Abstract:
Design advances for improving the performance of a plasma torch. The use of one or more of various advances described herein can improve the efficiency and effectiveness of the torch, the reactor and the manufacturing process. The use of the torch with hydrogen plasma gas, natural gas feedstock, and carbon black production are also described.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 61/934,184 filed Jan. 31, 2014, the disclosures of which is expressly incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The field of art to which this invention generally pertains is methods and apparatus for making use of electrical energy to affect chemical changes. 
       BACKGROUND 
       [0003]    There are many processes that can be used and have been used over the years to produce carbon black. The energy sources used to produce such carbon blacks over the years have, in large part, been closely connected to the raw materials used to convert hydrocarbon containing materials into carbon black. Residual refinery oils and natural gas have long been a resource for the production of carbon black. Energy sources have evolved over time in chemical processes such as carbon black production from simple flame, to oil furnace, to plasma, to name a few. As in all manufacturing, there is a constant search for more efficient and effective ways to produce such products. Varying flow rates and other conditions of energy sources, varying flow rates and other conditions of raw materials, increasing speed of production, increasing yields, reducing manufacturing equipment wear characteristics, etc. have all been, and continue to be, part of this search over the years. 
         [0004]    The systems described herein meet the challenges described above, and additionally attain more efficient and effective manufacturing process. 
       BRIEF SUMMARY 
       [0005]    A plasma torch is described including at least two cylindrical, graphite electrodes nested inside one another and coaxially aligned; the plasma torch described above where of claim  1  wherein the inner electrode is hollow; the plasma torch described above where the inner electrode is a solid cylinder; the plasma torch described above useable with plasma gas which is at least about 60% H 2  by volume; the plasma torch described above useable with plasma gas containing at least one of CO, C 2 H 2 , HCN, CH 4 , C 2 H 6 , N 2 , polycyclic aromatic hydrocarbons, monoaromatic hydrocarbons, and/or Ar gas; present in at least 10 ppm (parts per million); the plasma torch described above where the gap between the concentric electrodes is not less than about 4 mm and not more than about 20 mm; the plasma torch described above containing a tip wherein the gap distance, electrode thickness, and/or surface area of the tip remains substantially constant during wear; the plasma torch described above additionally containing at least one annulus between electrodes particularly adapted for the flow of plasma gas; the plasma torch described above additionally containing an upper annulus and a lower annulus between electrodes wherein the upper annulus is wider than the lower annulus; the plasma torch described above additionally containing a power supply capable of supplying an about 300 to about 1500V operating voltage and an open circuit voltage up to about 4500V; the plasma torch described above where at least one electrode has a tip and the torch additionally contains a magnetic field generating component capable of providing a magnetic field at the tip of the electrode with an axial component of between about 10 and about 100 mT; the plasma torch described above containing an upper cathode and a lower cathode and an upper anode and a lower anode, wherein the upper cathode is connected to the lower cathode to make one electrically conductive electrode and the upper anode is connected to the lower anode to make one electrically conductive electrode and each of these connections are made at an electrically conductive electrode junction; the plasma torch described above where conical threads are used to connect upper and lower electrodes; the plasma torch described above containing an annulus between electrodes wherein the lower electrode has a more narrow annulus than the upper electrode; the plasma torch described above where the lower electrodes are considered consumable; the plasma torch described above where multiple consumable electrodes are attached to an upper electrode; the plasma torch described above containing ring thicknesses where the ring thickness of the lower electrodes is within 10% of each other; the plasma torch described above including an electrode tip area where the surface area of the electrode tip is greater than 2:3 but less than 4:1 when the surface area of the outer electrode is compared to the surface area of the inner electrode; the plasma torch described above where at least one of the electrodes has a substantially barrel stave design; the plasma torch described above where at least 5 staves are used to create a hollow concentric ring; the plasma torch described above containing axial grooves cut into the electrodes to provide relief of thermal stress and/or provide controlled thermal cracking; the plasma torch described above including an electrode tip area wherein the cylindrical electrodes comprise cylindrical rods capable of being held at the same electric potential that approximate a hollow cylinder at the tip; the plasma torch described above where the inner electrode comprises a shower head design; the plasma torch described above including an annulus for the flow of shield gas; the plasma torch described above including at least one channel for the flow of plasma gas through one or more than one of the following: an annulus, a shield gas channel, a shower head in a central electrode, through the body of hollow concentric electrodes, and/or through the center of hollow concentric electrodes; the plasma torch described above including at least one magnet to generate and tailor a magnetic field; and the plasma torch described above including a conductive mechanical connector connecting the anode to the cathode and providing a conductive path for initiation of arc. 
