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1 Powder Technology 201 (2010) Contents lists available at ScienceDirect Powder Technology journal homepage: Aeration and mixing behaviours of nano-sized powders under sound vibration P. Ammendola, R. Chirone Istituto di Ricerche sulla Combustione CNR, P.le V. Tecchio 80, Napoli Italy article info abstract Article history: Received 14 January 2009 Received in revised form 26 August 2009 Accepted 1 March 2010 Available online 10 March 2010 Keywords: Nanoparticles Fluidization Mixing Aeration Acoustic fields The aeration and mixing behaviours of two nano-sized powders, Al 2 O 3 (40 nm) and CuO (33 nm), have been investigated in a laboratory scale fluidized bed. The fluidization quality of both powders is very poor without application of acoustic fields. Sound intensities larger than 135 db and frequencies in the range Hz improve their fluidization quality resulting into a homogeneous fluidization regime with high bed expansion. Under the effect of sound, mixing between powders has been qualitatively characterized by the visual observation of the bed and the SEM analysis of captured samples. Under the operating conditions tested, mixing between aggregates of the two powders takes only few minutes. However, mixing also occurs inside aggregates but this process requires larger times, of the order of min Elsevier B.V. All rights reserved. 1. Introduction Ultrafine powders, in particular nanoparticles (b100 nm), have received increased attention with regards to the manufacturing of semiconductors, drugs, cosmetics, foods, plastics, catalysts, paints, sunscreens, biomaterial, due to special physical and chemical properties arising from their extremely small primary particle size and very large surface area per unit mass [1]. In this framework, it has become increasingly important to understand how these nanoparticles can be handled and processed [2]. Nevertheless, handling and processing of ultrafine powders is very challenging being extremely cohesive and in a gas flow they tend to form hard aggregates because of strong interparticle forces [1,3 5]. Recent literature reports examples of fluidization of nanoparticle aggregates [1 8] which can be classified according to their fluidization behaviour as either Agglomerate Particulate Fluidization (APF) or Agglomerate Bubbling Fluidization (ABF) [6]. APF is characterized by large bed expansion (up to few times the original bed height), smooth fluidization, like Geldart group A micron-size particles, and very low minimum fluidization velocities. ABF shows little bed expansion, bubbling, and the bed behaves more like Geldart group B micron-size particles. To overcome possible difficulties arising during fluidization of nanoparticles, i.e. channelling or slugging, different assisting methods, such as acoustic fields [3,9], centrifugal fields (rotating fluidized beds) [10,11], mechanical vibration [4], electric fields [12] or magnetic fields [13 15], have been adopted. In particular, under the influence of appropriate acoustic fields, channelling or slugging tends to disappear Corresponding author. Tel.: address: paola.ammendola@irc.cnr.it (P. Ammendola). and the beds expand uniformly [16]. Moreover, in the case in which a fluidization regime has been reached, the aid of sound application results into a significative reduction of the minimum fluidization velocity. Zhu et al. [16] reported that the optimal sound frequency range is Hz; in particular, from 200 to 600 Hz bubbling fluidization has been observed. On the other hand, Guo et al. [3,9,17] reported that the optimal sound frequency range is Hz and the fluidization behaviour appears similar to that of Geldart group A particles with no bubbles and negligible elutriation. At sound frequency exceeding these respective ranges, sound has almost no impact on the fluidization: the aeration process encounters slugging and is accompanied by much elutriation. As regards the effect the sound pressure level, below a critical value, 115 db [16] or 100 db [3,9,17], there is no effect. Moreover, the fluidization quality generally improves as the sound pressure level increases [3,9,16,17]. Only few studies can be found in literature on mixing behaviour of different nanopowders [11,18]. In particular, Nakamura and Watano [11] analyzed the mixing of two nanopowders, SiO 2 and Al 2 O 3, under the effect of a centrifugal acceleration. They found that the mixing occurred at a micron-scale. Present work is focused on the characterization of two different nanopowders, Al 2 O 3 (40 nm) and CuO (33 nm), during aeration under the application of acoustic fields of different intensities and frequencies. In particular, experimental tests have been carried out in a laboratory scale sound assisted fluidized bed to highlight the role of sound on the quality of fluidization and on the quality and degree of mixing between the two nanopowders. Outputs of experimental activity were: i) the ranges of acoustic field intensities and frequencies most effective to reach a good fluidization regime; ii) a preliminary assessment of the efficiency of sound application to promote nanoparticles mixing. As regards the last point, mixing between different /$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.powtec

