Patent Publication Number: US-2011062014-A1

Title: Plasma reactor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation of U.S. patent application Ser. No. 10/560,439 filed Jul. 4, 2006, which was the National Stage Entry of and claims priority to PCT/U.S. 04/19589, filed Jun. 18, 2004, which further claims the benefit of U.S. Provisional Patent Application Ser. No. 60/480,132 filed Jun. 20, 2003, each of which is incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a plasma reactor and process for the production of hydrogen-rich gas from light hydrocarbons. 
     BACKGROUND 
     Improving the efficiency of energy production remains an important technological goal, owing to the significant economic benefits that result in almost every sector of the economy. One potential method for improving the efficiency of energy production is to provide an energy efficient method of converting light hydrocarbons to hydrogen-rich gas, to thereby increase energy production from natural gas. 
     Plasma fuel converters such as plasmatrons are known to reform hydrocarbons to produce hydrogen-rich gas. DC arc plasmatrons, for example, are disclosed in U.S. Pat. Nos. 5,425,332 and 5,437,250. DC arc plasmatrons generally operate at low voltage and high current. As a result, these plasmatrons are particularly susceptible to electrode erosion and/or melting. DC arc plasmatrons also require relatively high power inputs of 1 kW or more and relatively high flow rates of coolant to keep the temperature in check. 
     Other conventional methods for the conversion of light hydrocarbons to hydrogen-rich gas are generally energy inefficient and, as a result, in many small-scale applications, such as the production of hydrogen for fuel cells, the cost of hydrogen gas made by these methods is not competitive. Thus, there is a need in the art for a more energy efficient process for the conversion of light hydrocarbons to hydrogen-rich gas. 
     U.S. Pat. Nos. 5,993,761 and 6,007,742 (Czernichowski et al.) describe processes for the conversion of light hydrocarbons to hydrogen-rich gas using gliding arc electric discharges in the presence of oxygen and, optionally, water. In the process, two electrodes having flat sheet geometry are arranged for arc ignition and subsequent gliding of the arc. The distance between the cathode and anode gradually increases to a point that no longer supports the gliding arc. As a result, the gliding arc disappears at one end of the electrodes, creating pulsed plasma wherein the properties of the plasma change with time. Due to the use of pulsed plasma, the process is relatively unstable over time. Reagents and oxygen are preheated using an external heat source. As a result of the preheating of the reagents and oxygen using an external heat source, the process suffers from poor energy efficiency. A premixed feed gas including hydrocarbons and oxygen is introduced to the reactor located at the central axis of the reactor. 
     U.S. Pat. No. 5,887,554 (Cohn et al.) also discloses a system for the production of hydrogen-rich gas from light hydrocarbons. The system includes a plasma fuel converter for receiving hydrocarbon fuel and reforming it into hydrogen-rich gas. The plasma fuel converter can be operated using either pulsed or non-pulsed plasma and can utilize arc or high frequency discharges for plasma generation. Products from the plasma fuel converter are employed to preheat air input to the fuel converter. In one embodiment shown in  FIG. 6 , residence time in the reactor is increased by providing a centralized anode and a plurality of radial cathodes to thereby cause the arc to glide towards the center of the reactor under the influence of gas flowing in the same direction as the gliding arc. 
     U.S. Pat. No. 6,322,757 (Cohn et al.) discloses a plasma fuel converter which employs a centralized electrode and a conductive reactor structure which acts as the second electrode for creation of a plasma discharge. Reagents are fed to the reactor just below the smallest gap between the electrodes and flow in the same direction as the gliding arc to thereby produce hydrogen-rich gas. In alternative embodiments, air and/or fuel are preheated by counter-flow heat exchange with the products of the reforming reaction and fed to the reactor either above or just below the smallest gap between the electrodes. 
     Although some improvements in the energy efficiency of plasma fuel converters have been achieved, there remains a need for higher energy efficiencies for use of non-equilibrium low temperature plasma. 