         [0006]    Additional embodiments include: a plasma reactor containing a plasma chamber where the walls of the reactor include gas flow channels that can transport heat away from the plasma chamber; the plasma reactor described above where the channels are designed so as to allow at least some of the heated gas to be redirected as plasma gas; the plasma reactor described above including the torch described where the walls of the reactor narrow after the torch section to form a throat section and then the walls diverge after the throat section; the plasma reactor described above including hydrocarbon feedstock injectors in the throat section; the plasma reactor described above including hydrocarbon feedstock injectors within  5  diameters of the throat in either the upstream or downstream direction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIGS. 1 ,  2  and  3  show schematic representations of typical plasma torches as described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    The particulars shown herein are by way of example and for purposes of illustrative discussion of the various embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
         [0009]    The present invention will now be described by reference to more detailed embodiments. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
         [0010]    Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for describing particular embodiments only and is not intended to be limiting of the invention. As used in the description of the invention and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. All publications, patent applications, patents, and other references mentioned herein are expressly incorporated by reference in their entirety. 
         [0011]    Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches. 
         [0012]    Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Every numerical range given throughout this specification will include every narrower numerical range that falls within such broader numerical range, as if such narrower numerical ranges were all expressly written herein. 
         [0013]    Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
         [0014]      FIGS. 1 ,  2  and  3  are schematic  2 -dimensional representations of variants of typical torches described herein. For example,  FIG. 1  shows plasma gas flowing through plasma gas inlets ( 103 ) between an inner electrode ( 108 ), typically the cathode and the outer electrode ( 107 ), typically the anode. Each electrode is comprised of an upper electrode section ( 104 ) and lower electrode section ( 106 ) that are connected at the electrode junction ( 105 ). The electrode junction can use conical threads to ensure that weight distribution of the lower electrode is even across the junction and that thermally induced stress fractures are minimized. Other methods of joining the upper and lower electrodes could also be employed including hook and latch, tongue and groove, mechanical bolts, as non-limiting examples. The annulus is the space between the concentric electrodes where some but not necessarily all of the plasma gas is passed before reaching the plasma region ( 111 ). The width of the annulus is described as the average distance between the anode and cathode and the gap or the tip distance is described as the closest distance at the tip of the cathode to the tip of the anode. The electrode holders ( 101 ) are connected to the upper electrodes at the holder/electrode junction ( 102 ). The electrode holders enable the electrical isolation of both the anode and cathode and additionally supply the electrical connections to the anode and cathode. The connection of the power supply to either or both the anode and cathode can be accomplished through other means, but is convenient when accomplished with the electrode holders which serve multiple functions. The use of various materials at the electrode holders, including Teflon™ polymer and various ceramics amongst others, are included to provide adequate thermal and electrical isolation. These materials additionally allow for pre-heated plasma gas to flow in close proximity to the electrode holders which can optionally be water cooled. The plasma gas is transported through the arc ( 110 ) and serves as heat dissipation into the plasma zone or region ( 111 ) which ideally is the hottest part of the reactor. The wider upper annulus in  FIG. 1  (in the upper electrode section above lower annulus  109 ) allows for reduced probability of arcing between the electrodes in this area of the torch. Additionally, the lower electrode is designed to be consumable in nature and allow for facile, rapid, inexpensive replacement without the need to replace the upper electrodes and the electrode holders. 
         [0015]      FIG. 2  shows several different possible flow paths for the plasma gas. The plasma gas can flow around the outer electrode to act as a shield gas ( 204 ). This will protect the electrodes and provide for longer service life and maximize the utility of the heat load supplied by the torch. The electrodes can also have a shower head design ( 202 ) wherein the inner electrode is not a hollow ring, but rather a solid electrode with hollow shafts that allow for heat dissipation ( 203 ) at the inner electrode and maximization of the utility of the heat load of the torch. Additionally, the plasma gas can flow through the annulus ( 201 ) or it can flow through the walls of the hollow concentric inner and outer electrodes depicted in  FIG. 1  for example through tubular shafts drilled axially through the electrodes. 
