permeable membrane  48  is included on each of the fuel plates  52  to define a portion of fuel passages  54 . Fuel enters through an inlet  56  and exists through an outlet  58 . An opening  60  is open to a vacuum source  62 . Fuel  30  passes within the fuel passages  54  defined by the stacked fuel plates  52 . The fuel plates  52  are disposed within the housing  55  that defines the inlet  56  and the outlet  58 . The use of the fuel plates  52  allows for the adaptation of the fuel deoxygenator  22 ″ to various applications by the addition or subtraction of fuel plates  52 . Although embodiments of fuel deoxygenators are shown and described, a worker skilled in the art with the benefit of this application would understand that other configurations of fuel deoxygenators are within the contemplation of this invention. 
         [0029]    Referring to  FIG. 4 , the composite permeable membrane  48  is shown in cross-section and preferably includes a permeable layer  62  disposed over a porous backing  64 . The porous backing  64  supplies the required support structure for the permeable layer  62  while still allowing maximum oxygen diffusion from fuel. The permeable layer  62  is coated on to the porous backing  64  and a mechanical bond between the two is formed. The permeable layer  62  is preferably a 0.5-20 μm thick coating of Teflon AF 2400 over a 0.005-in thick porous backing  64  of polyvinylidene fluoride (PVDF) with a 0.25 μm pores size. Other supports of different material, thickness and pore size can be used that provide the requisite strength and openness. 
         [0030]    Preferably the permeable layer  62  is Dupont Teflon AF amorphous fluoropolymer however other materials known to workers skilled in the art are within the contemplation of this invention, such as Solvay Hyflon AD perfluorinated glassy polymer and Asahi Glass CYTOP polyperfluorobutenyl vinyl ether. Each composite permeable membrane  48  is supported on a porous substrate  66 . The porous substrate  66  is in communication with the vacuum source  62  to create an oxygen partial pressure differential across the composite permeable membrane  48 . 
         [0031]    In operation a partial pressure differential is created by the vacuum source  62  between a non-fuel side  68  of the permeable membrane  48  and a fuel side  70 . Oxygen indicated at arrows  72  diffuses from fuel  30  across the composite permeable membrane  48  and into the porous substrate  66 . From the porous substrate  66  the oxygen  72  is vented out of the fuel system  20 . 
         [0032]    Referring to  FIG. 5 , the catalyst module  24  is mounted within a housing  19  of the propulsion system  10 . Heat generated by the propulsion system  10  heats the catalytic material  36  and fuel  30  flowing therethrough to temperatures promoting the desired catalytic reactions. The catalytic reaction of the fuel increases the heat absorption capability of the fuel and produces favorable combustible materials. 
         [0033]    At lower temperatures, such as during initial start up of the propulsion system  10 , coke formation is prevented by the removal of dissolved oxygen in the deoxygenator  22 . As appreciated, without removing dissolved oxygen from fuels, coke deposits would form on the internal components of the fuel system  20 . This includes the honeycomb structures  38  within the catalytic module  24 . It is for this reason that widespread use of catalysts to provide endothermic decomposition fuels has not been practical. The use of the fuel deoxygenator  22  prevents fouling of the catalyst material  36  at lower temperatures, enabling beneficial catalytic reactions at elevated temperatures that increase the heat sink capability of an endothermic fuel. The increased heat sink capability enables operation of the propulsion system  10  at greater temperatures and greater efficiencies. 
         [0034]    The foregoing description is exemplary and not just a material specification. The invention has been described in an illustrative manner, and should be understood that the terminology used is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications are within the scope of this invention. It is understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.