     SUMMARY 
     Accordingly, it is an object of certain embodiments of the invention to provide a plasma fuel converter and a process for the conversion of light hydrocarbons to hydrogen-rich gas using a low temperature, non-equilibrium plasma. 
     It is another object of certain embodiments of the invention to provide a plasma fuel converter and a process for the conversion of light hydrocarbons to hydrogen-rich gas using a low temperature, non-equilibrium plasma that has a relatively high energy efficiency. 
     In order to achieve the above and other objects of the invention, a plasma reactor for conversion of light hydrocarbons to hydrogen-rich gas is disclosed. In a first aspect, the plasma reactor has a wall defining a reaction chamber. The plasma reactor also has an outlet. The plasma reactor has a reagent inlet fluidly connected to the reaction chamber for creating a vortex flow in the reaction chamber. The plasma reactor also has a first electrode and a second electrode connected to a power source for generating a sliding arc discharge in the reaction chamber. 
     In another aspect of the invention, a method for plasma conversion of light hydrocarbons to hydrogen-rich gas is provided. In the method, a plasma reactor is provided. The plasma reactor has a wall defining a reaction chamber, an outlet, and a reagent inlet fluidly connected to the reaction chamber for creating a vortex flow in the reaction chamber. The plasma reactor also has a first electrode and a second electrode connected to a power source for generating a sliding arc discharge in the reaction chamber. The method includes introducing a gas selected from the group consisting of one or more light hydrocarbons, oxygen, an oxygen containing gas, and mixtures thereof, into the reaction chamber in a manner that creates a vortex flow in the reaction chamber. The method also includes processing the light hydrocarbons using a plasma assisted flame; and recovering hydrogen-rich gas from the reactor. 
     These and various other advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a vortex reactor in accordance with the present invention showing the circumferential flow component of the first gas. 
         FIG. 2  is a schematic representation of a vortex reactor in accordance with the present invention showing the axial flow component of the gases in the reaction chamber. 
         FIG. 2   b  is a schematic representation of a vortex reactor showing a second swirl generator. 
         FIG. 3  is a schematic representation of a vortex reactor in accordance with the present invention and having a third gas inlet. 
         FIG. 4  is a schematic representation of a vortex reactor in accordance with the present invention provided with a counter-current heat exchanger. 
         FIG. 4   b  is a schematic representation of a vortex reactor with two heat exchangers employed. 
         FIG. 5  is a schematic representation of an alternative embodiment of a heat exchanger which may be used in accordance with the present invention. 
         FIG. 6  is a schematic representation of a vortex reactor in accordance with the present invention showing the movable circular ring electrode in the ignition position. 
         FIG. 7   a  is a schematic representation of a vortex reactor in accordance with the present invention showing the movable circular ring electrode in the reactor operating position. 
         FIG. 7   b  is schematic representation of a vortex reactor showing a circular ring electrode supported by supporting wires. 
         FIG. 8  is a schematic representation of a vortex reactor in accordance with the present invention provided with a spiral electrode. 
         FIG. 9  is a schematic representation of a vortex reactor in accordance with the present invention provided with both a spiral electrode and a circular ring electrode. 
         FIG. 10  is a schematic representation of a vortex reactor in accordance with the present invention provided with a circular ring electrode which forms part of the bottom of the reactor. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The present invention relates to a device and process for conversion of light hydrocarbons to hydrogen-rich gas using a low temperature, non-equilibrium plasma. The term “light hydrocarbons” as used herein refers to C 1  to C 4  hydrocarbons, which may be saturated or unsaturated, branched or unbranched, and substituted or unsubstituted with one or more oxygen, nitrogen, or sulfur atoms. 
     In general, dimensions, sizes, tolerances, parameters, shapes and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. In general, a dimension, size, parameter, shape or other quantity or characteristic is “about” or “approximate” as used herein, whether or not expressly stated to be such. 
