Abstract:
Systems useful for superadiabatic combustion generation of a reducing atmosphere for metal heat treatment include a superadiabatic reactor which supplies a reducing atmosphere to a metal heat treatment apparatus. In one aspect, the reactor includes a porous medium and a start-up heater in the flow path of the gas that is to be heated. In another aspect, the gas is passed through a porous medium in alternating, opposite directions by the control of valves which lead the gas to and from the medium. In yet another aspect, a gas inflow pipe leads into an insulated porous bed from which the gas flows in a counter flow manner around the inflow pipe.

Description:
BACKGROUND OF THE INVENTION 
     1 Field of the Invention 
     The present invention relates to the generation of a reducing atmosphere for heat treatment of metals, and more particularly to generating a reducing atmosphere for heat treatment of metals from superadiabatic combustion. 
     2. Brief Description of the Related Art 
     Heat treatment of metals has been utilized to improve the properties of metals. For example, U.S. Pat. Nos. 5,284,526, 5,298,090, and 5,417,774, all issued to Garg et al., describe processes for annealing metals in which nitrogen and residual oxygen are passed through a platinum-group catalyst reactor to convert the oxygen to water, and then passing this reaction product along with a hydrocarbon into the heating zone of a continuous furnace. According to Garg, the water is converted to carbon dioxide and hydrogen by water gas shift reaction, and a reducing atmosphere is produced for the heat treatment of metal in the furnace. 
     Such prior processes suffer from several disadvantages. The requirement for a catalyst in order for the reaction to proceed adds additional costs to the process and apparatus. Furthermore, for many prior processes, the reaction gases must be heated, which further complicates the process and makes the overall process less efficient and significantly more costly. These prior processes are generally concerned with combustion in a fuel-lean reaction. 
     Metal heat treatment in a controlled atmosphere has previously been described. See, for example, U.S. Pat. Nos. 4,992,113, 5,057,164, 5,069,728, 5,207,839, and 5,242,509, each of which is incorporated in its entirety herein by reference. 
     SUMMARY OF THE INVENTION 
     In accordance with a first exemplary embodiment in accordance with the present invention, a process of heat treating metal comprises the steps of superadiabatically reacting a hydrocarbon with oxygen to produce hydrogen, and exposing the metal to the hydrogen. 
     In accordance with a second exemplary embodiment in accordance with the present invention, a system useful for heat treating metal with a reducing atmosphere comprises a superadiabatic reactor having a product gas outlet, and a metal heat treatment apparatus having an inlet in fluid communication with said reactor gas outlet. 
     Still other objects, features, and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of embodiments constructed in accordance therewith, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention of the present application will now be described in more detail with reference to preferred embodiments of the apparatus and method, given only by way of example, and with reference to the accompanying drawings, in which: 
     FIG. 1 diagrammically illustrates a system in accordance with the present invention; 
     FIG. 2 schematically illustrates a first exemplary embodiment of a superadiabatic reactor usable in the system of FIG. 1; 
     FIG. 3 schematically illustrates a second exemplary embodiment of a superadiabatic reactor usable in the system of FIG. 1; 
     FIG. 4 illustrates a graph of a temperature profile of a portion of the reactor of FIG. 3 achievable in accordance with the present invention; 
     FIG. 5 illustrates a third exemplary embodiment of a superadiabatic reactor usable in the system of FIG. 1; and 
     FIG. 6 illustrates a chart of the product distribution for methane conversion as a fractional percent, achievable in accordance with the present invention, for two feed flow rates. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawing figures, like reference numerals designate identical or corresponding elements throughout the several figures. 
