This invention is generally related to an apparatus and method of capturing, and storing hydrocarbon gas and greenhouse gas species within a carbon nanotube matrix. Additionally, methods are directed to the production of hydrogen without a substantial carbon dioxide byproduct. More specifically, this invention is related to a method that utilizes an initial amount of carbon nanotubes as a substrate to capture a hydrocarbon gas and/or a greenhouse gas, forming nanotubes saturated with a hydrocarbon gas or a greenhouse gas, or both. The nanotubes saturated with hydrocarbon gas or greenhouse gas can then be placed under a vacuum and exposed to microwave energy generating: (a) a release of hydrogen into a storage vessel; and (b) de novo synthesis of carbon nanotubes from the recycled carbon elements of the hydrocarbon gas or the greenhouse gas. The newly synthesized carbon nanotubes, in turn, become saturated with hydrocarbon gas or greenhouse gas, which can be exposed to microwave radiation and produce even more newly synthesized carbon nanotubes. This process represents an apparatus and method of making hydrogen with an industrial valuable byproduct, carbon nanotubes. Additionally, the process is substantially free from carbon contaminants and carbon dioxide production.
Hydrocarbon and Greenhouse Gas Production. Many chemical compounds found in the Earth's atmosphere act as “greenhouse gases.” These gases allow sunlight to enter the atmosphere freely. When sunlight strikes the Earth's surface, some of it is reflected back towards space as infrared radiation (heat). Greenhouse gases absorb this infrared radiation and trap the heat in the atmosphere. Over time, the amount of energy sent from the sun to the Earth's surface should be about the same as the amount of energy radiated back into space, leaving the temperature of the Earth's surface roughly constant. However, there is growing concern in the scientific community that greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and sub-surface ocean temperatures to rise. The concern that increases in global temperature over the past few decades are due to human activities directly. Additionally, increases in global temperatures have prompted international governments to monitor and/or reduce the amount of greenhouse gas emissions that are produced by industrialized nations.
Many gases exhibit “greenhouse” properties. Some of them occur in nature (e.g. water vapor, carbon dioxide, methane, and nitrous oxide), while others are exclusively human-generated (e.g. gases used for aerosols, HFCs, PFCs and SF6). Most of the human-generated greenhouse gas emissions of carbon dioxide produced in the United States are a result of energy production, more specifically, energy-related usage of petroleum and natural gas. Another greenhouse gas emission, methane, comes from landfills, coal mines, oil and gas operations, and agriculture. Nitrous oxide is also emitted from burning fossil fuels and through the use of certain fertilizers and industrial processes.
Hydrogen Energy Production. There is a currently a need for hydrogen to play a greater role in the energy market because of the increasing demand for fuel cell systems and the growing demand for reduction of greenhouse gases and zero-emission fuels. Hydrogen production must keep pace with this growing market demand, but there are still some technical and infrastructure hurdles that first need to be overcome. Although hydrogen is the most abundant element on the planet, it is bound to other elements from which it must be separated before it can be used in energy production or as a chemical feedstock, etc. Thermo-chemical and electrochemical methods for hydrogen generation have been developed, however these processes are generally costly, energy-intensive, produce carbon dioxide, and not always environmentally friendly. Thus, hydrogen production in the United States is not generally used for energy production. For example, approximately 95% of the hydrogen produced in the United States today comes from carbonaceous raw materials, primarily fossil in its origin. However, only a fraction of the hydrogen produced is currently employed for energy purposes. The bulk of this hydrogen is used as chemical feedstock for petrochemical, food, electronics and metallurgical industries.
In the future, increased hydrogen production will most likely be met by conventional technologies, such as natural gas reformation. In these processes, hydrogen is produced and the carbon is converted to carbon dioxide and released to the atmosphere. With the growing concern of global climate change, alternatives to the atmospheric release of carbon dioxide are needed. Sequestration of carbon dioxide is an option but it is also energy intensive and expensive. Better methods of hydrogen production are needed, including environmentally friendly methods that do not produce carbon dioxide.
Reducing the demand for fossil resources remains a significant concern for most industrialized nations. Renewable resource based processes including solar or wind driven electrolysis and photolytic water splitting hold promise for clean hydrogen production. Such processes are desirable but considerable advance must be made before these processes are technologically feasible and economically competitive.
