Patent Publication Number: US-10322832-B2

Title: Systems for producing solid carbon by reducing carbon oxides

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/775,939, filed Sep. 14, 2015, which issued as U.S. Pat. No. 9,586,823 on Mar. 7, 2017, which application is a national phase entry under 35 U.S.C. § 371 of International Patent Application PCT/US2014/026631, filed Mar. 13, 2014, designating the United States of America and published in English as International Patent Publication WO 2014/151898 A1 on Sep. 25, 2014, which claims the benefit under Article 8 of the Patent Cooperation Treaty to the filing date of U.S. Provisional Patent Application Ser. No. 61/788,474, filed Mar. 15, 2013, for “Systems for Producing Solid Carbon by Reducing Carbon Oxides,” the contents of each of which are incorporated herein by this reference. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present disclosure relate to reactors, systems, and methods for forming products from a reactive gas in the presence of a catalyst. For example, methods disclosed herein may be used to form various allotropes and morphologies of solid carbon, including carbon nanotubes. 
     BACKGROUND 
     Continuous production of solid products from reactive gases typically entails reacting the reactive gases in the presence of a catalyst in such a way that the solid products are elutriated from the surface of the catalyst. The solid products are thus swept away from the catalyst and subsequently harvested in a suitable process or series of processes for removing particulates from reaction tail-gases. Reactors conventionally used for such continuous solid production include fluidized-bed reactors, fixed bed reactors, pulsed bed reactors, shaft kilns, etc. Particulate removal devices typically include cyclones, bag houses, electrostatic precipitators, scrubbers, etc. 
     Various morphologies and allotropes of carbon are used industrially, such as for fuel, as reducing agents and electrodes in metallurgical processes, as corrosion-resistant materials in furnaces and heat exchangers, as carbon electrodes, as fillers and colorants in plastics, rubbers and inks, and as strengtheners in many polymer formulations including tires and hoses. High-purity carbon in many allotropes and morphologies is a bulk commodity chemical widely used in industry. Carbon nanotubes (CNTs) may be particularly valuable. 
     CNTs may be used for various emerging applications because of their remarkable material properties. CNT use in various manufacturing processes potentially adds strength, toughness, impact resistance, electrical conductivity, and/or thermal conductivity to a product. Currently, CNTs are one of the more expensive allotropes of carbon, and the cost, availability, and quality limit broad industrial uses of this material. Methods for producing CNTs that reduce costs and improve availability and quality would therefore be useful. 
     CNTs may be produced in a variety of sizes and types. Background information on carbon nanotubes is included in U.S. Patent Application Publication No. 2012/0034150 A1, titled “Method for Producing Solid Carbon by Reducing Carbon Oxides,” and published on Feb. 9, 2012, the contents of which are incorporated by this reference. There are two main classes of carbon nanotubes: single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). CNTs are typically produced using methods that form primarily SWNTs or primarily MWNTs of various lengths and diameters. One CNT manufacturing process, described in U.S. Patent Application Publication No. 2012/0034150, may produce a mixture of both MWNTs and SWNTs. 
     BRIEF SUMMARY 
     In some embodiments, an apparatus for producing carbon nanoparticles by reducing carbon oxide compounds using a reducing agent includes a reactor configured to receive a dried recycle gas stream, a fresh feed gas stream comprising carbon oxide, and a gaseous reducing agent. The reactor includes at least one mixing means configured to mix the reagents to form a combined gaseous feed stream, a first heat exchanger configured to heat the combined gaseous feed stream to a first temperature, at least one heater configured to further heat the combined gaseous feed stream to a temperature higher than the first temperature, and a reaction vessel configured to receive the heated combined gaseous feed stream. The reaction vessel is configured to maintain predetermined reaction conditions of temperature and pressure, and has an output configured to deliver a tail gas stream to the first heat exchanger. The reaction vessel and other associated vessels and piping that may be subjected to metal dusting conditions in the course of normal service may be designed to prevent metal dusting such as, for example, by using high-chromium steel or a ceramic lining. 
     The system also includes a product separator configured to separate the tail gas stream into a products stream and the clean tail gas stream. The products stream includes carbon nanoparticles. The product separator is configured to pass the products stream through a cooling heat exchanger lock drum. The system also includes a water separation unit configured to receive the clean tail gas stream from the product separator. The water separation unit includes a second heat exchanger configured to transfer heat from the clean tail gas stream to the recycle gas stream, a first condenser configured to receive the clean tail gas stream and to cool the clean tail gas stream to condense water from the clean tail gas stream, and a second condenser to further cool the clean tail gas stream and form the dried recycle stream. A product receiving unit (e.g., a product packaging unit) is configured to receive the products stream from the cooling heat exchanger lock drum. 
     In some embodiments, product separator and water separator systems may be combined as a scrubber suitable for condensing at least a portion of the water vapor in the tail gas stream and for removing the solid carbon entrained in the tail gas stream. In such cases, the solid carbon becomes entrained in the water stream exiting the scrubber. The wet product may be subsequently removed from the water and dried to form a dry product that can then be transferred to a product-receiving unit, as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 a  and 1 b    are simplified schematic diagrams of a system for producing solid carbon and water using carbon oxides and a reducing agent, and showing two alternative flow paths; 
         FIG. 2  is a simplified schematic diagram of a system for producing solid carbon of various morphologies, and showing the flow of materials between unit operations; 
         FIG. 3  is a simplified schematic diagram of a gas supply subsystem for the system of  FIG. 2 ; 
         FIG. 4  is a simplified schematic diagram of a gas handling subsystem for the system of  FIG. 2 ; 
         FIG. 5  is a simplified schematic diagram of a reactor subsystem for the system of  FIG. 2 ; 
         FIG. 6  is a simplified schematic diagram of a product separation subsystem for the system of  FIG. 2 ; 
         FIG. 7  is a simplified schematic diagram of a water separation subsystem for the system of  FIG. 2 ; 
         FIG. 8  is a simplified schematic diagram of a water treatment subsystem for the system of  FIG. 2 ; 
         FIG. 9  is a simplified schematic diagram of a product packaging subsystem for the system of  FIG. 2 ; 
         FIG. 10  is a simplified schematic diagram of a utilities subsystem for the system of  FIG. 2 ; 
         FIG. 11  is a simplified schematic diagram of another system for producing carbon of various morphologies using, for example, hydrogen and carbon oxide; 
         FIG. 12  is a simplified schematic diagram of a gas supply subsystem for the system of  FIG. 11 ; 
         FIG. 13  is a simplified schematic diagram of a gas handling subsystem for the system of  FIG. 11 ; 
         FIG. 14  is a simplified schematic diagram of a fluidized-bed reaction subsystem for the system of  FIG. 11 ; 
         FIG. 15  is a simplified schematic diagram of a product separation and packaging subsystem for the system of  FIG. 11 ; 
         FIG. 16  is a simplified schematic diagram of a water treatment subsystem for the system of  FIG. 11 ; 
         FIG. 17  is a simplified schematic diagram of a venting subsystem for the system of  FIG. 11 ; 
         FIG. 18  is a simplified schematic diagram of another system for producing solid carbon; 
         FIG. 19  is a simplified schematic diagram of a gas supply subsystem for the system of  FIG. 18 ; 
         FIG. 20  is a simplified schematic diagram of a gas handling subsystem for the system of  FIG. 18 ; 
         FIG. 21  is a simplified schematic diagram of a fluidized-bed reaction subsystem for the system of  FIG. 18 ; 
         FIG. 22  is a simplified schematic diagram of a product separation subsystem for the system of  FIG. 18 ; 
         FIG. 23  is a simplified schematic diagram of a water separation subsystem for the system of  FIG. 18 ; 
         FIG. 24  is a simplified schematic diagram of a water treatment subsystem for the system of  FIG. 18 ; 
         FIG. 25  is a simplified schematic diagram of a product packaging subsystem for the system of  FIG. 18 ; and 
         FIG. 26  is a simplified schematic diagram of a utilities subsystem for the system of  FIG. 18 . 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes a reactor, system, and method for the production of solid carbon products from reactive gases using a fixed plate reactor, a fluidized-bed reactor, or another type of reactor, along with separation equipment for solid carbon and water. The products are formed from the reactive gases in contact with a catalyst material. The reactor, system, and method reduce the capital and operating costs associated with production of solid carbon and water. By adjusting operating parameters of the system, a variety of carbon allotropes of various morphologies can be produced using the system. For example, the systems may be used to form carbon nanotubes (CNTs), buckminsterfullerenes, graphite, carbon nanofibers, graphene, diamond, and amorphous carbon including coke and carbon black. In particular, the methods and systems disclosed herein generally use carbon oxides and a reducing agent to form products and water in a wide variety of reactor designs including, but not limited to, fixed-plate reactors, aerosol reactors, shaft kiln reactors, fixed bed reactors, counter-flow bed reactors, or fluidized-bed reactors. The system may be scaled to various production capacities. 
     As used herein, the terms “product,” “products,” “solid products,” “carbon products,” and “solid carbon products” are synonymous and mean and include any dusty, friable, powdery, sandy, chunky, or porous material that may occur as solid granules or agglomerations of solid material, with or without voids. Such materials specifically include nanoparticles of various chemistries and morphologies including carbon nanotubes, carbon nanofibers, and nanodiamonds. 
     As used herein, the term “carbon oxide” means and includes carbon dioxide, carbon monoxide, and mixtures thereof. As used herein, the term “reducing agent” means and includes a material capable of reducing a carbon oxide. Reducing agents include, for example, hydrogen, a hydrocarbon gas such as methane, an alcohol such as ethyl alcohol, and mixtures thereof. 
     As used herein, the terms “catalyst” and “catalyst material” mean and include a material formulated to promote one or more reactions, resulting in the formation of a product. A portion of a catalyst may be removed from a surrounding portion of the catalyst during the reaction and contained in or adhered to the product. Thus, some of the catalyst may be physically removed during the reaction, and the catalyst may be continually replenished. The portion of the catalyst may not therefore be considered a catalyst in the classical sense, but is nonetheless referred to herein and in the art as a “catalyst” if the reaction is not believed to alter chemical bonds of the material forming the catalyst. Particularly useful catalysts for forming products include, for example, iron, nickel, cobalt, etc., or other elements of Groups 2 through 15 of the periodic table, and alloys and mixtures thereof. For example, catalysts may be selected from Groups 5 through 10 of the periodic table. Note that the periodic table may have various group numbering systems. As used herein, group 2 is the group including Be, group 3 is the group including Sc, group 4 is the group including Ti, group 5 is the group including V, group 6 is the group including Cr, group 7 is the group including Mn, group 8 is the group including Fe, group 9 is the group including Co, group 10 is the group including Ni, group 11 is the group including Cu, group 12 is the group including Zn, group 13 is the group including B, group 14 is the group including C, and group 15 is the group including N. In some embodiments, commercially available metals are used without special preparation. Catalysts may be granular, fibrous, particulate, zeolite, or in any other form compatible with the reactor design and catalyst-handling methods disclosed herein. For example, catalyst includes steel sand-blasting grit, steel wool, chopped steel wool, and similar materials. 
     304 stainless steel appears to catalyze the formation of CNTs under a wide range of temperatures, pressures, and gas compositions. However, the rate of formation of CNTs on 304 stainless steel appears to be relatively low, such that 304 stainless steel may be used effectively as a construction material for process equipment, with minimal deposition on surfaces thereof in normal operations. 316L stainless steel, in contrast, appears to catalyze the formation of solid carbon at significantly higher rates than 304 stainless steel, but may also form various morphologies of carbon. Thus, 316L stainless steel may be used as a catalyst to achieve high reaction rates, but particular reaction conditions may be maintained to control product morphology. Catalysts may be selected to include Cr, such as in amounts of about 22% or less by weight. For example, 316L stainless steel contains from about 16% to about 18.5% Cr by weight. Catalysts may also be selected to include Ni, such as in amounts of about 8% or more by weight. For example, 316L stainless steel contains from about 10% to about 14% Ni by weight. Catalysts of these types of steel have iron in an austenitic phase, in contrast to alpha-phase iron used as a catalyst in conventional processes. Given the good results observed with 316L stainless steel, the Ni and/or Cr may have a synergistic effect with Fe. 
     Solid carbon is formed using a catalytic conversion reactor to reduce carbon oxides using a reducing agent in the presence of a catalyst. By adjusting operating parameters of the system (e.g., reaction gas mixture, type and form of the catalyst, temperature, or pressure), various carbon allotropes and morphologies can be produced. 
     Oxidation and subsequent reduction of the catalyst surface alter the grain structure and grain boundaries. Without being bound by any particular theory, oxidation appears to alter the surface of the metal catalyst in the oxidized areas. Subsequent reduction may result in further alteration of the catalyst surface. Thus, the grain size and grain boundary of the catalyst may be controlled by oxidizing and reducing the catalyst surface and by controlling the exposure time of the catalyst surface to the reducing gas and the oxidizing gas. The oxidation and/or reduction temperatures may be in the range from about 500° C. to about 1,200° C., from about 600° C. to about 1,000° C., or from about 700° C. to about 900° C. The resulting grain size may range from about 0.1 μm to about 500 μm, from about 0.2 μm to about 100 μm, from about 0.5 μm to about 10 μm, or from about 1.0 μm to about 2.0 μm. In some embodiments, the catalyst may be an oxidized metal (e.g., rusted steel) that is reduced before or during a reaction forming solid carbon. Without being bound to any particular theory, it is believed that removal of oxides leaves voids or irregularities in the surface of the catalyst material, and increases the overall surface area of the catalyst material. 
     Catalysts may be in the form of nanoparticles or in the form of domains or grains and grain boundaries within a solid material. Catalysts may be selected to have a grain size related to a characteristic dimension of a desired diameter of the product (e.g., a CNT diameter). Catalyst powder may be formed in or near the reaction zone by injecting an aerosol solution such that upon evaporation or a carrier solvent, a selected particle size distribution results. Alternatively, powdered catalyst may be entrained in a carrier gas and delivered to the reactor. By selecting the catalyst and the reaction conditions, the process may be tuned to produce selected morphologies of product. In some embodiments, the catalyst may be formed over a substrate or support, such as an inert oxide that does not participate in the reactions. However, the substrate is not necessary; in other embodiments, the catalyst material is an unsupported material, such as a bulk metal or particles of metal not connected to another material (e.g., loose particles, shavings, or shot, such as may be used in a fluidized-bed reactor). 
     In certain embodiments, a broad range of inexpensive and readily available catalysts, including steel-based catalysts, are described, without the need for activation of the catalyst before it is used in a reaction. Iron alloys, including steel, may contain various allotropes of iron, including alpha-iron (austenite), gamma-iron, and delta-iron. In some embodiments, reactions disclosed herein advantageously utilize an iron-based catalyst, wherein the iron is not in an alpha phase. In certain embodiments, a stainless steel containing iron primarily in the austenitic phase is used as a catalyst. 
     Catalysts, including an iron-based catalyst (e.g., steel, steel wool), may be used without a need for an additional solid support. In certain embodiments, reactions disclosed herein proceed without the need for a ceramic or metallic support for the catalyst. Omitting a solid support may simplify the setup of the reactor and reduce costs. 
     A reactor may be coupled with heating and cooling mechanisms to control the temperature of the reactor. For example, a reactor may be configured such that products and excess reactant are recycled through a cooling mechanism to condense water vapor. The products and/or excess reactant may then be reheated and recycled through the reactor. By removing some of the water vapor in the recycled gases, the morphology of formed may be controlled. Changing the partial pressure of water vapor changes the carbon activity of a mixture. The reactor may also be coupled to a carbon collector in which water and unreacted reactants are separated from the carbon products. The separated products are collected and removed from the system. 
     In certain embodiments hereof, the partial pressure of water in the reaction is regulated by various means, including recycling and condensation of water, to influence, for example, the structure or other aspects of the composition of products produced. The partial pressure of water appears to assist in obtaining certain desirable carbon allotropes. 
     The system typically includes various subsystems, such as a gas supply subsystem, a gas handling subsystem, a heating subsystem, a reactor subsystem, a product separation subsystem, a water separation subsystem, a water treatment subsystem, a product packaging subsystem, and a utilities subsystem. The subsystems may be arranged in many combinations, and may be replaced with other unit operations that perform similar or related functions. Various arrangements of these subsystems are described herein. The characterization of components of the system as distinct subsystems is for convenience of explanation only, and various other groupings of the unit operations described may be selected based on the principles disclosed herein. 
     The subsystems may be arranged to accommodate a variety of gas and product flows. In general, gas flows from the gas handling subsystem to the reactor subsystem. Recycled gas flows from the water removal subsystem to the gas handling subsystem, and water flows from the water removal subsystem to the water treatment subsystem. Other flows may be directed to accommodate selected design specifications. Product may be removed by a product removal subsystem either directly from the reactor (e.g., as in a shaft-kiln or counter-flow packed bed reactor) or from the tail gas stream exiting the reactor (e.g., as in a fluidized bed or pulsed bed reactor) prior to entering the water removal subsystem. 
     For example,  FIG. 1 a    illustrates a simplified block flow diagram of a system  1  for producing solid carbon and water.  FIG. 1 a    includes process flow lines indicating the movement of material through the system  1 . In the system  1 , reaction gases flow from a gas supply subsystem  10  to a gas handling subsystem  12 , then to a heating subsystem  14  on the way to a reactor subsystem  16 . 
     As further depicted by dashed lines in  FIG. 1 a   , the gas supply subsystem  10  may optionally provide reaction gases at various points within the system  1 . For example, the gas supply subsystem  10  may supply gases directly to the heating subsystem  14 , where the reaction gases may mix with recycled gases from the gas handling subsystem  12 . Alternatively, the gas supply subsystem  10  may provide the reaction gases directly into the reactor sub system  16 . 
     Products may be removed before cooling. In such embodiments, hot reaction gases with entrained products flow directly from the reactor subsystem  16  to the product removal subsystem  24 . After the products are removed, remaining hot gases pass to the heating subsystem  14 . The remaining hot gases are counter-flowed through one or more heat exchangers in the heating subsystem  14  to provide at least a portion of the heat required to bring the reaction gases to a predetermined reaction temperature. The resulting cooled gases flow to the water removal subsystem  20 . The water removal subsystem  20  separates water from the gases. The water passes to a water treatment subsystem  22 , and the remaining gases are recycled to the gas handling subsystem  12 . A utilities subsystem provides support to other subsystems, such as for cooling water, chilled glycol, an inert gas purge, a vent stack, and/or a flare. 
     An alternate system  1 ′ illustrating another way in which the subsystems may be combined, is depicted in  FIG. 1 b   . The system  1 ′ has the same subsystems as system  1 , but the tail gases and products have different flow paths. Hot tail gases with entrained products flow from the reactor subsystem  16  directly to the heating subsystem  14 , where at least a portion of the heat in the hot tail gases is transferred to the reaction gases entering the reactor subsystem  16 . After the heat exchange, the cooled tail gases flow to the product removal subsystem  24 , where the product is removed. The remaining tail gases then flow to the water removal subsystem  20 . 
     The choice of flow paths for gases, liquids, and solids is a design choice. If pressurizing the reaction gases is required, the reaction gases may flow from the gas supply subsystem  10  to the gas handling subsystem  12 , and from there to the heating subsystem  14 . If compression is not desired (e.g., if the gas supply subsystem  10  provides reaction gases at sufficient pressure), the reaction gases may flow to the heating subsystem  14  or directly to the reactor sub system  16 . 
     In some embodiments, the gas supply subsystem  10  provides a fresh feed gas stream of process gases, including at least one carbon oxide and at least one reducing agent. A portion of the carbon oxide fresh feed gas stream and a portion of the reducing agent fresh feed gas stream are added to the recirculating gas mixture separately or in combination. 
     Typically, adding the gases separately to the recirculating gas mixture just prior to the reactor subsystem  16  greatly reduces the risk of reaction during heating. However the fresh feed gas streams can be added at any point to the recirculating gas stream, including prior to the gas handling subsystem  12 . Depending on where the fresh feed streams are added, the fresh feed gas streams may be preheated to avoid lowering the temperature of the recirculating gas mixture below the capacity of the system  1  to maintain selected conditions. In one embodiment, hydrogen is added before the gas handling subsystem  12  without preheating and carbon oxide is preheated and added just before the reactor subsystem  16 . In another embodiment, the hydrogen and the carbon oxide are each preheated and added separately just before the reactor subsystem  16 . 
     The gas supply subsystem  10  may include means for storing a supply of reaction gases and means for regulating the pressures, temperatures, and flow rates of the gases. The gas supply subsystem  10  may also contain a control system feedback loop to monitor and control the flow rate of at least one of the fresh feed gases to maintain system pressure or reaction gas composition in the system  1 . An effective control system may use the system pressure (e.g., the inlet or outlet pressure of the compressor or process gas blower) to control the flow rate of fresh reducing agent and the reaction gas composition to control the carbon oxide flow rate. Parameters that may be controlled include the carbon activity and the oxygen potential of the reaction gas mixture in the reactor. The partial pressure of water is one factor that appears to affect the type and character (e.g., morphology) of solid carbon formed, as well as the kinetics of carbon formation. 
     Carbon activity (A c ) can be used as an indicator of whether solid carbon will form under particular reaction conditions (e.g., temperature, pressure, reactants, concentrations). Without being bound to any particular theory, it is believed that carbon activity is the key metric for determining which allotrope of solid carbon is formed. Higher carbon activity tends to result in the formation of CNTs, lower carbon activity tends to result in the formation of graphitic forms. 
     Carbon activity for a reaction forming solid carbon from gaseous reactants can be defined as the reaction equilibrium constant times the partial pressure of the reactants, divided by the partial pressure of gaseous tail gas constituents. For example, in the reaction, CO (g) +H 2(g) ⇄C (s) +H 2 O (g) , with a reaction equilibrium constant of K, the carbon activity A c  is defined as K·(P CO ·P H2 /P H2O ). Thus, A c  is directly proportional to the partial pressures of CO and H 2 , and inversely proportional to the partial pressure of H 2 O. Higher P H2O  tends to inhibit CNT formation. The carbon activity of this reaction may also be expressed in terms of mole fractions and total pressure: A c =K·P T (Y CO ·Y H2 /Y H2O ), where P T  is the total pressure and Y is the mole fraction of a species. Carbon activity generally varies with temperature because reaction equilibrium constants vary generally with temperature. Carbon activity also varies with total pressure for reactions in which a different number of moles of gas are produced than are consumed. Mixtures of solid carbon allotropes and morphologies thereof can be achieved by varying the catalyst and the carbon activity of the reaction gases in the reactor. 
     Similarly, oxygen potential can be used as an indicator of whether a catalyst material will oxidize under particular reaction conditions (e.g., temperature, pressure, reactants, and reactant concentrations). Oxidation of catalyst material may limit the ability of the catalyst material to subsequently catalyze reactions. Oxygen potential may be defined in terms of the equilibrium constant of the oxidation reaction. For example, iron metal may be oxidized to iron oxide: Fe+½O 2 ⇄FeO. The equilibrium constant is defined as K FeO =exp[−ΔIG FeO /(R g T)]=√(P O2 /P T ), where ΔG FeO  is the Gibbs free energy of the oxidation reaction, R g  is the gas constant, T is the absolute temperature, P O2  is the partial pressure of oxygen, and P T  is the total pressure. Using this equation, the partial pressure of oxygen that will initiate the oxidation of iron may be determined: P O2 &gt;Pr K FeO   2 . The partial pressure of oxygen can be obtained from the equilibrium of one or more fast reactions involving oxygen, such as: 
     
       
         
           
             
               
                 
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     In these reactions, K i  is the equilibrium constant for the decomposition of gas i. The partial pressure of oxygen may be controlled by the partial pressures of other gases. 
     The gas handling subsystem  12  typically includes an air-handling device (e.g., a compressor or process gas blower) suitable for moving reaction gases through the system by providing motive power to flow the reaction gases at desired flow rates and pressures through the system  1 . The gas handling subsystem  12  also includes associated controls and components. The gas handling subsystem  12  circulates the gas mixtures through the system  1 . Various compressor types suitable to the service and capacity of the system  1  may be selected including reciprocating, scroll, screw, hooked-screw and centrifugal compressors, or process gas blowers. In one embodiment, discharge from the water removal subsystem  20  feeds a compressor of the gas handling subsystem  12 . The gas handling subsystem  12  typically discharges the reaction gas stream to the heating subsystem  14 . The gas handling subsystem  12  may also include a control system feedback loop to monitor and control the flow rate of the reaction gas stream. 
     The heating subsystem  14  typically includes one or more heat exchangers, wherein hot gases from the outlet of the reactor subsystem  16  are counter flowed with reaction gases flowing to the inlet of the reactor subsystem  16 . One or more heaters may provide additional heat to the reaction gases to reach a selected temperature prior to entering the reactor subsystem  16 . The reaction gas stream may be heated to reaction temperatures in the heating subsystem  14 . The flow of gases through the heating subsystem  14  may allow recovery of at least a portion of the process heat from the reaction tail gas stream. The reactor subsystem  16  may also contain one or more heaters to further heat the reaction gas feed stream to a selected temperature prior to entering a reaction vessel. The heating subsystem  14  may also include a control system feedback loop to monitor and control the temperature of the reaction gas stream. 
     The reactor subsystem  16  includes a reactor, such as a fluidized bed, a packed bed, a shaft kiln, or any other appropriate design. The reactor subsystem  16  also typically includes associated controls and components, such as features for catalyst flow. A reaction vessel receives the heated combined feed stream and catalyst. CNT and water formation occur within the reaction vessel as the reaction gas stream contacts the catalyst. The reaction vessel outputs a reactor tail gas stream that may contain the reaction products (e.g., water and CNTs or other carbon products) and unreacted gases. The reaction vessel may also include a control system feedback loop to monitor and control reaction conditions in the reaction vessel and/or the heaters. 
     Various reactor types and configurations may be used in the reactor subsystem  16 , including, plate, fixed-bed, fluidized-bed, pulsed fluid-bed, shaft-kiln, and rotating-bed reactors. In one embodiment, catalyst material is formed into a series of plates in a plate reactor. For example, the plates may be concentric steel tubes. In another embodiment, the catalyst material is in the form of small particles in a fixed-bed reactor, and the catalyst material and product are periodically removed from the reactor and agitated to remove the product from the catalyst particles. The catalyst material is separated for reuse, and the product is removed for packaging. In yet another embodiment, a pulsed fluid-bed reactor is used, wherein a bed of catalyst particles is maintained as a fixed bed and periodically fluidized by pulses of the reaction gases (e.g., a temporary increase in the flow rate of reaction gases) so that the product is elutriated and removed from the reactor. 
     The product removal subsystem  24  typically includes means for separating products from a gas stream. Such means may include one or more cyclones (single-stage or multi-stage), filters, electrostatic precipitators, bag houses, or various combinations of these separation devices. The product removal subsystem  24  may include any suitable device(s), such as one or more cyclones operating in series, to remove various fractions of solid carbon from the tail gas stream. The product removal subsystem  24  may also include associated controls and components. The product removal subsystem  24  removes the product from the reactor tail gas stream to form a products stream and a clean tail gas stream. In embodiments in which the reactor subsystem  16  includes a shaft-kiln reactor, the product and catalyst are removed from the bottom of the reactor through a lock hopper system. In such embodiments, the product removal subsystem  24  may include additional means to separate the residual catalyst from the solid carbon and to classify the residual catalyst and recycle all or portions thereof to the catalyst feed system for the reactor. 
     In some embodiments, a lock drum passes the products stream from the product removal subsystem  24  to a product packaging subsystem  26 , while a clean tail gas stream outlet passes the clean tail gas stream to the heating subsystem  14 , to a heat exchanger within the reactor subsystem  16 , or to the water removal subsystem  20 . The function of a lock drum is to isolate the reaction gases in the product removal subsystem  24  from the product packaging subsystem  26 . A lock drum typically includes means for purging the reaction gases and may include means for cooling of the product prior to transfer to the product packaging subsystem  26 . The product packaging subsystem  26  may include means for handling and transfer of materials, such as pneumatic transfer, screw conveyor, or other such means as are commonly used in the handling of powdered or granular products. 
     The product removal subsystem  24  may be configured to operate at substantially the same temperature as the reactor, such as in embodiments in which the tail gases flow from the reactor subsystem  16  directly to the product removal subsystem  24  (e.g., in system  1  of  FIG. 1 a   ). In such embodiments, materials and design of the lock hopper may be selected to withstand the expected temperatures of the products. In embodiments in which the tail gases are cooled before flowing to the product removal subsystem  24  (e.g., in system  1 ′ of  FIG. 1 b   ), materials and design of the lock hopper may be less robust. That is, cooling the tail gases may allow the lock hopper to be formed from a wider range of materials. Whether to cool the reactor tail gas prior to or after product separation may influence the available product-separation options because some separation techniques may require particular temperature ranges. 
     In some embodiments, the product removal subsystem  24  or the water removal subsystem  20  may include a water-gas conversion reactor capable of capturing solid carbon material that bypasses the product removal subsystem  24 . Water is injected into the water-gas conversion reactor so that the water-gas reaction (i.e., C (s) +H 2 O→O+H 2 ) converts remaining solid carbon material back to carbon oxides and hydrogen. Oxidation of solid carbon may limit or prevent the fouling of downstream equipment, contamination of the water removed from the tail gas stream, or contamination of the vent gas streams. 
     In some embodiments, the water-gas conversion reactor may include a control system to regulate the flow of water based on the pressure drop across the water-gas reactor and the downstream water content of the clean tail gas stream. The water-gas conversion reactor may include filter materials selected to withstand the operating temperatures and water-gas reaction conditions (e.g., sintered metal or ceramic filters). In some embodiments, multiple filters may be employed, and reaction gases may alternate between the filters. In such embodiments, each filter may cycle between a filtering mode and a water-gas reaction mode. The filter may collect solid carbon in filtering mode. After a period of time, the filtering efficiency may decrease. The filter may then be switched to water-gas reaction mode. In the water-gas reaction mode, the filter or a supplemental reaction gas stream (e.g., all or a portion of the clean tail gas stream) is heated to increase reaction rates of the water-gas reaction. After a period of time, the filter may be free of built-up solid carbon, and the filter may be switched back to filtering mode. If two or more filters are present, one may operate in filtering mode while another operates in water-gas reaction mode, allowing continuous operation of the filtering system. 
     The reaction gas mixtures and conditions of the system  1  are commonly associated with metal dusting (i.e., deposition of solid carbon on metal surfaces). Thus, the materials and components selected for the high temperature portions of the system  1  may be selected to avoid metal dusting. An advantage of adding water in the water-gas reactor in excess of that required for the water-gas reaction is that water tends to passivate or protect common grades of stainless steel (e.g., 304, 310, etc.) from metal dusting. Excess water may reduce the risk of metal dusting and may be used downstream or removed. For example, if the product removal subsystem  24  includes a water-gas conversion reactor operating with excess water, the clean tail gas leaving the product removal subsystem  24  may flow to the heating subsystem  14  (see  FIG. 1 ). After flowing through the heating subsystem  14  or a portion thereof, the clean tail gas stream may flow through additional heat exchangers to further cool the clean tail gas stream and through one or more filters to remove additional product. The clean tail gas stream then flows through one or more heat exchangers or condensers within the water removal sub system  20 . 
     The water removal subsystem  20  typically includes means for condensing the water from the process gas stream after the product has been removed. The water removal subsystem  20  may include one or more condensers cooled by water, chilled brine, glycol, or air, as well as associated tanks, controls, and components. In some embodiments, the water removal subsystem  20  includes a two-stage condenser in which the first stage is water-cooled and the second stage is glycol- or brine-cooled to achieve a clean tail gas stream having a lower the dew point than is possible with water-cooling alone. 
     The water removal subsystem  20  may be operated to remove sufficient water and maintain a desired water concentration (e.g., partial pressure of water) in the dried gases exiting the water removal subsystem  20 . The water concentration influences the oxidation potential of reaction gases in the reactor. Therefore, controlling the water concentration may be important to control the deposition of the selected allotropes and morphologies of solid carbon and to avoid oxidation or fouling of the catalysts. The cooling medium (e.g., water, brine, glycol, air, etc.) for the condensers may be provided by a utilities subsystem. 
     The water removal subsystem  20  may have an output for condensed water and an output for the recycled gas stream. The recycled gas stream returns to the gas handling subsystem  12  for reuse or ventilation to a safe location. If the feed gases contain significant inert impurities, a purge stream may control the accumulation of the inert gases in the system  1 . The condensed water from the water removal subsystem  20  is transferred to the water treatment sub system  22 . 