     The present invention relates generally to the reaction of an oxidant, preferably oxygen, by introducing hydrocarbon gases, e.g., CH 4 , which produces a reducing atmosphere for metal heat treatment: 
     
       
         O 2 +2CH 4 →4H 2 +2CO 
       
     
     The reaction of a hydrocarbon fuel and oxygen has, in the prior art processes, been catalyzed and performed as a fuel-lean reaction at high temperatures, involving the further application of continuous external heating supplied to the reaction chamber. The present invention, in contrast, eliminates the need for both a catalyst and continuous external heating, and is preferably conducted fuel-rich. Thus, as in the patents to Garg, above, the fuel-lean reaction evolves carbon dioxide and water, while the fuel-rich reaction preferable in the present invention evolves carbon monoxide and (diatomic) hydrogen gas useful as a reducing atmosphere for metal heat treatment. 
     By using superadiabatic combustion, also termed excess enthalpy combustion, both the continuous external heating and the catalyst of the prior art can be eliminated from the reaction chamber. In general terms, once ignition starts with the assistance of a startup heater, the startup heater can be turned off and the temperature of the superadiabatic reactor of the present invention can be maintained at combustion temperature. 
     Excess enthalpy (superadiabatic) combustion has been well examined in the literature. See, e.g., Weinberg, F.,  Superadiabatic Combustion and Its Applications,  in International School-Seminar, Contributed Papers, Minsk, Belarus, Aug. 28-Sept. 1, 1995, pp. 1-20, which reviews superadiabatic combustion principles and describes several exemplary superadiabatic reactors. In general, the effect of superadiabatic combustion occurs when a mixture of gaseous fuel and an oxidizer, which mixture has an overall low caloric value (i.e., low adiabatic temperature) passes through an inert, solid, porous body having a high heat capacity. The intense heat exchange during oxidation of the fuel between the combustion gases and the porous body permits accumulation of energy from combustion in the body. Thus, the flame temperature achieved can be much higher than the adiabatic temperature of the feed fuel mixture, because of the effective heat transfer feedback to the feed gases from the porous body. Although superadiabatic reactors have been proposed for use in some applications, the present invention for the first time combines the advantages of excess enthalpy combustion with a metal heat treatment process and apparatus. 
     FIG. 1 illustrates a system in accordance with the present invention, which includes a superadiabatic reactor  100  connected to an exemplary metal treatment apparatus  10  by a flow pathway  102 . Metal to be treated (not illustrated) is exposed in apparatus  10  to a treatment gas supplied to the apparatus from reactor  100 . The details of apparatus  10  will be readily understood by one of ordinary skill in the art, and may be any of numerous metal treatment apparatus which have been or will be proposed, including those described in the aforementioned U.S. Pat. Nos. 4,992,113, 5,057,164, 5,069,728, 5,207,839, and 5,242,509, including high temperature furnaces. Accordingly, additional details of apparatus  10  are not included herein. 
     FIG. 2 schematically illustrates a first exemplary embodiment of a superadiabatic reactor usable as reactor  100  in the system of FIG. 1, reactor  104 . Reactor  104  includes a reactor vessel  106 , which is preferably insulated so that heat transfer from the vessel is controlled, and preferably minimized. Vessel  106  includes an entrance  108  which allows a feed gas or feed gas mixture to enter the vessel, and an exit  110  which allows a product gas or product gas mixture to exit the vessel. Preferably, exit  110  is in fluid communication with pathway  102 , illustrated in FIG.  1 . 
     Reactor  104  includes a porous solid medium  112  in vessel  106 , formed of a high temperature refractory, ceramic (e.g., aluminum oxide), or similar high temperature material, and includes gas pathways (not illustrated) therethrough, so that gas may readily flow through the medium  112 . Reactor  104  also includes a start-up heater  114 , simplistically illustrated in FIG. 2 as a box, which can be activated to heat up medium  112  to a temperature sufficient to ignite feed gas flowing through vessel  108 . As will be described in greater detail below, heater  114  can be deactivated or turned off once a superadiabatic reactor in accordance with the present invention is generating enough energy to maintain its own process, which can result in significant energy savings over prior systems which require continuous heating to produce metal treatment gas, as discussed elsewhere herein. In order to monitor reactor  104 , as well as other embodiments of reactor  100  described herein, the reactor is provided with temperature probes or thermocouples (not illustrated) mounted in heat transfer communication with the reactor, which provide data signals indicative of the temperature of the reactor. This temperature signal data can be used in an appropriate feedback control scheme, implemented in a manner well know to those skilled in the art, to control the temperature of the reactor and the combustion therein. 
