Pilot Plant Scale Study of the Influence of the ... - ACS Publications

Ind. Eng. Chem. Res. 2009, 48, 8407–8417

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Pilot Plant Scale Study of the Influence of the Operating Conditions in the Production of Carbon Nanofibers Vicente Jimeńez,*,† Antonio Nieto-Ma´rquez,† Jose´ Antonio Dıáz,† Rubi Romero,‡ Paula Sańchez,† Jose´ Luis Valverde,† and Amaya Romero§ Departamento de Ingenierıá Quı´mica, Facultad de Quı´micas, UniVersidad de Castilla-La Mancha, 13071 Ciudad Real, Spain, Facultad de Quı´mica, UniVersidad Autońoma del Estado de Me´xico, Paseo Coloń esq. Paseo Tollocan s/n 50120 Toluca, Estado de Me´xico, and Departamento de Ingenierıá Quı´mica, Escuela Técnica Agrícola, UniVersidad de Castilla-La Mancha, 13071 Ciudad Real, Spain

In this paper, the optimization of typical reaction variables for a pilot scale synthesis of carbon nanofibers (CNFs) using a fixed-bed reactor has been carried out to provide a more economically viable large scale production of these materials. Using a Ni/SiO2 catalyst (10 wt % Ni) and ethylene as the carbon source, the optimum value of temperature, space velocity, and H2/C2H4 ratio (v/v) in terms of carbon yield was 600 °C, 10000 h-1, and 1:4, respectively. The modification of these variables caused a significant change in the type and amount of solid carbon recovered. Carbon product characterization demonstrated that CNFs with mesoporous character, large external surface, and good thermal stability and crystallinity were obtained. Finally, results demonstrated a successful scale-up by a factor of 45 in the pilot plant scale; a CNFs yield of 106 gCNFs/gcatalytic metal could be obtained at optimal conditions during a reaction time of 60 min at optimal conditions in the pilot plant scale. For the same reaction conditions, only 80 gCNFs/gcatalytic metal were obtained in the laboratory reactor. 1. Introduction Carbon materials are found in a variety of forms such as graphite, diamond, fullerenes, carbon nanofibers (CNFs), and carbon nanotubes (CNTs). Within the five, the nanostructured carbon materials have sparked an increasing interest for chemists, physicists, and material scientists worldwide. The past decades have seen an exponential increase in patent filings and publications on these carbon nanomaterials.1 Carbon nanofibers have been recently innovated due to their unique properties, which have generated an interest in applications, including selective adsorption,2 hydrogen storage,3 polymer reinforcement,4 and catalysts support.5 Although the mechanical/chemical sectors have accounted for around 73.5% of total revenues in 2007, other key sectors such as energy and electronics are also very important.6 The great interest in carbon nanofibers in certain industry segments is because alternative materials are characterized by their limited performance or much higher unit prices1 (a good example is the utilization of CNFs as an alternative to CNTs for electron emitters in flat panel displays). The main problem associated to CNFs is that the lack of sufficient amount of material limits the development of more practical applications. For this reason it is of great importance to develop a low cost and high productivity method to synthesize CNFs, which would open new novel applications for these materials. To produce carbon nanostructures three main techniques are generally used: arc discharge, laser ablation, and chemical vapor deposition. However, scientists are looking for more economical ways to produce these materials. In the laser ablation technique, a high-power laser beam impinges on a volume of carboncontaining feedstock gas (methane or carbon monoxide). Laser * To whom correspondence should be addressed. E-mail address: [email protected]. † Facultad de Químicas, Universidad de Castilla-La Mancha. ‡ Universidad Autońoma del Estado de Me´xico. § Escuela Tècnica Agrícola, Universidad de Castilla-La Mancha.

ablation produces a small amount of clean nanotubes, whereas arc discharge methods generally produce large quantities of impure material. In general, chemical vapor deposition (CVD) results in carbon nanostructures that have a large diameter range, which can be poorly controlled. Nevertheless, this method has the advantage that it is comparatively simple and easy to control and has low cost.7 Moreover, the CVD method should be easy to scale up, which favors commercial production.8 The principle of the CVD method is the decomposition of a gaseous precursor (hydrocarbons) on the substrate forming the solid (carbon) deposition. Transition metal catalysts are often employed in CVD to drop the decomposition temperature of the carbon precursor and make the synthesis more economical. The properties of carbons derived from hydrocarbons highly depend on the catalyst composition, decomposition temperature, and carbon precursor. Transition metals, such as Ni, Co, and Fe, and their alloys have been traditionally used as the catalysts for the hydrocarbon decomposition.9 Especially, the Ni-based catalysts are active at lower temperatures and provide higher yields of deposited carbons from the decomposition of hydrocarbons.10 In the literature, many of the studies on the synthesis of high amounts of CNFs and CNTs are made using fluidized bed reactors because mass transfer limitations are reduced compared to a fixed bed reactor.11 The fluidized bed reactor provides a close idealization in gas-solid contactors for high heat transfer rates. Nevertheless, the principal problem related to carbon nanostructures synthesis is that carbon deposition is minimized, which can be attributed to the permanent circulation of catalyst particles, which favors the burning of the deposited carbon on the catalyst in the oxygen-rich zone of the catalyst bed. Moreover, catalysts used in fluidized bed reactors require a number of special characteristics, such as being stable at both high temperatures and pressures, being mechanically stron, and having suitable support/metal interaction and activity.11,12 On the contrary, carbon nanostructure synthesis using fixed-bed reactors has reached exceptional yields,13 although in this case,

