- Email: [email protected]
Carbon black processing by thermal plasma. Analysis of the particle formation mechanism
Chemical Engineering Science 56 (2001) 2123}2132
Carbon black processing by thermal plasma. Analysis of the particle formation mechanism FreH deH ric Fabry , Gilles Flamant *, Laurent Fulcheri Institut de Science et de Ge& nie des Mate& riaux et Proce& de& s, IMP-CNRS, Odeillo, B.P.5, 66125 Font-Romeu Cedex, France Ecole des Mines de Paris, Centre d'Energe& tique, B.P.207 F-06904 Sophia Antipolis, France Received 20 March 2000; received in revised form 31 July 2000; accepted 20 September 2000
Abstract An original plasma process for carbon black and hydrogen synthesis is presented. A pilot scale (100 kW) reactor is developed to obtain new carbon black grades from hydrocarbon cracking. Even if three populations of particles are observed, the plasma carbon black is mainly characterised by spherical nanoparticles with a high degree of turbostratic organisation. A particle formation mechanism is proposed on the basis of a multiscale analysis of the plasma reactor accounting for: heat and mass balance, RTD measurement and internal #ow modelling and particle characterisation by TEM. The mechanism is characterised by two steps: "rst, a low temperature particle growth and second, a high temperature annealing. As a result, particle structure is strongly related to internal mixing phenomena. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Carbon black; Plasma technology; Formation; Hydrocarbon cracking; Nanoparticle
1. Introduction Carbon nanoparticles are named `Carbon blacka which is actually a generic term for a family of products, which are usually referred according to the methods (or raw materials) used in their manufacture. For example, standard products are known as: `furnace blacka, `channel blacka, `thermal blacka, `acetylene blacka, etc. (International carbon black directory and source book, 1999). All these manufacturing processes operate at high temperature (typically in the range 1700}2500 K) and are based on the reaction of a hydrocarbon feedstock to produce carbon particles and gaseous by-products such as: H O, H , CH and some pollutants, CO , SO and NO . Generally, the reaction is combustion in the air, V except for acetylene black that is obtained by self-decomposition of the gas. In contrast with classical soot, which contains always a lot of inorganic contaminants and extractable organic residues, commercial blacks contain over 97}99% elemental carbon depending on the manufacturing process (Donnet, Bansal, & Wang, 1993).
* Corresponding author. Fax: #04-68-30-77-58. E-mail address: #[email protected] (G. Flamant).
The worldwide production of carbon black is about 6 million tons per year, i.e. over £4 billion per year are currently being spent on carbon black by consumers (this puts carbon black well ahead of sulphur, chlorine and other non-metallic elements in terms of global expenditure). The key properties of carbon black are: E rubber reinforcing mechanical properties: improves tear strength, wear resistance, modulus and fatigue characteristics (Schwob, 1994), E pigment properties: light absorption, tinting strength, and UV protection, E electrical properties: improves electrical conductance, enhances maintenance of electric charge, etc. Today, applications in tyre and rubber product manufacturing industry represent 90% of world carbon black production. The end-use properties of carbon black depend on: particle size distribution, morphology of particles and aggregates, speci"c surface area, elementary grain microstructure, surface chemical bonds, etc. Those characteristics are process dependent. For example, furnace black is composed of more or less spherical particles of colloidal size (10}100 nm) coalesced into aggregates and agglomerates; the particles in thermal
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Table 1 Characteristics of Industrial Carbon Blacks and Graphite Powder from Fulcheri, Schwob, and Flamant (1997) Chemical process Process (estimated name temperature level)
Industrial manufacturer (product)
Feedstock
Mean particle diameter
BET Speci"c area (m/g)
¸ ? (nm)
¸ ! (nm)
Typical application
Partial combustion (1400}1900 K)
Furnace
Cabot (N330)
Heavy oil Natural gas
26}30 nm
70}90
1.6
1.2
Rubber
Selfdecomposition (2500 K)
Acetylene
SNAA (Y 50 A)
Acetylene
30}40 nm
60}80
2.6
2.5
Battery
Pyrolysis (3000 K)
Acheson
TIMCAL (KS 44)
Coke
80%(44 m
10
'100
'100
Lubricant
¸ and ¸ are, respectively, the crystal size in the hk plane and c direction (graphite lattice). ? A
blacks are somewhat coarser, spherical, typically 120}200 nm and up to 500 nm. In order to compare the characteristics of some typical products with respect to their application, mean particle size, BET speci"c surface area, electrical resistivity and dimensions of elementary graphite crystals are listed in Table 1 for three industrial carbon particles (Fulcheri et al., 1997). This table shows clearly the large di!erence between carbon black and graphite powder, but also that small di!erences in characteristics a!ect strongly the end-use properties of the manufacturing products. For that reason, more than 100 di!erent grades of carbon blacks are currently o!ered by commercial manufacturers (Adamski, 1999). Accounting for both previous remarks, we have identi"ed a niche for a new high temperature process that permits to vary the carbon black characteristics and consequently a thermal plasma reactor was developed (Fulcheri, Flamant, Variot, & Badie, 1995). In opposition with existing industrial processes, in such systems, the temperature is not limited by chemical thermodynamics since the reaction enthalpy is supplied inside the reactor by an electric arc. Thermal plasma for particle processing are currently under development for powder densi"cation (Pfender, 1999) and "ne particle synthesis (Baronnet, 1992; Kong & Pfender, 1997), but there exists only few MW-sized industrial units in operation worldwide. The most famous is the TIOXIDE plant (UK) which produces 0.3 nm TiO particle by reacting TiCl with O in a plasma jet (Tioxide: Plasma process technology, 1988). This paper addresses the following objectives: E to demonstrate the possible synthesis of new carbon black grades by thermal plasma, E to exemplify a multiscale and multidisciplinary approach,
E to understand particle formation mechanism in such high temperature process. We propose a three-step approach to link together the overall process performances, the plasma carbon black structure and the internal #uid #ow.
2. The process The main innovations of the plasma systems with respect to classical processes are: E Clean process: no emission of CO , and other pollu tants (SO , NO , etc). 6 V E Two products: carbon black and hydrogen. E New products: the operating temperature (1000}10 000 K) is large enough to generate new carbon nanostructures. The complete system is schemed in Fig. 1. The plasma torch 1 supplied by a 100 kW 3-phase AC generator produced a large volume expanded plasma #ow inside a reaction chamber 3. This plasma torch, initially developed by Gold et al. (1981), used three graphite electrodes 13 mm in diameter. The chamber was a 2 m high, 0.28 m i.d. graphite cylinder inserted in a 0.40 m i.d. water-cooled stainless steel vessel. The feedstock was introduced in the reaction chamber either on the side 2 or at the top, in the annular zone between the three graphite electrodes and their sleeves. The solid and gaseous products were collected in the cooling chamber 4 and separated in the bag "lter 5. The following measurements were performed: E Current and voltage of the three phases (r.m.s.), E Plasma gas #ow rate,
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Table 2 Plasma generator characteristics for a 3 Nm/h plasma gas #ow rate Plasma gas
N
Ar
Ar}H (5%)
; (V) I (A) H (kWh/Nm) .
195 199 20.2
55 2030.6 2.8
71 198 5.2
H . is the plasma enthalpy. N
Fig. 1. Schema of the experimental system. (1) 3-phase AC plasma generator, (2) side injector of hydrocarbons, (3) 2-meter high reactor vessel, (4) cooling and conveying zone, (5) "lter for separation of carbon black (CB) and hydrogen (H ).
E Water #ow rates and temperature variations of the cooling system (electrode sleeves, roof and reactor vessel) to determine the heat loss for each subsystem, E feedstock gas #ow rate (natural gas or ethylene), E o!-gas composition by Gas Phase Chromatography (GPC), E internal wall temperature by optical pyrometry, E mean plasma temperature from Optical Emission Spectrometry (OES). In addition, particle characterisation was performed by XRD, TEM and XPS. A detailed presentation of the measurement and characterisation systems is proposed in Fabry (1999).
3. Overall process performances 3.1. Plasma characteristics The power supply was composed of a three-phase electric boosted transformer with serial inductances as shown in Fig. 2. The nominal power of the boosted transformer was 175 kVA with following outlet charac-
teristics: short circuit voltage: 760 V, maximum current: 266 A, frequency: 50 Hz. Five sets of variable inductance may be used but, it was generally "xed at 3.52 or 4.48 mH. Arc voltage, current and plasma enthalpy (Hp) are listed in Table 2 for three gases: Nitrogen, Argon and Argon-Hydrogen 5%. The plasma enthalpy corresponds to net values which account for the plasma torch overall e$ciency. This table illustrates the strong e!ect of the plasma gas on arc voltage and plasma enthalpy. Under these conditions, the plasma enthalpy varies from 2.8 to 20.2 kWh/Nm for Ar and N , respectively. 3.2. Heat balance over the reactor Heat losses were measured over each part of the reactor: electrode sleeves, roof and reactor vessel, and plasma gas. The results are shown in Fig. 3 for the operation parameters listed in Table 2. More than 40% of the heat losses took place at the roof because of the very high temperature in this zone. Measurement of plasma temperature using OES is presented by Fabry (1999). With an Ar}H (20%) mixture, it reached about 11 000 K in the arc zone. Furthermore, at the same position, the wall temperature ranged from 1500 to 2000 K depending on plasma gas. 3.3. Mass balance over the reactor The mass balance over the reactor was performed during a side injection of ethylene in an argon plasma
Fig. 2. Schema of the power supply.
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Fig. 3. Bar chart of the thermal losses over each plasma reactor sub-systems for Ar, Ar}H (5%) and N Plasmas.
gas. The global conversion e$ciency of C H and the carbon yield (ratio between carbon atoms in the particulate and carbon content in the feedstock) are shown in Fig. 4a as a function of time. Both values increase with time because the reactor temperature was still increasing during the injection. After 50 min the quasi-equilibrium values were, respectively, 75 and 60%. Because argon is the less e$cient gas, as shown in Table 2, this conversion e$ciency and this carbon yield may be considered as lowest performances for the process. At the exhaust, H , C H , C H and CH were iden ti"ed, their molar fractions were determined accounting for the following global chemical reactions: C H P2C#2H , (1) C H PC H #H , (2) C H PC#CH . (3) The results are plotted in Fig. 4b. After a 50 min run, exhaust gas was composed of H : 75%, C H : 15%, C H : 8%, CH : 2%. 3.4. First attempt to explain the particle formation mechanism A review of carbon black formation mechanisms was proposed by Donnet et al. (1993) in systems where carbon particulate can be obtained from hydrocarbons either by pyrolysis or by incomplete combustion. Though several theories have been proposed for the formation of carbon particulate, no one mechanism could explain the formation from all media regardless of their composition, temperature, etc. However, scientists are now agreed that the mechanism involves three di!erent stages: (1) nucleation, which corresponds to the transformation of a molecular system to a particulate system,
(2) aggregation due to collisions of nanometer-sized particles (results of the nucleation process) to form 10}50 nm spherical particles, (3) agglomeration of the previous spherical particles into chains up to about 1 mm in length.
Among these three stages, the nucleation stage is the least well understood. A variety of explanations have been proposed over the years for the initial growth mechanism resulting in a spherical morphology of the particle. For example, the C condensation theory, the acetylene theory, the polyacetylene theory and the polyaromatisation theory as discussed by Donnet et al. (1993), but each mechanism alone shows shortcomings: C condensation could only be important at high temperature, polyacetylene formation is very slow and presents some di$culties with rearrangement to a polycyclic structure, polycyclic concentration and rate of soot formation could vary in opposite trends. Nevertheless, a tentative to link all the previous explanations was made by Bolouri and Amouroux (1983) on the basis of a composition versus temperature analysis of the H/C chemical system at thermodynamic equilibrium. Even though soot formation is not an equilibrium process, this approach o!ers a "rst grid to associate the o!-gas composition with a mean temperature of carbon black formation. Assuming thermodynamic equilibrium, we have de"ned a mean decomposition temperature by "tting experimental data about o!-gas composition with the equilibrium composition predicts by thermodynamics using the software of Baronnet (1996). Results are shown in Fig. 5. This plot indicates that mean equilibrium decomposition temperature may range between 1300 and 1600 K, which is a rather low temperature with respect to the plasma temperature (10 000 K in the arc zone with Ar for the enthalpy listed in Table 2). In this temperature
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Fig. 4. Mass balance over the plasma reactor as a function of time. Argon plasma, feedstock: ethylene (0.12 m/h). Time t"0 is the starting time of hydrocarbon injection. (a) Global C H conversion e$ciency and carbon yield, (b) O!-gas composition.
range, the particle formation mechanism is governed by the reaction of polyaromatics with acetylene according to Abrahamson (1977). This "rst explanation must be completed by the characterisation of the solid product to validate the assumption, because the particle structure must be quasiamorphous if they are formed at low temperature. On the contrary, if high degree of crystal organisation is found, other high temperature phenomena surely dominate the particle formation.