     FIG. 2 illustrates an exemplary feed gas mixture entering entrance  108 , the mixture including nitrogen, oxygen, and a hydrocarbon. Preferably, the feed gas is fuelrich, i.e., the hydrocarbon fuel is present in the feed gas in an amount greater than the stoichiometric amount for the combustion reaction for that hydrocarbon. Hydrocarbons useful in the present invention include, but are not limited to, methane, hexane, propane, butane, and methanol; methane is used herein as an exemplary hydrocarbon from which a product gas, hydrogen, is produced. As will be readily appreciated by one of ordinary skill in the art, the stoichiometric ratio for oxidizing (combusting) methane is 2, as evident from the above balanced equation. Thus, fuel-rich combustion of methane, for example, involves a CH 4 /O 2  ratio greater than 2, while fuel-lean combustion of methane involves a ratio less than 2. 
     As seen from FIG. 2, the feed gas mixture enters vessel  106 , and passes through medium  112 . As startup heater  114  has already heated up medium  112  to a temperature sufficient to at least partially oxidize the methane, the methane is oxidized, producing carbon monoxide and hydrogen gas. The heat energy released by this exothermic reaction heats the medium  112 , which heats incoming feed gas by radiation heat transfer, conduction heat transfer, or both. As the incoming feed gas is therefore preheated by energy from the reaction downstream of it, a reaction heat feedback  116  is established. Further details of excess enthalpy or superadiabatic combustion are well reviewed in Weinberg, above, and will not be further detailed herein. 
     FIG. 3 schematically illustrates a second exemplary embodiment of a superadiabatic reactor usable as reactor  100  in the system of FIG. 1, reactor  130 . Reactor  130  includes a feed inlet  132 , a product outlet  134 , and an insulated porous solid medium  136 , similar to medium  112 . Medium  136  includes a start-up heater (not illustrated). First and second two-way valves  138 ,  140  are connected by fluid pathways  150 ,  152 , to ports  158 ,  160 , respectively, of medium  136 . Reactor  130  includes a feed inlet flow path which includes an upper branch  142  and a lower branch  144 . Upper branch  142  fluidly connects feed inlet  132  with valve  138 , and lower branch  144  fluidly connects the feed inlet with valve  140 . Reactor  130  also includes a product outlet flow path which includes an upper branch  146  and a lower branch  148 . Upper branch  146  fluidly connects product outlet  134  with valve  138 , and lower branch  148  fluidly connects the product outlet with valve  140 . 
     Valves  138 ,  140  can be switched between two positions each, which together determine the direction of flow of gas through reactor  130 . In a first set of positions of valves  138 ,  140 , a first flow path “A” is established. Feed gas is prevented from flowing along lower inlet branch  144  by valve  140  and is allowed to flow through upper inlet branch  142  to valve  138 . Valve  138  directs the flow of feed gas along pathway  150  into port  158  of medium  136 . As the feed gas passes through medium  136 , it is at least partially combusted to form a product gas, e.g., hydrogen, and the reaction products exit the medium at port  160 . The product gas passes along pathway  152  and is directed by valve  140  along lower branch  148  to product outlet  134 . When set in the first position, valve  138  prevents product gas from entering pathway  150  and reentering medium  136 . 
