WO2013090274A1

WO2013090274A1 - Methods and system for forming carbon nanotubes

Info

Publication number: WO2013090274A1
Application number: PCT/US2012/068980
Authority: WO
Inventors: Robert D. Denton; Dallas Noyes
Original assignee: Exxonmobil Upstream Research Company; Solid Carbon Products Llc
Priority date: 2011-12-12
Filing date: 2012-12-11
Publication date: 2013-06-20
Also published as: AR089097A1; TW201341609A

Abstract

Systems and a method for forming carbon nanotubes are described. A method includes forming carbon nanotubes in a first reactor, using a feed gas. The carbon nanotubes are separated from a reactor effluent to form a waste stream. The feed gas, a dry waste stream, or both, are heated with waste heat from the waste stream. The waste stream is chilled in an ambient temperature heat exchanger to condense water vapor, forming a dry waste stream.

Description

METHODS AND SYSTEM FOR FORMING CARBON NANOTUBES

RELATED APPLICATIONS

[0001] This application claims priority to United States Provisional Patent Application Serial No. 61 /569,494 by Denton and Noyes and titled "Methods and System for Forming Carbon Nanotubes," which was filed 12 December 201 1 .

FIELD

[0002] The present techniques relate to an industrial scale process for forming carbon fibers and carbon nanomaterials.

BACKGROUND

[0003] This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.

[0004] Materials formed predominately of solid or elemental carbon have been used in numerous products for many years. For example, carbon black is a high carbon content material used as a pigment and reinforcing compound in rubber and plastic products, such as car tires. Carbon black is usually formed by the incomplete thermal pyrolysis of hydrocarbons, such as methane or heavy aromatic oils. Thermal blacks, formed by the pyrolysis of natural gas, include large unagglomerated particles, for example, in the range of 200-500 nm in size, among others. Furnace blacks, formed by the pyrolysis of heavy oils, include much smaller particles, in the range of 10-100 nm in size, that agglomerate or stick together to form structures. In both cases, the particles may be formed from layers of graphene sheets that have open ends or edges. Chemically, the open edges form reactive areas that can be used for absorption, bonding into matrices, and the like.

[0005] More recent forms of elemental carbon, such as fullerenes, have been developed, and are starting to be developed in commercial applications. In contrast to the more open structures of carbon black, fullerenes are formed from carbon in a closed graphene structure, i.e., in which the edges are bonded to other edges to form spheres, tubes, and the like. Two structures, carbon nanofibers and carbon nanotubes, have numerous potential applications, ranging from batteries and electronics to the use in concrete in the construction industry. Carbon nanomaterials may have a single wall of graphene or multiple nested walls of graphene or form a fiber structure from a stacked set of sheets in a cup or plate form. The ends of the carbon nanotubes are often capped with hemispherical structures, in a fullerene-like configuration. Unlike for carbon black, large scale production processes have not been implemented for carbon nanomaterials. However, research has been conducted on a number of proposed production processes.

[0006] Arc-based, laser-based ablation techniques and chemical vapor deposition have classically been used to generate carbon nanotubes from a carbon surface. For example, techniques for generating carbon nanotubes are reviewed in Karthikeyan, et al., "Large Scale Synthesis of Carbon Nanotubes," E-Journal of Chemistry, 2009, 6(1 ), 1 -12. In one technique described, an electric arc is used to vaporize graphite from electrodes in the presence of metal catalysts, achieving production rates of about 1 gram/min. Another technique described uses laser ablation to vaporize carbon from a target electrode in an inert gas stream. However, the laser technique uses high purity graphite and high power lasers, but provides a low yield of carbon nanotubes, making it impractical for large scale synthesis. A third technique described by the authors, is based on chemical vapor deposition (CVD), in which a hydrocarbon is thermally decomposed in the presence of a catalyst. In some studies, these techniques have achieved production rates of up to a few kilograms/hour at a 70 % purity level. However, none of the processes described are practical for large scale commercial production.

[0007] Hydrocarbon pyrolysis is used in the production of carbon black and various carbon nanotube and fullerene products. Various methods exist for creating and harvesting various forms of solid carbon through the pyrolysis of hydrocarbons using temperature, pressure, and the presence of a catalyst to govern the resulting solid carbon morphology. For example, Kauffman et al. (US patent 2,796,331 ) discloses a process for making fibrous carbon of various forms from hydrocarbons in the presence of surplus hydrogen using hydrogen sulfide as a catalyst, and methods for collecting the fibrous carbon on solid surfaces. Kauffman also claims the use of coke oven gas as the hydrocarbon source.

[0008] In another study, a flame based technique is described in Vander Wal, R.L., et al., "Flame Synthesis of Single-Walled Carbon Nanotubes and Nanofibers," Seventh International Workshop on Microgravity Combustion and Chemically Reacting Systems, Aug. 2003, 73-76 (NASA Research Publication: NASA/CP— 2003-212376/REV1 ). The technique used the introduction of a CO or CO/C₂H₂ mixture into a flame along with a catalyst to form the carbon nanotubes. The authors noted the high productivity that could be achieved using flame based techniques for the production of carbon black. However, the authors noted that scaling the flame synthesis presented numerous challenges. Specifically, the total time for catalyst particle formation, inception of the carbon nanotubes, and growth of the carbon nanotubes was limited to about 100 ms.

[0009] International Patent Application Publication WO/2010/120581 , by Noyes, discloses a method for the production of various morphologies of solid carbon product by reducing carbon oxides with a reducing agent in the presence of a catalyst. The carbon oxides are typically either carbon monoxide or carbon dioxide. The reducing agent is typically either a hydrocarbon gas or hydrogen. The desired morphology of the solid carbon product may be controlled by the specific catalysts, reaction conditions and optional additives used in the reduction reaction. The process is conducted at a low pressure and uses a cryogenic chilling process to remove water from a feed stream.

[0010] While all of the techniques described can be used to form carbon nanotubes, none of the processes provide a practical method for bulk or industrial scale production. Specifically, the amounts formed and the process efficiencies are both low.

SUMMARY

[0011] An embodiment discussed herein provides a system for the production of carbon nanotubes. The system includes a feed gas heater configured to heat a feed gas with waste heat from a waste gas stream and a reactor configured to form carbon nanotubes from the feed gas. A separator is configured to separate the carbon nanotubes from the reactor effluent stream forming the waste gas stream. The system includes a water removal system that has an ambient temperature heat exchanger and a separator configured to separate the bulk of the water from the waste gas stream to form a dry waste gas stream.

[0012] Another embodiment provides a method for forming carbon nanotubes. The method includes forming carbon nanotubes in a first reactor, using a feed gas, and separating the carbon nanotubes from a reactor effluent to form a waste stream. The feed gas, a dry waste gas stream, or both, are heated with waste heat from the waste stream. The waste stream is chilled in an ambient temperature heat exchanger to condense water vapor, forming the dry waste gas stream.

[0013] Another embodiment provides a reaction system for forming carbon nanotubes. The reaction system includes two or more reactors configured to form carbon nanotubes from gas streams including methane and carbon dioxide. In the reaction system, an effluent from each reactor, before a final reactor, is used as a feed stream for a downstream reactor, and an effluent stream from the final reactor comprises a reactant depleted waste stream. A separation system is located

downstream of each reactor, wherein each separation system is configured to remove carbon nanotubes from the effluent from the reactor. A feed heater is located downstream of each separation system, wherein each feed heater comprises a heat exchanger configured to heat a feed gas stream for a following reactor using waste heat from the effluent from the reactor, and wherein a feed heater downstream of the final reactor is configured to heat a gas stream for the first reactor. An ambient temperature heat exchanger is located downstream of each feed heater, wherein each ambient temperature heat exchanger is configured to remove water from the effluent, forming the feed stream for the following reactor. A compressor is configured to increase the pressure of the reactant depleted waste stream. An ambient temperature heat exchanger, located downstream of the compressor, is configured to remove water from the reactant depleted waste stream. A gas fractionation system is configured to separate the reactant depleted waste stream into a methane enriched stream and a carbon dioxide enriched stream. A mixer is configured to blend the methane enriched stream or the carbon dioxide enriched stream into an initial feed stream.

DESCRIPTION OF THE DRAWINGS

[0014] The advantages of the present techniques are better understood by referring to the following detailed description and the attached drawings, in which:

[0015] Fig. 1 is a block diagram of a reaction system that generates carbon nanotubes, for example, as a by-product of a carbon dioxide sequestration reaction;

[0016] Fig. 2 is a C-H-O equilibrium diagram of the equilibria between carbon, hydrogen, and oxygen, indicating species in equilibrium at various temperature conditions;

[0017] Fig. 3 is a simplified process flow diagram of a reaction system for making carbon nanotubes from a gas feed that includes carbon dioxide and methane;

[0018] Figs. 4A, 4B, and 4C are a simplified process flow diagram of another reaction system for making carbon nanotubes from a gas feed that includes carbon dioxide and methane; [0019] Figs. 5A, 5B, and 5C are a simplified process flow diagram of another reaction system for making carbon nanotubes from a gas feed that includes carbon dioxide and methane;

[0020] Fig. 6 is a drawing of a fluidized bed reactor for forming carbon nanotubes;

[0021] Fig. 7 is a schematic of a catalytic reaction for the formation of carbon nanotubes on a catalyst bead;

[0022] Fig. 8 is a simplified process flow diagram of a gas fractionation process that can be used in a reactor system for the production of carbon nanotubes;

[0023] Fig. 9 is a simplified process flow diagram of another gas fractionation process that can be used in a reactor system for the production of carbon nanotubes;

[0024] Fig. 10 is a simplified process flow diagram of a separation system that can separate carbon nanotubes from a reactor effluent stream; and

[0025] Fig. 1 1 is a method for generating carbon nanotubes from a feed gas that includes methane and carbon dioxide.

DETAILED DESCRIPTION

[0026] In the following detailed description section, specific embodiments of the present techniques are described. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present techniques, this is intended to be for exemplary purposes only and simply provides a description of the exemplary embodiments. Accordingly, the techniques are not limited to the specific embodiments described below, but rather, include all alternatives, modifications, and equivalents falling within the spirit and scope of the appended claims.

[0027] At the outset, for ease of reference, certain terms used in this application and their meanings as used in this context are set forth. To the extent a term used herein is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Further, the present techniques are not limited by the usage of the terms shown below, as all equivalents, synonyms, new developments, and terms or techniques that serve the same or a similar purpose are considered to be within the scope of the present claims.

[0028] Carbon fibers, nanofibers, and nanotubes are allotropes of carbon that have a cylindrical nanostructure. Carbon nanofibers and nanotubes are members of the fullerene structural family, which includes the spherical carbon balls termed "fullerene." The walls of the carbon nanotubes are formed from sheets of carbon in a graphene structure. As used herein, nanotubes may include single wall nanotubes and multiple wall nanotubes of any length. It can be understood that the term "carbon nanotubes" as used herein and in the claims, includes other fullerene allotropes of carbon, such as carbon fibers, carbon nanofibers, and other carbon nanostructures.

[0029] A "compressor" is a device for compressing a working gas, including gas- vapor mixtures or exhaust gases, and includes pumps, compressor turbines,

reciprocating compressors, piston compressors, rotary vane or screw compressors, and devices and combinations capable of compressing a working gas. In some

embodiments, a particular type of compressor, such as a compressor turbine, may be preferred. A piston compressor may be used herein to include a screw compressor, rotary vane compressor, and the like.

[0030] As used herein, a "plant" is an ensemble of physical equipment in which chemical or energy products are processed or transported. In its broadest sense, the term plant is applied to any equipment that may be used to produce energy or form a chemical product. Examples of facilities include polymerization plants, carbon black plants, natural gas plants, and power plants.

[0031] A "hydrocarbon" is an organic compound that primarily includes the elements hydrogen and carbon, although nitrogen, sulfur, oxygen, metals, or any number of other elements may be present in small amounts. As used herein, hydrocarbons generally refer to components found in natural gas, oil, or chemical processing facilities. [0032] As used herein, the term "natural gas" refers to a multi-component gas obtained from a crude oil well or from a subterranean gas-bearing formation. The composition and pressure of natural gas can vary significantly. A typical natural gas stream contains methane (CH₄) as a major component, i.e., greater than 50 mol % of the natural gas stream is methane. The natural gas stream can also contain ethane (C₂H₆), higher molecular weight hydrocarbons (e.g., C₃-C₂o hydrocarbons), one or more acid gases (e.g., hydrogen sulfide), or any combination thereof. The natural gas can also contain minor amounts of contaminants such as water, nitrogen, iron sulfide, wax, crude oil, or any combination thereof. The natural gas stream may be substantially purified prior to use in embodiments, so as to remove compounds that may act as poisons.

[0033] A "low-BTU natural gas" is a gas that includes a substantial proportion of C0₂ as harvested from a reservoir. For example, a low BTU natural gas may include 10 mol % or higher C0₂ in addition to hydrocarbons and other components. In some cases, the low BTU natural gas may include mostly C0₂.

[0034] "Substantial" when used in reference to a quantity or amount of a material, or a specific characteristic thereof, refers to an amount that is sufficient to provide an effect that the material or characteristic was intended to provide. The exact degree of deviation allowable may in some cases depend on the specific context.

Overview

[0035] Embodiments described herein provide systems and methods for making carbon fibers, nanofibers, and nanotubes (CNTs) on an industrial scale using

feedstocks that can include nearly stoichiometric mixtures of carbon dioxide and methane, among others. In some embodiments, the feedstocks are higher in CH₄, while in other embodiments, the feedstocks are higher in C0₂. The process is conducted under high temperature and pressure conditions using a Bosch reaction, as discussed with respect to Fig. 2. The process may be energy neutral to slightly endothermic. At least a portion of the heat from the reaction can be recovered and used to heat the feed gases, providing a portion of the heat used by the process during continuous operations. As a high pressure process is used, an ambient temperature heat exchanger is sufficient for the removal of water vapor from the product stream, without using cryogenic coolers. After separation of the product and water formed during the reaction, a gas fractionation system is used to separate any remaining amounts of the limiting reagent from a waste gas mixture and recycle this reagent to the process.

