Abstract:
A reaction vessel and a method for efficiently producing and collecting nanoparticles in the reaction vessel using cyclonic gas flow. Gas is injected through an inlet tangentially positioned relative to the axis of the vessel to cause the gas to form an outer helical vortex flow and an inner helical vortex flow within the vessel. A reaction synthesizer is operated within the inner helical vortex to produce nanoparticles and larger byproduct particles from precursor materials. The double helical vortex conveys the byproduct particles to an outlet for collection responsive to the cyclonic flow and gravity and an outlet downstream of the inner helical vortex collects the nanoparticles. The vortices formed in the reaction vessel minimize buildup on walls of the reaction vessel and provide a way to rapidly move the synthesized nanoparticles out of the reaction vessel.

Description:
TECHNICAL FIELD 
   The present invention relates in general to reaction vessels and, in particular, to reaction vessels with cyclonic gas flow. 
   BACKGROUND OF THE INVENTION 
   Synthesis of nanopowder can be conducted within a closed vessel using various processes. U.S. Pat. No. 6,777,639 (“the &#39;639 patent”) describes a repetitive pulsed power nanopowder synthesis technique. In this process, a high-magnitude current pulse creates a high-density plasma from a quantity of precursor material that results in the production of nanoparticles held in a gas suspension. U.S. Pat. No. 5,460,701 describes a continuous arc process used to produce gas suspensions of nanopowder. A reaction vessel is needed that aids in the collection of the particles from the gas in which they are suspended. 
   In the above nanoparticle synthesis devices, a continuous flow of gas (a “quench gas”) serves as the cooling medium for the hot plasma from which the nanoparticles are synthesized. For a given nanoparticle production rate, the flow rate of this gas needs to cool the reaction products to a temperature below the rated temperature of components downstream of the reaction vessel such as piping, filters, and blowers. This quench gas is also used to convey suspended nanoparticles from the reaction vessel into a powder collection device such as a filter or electrostatic precipitator. The gas flow rate needs to clear the reaction zone of suspended nanoparticles, because suspended particles lingering in the production zone can serve as nucleation sites for the growth of larger, undesirable particles. It is therefore desirable to design the reaction vessel in such a way as to promote the smooth flow of suspended nanoparticles out of the reaction zone. 
   During operation, the pulsed-power process (as described in the &#39;639 patent) generates a plasma that violently expands and mixes with the gases within the vessel. It is desirable to contain this rapid expansion within a relatively large reaction vessel to minimize contact between the walls of the reaction vessel and the hot expanding reaction products. Using a large reaction vessel prevents the vessel walls from physically interfering with the nanoparticle synthesis process and prevents damage to the reaction vessel walls. However, the large size of the vessel can lead to large zones of relatively slow-moving gas within the reaction vessel, especially if the diameter of the reaction vessel is significantly larger than the gas inlet piping. These stagnant zones allow suspended particles to linger within the reaction vessel, interfering directly with the synthesis process as well as forming opaque clouds that may interfere with optical instrumentation used to monitor the process. To sweep the particles out faster it is often necessary to increase the gas flow rate to well above the rate required to cool the reaction products. This potentially can interfere with the synthesis of the reaction products. Moreover, increasing the quench gas flow rate requires the added expense of using larger gas handling components (piping, blowers, collection filters, heat exchangers, etc.). Thus, it is desirable to employ a reaction vessel geometry that minimizes the gas flow rate required to remove the suspended nanopowder from the reaction vessel. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the foregoing needs by providing a method, apparatus, and system for conducting reactions within a reaction vessel having cyclonic gas flow. One embodiment of the present invention is a method for synthesizing a material, such as nanopowder. For the purposes of the present invention, Applicants&#39; discussion is directed toward nanopowders. The invention is further applicable to other-sized materials that are suspended within the reaction vessel. The method includes creating a cyclonic flow of a gas within a reaction vessel. The cyclonic flow of gas includes an outer helical vortex and an inner helical vortex. The method includes synthesizing a plurality of nano-sized particles and collecting the nano-sized particles from an outlet nozzle positioned downstream of the inner helical vortex gas flow. 