     The water treatment subsystem  22  typically includes one or more filters for minimizing product discharge in the water. The water treatment subsystem  22  may also include one or more aeration tanks, overflow tanks, and associated controls and components. The water treatment subsystem  22  may be operable to limit or minimize the release of reaction gases or product with the water. The water treatment subsystem  22  may contain a water tank and at least one filter to remove product impurities before discharging condensed water to a drain. The water treatment subsystem  22  may also include air sparging or other treatment of the water to remove dissolved reaction gases from the condensed water. 
     The product packaging subsystem  26  typically includes product storage and handling equipment to transfer product to containers in a manner that limits or minimizes worker and environmental contamination from airborne solid carbon particles. For example, the product packaging subsystem  26  may include an automated drum load-out station configured to operate with negative pressure, HEPA and ULPA filtration systems, and associated controls and components. The product packaging subsystem  26  may be physically proximate to the product removal subsystem  24  and may include a silo, a vent, and a conveyor belt to move product to enclosed load-out units. The product packaging subsystem  26  may be configured to load containers such as barrels. The product packaging subsystem  26  may also include a blower and filter to provide negative pressurization to load-out units and to remove impurities from air within the product packaging subsystem  26 . The product packaging subsystem  26  may also provide access to a product shipping area. 
     The utilities subsystem typically includes plant services for the operation of the system  1 . Such services may include plant air, cooling water supply, chilled brine or glycol supply, vent system (e.g., a stack and flare system), inert purge gas supply and handling, and associated controls and components. The utilities subsystem may provide a cyclic cooling water supply and return (e.g., a cooling tower) and a cyclic glycol supply and return (e.g., a chiller) for the condensers, and at least one vent or flare to release filtered gases. 
     The system  1  includes various control mechanisms. For example, the system  1  may include means to determine gas composition, reactor pressure, reactor temperature, and reaction gas flow rates. The system  1  may also include means to control such parameters, such as valves, heaters, etc. Controlling such parameters may allow an operator to select products of a given particular quality and morphology. 
     Gas composition within the system  1  may be controlled by providing the carbon oxide make-up gas stream at a rate selected to maintain a desired carbon activity in the tail gas streams and by providing the reducing agent make-up gas stream to maintain the pressure in the system  1 . Because the reducing agent is consumed in the reaction at a stoichiometric ratio with the carbon oxides, the pressure in the system falls as the reaction gases are consumed. The flow of the reducing gas may be controlled to maintain the pressure, and the carbon oxide stream may be controlled to maintain the carbon activity in the tail gas streams. 
     In some embodiments, the partial pressure of water vapor is controlled within the system  1 . Controlling water vapor partial pressure is useful to form a desired allotrope and morphology of solid carbon. Controlling water vapor partial pressure is also useful to maintain the catalyst in a condition for promoting reactions (e.g., by limiting oxidation of the catalyst). Control of the water vapor partial pressure may be achieved through various means, such as controlling the condensation temperature in the water removal subsystem  20 , which determines the partial pressure of water vapor in the dry gas leaving the condensers. Control of the water vapor partial pressure may also be achieved through the control of the carbon oxide make-up gas stream, which determines the rate of water formation in the reactor subsystem  16 . The reduction of the carbon oxide by the reducing agent in the reactor subsystem  16  results in the formation of water vapor in the reactor subsystem  16 . 
     The water vapor formed in the reactor subsystem  16  is believed to be an important contributor to the allotropes and morphologies of carbon formed. As the oxygen potential of the reaction gases in the reactor subsystem  16  increases, allotropes and morphologies of solid carbon having higher Gibbs free energies tend to preferentially oxidize in the water-gas reaction. By controlling the oxygen potential of the reaction gases in the reactor subsystem  16 , the selectivity of the carbon allotropes and morphologies produced can be improved. 
     The allotropes and morphologies of carbon formed in the reactor subsystem  16  depend on the carbon activity, oxygen potential, temperature, pressure, and type of catalyst. Suitable conditions for various types of solid carbon can be readily determined by experiment. 
     Carbon activity may be controlled by heating components of the system  1  to operating conditions in the presence of pure hydrogen (e.g., a heated hydrogen stream). Once the system  1  is at operating temperature and pressure, the carbon oxide make-up gas stream is introduced. The flow of the carbon oxide make-up gas stream is controlled to maintain a selected reaction gas composition or tail gas composition. The reducing agent make-up gas stream may be similarly controlled to maintain the desired system pressure. 
     Thus, the pressure at any point in the system  1  may be used as the process control variable. For example, the compressor outlet pressure may be used as the process control variable to control the reducing agent make-up gas stream flow rate. In other words, the outlet pressure may be maintained at a constant value by changing other parameters. Many variations may also be used to control make-up gas flow, such as safety interlocks, positive feedback control of the carbon activity and oxygen potential by controlling the flow of the carbon oxide make-up gas stream, condenser temperature control in the water removal subsystem  20 , etc. 
     A relatively stable reaction pressure, which may be provided as described above, may be important because the deposition of products is typically a diffusion-limited process. Without being bound to a particular theory, higher pressure appears to correlate with a higher rate of carbon formation. Furthermore, the morphology of the solid carbon may vary with pressure. To maintain consistent carbon morphology, the carbon activity and oxygen potential may be adjusted to compensate for the increased pressure. The carbon activity and oxygen potential can be adjusted by a combination of changing the reaction gas mixture, changing the temperature of the reactor subsystem  16 , and changing the temperature of the condensers in the water removal subsystem  20 . 
     In some embodiments, start-up of the system  1  is performed under an atmosphere in which carbon formation is not possible in any of the equipment. For example, the start-up atmosphere may typically be an inert atmosphere such as nitrogen or argon, or a reducing atmosphere such as hydrogen. Start-up often includes a period during which the reactor is at a temperature of at least approximately 500° C. and in an atmosphere of flowing hydrogen or other reducing gas that reduces any metal oxides prior to introducing the reaction gases. The start-up may continue for a suitable time for catalyst reduction, which depends on the catalyst composition and physical form. The flow of reducing gases during the start-up process transports water vapor formed by the catalyst reduction to the water removal subsystem  20 , where the water vapor can be removed from the system  1 . 
     Thus, the process heater of the heating subsystem  14  may be divided into two sections: a first high-power start-up heater used for start-up heating only and a second lower-power operating heater for continuous operation. The start-up heater may be isolated from the system during normal operation or may be located in an otherwise cool section of the heating system where the carbon activity is sufficiently low that carbon will not deposit. The operating heater is typically located adjacent the reactor subsystem  16  so that the heated reaction gases flow directly into the reactor vessel from the operating heater. 
     The operating heater of the heating subsystem  14  can be simplified if direct combustion is used to bring the gases to reaction temperature. If oxygen is introduced into the recycle gas mixture, some of the gases may oxidize, releasing heat and forming CO, CO 2 , and water in various proportions depending on the composition of the recycle gas mixture. This direct combustion may provide sufficient energy to heat the reaction gases to the reaction temperature. The oxygen may be supplied from a water electrolysis device that generates both oxygen and hydrogen. Both of the generated gases may be subsequently used as feed gases to the process, with the water used for the direct-combustion heating and the hydrogen added as a reducing gas. Direct-combustion heating is described in International Patent Application No. PCT/US14/25065, filed Mar. 12, 2014, titled “Direct-Combustion Heating,” the entire disclosure of which is incorporated herein by this reference. 
     Reaction gas temperature can be controlled by the heating subsystem  14 . If all or a portion of the make-up gas stream is injected at the entrance to the reactor vessel, downstream of the operating heater, a make-up heater may be added to this make-up gas stream. The make-up heater can be controlled based on the mixed gas temperature of the make-up gas stream and the reaction gas stream. The make-up gas heater or heaters may use a portion of the hot tail gases to provide energy for the make-up gas heating. 
     Reaction gas flow rate control can be achieved through a variety of means. Generally, components of the system  1  may be protected from low-flow conditions by interlocking flow controls. For example, when low-flow conditions occur, the carbon oxide make-up gas stream may be terminated and the process heaters shut off. 