[0038] As used herein, an ambient temperature heat exchanger can include water chillers, air coolers, or any other cooling system that exchanges heat with a source that is at substantially ambient temperature. It can be understood that ambient temperature is substantially the temperature of the outside air at the location of the facility, e.g., ranging from about -40 ^QC to about +40 ^QC, depending on the location of the facility. Further, different types of ambient temperature heat exchangers may be used

depending on current ambient temperature. For example, a facility that uses water chillers in a summer season may use air coolers in a winter season. It can be

understood that an appropriate type may be used at any point describing an ambient temperature heat exchanger herein. The ambient temperature heat exchangers may vary in type across the plant depending on the amount of cooling needed.

[0037] Embodiments described herein can be used to produce industrial quantities of carbon products such as fullerenes, carbon nanotubes, carbon nanofibers, carbon fibers, graphite, carbon black, and graphene, among others, using carbon oxides as the primary carbon source. The balance of the possible products may be adjusted by the conditions used for the reaction, including catalyst compositions, temperatures, pressures, feedstocks, and the like. In a reactor system, the carbon oxides are catalytically converted to solid carbon and water. The carbon oxides may be obtained from numerous sources, including the atmosphere, combustion gases, process off- gases, well gas, and other natural and industrial sources. [0038] The present process uses two feedstocks, a carbon oxide, e.g., carbon dioxide (C0₂), and a reducing agent, e.g., methane (CH₄). The reducing agent may include other hydrocarbon gases, hydrogen (H₂), or mixtures thereof. A hydrocarbon gas can act as both an additional carbon source and as the reducing agent for the carbon oxides. Other gases, such as syngas, may be created as intermediate compounds in the process or be contained in the feed, and can also be used as the reducing agent. Syngas, or "synthetic gas," includes carbon monoxide (CO) and hydrogen (H₂) and, thus, includes both the carbon oxide and the reducing gas in a single mixture. Syngas may be used as all or portion of the feed gas.

[0039] Carbon oxides, particularly carbon dioxide, are abundant gases that may be extracted from exhaust gases, low-BTU well gas, and from some process off-gases. Although carbon dioxide may also be extracted from the air, other sources often have much higher concentrations and are more economical sources from which to harvest the carbon dioxide. Further, carbon dioxide is available as a by-product of power generation. The use of CO₂ from these sources may lower the emission of carbon dioxide by converting a portion of the CO₂ into carbon products.

[0040] The systems described herein may be incorporated into power production and industrial processes for the sequestration of carbon oxides, allowing their conversion to solid carbon products. For example, the carbon oxides in the combustion or process off-gases may be separated and concentrated to become a feedstock for this process. In some cases these methods may be incorporated directly into the process flow without separation and concentration, for example as an intermediate step in a multi-stage gas turbine power station.

[0041] Fig. 1 is a block diagram of a reaction system 100 that generates carbon structures, for example, as a by-product of a carbon dioxide sequestration reaction. The reaction system 100 is provided a feed gas 102, which is a mixture of CO₂ and CH₄. In some embodiments, the reaction may allow for sequestration of CO₂ from exhaust streams of power plants and the like. In other embodiments, the CH₄ is at a higher concentration, for example, in a low-BTU gas stream from a natural gas field. Other components may be present in the feed gas 102, such as C₂H₆, C₂H₄, and the like. In an embodiment, the feed gas 102 has been treated to remove these

components, for example, for sale as product streams.

[0042] The feed gas 102 is passed through a heat exchanger 104 to be heated for reaction. During continuous operation, the heating is performed using heat 106

recovered from the reaction. During start-up, an auxiliary heater is used to provide the initial heat as described further below. The heated feed gas 108 is fed to a reactor 110.

[0043] In the reactor 110, a catalyst reacts with a portion of the heated feed gas 108 to form carbon nanotubes 112. As described in more detail below, the reactor 110 can be a fluidized bed reactor that uses any number of different catalysts, including, for example, metal shot, supported catalysts, and the like. The carbon nanotubes 112 are separated from the flow stream 114 out of the reactor 110, leaving a waste gas stream 116 containing excess reagents and water vapor. The heat from the flow stream 114 is used to form the heated feed gas 108 prior to the flow stream 114 entering the chiller as waste gas stream 116.

[0044] The waste gas stream 116 is passed through an ambient temperature heat exchanger, such as water chiller 118, which condenses out the water 120. The resulting dry waste gas stream 122 is used as a feed stream for a gas fractionation system 124. It can be understood that a dry waste gas stream, as used herein, has the bulk of the water removed, but may still have small amounts of water vapor. For example, the dew point of a dry waste gas stream may be greater than about 10 ^QC, greater than about 20 ^QC, or higher. A dryer may be used to lower the dewpoint, for example, to -50 ^QC or lower, prior to gas fractionation.

[0045] The gas fractionation system 124 removes a portion of the lowest

concentration reagent and recycles it to the process, for example, by blending the recycle 126 with the feed gas 102. The higher concentration gas, or excess feed 128, can be disposed, for example, by sales to downstream users. As an example, if CO₂ is the highest concentration gas in a blend with CH₄, the gas fractionation system 124 can be used to remove any CH₄ remaining in the waste gas stream, and send it back into the process as recycle 126. The process functions as an equilibrium reaction between the reagents and solid carbon, as discussed further with respect to Fig. 2.

[0046] Fig. 2 is a C-H-0 equilibrium diagram 200 of the equilibria between carbon 202, hydrogen 204, and oxygen 206, indicating species in equilibrium at various temperature conditions. There is a spectrum of reactions involving these three elements in which various equilibria have been named as reactions. The equilibrium lines at various temperatures that traverse the diagram show the approximate regions in which solid carbon will form. For each temperature, solid carbon will form in the regions above the associated equilibrium line, but will not form in the regions below the equilibrium line.

[0047] Hydrocarbon pyrolysis is an equilibrium reaction between hydrogen and carbon that favors solid carbon production, typically with little or no oxygen or water present, e.g., along the equilibrium line 208 from higher hydrogen 204 content to higher carbon 202 content. The Boudouard reaction, also called the carbon monoxide disproportionation reaction, is an equilibrium reaction between carbon and oxygen that favors solid carbon production, typically with little or no hydrogen or water present, and is along the equilibrium line 210, from higher oxygen 206 content to higher carbon 202 content.

[0048] The Bosch reaction is an equilibrium reaction where all of carbon, oxygen, and hydrogen are present that favors solid carbon production. In the C-H-0 equilibrium diagram 200, the Bosch reactions are located in the interior region of the triangle where equilibrium is established between solid carbon and reagents containing carbon, hydrogen, and oxygen in various combinations. Numerous points in the Bosch reaction region favor the formation of CNTs and several other forms of solid carbon product. The reaction rates and products may be enhanced by the use of a catalyst such as iron. The selection of the catalysts, reaction gases, and reaction conditions may provide for the control of the type of carbon formed. Thus, these methods open new routes to the production of solid carbon products such as CNTs.

