PRODUCTION OF CARBON NANOMATERIALS FROM
HYDROCARBON FEEDSTOCKS BY MEANS OF AN ARC DISCHARGE WITH RECESSED ELECTRODES.
Guzel Rafikovna Ganieva1, Boris Akhunovich Timerkaev2, Gulnara Damirovna Yalaltdinova3
1Kazan Federal University
2 General Physics Department, Kazan National Research Technical University – KAI, 420111, K. Marx St., 10, Kazan, Russia
3 Kazan National Research Technical University – KAI, 420111, K. Marx St., 10, Kazan, Russia e-mail (guzel1003@rambler.ru):
ABSTRACT
In this paper, they proposed an experimental unit for the decomposition of heavy hydrocarbon raw materials in a recycled electric arc plasma in order to produce finely dispersed soot and carbon nanotubes.
The advantage of this unit is that the electrodes are placed directly in raw material. The discharge is ignited in the thickness of the hydrocarbon raw material. Unlike traditional methods of carbon nanotube obtaining by the evaporation of graphite electrodes in an arc discharge, the proposed method uses hydrocarbon raw materials as the source of atomic carbon, for example, fuel oil, oil, bitumen, and the arc discharge ignites in the thickness of this raw material. In this case, carbon nanotubes are formed on the electrodes. The paper presents electron microscopic analysis of carbon deposits formed on electrodes during an experiment.
Keywords: heavy hydrocarbon raw materials, fuel oil, recessed arc discharge, plasma chemical method, copper electrodes, carbon nanotubes.
INTRODUCTION
Oil is the most popular mineral in the world in recent years. Without it, it is impossible to imagine almost any branch of modern industry. The need for oil increases every year, therefore, production should also be increased. Despite the increasing volumes of oil production in the world, the reserves of readily available oil in the earth depths are limited. The analysis of oil product consumption all over the world and the assessment of residual stocks of traditional oil indicate that it is necessary to use hard-to-recover oil reserves, which are the alternative sources of hydrocarbons. However, hard-to-recover reserves are more
"expensive" resources than traditional ones. Since hard-to-recover reserves (heavy oil and natural bitumen) are characterized by a high content of aromatic hydrocarbons, resinous asphaltene substances, a high concentration of metals, mechanical impurities and sulfur compounds, high density and viscosity, an increased coking ability, which leads to high production costs, they can not be pump through the existing oil pipelines and their processing is unprofitable according to the classical schemes. The extraction of heavy highly viscous oils and natural bitumen seems to be expedient only through the development and the application of efficient technologies for their processing. Therefore, the most complete use of oil by its processing deepening will remain an urgent problem throughout the world for many decades. The production of valuable products, such as carbon nanomaterials, from oil and refinery waste through the use of plasma methods is of particular interest.
METHODS
Plasma-chemical processing of oil and oil refining residues is one of the promising methods. Several directions are worth mentioning here: plasma-chemical decomposition of heavy oils to obtain light fractions, desulfurization of oil, the production of carbon nanomaterials from oil and from oil refinery residues.
There are many ways of carbon nanotube obtaining according to literature. The production of carbon nanotubes by an electric arc method from graphite, laser ablation and chemical deposition from the gas phase are considered to be the most common methods. However, in recent years, works appeared on the production of carbon nanomaterials in an electric discharge from carbon-containing raw materials in vacuum [1-7]. Liquid and gaseous hydrocarbons are used as raw material, which are affected by a glow discharge. There are also the works in which a discharge is organized near a working fluid surface. The interaction of a discharge with the liquid surface is maintained by a magnetic field [8,9]. Surface boiling is observed during this process, which leads to a surface evaporation of the hydrocarbon raw material. There are the studies on the interaction of an electric arc argon and nitrogen plasma with fuel oil and oil in order to obtain light oil fractions [10]. Along with the advantages associated with the acceleration of chemical reactions and a deeper processing of raw materials, this method has a number of drawbacks: the complexity of plasmatron design, the short life of electrodes, and the need to prepare sprayed hydrocarbon feedstock by its preheating.
Plasma-chemical technologies are more promising and simple in the organization of a discharge in the thickness of hydrocarbon raw materials [11-13]. Unlike traditional methods of hydrocarbon raw material processing, this approach to a discharge organizing directly into raw materials simplifies the process of a working fluid preparation. When the processed feed is exposed to a gas-discharge plasma, an electric energy is transferred to the electrons. During the interaction with hydrocarbon molecules, these fast electrons are able to break carbon bonds. Thus, when plasma interacts with hydrocarbon molecules, a plasma-chemical conversion of the hydrocarbon feed takes place.
In order to study the results of an arc discharge action on heavy hydrocarbon feedstock [9,14,16,17], an experimental unit has been developed and created, including a ceramic tank with hydrocarbon feed, connecting wires, a high-current source of electrical power, measuring devices and a fume hood.
In this paper, fuel oil is used as a hydrocarbon feedstock. The production of carbon nanomaterials occurs in a ceramic tank filled in with hydrocarbon raw materials, in which two copper electrodes are present at a certain depth. The installation scheme is shown on Fig. 1.