         [0016]      FIG. 3  depicts the torch and the downstream reaction zone. The plasma gas ( 301 ) flows downstream of the plasma zone through the inner electrode ( 302 ) and the outer electrode ( 303 ) into a converging or narrowing region ( 304 ) and into the throat ( 305 ) and then out of the throat into a diverging reactor ( 306 ). This creates a large amount of turbulence and this configuration will provide for optimal mixing with hydrocarbon feedstock. The figure also shows recycle plasma gas ( 307 ) flowing around the lower and mid portion of the plasma zone to serve two functions (1)—cool the walls of the plasma zone and (2)—pre-heat the plasma gas prior to entering the plasma chamber to more effectively use the heat load of the plasma torch and prolong the service life of the plasma chamber. 
         [0017]    While all of these figures show the torch/reactor in a vertical state with downward flow, it is also possible to have an upward flow, or a horizontal reactor. For the particular torch/reactor designs shown, vertical reactor orientation with downward flow is preferred. 
         [0018]    Carbon black has been made from a variety of processes over the years, however, commercial development of a plasma based process has never been successful. In the past, plasma generator designs for the production of carbon black have not possessed adequate heating rates, resistance to corrosion, economical plasma gas, rapid mixing, and sufficient economics of manufacture to survive when pitted against the incumbent furnace process. The plasma torch described herein enable continuous operation and production of high quality carbon black from the plasma process where others have failed. 
         [0019]    Plasma jets for various industrial processes are normally produced by plasma generators comprising a discharge chamber and mutually insulated electrodes. An electric arc discharge is initiated in the discharge chamber between the electrodes, in the flow of a medium. The medium, typically a gas, is heated in the discharge to the plasma state and flows out of the generator in the form of a plasma jet. 
         [0020]    Of all plasma generator components, electrodes, or rather their surfaces exposed to the electric arc, “arc-spots”, are exposed to the most extreme thermal flux. The thermal flux in these areas can exceed 10 5  W/cm 2  (Watts per square centimeter) and this environment will melt or erode all known metals. Cooling of the plasma components is typically achieved via jacketed cooling techniques with thermal exchange agents. 
         [0021]    In the plasma reactor described herein, the power supply, the control of arc placement, the distance between the electrodes, the gas flow rate amongst other factors are controlled with high precision to ensure performance. The power supply is connected to the electrodes and provides for very high open circuit voltage to deal with high voltage spikes. The power supply can be capable of supplying 500-1500 V (volts) or greater typical operating voltage. The power supply has an open circuit voltage that can be 1.5 to 3.0 times the operating voltage. These voltage ranges have been found to be optimal for the manufacture of carbon black at specific plasma gas flow rates in combination with hydrocarbon feedstock flow rates, a plasma gas comprised of greater than 60% hydrogen, and the gap distance of between 4 and 20 mm (millimeters). 
         [0022]    For the given gap distance, voltage, plasma gas flow rates, and surface area of electrode tips, an optimal power density of the electrode tip is from 0.1 to 2 kW/cm 2  (kilowatts per square centimeter). Below this range power output would be too low for efficient carbon black production and above this range the torch would rapidly decompose resulting in inefficient carbon black production due to electrode wear. 
         [0023]    The plasma gas is the gas that has passed through the plasma torch region and may have sufficiently interacted to be deemed in the plasma state. Plasma gas as used herein can mean the excited gas and can also mean any gas passing through the plasma torch area that could have been induced into the plasma state, but for whatever reason has not been induced. 
         [0024]    The components of the plasma gas for the highly efficient plasma reactors described herein are comprised of at least about 60% hydrogen up to about 100% hydrogen and can further comprise up to about 30% nitrogen, up to about 30% CO, up to about 30% CH 4 , up to about 10% HCN, up to about 30% C 2 H 2 , and up to about 30% Ar. Additionally, the plasma gas can also be comprised of polycyclic aromatic hydrocarbons such as anthracene, naphthalene, coronene, pyrene, chrysene, fluorene, and the like. In addition, the plasma gas can have benzene and toluene or similar monoaromatic hydrocarbon components present. A more typical composition can comprise 90% or greater hydrogen, and 0.2% nitrogen, 1.0% CO, 1.1% CH 4 , 0.1% HCN, 0.1% C 2 H 2 , or thereabouts. The plasma gas can also comprise about  80 % hydrogen and the remainder can comprise some mixture of the afore mentioned gases, polycyclic aromatic hydrocarbons, monoaromatic hydrocarbons, and other components. 