Reaction Systems

[0049] Fig. 3 is a simplified process flow diagram of a reaction system 300 for making carbon products from a gas feed that includes carbon dioxide and methane. As shown this reaction system 300 can be used for feed gas 302 that is higher in C0₂ or higher in CH₄. More specific reactor systems are discussed with respect to Fig. 4 for a higher C0₂ content feed gas and Fig. 5 for a higher CH₄ content feed gas. In the reaction system 300, the feed gas 302 is combined with a recycle gas 304 that has an enhanced concentration of the lesser gas. This may be done using a static mixer 306.

[0050] The combined gas stream 308 is passed through a heat exchanger or set of heat exchangers in series 310 to be heated by a reactor effluent stream. The temperature can be raised from about 90 ^QF (about 32.2 ^QC) to about 1400 ^QF (about 760 ^QC) for the heated gas stream 312. This temperature is likely to be sufficient for maintaining the reaction during continuous operations. During start-up, all or part of the heat may be provided by a package heater 314. The hot gas stream 316 is then introduced into a first fluidized bed reactor 318. A general fluidized bed reactor that may be used in embodiments is discussed with respect to Fig. 6. In the first fluidized bed reactor 318, carbon nanotubes are formed on catalyst particles. The catalyst particles and reactions are further discussed with respect to Fig. 7.

[0051] The carbon nanotubes are carried from the first fluidized bed reactor 318 in a reactor effluent stream 320. The reactor effluent stream 320 may be at a temperature of about 1650 ^QF (about 899 ^QC) and may be cooled, for example, providing some or all of the heat used to heat the reactants. Either prior to or after cooling, the reactor effluent stream 320 is passed through a separation device, such as a first lock hopper 322, to remove the carbon nanotubes. The resulting waste gas stream 324 is used to provide heat in a heat exchanger 326. The carbon may also be removed in secondary separation devices (not shown) at lower temperatures than the waste gas stream 324. This is particularly easy to do where multiple heat exchangers in parallel may be used to cool the waste gas stream 324 while heating the feed gas to the next reactor 336.

Normally, all of the carbon solids will be removed by separation device(s) prior to the condensation of any of the water vapor present in the waste gas stream 324. The cooled waste gas stream 328 is then passed through an ambient temperature heat exchanger 330, which further cools the cooled waste gas stream 328 and results in the bulk of the water formed condensing as a liquid, which is then fed to a separation vessel 332.

Water 334 is removed from the separation vessel, and a reactant stream 336 exits the top of the first separation vessel 332 at about 100 ^QF (about 38 ^QC).

[0052] The reactant stream 336 passes through the heat exchanger 326 and is heated by waste heat from the waste gas stream 324. The heated stream 338 is the fed to a second fluidized bed reactor 340 in which additional carbon nanotubes are formed. However, the heated stream 338 may not be at a sufficiently high temperature, e.g., greater than about 1600 ^QF (about 871 ^QC), to form carbon nanotubes in the second fluidized bed reactor 340. To increase the temperature of the heated stream 338, a second package heater 341 may be used. In some embodiments, a second reactor effluent stream 342 is used to provide heat to the second reactant stream 336. The second reactor effluent stream 342 is then fed to a second lock hopper(s) 344 to separate carbon products from the second reactor effluent stream 342. The resulting waste gas stream 346 is used to provide heat to the combined gas stream 308 as it passes through the heat exchanger 310.

[0053] Although only two fluidized bed reactors 318 and 340 are shown, the reaction system 300 may contain more reactors if desired. The determination of the number of reactors is based on the concentration of the feedstocks and the desired remaining amount of each feedstock. In some circumstances, three, four, or more reactors may be used in sequence, in which an effluent stream from each reactor provides heat to a feed gas for the next reactor in the sequence. Further, the reactors do not have to be fluidized bed reactors, as other configurations may be used in embodiments. For example, a fixed bed reactor, a tubular reactor, a continuous feed reactor, or any number of other configurations may be used.

[0054] After providing heat to the combined gas stream 308, the cooled waste stream 348 is passed through an ambient temperature heat exchanger 350 and then fed to a separation vessel 352. Water 354 settles in the separation vessel 352 and is removed from the bottom. The resulting gas stream 356 is at around 100 ^QF (about 38 ^QC) and at a pressure of about 540 psia (about 3,720 kPa). In one embodiment the gas is then dried to a low dew point in a drier (not shown). The stream enters a compressor 358 that increases the pressure of the gas stream 356 to about 1050 psia (about 7,240 kPa) forming a high pressure stream 360 which is passed through another ambient temperature heat exchanger 362. From the ambient temperature heat exchanger 362, the high pressure stream 360 is fed to a separation vessel 364 for removal of any remaining water, if a drier has not been used.

[0055] The dried gas stream 366 is then sent to a gas fractionation system 368, which separates the excess feed 370 from the recycle gas 304. In reaction systems 300 based on a proportionate excess of C0₂, the excess feed 370 may primarily include C0₂ and the recycle gas may primarily include CH₄. In reaction systems 300 based on a proportionate excess of CH₄, the excess feed 370 may primarily include C0₂ and the recycle gas may primarily include CH₄. In some embodiments, a portion of the excess feed 370, the recycle gas 304, or both may be tapped to provide a fuel gas stream, a purge gas stream, or both for use in the plant.

[0056] The reaction conditions used can cause significant degradation of metal surfaces, as indicated by choice of the catalyst itself, which may include stainless steel beads. Accordingly, the process may be designed to decrease the amount of metal exposed to the process conditions, as discussed further with respect to the following figures.

[0057] Figs. 4A, 4B, and 4C are a simplified process flow diagram of another reaction system 400 for making carbon nanotubes from a gas feed that includes carbon dioxide and methane. In Fig. 4, like number items are as described with respect to Fig. 3. The numbered diamonds in the process correspond to simulated process values, as provided in Table 1 for a higher C0₂ content feed gas 402. A second set of simulated values is provided in Table 2. As shown in the second simulation, some results indicate that the overall process may be slightly endothermic under some conditions. In this case, the extra heat provided by the second package heater 341 may be useful for enhancing carbon nanotube production while lowering the production of other products, such as amorphous carbon. As for Fig. 3, the feed gas 402 passes through a static mixer 306 where it is combined with a high methane recycle gas 404. The combined gas stream 308 is passed through a heat exchanger 310, for example, including multiple shell and tube heat exchangers 406. The main difference between the more detailed process flow diagram of Fig. 4 and that of Fig. 3 is the use of heat exchangers to cool the reactor effluent streams 320 and 342 prior to separating the CNTs from the reactor effluent streams 320 and 342.

[0058] In this embodiment, the heated gas stream 312 is raised to a temperature of about 800 ^QF (about 427 ^QC) in the heat exchanger 310 prior to flowing through a second heat exchanger 408. In the second heat exchanger 408, the heated gas stream 312 flows through a first ceramic block heat exchanger 410, as indicated by arrows 412. Heat stored in the first ceramic block heat exchanger 410 is exchanged to the heated gas stream 312 and may increase the temperature to about 1540 ^QF (838 ^QC).

