RU2489350C2 - Method of producing carbon nanomaterials and device for its implementation - Google Patents

Abstract

FIELD: nanotechnology.

SUBSTANCE: invention relates to the field of nanotechnology, and can be used to produce nanotubes and fullerenes. The carbonaceous material is evaporated in the volumetric thermal plasma, and condensed on the outer surface of the anode 2 and the inner surface of the cathode 3. A glow-discharge plasma is used, which is set by electrical voltage supply sufficient to break the inter-electrode gap between the coaxial hollow cathode 3 having a shape of a cup with a permeable bottom 4, and the anode 2 located with the ability to move axially. The carbonaceous material is fed through the permeable bottom 4 of the cathode 3 and is selected from the group consisting of methane, propane, butane for gaseous carbonaceous material or from the group consisting of crude oil, oil fuel, benzene, gas oil for liquid carbonaceous material. The bottom 4 is made in the form of a grid or a membrane.

EFFECT: invention enables to reduce the power consumption of the process, to broaden the types of the used hydrocarbon crude, to simplify the design of the device and to provide the ecological compatibility of the process and its high performance.

4 cl, 1 dwg, 2 ex

Description

The invention relates to the field of nanotechnology and can be used to obtain nanocarbon materials, including nanotubes and fullerenes.

A known method for producing fullerenes (patent application RU No. 2005141129 "Method for producing fullerenes", IPC C01B 31/02, published June 10, 2006), in which a hydrocarbon-containing gas and an oxygen-containing gas are discharged from the outlet section located in the fullerene reactor into the fullerene reactor and burn.

The disadvantages of this method is the low fraction of the conversion of hydrocarbons to fullerenes, the high oxygen content in fullerene products, as well as the environmental friendliness of the process due to the large amount of waste and emissions of combustion products into the atmosphere.

A known method (patent application RU 97115694 "Method and device for producing fullerenes", IPC C01B 31/02, publ. 07/10/1999), in which fullerenes are obtained by exposure of a hydrocarbon to an electric arc, and liquid hydrocarbon is used as the carbon-containing material.

The disadvantages of this method are the high energy intensity of the process associated with the use of an electric arc, as well as the low processability of the process, due to the fact that liquid hydrocarbon fills the housing above the level of the electrodes, which makes it difficult to introduce hydrocarbon into the discharge chamber and collect the finished product.

A known method of producing carbon nanotubes (patent RU 2414418 "Method for producing hydrogen and carbon nanotubes from hydrocarbon gas", IPC C01B 3/26, C01B 31/02, B82B 3/00, publ. 03/20/2011), in which previously in an inert medium gas, the catalyst is sprayed to nanosized particles by evaporation of the anode graphite electrode, inside which a metal wire is installed, which is used as a catalyst, with a diameter of 0.5 mm or less. Then, the inert gas is pumped out, the AC electric arc is ignited by the method of touching the electrodes, followed by increasing the interelectrode distance to 0.3–0.5 mm, and high-temperature pyrolysis of the hydrocarbon gas is carried out in the plasma of the electric discharge at a pressure in the reactor of 0.5–2 atm to produce hydrogen and carbon nanostructures. The growth of carbon nanostructures, which are predominantly single and multilayer nanotubes without impurities of other carbon structures of carbon, occurs on synthesized catalyst particles. Helium is used as an inert gas. Methane, associated petroleum gas, acetylene, propane, butane, and natural gas are used as hydrocarbon gas. As a catalyst, palladium, iron, nickel, cobalt are used.

The disadvantage of this method is the high energy intensity associated with the use of an electric arc.

Closest to the claimed is the method presented in patent RU 2419585 "Method and reactor for the production of carbon nanotubes", IPC C01B 31/02, B82B 3/00, H05H 1/50, publ. 05/27/2011. In this method, the carbon-containing material is vaporized / decomposed in a bulk thermal plasma generated by rotating an electric arc using an external magnetic field, and said vaporized / decomposed carbon-containing material is condensed on surfaces or on particles in a gas stream. It is preferred that said carbonaceous material can be recycled through volumetric plasma. Surfaces can be, for example, an electrode or substrate. In a further embodiment, the placement and rotation of the arc is achieved by adjusting the distribution, direction and intensity of the applied magnetic field. This embodiment avoids the solution in which the magnets are installed inside the hot reactor, which requires cooling of the magnets due to the high temperature in the reactor and, especially, in the plasma field. Magnets located outside are protected from plasma heat. Also, the mechanical rotation of one or both electrodes, as described in the aforementioned publications on the arc method, does not provide rotation of the electric arc. The carbonaceous material may be a gas, liquid, or solid, and may be selected from the group consisting of carbon black, graphite powder, coal, natural gas, hydrocarbons, and petroleum products. The presence of carbonaceous material can, alternatively, be achieved by adding or vaporizing carbonaceous electrodes. Along with the aforementioned carbon-containing material, a catalyst can be added either with the introduced plasma gas, or by application to the said surfaces. The catalyst can be selected from the group consisting of Ni, Co, Fe, Y, salts and organometallic compounds of Ni, Co, Fe, Y, suspensions of Ni, Co, Fe, Y and said salts, said compounds and their combinations. As the plasma gas, hydrogen, helium, nitrogen, argon, carbon monoxide or mixtures thereof, or a chemical substance (preferably gas) can be used with which one or more of these gases can be obtained by heating.

The known method has the following disadvantages:

- high energy intensity of the process associated with the need to use an electric arc;

- the complexity of the process due to the use of a rotating electric arc;

- high costs for the behavior of the method, due to the need to use expensive catalysts (cobalt, nickel, etc.);

- restriction in raw materials, the predominant use of gas as a hydrocarbon.

A device is known (patent application RU No. 97115694 "Method and device for producing fullerenes", IPC C01B 31/02, publ. 07/10/1999), comprising a housing in which are placed bipolar graphite electrodes separated by insulation, while the insulating element is made in in the form of a horizontally located plate with through holes, in each of which a movable electrode is freely placed, and bipolar graphite electrodes are strengthened on the bottom side of the insulating plate, blocking part of each hole and preventing falling out movable electrodes, the housing being filled with liquid hydrocarbon above the level of the electrodes.

The disadvantage of this device is that its design makes it difficult to collect fullerene soot, which is caused by the fact that the electrodes are coated with liquid hydrocarbon feed.

A device is known (patent RU No. 2425795 "Installation for producing hydrogen and carbon nanomaterials and structures from hydrocarbon gas, including associated petroleum gas", IPC C01B 3/26, C01B 31/00, B82B 3/00, publ. 08/10/2011), in which the installation for producing carbon material and hydrogen consists of a plasma-arc reactor, including a vacuum chamber with coaxially arranged graphite electrodes, one of which is movable, a vacuum pumping system, an electrical power system with an alternating current source, a water cooling system, a system gas supply and discharge, measuring systems for monitoring pressure and electrical discharge parameters, the installation further comprises a palladium filter, a compressor pump and a reservoir for collecting hydrogen, the vacuum chamber of the plasma-arc reactor further comprises a silicon plate in the form of an annular washer on which catalyst particles are deposited moreover, the plate is mounted on a low-consuming electrode, in the form of a tablet, the diameter of which is larger than the diameter of the other electrode.

To produce hydrogen and carbon nanotubes in this installation, the reactor vacuum chamber is filled with hydrocarbon gas selected from the series: methane, associated petroleum gas, acetylene, propane, butane, natural gas, and an alternating current electric arc is ignited by touching electrodes and at a pressure of 0.5- 2 atm carry out high-temperature pyrolysis. Hydrocarbon gas is continuously supplied to the reactor chamber. The growth of carbon nanostructures, which are predominantly single and multilayer nanotubes, without the formation of other carbon structures, occurs directly on the plate on catalytic metal particles. The resulting hydrogen is removed through a palladium filter and pumped into the tank using a compressor pump.

The disadvantage of this device is its structural complexity, due to the need to cool the electrodes.

It is also known the device closest to the proposed (patent RU No. 2419585 "Method and reactor for the production of carbon nanotubes", IPC C01B 31/02, B82B 3/00, H05H 1/50, publ. 05.27.2011), in which the reactor for the production of carbon nanotubes or other carbon nanostructures, for example, cones, includes an electrode and a hollow counter electrode, the electrodes being mounted axially opposite each other, and at least one magnet. The counter electrode may be a pipe or a pipe, part of which has a conical shape. The counter electrode is provided with openings for the passage and recirculation of gases and particles.

