CC BY
1УДК 541.16 Babayev R.K., Rzayeva Z.J.
Babayev R.K.
Ph.D., Associate Professor Azerbaijan State University of Oil and Industry (Baku, Azerbaijan)
Rzayeva Z.J.
Master's degree student Azerbaijan State University of Oil and Industry (Baku, Azerbaijan)
STUDY OF THE KINETICS OF CNT SYNTHESIS PROCESSES
Аннотация: the article is devoted to the study of the kinetics of the synthesis process of carbon nanotubes. The kinetic dependences of the CNT synthesis process on the feed rate of the initial carbon-containing substances were determined, obtained at different temperatures in the reaction zone. A mathematical description of the mass transfer kinetics of the CNT synthesis process in a capacitive reactor has been developed. The functional dependence of the change in CNT mass over time has been obtained.
Ключевые слова: carbon nanotubes, kinetics, mass transfer, mathematical model.
The creation of effective and highly economical structural materials is one of the main tasks of modern materials science. This problem is best solved by creating composite materials, in which the joint work of dissimilar components is tantamount to obtaining a new material, the properties of which are qualitatively and quantitatively different from the properties of each of the components [1-3].
One of these promising composite materials is composites in which the reinforcing filler is various nanostructures. Among them, carbon nanotubes (CNTs) occupy a special place. Currently, more and more data are emerging on the effect of nanotubes on the mechanical properties of composite materials obtained on their basis.
Work on the creation and study of the properties of such composite materials will be carried out more widely if mass production of cheap, defect-free nanotubes is established.
There are many experimental methods for producing CNTs. But due to insufficient knowledge of the growing patterns nanotubes, their cost is quite high. The creation of carbon fiber with the addition of carbon nanotubes will greatly improve the performance of structural materials [4-6].
A CNT synthesis reactor with a heated substrate was chosen as an experimental setup (Fig. 1). This equipment implements the method of gas-phase chemical deposition of crystalline nanocarbon on a metal-metal oxide catalyst.
Carbon-containing gas (propane-butane mixture 1) and inert gas (argon, cylinder 2) enter adsorption filters 3, where they are dried. Next, the gas components enter reactor 4. But before this, the carbon-containing gas undergoes preheating. NiMgO catalyst particles measuring 86...140 ^m in a monolayer are preliminarily applied to the substrate. The reacted gaseous products are released into the atmosphere.
Fig.1 Experimental setup for CNT synthesis.
Experimental studies aimed at studying the influence of technological parameters on qualitative and quantitative characteristics and finding the kinetic curves of the process of CNT synthesis from a propane-butane mixture consisted of several series of experiments carried out in an industrial CNT synthesis reactor with a heated substrate. Each series differed in the temperature of the process. Technological samples
were taken from each experiment to determine the structure and qualitative characteristics of CNTs.
Series of experiments were also carried out at a reaction zone temperature of 610-640 ° C, a carbon-containing gas volume flow rate of 5,10.15 and 20 l/min. The time of oxygen displacement from the reaction zone and the exit of the CNT synthesis reactor to the mode is 2.5 hours. The duration of the synthesis process ranged from 1 to 90 minutes. The third series of experiments was carried out at a temperature in the reaction zone of 670 ° C, a volume flow rate of carbon-containing gas of 5.10.15 and 20 l/min. The time of oxygen displacement from the reaction zone and the exit of the CNT synthesis reactor to the mode is 2.5 hours. The duration of the synthesis process ranged from 1 to 90 minutes with a discreteness of 1, 5, 10, 20, 40, 90.
For all these experiments, kinetic dependencies of the material mass gain on the time of the CNT synthesis process were obtained.
The processes of formation of carbon structures on the surface of a metal catalyst during the thermal decomposition of hydrocarbons are determined by a set of macroparameters (temperature, pressure, concentration), and the rate of the process is determined by the limiting stage - external mass transfer from the gas flow to the surface of the catalyst. Within the framework of the macrokinetic approach, using experimental data, the kinetic coefficients are determined [7-8]:
• coefficient of effective mass transfer from the gas flow to the catalyst surface, Pc [m/s], which determines the intensity of the CNT growth process,
• catalyst surface activity coefficient, equal to the fraction of the catalyst surface involved in the synthesis process compared to the initial one, KFm, which determines the process of catalyst deactivation.
