catalytic modification of flammable atmosphere in aircraft fuel tanks

Catalytic Modification of Flammable Atmosphere in Aircraft Fuel Tanks Thesis by Inki Choi In Partial Fulfillment of the Requirements for the Engineer’s Degree California Institute of Technology Pasadena, California 2010 (Submitted June 7, 2010) ii c  2010 Inki Choi All Rights Reserved iii Acknowledgments I would like to show my appreciation to my academic advisor Professor Joseph Shepherd. He advised me with his abundant experience and knowledge to overcome obstacles whenever I lost my way. His patience especially allowed me to learn lots of experimental knowledge and gave me a scientific attitude. My thesis committee members, Professors Meiron and Blanquart gave me very instructive com- ments. Their comments were of great help for me to complete a more integrated thesis. I am also thankful to my laboratory members. They kept helping me adapt myself to life at Caltech. I appreciate particularly Philipp Boettcher and Sally Bane who helped me in various ways to fulfill my goal with experimental experiences and techniques. Finally, I really want to show my deepest thanks to my wife who assisted me sincerely throughout the years here at Caltech. She was strong and very supportive even though it was very difficult time for us to stay here. The work was carried out in the Explosion Dynamics Laboratory of the California Institute of Technology and was supported by the Boeing Company through the Strategic Research and Development Relationship agreement CT-BA-GTA-1. I would like to thank Ivana Jojic, Leora Peltz, and Shawn Park at the Boeing Company, and Prof. Sossina Haile, Sinchul Yeom, Taesik Oh, Steve Ballard, Richard Gerhart, and Thomas Brennan for making this experiment possible. iv Abstract A facility for investigating catalytic combustion and measurement of fuel molecule concentration was built to examine catalyst candidates for inerting systems in aircraft. The facility consists of fuel and oxygen supplies, a catalytic-bed reactor, heating system, and laser-based diagnostics. Two supplementary systems consisting of a calibration test cell and a nitrogen-purged glove box were also constructed. The catalyst under investigation was platinum, and it was mixed with silica particles to increase the surface area available to react. The catalyst/silica mixture was placed in a narrow channel section of the reactor and supported from both sides by glass wool. The fuels investigated were n-octane and n-nonane because their vapor pressure is sufficiently high to create flammable gaseous mixtures with atmospheric air at room temperature. Calibration experiments were per- formed to determine the absorption cross-section of the two fuels as a function of temperature. The cross-section values were then used to determine the fuel concentration before the flow entered the reactor and after exposure to the heated catalyst. An initial set of experiments was performed with the catalytic-bed reactor at two temperatures, 255 and 500 ◦ C, to investigate pyrolysis and oxidation of the fuel. The presence of the catalyst increased the degree of pyrolysis and oxidation at both temperatures. The results show that catalytic modification of flammable atmospheres may yield a viable alternative for inerting aircraft fuel tanks. However, further tests are required to produce oxidation at sufficiently low temperature to comply with aircraft safety regulations. v Contents Abstract iv 1 Introduction 1 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Theory 4 2.1 Catalytic Combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2 Types of Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.3 Ceramic Supporter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.4 Catalyst Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.5 Laser Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3 Experimental Setup 7 3.1 Piping System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2 Gaseous Fuel Generating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.3 Heating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.3.1 Pipe Heating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.3.2 Flange Heating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.4 Catalyst-Packed Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.5 Laser Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.6 Auxiliary Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.6.1 Calibration System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.6.2 Glove Box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4 Experimental Procedures 20 4.1 Catalyst Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.2 Catalyst Packing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.3 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 vi 4.4 Catalytic Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.5 Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 5 Results and Discussion 26 5.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5.2 Catalytic Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 5.2.1 Effect of Packing the Reactor on the Flow Rate . . . . . . . . . . . . . . . . . 28 5.3 Empty Reactor with n-Nonane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 5.3.1 Reactor Filled with Glass Wool . . . . . . . . . . . . . . . . . . . . . . . . . . 29 5.3.2 Reactor Filled with Glass Wool and Silica . . . . . . . . . . . . . . . . . . . . 