2 50 P. Ammendola, R. Chirone / Powder Technology 201 (2010) nanopowders during aeration, promoted by the application of a suitable acoustic field, has been characterized by experiments carried out using a powder as a tracer in a bed made of the other powder. In particular, both the global mixing between two different powders (i.e. the uniform mixing of different aggregates) and the local mixing (i.e. the presence of mixed aggregates made of two powders) have been investigated by the visual observation of the bed and the analysis of samples of the fluidized materials taken from the upper part of the bed at different times by means an ad hoc non-destructive sampling procedure. 2. Experimental 2.1. Apparatus and materials The experimental apparatus is sketched in Fig. 1. It consists of a fluidization column (41 mm ID and 1000 mm in height), made of quartz, equipped with a sintered bronze porous plate gas distributor, a set of filters for the collection of elutriated fines at the column exit, a pressure transducer (Hartmann & Braun) installed at 0.02 m above the gas distributor to measure the pressure drop across the bed, a sound wave guide at the top of the freeboard, a sound-generation system and a data acquisition system for recording the sound frequency (f) and intensity (Sound Pressure Level=SPL), the pressure drop across the bed (ΔP), the superficial gas velocity (u) and the actual bed height (H). The sound-generation system is made of a digital signal generator to obtain an electric sine wave of specified frequency whose signal is amplified by means of a power audio amplifier rated up to 400 W. The signal is then sent to a 8 W woofer loudspeaker placed downstream the sound wave guide. The wind box (41 mm ID and 600 mm high), made of stainless steel, is filled by ceramic rings to ensure a uniform distribution of gas flow. To minimize any effect of humidity on the nanoparticle fluidization, pure dry nitrogen from a compressed tank is used as the fluidizing gas. The flow rate of gas is controlled by a mass flow controller (Bronkhorst). Both the fluidization and the mixing tests have been carried out at room temperature and ambient pressure conditions. The behaviour of two different nano-sized powders, Al 2 O 3 and CuO (Sigma-Aldrich), has been investigated during aeration. The powders have primary particle average sizes of 40 nm and 33 nm and densities of 3973 kg/m 3 and 6315 kg/m 3, respectively. Fig. 2 reports the cumulative size distributions of the powders as received measured by using a laser granulometer (Mastersizer 2000 Malvern Instruments) after the dispersion of the powders in water under mechanical agitation of the suspension and with or without the application of ultrasound (US). Analysis of data shows that both the powders form relatively large aggregates. In particular, under the weak effect of mechanical agitation the Al 2 O 3 aggregates are larger than the CuO ones. The application of ultrasound involves the break-up of large aggregates in smaller ones for both powders, even if this action is more effective for Al 2 O Procedure for characterization of fluidization quality Aeration tests have been performed in correspondence of an initial bed height (H 0 ) fixed at about 15 cm, corresponding to a bed of about 48 and 35 g for Al 2 O 3 and CuO, respectively. Pressure drop and bed expansion curves have been obtained in two different experimental conditions: i) aeration with a fluidizing gas; ii) aeration with both a fluidizing gas and the application of acoustic fields of different intensities ( db) and frequencies ( Hz). The ranges of sound intensity and frequency are similar to those used for sound assisted fluidization of cohesive Geldart group C powders [19] and nanoparticles [3,16,20]. Apart from a direct observation of the bed, the ranges of sound intensities and frequencies most effective to reach a good fluidization quality have been identified by comparing the different experimental curves. The experimental pressure drop curves have been also worked out to calculate the minimum fluidization velocities (u mf ) under the investigated experimental conditions. The size, density and voidage of fluidizing nanoparticle aggregates have been calculated by working out the experimental bed expansion data according to three different methods, proposed by Wang et al. [6], Zhu et al. [1] and Nam et al. [4], all based on the Richardson Zaki equation [21] Procedure for characterization of mixing quality Mixing tests have been carried out both without and with the application an acoustic field (140 db 120 Hz) able to fluidize the powders. The superficial gas velocity has been fixed at about 0.45 cm/s, Fig. 1. Experimental apparatus: 1) N 2 tank; 2) mass flow controller; 3) display; 4) pressure transducer; 5) quartz column; 6) filter; 7) microphone; 8) sound guide; 9) loudspeaker; 10) wind box. Fig. 2. Cumulative size distributions of powders.