An evaporation chamber and a growth substrate with controlled cooling and a variable distance to the arc zone are installed in the central part of the reactor. Several auxiliary inlets are also installed for introducing various reaction components. The reactor is sized so that it can be operated at reduced pressures up to 0.1 bar, but it can also function at elevated pressures up to 3 bar. To control the rotational movement of the arc, as well as the vertical position of the arc in the reactor, magnetic coils are used. In one embodiment, there are four independent direct current magnetic coils located around the reactor at various levels from top to bottom, which gives flexibility to control the arc in terms of arc rotation and its vertical position relative to the electrodes. The arc current in this embodiment is supplied by means of a DC power source connected between the two electrodes, and the arc burning between these electrodes is rotated by an external magnetic field controlled by magnetic coils.

The disadvantages of the known device are:

- design complexity associated with the need to use a water cooling system of electrode elements and the use of magnetic coils;

- the difficulty of using liquid hydrocarbons as raw materials, since the inlets are in the upper part of the reactor, above the electrodes.

The objective of the claimed invention is to create a method and device for its implementation, which allows to achieve technical results, which include reducing the energy intensity of the process, expanding the types of raw materials used and simplifying the design.

The invention consists in the following.

In a method for producing carbon nanomaterials, including vaporization / decomposition / dissociation in a bulk thermal plasma of carbon-containing material and condensation on the surface of the anode and cathode of carbon nanomaterials, a glow discharge plasma is used as the bulk thermal plasma. In this case, a glow discharge is set by applying an electric voltage sufficient to break the interelectrode gap in the region formed by the anode located in the cavity of the hollow cathode coaxially with it and the walls of the hollow cathode. The anode is located in the cavity of the hollow cathode with the possibility of moving along the axis in order to establish the distance necessary for breakdown to occur, as well as to establish the required distribution of electric potential, field, current density and charges in the discharge gap, and the hollow cathode has the shape of a glass with a permeable bottom, providing the ability to feed through it carbon-containing raw materials. The carbon-containing raw materials used can be in a gaseous or liquid state, gaseous (methane, propane, butane, etc.) and liquid (oil, fuel oil, gas oil, etc.) hydrocarbons can serve as hydrocarbon raw materials. The surface on which the resulting carbon nanomaterial accumulates is the outer surface of the anode and the inner surface of the cathode cavity.

In a device for producing carbon nanomaterials, which includes a discharge chamber in which an electrode and a hollow counter electrode are placed, the electrodes are aligned, the hollow counter electrode is a cathode having the form of a glass with a permeable bottom, and the anode is located inside the cathode cavity with the possibility of moving along them axis. In this case, the permeable bottom of the cathode is made either in the form of a lattice when using liquid hydrocarbon feedstocks, or in the form of a membrane when using gaseous hydrocarbon feeds. And the anode material is made, for example, of copper or graphite.

The claimed invention is characterized by the following features. In the inventive method, a glow source is used as a plasma source, which relates to discharges with a cold cathode, that is, thermionic emission processes do not play a significant role in maintaining the discharge, in contrast to the method implemented in the prototype. In the method of the prototype, an arc discharge is used as a plasma source, in which thermionic emission, that is, the separation of electrons from a cathode heated to a high temperature, plays one of the main mechanisms of electron emission, which ensures the maintenance of an independent discharge. Also one of the features of the electric arc is the high current strength, which requires the use of powerful power sources. All these factors lead to high energy costs, a significant part of which is not spent on the target decomposition of hydrocarbon molecules. In addition, during an arc discharge, the cathode is heated, which requires measures for its cooling, which also leads to additional energy costs for pumping coolant and complicating the design of the device.

A glow discharge has a significant thermal disequilibrium: a high electron temperature (of the order of tens of thousands of kelvin) is combined with a low temperature of ionic and neutral gas (400-800 K). Under the conditions of a glow discharge, the energy of the electric field from the source is transferred to free electrons, and then, in collisions with hydrocarbon molecules, this energy is spent on ionization, dissociation and excitation. This is the fundamental difference between the use of a glow discharge for the decomposition of hydrocarbons and the subsequent synthesis of nanocarbon material from an arc discharge. The breaking of bonds of long hydrocarbon molecules and the appearance of atomic carbon and hydrogen occurs due to the "bombardment" of electrons, and not due to evaporation / decomposition in thermal plasma, which significantly reduces the energy intensity of the process.

The prototype indicates that thermal plasma is used, which is primarily used as a heat source for the evaporation of solid carbon or the dissociation of gaseous carbon compounds. It is also noted there that in thermal plasmas, electrons and ions have the same temperature - usually 5000-50000 K.

