Abstract:
A demisting flame arrestor provides a border between an electrolytic reaction vessel and an intake manifold of an engine, and includes a composite material having hydrophilic zones and hydrophobic zones constructed to form multiple pathways for permitting gaseous flow. The hydrophilic zones promote trapping and condensation of water vapor and in the event of fire disperse flame into the multiple pathways to arrest the flame front. The hydrophobic zones repel condensed water to return the water to the reaction vessel through force of gravity.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/322,252, which was filed Apr. 8, 2010, and which is fully incorporated herein by reference. 
     
    
     1. Field of the Invention 
       [0002]    The present invention relates generally to flame arrestors and demisters used in reaction cells, and more specifically to a device that functions as both a flame arrestor and a demister for use in an electrolytic hydrogen generator. 
         [0003]    2. Description of Related Art 
         [0004]    Water electrolyzers have been used for the purpose generating and supplying a small stream of hydrogen and oxygen gas to internal combustion engines. The hydrogen/oxygen gas stream is usually only a fraction of a percent of the intake combustion air flow but evidence has been presented to show that this small stream can reduce emissions of particulates from diesel engines and in some cases also reduce emissions of NOx and provide small increases in engine fuel efficiency. Typically these electrolyzers are powered by electrical current from the vehicle battery or alternator and are fitted to the back of the cab compartment of a truck or under the hood of a car. To further improve the effectiveness of these devices it is necessary to minimize their volume and mass and maximize their energy efficiency for a given intended hydrogen output. Furthermore it is important to operate an electrolyzer that can function reliably for thousands of hours in extremes of temperature, the presence of continuous shock and vibration and road grime and grit. 
         [0005]    The science and engineering principles behind the design and operation of water electrolyzers are well known and understood. Electrolysis of water is the decomposition of water (H 2 O) into oxygen (O 2 ) and hydrogen gas (H 2 ) due to an electric current being passed through the water. An electrical power source is connected to two electrodes, or two plates, (typically made from some inert metal such as platinum or stainless steel) which are placed in the water. Hydrogen will appear at the cathode (the negatively charged electrode, where electrons are transferred to water molecules), and oxygen will appear at the anode (the positively charged electrode where electrons are transferred from water molecules to the electrode). The generated amount of hydrogen is twice the amount of oxygen, and both are proportional to the total electrical charge that was sent through the water. Electrons carry the current in the circuit external to the electrolysis cell and in the electrodes, while charged ions carry electric current through the water or electrolyte solution. 
         [0006]    In the water at the negatively charged cathode, a reduction reaction takes place, with electrons (e − ) from the cathode being given to water molecules to form hydrogen gas:
       Cathode (reduction):  2 H 2 O( 1 )+ 2 e − →H 2 (g)+ 2 OH − (aq)       
 
         [0008]    At the positively charged anode, an oxidation reaction occurs, where water is oxidized to generate oxygen gas and giving electrons to the anode.
       Anode (oxidation): H 2 O( 1 ) → 1 / 2 O 2 (g)+ 2 H + (aq)+ 2 e −         
 
         [0010]    Combining these two reactions with
       H 2 O( 1 )→ 2 H + (aq)+ 2 OH − (aq)
 
yields the overall decomposition of water into oxygen and hydrogen:
   Overall reaction:  2 H 2 O( 1 )→ 2 H 2 (g)+O 2 (g)       
 
         [0013]    For every two electrons the number of hydrogen molecules produced is twice the number of oxygen molecules. Assuming equal temperature and pressure for both gases, the produced hydrogen gas has therefore twice the volume of the produced oxygen gas. 
         [0014]    In acid solution the reactions and standard electrode potentials are
         2 H + (aq)+ 2 e − →H 2 (g) E 0 = 0 . 00 V     1 / 2 O 2 (g)+ 2 H + (aq)+ 2 e − →H 2 O( 1 ) E 0 = 1 . 23 V giving a Standard EMF of  1 . 23  V.       
 