3 P. Ammendola, R. Chirone / Powder Technology 201 (2010) enough to fluidize the materials under the application of the above acoustic field. For all the experimental conditions tested, the starting point was obtained by feeding 30 g of Al 2 O 3 nanopowder (white powder) and 5 g of CuO nanopowder (black powder), used as a tracer. Two different ways of introducing initially the powders inside the column have been considered as schematically reported in Fig. 3. In particular the starting points have been obtained by feeding the Al 2 O 3 nanopowder and then the CuO nanopowder (Fig. 3A) or, alternatively, the CuO nanopowder and then the Al 2 O 3 nanopowder (Fig. 3B). Each test has been carried out for about 120 min. The visual observation of the bed, also recorded by a video camera, gave some preliminary rough information on the uniformity of mixing and the mixing characteristic time at a global scale. During these experiments, samples of the fluidized materials have been taken at different times by means an ad hoc nondestructive sampling procedure, similar to that reported by Nam et al. [4]. In particular, a probe made of a silicon tube linked to the adhesive sample disk used for SEM analysis has been inserted from the top of the reactor, so that the fluidized materials present in the upper part of the bed sticked on it. The different samples have been analyzed by SEM/EDS analysis in order to determine the shape and the chemical composition of aggregates. The latter analysis gave additional information on the aggregates mixing degree and on the time required to reach a rather stationary condition in aggregate mixing. 3. Results and discussion 3.1. Characterization of fluidization quality Fig. 4 reports the dimensionless pressure drop ΔP/ΔP 0, where ΔP 0 is the pressure drop equal to buoyant weight of particles per unit area of bed, as a function of the superficial gas velocity u obtained without the application of sound both increasing (Up curve) and decreasing the superficial gas velocity (Down curve) step by step for Al 2 O 3 (A) and CuO (B), respectively. After loading, the beds of both powders have been aerated for several minutes at a superficial gas velocity (about 7 and 5 cm/s for Al 2 O 3 and CuO, respectively), low enough to avoid the elutriation of large quantities of powder out of the bed. The Down curve has been firstly obtained and then the Up curve. Analysis of the curves shows that the fluidization quality of both beds is very poor. Even if pressure drop curves approach the unity, both types of curves are irregular. The presence of channels across the bed and the irregular surface of the beds together with the absence of particle motion confirm the poor quality of the fluidization. However some bed expansions have been observed for both powders. These evidences highlight that under the effect of a fluidizing gas and without the application of an acoustic field both the nanopowders were not Fig. 3. Initial conditions of mixing tests. Fig. 4. Dimensionless pressure drop (ΔP/(ΔP 0 ) as a function of superficial gas velocity during aeration without the application of acoustic fields for Al 2 O 3 (A) and CuO (B). able to reach a really fluidized state but only an aerated state. However, it is possible to individuate a critical value of the superficial gas velocity (about 4 cm/s for both powders), above which the dimensionless pressure drop approaches the unity. The application of acoustic fields of different intensities and frequencies generally results in more regular pressure drop and bed expansion curves for both the powders. However there are some differences that make possible to identify the ranges of intensities and frequencies of acoustic fields most effective to stabilize a good fluidization regime. In particular, sound intensities lower than 135 db did not prove significant influences on bed behaviour, i.e. the fluidization quality of powders is low. In this case the dimensionless pressure drop ΔP/ΔP 0 and the bed expansion ratio H/H 0 are less than 0.6 and 1.1 and hysteresis phenomena have been also observed. On the other hand, sound intensities higher than 135 db are required in order to improve the fluidization quality of both powders. Fig. 5 reports the pressure drop and the bed expansion curves obtained with the application of an acoustic field of SPL=140 db and f=100 Hz for Al 2 O 3 (A and B) and CuO (C and D). Typical ideal fluidization curves have been obtained for both powders: the dimensionless pressure drop across the bed and the bed expansion ratio reached high values (about 1 and 1.8, respectively). Similar behaviours have been also observed for higher sound intensities and for all the investigated frequencies. With regards to the bed expansion curves it can be noted that both the powders showed some bed expansion even for superficial velocities lower than their respective u mf. In addition, both the materials showed a typical APF fluidization behaviour: homogeneous