4. Plasma carbon black structure Plasma-processed carbon black was examined by Transmission Electron Microscopy to identify the main
characteristics of the product such as, shape, morphology, size, and degree of organisation. 4.1. TEM examination of plasma carbon black For a side injection of hydrocarbon, the solid product is composed of three populations with respect to mean size and structure. The three populations are shown in Fig. 6: E Population 1 (P1) corresponds to the main part of the product. This population is composed of particles with a high degree of organisation and poorly aggregated. The lattice distance in the c direction (d ) is in the range 0.338}0.342 nm. By comparison with graphite,
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Fig. 5. Thermodynamic equilibrium of the C}H system with H/C"2 (P"0.84 atm, pressure at IMP-CNRS laboratory) and "tted temperature domain of hydrocarbon decomposition.
for which d "0.335 nm, the former result indicates that the particles are partially graphitised. But amorphous carbon layers are observed at the surface. For that reason, we think that the particles are transported in both low and high temperature regions during their time of residence in the reactor. E Population 2 (P2) is composed of nearly spherical amorphous particles, which indicates a formation mechanism at low temperature. The lattice distance d varies from 0.35 to 0.36 nm. E Population 3 (P3), the less abundant part of the product, looks like acetylene black aggregates (d "0.342 nm) suggesting a formation at high tem perature (Bourrat, 1987). The size distribution of plasma carbon black aggregates does not vary with the gaseous feedstock (CH or C H ), but Fig. 7 shows a strong e!ect of the injection location. The injection of the hydrocarbon in the annular zone between the electrode and the sleeve (case 2 in Fig. 8) gives smaller aggregates composed of more than 40% of the population with 50 nm diameter. On the contrary, side injection corresponds to distribution centred at dM "200 nm. ?
Fig. 6. TEM observation of the plasma carbon black and identification of the three populations (feedstock: ethylene).
4.2. Second attempt to explain the particle formation mechanism The examination of the plasma carbon black at the nanometer scale gives information in contradiction with the "rst interpretation about the mean temperature of formation suggested previously. The high degree of crystalline organisation observed for the main fraction of the
product (Population 1) cannot be explained by a low temperature formation mechanism (1300}1600 K). High magni"cation TEM image of P1 plasma carbon black is presented in Fig. 8. It shows an inhomogeneous structure of the single particle. The particle core exhibits a polygon-shaped morphology with two or three annular
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Fig. 7. Diameter distribution of the plasma carbon black aggregates. Injection of C H and CH as parameter and injection location of C H as parameter.
a three-step formation mechanism: Step 1: primary growth of the particle at low temperature (+1500 K), in agreement with the o!-gas composition, Step 2: particle recirculation in high temperature regions of the reactor (arc zone) causing the partial graphitisation of the carbon surface layer. Steps 1 and 2 may be repeated few times, Step 3: secondary growth of the particle in low temperature region of the reactor before exit. On that account the carbon surface layer is amorphous. This formation mechanism is based on the existence of recirculation phenomena inside the plasma reactor. This assumption is validated by a study of internal #uid #ow. It is based on both RTD measurement and numerical simulation.
5. Relation between reactor internal 6uid 6ow and particle formation mechanism Fig. 8. TEM observation of a particle of the P1 population. (1) Primary growth followed by a high temperature treatment, (2) secondary growth layer.
Fluid #ow was characterised by RTD experiments and #uid dynamics modelling. 5.1. Residence time distribution study
shells of partially graphitised material (d "0.338 nm) surrounded by a quasi-amorphous carbon layer. This concentric morphology is the sign of a formation process at low temperature (Bourrat, 1987), but particle growth at this temperature never forms such high degrees of carbon layer organisation. As a consequence, we assume
The RTD experiments were performed by side injection (at the same position as the hydrocarbon) of hydrogen in argon plasma. Step type injection was used. Hydrogen (1.2 Nm/h) was injected in argon plasma (3 Nm/h) and the hydrogen concentration variation at the reactor outlet was measured by GPC. A typical exit
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Fig. 9. Residence time distribution in the plasma reactor. (䉬) Experimental data, (*) tanks-in-series model.
age distribution (E) is plotted in Fig. 9. The RTD curve exhibits a characteristic shape: a main maximum at dimensionless time "1 ("t/, with "1.4 min) and two secondary maxima. This characteristic #ow pattern plot may be associated with recirculation phenomena (Villermaux, 1982). Plot "tting with a tanks-in-series model gives N"15 (N is the equivalent number of perfectly mixed reactors). Thanks to these experiments, we have obtained a "rst proof of the existence of recirculation zones in the plasma reactor but we need other information: what are the location and the temperature level of these zones? To answer this question, we performed a numerical simulation of the internal #uid #ow. 5.2. Fluid dynamics simulation results CFD modelling of the plasma #ow was developed by Ravary (1998) and Ravary, Fulcheri, Bakken, Flamant and Fabry (1999) assuming an axisymetrical #ow. The model is based on the association of an electric arc sub-model with the FLUENT2+ computing code. The simulation results show the formation of recirculation zones in the top region of the reactor. Recirculation phenomena are mainly due to the electromagnetic forces at the electrode tips (Ravary et al., 1999). Fig. 10 gives an example of velocity and temperature "elds at the reactor top for argon plasma. The velocity distribution shows evidence of two high temperature recirculation zones: a very large one in the arc region (the electrodes are drawn as a right-angled box) and a small one near the top corner of the graphite wall. In addition,
in the arc zone, the computed temperature is about 11 000 K, in agreement with the measurements of Fabry (1999). 5.3. Explanation of the particle formation mechanism According to the numerical simulation results and the previous TEM observations, the proposed particle formation mechanism for the three plasma carbon black populations is the following (the positions are marked as letters in Fig. 10). E A and A * Regions of hydrocarbons cracking. We propose the following process: (i) primary growth of particles at low temperature (1300}1600 K) to form spherical amorphous carbon black looking like `thermal blacka (P2 population with an initial population equal to a#b#c, with a(b(c), (ii) gaseous byproducts formation as identi"ed by GPC, in particular acetylene (initial quantity of C H is denoted as d#e#f with d'e'f ). E B * Exit region of the a part of the P2 population and of the d part of acetylene and of the other gaseous by-products. E C * Moving up of products along the wall, the b and the c parts of P2, and the e and the parts of C H . At the same time, acetylene decomposes progressively to form strongly branched `acetylene carbon blacka (P3 population) according to the process proposed by Hudson and Heicklen (1968). E D * Moving down to the reactor exit of the e part of P3 and the b part of P2 after thermal annealing at high temperature.
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Fig. 10. Results of the #uid #ow modelling (Ar, 3 Nm/h, useful thermal power: 15 kW) and identi"cation of the various steps of the particle formation mechanism. The temperatures are in K.
E E * Circulation of the rest of P2 and P3 families (c and f parts) in the electric arc zone at very high temperature (¹'10 000 K), the estimated residence time is 10\ to 2;10\ s. We identify the following processes: (i) thermal treatment of the P2 large spherical particles to form polygonal well organised carbon black, we have named the P1 population (most abundant), and which presents an original structure, (ii) thermal treatment of some P3 aggregates and vaporisation of the small particles followed by the growth of solid carbon from atoms and carbon clusters (Zhang et al., 1986).
E F * Downward #ow of the previous particles (identi"ed at position E) and secondary growth of amorphous carbon during their residence time in the low temperature regions A}A. E G and H * Exchange regions between the two zones of recirculation. This scenario gives a coherent interpretation of the gaseous and solid products produced in the plasma reactor. It agrees with the exhaust gas composition, the number and the structure of carbon black populations and the #ow pattern inside the plasma reactor.
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6. Conclusion
References
Plasma technology gives the opportunity to process new grades of carbon black but the formation mechanism analysis shows the strong importance of internal #uid #ow on the resulting structure of carbon particles. The main part of carbon black grows at temperature in the range 1300}1600 K and is then annealed in the region of electric arcs due to #uid recirculations. TEM analysis of the solid product indicates the existence of three plasma carbon black populations whatever the gaseous feedstock, even if there exists a dominant population (the most original) consisting of partially graphitised spherical particles (d "0.338 nm) with mean diameter in the range 150}200 nm. Feedstock injection inside the electrode sleeves gives rise to smaller aggregates (mean diameter of 50 nm). This product looks like `acetylene blacka, but electric arc instabilities are observed in this situation. We demonstrated the necessity of a multiscale approach to understand, on the one hand, the structure (i.e. the end-use properties) of carbon black at the nanometer scale and, on the other hand, the overall process performances such as exit gas composition. Such an approach is essential for plasma reactor because of the existence of regions with very large temperature di!erences (as large as 10 000 K). This study is a "rst step for the design of a new clean industrial plasma process for carbon black and hydrogen synthesis. Research work is now in progress in the following directions:
Abrahamson, J. (1977). Saturated platelets are new intermediates in hydrocarbon pyrolysis and carbon formation. Nature, 266, 323}327. Adamski, R. (1999). In International carbon black directory and source book. Portland, ME, USA: Intertech Publications. Baronnet, J. M. (1992). Thermal plasma synthesis of nanometric powders. High Temperature Chemical Processes, 1(4), 577}597. Baronnet, J. M. (1996). ALEX Software, Laboratoire de Chimie des Plasmas, University of Limoges. Bolouri, K. S., & Amouroux, J. (1983). Analyse des processus de formation de noir de carbone: une correlation entre le meH canisme de formation du noir de carbone et les calculs des eH quilibres chimiques hydrogene-Carbone. Bulletin de la Socie& te& Chimique de France, (5}6), I.101}I.109. Bourrat, X. (1987). Contribution a% l+e& tude de la croissance du carbone en phase vapeur. Ph.D. thesis, September, University of Pau. Donnet, J. B., Bansal, R. C., & Wang, M. J. (1993). Carbon black. New York: Marcel Dekker, ISBNO-8247-8975-X. Fabry, F. (1999). Etude d 'un proce& de& plasma pour la synthe% se de noirs de carbone structure& s par pyrolyse d'hydrocarbures a% haute tempe& rature et caracte& risation des produits. Ph.D. thesis, 6 July, University of Perpignan. Fulcheri, L., Flamant, G., Variot, B., & Badie, J. M. (1995). Characterisation of a 3-phase AC plasma reactor for carbon black synthesis from natural gas. In J. Heberlin (Ed.), Proceedings of International Symposium on plasma chemistry*ISPC, vol. III (pp. 1159}1162). Minneapolis, USA: University of Minnesota. Fulcheri, L., Schwob, Y., & Flamant, G. (1997). Comparison of new carbon nanostructures produced by thermal plasma with industrial Carbon black grades. Journal de Physique III, France, 7, 491}503. Gold, D., Bonet, C., Chauvin, G., Mathieu, A. C., Geirnaert, G., & Millet, J. (1981). A 100 kW three phase AC plasma furnace for spheromK disation of aluminium silicate particles. Plasma Chemistry and Plasma Processing, 1(2), 161}177. Hudson, J. L., & Heicklen, J. (1968). Theory of carbon formation in vapor-phase pyrolysis: I. Constant concentration of active species. Carbon, 6, 405}418. Kong, P. C., & Pfender, E. (1997). Plasma processes. In A. W. Weiner (Ed.), Carbide, nitride and boride materials synthesis and processing. London: Chapman & Hall (Chapter 14). Pfender, E. (1999). Thermal plasma technology: Where do we stand and where are we going. Plasma Chemistry and Plasma Processing, 19(1), 1}31. Ravary, B. (1998). Mode& lisation thermique et hydrodynamique d 'un re& acteur plasma triphase& . Contribution a% la mise au point d 'un proce& de& industriel pour la fabrication de noir de carbone. Ph.D. thesis, Ecole des Mines de Paris. Ravary, B., Fulcheri, L., Bakken, J. A., Flamant, G., & Fabry, F. (1999). In#uence of the electromagnetic forces on momentum and heat transfer in a 3-phase AC plasma reactor. Plasma Chemistry and Plasma Processing, 19(1), 69 }89. Schwob, Y. (1994). ProprieH teH s renforc7 antes du carbon black. Caoutchouc et Plastiques, 733, 70}74. Tioxide: Plasma process technology (1988). UK: Tioxide Greatham. Villermaux, J. (1982). GeH nie de la reH action chimique. In Lavoisier Paris, (Ed.), Techniques et Documentation. Zhang, Q. L., O'Brien, S. C., Heath, J. R., Liu, Y., Curl, R. F., Kroto, H. W., & Smalley, R. E. (1986). Reactivity of large carbon clusters: SpheromK dal carbon shells and their possible relevance to the formation and morphology of soot. Journal of Physical Chemistry, 90(4), 525}528.
E design of a new internal reactor geometry to limit recirculation phenomena, and consequently, the dispersion of particles properties, E numerical simulation of feedstock mixing with plasma #ow and of radiative heat transfer accounting for the particles, E pertinent reactor temperature monitoring to control the structure of solid product, E analysis of the e!ect of feedstock and plasma gas on the product quality.
Acknowledgements Authors acknowledge F. Sandiumenge (ICMAB, Barcelona) and X. Bourrat (CNRS, Pessac) for the TEM micrographs. This work was supported by TIMCAL, EDF and GDF, and by EC Joule project JOE 3-CT970057.
Carbon black processing by thermal plasma. Analysis of the particle formation mechanism FreH deH ric Fabry , Gilles Flamant *, Laurent Fulcheri Institut de Science et de Ge& nie des Mate& riaux et Proce& de& s, IMP-CNRS, Odeillo, B.P.5, 66125 Font-Romeu Cedex, France Ecole des Mines de Paris, Centre d'Energe& tique, B.P.207 F-06904 Sophia Antipolis, France Received 20 March 2000; received in revised form 31 July 2000; accepted 20 September 2000
Abstract An original plasma process for carbon black and hydrogen synthesis is presented. A pilot scale (100 kW) reactor is developed to obtain new carbon black grades from hydrocarbon cracking. Even if three populations of particles are observed, the plasma carbon black is mainly characterised by spherical nanoparticles with a high degree of turbostratic organisation. A particle formation mechanism is proposed on the basis of a multiscale analysis of the plasma reactor accounting for: heat and mass balance, RTD measurement and internal #ow modelling and particle characterisation by TEM. The mechanism is characterised by two steps: "rst, a low temperature particle growth and second, a high temperature annealing. As a result, particle structure is strongly related to internal mixing phenomena. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Carbon black; Plasma technology; Formation; Hydrocarbon cracking; Nanoparticle
1. Introduction Carbon nanoparticles are named `Carbon blacka which is actually a generic term for a family of products, which are usually referred according to the methods (or raw materials) used in their manufacture. For example, standard products are known as: `furnace blacka, `channel blacka, `thermal blacka, `acetylene blacka, etc. (International carbon black directory and source book, 1999). All these manufacturing processes operate at high temperature (typically in the range 1700}2500 K) and are based on the reaction of a hydrocarbon feedstock to produce carbon particles and gaseous by-products such as: H O, H , CH and some pollutants, CO , SO and NO . Generally, the reaction is combustion in the air, V except for acetylene black that is obtained by self-decomposition of the gas. In contrast with classical soot, which contains always a lot of inorganic contaminants and extractable organic residues, commercial blacks contain over 97}99% elemental carbon depending on the manufacturing process (Donnet, Bansal, & Wang, 1993).
* Corresponding author. Fax: #04-68-30-77-58. E-mail address: #[email protected] (G. Flamant).
The worldwide production of carbon black is about 6 million tons per year, i.e. over £4 billion per year are currently being spent on carbon black by consumers (this puts carbon black well ahead of sulphur, chlorine and other non-metallic elements in terms of global expenditure). The key properties of carbon black are: E rubber reinforcing mechanical properties: improves tear strength, wear resistance, modulus and fatigue characteristics (Schwob, 1994), E pigment properties: light absorption, tinting strength, and UV protection, E electrical properties: improves electrical conductance, enhances maintenance of electric charge, etc. Today, applications in tyre and rubber product manufacturing industry represent 90% of world carbon black production. The end-use properties of carbon black depend on: particle size distribution, morphology of particles and aggregates, speci"c surface area, elementary grain microstructure, surface chemical bonds, etc. Those characteristics are process dependent. For example, furnace black is composed of more or less spherical particles of colloidal size (10}100 nm) coalesced into aggregates and agglomerates; the particles in thermal
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Table 1 Characteristics of Industrial Carbon Blacks and Graphite Powder from Fulcheri, Schwob, and Flamant (1997) Chemical process Process (estimated name temperature level)
Industrial manufacturer (product)
Feedstock
Mean particle diameter
BET Speci"c area (m/g)
¸ ? (nm)
¸ ! (nm)
Typical application
Partial combustion (1400}1900 K)
Furnace
Cabot (N330)
Heavy oil Natural gas
26}30 nm
70}90
1.6
1.2
Rubber
Selfdecomposition (2500 K)
Acetylene
SNAA (Y 50 A)
Acetylene
30}40 nm
60}80
2.6
2.5
Battery
Pyrolysis (3000 K)
Acheson
TIMCAL (KS 44)
Coke
80%(44 m
10
'100
'100
Lubricant
¸ and ¸ are, respectively, the crystal size in the hk plane and c direction (graphite lattice). ? A
blacks are somewhat coarser, spherical, typically 120}200 nm and up to 500 nm. In order to compare the characteristics of some typical products with respect to their application, mean particle size, BET speci"c surface area, electrical resistivity and dimensions of elementary graphite crystals are listed in Table 1 for three industrial carbon particles (Fulcheri et al., 1997). This table shows clearly the large di!erence between carbon black and graphite powder, but also that small di!erences in characteristics a!ect strongly the end-use properties of the manufacturing products. For that reason, more than 100 di!erent grades of carbon blacks are currently o!ered by commercial manufacturers (Adamski, 1999). Accounting for both previous remarks, we have identi"ed a niche for a new high temperature process that permits to vary the carbon black characteristics and consequently a thermal plasma reactor was developed (Fulcheri, Flamant, Variot, & Badie, 1995). In opposition with existing industrial processes, in such systems, the temperature is not limited by chemical thermodynamics since the reaction enthalpy is supplied inside the reactor by an electric arc. Thermal plasma for particle processing are currently under development for powder densi"cation (Pfender, 1999) and "ne particle synthesis (Baronnet, 1992; Kong & Pfender, 1997), but there exists only few MW-sized industrial units in operation worldwide. The most famous is the TIOXIDE plant (UK) which produces 0.3 nm TiO particle by reacting TiCl with O in a plasma jet (Tioxide: Plasma process technology, 1988). This paper addresses the following objectives: E to demonstrate the possible synthesis of new carbon black grades by thermal plasma, E to exemplify a multiscale and multidisciplinary approach,
E to understand particle formation mechanism in such high temperature process. We propose a three-step approach to link together the overall process performances, the plasma carbon black structure and the internal #uid #ow.
2. The process The main innovations of the plasma systems with respect to classical processes are: E Clean process: no emission of CO , and other pollu tants (SO , NO , etc). 6 V E Two products: carbon black and hydrogen. E New products: the operating temperature (1000}10 000 K) is large enough to generate new carbon nanostructures. The complete system is schemed in Fig. 1. The plasma torch 1 supplied by a 100 kW 3-phase AC generator produced a large volume expanded plasma #ow inside a reaction chamber 3. This plasma torch, initially developed by Gold et al. (1981), used three graphite electrodes 13 mm in diameter. The chamber was a 2 m high, 0.28 m i.d. graphite cylinder inserted in a 0.40 m i.d. water-cooled stainless steel vessel. The feedstock was introduced in the reaction chamber either on the side 2 or at the top, in the annular zone between the three graphite electrodes and their sleeves. The solid and gaseous products were collected in the cooling chamber 4 and separated in the bag "lter 5. The following measurements were performed: E Current and voltage of the three phases (r.m.s.), E Plasma gas #ow rate,
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Table 2 Plasma generator characteristics for a 3 Nm/h plasma gas #ow rate Plasma gas
N
Ar
Ar}H (5%)
; (V) I (A) H (kWh/Nm) .
195 199 20.2
55 2030.6 2.8
71 198 5.2
H . is the plasma enthalpy. N
Fig. 1. Schema of the experimental system. (1) 3-phase AC plasma generator, (2) side injector of hydrocarbons, (3) 2-meter high reactor vessel, (4) cooling and conveying zone, (5) "lter for separation of carbon black (CB) and hydrogen (H ).
E Water #ow rates and temperature variations of the cooling system (electrode sleeves, roof and reactor vessel) to determine the heat loss for each subsystem, E feedstock gas #ow rate (natural gas or ethylene), E o!-gas composition by Gas Phase Chromatography (GPC), E internal wall temperature by optical pyrometry, E mean plasma temperature from Optical Emission Spectrometry (OES). In addition, particle characterisation was performed by XRD, TEM and XPS. A detailed presentation of the measurement and characterisation systems is proposed in Fabry (1999).
3. Overall process performances 3.1. Plasma characteristics The power supply was composed of a three-phase electric boosted transformer with serial inductances as shown in Fig. 2. The nominal power of the boosted transformer was 175 kVA with following outlet charac-
teristics: short circuit voltage: 760 V, maximum current: 266 A, frequency: 50 Hz. Five sets of variable inductance may be used but, it was generally "xed at 3.52 or 4.48 mH. Arc voltage, current and plasma enthalpy (Hp) are listed in Table 2 for three gases: Nitrogen, Argon and Argon-Hydrogen 5%. The plasma enthalpy corresponds to net values which account for the plasma torch overall e$ciency. This table illustrates the strong e!ect of the plasma gas on arc voltage and plasma enthalpy. Under these conditions, the plasma enthalpy varies from 2.8 to 20.2 kWh/Nm for Ar and N , respectively. 3.2. Heat balance over the reactor Heat losses were measured over each part of the reactor: electrode sleeves, roof and reactor vessel, and plasma gas. The results are shown in Fig. 3 for the operation parameters listed in Table 2. More than 40% of the heat losses took place at the roof because of the very high temperature in this zone. Measurement of plasma temperature using OES is presented by Fabry (1999). With an Ar}H (20%) mixture, it reached about 11 000 K in the arc zone. Furthermore, at the same position, the wall temperature ranged from 1500 to 2000 K depending on plasma gas. 3.3. Mass balance over the reactor The mass balance over the reactor was performed during a side injection of ethylene in an argon plasma
Fig. 2. Schema of the power supply.
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Fig. 3. Bar chart of the thermal losses over each plasma reactor sub-systems for Ar, Ar}H (5%) and N Plasmas.
gas. The global conversion e$ciency of C H and the carbon yield (ratio between carbon atoms in the particulate and carbon content in the feedstock) are shown in Fig. 4a as a function of time. Both values increase with time because the reactor temperature was still increasing during the injection. After 50 min the quasi-equilibrium values were, respectively, 75 and 60%. Because argon is the less e$cient gas, as shown in Table 2, this conversion e$ciency and this carbon yield may be considered as lowest performances for the process. At the exhaust, H , C H , C H and CH were iden ti"ed, their molar fractions were determined accounting for the following global chemical reactions: C H P2C#2H , (1) C H PC H #H , (2) C H PC#CH . (3) The results are plotted in Fig. 4b. After a 50 min run, exhaust gas was composed of H : 75%, C H : 15%, C H : 8%, CH : 2%. 3.4. First attempt to explain the particle formation mechanism A review of carbon black formation mechanisms was proposed by Donnet et al. (1993) in systems where carbon particulate can be obtained from hydrocarbons either by pyrolysis or by incomplete combustion. Though several theories have been proposed for the formation of carbon particulate, no one mechanism could explain the formation from all media regardless of their composition, temperature, etc. However, scientists are now agreed that the mechanism involves three di!erent stages: (1) nucleation, which corresponds to the transformation of a molecular system to a particulate system,
(2) aggregation due to collisions of nanometer-sized particles (results of the nucleation process) to form 10}50 nm spherical particles, (3) agglomeration of the previous spherical particles into chains up to about 1 mm in length.
Among these three stages, the nucleation stage is the least well understood. A variety of explanations have been proposed over the years for the initial growth mechanism resulting in a spherical morphology of the particle. For example, the C condensation theory, the acetylene theory, the polyacetylene theory and the polyaromatisation theory as discussed by Donnet et al. (1993), but each mechanism alone shows shortcomings: C condensation could only be important at high temperature, polyacetylene formation is very slow and presents some di$culties with rearrangement to a polycyclic structure, polycyclic concentration and rate of soot formation could vary in opposite trends. Nevertheless, a tentative to link all the previous explanations was made by Bolouri and Amouroux (1983) on the basis of a composition versus temperature analysis of the H/C chemical system at thermodynamic equilibrium. Even though soot formation is not an equilibrium process, this approach o!ers a "rst grid to associate the o!-gas composition with a mean temperature of carbon black formation. Assuming thermodynamic equilibrium, we have de"ned a mean decomposition temperature by "tting experimental data about o!-gas composition with the equilibrium composition predicts by thermodynamics using the software of Baronnet (1996). Results are shown in Fig. 5. This plot indicates that mean equilibrium decomposition temperature may range between 1300 and 1600 K, which is a rather low temperature with respect to the plasma temperature (10 000 K in the arc zone with Ar for the enthalpy listed in Table 2). In this temperature
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Fig. 4. Mass balance over the plasma reactor as a function of time. Argon plasma, feedstock: ethylene (0.12 m/h). Time t"0 is the starting time of hydrocarbon injection. (a) Global C H conversion e$ciency and carbon yield, (b) O!-gas composition.
range, the particle formation mechanism is governed by the reaction of polyaromatics with acetylene according to Abrahamson (1977). This "rst explanation must be completed by the characterisation of the solid product to validate the assumption, because the particle structure must be quasiamorphous if they are formed at low temperature. On the contrary, if high degree of crystal organisation is found, other high temperature phenomena surely dominate the particle formation.