10.1021/ie9005386 CCC: $40.75  2009 American Chemical Society Published on Web 07/31/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 18, 2009

Figure 1. Scheme of the experimental setup employed.

the amount of deposited carbon depends on the mass transport to active catalyst sites (the solid product coats the catalyst particles retarding further heat and mass transfer processes).14 The aim of the present paper is the scale-up of CNFs production process for application in catalysis, fuel cells, and H2 storage. This work report the synthesis of carbon nanofibers by the catalytic decomposition of a mixture of ethylene and hydrogen over a conventional nickel catalyst supported on silica, using a bench scale plant based on a fixed bed reactor, in which an effort to minimize heat and mass transport limitations has been done. To minimize these limitations, small Ni metal particles deposited on a porous support (SiO2) were used to increase the gas-solid interface. Optimization of the growth conditions is targeted in order to obtain the maximum CNFs yield, while the structure and diameter of the CNFs are controlled. 2. Experimental Section 2.1. Catalyst Preparation. The supported catalyst (Ni/SiO2) with 10% w/w Ni was prepared by impregnation as follows: several grams of silica (Aldrich) were placed in a glass vessel and kept under vacuum at room temperature for 2 h to remove water and other compounds adsorbed on the structure. A known volume of an aqueous Ni(NO3)2 (Panreac) solution (the minimum amount required to wet the solid) was then poured over the sample. After 2 h, the solvent was removed by evaporation under vacuum at 90 °C. The prepared catalyst precursors were dried at 110 °C overnight. After drying, the catalyst was calcined in static air at 600 °C using a 10 °C/min ramp and kept at that temperature for 4 h. Before using, the catalyst precursors were sieved into a batch of 249 µm average particle diameter. 2.2. Experimental Apparatus for the CNFs Synthesis. Synthesis experiments were carried out in the experimental set up schematically shown in Figure 1. The fixed-bed reactor, consisting of a quartz tube of 9 cm diameter and 100 cm length, was located in a horizontal electric furnace (JH Hornos) with an effective heating zone of 80 cm. Thermocouple type K was used for monitoring the temperature of the bed. Hydrogen and ethylene flow rates were controlled by mass controllers (Brooks

Instruments, model 5850). Supported catalyst was taken in a quartz boat, which was kept inside the heating zone during the experiment. 2.3. Operating Procedure. Carbon nanofibers were grown at atmospheric pressure at different temperatures, space velocities, and H2/C2H4 (v/v) ratios. In each synthesis run, 5 g of the prepared catalyst (Ni/SiO2) was placed in the center of the reactor and activated by heating (10 °C min-1) in a flow of dry 20% v/v H2/He at the desired reaction temperature. The reduced activated catalyst was thoroughly flushed with dry He for 1 h before introducing the H2/C2H4 ratio desired feed. The growth time was varied in the different experiments. Silica supports were subsequently separated from the carbon product by leaching the primary product in hydrofluoric acid (48%) for 15 h with vigorous stirring followed by filtration and washing with deionized water. After decomposition of ethylene, the weight of carbon deposited and the deactivation function was calculated in each synthesis using the following equations: yields of CNFs (gCNFs /gcatallytic metal) ) (mtot - mcat)/(mcat0.1) deactivation function )

CNFs growth rate at t ) t CNFs growth rate at t ) 0

where mcat was the initial amount of the catalyst (before reaction) and mtot was the total weight of the sample after reaction. 2.4. Characterization Techniques. Surface area/porosity measurements were carried out using a Micromeritics ASAP 2010 sorptometer apparatus with N2 at 77 K as the sorbate. The samples were outgassed at 453 K under vacuum (6.6 × 10-9 bar) for 16 h prior to analysis. Specific surface areas were determined by the multipoint BET method; total pore volume and sizes were evaluated using the standard BJH treatment, and micropore volumes were evaluated using the t-plot equation. Temperature-programmed desorption of ammonia (TPDA) profiles were recorded in order to determine total acid density of the catalyst on a Micromeritics AutoChem 2950 HP apparatus. The sample was first heated at 10 °C · min-1 under a flow of helium from room temperature to the calcination


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Table 1. Metal Content, Textural Characteristics, Acidity, and Ni Particle Size of the Reduced Catalyst (20% v/v H2/He at 600 °C) Used for Synthesis of the CNFs Ni content (XDR) Ni (TEM) Ni particle BET surface mesopore micropore total pore average pore acidity (wt %) particle size (nm)a size (nm)b area (m2/g) area (m2/g) volume (cm3/g) volume (cm3/g)c diameter (nm) ((mmol of NH3)/g) SiO2 Ni/SiO2

10

11.6

11.5;60

370.0 268.3

314 (85%) 245 (92%)

0.021 0.006

0.667 1.164

5.9 13.5

0.211

a Average Ni particle sizes determined by XRD for the half-width of the main Ni peaks. b Average Ni particle size (particle size < 25 nm; 25 < particle size < 150) determined by counting 200 particles on TEM images. c Cumulative pore volume obtained using both the Horvath-Kawazoe (micropore volume) and the BJH method (mesopore volume).

temperature, holding this temperature for 30 min. After reducing the catalysts under H2 flow, the system was cooled to 180 °C. Ammonia was then allowed to flow over the sample for 15 min. Later, the sample was purged with helium to eliminate physisorbed species. The temperature was ramped at 10 °C · min-1 from 180 to 600 °C; TPDA data were acquired. The total acidity was obtained by integration of the area under the curve. Temperature-programmed reduction (TPR) measurements of the catalysts were carried out with the same apparatus described above. After loading, the samples were outgassed by heating at 10 °C · min-1 in an argon flow. The temperature was raised to the calcination temperature of the samples and kept constant at this temperature for 30 min. Next, the temperature was lowered to room temperature and the samples were stabilized under an Ar/H2 flow (g99.9990% purity, 83/17 volumetric ratio). Temperature and detector signals were continuously recorded while heating at 10 °C · min-1 up to 700 °C. The liquids formed during the reduction process were retained in a cooling trap placed between the sample and the detector. TPR profiles were reproducible, with standard deviations for the temperature of the peak maxima of (2%. XRD analyses were carried out on a Philips X’Pert instrument using nickel-filtered Cu KR radiation through a primary monochromator. The samples were scanned at a rate of 0.02° step-1 over the range 5° e 2θ e 80° (scan time ) 2 s step-1). The metal particle size was determined after the catalyst reduction in a 20% v/v H2/He mixture for 3 h. Once the sample was cooled, it was passivated in a 1% v/v O2/He mixture to prevent bulk oxidation. The mean Ni metal particle size (∑inidi/∑ini where ni represents the number of particles of diameter di) and its distribution and the morphology and diameter distribution of the CNFs were probed by transmission electron microscopy (TEM) using a Philips Tecnai 20T, operated at an acceleration voltage of 200 keV. Suitable specimens were prepared by ultrasonic dispersion in acetone with a drop of the resultant suspension evaporated onto a holey-carbon-supported grid. The diameter distribution was measured by counting ∼200 samples on the resulted TEM images. Temperature-programmed oxidation (TPO) was used to determine the CNFs crystallinity. Analyses were performed on 10 mg samples using a Perkin-Elmer TGA7 termogravimetric analyzer with a flow of 50 mL min-1 of 20% v/v O2/He mixture and with a heating rate of 5 °C min-1 up to 1000 °C. The Ni metal loading was determined (to better than (1%) by atomic absorption (AA) spectrophotometry, using a SPECTRA 220FS analyzer. Samples (ca. 0.5 g) were treated in 2 cm3 HCl, 3 cm3 HF, and 2 cm3 H2O2 followed by microwave digestion (523 K). 3. Results and Discussion 3.1. Catalyst Characterization. The preparation of a highly loaded and well-dispersed Ni catalyst is essential for synthesizing high yields of CNFs. Table 1 shows the general character-

ization results obtained with the reduced catalyst (20% v/v H2/ He at 600 °C). The nitrogen adsorption isotherm of the catalyst (not shown) was consistent with the type IV according to the IUPAC classification15 corresponding to mesoporous materials with capillary condensation. It can be seen that a slight decrease of BET surface area and micropore volume was observed for the supported catalysts (Ni/SiO2) with respect to the support (SiO2), which have been attributed to the closure of the pores as consequence of the consumption of the surface hydroxyl groups of the silica by reaction with the active phase during the impregnation stage.16 The addition of Ni to the support led to an increase of the pore size, which has been observed elsewhere.17 So, an increase in the total pore volume took place after the Ni incorporation. It is important to note that the catalyst was largely unaffected by changes in the reduction temperature (not shown). As can be observed in TEM pictures and from the analysis of the particle size distribution (Figure 2), reduction of the catalyst generated a metal phase that exhibits highly dispersed very small Ni particles of around 15 nm but also Ni aggregates resulting in the formation of large metal crystallites with a wider size distribution. TPR profile of the Ni/SiO2 catalyst (Figure 3) showed a main reduction peak positioned at 270-500 °C, with a maximum at around 400 °C; a reduction profile indicating bulk-like NiO (TPR profile for pure NiO -not shownconsisted on a unique reduction peak centered at 400 °C).18,19 Moreover, different Ni species were formed during the thermal process as the presence of two peaks centered at around 320 and 440 °C indicated. These peaks, located before and after the maximum exhibited by the pure NiO, have been attributed to the reduction of NiO species having different interactions with the support.20,21 XDR analysis (Figure 4) of the passivated catalyst showed no evidence of NiO, being the profile characterized by three peaks (44.5°, 51.8°, and 76.1°) corresponding to (111), (200), and (220) planes of metallic Ni (cubic Ni symmetry), respectively. For comparison, the XRD analysis of the nonpassivated catalyst has been also included, showing how the characteristic peaks of NiO (37.2°, 43.2°, 62.8°, and 75.3°) are, in this case, present. 3.2. Optimization of the Synthesis Conditions: Pilot Plant Scale. The growth rate maximization strategy involved stepwise changes in reaction temperature, space velocity, and H2 content in the feed. The reaction conditions considered are summarized in Table 2. 3.2.1. Effect of Synthesis Temperature. To study the effect of temperature on the carbon yield, the synthesis temperature was changed from 450 to 850 °C, but the space velocity and H2/C2H4 ratio were kept unchanged at 25000 h-1 and 1:4, respectively. The results, which are shown in Table 2, demonstrate that the CNFs capacity (i.e., maximum gravimetric carbon yield) increased as the temperature was raised from 450 to 600 °C. The maximum carbon yield after 1 h of reaction, obtained at 600 °C, was of 88 gc/gcatalyst, the CNFs capacity decreasing

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Figure 2. Representative TEM images and Ni particle size distribution of passivated Ni/SiO2. Reduction conditions: 20% v/v H2/He at 600 °C.

Figure 3. TPR profile of the Ni/SiO2 catalyst after calcination.

at higher temperatures. Such a temperature maximum has been noted elsewhere and it is related to the nature of the catalyst and feedstock.22 To explain the results, it is necessary to have into account the generally accepted mechanism of CNFs growth. According to it, surface reactions lead to formation of surface

carbon, dissolution and segregation, diffusion of carbon through bulk metal from the gas-solid side to solid support one, and precipitation on the support side of the metal particles.10 The increase of the carbon yield with the reaction temperature could be due to an increase of the surface reaction rates, leading


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Figure 4. XRD pattern of the reduced (20% v/v H2/He at 600 °C) and passivated Ni/SiO2 catalyst. Table 2. Reaction Conditions and Physical Parameters for the Pilot Scale Synthesis of CNFs temperature space velocity H2/C2H6 reaction carbon yield BET surface micropore area pore volume max. weigh average CNFs (°C) (h-1) ratio (v/v) time (h) (gc/gcatalytic metal)a area (m2/g) (m2/g)b (cm3/g)c npgd loss Ta (°C) diameters (nm) 450 550 600 750 850 600 600 600 600 600 600 600 600 600 600 600 600 600

25000 25000 25000 25000 25000 100 1000 10000 25000 10000 10000 10000 10000 10000 10000 10000 10000 10000

1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 only C2H4 0.5:4 1:4 2:4 3:4 1:4 1:4 1:4 1:4

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 3 5

36 48 88 80 60 2 22 106 88 40 45 106 69 67 106 140 145 145

286 208 202 109 68 99 143 200 202 123 147 200 129 114 200 172 102 93

57 (20%) 38 (18%) 25 (13%) 9 (8%) 2 (3%) 10 (10%) 19 (13%) 25 (13%) 25 (13%) 14 (11%) 14 (10%) 25 (13%) 14 (11%) 12 (11%) 25 (13%) 17 (10%) 7 (7%) 4 (4%)

1.60 1.08 0.98 0.36 0.30 0.43 0.92 0.94 0.98 0.62 0.77 0.94 0.67 0.55 0.94 0.87 0.47 0.38

7.7 7.2 8.6 12.3 12.3 12.2 10.8 8.5 8.6 10.6 9.5 8.5 10.7 10.8 8.5 9.5 9.5 10.7

519 540 546 636 640 664 580 550 546 559 564 550 570 543 550 540 540 550

37 40 44 51 65 65 61 40 44 62 58 40 62 54 40 57 66 91

a Carbon yield after 1 h of reaction. b In brackets, percentage of micropore area with respect to the total surface area. c Cumulative pore volume obtained using both the Horvath-Kawazoe (micropore volume) and the BJH method (mesopore volume). d Number of grapheme planes in the crystallites (npg ) Lc/d002).

to a high CNFs growth rate.23 Conversion of ethylene increased at high temperatures, which lead to a higher percentage conversion into carbon species (at low temperatures hydrogenation to ethane was the favored reaction). Furthermore, at higher temperatures, the carbon solubility and diffusion are much higher24 and thus, carbon diffusion through the metal particles is also faster, which is preferred for catalyst activation and CNFs formation. The decrease of the CNFs capacity at higher temperatures has been related to a more rapid catalyst deactivation which has been explained by multiple mechanisms including blocking of catalyst sites and catalyst fragmentation.23 CNFs capacity at three representative temperatures is shown as a function of time-on-stream in Figure 5a whole CNFs growth rate is plotted in Figure 5b. It is clear that the carbon deposition rate declined with increasing reaction times.18 It fell very quickly for temperatures higher than 600 °C. The deactivation behavior of the catalyst at different temperatures is shown in Figure 5c. According to different authors23 three different periods can be observed in the CNFs deactivation mechanism:

Induction: reaction period very short in time corresponding to the initial growth rate values. It is not always observed.25 Steady-state: period in which almost no deactivation exists (or it is very slow). Deactivation: period in which an important decrease in the growth rate takes place as a consequence of the catalyst deactivation. It is important to note that the initial period (induction) was not observed in our study, because the first data point was recorded after 10 min of reaction. Results showed that at synthesis temperatures higher than 600 °C, no stabilized growth region was observed, the catalyst being deactivated continuously. Nevertheless, at lower temperatures a relatively stable steadystate period exists, being followed by a rapid deactivation. Considering that the formation of encapsulating catalyst is the main cause of deactivation, these results would confirm that the higher temperature leads to greater carbon coverage, leading to higher deactivation rates. 3.2.2. Effect of the Space Velocity. To gain a deeper understanding of the effect of the space velocity, the carbon

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Figure 6. (a) Carbon yield, (b) growth rate, and (c) deactivation function as a function of time on stream at different space velocities. Flow conditions: temperature ) 600 °C, H2/C2H4 ratio (v/v) )1:4. Figure 5. (a) Carbon yield, (b) growth rate, and (c) deactivation function as a function of time on stream at three different temperatures. Flow conditions: space velocity ) 25000 h-1, H2/C2H4 ratio (v/v) ) 1:4.

production after 1 h of reaction (synthesis temperature of 600 °C and H2/C2H4 rate of 1:4) is shown in Table 2. Despite the fact that the increase in the space velocity reduces the hydrogen concentration in the outlet gas, which in turn is related to a more rapid catalyst deactivation (as it will be explained below), it is worth noting that the carbon yield and the growth rate increased significantly (Figure 6a,b). In other words, although a higher growth rate was obtained at lower residences time, catalyst deactivation was faster (Figure 6c). As commented above, the very fast deactivation rate at high values of space velocity may be caused by the high carbon formation rate on

the metal surfaces, which overcoats the catalyst particle and deactivates it. As a consequence, an optimum space velocity with respect to carbon yield close to 10000 h-1 could be established. For economical considerations, there is always a need to choose the optimum space velocity in industrial processes to look for an optimum condition in which the formation rate of carbon atoms on the metal surfaces and their diffusion rate in the catalyst are in equilibrium for the longer time.26,27 3.2.3. Effect of H2/C2H4 (v/v) Ratio. To examine the effect of H2 on the synthesis of CNFs, the H2/C2H4 ratio was varied from only ethylene to 3:4, keeping the space velocity and synthesis temperature constant at 10000 h-1 and 600 °C, respectively.


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Figure 8. Comparison between the carbon yields obtained at pilot and laboratory scale. [1] Flow conditions: temperature ) 600 °C, space velocity ) 25000 h-1, H2/C2H4 ratio (v/v) ) 1/4, reaction time ) 1 h. [2] Flow conditions: temperature ) 600 °C, space velocity ) 10000 h-1, H2/C2H4 ratio (v/v) ) 1/4, reaction time ) 1 h. [3] Flow conditions: temperature ) 600 °C, space velocity ) 10000 h-1, H2/C2H4 ratio (v/v) ) 1/4, reaction time ) 2 h.

Figure 7. (a) Carbon yield, (b) growth rate, and (c) deactivation function as a function of time on stream at different H2/C2H4 ratios (v/v). Flow conditions: temperature ) 600 °C, space velocity ) 10000 h-1.

H2 has been reported to both accelerate and suppress CNFs formation and has an important role on the kinetics of CNFs growth. An H2/C2H4 ratio of 1:4 (v/v) induced an increase in carbon yield up to 2.5 times (after 1 h of reaction) if compared with that obtained with no H2 in the feed, resulting in a higher C2H4 conversion (Table 2 and Figure 7a). This particular behavior observed at relatively low temperatures (600 °C)28 shows how, at low concentration, hydrogen might help to remove the carbon layer which encapsulates the catalyst particle, enhancing CNFs formation.29 Increasing H2 concentration up to an H2/C2H4 ratio of 2:4 (v/v) implied a reduction in the carbon yield because of the competition between the reactants for the metallic surface sites, causing a decrease in the amount of both carbon nanofibers and encapsulating carbon species formed (H2

suppresses C2H4 adsorption and dissociation, lowering the carbon yield).30 According to different authors,31,32 the negative effect of hydrogen can also be explained as a thermodynamic effect: the presence of hydrogen in the feed could drive the reverse reaction and promote the partial gasification of the deposited carbon. Anyway, an optimum H2 content in the feed, which depends on the nature of the catalyst and the operating conditions,33 can be observed (H2/C2H4 ) 1:4), for which the growth rate decreases (Figure 7b). The deactivation function showed in Figure 7c would indicate that at high H2 content (H2/C2H4 ) 2:4) the deactivation was less pronounced. 3.3. Lab and Pilot Scale Experiments Comparison in Terms of Carbon Yield. A semiquantitative comparison between the results obtained at different scales has been considered. Results are shown in Figure 8. The scale of the CNFs production pilot plant reactor is around 40-50 g per batch (using 5 g of catalyst) compared with about 1.2 g of CNFs that can be obtained in the laboratory scale reactor (using 0.2 g of catalyst). Our findings demonstrate a successful scale-up by a factor of 45. A carbon yield of 106 gC/gcatalytic metal was obtained during a reaction time of 60 min at optimal conditions in the pilot plant scale. For the same reaction conditions (at which the maximum carbon yield was obtained in both systems), only 80 gC/gcatalytic metal were obtained in the lab-scale reactor. Thus, a substantial improvement was reached with the pilot plant system. 3.4. CNFs Characterization. Structural properties of the synthesized CNFs were studied through the mean number of graphene planes in the crystallites (ngp) (see Table 2), calculated using the formula npg ) Lc/d002 where Lc is the average crystal domain size along a direction perpendicular to the basal planes in a graphitic-type structure, and d002 is the average interlayer spacing. npg values increased with the synthesis temperature, indicating more graphitic carbon materials, keeping a nearly constant value at temperatures around 750 °C. At low reaction temperatures, carbon was deposited as highly disordered carbon, usually as long nanofibers emerging from Ni particles. Nevertheless, at higher temperatures, carbon atoms were deposited as a well-ordered, almost graphitic carbon usually associated with coatings of uniform thickness on Ni particles, which lead to a fast catalyst deactivation. On the other hand, the effects of the space velocity on the npg values were also clear. As can be observed, npg values decreased when the space velocity was raised indicating less graphitic nature of the carbon materials.

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Figure 10. TEM images of typical CNFs synthesized in the pilot scale plant at different temperatures: (a) 450, (b) 600, (c) 750 °C.

Figure 9. TPO patterns of CNFs synthesized at (a) different temperatures, (b) different space velocities, and (c) different H2/C2H4 ratios (v/v).

Finally, npg values passed though a minimum in varying the H2/C2H4 (v/v) ratio. TPO is a convenient and reliable way to evaluate the extent of carbon structural order, that is, amorphous and/or graphitic nature. Figure 9 shows the oxidation profiles of CNFs produced at different temperatures, space velocity, and H2 content. Table 2 presents the maximum weight loss temperature. Logically, CNFs are oxidized at a lower temperature than graphite since they have higher surface area, more defects, and exposed edge planes, and C-H/O-H groups, which altogether make CNFs much more easily attacked by O2.34 On the basis of TPO response, it is evident that an