30 5.3.3 Reactor Filled with Glass Wool, Silica, and a Platinum Catalyst . . . . . . . 31 6 Conclusions 33 Bibliography 35 A Cross-Section Measurements 36 B Heat Transfer Calculations 37 C Experiment Checklist 39 vii List of Figures 1.1 Catalytic modification of the flammable atmosphere in the aircraft fuel tank . . . . . 2 3.1 Schematic diagram of the experimental setup . . . . . . . . . . . . . . . . . . . . . . . 7 3.2 The mass flow controllers connected to the piping system . . . . . . . . . . . . . . . . 8 3.3 Gas exhaust fan and exhaust lines and condensed fuel removal system in the exhaust line 9 3.4 The fuel vessel, with stirring bar, and bubbler setup for creating fuel-air mixtures . . 9 3.5 Gaseous fuel generation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.6 Control panel for the pipe heating system . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.7 Circuit diagrams for the piping heating system . . . . . . . . . . . . . . . . . . . . . . 12 3.8 Heating and insulation system for the gas piping system . . . . . . . . . . . . . . . . . 13 3.9 Control panel for the flange heating system . . . . . . . . . . . . . . . . . . . . . . . . 13 3.10 Circuit diagrams for the flange heating system . . . . . . . . . . . . . . . . . . . . . . 14 3.11 Heating and insulation system for the reactor flanges . . . . . . . . . . . . . . . . . . . 14 3.12 Reactor assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.13 Catalytic bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.14 Optical system for laser-based fuel sensing . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.15 Schematic view of the optical setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.16 Screen shot of the LabVIEW virtual instrument used for fuel concentration measurements 17 3.17 Infrared filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.18 Calibration system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.19 Glove box used when handling the catalyst . . . . . . . . . . . . . . . . . . . . . . . . 19 3.20 Catalyst packing system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.1 Initial catalyst mixture preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.2 Final catalyst mixture preparation and the mixture under the microscope . . . . . . . 23 4.3 Catalyst packing procedures and enlarged narrow section filled with mixture . . . . . 24 4.4 Logarithmic plot of the intensity ratio versus fuel concentration, with the slope equal to the absorption cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.1 Cross-section of n-octane as a function of temperature. . . . . . . . . . . . . . . . . . 27 viii 5.2 Cross-section of n-nonane as a function of temperature. . . . . . . . . . . . . . . . . . 27 5.3 Variation of fuel molar density and temperature at the inlet and outlet flanges for n-nonane in the empty reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 5.4 Variation of fuel molar density and temperature at the inlet and outlet flanges for n-nonane in the reactor filled with glass wool . . . . . . . . . . . . . . . . . . . . . . . 30 5.5 Variation of fuel molar density and temperature at the inlet and outlet flanges for n-nonane in the reactor filled with glass wool and silica . . . . . . . . . . . . . . . . . 31 5.6 Variation of fuel molar density and temperature at the inlet and outlet flanges for n-nonane in the reactor filled with glass wool, silica, and the platinum (Pt) catalyst . 32 B.1 Reactor schematic for heat transfer calculations . . . . . . . . . . . . . . . . . . . . . . 37 ix List of Tables 2.1 Definitions and units of symbols in Equation 2.3 . . . . . . . . . . . . . . . . . . . . . 6 A.1 Cross section measurement of each fuel at selected temperatures . . . . . . . . . . . . 36 B.1 Heat transfer calculation variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 B.2 Calculated temperature distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 1 Chapter 1 Introduction 1.1 Background A major concern in aviation safety and aircraft design is the possibility of accidental ignition of flammable mixtures. This explosion risk can be mitigated by eliminating all sources of ignition, which may be practically impossible, or by ensuring that the mixture composition cannot be ignited by any source. The gas in the fuel tank ullage is one of the main concerns. For example, the National Transport Safety Board investigation pointed out that the explosion of the center wing fuel tank resulting from the ignition of the flammable atmosphere in the tank was the probable cause of the TWA Flight 800 accident in 1996 (NTSB, 2000). Currently, the installation of an inert atmosphere generation system on the fuel tank is required by the Federal Aviation Administration (FAA, 2008). One inerting system currently in use is a hollow fiber membrane, which operates on the principle of selective permeability creating an output flow that is highly enriched with nitrogen (Air Weekly, 2010). The output stream is directed into the fuel tank, displacing the potentially flammable mixture in the fuel tank ullage and thereby lowering the oxygen concentration below the flammability limit. A single unit of the hollow fiber membrane system weighs approximately 400 lbs and requires either engine bleed air or a separate compressor for the high-pressure input into the bundle of membrane fibers (Air Weekly, 2010). The use of engine bleed air requires a heat exchanger and ductwork carrying air from the engine to the separation unit. These requirements stand in contrast to the goal in current aircraft design, which aims to reduce weight and complexity by eliminating heat exchangers and duct work by using electrical systems instead of bleed air. Hence, alternative methods for inerting fuel tanks are in development. One such alternative is low-temperature catalytic oxidation, which converts the flammable fuel- air mixtures into inert products. The key idea is to use catalysts to initiate reactions between hydrocarbon and oxygen molecules producing carbon dioxide and water vapor, which are fed back into the fuel tank displacing the vapor in the ullage. In this manner, the overall composition in the fuel tank is moved outside the flammability region, which is a function of the relative proportions of [...]