4. Plasma carbon black structure Plasma-processed carbon black was examined by Transmission Electron Microscopy to identify the main
characteristics of the product such as, shape, morphology, size, and degree of organisation. 4.1. TEM examination of plasma carbon black For a side injection of hydrocarbon, the solid product is composed of three populations with respect to mean size and structure. The three populations are shown in Fig. 6: E Population 1 (P1) corresponds to the main part of the product. This population is composed of particles with a high degree of organisation and poorly aggregated. The lattice distance in the c direction (d ) is in the range 0.338}0.342 nm. By comparison with graphite,
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Fig. 5. Thermodynamic equilibrium of the C}H system with H/C"2 (P"0.84 atm, pressure at IMP-CNRS laboratory) and "tted temperature domain of hydrocarbon decomposition.
for which d "0.335 nm, the former result indicates that the particles are partially graphitised. But amorphous carbon layers are observed at the surface. For that reason, we think that the particles are transported in both low and high temperature regions during their time of residence in the reactor. E Population 2 (P2) is composed of nearly spherical amorphous particles, which indicates a formation mechanism at low temperature. The lattice distance d varies from 0.35 to 0.36 nm. E Population 3 (P3), the less abundant part of the product, looks like acetylene black aggregates (d "0.342 nm) suggesting a formation at high tem perature (Bourrat, 1987). The size distribution of plasma carbon black aggregates does not vary with the gaseous feedstock (CH or C H ), but Fig. 7 shows a strong e!ect of the injection location. The injection of the hydrocarbon in the annular zone between the electrode and the sleeve (case 2 in Fig. 8) gives smaller aggregates composed of more than 40% of the population with 50 nm diameter. On the contrary, side injection corresponds to distribution centred at dM "200 nm. ?
Fig. 6. TEM observation of the plasma carbon black and identification of the three populations (feedstock: ethylene).
4.2. Second attempt to explain the particle formation mechanism The examination of the plasma carbon black at the nanometer scale gives information in contradiction with the "rst interpretation about the mean temperature of formation suggested previously. The high degree of crystalline organisation observed for the main fraction of the
product (Population 1) cannot be explained by a low temperature formation mechanism (1300}1600 K). High magni"cation TEM image of P1 plasma carbon black is presented in Fig. 8. It shows an inhomogeneous structure of the single particle. The particle core exhibits a polygon-shaped morphology with two or three annular
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Fig. 7. Diameter distribution of the plasma carbon black aggregates. Injection of C H and CH as parameter and injection location of C H as parameter.
a three-step formation mechanism: Step 1: primary growth of the particle at low temperature (+1500 K), in agreement with the o!-gas composition, Step 2: particle recirculation in high temperature regions of the reactor (arc zone) causing the partial graphitisation of the carbon surface layer. Steps 1 and 2 may be repeated few times, Step 3: secondary growth of the particle in low temperature region of the reactor before exit. On that account the carbon surface layer is amorphous. This formation mechanism is based on the existence of recirculation phenomena inside the plasma reactor. This assumption is validated by a study of internal #uid #ow. It is based on both RTD measurement and numerical simulation.
5. Relation between reactor internal 6uid 6ow and particle formation mechanism Fig. 8. TEM observation of a particle of the P1 population. (1) Primary growth followed by a high temperature treatment, (2) secondary growth layer.
Fluid #ow was characterised by RTD experiments and #uid dynamics modelling. 5.1. Residence time distribution study
shells of partially graphitised material (d "0.338 nm) surrounded by a quasi-amorphous carbon layer. This concentric morphology is the sign of a formation process at low temperature (Bourrat, 1987), but particle growth at this temperature never forms such high degrees of carbon layer organisation. As a consequence, we assume
The RTD experiments were performed by side injection (at the same position as the hydrocarbon) of hydrogen in argon plasma. Step type injection was used. Hydrogen (1.2 Nm/h) was injected in argon plasma (3 Nm/h) and the hydrogen concentration variation at the reactor outlet was measured by GPC. A typical exit
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Fig. 9. Residence time distribution in the plasma reactor. (䉬) Experimental data, (*) tanks-in-series model.
age distribution (E) is plotted in Fig. 9. The RTD curve exhibits a characteristic shape: a main maximum at dimensionless time "1 ("t/, with "1.4 min) and two secondary maxima. This characteristic #ow pattern plot may be associated with recirculation phenomena (Villermaux, 1982). Plot "tting with a tanks-in-series model gives N"15 (N is the equivalent number of perfectly mixed reactors). Thanks to these experiments, we have obtained a "rst proof of the existence of recirculation zones in the plasma reactor but we need other information: what are the location and the temperature level of these zones? To answer this question, we performed a numerical simulation of the internal #uid #ow. 5.2. Fluid dynamics simulation results CFD modelling of the plasma #ow was developed by Ravary (1998) and Ravary, Fulcheri, Bakken, Flamant and Fabry (1999) assuming an axisymetrical #ow. The model is based on the association of an electric arc sub-model with the FLUENT2+ computing code. The simulation results show the formation of recirculation zones in the top region of the reactor. Recirculation phenomena are mainly due to the electromagnetic forces at the electrode tips (Ravary et al., 1999). Fig. 10 gives an example of velocity and temperature "elds at the reactor top for argon plasma. The velocity distribution shows evidence of two high temperature recirculation zones: a very large one in the arc region (the electrodes are drawn as a right-angled box) and a small one near the top corner of the graphite wall. In addition,
in the arc zone, the computed temperature is about 11 000 K, in agreement with the measurements of Fabry (1999). 5.3. Explanation of the particle formation mechanism According to the numerical simulation results and the previous TEM observations, the proposed particle formation mechanism for the three plasma carbon black populations is the following (the positions are marked as letters in Fig. 10). E A and A * Regions of hydrocarbons cracking. We propose the following process: (i) primary growth of particles at low temperature (1300}1600 K) to form spherical amorphous carbon black looking like `thermal blacka (P2 population with an initial population equal to a#b#c, with a(b(c), (ii) gaseous byproducts formation as identi"ed by GPC, in particular acetylene (initial quantity of C H is denoted as d#e#f with d'e'f ). E B * Exit region of the a part of the P2 population and of the d part of acetylene and of the other gaseous by-products. E C * Moving up of products along the wall, the b and the c parts of P2, and the e and the parts of C H . At the same time, acetylene decomposes progressively to form strongly branched `acetylene carbon blacka (P3 population) according to the process proposed by Hudson and Heicklen (1968). E D * Moving down to the reactor exit of the e part of P3 and the b part of P2 after thermal annealing at high temperature.
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Fig. 10. Results of the #uid #ow modelling (Ar, 3 Nm/h, useful thermal power: 15 kW) and identi"cation of the various steps of the particle formation mechanism. The temperatures are in K.