     Valves  138 ,  140  can be positioned to establish a second flow path “B”, which is, in one sense, opposite flow path “A”. Feed gas is prevented from flowing along upper inlet branch  142  by valve  138  and is allowed to flow through lower inlet branch  144  to valve  140 . Valve  140  directs the flow of feed gas along pathway  152  into port  160  of medium  136 . As the feed gas passes through medium  136 , it is at least partially combusted to form a product gas, e.g., hydrogen, and the reaction products exit the medium at port  158 . The product gas passes along pathway  150  and is directed by valve  138  along upper branch  146  to product outlet  134 . When set in the second position, valve  140  prevents product gas from entering pathway  152  and reentering medium  136 . 
     Thus, when valves  138 ,  140  are set to establish path “A”, gas flows through medium  136  in the direction indicated by arrow  154 , and the high temperature volume of medium  136 , e.g., the flame front from combustion of methane, expands or moves in the direction indicated by arrow  156 . Similarly, when valves  138 ,  140  are set to establish path “B”, gas flows through medium  136  in the direction indicated by arrow  156 , and the high temperature volume of medium  136 , e.g., the flame front from the combustion of methane, expands or moves in the direction indicated by arrow  154 . To maintain the flame front within the medium  136 , and therefore to prevent the flame from flashing back into the feed gas supply, and also to trap heat in the porous medium, valves  138 ,  140  are switched between the first and second sets of positions, which reverses the flow as described above. By reversing the flow directions through medium  136 , the flame front can be caused to move back and forth within the medium to maintain the medium at a very high temperature, thus allowing superadiabatic combustion to continuously occur. 
     FIG. 4 illustrates a graph of a temperature profile medium  136  achievable in accordance with the present invention. As illustrated in FIG. 4, the average temperature of the medium at the inlet (left endpoint) and outlet (right endpoint) can be maintained around 30° C., while average temperatures within the porous solid medium can reach 800° C. by timing the flow reversal to occur when the heat wave nearly reaches the ports  158 ,  160 . The excess enthalpy and heat transfer from combustion at this temperature is sufficient to maintain combustion in the porous medium without the need for an additional, external heater or catalyst. 
     FIG. 5 illustrates a third exemplary embodiment of a superadiabatic reactor usable for reactor  100  in the system of FIG. 1, reactor  180 . Reactor  180  is a recuperative-type reactor. Reactor  180  includes a bed of a porous solid medium  182  in which excess enthalpy combustion of the hydrocarbon fuel occurs. As illustrated in FIG. 5, porous bed  182  has an exposed top surface  190 , and a feed tube  186  extends into the bed through the top surface. Porous bed  182  is otherwise closed off and, as in the other embodiments herein, is insulated and provided with a start-up heater (not illustrated). Thus, feed gas  184  is supplied through feed tube  186  into porous bed  182  where it reacts. Product gas  188  leaves the porous bed and flows around the feed tube. The portions of the porous bed which surround the feed tube, as well as the hot product gas, transfer heat to the feed tube and the feed gas therein, thus assisting in maintaining excess enthalpy combustion in reactor  180 . 
     Reactor  180  can optionally further be provided with a carrier gas tube  192  (illustrated in phantom) inside feed tube  186 , which can supply a non-reactive carrier gas into medium  182 . The further provision of carrier gas tube  192  permits the total mass flow rate into reactor  180  to be controlled by controlling the mass or volume flow rate of the carrier gas flowing through the carrier gas tube, which in turn controls the temperature of the reactor. 
     FIG. 6 illustrates a chart of the product distribution for methane conversion, as a fractional percent, achievable with the reactor of FIG. 5, for two feed gas flow rates. For both flow rates, the ratio of hydrocarbon (methane) to oxygen was 1.40 (fuel lean). As demonstrated by the data represented in FIG. 6, the relatively slow mass flow rate (0.17 g/sec) produced a greater fractional percent of hydrogen than the fast mass flow rate (0.20 g/sec), which can be attributed to a higher combustion temperature because of the longer residence time of the reaction gas in the reactor. 
     Each of the aforementioned U.S. Patents and literature references is incorporated by reference herein in its entirety. 
     While the invention has been described in detail with reference to preferred embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention.