Formation of carbon black from Freons was studied in RF thermal plasma conditions. Equilibrium

compositions of the products were calculated by computer code ASTRA 4.0. The

experiments on thermal decomposition were performed in an RF thermal plasma reactor.

By comparing the calculated and experimental results a model was constructed on carbon

black formation in RF thermal plasma reactor

Babievskaya, I. V.

Főglein, K. A.

Gubicza, Jenő

Szépvölgyi, János

English

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 Theoretical and Experimental Aspects of Carbon Formation in Thermal Plasma  K. A. Főglein1, J. Gubicza3, I. Z. Babiewskaya4, J. Szépvölgyi1,2  1Institute of Materials and Environmental Chemistry  Chemical Research Center, Hungarian Academy of Sciences, Budapest, Hungary 2 Research Institute of Chemical and Process Engineering, University of Pannonia, Veszprém, Hungary 3Department of Solid State Physics, Eötvös University, Budapest, Hungary  4Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, Russia  Abstract Formation of carbon black from Freons was studied in RF thermal plasma conditions. Equi-librium compositions of the products were calculated by computer code ASTRA 4.0. The experiments on thermal decomposition were performed in an RF thermal plasma reactor. By comparing the calculated and experimental results a model was constructed on carbon black formation in RF thermal plasma reactor.   Keywords: Freons, carbon black, RF thermal plasma, modeling  1.  Introduction Decomposition of chlorine and fluorine containing hy-drocarbons (Freons) is in the focus of research for quite a time. It is reasoned by the ozone depletion capability of particular compounds on one hand, and their strong greenhouse effects on the other. Catalytic methods [1] and plasma processing [2] seem to be suitable technologies for the decomposition of halogenated methane derivatives. It has been established that decomposition of particular compounds in neutral or reactive atmosphere led to for-mation of different solid carbon species [3-5]. These find-ings may open new vistas in the processing of Freons. However, mechanism and kinetics of carbon formation during thermal decomposition is poorly understood yet.  In thermal plasmas the materials are treated in special circumstances such as very high temperature, intensive heat and mass transfer, very rapid cooling of reaction products and local equilibrium conditions. Thus, Freons can be completely decomposed in thermal plasmas along with the production of soot and carbon black (CB). In this paper results of thermodynamic calculations and experi-ments on the decomposition of three Freons (CCl4, CHCl3 and CFCl3) are discussed.   2.  Thermodynamic calculations In order to get information on the equilibrium composi-tions during decomposition of Freons in thermal plasmas we performed thermodynamic calculations in the tem-perature range of 298 - 6000 K by computer code ASTRA 4.0 based on the minimization of Gibbs free enthalpy.  It is known that in thermal plasma decompositions the first condensed material that appears on cooling of gase-ous species will be the main reaction product.  In the calculations we assumed that even solid carbon participates in the equilibrium. According to our calcula-tions, in order to get solid carbon as main reaction prod-uct the temperature of gaseous species should be de-creased with a high rate from 1000 to 298 K (Fig. 1). The carbon yields in equilibrium conditions are changing in the following order:  CFCl3 < CCl4 ≈ CHCl3 at 1000 K and CFCl3  ≈ CCl4 < CHCl3 at 298 K.  Fig. 1 Thermodynamic calculations on the solid carbon yields in argon atmosphere  In order to have a deeper insight into decomposition processes we performed thermodynamic calculations by taking into consideration the gaseous carbon radicals such as C, C2, C3, C4 and C5, as well. The calculations (Fig. 2) show that gaseous carbon radicals formed from CFCl3 are preserved at much lower temperatures than the same radi-cals from other precursors. Below 4000 K concentration of gaseous carbon originated from CFCl3, is much higher compared to the other ones which leads to formation of higher amount of soot and hence, formation of more CB in the CFCl3 case.    3.  Experimental The experiments were carried out in an RF thermal plasma reactor made of quartz glass. The RF generator operated at 27 MHz with plate powers of 1-4 kW [6]. All model compounds (CCl4, CHCl3 and CFCl3) were Aldrich products. Gases from Linde Co were used as cen- tral, sheath and carries gases, respectively. The initial gas compositions are listed in Table 1.   