E E * Circulation of the rest of P2 and P3 families (c and f parts) in the electric arc zone at very high temperature (¹'10 000 K), the estimated residence time is 10\ to 2;10\ s. We identify the following processes: (i) thermal treatment of the P2 large spherical particles to form polygonal well organised carbon black, we have named the P1 population (most abundant), and which presents an original structure, (ii) thermal treatment of some P3 aggregates and vaporisation of the small particles followed by the growth of solid carbon from atoms and carbon clusters (Zhang et al., 1986).
E F * Downward #ow of the previous particles (identi"ed at position E) and secondary growth of amorphous carbon during their residence time in the low temperature regions A}A. E G and H * Exchange regions between the two zones of recirculation. This scenario gives a coherent interpretation of the gaseous and solid products produced in the plasma reactor. It agrees with the exhaust gas composition, the number and the structure of carbon black populations and the #ow pattern inside the plasma reactor.
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6. Conclusion
References
Plasma technology gives the opportunity to process new grades of carbon black but the formation mechanism analysis shows the strong importance of internal #uid #ow on the resulting structure of carbon particles. The main part of carbon black grows at temperature in the range 1300}1600 K and is then annealed in the region of electric arcs due to #uid recirculations. TEM analysis of the solid product indicates the existence of three plasma carbon black populations whatever the gaseous feedstock, even if there exists a dominant population (the most original) consisting of partially graphitised spherical particles (d "0.338 nm) with mean diameter in the range 150}200 nm. Feedstock injection inside the electrode sleeves gives rise to smaller aggregates (mean diameter of 50 nm). This product looks like `acetylene blacka, but electric arc instabilities are observed in this situation. We demonstrated the necessity of a multiscale approach to understand, on the one hand, the structure (i.e. the end-use properties) of carbon black at the nanometer scale and, on the other hand, the overall process performances such as exit gas composition. Such an approach is essential for plasma reactor because of the existence of regions with very large temperature di!erences (as large as 10 000 K). This study is a "rst step for the design of a new clean industrial plasma process for carbon black and hydrogen synthesis. Research work is now in progress in the following directions:
Abrahamson, J. (1977). Saturated platelets are new intermediates in hydrocarbon pyrolysis and carbon formation. Nature, 266, 323}327. Adamski, R. (1999). In International carbon black directory and source book. Portland, ME, USA: Intertech Publications. Baronnet, J. M. (1992). Thermal plasma synthesis of nanometric powders. High Temperature Chemical Processes, 1(4), 577}597. Baronnet, J. M. (1996). ALEX Software, Laboratoire de Chimie des Plasmas, University of Limoges. Bolouri, K. S., & Amouroux, J. (1983). Analyse des processus de formation de noir de carbone: une correlation entre le meH canisme de formation du noir de carbone et les calculs des eH quilibres chimiques hydrogene-Carbone. Bulletin de la Socie& te& Chimique de France, (5}6), I.101}I.109. Bourrat, X. (1987). Contribution a% l+e& tude de la croissance du carbone en phase vapeur. Ph.D. thesis, September, University of Pau. Donnet, J. B., Bansal, R. C., & Wang, M. J. (1993). Carbon black. New York: Marcel Dekker, ISBNO-8247-8975-X. Fabry, F. (1999). Etude d 'un proce& de& plasma pour la synthe% se de noirs de carbone structure& s par pyrolyse d'hydrocarbures a% haute tempe& rature et caracte& risation des produits. Ph.D. thesis, 6 July, University of Perpignan. Fulcheri, L., Flamant, G., Variot, B., & Badie, J. M. (1995). Characterisation of a 3-phase AC plasma reactor for carbon black synthesis from natural gas. In J. Heberlin (Ed.), Proceedings of International Symposium on plasma chemistry*ISPC, vol. III (pp. 1159}1162). Minneapolis, USA: University of Minnesota. Fulcheri, L., Schwob, Y., & Flamant, G. (1997). Comparison of new carbon nanostructures produced by thermal plasma with industrial Carbon black grades. Journal de Physique III, France, 7, 491}503. Gold, D., Bonet, C., Chauvin, G., Mathieu, A. C., Geirnaert, G., & Millet, J. (1981). A 100 kW three phase AC plasma furnace for spheromK disation of aluminium silicate particles. Plasma Chemistry and Plasma Processing, 1(2), 161}177. Hudson, J. L., & Heicklen, J. (1968). Theory of carbon formation in vapor-phase pyrolysis: I. Constant concentration of active species. Carbon, 6, 405}418. Kong, P. C., & Pfender, E. (1997). Plasma processes. In A. W. Weiner (Ed.), Carbide, nitride and boride materials synthesis and processing. London: Chapman & Hall (Chapter 14). Pfender, E. (1999). Thermal plasma technology: Where do we stand and where are we going. Plasma Chemistry and Plasma Processing, 19(1), 1}31. Ravary, B. (1998). Mode& lisation thermique et hydrodynamique d 'un re& acteur plasma triphase& . Contribution a% la mise au point d 'un proce& de& industriel pour la fabrication de noir de carbone. Ph.D. thesis, Ecole des Mines de Paris. Ravary, B., Fulcheri, L., Bakken, J. A., Flamant, G., & Fabry, F. (1999). In#uence of the electromagnetic forces on momentum and heat transfer in a 3-phase AC plasma reactor. Plasma Chemistry and Plasma Processing, 19(1), 69 }89. Schwob, Y. (1994). ProprieH teH s renforc7 antes du carbon black. Caoutchouc et Plastiques, 733, 70}74. Tioxide: Plasma process technology (1988). UK: Tioxide Greatham. Villermaux, J. (1982). GeH nie de la reH action chimique. In Lavoisier Paris, (Ed.), Techniques et Documentation. Zhang, Q. L., O'Brien, S. C., Heath, J. R., Liu, Y., Curl, R. F., Kroto, H. W., & Smalley, R. E. (1986). Reactivity of large carbon clusters: SpheromK dal carbon shells and their possible relevance to the formation and morphology of soot. Journal of Physical Chemistry, 90(4), 525}528.
E design of a new internal reactor geometry to limit recirculation phenomena, and consequently, the dispersion of particles properties, E numerical simulation of feedstock mixing with plasma #ow and of radiative heat transfer accounting for the particles, E pertinent reactor temperature monitoring to control the structure of solid product, E analysis of the e!ect of feedstock and plasma gas on the product quality.
Acknowledgements Authors acknowledge F. Sandiumenge (ICMAB, Barcelona) and X. Bourrat (CNRS, Pessac) for the TEM micrographs. This work was supported by TIMCAL, EDF and GDF, and by EC Joule project JOE 3-CT970057.