increase in the reaction temperature leads to more crystalline structures. All CNFs exhibited only a single oxidation peak from the DTA curve (not shown), again indicating high product purity.35 On the contrary, an increase in the space velocity made CNFs structure less crystalline. Finally, no substantial differences were encountered in CNFs oxidation characteristics at different H2/C2H4 ratios. Synthesized CNFs were characterized by means of N2 adsorption at 77 K. Typical isotherms (not shown) can be clearly assigned to the type IV IUPAC classification, characteristic of mesoporous materials. Additionally, these isotherms present a hysteresis loop, commonly related to capillary condensation in laminar mesopores.15Textural parameters obtained by applying the BET equation to the respective isotherms are shown in Table 2. It is observed that as the operating temperature increased, the specific BET surface area and the total pore volume of the deposited carbon decreased, in agreement with the results obtained by others authors.36 An opposite trend was found as the space velocity was increased. Thus, the specific surface area and pore volume increased with the space velocity until a value of 10000 h-1 was reached. Finally, surface area passed though a maximum in varying the H2 content in the feed. Low specific surface areas of carbonaceous materials are typically assigned to highly order graphitic carbon and high specific surface areas with highly disordered carbons. It is concluded that the effect of the reaction variables on the textural properties is in agreement with the XRD and TPO results. The specific surface area of the synthesized CNFs ranged between 100 and 250 m2/g depending on the reaction


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Figure 12. TEM images of typical CNFs synthesized in the pilot scale plant at different space velocities (H2/C2H4 ) 1:4) and H2/C2H4 (v/v) ratios (space velocity ) 10000 h-1): (a) 1000 h-1 and (b) 10000 h-1, (c) H2/C2H4 ) 0.5:4.

Figure 11. Diameter distribution of typical CNFs synthesized in the pilot scale plant at (a) different temperatures, (b) different space velocities, and (c) different H2/C2H4 (v/v) ratios.

parameters, in good agreement with those reported in the literature for similar materials.37,38 These high values of surface area are related to the presence of graphite edges, which act as adsorption sites. The diameter and structure of the CNFs were determined in a TEM study. Representative TEM images and the diameter distribution of CNFs obtained at three different reaction temperatures, 450, 600, and 750 °C, are shown in Figure 10a and 11a, respectively. As observed, an increase in reaction temperature generated a wide range of CNFs diameters, which can be attributed to thermal sintering of the Ni particles. It has been previously reported that the diameter of the CNFs is governed by the size of the metal particle.39 In this study, CNFs with an average diameter that exceed the initial Ni particle size (before reaction) were obtained (Table 2), indicating particle reconstruction in the initial stages of CNFs synthesis. Moreover,

Figure 13. TEM images of typical CNFs synthesized in the pilot-scale plant after different reaction times: (a, c) 2 h and (b, d) 5 h.

the length of the CNFs was considerably higher at lower temperatures, which can be attributed to the duration of the reaction.18 As the temperature increases, Ni particle deactivation took place in a shorter time and consequently, CNFs with a

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Table 3. Comparison between Some Physical-Chemical Characteristics of CNFs Obtained at Pilot and Laboratory Scale BET surface area (m2/g)

npga

micropore area (m2/g)

flow conditionsb

1

2

3

1

2

3

1

2

3

pilot plant CNFs lab CNFs

202 120

200 125

161 103

25 17

25 19

54 12

8.6 10.5

8.5 10.7

9.5 10.7

Number of grapheme planes in the crystallites (npg ) Lc/d002). b Flow conditions: (1) temperature ) 600 °C, space velocity ) 25000 h-1, H2/C2H4 ratio (v/v) ) 1:4, reaction time ) 1 h; (2) temperature ) 600 °C, space velocity ) 10000 h-1, H2/C2H4 ratio (v/v) ) 1:4, reaction time ) 1 h; (3) temperature ) 600 °C, space velocity ) 10000 h-1, H2/C2H4 ratio (v/v) ) 1:4, reaction time ) 2 h. a