         [0017]    In base solution the reactions and standard electrode potentials are
         2 H 2 O( 1 )+ 2 e − →H 2 (g)+ 2 OH − (aq) E 0 =− 0 . 83 V     1 / 2 O 2 (g)+ 2 H 2 O( 1 )+ 2 e − ♯ 2 OH − (aq) E 0 = 0 . 40 V giving a Standard EMF of 1.23 V.       
 
         [0020]    Electric current is carried through the electrolyte solution by way of movement of ions such as H + (aq) or OH − (aq) . However in pure water these ions are in very low concentration so an additional electrolyte must be added to allow practical values of current to flow. Typically an alkalis such as Sodium Hydroxide (Na 0 H) or Potassium Hydroxide (KOH) is added in quite high concentrations. A typical value for KOH would be about 30 wt%, the concentration at which the electrical conductivity reaches a maximum. 
         [0021]    Faraday&#39;s Law provides the relationship between the current and the rate of electrolysis, where N the number of moles of gas released by a current I in time t is given by
       N=I*t/(n*F) ( 1 )       
 
         [0023]    N is the number of electrons required to deliver one mole of gas, for hydrogen n= 2  for oxygen n= 4 . Thus the rate of hydrogen production is given by
       ΔI/Δt=I/( 2 *F) ( 2 )       
 
         [0025]    The minimum voltage required to electrolyze water is 1.23 V but higher voltages must be applied in order to increase the current. Voltage drops occur at the electrodes due to overpotential and across the electrolyte gap between to two electrodes. 
         [0026]    Overpotential (η) refers to the difference between the applied potential necessary to produce a current i and the equilibrium potential E 0  at zero current,
       η=E−E 0  ( 3 )       
 
         [0028]    For the anode where oxygen is produced the Overpotential is related to the current density by
       i=i 0 exp(−b η)( 4 )   where b=αnF/RT and i=I/A ( 5 ) and where i 0  is a constant relating to the particular electrode reaction and the surface on which it occurs say platinum in KOH, a is a constant usually with a value of 0.5. F is the Faraday constant, R the gas constant and T temperature in K. A is the active area of the electrode and I is the actual measured current. A similar equation exists for the cathode but with different values of i 0  and a. These equations can also be expressed in  terms  of the overvoltage as   η=Bln i 0 −Blni=Bln i 0 /I ( 6 )       
 
         [0032]    where B= 1 /b. 
         [0033]    The gap between to two electrodes is filled with conducting electrolyte but does incurred a potential drop. This potential drop is given by
       V electrolyte =I*R ( 7 )       
 
         [0035]    where I is the current and R the resistance of the electrolyte, given by:
       R=A/d*κ( 8 )       
 
         [0037]    A is the effective electrode area and d the electrode separation, κ is the conductivity of the electrolyte which is a function of the electrolyte composition and concentration and temperature. 
         [0038]    The Current Voltage Characteristic for a single cell is therefore given by
       V=E 0  =η anode −η cathode +IR ( 9 )   V=E 0  B anode ln i 0 /i−B cathode ln i 0 /i+IR ( 10 )       
 
         [0041]    It can be seen that for a given current the voltage can be reduced by increasing the effective surface area of the electrodes, reducing the electrode separation, increasing the concentration and temperature of the electrolyte and by catalyzing the electrodes which has the effect of increasing the value of i 0 . 
         [0042]    The maximum efficiency of an Electrolyzer c, is given by,
       ε=ΔH 0 /ΔG 0         
 
         [0044]    where ΔG 0  and ΔH 0  are the standard Cibbs Energy Change and standard Enthalpy change for the electrolysis reaction. For water electrolysis the maximum efficiency is 120% this is greater than 100 because in principle the reaction can extract heat from the surroundings, in practice however the efficiency is below 100% because the driving voltage is always greater than 1.23 V. 
         [0045]    The actual efficiency is given by
       ε=ΔH 0 /(nFV) where V is the cell operating voltage at a given current. V is given by equation ( 10 ).       
 