Fig. 1. Schematic diagram of the experimental setup: 1 - container with the raw hydrocarbons, 2 - electrodes, 3 – connecting wire, 4 - static voltmeter, 5 - ammeter, 6 –ballast resistance box, 7 - the power
supply, 8 – stand.
Before the beginning of the experiment, the ceramic container 1 is placed under the hood on a special
distance to ensure stable burning of the arc discharge. The values of current and voltage are measured using a voltmeter 4 and an ammeter 5.
Under the influence of an electric arc on hydrocarbon raw materials, the decomposition of complex hydrocarbon molecules into simple fractions takes place. Due to a high temperature of an arc, some pressure is created that is able to support the plasma region within the hydrocarbon raw material. The edges of this area are in contact with fuel oil. Because of a high temperature, high-boiling fractions of complex hydrocarbons find themselves in the plasma region and, under the action of fast electrons and high-energy ions, they break up into small fractions.
When an electric arc discharge is arranged in the hydrocarbon fluid at a certain depth, an electric arc burns in hydrocarbon vapor evaporated into the discharge area from an inner surface of the gas-vapor bubble.
Some pressure is set in an electric arc, deepened in fuel oil, which corresponds to the depth of an arc immersion. When an arc burns in fuel oil, an electric arc channel will be filled with gases and hydrocarbon vapors of a wide variety of fractions, a large number of which contains gasoline and other light fractions, which are dissolved partially in fuel oil. That part of the fuel oil that contacts the arc directly will be in a boiling state, supplying various oil fractions to the discharge area. Surface boiling and relatively low thermal conductivity of fuel oil prevent overheating of the main mass of fuel oil and its coking.
Hydrocarbon molecules will be attacked by fast electrons and discharge ions, as well as by excited atoms and the molecules of hydrocarbon gases in the electric discharge region. Atomic carbon will also be formed in the volume of the plasma, which is deposited on the electrodes.
RESULTS
The carbon outgrowths formed on electrodes were subsequently studied in various ways. Figure 2 shows the photograph of carbon outgrowths on the electrodes. Immediately after the experiment, they also have residual fuel oil.
Fig.2. Carbon build-up on electrodes formed during the decomposition of heavy hydrocarbon feedstocks.
In order to remove fuel oil, the growths were thoroughly washed in gasoline first, then they were heated in an oven at 800 °C. An electron-scanning microscope was used to analyze the formed growths. A cleaned sample was placed on a mesh (a thin metal disk) and then into the sluice compartment of the microscope.
The results obtained by Grossbeam Zeiss Auriga electron-scanning microscope are presented on Figures 3-3.3.
Fig.3. Electron microscopik image. Cathode formation. Enlargement 200000х.
Fig.3.1. Electron microscopik image. Anode formation. Enlargement70000х.
Fig.3.2. Electron microscopik image. Cathode formation. Enlargement 125000х.
Fig.3.3. Electron microscopik image. Anode formation. Enlargement 260000х.
Electron microscopic analysis of soot showed that the deposits contain a large number of carbon nanotubes (fibers) of different length and structure. Nanostructures formed in a chaotic order in the form of tightly intertwined threads. Since nanotubes are twisted together, it can be assumed that they have a complex structure. On (Fig.3.2.) we can see multilayered nanotubes of "Russian matryoshka" type. A nanotube has a diameter of 44.04 nm, a nanotube similar to it, which is inside it, has the diameter of 18.46 nm. Figure 3.3 shows that there are nanotubes wrapped in a graphene sheet. Apparently in the course of the experiment, the conditions are created for the growth of multilayer nanotubes of different configurations, which can be very valuable for structural applications during the creation of composite materials using carbon nanotubes.
DISCUSSION
The evaluation of possible use for this type of discharge with recessed electrodes for a deep processing of petroleum products is relevant from the standpoint of search for optimal options to obtain valuable light fractions and nanomaterials. Thus, the production of carbon nanotubes from liquid hydrocarbons makes it possible to obtain carbon nanostructures. An average diameter of single-walled nanotubes ranges from 11.81 nm to 18.46 nm, the diameter of multilayered ones ranges from 25.72 nm to 44.04 nm. The torsion of carbon nanotubes does not allow one to know their exact length.
In the process of heavy hydrocarbon raw material decomposition, the problem of electrode cooling is also solved, since the raw material serves as a coolant.
CONCLUSIONS
Thus, after complex studies of the plasma chemical effect on heavy hydrocarbon feedstock, it should be noted that an experimental unit was designed and built; Optimal conditions for an arc discharge burning in
interaction with a hydrocarbon feedstock is studied; they showed the efficiency of such a discharge organization for to obtain multilayered carbon nanotubes.
SUMMARY
The decompositions of heavy hydrocarbon raw materials by recessed electric arc plasma make it possible to obtain high-quality carbon nanotubes with a low content of impurities. Also, the development and the creation of new efficient technologies for the plasma chemical processing of heavy non-traditional hydrocarbon raw materials will make it possible to improve the reproduction of Russia raw material base significantly through economically viable involvement in the development of high-viscosity oil fields [18].
ACKNOWLEDGEMENTS
The work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University.
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