[0059] While the first ceramic block heat exchanger 410 is used to heat the heated gas stream 312, a second ceramic block heater 414 is used to cool the second reactor effluent stream 342 by flowing this stream through the second ceramic block heater 414, as indicated by arrows 416. When the second ceramic block heat exchanger 414 reaches a selected temperature, or the first ceramic block heat exchanger 410 drops to a selected temperature, the positions of the inlet valves 418 and outlet valves 420 are changed. In other words, open valves are closed and closed valves are opened. The change in the positions of the valves changes which ceramic block heat exchanger 410 or 414 is being heated by the flow from the reactor 340 and which ceramic block heat exchanger 414 or 410 is used to heat the heated gas stream 312. After flowing through the ceramic block heat exchanger 410 or 414, the flow is as described with respect to Fig. 3.

[0060] The second heat exchanger 326 may also include shell-and-tube heat exchangers 422 that, in this case, raise the temperature of the second reactant stream 336 from about 100 ^QF (about 37.8 ^QC), at point 1 1 , to about 715 ^QF (about 379.4 ^QC) at point 12. The second reactant stream 336 is then passed through another heat exchanger 424, which includes two ceramic block heat exchangers 426. These ceramic block heat exchangers 426 are configured to have an exchanged flow, as shown for the second heat exchanger 408 discussed above. Other portions of the system 400 are similar to that described with respect to Fig. 3, although the process values can differ and the relevant process values for the system are shown in Table 1 or Table 2 for the additional simulation. Further, more than two reactor systems may also be used in this embodiment.

[0061] In this embodiment, the separation systems 426 for the CNTs include cyclonic separators 428, lock hoppers 430, and filters 432. After the majority of the CNTs are removed by the cyclonic separators 428 and deposited into the lock hoppers 430, the filters 432 are used to remove remaining CNTs from the waste gas streams 324 and 346. This may help to prevent plugging, or other problems, caused by residual CNTs in the waste gas streams 324 and 346. The filters 432 can include bag filters, among other types. The CNT separation systems 426 are discussed in further detail with respect to Fig. 10. TABLE 1 : Process values for Higher Carbon Dioxide Feed

TABLE 2: Process values for Higher Carbon Dioxide Feed

[0064] After the removal of a final aliquot of water from the high pressure stream 360 in the third separation vessel 364, the dried gas stream 366 is sent to a gas

fractionation system 434, which can remove a high methane recycle gas 404 from a C0₂ waste stream 436. The gas fractionation system 434 is discussed further with respect to Fig. 8. The individual streams 404 and 436 can be used to supply other gases for the process. For example, a fuel gas stream 438 may be removed from the high methane recycle gas 404 and used for powering turbines, boilers, or other equipment, for example, to provide power to the system 400. Further, a purge gas stream 440 may be removed from the C0₂ waste stream 436. The purge gas stream 440 may be used for cooling and purging the CNTs, as described with respect to Fig. 10. The purge gas may also be used for various cleaning functions in the plant, such as blowing residual CNTs out of a ceramic heat exchanger 410, 414, or 426 when flow is reversed.

[0065] The process conditions shown in Tables 1 and 2 are merely intended to be examples of conditions that may be found in a plant, as determined by simulations. The actual conditions may be significantly different and may vary significantly from the conditions shown. A similar plant configuration may be used for a high methane feed gas, as discussed with respect to Fig. 5. Further, the recycle and effluent waste streams can contain substantial quantities of hydrogen and carbon monoxide, e.g., greater than about 5 mol % each, 10 mol % each, or even 20 mol % each. These components will generally be present in the feed and all non-C0₂ product streams, i.e., the recycle methane will always contain some CO and H₂.

[0066] Figs. 5A, 5B, and 5C are a simplified process flow diagram of another reaction system 500 for making carbon nanotubes from a gas feed that includes carbon dioxide and methane. In Fig. 5, like number items are as described with respect to Figs. 3 and 4. Further, some numbers are not shown in order to simplify the drawing. In this embodiment, the feed gas may be higher in methane than in carbon dioxide, for example, at around 80 mol % CH₄ and 20 mol % CO₂, although any ratios may be used. Similarly, the recycle gas 504 will be higher in C0₂ than in CH₄, resulting in a reactor feed gas that may be around 51 mol % C0₂ and 49 mol % CH₄. The rest of the process will be similar to the system 400 discussed with respect to Fig. 4. However, since the CH₄ waste stream 506 may be commercially sold to energy markets, a gas fractionation system 508 that is configured to generate a much higher purity CH₄, e.g., about 99 mol % CH₄ or higher may be used. A gas fractionation system 508 that may be used is discussed further with respect to Fig. 9. As for the system 400 discussed with respect to Fig. 4, a purge gas stream 510 may be taken from the recycle gas 504 and a fuel gas stream 512 may be taken from the CH₄ waste stream 506. It will be understood that the waste streams are only with respect to the process. Both the CH₄ waste stream 506 of Fig. 5 and the C0₂ waste stream 436 of Fig. 4 can be commercially sold, for example, to a pipeline operator.

[0067] It can be understood that the systems for the formation of carbon nanotubes may include any number of reactors, of any number of types, including the fluidized bed reactors shown. In one embodiment, only a single reactor is used to form the carbon nanotubes.

Reactor Systems

[0068] Fig. 6 is a drawing of a fluidized bed reactor 600 for forming carbon

nanotubes 602. A hot gas feed stream 604 is fed through a line 606 into the bottom of the fluidized bed reactor 600. A control valve 608 may be used to regulate the flow of the hot gas feed stream 604 into the reactor. The hot gas feed stream 604 flows through a distributor plate 610 and will fluidize a bed of catalyst beads 612 held in place by the reactor walls 614. As used herein, "fluidize" means that the catalyst beads 612 will flow around each other to let gas bubbles through, providing a fluid-like flow behavior. As discussed herein, the reaction conditions are very harsh to any exposed metal surface, as the metal surface will perform as a catalyst for the reaction. Thus, the reaction will result in the slow degradation of an exposed metal surface. Accordingly, the interior surface of the reactor, including the reactor walls 614 and heads 615, as well as the distributor plate 610, and other parts, can be made of a ceramic material to protect the surfaces.

[0069] As the hot gas feed stream 604 flows through the fluidized bed of catalyst particles 612, CNTs 602 will form from catalyst beads 612. The flowing hot gas feed stream 604 carries the CNTs 602 into an overhead line 616 where they are removed from the reactor 600. Depending on the flow rate, for example, as adjusted by the control valve 608, some amount of catalyst beads 612, or particles fragmented from the catalyst beads 612, may be carried into the overhead line 616. Accordingly, a catalyst separator 618 may be used to separate catalyst beads 612, and larger particles, from a reactor effluent stream 620 and return them to the reactor 600 through a recycle line 622. Any number of configurations may be used for the catalyst separator 618,

including a cyclonic separator, a settling tank, a hopper, and the like. The reactions that take place in the fluidized bed are discussed in more detail in Fig. 7.