   Another embodiment of the present invention is a reaction chamber for synthesizing nanopowder. The reaction chamber includes a portion such as a cylindrical portion with an inner wall. The reaction chamber includes an inlet through which gas flows in a tangential direction along the inner wall to form an outer helical vortex. The embodiment includes a means for synthesizing nanopowder that generally creates both nano-sized particles and, as a byproduct, larger particles. The embodiment includes an outlet positioned downstream of the inner helical vortex gas flow to collect the nano-sized particles. The embodiment can also include a second outlet for collecting the larger particles created as a byproduct. In another embodiment, the reaction vessel has a conical-shaped portion attached to the cylindrical portion for enhanced collection and separation of particles. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features of the invention will be described hereinafter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, refer to the following descriptions and the accompanying drawings, in which: 
       FIG. 1  is a diagram of a reaction vessel having cyclonic flow and one outlet nozzle in accordance with an embodiment of the present invention; 
       FIG. 2  is a plan view of the reaction vessel from  FIG. 1  having cyclonic flow and one outlet nozzle in accordance with an embodiment of the present invention; 
       FIG. 3  is an elevation view of the reaction vessel from  FIG. 1  having cyclonic flow and one outlet nozzle in accordance with an embodiment of the present invention; 
       FIG. 4  is an elevation view of a reaction vessel having a cone-shaped section and cyclonic flow in accordance with another embodiment of the present invention; 
       FIG. 5  is a cut-away view of a reaction vessel with an axial discharge electro-thermal gun operable in accordance with an embodiment of the present invention; and 
       FIG. 6  is a cut-away view of a reaction vessel with a radial gun operable for synthesizing nanopowder in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   In the following description, numerous specific details are set forth regarding the production of nanopowder and nanoparticles to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced in other contexts than the production of nanoparticles and nanopowder. In other instances, well-known systems have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details concerning controlling gas flow rates, controlling the reaction within the reaction vessel, and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present invention. Further, such details are within the skills of persons of ordinary skill in the relevant art. Also, the present invention may be practiced using means for synthesizing nanopowders other than the means described in this patent. 
   The following terms shall have the definitions given below when used in either lower case or with capitalization in this specification: 
   “Nanopowder” shall mean nanomaterial primarily comprised of nanoparticles that are of a size of 1-500 nanometers (nm). 
   “Precursor Material” shall mean material that is processed to produce nanopowder. 
   “Byproducts” shall mean material removed from a body of precursor material generally having a size of 10 micron (μm) or larger. 
   “Ablative Material” shall mean material removed from a body of precursor material due to the combined effects of heat transfer and mechanical shear caused by plasma velocities. 
   In general, larger reaction vessels require a greater flow rate of gas to sweep out particles for a given time period. Also, increased flow rates require larger blowers, piping, and related equipment. Therefore, to minimize costs, it is desirable to direct the gas through the reaction vessel in such a way as to minimize the volume flow rate of gas required to sweep the reaction products out of the tank. In addition, if the inlet pipe is considerably smaller in diameter than the reaction vessel itself, a jet of gas flowing into the relatively stagnant tank can cause considerable random mixing within the tank that can delay the complete removal of particles from the tank. This mixing effect is therefore undesirable. This mixing effect can be minimized by employing an inlet diffuser, which slows and expands the jet of incoming gas so that it flows uniformly from one end of the reaction vessel to the other. Such a diffuser could consist of a long, gently tapering section of inlet pipe that gradually expands from the diameter of the inlet pipe to the diameter of the reaction vessel. Alternatively, the diffuser could consist of a matrix of baffles or other flow redirecting elements located within the reaction vessel. Either such diffuser is likely to be large and expensive compared to the size and cost of the reaction vessel itself. Therefore, the vessel size and manner of introducing gas into a reaction vessel must be carefully considered when designing for cost efficiency. 
   The present invention provides a method and apparatus that minimizes random mixing and is less expensive than alternatives by taking advantage of the flow physics associated with nanoparticles. Cyclone technology is used in many industries to remove particles from gas suspensions. However, cyclones are inefficient in separating nanoparticles from a gas stream because their size and mass allow them to follow the gas streamlines and remain within the vortices. This aspect is exploited in the current invention. The current invention has a tangential inlet pipe and central outlet pipe, which allows for an inner helical vortex and an outer helical vortex. In one embodiment, the inlet pipe diameter is between about ⅕ and ⅓ the diameter of the reaction vessel and it is located at the upper periphery of the reaction vessel. Further, the inlet pipe is aligned to cause the incoming gas to flow tangentially along the inner wall of the reaction vessel. This creates a flow pattern in which, due to the conservation of angular momentum, flow is divided into an outer, downward-flowing vortex and an inner, upward-flowing vortex between which there is relatively little mixing. Nanoparticles formed in the center of the tank are swept directly out of the tank via the center vortex rather than being dispersed randomly throughout the tank. Nanopowder produced closer to the reactor walls are swept down in the outer vortex and then up the inner vortex. This not only minimizes powder in the reactor, but also minimizes the amount of powder suspended near the tank walls, since the gas flowing along the tank walls consists almost entirely of clean (powder-free) gas arriving from the inlet pipe that remains at a high velocity. This design also minimizes the overall optical opacity of the gas/powder mixture within the tank and minimizes powder buildup on the surface of any viewing windows or external ports. The tangential inlet is also much less expensive to construct and is more compact than either a tapered diffuser or a baffled inlet diffuser. 