     Gaseous hydrocarbons and oxygen (pure oxygen or oxygen in air, or oxygen in enriched air) are the reagents in the process of the present invention. The conversion process consists of two steps as illustrated below using methane as the light hydrocarbon reagent: 
       CH 4 +2O 2 .→2H 2 O+CO 2    (I)
 
       2CH 4 +CO 2 +2H 2 O→6H 2 +3CO   (II)
 
     Step (I) is exothermic, whereas step (II) is endothermic and tends to be the rate-determining step. 
     Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to  FIG. 1 , a schematic view of a vortex reactor  10  of the present invention is depicted. Vortex reactor  10  includes a reaction chamber  12 . At or near the top of vortex reactor  10 , there are one or more nozzles  14  for feeding a first gas to vortex reactor  10 . Nozzles  14  may be located about the circumference of vortex reactor  10  and are preferably spaced evenly about the circumference. Preferably, at least four nozzles  14  are employed. The first gas is introduced to reaction chamber  12  via nozzles  14  which are oriented tangential relative to wall  13  of reaction chamber  12 . The tangential orientation of nozzles  14  imparts a circumferential velocity component  16  to the first gas as it is introduced to reaction chamber  12 . The set of nozzles  14  for the first gas feeding will be referred to as the first swirl generator. Optionally, a second swirl generator comprising nozzles  15 , shown in  FIG. 2   b  can be installed along the length of the chamber. Multiple swirl generators, i.e. more than two, can be installed for introduction of multiple gases as desired. Preferably all swirl generators rotate gas in the chamber in the same direction. Products leave reaction chamber  12  via outlet  20  located at or near the top of reaction chamber  12 . 
     One embodiment of the present invention employs a flange  30  with a circular opening  32  located substantially at the center of flange  30  to form a reverse vortex flow. Flange  30  is located proximate to the first swirl generator with nozzles  14 . The opening  32  in the flange  30  is preferably circular, but may be other shapes such as pentagonal or octagonal. The size of circular opening  32  is important to determining the flow pattern in reaction chamber  12 . The diameter of opening  32  in flange  30  should be from about 70% up to 95% of the diameter of reaction chamber  12  to form the reverse vortex flow similar to that shown in the  FIG. 2  without a considerable recirculation zone. To form the reverse vortex flow with a considerable recirculation zone  110  ( FIG. 2   b ), the diameter of opening  32  in flange  30  should be from about 10% up to 75% of the diameter of reaction chamber  12 . 
     The reverse vortex flow in reaction chamber  12  causes the reagents to swirl around a region of plasma and flame  80 , shown, for example, in  FIG. 7   a , in reaction chamber  12 . This provides heating of the reagents as they move downwardly around central core region  24 . Also, the reverse vortex flow increases the residence time of reactants inside reaction chamber  12 . Increased residence time helps to complete the second step (II) of the conversion reaction. Large recirculation zone  110  also promotes completion of the conversion process especially by decreasing ignition time (initiation of the first step (I) of the conversion reaction). 
     Reverse vortex flow in this invention means that the vortex flow has axial motion initially from the swirl generator to the “closed” end of reaction chamber  12  (along wall  13  of the chamber), and then the flow turns back and moves along the axis to the “open” end of the chamber, where a swirl generator may be placed. This flow is similar to the flow inside a dust separation cyclone, or inside a natural tornado. This flow has very interesting and useful properties. For example, gas dynamic insulation of the central (axial) zone: walls of the chamber do not “feel” what is going on in the center. It can be cold or extremely hot (flame or plasma) in the center of reaction chamber  12 . Primarily the temperature of incoming gas defines the temperature of wall  13 . For the process pf hydrocarbon conversion it means that the zone of combustion is separated from wall  13 . 
     Without the reverse vortex flow, the reagents would enter reaction chamber  12  through inlet  18  and pass between the electrodes forming the plasma and leave reaction chamber  12  at a relatively high velocity, and, at least in a small reactor, incomplete conversion of the reagents of the conversion reaction would likely occur. The present invention provides an increased residence time in reaction chamber  12 , by causing the reactants to travel a greater distance in the reactor by imparting a circumferential velocity component to the reagents. Residence times can be increased by an order of magnitude using a preferred form of the reverse vortex flow. This helps to ensure complete conversion of the reactants to products of the conversion reaction. 