[0070] Fig. 7 is a schematic of a catalytic reaction 700 for the formation of carbon nanotubes on a catalyst bead 702. An initial reaction 704 between a portion of the CH₄ and the C0₂ in the hot gas feed stream 706 results in the formation of CO and H₂ in stoichiometric amounts. Excess amounts of the source gases 706 continue to flow through the reactor, helping to fluidize the bed and carrying away CNTs 708 and catalyst particles 710.

[0071] The reactions that form the CNTs 708 take place on the catalyst bead 702. The size of the CNTs 708, and the type of CNTs 708, e.g., single wall or multiwall CNTs 708, may be controlled by the size of the grains 712. In other words, a nucleus of iron atoms of sufficient size at the grain boundary forms the nucleating point for the growth of the carbon products on the catalyst bead 702. Generally, smaller grains 712 will result in fewer layers in the CNTs 708, and may be used to obtain single wall CNTs 708. Other parameters may be used to affect the morphology of the final product as well, including reaction temperature, pressure, and feed gas flow rates. [0072] The CO and H₂ react at grain boundaries 714, lifting active catalyst particles 716 off the catalyst bead 702, and forming H₂O 718 and the solid carbon of the CNTs 708. The CNTs 708 break off from the catalyst bead 702 and from the catalyst particle 710. Larger catalyst particles 710 can be captured and returned to the reactor, for example, by the catalyst separator 618 discussed with respect to Fig. 6, while very fine catalyst particles 710 will be carried out with the CNTs 708. The final product will include about 95 mol % solid carbon and about 5 mol % metal, such as iron. The CNTs 708 will often agglomerate to form clusters 720, which are the common form of the final product. Some amount of the CO and H₂ passes through the reactor without reacting and are contaminants in the reactor effluent streams.

[0073] As the reaction proceeds, the catalyst bead 702 is degraded and finally consumed. Accordingly, the reaction can be described as a metal dusting reaction. In some embodiments, metal surfaces are protected from attack by a ceramic lining, since the metal surfaces in contact with the reaction conditions would not only degrade, but may also result in the formation of poorer quality products.

[0074] The catalyst bead 702 can include any number of other metals, such as nickel, ruthenium, cobalt, molybedenum, and others. However, the catalytic sites on the catalyst beads 702 are principally composed of iron atoms. In one embodiment, the catalyst bead 702 includes metal shot, for example, about 25-50 mesh metal beads that are used for shot blasting. In one embodiment, the catalyst may be a stainless ball bearing, and the like.

Gas Fractionation Systems

[0075] Fig. 8 is a simplified process flow diagram of a gas fractionation process 800 that can be used in a reactor system for the production of carbon nanotubes. The gas fractionation system 800 is a bulk fractionation process that may be used with a high CO₂ reactor system, such as that discussed with respect to Fig. 4. In the gas

fractionation system 800, the feed gas 802 is fed to a dryer 804 to reduce the dew point to about -70 ^QF (about -56.7 ^QC), or lower. The feed gas 802 can correspond to the dried gas stream 366 discussed with respect to Figs. 3-5. The dryer 804 can be a fixed or fluidized dryer bed, containing an adsorbent, such as molecular sieves, desiccants, and the like. Other dryer technologies may also be used, such as cryogenic drier systems. In some embodiments, the dryer can be located prior to the compressor 358, which may eliminate the need for the ambient temperature heat exchanger 362.

[0076] The dry gas feed 806 is then fed through a cryogenic chiller 808 to reduce the temperature in preparation for the separation. As CO₂ will condense from the gas at about -77 ^QF (about -61 ^QC), a multistage chilling system 810 may be used to reduce the temperature to around this level. The multistage chilling system 810 may include a heat recovery system 812 used to heat the outlet gas with energy 813 from the dry feed gas 806.

[0077] The chilled feed 816 is fed to a separation vessel 818 to separate a liquid stream 820 and a vapor stream 822. The vapor stream 822 is passed through an expander 824 to lower the temperature by generating mechanical work 826 in an adiabatic expansion process. In one embodiment, the mechanical work 826 is used to drive a generator 828, which may provide a portion of the electricity used in the plant. In another embodiment, the mechanical work 826 is used to drive a compressor, for example, for compressing a refrigerant stream for the multistage chilling system 810. The expansion can result in a two phase stream 830.

[007S] The liquid stream 820 and the two phase stream 830 are fed to a separation column 832, for example, at different points along the separation column 832. Heat is supplied to the separation column 832 by a reboiler 834. The reboiler 832 is heated by a stream from a heat exchanger 836. The heat exchanger 836 may be part of a chiller system that is warmer than the separation column 832, although below ambient temperature. The column bottom stream 838 is passed through the reboiler 834 and a portion 840 is reinjected after being warmed. An outlet stream 842 from the reboiler 834 provides the CO₂ product 844. A portion 846 of the CO₂ product 844 may be recycled through the heat exchanger 836 to carry energy to the reboiler 834. [0079] The overhead stream 848 from the separation column 832 is a methane enhanced stream, for example, including about 73 mol % CH₄ and about 23 mol % C0₂. As noted, the overhead stream 848 may be used in a chiller system 812 to cool the dry gas feed 806, warming the overhead stream 848 to form the recycle gas 850. Other components may be present in the recycle gas 850 including, for example, about 3.5 mol % CO and H₂. If the methane is intended for sale, such as in the high methane reaction system discussed with respect to Fig. 6, a higher purity separation system may be used, as discussed with respect to Fig. 9.

[0080] Fig. 9 is a simplified process flow diagram of another gas fractionation process 900 that can be used in a reactor system for the production of carbon

nanotubes. In Fig. 9, like numbered items are as discussed with respect to Fig. 8. In this gas fractionation process 900, the chilled feed 816 can be directly fed to a first separation column 902, which separates the CO₂. The CO₂ exits the first separation column 902 in a bottoms stream 904. A portion of the bottoms stream 904 is passed through a reboiler 906, which adds heat. The heated stream 908 is then reinjected into the first separation column 902. The remainder of the bottoms stream 904 forms the CO₂ product 910, which is recycled, e.g., as recycle gas 504 discussed with respect to Fig. 5.

[0081] The overhead stream 912 from the first separation column 902 is sent to a second separation column 914 for further purification of the methane product. The bottoms stream 916 from the second separation column 914 is pressurized by a pump 918 and returned to the first separation column 902 as a reflux stream 920. The overhead stream 922 from the second separation column 914 is passed through a chiller 924, which can use a nitrogen refrigeration unit 926 to achieve much lower temperatures. The chilled stream is then flashed into a separation vessel 928. The overhead stream 930 from the separation vessel 928 provides the CH₄ enhanced product. The overhead stream 930 may be used to provide cooling for the dry gas feed 806, for example, by being fed through a common chilling system 812. The bottoms stream 934 from the separation vessel 928 is pressurized by a pump 936 and returned to the second separation column 914 as a reflux stream 938.

[0082] The configurations and units discussed with respect to Figs. 8 and 9 are merely exemplary. Any number of variations may be made to these systems. Further, other gas separation systems may be used in embodiments, so long as flow rates and purity levels can be achieved.