         [0025]    The material of construction for the electrodes in this invention should have high thermal conductivity over 100 W/m-K (watts per meter kelvin) and electrical resistivity less than 10e-2 ohm-m (meter). Materials that fit this description include graphite and silicon carbide, although graphite is preferable. The material should be resistant to chemical erosion in highly reactive hydrogen free radical atmosphere. 
         [0026]    The use of conical threads in joining electrodes reduces stress concentration and potential of electrode cracking can be employed. Preferable tapers comprise those with an about 1 in 3 angle can be used, although tapers down from about 1 in 2 to about 1 in 20 can also be used. To prevent the unscrewing of electrodes through vibration, a hole can be drilled through the threaded section and a pin can be inserted. 
         [0027]    The ideal gap distance between concentric electrodes is about  4  millimeters to about 20 millimeters (mm) based on desired operating voltage, current, and electrode wear. Gap size may change operating voltage anywhere from about 500V up to about 1200V. A gap of about 8 millimeters to about 14 millimeters is the preferred gap size that provides for optimal arcing in this voltage range with minimal electrode wear, optimal thermal transfer, minimal undesired arcing, and minimal blow-out due to arc “lift-off” of the arc (loss of arc). 
         [0028]    Additionally, the electrode length can be controlled in order to control heat distribution in the electrode. An increase in electrode length to reduce losses in water cooled holders. For instance, a preferable range for length for a 750 kW torch is between about 600 mm and about 1500 mm, wherein the 1500 mm electrode length will provide for the most gradual thermal distribution. It can be readily realized by those skilled in the art that the increased length will not only distribute heat, but also provide for more surface to allow radiative heat loss and convective heat loss from electrode to gas flowing in or around the annulus. This will, of course, be balanced against weight load requirements to gain optimal advantage of both heat management and electrode integrity (reduction of cracks, etc.) 
         [0029]    Additional methods to control or optimize weight load vs. thermal stress is to make the concentric rings out of cylindrical electrodes with touching tips that allow for the passage of electricity through concentric tubes that still allow for an annulus between the anode and cathode. The electrodes in this type of embodiment could also be rectangular in nature with groove and tongue type connections to allow for electrical conductivity and weight load support. 
         [0030]    To address thermal stress cracking in very large diameter hollow cylindrical electrodes, use of a barrel stave design where the sections are held together with features commonly employed in such a design which allows different sections to flex based on thermal gradient, or the use of a solid piece of material that has axial slots cut into it to relieve thermal stress. The axial slots can be referred to as the barrel stave design. In the barrel stave design, at least 5 staves or sections would be required to create the concentric ring or barrel. 
         [0031]    Another alternative would be the use of distinct pieces to simulate a cylinder, for example a ring of solid rods. This configuration also possesses advantages in material availability and ease of replacement. 
         [0032]    Elongating the life of the electrodes is largely dependent on the ability to minimize the thermal effect of the electric arc on the electrodes, as well as adequate protection of the electrode surface against the erosive medium. This can partially be achieved by applying an electromagnetic field to reduce the effects of the arc spots by moving the arc spots rapidly over the electrode surface, whereby the mean thermal flux is reduced in density to the areas of contact between the electrodes and electric arc. The magnetic field is provided for through the use of an annular magnetic coil located outside of the electrodes. The field can also be provided for by the use of a permanent magnet as long as the field is oriented in such a way that the arc is rotated around the central axis of the torch which facilitates the rotation of the arc around said axis. 
         [0033]    Additionally, the magnetic field will push the plasma outside of the confines of the immediate space between the two electrodes. This means that the erosive medium (superheated H 2  and hydrogen radical) will be largely separated from the electrode itself. In one embodiment, the method includes the use of a rotating arc discharge created through the application of a magnetic field to the electrodes, as measured at the annulus at the tip of the torch, of about 20 millitesla (mT) to about 100 millitesla, measured in the axial direction, hereafter referred to as axial component. A value of about 30 millitesla to about 50 millitesla can typically be used. The typical radial component of the magnetic field can be between about 3 and about 15 mT. 