4 52 P. Ammendola, R. Chirone / Powder Technology 201 (2010) Fig. 5. Dimensionless pressure drop (ΔP/ΔP 0 ) and bed expansion ratio (H/H 0 ) as a function of superficial gas velocity during aeration with the application of an acoustic field (SPL=140 db, f=100 Hz) for Al 2 O 3 (A and B) and CuO (C and D). fluidization regime like Geldart's group A powders, characterized by absence of bubbles and with a clear bed surface. Moreover, the application of acoustic fields plays a beneficial role not only in stabilizing the bed aeration but also on elutriation of single fines or smaller aggregates, as evidenced by the fact that the filter for exhaust gas remained clean after many hours of aeration test. Pressure drop data have been analyzed to obtain the experimental values of the minimum fluidization velocities (u mf ) under different conditions for Al 2 O 3 and CuO. For both powders the values of u mf are quite insensitive in the range of sound intensity and frequency tested, being about and cm/s for Al 2 O 3 and CuO, respectively. These values are several orders of magnitude higher than the u mf of the primary nanoparticles, in spite of the application of acoustic fields. This result suggests the fluidization of both nanopowders in the form of aggregates even under the effect of very intense acoustic fields. The maximum values of dimensionless pressure drop (ΔP max /ΔP 0 ) and of the bed expansion ratio (H max /H 0 ), derived from experimental pressure drop and bed expansion curves could be considered as an index of the fluidization quality, i.e. higher values of ΔP max /ΔP 0 and H max /H 0 suggest a better fluidization quality. Fig. 6 reports the values of ΔP max /ΔP 0 and H max /H 0 as functions of SPL at fixed frequency f=120 Hz (A) and as a function of f at fixed sound pressure level SPL=140 db (B), for Al 2 O 3 and CuO. The fluidization quality of both powders improves, i.e. both the ΔP max /ΔP 0 and H max /H 0 increase, as the sound intensity increases. However, under the experimental conditions tested, sound intensities at the top of the bed higher than 135 db are required to obtain values of ΔP max /ΔP 0 close to unity and of H max /H 0 higher than 1.6. It must be noted that this critical value of sound intensity is larger than those found by other Authors, 100 db [3,9,17] or 115 db [16]. It is likely that both the differences in particle size range of tested nanoparticles or of their aggregates and sound attenuations in deeper beds can account for such differences. On the other hand, the effect of frequency is not monotone, according to literature data [3,9,16,17], and an optimum condition, in terms of a highest values of ΔP max /ΔP 0 and H max /H 0, has been found, whatever the powder, for 100 Hzbfb125 Hz. The explanation of this trend is not straightforward; it could be related to the ability of sound to penetrate the bed as well as to promote aggregates reduction into a scale which depends also on powder structure. For frequencies higher than about 125 Hz the acoustic field is not able to propagate inside the bed, while for frequencies lower than about 100 Hz the relative motion between smaller and larger aggregates, i.e. the break-up of aggregates, is practically absent. Between these ranges there is a range of optimal frequencies able to promote a relative motion between aggregates and then their break-up. The characteristic properties, i.e. size, density and voidage, of fluidizing nanoparticles aggregates have been calculated by working out the experimental bed expansion data on the basis of three different methods proposed in literature by Wang et al. [6], Zhu et al. [1] and Nam et al. [4], valid for APF fluidization. The values obtained with the three methods were very close to each other for both Al 2 O 3 and CuO. The calculated size, density and voidage of Al 2 O 3 aggregates are in the ranges μm, kg/m 3 and , respectively. The same properties of CuO aggregates are in the ranges μm, 240

5 P. Ammendola, R. Chirone / Powder Technology 201 (2010) Fig. 6. A) Effect of sound intensity (f=120 Hz) and B) effect of sound frequency (SPL=140 db) on ΔP max /ΔP 0 and H max /H 0 for Al 2 O 3 (full symbols) and CuO (open symbols). 260 kg/m 3 and These sizes are more than 1000 times larger than the primary nanoparticles and also aggregate densities are larger than the bulk densities of powders (273 and 177 kg/m 3, respectively) Characterization of mixing quality The possibility that there is an efficient mixing between different nanopowders during aeration, promoted by the application of a suitable acoustic field, has been highlighted by experiments carried out using a powder (CuO) as a tracer in a bed made of the other powder (Al 2 O 3 ). In particular, both the global mixing between two different powders (i.e. uniform mixing of different aggregates) and the local mixing (i.e. presence of mixed aggregates made of two powders) have been investigated. The visual observation of the bed, also recorded by a video camera, gave some preliminary qualitative information on the uniformity of mixing and the mixing characteristic time at a global scale. Fig. 7 refers to the A case of Fig. 3: the starting point has been obtained by feeding 30 g of the Al 2 O 3 nanopowder (white powder) and then 5 g of copper oxide nanopowder (tracer), which forms a black layer on the top of the bed (Fig. 7A). Fig. 7B refers to 1 min bed aeration with a nitrogen flow rate (u =0.45 cm/s) without the application of an acoustic field. Fig. 7C and D are relative to bed aeration assisted by the application of the acoustic field (SPL=140 db, f=120 Hz) for one and five minutes, respectively. Analysis of the figures shows that without the application of acoustic fields no mixing occurs (Fig. 7B). Channelling is present and, as a result of stratification of particles of different densities, there is formation of one or more channels with transport of lighter powder on the top of the bed. The application of the acoustic field results into a relatively large bed expansion and solid mixing (Fig. 7C). After few minutes the entire bed turned grey and appeared well mixed (Fig. 7D). At a macroscopic scale, the visual observation of the bed highlighted the effectiveness of sound application in promoting the fluidization and the global mixing of nanopowders within few minutes. Even exchanging the order of placement of powders (Fig. 3B), similar mixing was approached in few minutes of aeration under the same experimental conditions. Nam et al. [4] obtained similar behaviour during mechanical vibro-fluidization of nanoparticles. Even if both analysis are based only on a qualitative observation of the bed along time, sound assisted fluidization seems to be as effective as mechanical vibration to promote particle mixing and bed expansion. In order to obtain additional information, during these experiments samples of the fluidized materials have been taken from the upper part of the bed at different times by means of the ad hoc non-destructive sampling procedure described in the experimental section. The different samples have been then characterized by SEM/EDS analysis in order to determine the shape and the chemical composition of aggregates. On the basis of the initial bed composition (30 g of Al 2 O 3 and 5 g of CuO), the Al and Cu weights correspond to 80% and 20%, respectively. These values represent the theoretical limit corresponding to a complete mixing both at a global scale (average composition of the bed) and at a local scale (average composition of aggregates). The EDS analysis performed on entire areas of bed samples showed that the Al weight composition was about 83 86% already after 1 min of sound assisted aeration whatever the order of placement of powders (Fig. 3) of the powders and after 120 min this value reduces Fig. 7. Mixing experiments using CuO as a tracer powder. A bed made of 30 g of Al 2 O 3 (white) and 5 g of CuO (black). B bed aerated for 1 min without application of sound (u=0.45 cm/s). C bed aerated for 1 min with sound application (u=0.45 cm/s, SPL=140 db, f =120 Hz). D bed aerated for 5 min with sound application (u=0.45 cm/s, SPL =140 db, f =120 Hz).