         [0047]    At a practical current density of about  2  A cm −2  the cell voltage is about 2 V, this would give an efficiency of 74%. 
         [0048]    Electrolyzer Design 
         [0049]    From the above descriptions and equations it can be shown that the most energy efficient electrolyzer is one that minimizes the overall cell impedance. For an electrolyzer supplied with current from a vehicle alternator operating at a constant voltage if ˜13 V and with a current draw limited to 20-30 A, the impedance of the electrolyzer is minimized by reducing the electrode gap, increasing the electrolyte conductivity and increasing the number of cells in series, usually to six. Under these circumstances the electrode area is optimized to reduce cell impedance but to remain within the chosen current draw the alternator. 
         [0050]    If the electrolyte is potassium hydroxide the maximum conductivity occurs at about 28% by weight potassium hydroxide. 
         [0051]    An electrolyzer with  6  cells in series with stainless steel electrodes of 200 cm 2  and a spacing of 1 cm immersed in 28% KOH will operate at 12 V and about 30 A and produce about 1.3 L/min of hydrogen. This electrolyzer would require a minimum volume of about 1.5 L of electrolyte or about 1 L of water. This amount of water would be consumed in 16.5 hours. 
         [0052]    During electrolyzer operation, water vapor or tiny droplets of water can become entrained in the flow of gasses out of the electrolyzer. This process leads to an undesirable loss of water volume from the electrolyzer vessel, and an unwanted introduction of water into the engine manifold. To minimize these problems, a water trap or other means for separating water from the gasses generated by the electrolyzer may be incorporated into the electrolyzer design. 
         [0053]    The production of a volatile gas such as hydrogen raises safety concerns, especially if the hydrogen is allowed to accumulate under pressure during equipment malfunction or under some other accident scenario. To mitigate damage in the event that the hydrogen ignites, it is prudent to incorporate safety measures such as a flame arrestor or other means for flame suppression in some part of the system. Ideally, the flame suppression should prevent a flame that originates in the engine from propagating into the electrolysis vessel, and vice-versa. 
         [0054]    For the present inventors, a design objective for an on-board hydrogen/oxygen generator is the minimization of cost and complexity through the use of passive controls. The problem being solved by the present invention is how to design a filtration device between the electrolytic reactor and the intake manifold of the engine, so that the device acts as both a flame arrestor and a demister. 
       SUMMARY OF THE INVENTION 
       [0055]    The present invention provides an engineering design for a demisting flame arrestor that may be installed, for example, in an on-board electrolytic hydrogen generator. The demisting flame arrestor provides a border between a reaction vessel of the generator and an intake manifold of an internal combustion engine. 
         [0056]    In one embodiment, the demisting flame arrestor includes a composite material having hydrophilic zones and hydrophobic zones constructed to form multiple pathways for permitting gaseous flow. The hydrophilic zones may be configured to promote trapping and condensation of water vapor during normal operation, and to disperse flame into the multiple pathways to arrest the flame front in the event of an accidental explosion. The hydrophobic zones may be configured to repel condensed water to return the water to the reaction vessel through force of gravity. The hydrophilic zones and hydrophobic zones may be evenly distributed throughout the composite material, or they may be distributed in a graduated, layered, or random arrangement. The composite material may be constructed so that the hydrophilic material forms a structural base for hydrophobic material, or so that the hydrophobic material forms a structural base for hydrophilic material, or so that both materials contribute to the structural integrity of the composite. In one implementation, the hydrophilic zones comprise a metal such as nickel and the hydrophobic zones comprise a polymer such as Teflon. 