     In the embodiment of  FIG. 1 , the reagents are premixed and introduced to reaction chamber  12  via nozzles  14 . This creates a full volume of flame in reaction chamber  12  causing reactor wall  13  to become very hot, indicating a significant energy loss to the environment from reactor  10 . As a result of this condition, care must be taken to provide safe conditions for ignition of the flame and to prevent combustion of the reagents prior to their entry into reaction chamber  12 . These factors indicate that the embodiment of  FIG. 1 , wherein the reagents are premixed and fed to reaction chamber  12  via nozzles  14 , is a less preferred embodiment of the invention. Typical inlet velocities for feeding gas into reaction chamber  12  via nozzles  14  is from about 10 m/s to about 50 m/s. 
     In order to reduce heat loss to the environment and minimize the risk of unwanted combustion outside the reactor, two separate gases or gaseous mixtures that both are non-flammable and that form together a flammable gas mixture, can be fed to reaction chamber  12  via different inlets as depicted in  FIG. 2 . In the present invention, non-flammable means non-combustible under the conditions existing at the specified location (in this embodiment, outside the reactor). In this embodiment, a second gas is fed from the bottom of reaction chamber  12  via gas inlet  18  co-directionally with an upward axial flow component of the first gas in reaction chamber  12  accelerating this axial flow component. In this manner, the present invention ensures a sufficiently high axial velocity in reaction chamber  12  to move a gliding arc axially upwardly for plasma creation. The reverse vortex flow also helps to mix the first and second gases in the reaction chamber  12 . 
     In order to minimize the risk of unwanted combustion outside the reactor, two separate gases or gaseous mixtures that both are non-flammable and that form together flammable gas mixture, can also be fed to the reaction chamber via different swirl generators (made of nozzles  14  and nozzles  15  as depicted in  FIG. 2   b ). 
     A preferred ratio of the tangential flow velocity to the axial flow velocity is about 4.0. This ratio of flow velocities causes the reverse vortex flow to follow approximately a 15 degree slope in reaction chamber  12 . Preferably, in this embodiment, the hydrocarbon-rich feed gas is introduced to reaction chamber  12  via nozzles  14  and an oxygen-rich gas is introduced to reaction chamber  12  through inlet  18 . In this manner, the flame in reaction chamber  12  can be maintained at a distance from wall  13  of reactor  10 , thereby keeping the wall of reactor  10  relatively cool. This is achieved as a result of the downward flow of the hydrocarbon-rich gas from nozzles  14  along wall  13  of reaction chamber  12 , which provides insulation between the plasma and flame and reactor wall  13 . In this manner, heat loss to the environment can be reduced thereby further improving the efficiency of reactor  10 . However, it is also possible to achieve acceptable results by feeding the hydrocarbon-rich feed gas to reaction chamber  12  via inlet  18  and the oxygen-rich gas via nozzles  14 . 
     Referring to  FIG. 3 , there is shown another embodiment of reactor  10  of the present invention which further includes a third inlet  26  at the top of reaction chamber  12  for introduction of a third gas to reaction chamber  12 . The third gas may be employed, as necessary, to assist the flame in t reaction chamber  12 . Preferably, the third gas is oxygen-rich gas. 
     In another embodiment of the invention shown in  FIG. 4 , a heat exchanger  40  is employed to preheat the at least one feed gas for reactor  10 . Preferably, when employing two or more inlets to feed gas to reactor  10 , at least two of the feed gases are preheated in heat exchanger  40 . More preferably, both the hydrocarbon-rich gas fed via nozzles  14  and the oxygen-rich gas fed via inlet  18  are preheated in heat exchanger  40 . Also, it is preferred to preheat the feed gases by counter-current heat exchange with the product stream from reactor  10  as shown in  FIG. 4 . This reduces the amount of energy input to the system for preheating the feed gases, and cools the product stream, which is also desirable in the process of the invention. 