Thermal nonequilibrium also affects the increase in the percentage of nanocarbon material, primarily fullerenes. The thermodynamics of the synthesis of fullerene molecules are such that the carbon atoms obtained during the decomposition of hydrocarbons require rapid cooling. For this purpose, buffer gas and special cooled surfaces are often used. In the case of a glow discharge, the plasma region itself is “cold”.

The use of a hollow cathode in the inventive device for creating a glow discharge by a plasma method increases the concentration and average energy of electrons due to the fact that the electrons oscillate in the cavity of the hollow cathode, repeatedly reflecting from the cathode barrier. Due to this, their lifetime inside the cavity of the hollow cathode increases, which leads to more efficient ionization and excitation of hydrocarbon gas molecules. With increasing electron energy, the excitation cross section for the energy levels of hydrocarbon molecules grows rapidly, which significantly increases the probability of decay of molecules, including into carbon and hydrogen atoms, which leads to an increase in the efficiency of plasma-chemical processes.

In the claimed invention, the supply of hydrocarbon materials is carried out through the permeable lower wall of the hollow cathode, which allows to solve a whole range of problems.

When passing through the cathode wall, the hydrocarbons are preheated, which increases the efficiency of plasma-chemical processes. Also, the supply of hydrocarbon feedstock through the permeable lower wall of the hollow cathode allows solving the problem of cooling the cathode material, which in industrial plants is not as significant as in electric arc processes, but can still be heated. One of the main advantages of using a permeable wall of a hollow cathode is the possibility of using not only gaseous, but also liquid hydrocarbon, depending on the permeability of the cathode wall. In this case, a new mechanism of plasma-chemical processes in the reactor appears: the decomposition of hydrocarbon materials and the synthesis of nanocarbon materials occurs not only in the plasma volume, but also on the cathode surface. When using liquid hydrocarbon feed, the cathode surface is covered with a thin layer of liquid in which, under the influence of an electric discharge, the processes of evaporation, decomposition, and dissociation of molecules occur. In this case, part of the molecules decomposes to atomic hydrogen and carbon directly in the cathode region, and part of the vaporized material dissociates the volume of the plasma.

In addition to the fact that during the implementation of the method, fullerenes and nanotubes are produced, an additional valuable product is formed in the reactor - hydrogen.

Figure 1 shows a device for producing carbon nanomaterials. The device comprises a reactor 1, coaxially located anode 2 and cathode 3. The cathode 3 has the shape of a cup with a permeable bottom 4. Permeability is ensured by placing a hollow cathode 3 in the bottom 4 using either a liquid hydrocarbon feedstock or a porous membrane for gaseous feedstock. Anode 2 is located inside the cavity of the cathode. The cathode 3 is fixed, the anode 2 has the ability to move along the axis in order to establish the distance necessary for the breakdown to occur, as well as to establish the required distribution of electric potential, field, current density and charges in the discharge gap. The material of the anode 2 may be copper or graphite. The cathode 3 is made of metal, for example copper, the lattice placed in the bottom 4 is also made of copper (for liquid hydrocarbon raw materials), and the membrane is made of copper made by powder metallurgy (for gaseous raw materials). The hydrocarbon feed system 5 is located under the cathode 3 and is in contact with the bottom wall 5 of the cathode 3. The feed system 5 includes a pump 6 (gas for supplying gaseous, liquid for supplying liquid hydrocarbon feed). The DC power supply 6 is connected by its positive pole to the anode 2, and the negative to the cathode 3. The supply of buffer gas (argon or helium) is carried out through pipeline 7. The role of the buffer gas is to cool the fragments of graphite, as well as in the removal of energy that is released when connecting fragments into fullerenes and nanotubes. Gaseous reaction products are discharged through line 8.

To implement the method of producing carbon nanomaterials from hydrocarbon feedstocks, a vacuum is created in the reactor 1, then buffer gas is supplied through the pipeline 7. Next, the hydrocarbon feedstock is supplied through the permeable wall 4 using the pump 6 of the gas supply system 5. An electric voltage is supplied from the power supply 6 to the anode 2 and cathode 3, sufficient for the breakdown of the interelectrode gap and the establishment of a glow discharge. When a glow discharge plasma interacts with a hydrocarbon feed, bonds between long hydrocarbon molecules break and atomic carbon and hydrogen appear. In an electric discharge, carbon atoms attach electrons and, as negatively charged particles, move to anode 2 and accumulate on it in the form of nanocarbon material containing fullerene compounds and nanotubes. In a glow discharge created in the gap between the anode 2, located in the cavity of the hollow cathode 3, the average energy and electron concentration increase, which leads to more efficient ionization and excitation of hydrocarbon molecules, and, accordingly, to a more efficient synthesis of fullerenes and nanotubes from it.