         [0057]    Another embodiment of the invention provides a method for manufacturing a demisting flame arrestor. The method prescribes salient steps for preparing an emulsion of polymer, soaking a metal mesh in the emulsion, removing the metal mesh from the emulsion, draining the metal mesh, applying heat to the metal mesh to sinter the polymer onto the metal mesh, and compressing the sintered metal mesh into a desired form. The metal mesh may be nickel mesh and the polymer may be Teflon particles. In another embodiment, a method for forming a demisting flame arrestor may include steps for soaking sintered metal particles within a polymer emulsion, removing and drying the particles, then heating and compressing them into a desired form. In another embodiment, a method for forming a demisting flame arrestor may include steps for mixing polymer particles such as Teflon particles with a metallic powder such as nickel powder, then applying heat and compression to achieve a desired form. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0058]    Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. Component parts shown in the drawings are not necessarily to scale, and may be exaggerated to better illustrate the important features of the invention. Dimensions shown are exemplary only. In the drawings, like reference numerals may designate like parts throughout the different views, wherein: 
           [0059]      FIG. 1  is a cross sectional diagram of a reaction cell incorporating one embodiment of a demisting flame arrestor according to the invention. 
           [0060]      FIGS. 2 through 9  each show a cross sectional diagram of a design for a demisting flame arrestor having both hydrophobic and hydrophilic properties according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0061]    The following disclosure presents an exemplary embodiment of the invention for a demisting flame arrestor for use in a reaction cell. In one application, the demisting flame arrestor is designed for installation in an on-board electrolytic hydrogen generating system to form a boundary in the gaseous flow path between a reactor such as an electrolysis vessel and an outflow conduit such as an engine manifold. 
         [0062]      FIG. 1  shows cross sectional view of a reaction cell  10  equipped with an exemplary embodiment of a demisting flame arrestor  12  of the present invention. Reaction cell  10  may be an electrolysis vessel containing a volume of an electrolyte  14 , such as KOH. An anode  16  and a cathode  18  may be suspended at least partially within the electrolyte  14 , and each of these electrodes may be conductively connected, respectively, to positive and negative terminals  20  and  22 . Terminals  20  and  22  are preferably located outside of the vessel  10  at any location that is convenient for connection to an external source of electrical power, such as a vehicle battery, battery charging system, or other DC source. Energization of terminals  20  and  22  creates an electric field between anode  16  and cathode  18  to cause electrolysis of the electrolyte. For an aqueous electrolyte  14 , hydrogen and oxygen gas will be produced according to the oxidation and reduction reactions presented above. 
         [0063]    The hydrogen and oxygen gases will form initially on the electrodes  16  and  18  and rise, along with some amount of water vapor, through the surface of the electrolyte  14  and into the air gap  24 . Under the proper operating conditions, a pressure differential will occur across the demisting flame arrestor  12 , that is, between the air gap  24  and a conduit  26 . For example, in an application where conduit  26  is connected to an intake manifold of an internal combustion engine, a vacuum in the manifold will draw the gases and water vapor through the demisting flame arrestor  12  and into the manifold. 
         [0064]    The demisting flame arrestor  12  is shown at a location directly above the surface of electrolyte  14 . In other embodiments, the demisting flame arrestor may be located a greater distance above the electrolyte, or a considerable distance away from reaction cell  10 , such as further within conduit  26 , or at an interface between conduit  16  and the intake manifold of an engine or other apparatus. In any case, it may be desirable to locate the demisting flame arrestor out of range of electrolyte “slop”, particularly in on-board applications where one would expect the electrolyte surface to rise and fall as a result of the acceleration or travel of a vehicle. 
         [0065]    According to the invention, the demisting flame arrestor  12  has both hydrophilic and hydrophobic properties, so that the collection of water vapor may be promoted by the hydrophilic property, and the repulsion of water droplets may be promoted by the hydrophobic property. Generally speaking, hydrophilic and hydrophobic properties are mutually exclusive properties within any homogeneous material. It is therefore an object of the invention to construct the flame arresting demister as a composite material, or as an assembly of hydrophilic and hydrophobic materials, in such a way that hydrophilic and hydrophobic zones are distributed throughout the composition. 