   The cyclonic flow within the reaction vessel helps to prevent large (&gt;10 micron) undesirable by-product particles (“slag”) from exiting the reaction vessel through the same outlet as the desirable nanopowder or other reaction products. This is because large particles are subjected to centrifugal particle separation, which is commonly employed throughout many industries. In one embodiment of the present invention, the reaction vessel can be constructed with a conical bottom and a hopper into which these large particles are driven by the cyclonic flow and by gravity. These slag particles can then be removed from the hopper for disposal. 
   Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements may be designated by the same reference numeral through the several views. 
     FIG. 1  is a diagram of a reaction vessel  100  with cyclonic flow  110  in accordance with an embodiment of the present invention. Item  104  represents an inlet nozzle positioned so that gas  114  flows tangentially into a cylindrical portion of the vessel shown as item  108 . The gas  114  flowing tangentially into vessel  108  results in a cyclonic flow. Gas is drawn out of the vessel  100  through outlet nozzle  102 . Outlet nozzle  102  is coupled to opening  116  which is positioned near the center of the cylinder  108  and is positioned to help create the inner vortex  112 . The outlet  102  is shaped to efficiently redirect the flow into the downstream piping. Therefore, as shown (and oriented) in  FIG. 1 , inlet nozzle  104  and outlet opening  116  are positioned to create an outer helical vortex (shown as item  110 ) flowing downward and an inner helical vortex  112  flowing upward. A means for synthesizing nanopowder (such as disclosed in the &#39;693 patent but not shown in  FIG. 1 ) is placed inside the reaction vessel  100  and operated to create nano-sized particles and possibly larger particles as byproducts. The means for synthesizing nanopowder is generally operated within the inner vortex  112 . The nano-sized particles are held in suspension within the gas and rapidly carried out of the vessel by the inner vortex  112 . Any larger particles created as a byproduct to synthesizing nanopowder tend to be flung outward into the outer vortex where they flow downward with the assistance of gravity. The outer helical vortex has an added benefit of sweeping the walls of the cylinder  108  clean and preventing particles from attaching to the cylinder walls. In  FIG. 1 , the inlet nozzle  104  and the outlet nozzle  102  are located in the upper portion of cylinder  108  to promote the formation of the inner helical vortex  112  and the outer helical vortex  110 . Reaction vessel  100  may also contain additional inlet and outlet ports (not shown) for placing precursor material within the cylinder  108 . For clarity, such ports are omitted from  FIGS. 1-4 . 
     FIG. 2  is a plan view of the reaction vessel as shown in  FIG. 1 . The plan view shown in  FIG. 2  shows a gas flowing through inlet nozzle  104  and flowing tangentially along the wall of the cylinder  108 . Item  110  represents an outer helical vortex that, as shown, flows simultaneously clockwise and into the page. Item  112  represent an inner helical vortex that, as shown, flows simultaneously clockwise and out of the page. Items  102 ,  104 ,  108 ,  110 , and  112  from  FIG. 2  correspond to like-numbered items in  FIG. 1 . 
     FIG. 3  represents an elevation view of the reaction vessel as shown in  FIGS. 1-2 . Items  102 ,  104 ,  108 ,  112 ,  114 , and  116  from  FIG. 3  correspond to like-numbered items in  FIGS. 1-2 . Gas flows through inlet nozzle  104  to form an outer helical vortex (not shown) within cylinder  108 . An inner helical vortex  112  is formed and the corresponding gas flows through opening  116  and out nozzle  102 . A means for synthesizing nanopowder (not shown) is operated within the reaction vessel resulting in a suspension of nanoparticles in the inner helical vortex  112  that flows out the reaction vessel through nozzle  102 . Larger particles may be formed as byproducts of synthesizing the nanopowder. The larger particles, with the assistance of gravity, fall and are collected in the bottom of the reaction vessel. 