     EXAMPLE 1 
       FIG. 2  depicts one embodiment of a system  100  for production of solid carbon products. Aspects of this embodiment are shown in further detail in  FIGS. 3 through 10 . The system  100  typically includes a gas supply subsystem  200 , a gas handling subsystem  300 , a reactor subsystem  400 , a product separation subsystem  500 , a water separation subsystem  600 , a water treatment subsystem  700 , a product packaging subsystem  800 , and a utilities subsystem  900  (each shown in more detail in  FIGS. 3 through 10 ). In  FIGS. 2 through 10 , solid arrows indicate the flow of materials (gas, liquid, etc.). In the system  100 , information also flows within and between various subsystems. The subsystems may be modified or combined, or may be added to additional subsystems as fitting desired production processes. The subsystems may be positioned to improve operating economics and to make efficient use of space in a process facility. The subsystems may also include various instruments or equipment, such as valves, transducers, flow meters, switches, controllers, computers, etc. The gases referenced in this description are for example only and may be substituted for, and/or added to other gases. The assignment of specific mechanics to specific subsystems within the system is also for example only. 
     As depicted in  FIG. 3 , gas supply subsystem  200  includes at least a supply of liquefied carbon dioxide (CO 2 ) or other carbon oxide  201  (e.g., a tank, pipe, etc.), and a supply of at least one reducing gas  203  such as methane (CH 4 ) of any selected purity. When conditions such as pressure and temperature are appropriate, the carbon oxide  201  passes through a vaporizer  202 , which evaporates the carbon oxide  201  to form carbon oxide gas. The carbon oxide gas is split into two portions  206 ,  207  that pass into the gas handling subsystem  300  (see  FIG. 4 ). One or more valves  204 ,  205  may control the flow of gases. A reducing agent  203  is provided to subsequent processes through reducing gas stream  208 . 
     In the gas handling subsystem  300 , shown in  FIG. 4 , one portion of the carbon oxide gas  207  is mixed with the reducing gas stream  208 . The combined gas stream may optionally be mixed with a recycle stream  609  containing dry tail gas supplied by the water separation subsystem  600  (see  FIG. 7 ). The combined gases may enter a first tank  302  before being compressed in a compressor or process gas blower  303 . The compressed gas is transferred to a second tank  304  for holding or further processing. The compressed gas from the second tank  304  is mixed with a portion  206  of the carbon oxide gas to form a combined feed gas stream  305  under conditions selected for particular reactions, such as temperature, pressure, concentration, and density. The conditions typically depend on the desired end-product; for example, carbon nanotubes, graphite, and carbon black each form under different reaction conditions. The combined gas feed stream  305  is analyzed by at least one analyzer  306  to obtain information useful for controlling other components of the system  100  (e.g., concentration of reactants in the combined feed gas stream  305 , temperature, or pressure). 
     As shown in  FIG. 5 , the combined gas feed stream  305  flows to the reaction subsystem  400  and to at least one heat exchanger  401  therein. The heat exchanger  401  is typically a counter-flow heat exchanger configured for gas-gas interchange, and the combined gas feed stream  305  receives heat from a warmer clean tail gas stream  503  from a cyclone  501  within the product separation subsystem  500  (see  FIG. 6 ). Multiple heat exchangers may be used to heat the combined gas feed stream; for example, two heat exchangers  401  are shown in  FIG. 5 . The heat exchangers  401  typically include materials selected for the process temperatures and to be resistant to metal dusting corrosion. In some embodiments, a small amount of water is added to the gas flow streams before the first heat exchanger  401  to cause a deposit of metal oxide on surfaces of the heat exchangers  401  to limit or prevent metal dusting. The heat exchangers  401  cool the clean tail gas stream  503  to form a cooled clean tail gas stream  405 , which flows to the water separation subsystem  600  (see  FIG. 7 ). 
     Within the reaction subsystem  400 , the combined gas feed stream  305  flows to at least one heater  402  (e.g., an electric heater) to bring the reactants up to reaction conditions, then to a reactor  403 . The heater  402  receives reaction feedback information from the reactor  403  via one or more sensors. The reactor  403  may be lined with a ceramic material to mitigate metal dusting corrosion therein. The reactions that occur within the reactor  403  may be Bosch-type reactions at conditions that produce selected morphologies of solid carbon, such as those disclosed in U.S. Patent Application Publication No. 2012/0034150. 
     A reactor tail gas stream  404  flows from the reactor  403  to a cyclone  501  within the product separation subsystem  500  ( FIG. 6 ). The cyclone  501  and flow lines connecting the reactor  403  to the cyclone  501  may be ceramic-lined to limit or prevent metal dusting corrosion. The cyclone  501  separates the reactor tail gas stream  404  into a products stream  502 , such as CNTs, and the clean tail gas stream  503 . The products stream  502  includes particles smaller than a selected size. For example, the cyclone  501  may be configured to separate particles smaller than about 12 microns into the products stream  502 . The size of particles separated into the products stream  502  depends on operating conditions and particle properties (e.g., shape and density). 
     The products stream  502  passes through a cooling apparatus  504 . Some residual gases  505 , such as gases containing carbon oxides and potentially explosive reducing agents such as hydrogen and methane may pass through a filter  506  and vent to a safe location (e.g., a flare, an exhaust stack). After cooling, the products stream  502  is transferred to the product packaging subsystem  800  (see  FIG. 9 ). 
     After leaving the cyclone  501 , the clean tail gas stream  503  returns to the reaction subsystem  400  ( FIG. 5 ) wherein the at least one heat exchanger  401  transfers heat from the clean tail gas stream  503  to the combined feed gas stream  305 , as described above. Alternatively, the reactor tail gas stream  404  may be first passed through a heat exchanger  401  before the cyclone  501 . Metal dusting corrosion of the heat exchanger  401  may be reduced by injecting a small amount of water into the clean tail gas stream  503  before the heat exchanger  401 . The water causes a layer of metal oxide to form on internal surfaces of the heat exchanger  401 , protecting the heat exchanger  401  from metal dusting. 
     As shown in  FIG. 7 , the cooled clean tail gas stream  405  flows to another heat exchanger  601  before entering a condenser  602  (e.g., a one-stage water-cooled condenser) in the water separation subsystem  600 . The cooled clean tail gas stream  405  passes through at least one filter  603 , where liquid water may be diverted into a holding tank  604 . After the clean tail gas stream  405  passes through the filter  603 , a portion  605  of the clean tail gas stream  405  may be safely vented, and the remainder of the clean tail gas stream  405  flows into a second condenser  606 . 
     The second condenser  606  may be a two-stage condenser, in which a first stage is water-cooled and a second stage is glycol-cooled. After passing through the two-stage condenser  606 , condensed water may be held in a holding tank  607  for a period of time before mixing with the output of the holding tank  604 . The condensed water  608  flows to the water treatment subsystem  700  (see  FIG. 8 ) for further processing. As water is collected in the holding tank  607 , recycled gases  609  flow back to the heat exchanger  601 , and finally to the tank  302  within the gas handling subsystem  300  ( FIG. 4 ). Cooling water and glycol for the condensers  602 ,  606  may be supplied by the utilities subsystem  900  (see  FIG. 10 ). In such embodiments, cooling water  903 ,  904  may flow to the first condenser  602  and a first set of coils of the second condenser  606 , respectively. The cooling water returns  610 ,  611  flow back to the utilities subsystem  900 . Chilled glycol  907  flows to a second set of coils of the second condenser  606 , and glycol return  612  flows back to the utilities subsystem  900 . 
     Within the water treatment subsystem  700  ( FIG. 8 ), a tank  701  accumulates the condensed water  608  from the water separation subsystem  600 . The tank  701  may systematically release water to a pump  702 , such as through a valved port in or near the bottom of the tank  701 . The pump  702  directs the condensed water  608  through one or more filters  703  to form a purified water stream  704 . The purified water stream  704  then passes to storage, municipal waste, or another safe and permitted disposal means. 
     The product packaging subsystem  800 , shown in  FIG. 9 , transfers the products stream  502  to a shipping package  801 , such as a barrel, box, or other container. The shipping package  801  may be selected to meet requirements for safe and effective transportation and delivery. The process of packaging may occur in a negative-pressure enclosure so that products do not contaminate workers or the workspace, for example, by flowing residual gases  804  through a fan or blower  802  and a HEPA/ULPA filter  803  before venting. After the products are packaged, shipping packages  801  may be moved to a shipping area  805 , such as a loading dock. 