As a result of approximation of experimental kinetic dependencies, a linear mathematical model of the functional dependence of the change in CNT mass over time was obtained.
Y = -0,00921-x2+7,268-x+11,018 (1)
The equation is used to calculate the effective mass transfer factor values:
Pc "W (2)
where m (0) is the mass flow at time t = 0, kg/s, Fi - Initial catalyst surface (t = 0), m2, Cp - hydrocarbon concentration in the stream, kg/kgsm, c*-equilibrium concentration of hydrocarbon on the catalyst surface, kg/kgsm, cc - mass fraction of chemically bound carbon in hydrocarbon.
The results of calculating the values of the effective mass transfer coefficients at various temperatures and volumetric flow rates of a mixture of propane and butane are presented in Table. 1.
Table 1. Calculated values of effective mass transfer coefficients.
Temperature in the reaction zone, С Volume consumption of hydrocarbon raw materials, L/min Effective mass transfer coefficient kq/sec m2
610 10 1,13
15 1,65
640 5 0,57
10 1,32
15 1,54
20 1,76
670 5 0,58
10 1,23
15 1,36
20 1,63
Taking into account the initial condition, solving this equation allows us to obtain a dependence, which is a mathematical description of the kinetics of this process. Accordingly, the problem arises of calculating the kinetic coefficient Pc and the value of the maximum specific yield for a given temperature and feed rate of a carbon-containing substance.
The value of the activity coefficient of the catalyst surface is determined as a functional dependence on the specific characteristic of the catalyst performance (specific carbon yield, Ku), numerically equal to the ratio of the mass of the obtained CNT to the mass of the catalyst involved in the synthesis process
KF =
K11*M
(cp-c*)*Fi*ft
(3)
Results of calculating the dependence of the activity coefficient catalyst surface versus specific carbon yield are shown in Fig.2.
Fig.2 Dependence of the activity coefficient of the catalyst surface
at a temperature of 610 C.
The resulting dependence of the change in CNT mass on time is presented in Fig.3.
12 10 ^ 8
3 6
*
i 4
2 0
time, s
Fig. 3 Dependence of change in CNT mass on time.
СПИСОК ЛИТЕРАТУРЫ:
1. Nanotechnology in the next decade. Forecast of development direction. Per. from English / Ed. M.K. Rocko, R.S. Williams and P. Alivisatos. M: Mir, 2002;
2. Eklund, P.C. Large-scale production of single-walled carbon nanotubes using ultrafast pulses from a free electron laser / P.C. Eklund et al. // NanoLetters. - 2012. - V. 2. - I. 6. - P. 561-566;
3. Yu L., Lv Y., Zhao Y., Chen Z. Scalable preparation of carbon nanotubes by thermal decomposition of phenol with high carbon utilizing rate //Mater. Letters, 2017, vol. 64, p. 2145-2147;
4. Redkin A.N., Kipin V.A., Malyarevich L.V. Synthesis of carbon fiber nanomaterials from ethanol vapor on a nickel catalyst / Inorg. materials, 2006, T. 42, No. 3, pp. 284-287;
5. Wang, G. Simultaneous production of hydrogen and multi-walled carbon nanotubes by ethanol decomposition over Ni/Al2O3 catalysts / G. Wang, H. Wang, Z. Tang, W. Li, J. Bai // Appl. Catal. B: Environmental, 2019, 88, 142-151;
6. Rodriguez, N.M. Carbon nanofibers: a unique catalyst support medium / N.M. Rodriguez, MS. Kim, RTX Baker. // Journal of Physical Chemistry. - 2013. - Vol. 98. - № 10. - P. 1310813111;
7. Synthesis of carbon nanotubes from acetone / A.V. Melezhik, M.A. Smykov, E.Yu. Filatova, A.V. Shuklinov, R A. Stolyarov, I.S. Larionova, A. G. Tkachev // Chemical technology, No. 4, 2012 P.197 - 206;
8. Promising nanomaterials based on carbon L.V. Kozhitov, I.V. Zaporotskova, V.V. Kozlov Bulletin of VolSU. Episode 10. Vol. 4.2019