     FIG. 4  is an elevation view of a reaction vessel  400  with a conical-shaped portion  418  and cyclonic flow in accordance with an embodiment of the present invention. Gas flows through inlet nozzle  404  and flows tangentially through cylinder  408  in a downward-flowing, outer helical vortex (not shown). An upward-flowing, inner helical vortex is formed in the innermost portion of cylinder  408  and is shown as item  412  flowing through opening  420  and out nozzle  402 . Within the reaction vessel  400 , a means for synthesizing nanopowder (not shown) is operated and produces a suspension of nanoparticles that flow out nozzle  402 . Larger particles may be produced as a byproduct of the synthesis of nanopowder. The larger particles may be directed by conical section  418  to flow out of the reaction vessel through outlet nozzle  406  as flow  416 . A reaction vessel  400  is similar to the reaction vessel  100  ( FIG. 1 ) with the addition of a conical shaped portion below the cylindrical shaped portion  108  ( FIG. 1 ).  FIG. 4  represents an elevation view of reaction vessel  400 , and a plan view of reaction vessel  400  is omitted because it would be essentially the same as  FIG. 2 . Reaction vessel  400  may also contain additional inlet and outlet ports (not shown) for placing precursor material, sensors, and instrumentation within the cylinder  408  or to gain access to or to view the interior of cylinder  408 . Further, reaction vessel  400  may have one or more viewing ports (not shown) for allowing a technician to view the interior of the reaction vessel to inspect the equipment and nanopowder synthesis. Also, the effectiveness of the separating characteristics of the reaction vessel  400  can be viewed through the viewing window. Further, the viewing window can be used to determine whether particles are collecting on the walls of cylinder  408  or the conical-shaped portion  418 . For clarity, such access and viewing ports are also not shown in  FIG. 4 . 
   While the present invention uses a conical shaped bottom of the reactor vessel and an exit port to collect large particles, the invention is not limited to this geometry. For example, the exit port  406  could be attached to the bottom of reaction vessel  108  in  FIG. 1  and the same principles would apply, although in this case it might collect the large by-product particles with less efficiency. 
   There are numerous processes available for fabricating very small particles such as nanopowder. The present invention relates to reaction vessels used to produce, separate, and capture such nanopowder. For purposes of this patent, the term “nanopowder” is used; however, other terms may be used for the very small particles that make up nanopowder. For instance, the particles that make up nanopowder may also be referred to as “ultrafine” or “submicron” particles. Such particles may be 1-500 nanometers in size. Such particles may also be referred to as “nanoparticles,” “nanosized,” “nanocrystalline,” and the like. 
   In accordance with the present invention, the production of nanopowder can be accomplished through various mechanisms within a reaction vessel having a cyclonic flow of gas. For example, nanopowder may be produced within a reaction vessel by mechanical processing, chemical processing, or thermal processing. Also, nanopowder may be produced using plasma processing, exploding wires, electro-thermal guns, hybrid exploding wire, laser abation, plasma torch synthesis, transferred arc, combustion flame, resistive boat evaporation, microwave synthesis, spark erosion, electron beam evaporation, or sputtering, as well as other processes for producing nanopowder. The subject matter of the present invention is not limited to a particular process for producing nanopowder. For purposes of this patent, the present invention and synthesis is described as if it occurred in the reaction vessel by electro-thermal synthesis. However, the subject matter of the present invention includes synthesis of nanopowder by other means. 
     FIG. 5  shows a cutaway view of a reaction vessel  500  incorporating an apparatus for synthesizing nanopowder in accordance with the present invention. Item  506  represents a conical shaped portion of the reaction vessel  500  and item  526  represents a cylindrical portion of reaction vessel  500 . Gas  530  flows through inlet nozzle  516  into cylindrical portion  526 . Gas  530  flows tangentially along the inner wall of cylindrical portion  526  in a pattern comprising a helical vortex shown as item  534 . The helical vortex  534  also travels tangentially along the inner wall of the conical portion of the reaction vessel shown as item  506 . An inner helical vortex  536  is formed by the flowing gas as a result of the outlet position and shape of reaction vessel  500 . While flowing as inner helical vortex  536 , the gas collects nano-sized particles (nanopowder) from the expanding cloud shown as item  528  and carries the particles as item  512  through outlet nozzle  532 . Item  528  represents an expanding cloud of nanopowder and byproducts produced by synthesis means  524 . The nanopowder travels through outlet nozzle  532  while the byproducts fall through outlet nozzle  522  to be collected in the slag hopper shown as item  504 . As shown in  FIG. 5 , the nanopowder is produced by an axial discharge electro-thermal gun shown as item  524 . A high-power, pulsed electrical power supply shown as item  502  sends current down an electrical bus shown as item  508  and results in a high-velocity plasma jet shown as item  518  to create the expanding cloud of nanopowder and byproducts shown as item  528 . Though reaction vessel  500  is shown with axial-discharge electro-thermal gun  524  to synthesis the nanopowder, other means and apparatuses for synthesizing nanopowder may be incorporated into reaction vessel  500 . 