     The utilities subsystem  900 , shown in  FIG. 10 , may interact with and support other subsystems of the system  100 . For example, the utilities subsystem  900  may include a cooling tower  901  configured to supply cooling water to and receive cooling water returns from the water separation subsystem  600  ( FIG. 7 ). Heat is removed from the cooling water returns  610 ,  611  in the cooling tower  901 . The cooling water  903 ,  904  is recycled via a pump  902  back to the water separation subsystem  600 . A chiller  905  receives the glycol return  612  from the water separation subsystem  600 . A glycol pump  906  recycles the chilled glycol  907  to the water separation subsystem  600 . The utilities subsystem  900  may also include at least one flare or vent  908  designed for the safe venting of various process gases. For example, the vent  908  may be configured to receive the residual gases  505  from the product separation subsystem  500 , the portion of the tail gas stream  605  from the water separation subsystem  600 , and/or the residual gases  804  from the product packaging subsystem  800 . The utilities subsystem  900  may also include other operations, sensors, or controls, as necessary for the system  100  to function. 
     EXAMPLE 2 
       FIG. 11  depicts another system  1100  for producing solid carbon products of various morphologies. The system  1100  includes a gas supply subsystem  1200 , a gas handling subsystem  1300 , a fluidized-bed reaction subsystem  1400 , a product separation and packaging subsystem  1500 , a water treatment subsystem  1600 , and a venting subsystem  1700  (each shown in more detail in  FIGS. 12 through 17 ). In the system  1100 , solid arrows indicated the flow of materials (gas, liquid, etc.). In  FIGS. 11 through 17 , solid arrows indicate the flow of materials (gas, liquid, etc.). Information also flows within and between various subsystems. The subsystems may be modified or combined, or may be added to with additional subsystems as fitting desired production processes. The subsystems may be positioned to improve operating economics and to make efficient use of space in a process facility. The subsystems may also include various instruments or equipment, such as valves, transducers, flow meters, switches, controllers, computers, etc. The gases referenced in this description are for example only and may be substituted for, and/or added to other gases. The assignment of specific mechanics to specific subsystems within the system is also for example only. 
     As depicted in  FIG. 12 , a supply of liquefied carbon oxide (e.g., CO, CO 2 ) is provided from a source  1101  (e.g., a tank, pipe, etc.) within the gas supply subsystem  1200 . When conditions such as pressure and temperature are appropriate, the carbon oxide passes through a first vaporizer  1202 , which evaporates the carbon oxide to form carbon oxide gas  1203 . The gas supply subsystem  1200  also includes a reducing agent source  1204  of, for example, liquefied hydrogen (H 2 ). When conditions such as pressure and temperature are appropriate, the liquid reducing agent passes through a second vaporizer  1205 , which evaporates the liquid reducing agent to form reducing gas  1206 . 
     Both the carbon oxide gas  1203  and the reducing gas  1206  flow to the gas handling subsystem  1300 , shown in  FIG. 13 , where they mix to form a combined fresh feed gas stream. The gas handling subsystem  1300  includes a heat exchanger  1301  configured to receive and cool a clean tail gas stream  1503  from the product separation and packaging subsystem  1500  (see  FIG. 15 ). The clean tail gas stream  1503  passes through one or more filters  1302  and/or condensers  1303 . Condensed water  1304  leaves the condensers  1303  and flows to the water treatment subsystem  1600  (see  FIG. 16 ). A portion of the resulting dried clean tail gas stream may form a vent gas  1305  and flow to the venting subsystem  1700  (see  FIG. 17 ), and another portion passes through a pump  1306  and is mixed with the combined feed gas stream to form a reaction gas  1307 . The reaction gas  1307  receives heat from the clean tail gas stream  1503  in the heat exchanger  1301  before flowing to the fluidized-bed reaction subsystem  1400  shown in  FIG. 14 . 
     In the fluidized-bed reaction subsystem  1400 , the reaction gas  1307  stream passes through a heat exchanger  1401  (e.g., a cross-flow gas-gas interchange heat exchanger) to extract heat from a tail gas stream  1404 . Multiple heat exchangers may be utilized to achieve a selected amount of heat transfer. The heat exchanger  1401  includes materials appropriate for the temperatures and conditions therein, such as materials resistant to metal dusting corrosion. A heater  1402  provides additional heat to bring the reaction gas  1307  to reaction conditions prior to flowing into the reaction vessel  1403 . The reaction gas  1307  flows into a reaction vessel  1403 , wherein CNTs or other carbon products (e.g., nanodiamonds, graphite, carbon black, etc.) are formed. The reaction vessel  1403  may be ceramic-lined to mitigate the effects of metal dusting corrosion. The reactions that occur within the reaction vessel  1403  may be Bosch-type reactions at conditions that produce selected morphologies of solid carbon, such as those disclosed in U.S. Patent Application Publication No. 2012/0034150. 
     The tail gas stream  1404  flows to the product separation and packaging subsystem  1500 , as shown in  FIG. 15 . The tail gas stream  1404  enters a cyclone  1501 , which separates the tail gas stream  1404  into a products stream  1502  and the clean tail gas stream  1503 . The cyclone  1501  and flow lines connecting the reaction vessel  1403  to the cyclone  1501  may be ceramic-lined to limit or prevent metal dusting corrosion. 
     The products stream  1502  includes particles smaller than a selected size. For example, the cyclone  1501  may be configured to separate particles smaller than about 12 microns into the products stream  1502 . The size of particles separated into the products stream  1502  depends on operating conditions and particle properties (e.g., shape and density). In some embodiments, the cyclone  1501  may be configured to separate various morphologies of carbon formed in the reaction vessel  1403 . In some embodiments, the heat exchanger  1401  ( FIG. 14 ) may be within the product separation and packaging subsystem  1500 , such that the clean tail gas stream  1503  flows through the heat exchanger  1401 . 
     The products stream  1502  passes through a cooling apparatus  1504 . Some residual gases  1505 , such as gases containing carbon oxides and potentially explosive reducing agents (e.g., hydrogen, methane, hydrocarbon gases), may pass through a filter  1506  and vent to a safe location (e.g., at the top of an exhaust stack). 
     After cooling, the products stream  1502  may be packaged in a shipping container  1507 . In the process of packaging, residual gases  1508  may pass through a pump  1509  and a filter  1510  before venting. After the products are appropriately packaged for storage and shipping, the shipping containers  1507  may be moved to a shipping area  1511 , such as a loading dock. 
     The water treatment subsystem  1600 , shown in  FIG. 16 , receives the condensed water  1304  from the gas handling subsystem  1300  (see  FIG. 13 ). The condensed water  1304  may pass through one or more filters  1601  to form a purified water stream  1602 . The purified water stream  1602  may pass through one or more cooling devices  1603  (e.g., a cooling tower) en route to storage, municipal waste, or safe run-off. 
     The venting subsystem  1700 , shown in  FIG. 17  may include at least one vent  1701  in a safe location (e.g., at the top of an exhaust stack). For example, the vent  1701  may be configured to receive the vent gases  1305  from the gas handling subsystem  1300  ( FIG. 13 ) and/or the residual gases  1505 ,  1508  from the product separation and packaging subsystem  1500  ( FIG. 15 ). 
     EXAMPLE 3 
       FIG. 18  depicts another system  1800  for producing solid carbon products of various morphologies. The system  1800  includes a gas supply subsystem  1900 , a gas handling subsystem  2000 , a reactor subsystem  2100 , a product separation subsystem  2200 , a water separation subsystem  2300 , a water treatment subsystem  2400 , a product packaging subsystem  2500 , and a utilities subsystem  2600  (each shown in more detail in  FIGS. 19 through 26 ). In  FIGS. 18 through 26 , solid arrows indicate the flow of materials (gas, liquid, etc.). Information also flows within and between various subsystems. The subsystems may be modified or combined, or may be added to with additional subsystems as fitting desired production processes. The subsystems may be positioned to improve operating economics and to make efficient use of space in a process facility. The subsystems may also include various instruments or equipment, such as valves, transducers, flow meters, switches, controllers, computers, etc. The gases referenced in this description are for example only and may be substituted for, and/or added to other gases. The assignment of specific mechanics to specific subsystems within the system is also for example only. 