         [0066]    In one embodiment, a demisting flame arrestor  12  comprises a composite material that is between about 10% and about 90% hydrophilic, with the balance of the composite being hydrophobic. The hydrophilic material may be a metal that is readily wettable by the electrolyte but not corrodible. Metals such as nickel, stainless steel, noble metals, and plated metals—such as iron or steel plated with nickel or chromium—are examples of hydrophilic materials that are suitable for use in the demisting flame arrestor. The hydrophobic material may be a thermoplastic polymer such as Teflon or polypropylene, or some other material that is stable in the electrolyte and that possesses the desired water repulsion properties. 
         [0067]    In one embodiment, as shown in the figure, demisting flame arrestor  12  comprises a composite having a structural base composed of a hydrophilic material  28  that contains a regular or irregular distribution of hydrophobic materials  30  throughout the structure. In another embodiment, the hydrophobic material may serve as the structural base, and the hydrophilic material may be distributed throughout the hydrophobic material. In another embodiment, the two materials may be assembled so that both materials provide structural support to maintain the integrity of the assembly. The distribution of one material within the other material may be an even distribution (homogeneous) or an uneven distribution (random, graduated, or layered). 
         [0068]    Operating as a demister (during normal operation), the demisting flame arrestor  12  works as a filter that ideally traps water vapor while passing hydrogen and oxygen (or other) gases. The arrangement of hydrophilic and hydrophobic zones create tiny circuitous pathways through the demisting flame arrestor. The gases compelled by the pressure differential will easily work their way through these pathways and around the hydrophilic zones. The water vapor however, when contacting a hydrophilic zone, will tend to agglomerate there and condense to form a water droplet on the hydrophilic zone. As the droplet grows and becomes heavier, it will fall through the demisting flame arrestor under force of gravity, encountering hydrophobic zones along the way. The hydrophobic zones will repel the droplet, helping to accelerate its passage downward until it eventually drops back into the volume of electrolyte  14 . In this way, the combination of hydrophilic and hydrophobic materials according to the invention discourages the accumulation of water droplets that would otherwise clog the demisting flame arrestor and obstruct the passage of the electrolysis gases. 
         [0069]    Operating as a flame arrestor (during an accident), the demisting flame arrestor  12  works by absorbing heat and directing a flame front through multiple pathways that are too narrow to permit the continuance of the flame. The multiple pathways may be the same circuitous pathways that allow the passage of gases through the demisting flame arrestor. The multiple pathways are bordered, in part, by the metal construction of the hydrophilic zones, which provide a corresponding multiplicity of surface areas that are ideal for sinking heat. In the event of an explosion or fire originating on either side of the demisting flame arrestor, the metal material suppresses the flames by dispersing the flame front and by absorbing the heat. 
         [0070]    In one embodiment, the demisting flame arrestor may be formed from a combination of metal wool and small particles of polymer, such as Teflon balls. For example, the wool may be spread flat, the polymer balls may be arranged on the wool, and the wool may be rolled into a cylindrical or “jelly roll” form. 
         [0071]    In another embodiment, the demisting flame arrestor may be formed as a sintered metal disk or perforated filter, and partially filled with hydrophobic particles. 
         [0072]    A method of manufacturing a demisting flame arrestor according to the invention may include the following salient steps: A polymer emulsion may be prepared, for example, using  5  nm Teflon particles and water. A metal or wire mesh, such as nickel mesh, may then be soaked in the Teflon emulsion, then removed and drained. Heat and compression may then be applied to the wire mesh to sinter the polymer onto the metal. A temperature of about  350  degrees C. may be suitable for this purpose. The compression may be used to mold the mesh into a desired form. 