shorter length were obtained. The angles with respect to the fiber axis were similar in CNFs synthesized at temperatures below 650 °C (ranging from 25 to 40°) with a tendency to decrease at higher temperatures. Finally, the graphitic sheets of CNFs grown at low temperatures (450 °C) were coiled and tilted toward the tube axis probably due to shape changes in Ni particles that occurred during catalyzing the decomposition of ethylene, which were caused by the changes in surface energy resulting from the C2H4-adsorption on the Ni particles.22,40 The effect of space velocity on the both morphological appearance and the diameter distribution of the deposited carbon at different space velocities is shown in Figures 12b and 11b, respectively. As the space velocity increases, the samples appear much denser and entangled with each other because of the larger amount of deposited carbon (Table 2). Some different morphological features were also apparent. At low values of the space velocity, CNFs exhibited fishbone structure, with straight, well-defined, and crystallized graphite sheets with a narrow core in the center. In contrast, for the CNFs produced a higher values of space velocity, TEM images showed that they also have a fishbone specific stacking orientation, but the angle between the graphene layers and the axis of the CNFs is, in this case, higher. Moreover, the graphite edges are not so straight and no hollow core can be observed. CNF diameter distribution shows that the range of CNFs diameters tends to decrease from around 25-150 nm to about 5-75 nm as the space velocity was increased from 100 to 10000 h-1, which has also been observed previously.41,42 According to TEM analysis, there are not too many morphological differences between CNFs synthesized at different H2/C2H4 ratios. In all cases, fishbone structures were obtained. It can be appreciated, especially at low H2 content, that Ni particles were clearly located on the tip end of the as-grown CNFs (Figure 12c). CNFs synthesized without hydrogen were much shorter compared to fibers synthesized with H2 owing to the faster catalyst poisoning by the formation of coke on the metal surface.43 In addition, it is not possible to establish a clear tendency with respect to the CNFs diameter distribution, the average diameter being always in the range 40-60 nm (Figure 11c). Summarizing, it can be observed that an increase of CNF productivity deteriorated the CNF quality, no matter whether it is due to the increase of H2 content, space velocity, or synthesis temperature. Hence, a compromise between productivity and quality in CNF synthesis must be established, a fact which has also been concluded in other studies.22 Finally, Table 2 lists the physicochemical characterization parameters for the pilot scale synthesized CNFs as a function of the reaction time. The reaction time was varied between 1 to 5 h. In addition, TEM pictures shown in Figure 13 allowed the study of the morphological occurrence of these samples. the length of the CNFs increased with the reaction time; however, very thick fibers were observed only after long reaction times due to the fact that, on large Ni particles, slow CNFs formation takes place, although they tend to deactivate much more slowly. Thus, the very reactive hydrocarbon used (C2H4) caused initially

a very rapid growth of CNFs initially, followed by a rapid deactivation of small particles, and only thick fibers continued to grow after the longest time.44 Figure 13c shows a thick fiber formed by some dark zones in which carbon planes were densely piled, and some random and bright parts, mainly in the center of the fiber, in which the graphene layers only appeared near the edges of the fiber. The formation of this curious structure is probably related to the carbon diffusion in large metal particles.41 In these particles, carbon diffusion to the edge of the base would be faster than to the center because the diffusion path is shorter. Therefore, more graphene layers precipitate near the edge of the base of the metal particle, and less in the center, raising the formation of slits in the CNFs structure. On the other hand, at long reaction times, carbon onions with good graphitization were produced apart from large and thick CNFs (Figure 13d).45 According to Weber et al.,46 carbon onions were also formed because, with the reaction time prolonged, the Ni catalyst particles may be passivated and the catalytic activities of the surface of Ni nanoparticles decrease. The dissolving rate may be higher than diffusing and precipitation rates with the reaction time prolonged, then carbon atoms will accumulate on the surface of catalyst to form the carbon onions. Finally, the total BET surface area and pore volume decreased with increasing the reaction time due to the formation of wider carbon nanostructures that are more mesoporous and something more graphitic in nature. 3.5. Lab and Pilot Scale Comparison in Terms of CNFs Surface Characterization. A general comparison between some physical-chemical characteristic of CNFs synthesized at different scales has been considered. Results are shown in Table 3. As can be observed, higher values of total BET surface area and lower graphitization degrees were always observed for the pilot scale synthesis of CNFs. 4. Conclusions In this paper, the optimization of typical reaction variables for the pilot scale synthesis of CNFs using a fixed-bed reactor was carried out to more economically produce these materials on a large scale. Synthesis of large amounts of carbon nanostructures (CNFs or CNTs) relies on careful control of the operating conditions such as temperature, space velocity, and H2 content. Using a Ni/SiO2 catalyst (10 wt % Ni) and ethylene as carbon source, the optimum temperature in terms of carbon yield was of 600 °C, the optimum space velocity of 10000 h-1 and the optimum H2/C2H4 ratio (v/v) of 1:4. The alteration of these variables caused a significant change in the type and amount of carbon recovered. Carbon product characterization demonstrated that CNFs with mesoporous character, large external surface, good thermal stability, and good crystallinity were obtained. Finally, findings demonstrated a successful scaleup by a factor of 45. A CNFs yield of 106 gCNFs/gcatalytic metal could be obtained in the pilot plant at optimal conditions during a reaction time of 60 min. In the same reaction conditions, only 80 gCNFs/gcatalytic metal were reached in the laboratory reactor.


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ReceiVed for reView April 2, 2009 ReVised manuscript receiVed June 23, 2009 Accepted July 9, 2009 IE9005386

Pilot Plant Scale Study of the Influence of the ... - ACS Publications

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