       FIG. 4  shows reactor  10 , provided with a wall  13 , nozzles  14 , inlet  18  and a product outlet  20 . Product stream  50  is fed from product outlet  20  to inlet  42  at a first end of heat exchanger  40 , through heat exchanger  40  to product outlet  43  of heat exchanger  40 . Product stream  50  leaves heat exchanger  40  as a hydrogen-rich cooled gas. At least one feed gas is fed to inlets  44 ,  46  located at a second end of the heat exchanger  40  for counter-current heat exchange with product stream  50 . In the embodiment of  FIG. 4 , first feed gas stream  52  is fed to inlet  44  of heat exchanger  40  and leaves heat exchanger  40  via first gas outlet  45 , whereupon first feed gas stream  52  is fed to nozzles  14  of reactor  10 . Second feed gas stream  54  is fed to inlet  46  of heat exchanger  40 , and leaves heat exchanger  40  via second gas outlet  47 , whereupon second feed gas stream  54  is fed to inlet  18  of reactor  10 . 
     In order to increase the heat exchange capacity of heat exchanger  40 , heat exchanger  40  may be filled with a heat conducting material, such as nickel pellets  48 . Other suitable heat conducting materials may be employed, though it is preferable to use nickel-based metals as the heat conducting material. In a more preferred embodiment, heat exchanger  40  is partially filled with a heat conducting material, such as nickel pellets  48 , as shown in  FIG. 5 . The remaining, unfilled portion  49  of heat exchanger  40  may be left as empty space. In a preferred embodiment, about half of the volume of heat exchanger  40  is filled with heat-conducting material. This serves to increase the residence time of intermediate products of product stream  50  in heat exchanger  40  to thereby improve conversion of the intermediate products to the final products via step (II) of the reaction given above. In this manner, significant conversion of intermediate products to final products can be realized in heat exchanger  40 . 
     In another embodiment of the invention shown in  FIG. 4   b , two or more heat exchangers,  40   a  and  40   b,  are employed to preheat the feed gases separately to desirable temperatures. Preferably, when employing one or more inlets to feed pure hydrocarbon gas to reactor  10 , this pure hydrocarbon gas should not be preheated to the temperature higher than the decomposition temperature (gaseous hydrocarbons decompose under the high temperature conditions to soot and hydrogen, for example for methane this decomposition start temperature is about 450.degree. C.). It is preferred to preheat the feed gases by counter-current heat exchange with the product stream from reactor  10 , and also to preheat oxygen-rich gas to higher temperature as shown in  FIG. 4   b . 
     In  FIG. 4   b , reactor  10  is provided with a wall  13 , nozzles  14 , inlet  18  and a product outlet  20 . Product stream  50  is fed from product outlet  20  to inlet  42   a  at a first end of heat exchanger  40   a,  through heat exchanger  40   a  to product outlet  43   a  of heat exchanger  40   a . Product stream  50  then enters inlet  42   b  at a first end of heat exchanger  40   b,  passes through heat exchanger  40   b  to product outlet  43   b  of heat exchanger  40   b.  Product stream  50  leaves heat exchanger  40   b  as a hydrogen-rich, cooled gas. At least one feed gas is fed to inlets  44 ,  46  located at the second ends of heat exchangers  40   b  and  40   a,  respectively, for counter-current heat exchange with product stream  50 . In the embodiment of  FIG. 4   b , first feed gas stream  52  is fed to inlet  44  of heat exchanger  40   b  and leaves heat exchanger  40   b  via first gas outlet  45 , whereupon first feed gas stream  52  is fed to nozzles  14  of reactor  10 . Second feed gas stream  54  is fed to inlet  46  of heat exchanger  40   a,  and leaves heat exchanger  40   a  via second gas outlet  47 , whereupon second feed gas stream  54  is fed to inlet  18  of reactor  10 . 