     As depicted in  FIG. 19 , a supply of liquefied carbon oxide, for example carbon dioxide (CO 2 ) or carbon monoxide (CO) is provided from a carbon oxide source  1901  (e.g., a tank, pipeline, etc.) within the gas supply subsystem  1900 . When conditions such as pressure and temperature are appropriate, the carbon oxide passes through a vaporizer  1902 , which evaporates the carbon oxide to form carbon oxide gas. The carbon oxide gas is split into three portions  1906 ,  1907 ,  1913 . A reducing gas source  1903  (e.g., a tank, pipeline, etc.) provides reducing gas stream  1904 , such as methane (CH 4 ) of any selected purity. The carbon oxide and the reducing gas stream  1904  typically flow from cryogenic storage tanks and may pass through heaters to vaporize the liquids. The reducing gas stream  1904  may pass through at least one compressor or process gas blower  1905  to achieve a selected pressure. The reducing gas stream  1904  and one portion  1907  of the carbon oxide gas are mixed and heated in a heater  1909  to form a combined feed gas stream  1910 . The combined feed gas stream  1910  and one portion  1906  of the carbon oxide gas flow to the gas handling subsystem  2000  (see  FIG. 20 ), and the other portion  1913  of the carbon oxide gas flows to the reactor subsystem  2100  (see  FIG. 21 ). One or more valves  1911 ,  1912  may control the flow of gases. 
     In the gas handling subsystem  2000 , shown in  FIG. 20 , the combined feed gas stream  1910  is mixed with a recycle stream  2309  containing carbon oxide gas supplied by the water separation subsystem  2300  (see  FIG. 23 ). The gases may be combined before or within a first tank  2001 , before being compressed in a compressor or process gas blower  2002  to a suitable pressure to accommodate the pressure drop through the overall reactor system that the gas handling subsystem  2000  is designed to support. The compressed gas is transferred to a second tank  2003  for holding or further processing. The compressed gas mixes with the portion  1906  of the carbon oxide gas from the gas supply subsystem  1900  ( FIG. 19 ) to form a combined feed gas stream  2004  under conditions such as temperature, pressure, concentration, and density selected for particular reactions. The conditions typically depend on the desired end product. The combined gas feed stream  2004  is analyzed by at least one analyzer  2005  to obtain information to control other components of the system  1800 . The combined gas feed stream  2004  flows to the reactor subsystem  2100  ( FIG. 21 ). 
     As shown in  FIG. 21 , the combined gas feed stream  2004  flows to the reactor subsystem  2100  and to at least one heat exchanger  2101  therein. The heat exchanger  2101  is typically a cross-flow heat exchanger configured for gas-gas interchange, and the combined gas feed stream  2004  receives heat from a warmer clean tail gas stream  2203  from a cyclone  2201  within the product separation subsystem  2200  (see  FIG. 22 ). Multiple heat exchangers may be used to heat the combined gas feed stream; for example, two heat exchangers  2101  are shown in  FIG. 21 . The heat exchangers  2101  typically include materials selected for the process temperatures and to be resistant to metal dusting corrosion. In some embodiments, a small amount of water is added to one or more gas flow streams before the first heat exchanger  2101  to cause a fine deposit of metal oxide on surfaces of the heat exchangers  2101  to reduce or prevent metal dusting. The heat exchangers  2101  cool the clean tail gas stream  2203  to form a cooled clean tail gas stream  2105 , which flows to the water separation subsystem  2300  (see  FIG. 23 ). 
     The combined gas feed stream  2004  flows to at least one heater  2102  (e.g., an electric heater) to bring the reactants up to reaction conditions. A portion of the carbon oxide gas  1913  from the gas supply subsystem  1900  ( FIG. 19 ) is mixed with the combined gas feed stream  2004  before the combined gas feed stream  2004  flows to a reactor  2103 . The heater  2102  receives reaction feedback information from the reactor  2103  via one or more sensors. The reactor  2103  may be lined with a ceramic material to mitigate metal dusting corrosion therein. The reactions that occur within the reactor  2103  may be Bosch-type reactions at conditions that produce selected morphologies of solid carbon, such as those disclosed in U.S. Patent Application Publication No. 2012/0034150. 
     In the reactor  2103 , the combined gas feed stream  2004  contacts catalyst material on a conveyor belt  2106 . The reactor  2103  has an input to receive catalyst material from at least one hopper  2107  in communication with the reactor  2103  and an output to release catalyst material into a tank  2108 . The conveyor belt  2106  moves catalyst material within the reactor  2013 . Residual gases may flow from the tank  2108  back to the reactor  2103 , or may form a vent gas  2109 . The hopper  2107  may also release a vent gas  2110 . 
     A reactor tail gas stream  2104  flows from the reactor  2103  to a cyclone  2201  within the product separation subsystem  2200  ( FIG. 22 ). The cyclone  2201  and flow lines connecting the reactor  2103  to the cyclone  2201  may be ceramic-lined to limit or prevent metal dusting corrosion. The cyclone  2201  separates the reactor tail gas stream  2104  into a products stream  2202 , such as CNTs, and the clean tail gas stream  2203 . The products stream  2202  includes particles smaller than a selected size. For example, the cyclone  2201  may be configured to separate particles smaller than about 12 microns into the products stream  2202 . The size of particles separated into the products stream  2202  depends on operating conditions and particle properties (e.g., shape and density). 
     The clean tail gas stream  2105  flows to the water separation subsystem  2300  ( FIG. 23 ), to another heat exchanger  2301  before entering a condenser  2302  (e.g., a one-stage water-cooled condenser). The clean tail gas stream  2105  passes through at least one filter  2303 , where liquid water may be diverted into a holding tank  2304 . After the clean tail gas stream  2105  passes through the filter  2303 , a portion  2305  of the clean tail gas stream  2105  may be safely vented, and the remainder of the clean tail gas stream  2105  flows into a second condenser  2306 . 
     The second condenser  2306  may be a two-stage condenser, in which a first stage is water-cooled and a second stage is glycol-cooled. After passing through the two-stage condenser  2306 , condensed water may be held in a holding tank  2307  for a period of time before mixing with the output of the holding tank  2304 . The condensed water  2308  flows to the water treatment subsystem  2400  (see  FIG. 24 ) for further processing. As water is collected in the holding tank  2307 , recycled gases  2309  flow back to the heat exchanger  2301 , and finally to the tank  2001  within the gas handling subsystem  2000  ( FIG. 20 ). The condensers  2302 ,  2306  in the water separation subsystem  2300  may be supplied by the utilities subsystem  2600  (see  FIG. 26 ). In such embodiments, cooling water  2603 ,  2604  may flow to the first condenser  2302  and a first set of coils of the second condenser  2306 , respectively. The cooling water returns  2310 ,  2311  flow back to the utilities subsystem  2600 . Chilled glycol  2607  flows to a second set of coils of the second condenser  2306 , and glycol return  2312  flows back to the utilities subsystem  2600 . 
     Within the water treatment subsystem  2400  ( FIG. 24 ), a tank  2401  accumulates the condensed water  2308  from the water separation subsystem  2300 . The tank  2401  may systematically release water to a pump  2402 , such as through a port in or near the bottom of the tank  2401 . The pump  2402  directs the condensed water  2308  through one or more filters  2403  to form a purified water stream  2404 . The purified water stream  2404  then passes to storage, municipal waste, or a safe run-off. 
     The product packaging subsystem  2500 , shown in  FIG. 25 , transfers the products stream  2202  to a shipping package  2501 . In the process of packaging, the products stream  2202  may flow to a hopper  2502 , through a valve  2503 , onto a conveyor belt  2504 , and into the shipping package  2501 . Residual gases  2505  may pass through a pump  2506  and filter  2507  before venting. After the products are packaged, shipping packages  2501  may be moved to a shipping area  2508 . 
     The utilities subsystem  2600 , shown in  FIG. 26 , may interact with and support other subsystems of the system  1800 . For example, the utilities subsystem  2600  may include a cooling tower  2601  configured to receive cooling water from the water separation subsystem  2300  ( FIG. 23 ). Heat is removed from the cooling water returns  2310 ,  2311  in the cooling tower  2601 . The cooling water  2603 ,  2604  is recycled via a pump  2602  back to the water separation subsystem  2300 . A chiller  2605  receives the glycol return  2312  from the water separation subsystem  2300 . A glycol pump  2606  recycles the chilled glycol  2607  to the water separation subsystem  2300 . The utilities subsystem  2600  may also include at least one vent  2608  in a safe location. For example, the vent  2608  may be configured to receive the residual gases  2205  from the product separation subsystem  2200 , the portion of the tail gas stream  2305  from the water separation subsystem  2300 , and/or the residual gases  2505  from the product packaging subsystem  2500 . The utilities subsystem  2600  may also include other operations, sensors, or controls as necessary for the system  1800  to function.