Carbon Nanotubes Science and Technology. Nanotechnology is based on a principle of building functional structures with chemistry and biology one atom at a time. The first report of a nanostructure was the Buckminsterfullerene, which is essentially a series of very large carbon molecules, the most common form of which is the C60 molecule. Carbon nanotubes were discovered in 1991 and consist of fullerene-related structures of graphene cylinders closed at either end with caps containing pentagonal rings. Carbon nanotubes are a special class of what is widely referred to as nanostructure or a man-made structure in the physical size range of 1 to 100 nanometers (“nm”). Bulk quantities of hollow carbon nanotubes can be produced using an arc-evaporation technique. The experimental variables for producing nanotubes include: ambient pressure, electrode size, gap size, power, and flux density. U.S. Pat. No. 5,346,683 issued to Green, et al., on Sep. 13, 1994, titled “Uncapped and Thinned Carbon Nanotubes and Process,” (“The '683 patent”) indicates one method of producing carbon nanotubes. For example, carbon nanotubes were prepared in the '683 patent using a carbon arc. An arc was struck between two electrolytic grade graphite rods, 8 mm O.D., 15 cm length having a purity >99%, in 100 Torr of helium using a dc voltage of 30V and a current of 180-200 A. The anodic graphite rod evaporated and the cathodic graphite rod increased in length. Approximately 20-30 percent of the carbon vaporized from the anode distilled onto the cathode, resulting in a gain in length of the cathode of about 3-4 cm. The nanotube material was found at the black central core of the cathodic rod.
A multi-walled carbon nanotube (“MWNT”) resembles a series of “pipes” within one another. Generally the outer tube of a MWNT is capped while the inner tubes are open. One MWNT can have a range of 2 to several hundred layers of pipes within one another. Even though the MWNT structures are interesting in a physical sense, in many ways their chemical and electronic properties are similar to common carbon structures. Although original MWNT were carbon structures, MWNTs have been formed from a wide variety of structures such as boron, nitride, and other compounds (e.g. U.S. Pat. Nos. 6,063,243, and 6,231,980). Since the discovery of the MWNT, an even more striking and profound discovery was made with the first observation of the single walled carbon nanotube (“SWNT”). The physical appearance of these structures are similar to the layered pipe structure of the MWNT, however, there is only one layer. One of the highlights of nanotube research has been the demonstration that SWNT can be opened and filled with a variety of materials ranging from single atoms (e.g. hydrogen fuel) to biological molecules.
Generally, in order to produce nanotubes, conditions of about 500 Torr and temperatures high enough to vaporize carbon should be met in an inert environment. One specific method for making SWNT is disclosed in U.S. Pat. No. 6,183,714 issued to Smalley, et al., on Feb. 6, 2001, titled “Method of Making Ropes of Single-Wall Carbon Nanotubes,” (“the '714 patent”). The '714 patent provides a method of making single-wall carbon nanotubes by laser vaporizing a mixture of carbon and one or more Group VIII transition metals. Single-wall carbon nanotubes preferentially form in the vapor and the one or more Group VIII transition metals catalyzed growth of the single-wall carbon nanotubes. In one embodiment of the '714 patent invention, one or more single-wall carbon nanotubes are fixed in a high temperature zone so that the one or more Group VIII transition metals catalyze further growth of the single-wall carbon nanotube that is maintained in the high temperature zone. In another embodiment, two separate laser pulses are utilized with the second pulse timed to be absorbed by the vapor created by the first pulse. The '714 patent is specifically incorporated herein by reference. Additionally, nanotubes can be produced commercially, and these methods are known by a person of ordinary skill in the art and incorporated herein by reference (e.g. Helixmaterial, located in Richardson, Tex.; CNI, Houston, Tex.; and Hyperion Catalysis International located in Cambridge, Mass.).
Since the discovery of carbon nanotubes, a significant amount of application driven research has taken place. A portion of this research has been directed toward the use of these structures as storage matrices for various gaseous species such as hydrocarbon gases and greenhouse gases, which are known to be harmful to the environment.
One method of enclosing foreign material in a carbon nanotube is described in U.S. Pat. No. 5,457,343 issued to Ajayan, et al., on Oct. 10, 1995, titled “Carbon Nanotubule Enclosing a Foreign Material.” (“the '343 patent”). The '343 patent provides a nanometer sized carbon tubule enclosing a foreign material. The carbon tubule comprises a plurality of tubular graphite monoatomic sheets coaxially arranged. The foreign material is introduced through a top portion of the carbon tubule. The introduction of the foreign material is accomplished after forming an opening at the top portion of the carbon tubule either by contacting the foreign material with the top portion of the carbon tubule together with a heat treatment or by an evaporation of the foreign material on the top portion of the carbon tubule together with the heat treatment. The foreign material is introduced only in a center hollow space defined by an internal surface of the most inner tubular graphite monoatomic sheet. The '343 patent is specifically incorporated herein by reference.
Microwave Energy. Microwaves are very short waves of electromagnetic energy that travel at the speed of light. Microwaves that are used in household microwave ovens are in the same family of frequencies as the signals used in radio and television broadcasting, and described in U.S. Pat. No. 2,495,429 issued to Spencer et al., on Jan. 24, 1950 and titled “Method of Treating Foodstuffs,” (“the '429 patent”). The heart of every microwave oven is the high voltage system that generates microwave energy. The high-voltage components accomplish this by stepping up AC line voltage to high voltage, which is then changed to an even higher DC voltage. This DC power is then converted to the RF energy. The nucleus of the high-voltage system is the magnetron tube. Generally, a magnetron is a diode-type electron tube which is used to produce the about 2450 MHz of microwave energy. It is classed as a diode because it has no grid as does an ordinary electron tube. A magnetic field imposed on the space between the anode (plate) and the cathode serves as the grid. While the external configurations of different magnetrons will vary, the basic internal structures are the same. These include the anode, the filament/cathode, the antenna, and the magnets. In this invention, a 500 Watt 2.45 GHz microwave source was used to quickly heat carbon nanotubes in a vacuum system, but similar devices that can heat carbon material quickly may also be utilized.
When nanotubes having stored hydrocarbon gases or greenhouse gases are heated quickly enough, the hydrogen will be stripped from the molecules and ejected from the nanotubes into the vacuum system where the hydrogen can then be placed in a storage tank for some other future use or to produce a plasma. In contrast, the carbon elements of the hydrocarbon gases or greenhouse gases are not ejected but tend to form newly synthesized carbon nanostructures, specifically SWNTs and MWNTs. For example, when a 500 Watt 2.45 GHz microwave source was used to heat nanotubes having stored hydrocarbon gases or greenhouse gases, hydrogen was released and the remaining carbon material become additional carbon nanotubes. The types of new nanotubes formed depend upon whether a catalyst material is present or not. For example, when nanotubes were produced without a catalyst material, MWNTs are formed. In microwave far field experiments, utilizing methane saturated carbon nanotubes without a catalyst material, C60 fullerenes were formed. In contrast, the addition of an iron catalyst leads to the formation of SWNTs. The present invention could also be used to produce cloned nanotubes from defective sites in an original carbon nanotube matrix.
NanoCarb-H2™ Currently, hydrogen production from biomass is receiving much attention. Unfortunately, most of these processes have significant drawbacks. Herein, we propose a microwave-assisted process (NanoCarb-H2)™ that converts organics, including but not limited to, methane, butane, propane, carbon monoxide, carbon dioxide etc., into hydrogen gas. The hydrogen gas can then be stored for use as a fuel. The carbon is converted into carbon nanotubes, which can be sold for use in industrial applications. The process obviates the necessity of having to sequester carbon dioxide since none is produced in the process. Thus, the process is substantially free of carbon dioxide production. The process is unique in a number of ways. Other processes require very high pressure and temperature in order to preclude the production of carbon dioxide. Such processes are generally energy intensive and complicated. In the current process, a specific diameter carbon nanotube was used as a substrate and is saturated with a hydrocarbon gas or greenhouse gas, and heated with a microwave generator under vacuum. The carbon from the organic compounds was cloned into additional carbon nanotubes having about the same diameter as the substrate nanotubes during the reaction, while hydrogen is split off, evolved and stored. For example, when methane is used as the reactant, a hydrogen to carbon ratio of approximately 4:1 is realized.