     If it is necessary to preheat the hydrocarbon-rich feed gas to the temperature higher than decomposition temperature, it is necessary to dilute the hydrocarbon gas with oxygen-rich gas, but this dilution should not result in formation of flammable mixture in feed gas stream. 
     The reactor of the present invention employs a plasma-assisted flame (PAF) in reaction chamber  12 . The PAF is produced by preheating reaction chamber  12  and the heat exchanger(s) with an inert gas such as nitrogen, or with a lean (leaner than the mixture of reagents for conversion) combustion mixture, and replacing the preheating gas with the feed gases which provide the reagents for the reactions (I) and (II). As the reagents mix in reaction chamber  12 , a flammable state is produced thereby resulting in the appearance of a flame in reaction chamber  12 . Finally, the oxygen concentration in reaction chamber  12  is reduced to a low level, which is at least sufficient to maintain a stable flame and to avoid soot formation. The oxygen concentration in reaction chamber  12  can alternatively be maintained at a level which provides a stoichiometric amount of oxygen for the reactions (I) and (II), as long as the flame is stable at this concentration. Thus, in a preferred embodiment, the number of oxygen atoms [O] in the sum of all feed gases that come to reaction chamber  12  is at least as large as the number of carbon atoms [C] in the same sum of all feed gases coming to reaction chamber  12 , as long as the flame is stable at this oxygen-rich gas feed. If the flame is stable using a stoichiometric concentration of oxygen ([O]/[C]=1), part of oxygen atoms can be fed to the reactor in the form of water vapor to produce more hydrogen via the reaction: 
       CH 4 +H 2 O→CO+3H 2  
 
     Using sliding arc plasma, as in the present invention, the soot-less flame can be maintained even with combinations of reactants which would normally be outside the limit of flammability or which can burn only with soot production, hence the term “plasma assisted flame” (PAF) appears. The PAF provides fast conversion of reagents to intermediate products, while keeping the energy input to the reactor at an efficient level (preferably less than 2% of the total chemical energy of the hydrocarbon gas), since the PAF consumes less electrical energy than sliding arc plasma alone. The PAF also permits the use of lower concentrations of oxygen in reaction chamber  12  to maintain the soot-less flame. This is desirable since lower oxygen concentrations tend to result in greater hydrogen production by minimizing the amount of water generated in reaction chamber  12  by reaction of oxygen with light hydrocarbons. 
     In one embodiment of the invention, the present invention utilizes a constant distance between electrodes to maintain a stable sliding arc in order to avoid the production of pulsed plasma, wherein the properties of the plasma constantly change with time. By maintaining the sliding arc with a constant distance between the electrodes, stable plasma is obtained and the properties of the plasma do not change significantly with time. 
     The stable sliding arc can be obtained, for example, using electrodes as shown in  FIG. 6 . In  FIG. 6 , a first electrode is provided in reaction chamber  12  in the form of a circular ring electrode  60 , supported by supporting wires  62  and connected to a power supply  64  via an electrical connection  66 . A second electrode  70  is preferably located in an upper portion of reaction chamber  12 . 
     Circular ring electrode  60  is mounted, via supporting wires  62  on a movable mount  68  for substantially vertical movement in reaction chamber  12 . Movable mount  68  is actuatable from outside reactor  10  to permit adjustment of the distance between circular ring electrode  60  and second electrode  70 . This arrangement permits circular ring electrode  60  to be positioned a first, minimum distance  69  from second electrode  70  for ignition of the sliding arc. Once the sliding arc is ignited, circular ring electrode  60  is moved vertically downwardly using movable mount  68  to position circular ring electrode  60  at a second, greater distance from second electrode  70 , as shown in  FIG. 7 . In this manner, a short distance between circular ring electrode  60  and second electrode  70  can be provided for ignition, and a longer distance between circular ring electrode  60  and second electrode  70  can be provided for operation of reactor  10 . The ability to adjust the distance between the electrodes also allows the optimization of the sliding arc plasma generation in t reaction chamber  12  by selection of the optimal distance between the electrodes for reactor operation. 