     FIG. 6  is a cut-away diagram of a reaction vessel  600  for synthesizing nanopowder in accordance with the present invention. Reaction vessel  600  comprises a cylinder portion  632  with an inlet nozzle  616 . Gas shown as item  634  enters inlet nozzle  616  and flows tangentially along the inner wall of cylinder portion  632  to form a helical vortex shown as item  630 . The gas could be any of the commonly known inert gases including helium, neon, argon, krypton, xenon and any other gas that is not a reactant when synthesizing the nanopowder (such as, for example, nitrogen for certain reactions). Alternatively, the gas could be a reactive gas such as oxygen, hydrogen, hydrocarbon, ammonia, silane, nitrogen oxide, and any other gas required to make the desired nanopowder (such as, for example, nitrogen for other certain reactions). Combinations of reactive gases and inert gases may also be used. Rods of metallic precursor material, shown as item  636 , are fed into the reaction vessel  600  using rod feeders, shown as item  626 . The tips of these form ablative electrodes that comprise the radial gun nanopowder synthesis device. The electrodes  636  are axially aligned with each other within the gas flowing through the reaction vessel  600 . Item  602  is the power supply for supplying a high-power pulsed electrical arc between the electrodes shown as item  636 . Item  634  represents the high-power pulsed electrical arc between the electrodes  636 . This arc ablates the metallic precursor material to create a rapidly expanding cloud of vapor from which the nanoparticles are formed. Item  638  represents a gas seal for preventing any gases inside the reaction vessel  600  from leaking around the metallic precursor material  636  to atmosphere. Item  628  represents a “gripper” for coupling the high-power pulsed electrical discharge to the precursor material  636 , and for maintaining the position of the precursor material  636  while the electrical pulse is supplied. Item  622  represents an outlet nozzle coupled to the lower portion of conical section  606 . Byproducts formed in the synthesis of nanoparticles are collected in slag hopper  604  after the byproducts fall through outlet nozzle  622 . Nanoparticles flow upward in the inner helical vortex shown as item  612 . The inner helical vortex  612  flows through outlet nozzle  618  located in the center of the cylindrical portion  632  of reaction vessel  600 . Item  608  represents high-current electrical bussing for transmitting the high power electrical discharge from the power supply  602  to the grippers  628 . Item  624  represents electrical insulators for insulating the nanopowder synthesis device from the grounded vessel. One distinct advantage of the current invention is the ability to provide convective cooling to the nanopowder synthesis system using moderate flow rates and without causing problems to the synthesis process. Convective cooling of the synthesis components requires high gas velocities. Generally this is accomplished using high flow rates, but this requires larger blowers and increases costs. Additionally the high velocities can often extinguish arcs resulting in problematic nanopowder synthesis. By using a cyclonic flow, high velocities can be achieved at lower flow rates without decreasing the reaction vessel diameter. This results in more efficient cooling of the internal components such as the gripper shown as item  628 , the electrical insulators shown as item  624 , and precursor material shown as item  636 . Additionally, the velocity at the center of the vortex is near zero. This can be exploited by placing the arc in this region so as to prevent the arc from being extinguished by the gas flow. Hence, efficient heat transfer can be accomplished without creating other problems for the nanopowder synthesis systems. 
   U.S. Pat. No. 5,460,701 teaches a continuous-arc plasma nanoparticle synthesis method that employs a cyclone immediately downstream of the reaction zone in which the nanoparticles are synthesized. In that method, the cyclone simply serves to enhance turbulent mixing of the newly synthesized nanoparticles with the surrounding quench gas to cool the nanoparticles and help prevent agglomeration of the nanoparticles. The present invention differs from this prior art method in that with the present invention the nanoparticles are synthesized in a reaction zone located within the central vortex of the cyclone rather than upstream of the entire cyclone. Rather than merely enhancing turbulent mixing of the nanoparticles with the quench gas, in the geometry of the present invention the cyclonic flow aids in sweeping nanoparticles out of the reaction zone and in clearing the reaction vessel of opaque aerosols, which aids the use of optical instrumentation. The cyclonic flow also helps sweep the reaction vessel wall clean of nanoparticles and other reaction products, aids in separating large-particle reaction by-products from the nanoparticles, and helps cool the nanoparticle synthesis equipment. 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.