Separation System

[0083] Fig. 10 is a simplified process flow diagram of a separation system 1000 that can separate carbon nanotubes from a reactor effluent stream. The separation system 1000 overlaps the lock hoppers 430 shown in Figs. 4 and 5, and is used to isolate the CNTs from the process for packaging. Each reactor in the system may have separate packaging trains 1002 and 1004. As the different reactors may be producing different amounts of CNTs, the equipment may be sized differently, although the functions may be the same. For example, in the first simulation, the amount of CNTs isolated by the first packaging train 1002 may be about 162.7 tons/day (148,000 kg/day), while the amount removed to the second packaging train 1004 may be about 57.5 tons/day (52,000 kg/day).

[0084] Each of the packaging trains 1002 and 1004 may have a sampling valve 1006 to remove CNTs from the lock hopper 430. The valve 1006 may be a rotary valve configured to allow a certain amount of CNTs and gas through during a portion of a rotation cycle. In some embodiments, the sampling valve 1006 may be a ball valve configured to open fully for a selected period of time to allow a selected amount of CNTs and gas through, prior to closing fully. The CNTs and gas are allowed to flow into a drum 1008 for purging and cooling.

[0085] After the sampling valve 1006 has closed, a purge stream 1010 may be opened into the drum 1008 to sweep out remaining gases, such as CO, H₂, H₂O, and CH₄. As noted, the purge stream 1010 may be taken from the CO₂ enriched side of the gas fractionation system, for example, as purge gas stream 440, discussed with respect to Fig. 4 or purge gas stream 510, discussed with respect to Fig. 5. The purge outlet stream 1012 will carry some amount of CNTs, and other fine particles, and may be passed through a filter 1014, prior to being sent back to the process as a purge return 1016. The filter 1014 may be a bag filter, cyclonic separator, or any other suitable separation system. After purging is completed, a packaging valve 1018 will open to allow a CNTs stream 1020 to flow to a filling station 1022 to be packaged in drums or tanks for sale.

[0086] The isolation system described above is merely exemplary. Any number of other systems may be used in embodiments. However, the CNTs have a very low density, of less than about 0.5 g/cc, depending on morphological distribution, and may best be packaged in a system configured to isolate them from the atmosphere to lower the amount lost to the plant environment.

Method

[0087] Fig. 1 1 is a method 1100 for generating carbon nanotubes from a feed gas that includes methane and carbon dioxide. The method 1100 begins at block 1102, at which a mixed C0₂ / CH₄ feedstock is obtained. The feed stock may be obtained from any number of sources. As mentioned, the feedstock may include a natural gas harvested from a sub-surface reservoir, an exhaust gas from a power generation plant, or any number of other gases from natural or plant sources.

[0088] At block 1104, the feedstock is combined with a recycle gas obtained from the wastes gases generated in the process. As described herein, the recycle gas may be obtained from the waste gases by cryogenic gas fractionation, as well as any number of other techniques. At block 1106, the combined gas stream is heated with waste heat recovered from the reaction process. After heating, at block 1108, the combined gas stream is reacted with a metal catalyst in a reactor to form the CNTs. At block 1110 the CNTs are separated from the waste gas. At block 1112, the separated CNTs are purged, cooled, and packaged to be sent to market. [0089] The waste gas is cooled to remove excess water formed during the reaction. As the process is conducted at high temperatures and pressures, an ambient temperature heat exchanger provides sufficient cooling to condense out the water vapor. The processes described at blocks 1106-1114 will be repeated for each sequential reactor in the reaction system.

[0090] At block 1116, the waste gas is fractionated into a C0₂ enriched stream and a CH₄ enriched stream. At block 1118, whichever stream contains the excess reagent can be sold, while the other stream can be recycled to block 1104 to be used in the process.

[0091] Still other embodiments of the claimed subject matter may include any combinations of the elements listed in the following numbered paragraphs:

1 . A system for the production of carbon nanotubes, including:

a feed gas heater configured to heat a feed gas with waste heat from a waste gas stream;

a reactor configured to form carbon nanotubes from the feed gas;

a separator configured to separate the carbon nanotubes from the reactor

effluent stream forming the waste gas stream; and

a water removal system, including an ambient temperature heat exchanger and separator configured to separate the bulk of the water from the waste gas stream to form a dry waste gas stream.

2. The system of paragraph 1 , wherein the ambient temperature heat exchanger includes a water chiller.

3. The systems of paragraphs 1 or 2, wherein the ambient temperature heat exchanger includes an air cooled heat exchanger.

4. The systems of paragraph 1 , 2, or 3, including a package heater configured to heat the feed gas for an initial startup of the system.

5. The systems of any of the preceding paragraphs, including: a heat exchanger configured to heat the dry waste gas stream with waste heat from the waste gas stream to form a second feed gas;

a second reactor configured to form carbon nanotubes from the second feed gas; a separator configured to separate the carbon nanotubes from an effluent stream from the second reactor forming a second waste gas stream, and wherein the waste gas stream used in the feed gas heater includes the second waste gas stream; and

a water removal system configured to separate water from the second waste gas stream using an ambient temperature heat exchanger to chill the second waste gas stream and remove the bulk of the water to form a second dry waste gas stream.

6. The system of paragraph 5, including:

a compressor configured to increase the pressure of the second dry waste gas stream; and

a final water removal system configured to remove water from the second waste gas stream.

7. The systems of paragraphs 5 or 6, including a gas fractionation system configured to separate a methane rich stream and a C0₂ rich stream from the second waste gas stream.

8. The systems of paragraphs 5, 6, or 7, including a mixing system

configured to mix the methane rich stream into the feed gas before the feed gas heater.

9. The systems of any of paragraphs 5-8, wherein the gas fractionation system includes a cryogenic condensation system configured to separate gases that are condensable at a temperature from gases that are non-condensable at the temperature.

10. The systems of paragraphs 1 or 5, wherein the reactor is a fluidized bed reactor using a counter-current flow of feed gas to fluidize a catalyst. 1 1 . The system of paragraph 10, wherein the catalyst includes metal shot- blasting beads.

12. The systems of paragraphs 10 or 1 1 , wherein the catalyst includes metal beads including iron and nickel, chromium, or any combinations thereof.

13. The systems of paragraphs 10, 1 1 , or 12, wherein the catalyst includes metal beads between about 25 mesh and 50 mesh in size.

14. The systems of any of the preceding paragraphs, wherein the reactor is lined with a material configured to prevent degradation of a metal shell.

15. The systems of any of the preceding paragraphs, wherein a piping connection between the reactor and a cross heat exchanger is lined with a refractory material configured to protect a metal surface from degradation.

16. The systems of any of the preceding paragraphs, wherein the feed gas heater includes a heat exchanger configured for use in a metal dusting environment.

17. A method for forming carbon nanotubes, including:

forming carbon nanotubes in a first reactor, using a feed gas;

separating the carbon nanotubes from a reactor effluent to form a waste stream; heating the feed gas, a dry waste gas stream, or both, with waste heat from the waste stream; and

chilling the waste stream in an ambient temperature heat exchanger to condense water vapor, forming the dry waste gas stream.