     Power consumption per unit length of the sliding arc for a fixed current is constant, and electrode spot energy is constant. Thus, by increasing the distance between circular ring electrode  60  and second electrode  70 , the power consumption in reaction chamber  12  can be substantially increased without increasing the current strength provided to the reactor. As a result, the sliding arc can be operated without overheating, melting, evaporation and droplet erosion of the electrode surface at the arc point. This provides a significantly improved life expectancy for the electrodes. 
     Circular ring electrode  60 , which forms the first electrode, can be interchanged with electrodes having other geometries. A circular geometry, for example, is desirable for a cylindrical reaction chamber  12 , such as that illustrated in the drawings since this geometry will maintain the sliding arc at a relatively constant distance from wall  13  of reactor  10 . Thus, for a cylindrical reaction chamber  12 , circular ring electrode  60  can be interchanged with, for example a flat circular disc, not shown. Second electrode  70  can also be in the form of a circular ring electrode or flat circular disc. In a more preferred embodiment, second electrode  70  also acts as a flow restrictor and thus may take the place of flange  30 , discussed above. 
     Referring to  FIG. 7 , there is shown reactor  10  of  FIG. 6  with circular ring electrode  60  in position to maintain a stable sliding arc for plasma generation. As shown in  FIG. 7 , the combination of the gas flows, electrode geometry and reagent mixture provide a PAF  80 . Reagents flow around PAF  80  in a reverse vortex flow pattern  82 , as shown. The stable sliding arc can be obtained, for example, using electrodes as shown in  FIG. 7   b . In  FIG. 7   b , a first electrode is provided in reaction chamber  12  in the form of a circular ring electrode  60 , connected to a power supply  64 . Second electrode  70  may be in the form of a circular ring electrode or flat circular disc. In a more preferred embodiment, second electrode  70  also acts as a flow restrictor and thus may take the place of flange  30 . Also shown in  FIG. 7   b  are swirl generators comprised of nozzles  15  and  14 . 
     The distance between the circular ring electrode and grounded cylindrical wall of the reactor is small enough to ensure electrical breakdown in cold gas. Once the breakdown takes place, the sliding arc is elongated and rotated by the gas flow and reaches the constant length, which is the largest possible length. 
     In another embodiment shown in  FIG. 8 , the present invention employs a spiral electrode  90  as the cathode for providing the sliding arc. The anode may again be a flat disc  70  or circular ring as in the previous embodiments. Spiral electrode  90  may be anchored to the reactor  10  at one end thereof by any suitable attachment mechanism  92 , such as a screw. Preferably spiral electrode  90  is of sufficient structural rigidity to support itself within reaction chamber  12 , as shown. Spiral electrode  90  produces an arc, which slides from free end  94  of spiral electrode  92  toward anchored end  93  of spiral electrode  90 . 
     The movement of the sliding arc is the result of reverse vortex flow  82  in reaction chamber  12 . Since the sliding arc moves around, the arc spot on the surface of spiral electrode  90  continuously moves to a new location, thus protecting the electrode material from excessive wear in a single location. This helps provide a longer life for spiral electrode  90 , and to prevent overheating, melting, evaporation and/or droplet erosion of the electrode surface at the arc point. Since the length of the sliding arc elongates by the reverse vortex flow, the arc reaches the maximal possible length, extinguishes and starts again once reactor  10  is running. Moreover, reverse vortex flow  82  of reagents in reaction chamber  12  helps provide easy breakdown conditions for the sliding arc in reactor  10 . 
     The shape of spiral electrode  90  can be optimized based on the flow conditions within reaction chamber  12 , and the type of power supply employed. For example, experimental flow visualization, numerical modeling and/or computerized flow simulation can be employed to help design the optimal shape for spiral electrode  90 . For the preferred shape for spiral electrode  90  the diameter of each successive spiral decreases relative to the previous spiral, as the distance from anode  70  increases. Also, it may be preferable for the bottom of the spiral to form a circular ring to provide a similar geometry to that shown below in  FIG. 9 . 