Fig. 2 Thermodynamic calculations on the yields of solid carbon and gaseous species, respectively in argon atmos-phere   Table 1 Initial gas compositions  The main reaction product was solid soot which mostly condensed on the air-cooled reactor wall. Further amounts of soot portions were separated in two collectors attached to the plasma reactor [6]. The solid soot contained differ-ent adsorbed hydrocarbons. To separate them and to get CB (C(s)) as main product, the soot was extracted by tolu-ene. The CB yield (YC(S)) was calculated as follows:   ) carbon·h  (molprecursor  of rateFeed)(mol·hformation blackcarbon  of RateY 1--1C(S) =              (1) The soot and CB samples were investigated by X-ray diffraction (XRD) in a Philips X’pert powder diffractome-ter using a CuKα radiation (λ=0.15418 nm) and a pyro-lytic graphite secondary monochromator.    4. Experimental design by STATISTICA code To get detailed information on the change of YC(S) against process conditions, the thermal plasma experi-ments were designed by STATISTICA code. In the ex-perimental design factors and levels should be deter-mined. The factors are parameters that change independ-ently of each other during the experiments, while the lev-els are the values of factors. In our work composition and feed rate of Freons and the plate power of RF generator were selected as factors. Each factor had three levels. The software generated a 3×3 Greco-Latin Square (Table 2). Thus, nine experiments should be performed to have as much information as possible on the effects of factors on YC(S).   Table 2 The Greco-Latin Square for Systems 1-3 Run No. Precursor Plate power (kW) Feed rate (mol·h-1) YC(S) 1 CCl4 1.6 0.15 10.8 2 CCl4 2.1 0.30 15.7 3 CCl4 2.6 0.45 29.7 4 CHCl3 1.6 0.30 35.6 5 CHCl3 2.1 0.45 36.2 6 CHCl3 2.6 0.15 34.3 7 CFCl3 1.6 0.45 43.7 8 CFCl3 2.1 0.15 28.7 9 CFCl3 2.6 0.30 36.6  From the plotting of experimental results (Fig. 2) we can establish that  - the carbon black yield depends on composition of precursors,  - the YC(S) values are changing in the following or-der: CCl4 < CHCl3 < CFCl3, - the plate power has a complex effect on YC(S) and   - increase of feed rate leads to higher YC(S) values.     Fig. 3 CB yield as plotted against the levels of different factors  The experimental results relating to the effect of pre-cursor composition on YC(S) were differing from those of thermodynamic calculations. In equilibrium conditions, the lowest YC(S) was obtained for CFCl3, while in the ex-periments the highest yields were detected exactly in this case. The reasons will be discussed below.  The mean plasma temperature and the mean residence time of reagents are increased with the plate power. It results in higher ionization and thus, more effective de-composition of precursors. However, the chance of sec-ondary reactions leading to undesirable products also in-creases. Decomposition of Freons to carbon and reactions of primary products are competitive processes which make the overall picture much more complicated.  Even the feed rate of Freons has an effect on YC(S). The higher the feed rate the higher the carbon concentration in the hot zone which leads to higher CB yields.   Gas composition (m%) Sys-tem Precursor  Precursor Ar H2  1 CCl4 0.93 99.07 - 2 CHCl3 1.15 98.85 - 3 CFCl3 1.15 98.85 - 4 CFCl3  3.52 96.45 0.03  5. Results and discussion Comparison of experimental data (Table 2) with the re-sults of thermodynamic calculations (Fig. 1) reveals that - all experimental YC(S) values are lower than the equilibrium yields calculated at 1000 K.  - In case of CFCl3 and CCl4 higher YC(S) values were obtained in the experiments as compared to the equilibrium ones at 298 K.  - Order of precursors in terms of YC(S) are different in the equilibrium and actual cases, respectively; the experimental YC(S) values increase in the fol-lowing order: CCl4 < CHCl3 ≈ CFCl3, while in thermodynamic equilibrium the relevant order is CFCl3 < CCl4 ≈ CHCl3 at 1000 K and CFCl3  ≈ CCl4 < CHCl3 at  298 K. The first finding as above refers to non-equilibrium conditions during the decomposition of given precursors in particular conditions. The precursors completely de-composed to smaller fragments. However, these interme-diates had a sort residence time in the hot zone. They cool down very rapidly and thus, may exist even at low tem-peratures.  To investigate the degree of CB crystallization against process conditions in the case of CFCl3-Ar-H2 system (Table 1, System 4) XRD measurements were carried out on soot and CB samples from different sections of the experimental set-up. From the diffractograms (Figs. 4 and 5), we determined the integrated area, the width and the height of peaks at 2Θ values of 20.20, 29.36 and 39.56 degrees, respectively (Table 3). The crystallite size was calculated as follows:    )2(cos9.0ΘΔ⋅Θ⋅= λD  (2) where D (nm) is the grain size of crytallites, λ (nm) is the wavelength of X-rays used for the measurements and Θ (rad) is the diffraction angle of the first strong X-ray peak. The nearest neighbor distance was calculated as follows:    Θ⋅⋅=sin29.0 λa  (3) 10 20 30 40 50 60 counts [a.u.]