         [0073]    Alternatively, one may start with sintered metal particles and soak them within a polymer emulsion. The particles may then be removed and dried, then heated and compressed into a desired form, again using a temperature around 350 degrees C. Alternatively, one may start with polymer particles such as Teflon particles, mix them with a metallic powder such as nickel powder, then apply heat and compression to achieve a desired form. 
         [0074]      FIGS. 2 through 9  show different exemplary embodiments of demisting flame arrestors according to the invention. Each may be characterized by its distribution of hydrophobic and hydrophilic materials, each material being complimentary to the other.  FIGS. 2 through 5  show embodiments wherein hydrophilic material  28  provides a structural base within which a plurality of hydrophobic zones  30  may be distributed.  FIGS. 6 through 9  show embodiments wherein the hydrophobic material  30  provides a structural base within which a plurality of hydrophilic zones  28  may be distributed. Round and triangular zones are shown for purposes of illustration only. Many geometries for zones  28  or  30  other than round and triangular may be employed in various embodiments of the invention. 
         [0075]      FIGS. 2 and 6  correspond to the general design of demisting flame arrestor  12  of reaction cell  10 .  FIG. 2  consists of a hydrophilic base material  28  within which a plurality of round or spherical hydrophobic zones  30  are embedded.  FIG. 6  consists of a hydrophobic base material  30  within which a plurality of round or spherical hydrophilic zones  28  are embedded. 
         [0076]      FIGS. 3 and 7  show embodiments of the demisting flame arrestor  12  in which either the hydrophobic zones  30  or hydrophilic zones  28  are more or less randomly distributed within the complimentary base material. The random placement of the plural zones  30  or  28  may result from the exploitation of randomness introduced by a manufacturing process, such as mixing. 
         [0077]      FIGS. 4 and 8  show an embodiment of the demisting flame arrestor  12  that includes a graduated distribution of hydrophobic or hydrophilic zones,  30  or  28 , within a complimentary structure. The graduated distribution may occur in the direction of gas flow or flame propagation, i.e. vertically with respect to the figures. For example, as shown in  FIG. 4 , a greater percentage of hydrophobic zones  30  may be arranged nearer to the bottom portion of the demisting flame arrestor to promote the rejection of water back into the reaction cell. The triangular shape of the zones  30  will promote such flow. The density of the zones  30  gradually decreases as we move toward the top of the device. However, in the embodiment shown in  FIG. 8 , a greater percentage of hydrophilic zones could be arranged nearer the top portion of the demisting flame arrestor to discourage flame propagation into the reaction cell from an explosion originating somewhere outside the reaction cell, such as in an engine. In this embodiment, the base of each triangular zone  28  faces toward the top surface of the demisting flame arrestor to increase flame resistance. The density of the zones  28  gradually decreases as we move toward the lower surface of the device. 
         [0078]    For the embodiments illustrated in  FIGS. 4 and 8 , the distribution of hydrophilic or hydrophobic zones could be gradual, so that the density of zones at any elevation within the demisting flame arrestor satisfies a desired distribution formula, such as a linear, polynomial, non-linear, or transcendental formula, or some other formula for statistical distribution. 
         [0079]    Alternatively, the distribution of zones throughout the demisting flame arrestor could be organized according to layers. This concept is illustrated in  FIGS. 5 and 9 , each of which shows a demisting flame arrestor  12  comprising multiple layers  31 ,  32 ,  33 . In these examples, the density of hydrophilic or hydrophobic zones could be made greatest only at a top-most or bottom-most layer of the demisting flame arrestor, with a corresponding lesser distribution used in the remaining layers to achieve one or more lower densities. 
         [0080]    Exemplary embodiments of the invention have been disclosed in an illustrative style. Accordingly, the terminology employed throughout should be read in a non-limiting manner. Although minor modifications to the teachings herein will occur to those well versed in the art, it shall be understood that what is intended to be circumscribed within the scope of the patent warranted hereon are all such embodiments that reasonably fall within the scope of the advancement to the art hereby contributed, and that that scope shall not be restricted, except in light of the appended claims and their equivalents.