     When a high potential, e.g. 3 kV/mm is applied across the electrodes, electrical breakdown ignites the gliding arc. The strong reverse vortex flow  82  in reaction chamber  12  forces the gliding arc to move around the longitudinal axis  100  of the reactor  10 . The arc thus elongates itself along spiral electrode  90  until it eventually reaches the end of spiral electrode  90  furthest away from anode  70 . Since the gliding arc is maintained in a central zone of t reaction chamber  12  by spiral electrode  90  as shown in  FIG. 8 , it is subjected to significantly less flow disturbances than it would be subjected to if the gliding arc extended closer to wall  13  of reactor  10 . Also, the area of the gliding arc is subjected to intensive convective cooling as a result of reverse vortex flow  82  and the gliding arc is thermally insulated from wall  13  of reactor  10  by this same reverse vortex flow  82 . These factors allow the provision of high plasma density, high power and high operating pressures, high electron temperatures, and relatively low gas temperatures. This combination of properties allows the selective stimulation of certain chemical processes within reactor  10 , if desired. 
     In another embodiment of the present invention, shown in  FIG. 9 , a combination of a spiral electrode  90  and a circular ring electrode  60  is employed. This embodiment combines the advantage of having the arc between circular ring electrode  60  and anode  70  during normal operation of reactor  10  with the ability to reignite the sliding arc without moving circular ring electrode  60 , if, for any reason, the arc should extinguish itself. Thus, in operation, the sliding arc is ignited at free end  94  of spiral electrode  90  and moves down spiral electrode  90  as described above. Once the sliding arc reaches circular ring electrode  60 , it is maintained between circular ring electrode  90  and anode  70 . Should the arc be extinguished, it will immediately reignite at free end  94  of spiral electrode  90  and the process will repeat itself. This arrangement adds additional stability to the plasma generation by minimizing the time that the arc is extinguished. 
     The arrangement shown in  FIG. 9  is for the case of DC or two-phase AC power. For three-phase AC power, multiple arrangements of electrodes as shown in  FIG. 9 , can be employed. 
     In yet another embodiment, shown in  FIG. 10 , a circular ring electrode  60  forms part of the bottom end of reactor  10 . 
     In yet another embodiment (not shown), spiral electrode  90  forms part of cylindrical wall  13  of reactor  10 . 
     It is to be understood that various features of the different embodiments shown in the drawings may be combined with one another in a vortex reactor in accordance with the present invention. For example, the various embodiments of heat exchanger  40  can be employed in any of the embodiments of the vortex reactor shown in the figures. 
     In a second aspect, the present invention relates to a method for the conversion of light hydrocarbons to hydrogen-rich gas in a vortex reactor. The method includes the steps of introducing at least one light hydrocarbon and oxygen into a reaction chamber, subjecting at least the light hydrocarbon feed gas to a reverse vortex flow, and converting said light hydrocarbons to hydrogen-rich gas with a plasma assisted flame (PAF). 
     In the method, the axial gas flow may be created by the steps of feeding gas in an axial direction into said reaction chamber and, optionally, accelerating said axial gas flow through a flow restriction. The circumferential gas flow may be created by the step of feeding gas into said reaction chamber in a direction tangential to a sidewall of said reaction chamber. In order to assist in the maintenance of the PAF, a third, oxygen-rich gas stream can optionally be introduced at the top of the reaction chamber. 
     The method includes generating plasma in said reaction chamber. Plasma generation may include the step of providing a sliding electrical arc in said reaction chamber, as discussed above. 
     The methods of the present invention may employ any of the reactors shown in the figures. In addition, each method of the present invention may optionally include the step of preheating one or more feed gases by counter-current heat exchange with the product stream from the vortex reactor. 
     If a vortex reactor with a movable electrode is employed, the method may further include the step of moving the electrode from a first, ignition position, to a second, operation position after ignition of the sliding arc in the reactor. In this method, operating conditions can be optimized, for example, by varying the distance between the movable electrode and the fixed electrode. 
     It is to be understood that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.