****** *********reactorcollector 1collector 2unidentified***  C6Cl6**************2 theta [deg]* Fig. 4 X-ray diffractograms of different soot samples  10 20 30 40 50 60collector 2collector 1reactor 2 theta [deg]counts [a.u.]unidentified Fig. 5 X-ray diffractograms of different carbon black samples   From the XRD investigations (Figs 4-5 and Table 3) the following conclusions can be drawn:     - soot collected in the reactor is amorphous, while products separated in collectors 1 and 2 are mainly crystalline materials; the main crystalline phases are hexachloro-benzene and an up-to-now unidenti-fied material.  - The crystalline fraction in C(S) increases with the distance from the plasma flame (reactor → collec-tor 1 → collector 2). The integrated area ratio of soot and CB samples normalized by their mass is about 0.5 (Table 3 (a)) which means that CB is more crystalline than the soot.  - The grain size ratio of the soot and CB is 1.5 (Ta-ble 3 (b)) indicating greater primary grains in the soot.  - The centre of the amorphous band shifted from 2Θ degree of 22.6 to higher values against the distance from the plasma flame (Table 3 (c)). The nearest neighbor distance calculated from the position of the centre of amorphous band decreased against the distance from plasma flame (Table 3 (d)), as well.  Based on above findings we constructed a model on carbon black formation in RF thermal plasma conditions. The soot particles have an inner core consisting of both amorphous and crystallized carbon, covered by a surface layer of organic compounds (such as C6Cl6). The mean crystal size of soot is 33.6 nm (Table 3 (b)). During tolu-ene extraction the surface layer is removed along with some portion of the bulk as it is justified by the decrease of mean grain size from 33.6 to 22.4 nm (Table 3 (b)).  The higher the distance from the plasma flame, the closer are the carbon atoms in the amorphous fractions. It results in a shifting of the amorphous hump in X-ray dif-fractogram (Table 3 (c)) to higher 2Θ values indicating the decrease of the nearest neighbor distance (Table 3 (d)). At greater distances from the plasma flame products being more crystallized even in the inner core were sepa-rated. We explain it by the longer condensation time of particular species as compared to particles condensed  immediately on the air-cooled reactor wall. The CB con-tent of the soot also depends on the distance from the plasma flame: the higher the distance the lower the CB content of the soot (Table 3 (e)).  Conclusions Effects of processing conditions on carbon black yield were determined during decomposition of CCl4, CHCl3 and CFCl3 in RF thermal plasma reactor. It was found that the CB yield mainly depended on the composition of Freons. The feed rate and the plate power affected the CB yield in less extent.  XRD investigations of the soot and the CB revealed that the soot particles have an inner core consisting of amor-phous and crystallized carbon which is covered by a sur-face layer of chlorine-containing organic molecules. Ratio of the surface layer and the inner core changes against the thermal history of materials i.e. against the distance of sampling from the plasma flame.  Table 3 Results of XRD investigations for System 4 XRD results XRD results Peak angle  (2Θ degree) a b Distance from the top of plasma flame (cm) c d e 20.20 0.482 33.6/22.4=1.5 47.5 (reactor) 22.6 0.354 82.57 29.36 0.482  82.5 (collector 1) 22.9 0.350 68.00 39.56 0.508  123.2 (collector 2) 23.0 0.348 55.36 Mean 0.491 33.6/22.4=1.5  a Integrated area ratios of the soot and C(S) samples normalized by their mass  b Grain size ratio of the soot and C(S) sample (D/nm)  c Angle of the center of the amorphous X-ray peak depending on the distance from the plasma flame (2Θ)  d Nearest neighbor distance depending on the distance from the plasma flame (a/nm)  e CB content of the soot (m/m%; mass ratio of CB and soot, after and before the extraction)   Acknowledgement This work has been supported by Hungarian Scientific Research Fund (OTKA) Grant Nos. F047057 and T047360.  References [1] Y. Takita, J. Moriyama, Y. Yoshinaga, H. Nishigu-chi, T. Ishihara, S. Yasuda, Applied Catalysis A-General 271(1-2), 55 (2004). [2] K. A. Főglein, P. T. Szabó, I. Z. Babievskaya, J. Szépvölgyi, Plasma Chemistry and Plasma Process-ing 25 (3), 289 (2005). [3] L. B. Avdeeva, T. V. Reshetenko, V. B Fenelonov, A. L. Chuvilin, Z. R. Ismagilov, Carbon 42(12-13), 2501 (2004). [4] J. J. Wang, M. Y. Zhu, R. A. Outlaw, X. Zhao, D. M. Manos, B. C. Holloway, Carbon 42(14), 2867 (2004). [5] U. Narkiewicz, W. Arabczyk, W. Konicki, Fullerenes Nanotubes and Carbon Nanostructures 13 Suppl. 1, 99 (2005). [6] K. A. Főglein, J. Szépvölgyi, P. T. Szabó, E. Mészáros, E. Pekker-Jakab, I. Z. Babievskaya, Plasma Chemistry and Plasma Processing 25(3), 275 (2005) 

Theoretical and experimental aspects of carbon formation in thermal plasma

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