... (b) Flammable region and the plan to (b) Flammable region and the plan to with the fuel tank & the fuel modifying treat the ullage gas in the tank reactor treat the ullage gas in the tank Figure 1.1: Catalytic modification of flammable atmosphere in the aircraft fuel tank reactor Fig 1 Catalytic modification of flammable atmosphere in the aircraft fuel tank Fig 1 Catalytic modification of flammable atmosphere. .. change the concentration of fuel molecules in the flow going into the reactor, the fuel must remain completely gaseous To ensure that the fuel is in the gas phase, the heating system is used to maintain the temperature of the piping system above the boiling point of the test fuel The heating system consists of four zones controlled independently using the heater control panel shown in Figure 3.6 A circuit... through the catalytic reactor The fuel only a function of concentration at an CO2 by When an aircraft of fuel in altitude, is concentrations initial point to point A in Figure 1.1b going concentration is gaining the vaportheconcentration of fueldoes not change because the vapor pressure of the fuel is constant under the fixed pressure and diluent gas are reducing because of the decreasing of the atmospheric... after fueling the plane the on a standard flammability diagram (Zabetakis, 1965) be used for the fuel modification in a Theseriesmoves to of aircraft; takeoff, flight and landing.with increased diluent gas tank gas of status point A in non -flammable region Firstly, the ullage gas in the composition may be flammable and the inerting systemcatalytic reactor The fuel concentration turned on The amount of diluent... concentrationsflammable region thereturn into the flammable as the By reducing the fuel pressure increases the the reactor, through mixture may ullage gas fuel concentration arriving point finally of the starting point because they are both at the is the same as that which is out flammable can come to thewhich the catalytic modification of flammable be re-engaged point,inert the region before system can region In. .. ground for landing, the increasing of the ambient pressure forces the ullage gas to move Therefore, the status of the concentration point is As the reactor because is in gas in the point C vented so should be modified again through the aircraft diluent gas to the the to ullage is This gas that ullage gas moves toof fuelB increased and goes downitconcentration ground for landing, the increasing of the ambient... modified into nonflammable condition through the flight Then, it will Firstly, the ullage gas series ofillustrates aircraft; takeoff,reactor.and landing return to the tank for a in the tank status of the process of initial inerting and composition changes Figure 1.1b typical flight may be in the flammable region at the ground before takeoff as shown as a starting point Figure 1-(b) is illustrating the... at the bottom of a tee built into the exhaust line The fuel can then be drained from the tee after the experiment, as shown in Figures 3.3a and 3.3b 3.2 Gaseous Fuel Generating System The fuel vessel is shown in Figure 3.4c A bubbler is made from quarter-inch tubing formed into a spiral with small holes approximately 1 mm diameter and is submersed in the fuel Bubbling 9 ! (a) Exhaust line tee and funnel... accounted for by takingsapphire interference patterns internal to at slight but this effect interference effect on reference windows When the flange temperature is increased, the expansions of sapphire windows measurements at each temperature The alignment of the infrared laser can be done by following the faint glow, while and alignment of thein the is done by following the faint plasmawindow and the path... stratification of multi-component mixtures and to ensure temperature uniformity in the fuel The stirring bar inside the vessel was manufactured with a ring to ensure stable rotation ! (a) Stirring bar with ring ! (b) Bubbler ! (c) Fuel vessel Figure 3.4: The fuel vessel, with stirring bar, and bubbler setup for creating fuel- air mixtures The liquid fuel temperature is regulated by immersing the vessel in an . Catalytic Modification of Flammable Atmosphere in Aircraft Fuel Tanks Thesis by Inki Choi In Partial Fulfillment of the Requirements for the Engineer’s Degree California Institute of Technology Pasadena,. is in the flammable region. By reducing the fuel concentrations through the reactor, the ullage gas can come to the arriving point finally which is out of flammable region. In this point, fuel. as that of the starting point because they are both at the same altitude; the ground level. Fig. 1. Catalytic modification of flammable atmosphere in the aircraft fuel tank   (b) Flammable