         [0034]    One or more magnetic coils can be employed to tailor the shape of the field used to manipulate the behavior of the arc. For example, a coil design may produce a diverging magnetic field with a 6 mT radial component and a 40 mT axial component at the tip of the torch. It is not possible to change one of these without the other using one coil, however, through the utilization of several coils the arc can be tailored to a specific radial and axial component. This may further reduce the capital cost of the magnetic coil and optimize the shape of the magnetic field to maximize the life of the electrode. 
         [0035]    As mentioned previously, the electrodes can be made up of upper and lower portions wherein the lower portion can be made to be replaced in an extremely rapid fashion with low cost graphite parts. The definition of consumable in this context means that more than 1 ton of carbon black but less than 100 tons of carbon black is produced per inch of axially oriented graphite erosion. Additionally, multiple consumable electrodes can be attached to the upper electrode e.g. 3 or 4 or more consumable electrodes that are removed or allowed to be consumed in place during carbon black production runs. This enables limited downtime of the torch and provides a sacrificial electrode that is both cheap and rapidly replaced. 
         [0036]    Use of different gas flow paths to affect the cooling of electrodes and change the flow profile in the arc region are shown, for example, in  FIG. 2 . Gas can pass through the annulus (default flow path), through an inner path ( 201 ) consisting of hole(s) drilled into the center electrode, through the hollow path inside the inner electrode, or through an outer path ( 204 ) around the outer electrode (referred to as shield gas). In  FIG. 2  this outer path is through a ring of holes in the upper electrode, but this flow entrance could also be an annular slot. The inner and outer flow paths help to cool the electrodes and transfer more heat to the gas. They also allow higher gas flow rates that might “blow out” the arc if directed entirely through the annulus. And the outer flow path acts as a shielding gas, helping to confine the plasma region and protect surrounding refractory. A typical flow split could be about 50% through the annulus and about 25% through each of the other two paths. Any combination of flow splits could be used to achieve different operating regimes and optimize for different goals (e.g. reducing wear, increasing operating voltage, etc.). 
         [0037]    Of further interest is the utilization of mechanical means of establishing electrical contact between the electrodes and initiating an arc. This eliminates the need for a high voltage starter and associated equipment and safety risks. This may consist of a moving rod made from an electrically conductive material such as graphite or copper that would touch the electrodes simultaneously, allowing current to flow, before being withdrawn. The starter rod could be a plunger that comes in through the outer electrode and strikes the arc in the annulus or it could be a rotating arm that strikes the arc at the electrode tips. 
         [0038]    As described herein, the reactor is separated into two sections or zones, a plasma zone and a reactor zone, with natural gas or other feedstock injection taking place in the area in-between. The throat is used not only to separate the two regions but to accelerate the plasma gas so that more intense mixing can take place in a smaller region. The throat is therefore defined as the narrowest section between the plasma zone and the reactor zone. The length of the throat can be several meters or as small as about 0.5 to about 2 millimeters. The narrowest point of the throat is defined as the most narrow diameter of the throat +20%. Any cross-section that is within about 10% of the most narrow cross-section is deemed to be within the scope of the throat. 
         [0039]    Preferable injection points into the reactor are about 5 diameters upstream of the throat and about 5 diameters downstream of the throat. One diameter is defined as the diameter of the throat at the most narrow point of the throat. Optionally the injection can occur within about +/−2 diameters or about +/−1 diameter of the throat. 
         [0040]    Acceptable hydrocarbon feedstock includes any chemical with formula C n H x  or C n H x O y . For example simple hydrocarbons such as: methane, ethane, propane, butane, etc. can be used. Aromatic feedstock such as benzene, toluene, methyl naphthalene, pyrolysis fuel oil, coal tar, coal, heavy oil, oil, bio-oil, bio-diesel, other biologically derived hydrocarbons, or the like. Also, unsaturated hydrodcarbon feedstocks can also be used, such as: ethylene, acetylene, butadiene, styrene and the like. Oxygenated hydrocarbons such as; ethanol, methanol, propanol, phenol, and similar are also acceptable feedstocks. These examples are provided as non-limiting examples of acceptable hydrocarbon feedstocks which can further be combined and/or mixed with other acceptable components for manufacture. Hydrocarbon feedstock referred to herein, means that the majority of the feedstock is hydrocarbon in nature. 
         [0041]    Thus, the scope of the invention shall include all modifications and variations that may fall within the scope of the attached claims. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.