When using liquid hydrocarbon raw materials, a new mechanism of plasma-chemical processes in the reactor appears: the decomposition of hydrocarbon raw materials and the synthesis of nanocarbon materials occurs not only in the plasma volume, but also on the cathode surface. When using liquid hydrocarbon feed, the cathode surface is covered with a thin layer of liquid in which, under the influence of an electric discharge, the processes of evaporation, decomposition, and dissociation of molecules occur. In this case, part of the molecules decomposes to atomic hydrogen and carbon directly in the cathode region, and part of the vaporized material dissociates the volume of the plasma.

Gaseous (methane, propane, butane, etc.) and liquid (oil, fuel oil, benzene, gas oil, etc.) hydrocarbons can serve as hydrocarbon feedstocks.

Hydrogen and other gaseous reaction products formed during the process are discharged through line 8.

Example 1

A device was developed for producing hydrogen and carbon nanomaterial, in which the plasma-chemical decomposition of the propane-butane mixture occurred under conditions of a glow discharge with a hollow cathode. The anode is graphite, with a diameter of 5 mm. The dimensions of the cathode made of copper were - diameter 60 mm, height 100 mm, the porosity of the permeable wall of the cathode made of copper and made by powder metallurgy, 0.25. The distance between the anode and cathode along the axis is 10 mm. The pressure in the chamber was 30 torr, the voltage was 4 kV, and the current was 150 mA. The productivity of carbon nanomaterial, broad-spectrum fullerenes (C60, C70, C84, C96, etc.) and multilayer nanotubes amounted to 100 grams per hour.

Example 2

A device for producing hydrogen and carbon nanomaterial, in which the decomposition of fuel oil, was implemented. The dimensions of the copper cathode were - diameter 60 mm, height 100 mm, the permeable wall is made in the form of a lattice of copper with a hole diameter of 0.3 mm. The anode is graphite, with a diameter of 5 mm. The distance between the anode and cathode along the axis is 6 mm. The pressure in the chamber was 40 torr, the voltage was 5 kV, and the current strength was 150 mA. The productivity of carbon nanomaterial, broad-spectrum fullerenes (C60, C70, C84, C96, etc.) and multilayer nanotubes amounted to 100 grams per hour.

The inventive method and device can reduce the energy intensity of the process, expand the types of hydrocarbons used and simplify the design of the device, as well as ensure the environmental friendliness of the process using inexpensive hydrocarbons, its high productivity (high yield of the final carbon nanomaterial to the feedstock).

Claims (4)

1. A method of producing carbon nanomaterials, including the evaporation / decomposition / dissociation in a volumetric thermal plasma of a carbon-containing material and its condensation on the surface, characterized in that a volumetric thermal plasma using a glow discharge plasma installed by applying an electric voltage sufficient to breakdown the interelectrode the gap between the coaxial hollow cathode, having the shape of a glass with a permeable bottom, and the anode located in the cavity of the cathode with the possibility of movement along the axis, Rich carbonaceous material is fed through the permeable bottom of the cathode and the surface is used as the anode outer surface and an inner surface of the cathode. 2. The method according to claim 1, characterized in that the carbon-containing material is in a gaseous or liquid state. 3. The method according to claim 2, characterized in that the carbon-containing material is selected from the group consisting of methane, propane, butane for gaseous carbon-containing material or from the group consisting of oil, fuel oil, benzene, gas oil for liquid carbon-containing material. 4. A device for producing carbon nanomaterials, comprising a reactor in which an electrode and a hollow counter electrode are placed, characterized in that the electrodes are aligned, the hollow counter electrode being a cathode having the shape of a glass with a permeable bottom, and the anode located inside the cathode cavity with the ability to move along their axis, while the permeable bottom of the cathode is made either in the form of a lattice when using liquid hydrocarbon material, or in the form of a membrane when using gaseous hydrocarbon wow stuff.