18. The method of paragraph 17, including:

feeding the dry waste stream to a second reactor;

forming another portion of carbon nanotubes in the second reactor;

separating the carbon nanotubes to form a second waste gas stream;

heating the feed with waste heat from the second waste stream; and

chilling the second waste stream in an ambient temperature heat exchanger to condense water vapor, forming a second dry waste stream.

19. The method of paragraph 18, including: compressing the second dry waste stream to form a compressed gas;

passing the compressed gas through an ambient temperature heat exchanger to condense and remove any remaining water vapor;

fractionating the compressed gas to separate methane and carbon dioxide; and adding the methane to the feed gas.

20. A reaction system for forming carbon nanotubes, including:

two or more reactors configured to form carbon nanotubes from gas streams including methane and carbon dioxide, wherein an effluent from each reactor, before a final reactor, is used as a feed stream for a downstream reactor, and wherein an effluent stream from the final reactor includes a reactant depleted waste stream;

a separation system downstream of each reactor, wherein the separation system is configured to remove carbon nanotubes from the effluent from the reactor;

a feed heater downstream of each separation system, wherein the feed heater includes a heat exchanger configured to heat a feed gas stream for a following reactor using waste heat from the effluent from the reactor, and wherein the feed heater downstream of the final reactor is configured to heat a gas stream for the first reactor;

an ambient temperature heat exchanger downstream of each feed heater,

wherein the ambient temperature heat exchanger is configured to remove water from the effluent, forming the feed stream for the following reactor; a compressor configured to increase the pressure of the reactant depleted waste stream;

an ambient temperature heat exchanger downstream of the compressor,

configured to remove water from the reactant depleted waste stream; a gas fractionation system configured to separate the reactant depleted waste stream into a methane enriched stream and a carbon dioxide enriched stream; and

a mixer configured to blend the methane enriched stream or the carbon dioxide enriched stream into an initial feed stream.

21 . The reaction system of paragraph 20, wherein a reactor includes a fluidized bed reactor using metal beads as a catalyst.

22. The reaction systems of paragraphs 20 or 21 , including a separation vessel downstream of each of the ambient temperature heat exchangers, wherein the separation vessel is configured to separate liquid water from a gas stream.

23. The reaction systems of paragraphs 20, 21 , or 22, including a package heater configured to heat the initial feed stream for plant startup.

24. The reaction systems of paragraphs 20, 21 , 22, or 23, wherein the package heater is used to heat a feed stream to a subsequent reactor.

25. The reaction systems of paragraphs 23 or 24, wherein the package heater is a heater configured to be field erected, or an electric power heater, a commercial heater configured for heating gases, or any combinations thereof.

26. The reaction systems of paragraphs 23, 24, or 25, wherein the package heater is configured to heat a reducing gas stream without substantial damage.

[0092] While the present techniques may be susceptible to various modifications and alternative forms, the embodiments discussed above have been shown only by way of example. However, it should again be understood that the techniques is not intended to be limited to the particular embodiments disclosed herein. Indeed, the present techniques include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.

Claims

CLAIMS What is claimed is:

1 . A system for the production of carbon nanotubes, comprising:

a reactor configured to form carbon nanotubes from the feed gas;

a separator configured to separate the carbon nanotubes from the reactor

effluent stream forming the waste gas stream; and

a water removal system, comprising an ambient temperature heat exchanger and separator configured to separate the bulk of the water from the waste gas stream to form a dry waste gas stream.

2. The system of claim 1 , wherein the ambient temperature heat exchanger comprises a water chiller.

3. The system of claim 1 , wherein the ambient temperature heat exchanger comprises an air cooled heat exchanger.

4. The system of claim 1 , comprising a package heater configured to heat the feed gas for an initial startup of the system.

5. The system of claim 1 , comprising:

a heat exchanger configured to heat the dry waste gas stream with waste heat from the waste gas stream to form a second feed gas;

a second reactor configured to form carbon nanotubes from the second feed gas; a separator configured to separate the carbon nanotubes from an effluent stream from the second reactor forming a second waste gas stream, and wherein the waste gas stream used in the feed gas heater comprises the second waste gas stream; and

6. The system of claim 5, comprising:

7. The system of claim 6, comprising a gas fractionation system configured to separate a methane rich stream and a C0₂ rich stream from the second waste gas stream.

8. The system of claim 7, comprising a mixing system configured to mix the methane rich stream into the feed gas before the feed gas heater.

9. The system of claim 7, wherein the gas fractionation system comprises a cryogenic condensation system configured to separate gases that are condensable at a temperature from gases that are non-condensable at the temperature.

10. The system of claim 1 , wherein the reactor is a fluidized bed reactor using a counter-current flow of feed gas to fluidize a catalyst.

1 1 . The system of claim 10, wherein the catalyst comprises metal shot- blasting beads.

12. The system of claim 10, wherein the catalyst comprises metal beads comprising iron and nickel, chromium, or any combinations thereof.

13. The system of claim 10, wherein the catalyst comprises metal beads between about 25 mesh and 50 mesh in size.

14. The system of claim 1 , wherein the reactor is lined with a material configured to prevent degradation of a metal shell.

15. The system of claim 1 , wherein a piping connection between the reactor and a cross heat exchanger is lined with a refractory material configured to protect a metal surface from degradation.

16. The system of claim 1 , wherein the feed gas heater comprises a heat exchanger configured for use in a metal dusting environment.

17. A method for forming carbon nanotubes, comprising:

forming carbon nanotubes in a first reactor, using a feed gas;

18. The method of claim 17, comprising:

feeding the dry waste stream to a second reactor;

forming another portion of carbon nanotubes in the second reactor;

separating the carbon nanotubes to form a second waste gas stream;

heating the feed with waste heat from the second waste stream; and

19. The method of claim 18, comprising:

compressing the second dry waste stream to form a compressed gas;

20. A reaction system for forming carbon nanotubes, comprising:

two or more reactors configured to form carbon nanotubes from gas streams comprising methane and carbon dioxide, wherein an effluent from each reactor, before a final reactor, is used as a feed stream for a downstream reactor, and wherein an effluent stream from the final reactor comprises a reactant depleted waste stream;

a feed heater downstream of each separation system, wherein the feed heater comprises a heat exchanger configured to heat a feed gas stream for a following reactor using waste heat from the effluent from the reactor, and wherein the feed heater downstream of the final reactor is configured to heat a gas stream for the first reactor;

an ambient temperature heat exchanger downstream of each feed heater,

an ambient temperature heat exchanger downstream of the compressor,

21 . The reaction system of claim 20, wherein a reactor comprises a fluidized bed reactor using metal beads as a catalyst.

22. The reaction system of claim 20, comprising a separation vessel downstream of each of the ambient temperature heat exchangers, wherein the separation vessel is configured to separate liquid water from a gas stream.

23. The reaction system of claim 20, comprising a package heater configured to heat the initial feed stream for plant startup.

24. The reaction system of claim 23, wherein the package heater is used to heat a feed stream to a subsequent reactor.

25. The reaction system of claim 23, wherein the package heater is a heater configured to be field erected, or an electric power heater, a commercial heater configured for heating gases, or any combinations thereof.

26. The reaction system of claim 23, wherein the package heater is configured to heat a reducing gas stream without substantial damage.