6 54 P. Ammendola, R. Chirone / Powder Technology 201 (2010) Fig. 8. Mixing of nanoparticles aggregates at a local scale. to 80 82%. These results confirm the information obtained by the visual observation of the bed, i.e. at a global scale the mixing of the bed really occurs within very short times. Even in the B case when the CuO has been fed at the bottom of the bed, the average composition of the bed approached the limit value within few minutes, highlighting that the bed is completely fluidized under the adopted operating conditions. The mixing at the global scale can be associated to two possibilities of mixing at a local scale, schematically represented in Fig. 8, where A and B indicate two initial aggregates only made of Al 2 O 3 and CuO nanoparticles, respectively. The sound assisted aeration could promote only a mixing of single aggregates (path I) or a mixing of subaggregates, due to a continuous break-up and re-forming of single aggregates, leading to the formation of mixed aggregates (path II). Fig. 9 reports the SEM images and the EDS analysis of aggregates taken at different times during both A and B mixing tests. Analysis of the figure shows that all aggregates are formed of both alumina and copper oxide, according to the path II of Fig. 8. Their compositions are variable in a wide range. Aggregates richer both in alumina and in copper oxide are present, but their shapes and sizes appear to be similar to each other, therefore it is not possible to distinguish the different composition of aggregates only on the basis of the morphological analysis. Fig. 10 reports the cumulative distribution of aggregates composition at different times obtained for A (Fig. 10A) and B (Fig. 10B) mixing tests. N denotes the number of aggregates having an Al weight composition lower than the x-axis value and N t represents the total number of aggregates. The analysis of the curves highlights that all aggregates are mixed even after 1 min of sound assisted aeration even if their composition is variable in a wide range. Aggregate composition data reported in Fig. 10 have been worked out to obtain the curves, reported in Fig. 11, representing the time dependence of aggregates mixing degree M(t) for both A and B mixing tests. The aggregate mixing degree M at a fixed time t has been defined as the ratio between the number of aggregates with the Al composition in the range 70 90% (i.e. near the limit value of 80%) and the total number of aggregates analyzed at time t. Each data series has been fitted with an exponential rise-to-maximum law: Mt ðþ= a 1 exp t b. The parameters a and b have been obtained by least squares minimization. The two dependence laws M A (t) and M B (t) obtained for A and B mixing test respectively are also reported in Fig. 11. The maximum aggregate mixing degree is the same (52 53%) for both A and B mixing tests, i.e. half of the aggregates have a composition close to the limit value. The analysis of the curves also shows that the dynamic phenomena of local mixing evolve in times longer than those characteristic of the global mixing. The order of placement of the two powders affects the time dependence of the aggregate mixing degree: in the B case (the CuO tracer is initially located under the Al 2 O 3 bed) the characteristic time of local mixing (about 150 min) required to reach the maximum mixing degree is twice as big as the one (about 80 min) obtained for the A case (the CuO tracer is initially located under the Al 2 O 3 bed). Fig. 9. SEM images and EDS analysis of nanoparticle aggregates. A) sample taken after 14 min during the A mixing test; B) sample taken after 30 min during the B mixing test.

7 P. Ammendola, R. Chirone / Powder Technology 201 (2010) Conclusions Aeration and mixing tests of two different nano-sized powders, Al 2 O 3 (40 nm) and CuO (33 nm), have been carried out in a laboratory scale fluidized bed at ambient temperature and pressure and using nitrogen as fluidizing gas. On the basis of the results obtained during the aeration of Al 2 O 3 and CuO nanoparticles with and without the application of different acoustic fields, the main results are the following: the fluidization quality of both Al 2 O 3 and CuO beds is very poor without application of acoustic fields even if some bed expansion has been found; the application of an acoustic field is generally required to stabilize a fluidization regime; an ideal fluidization regime, i.e. ideal like pressure drop curves and relatively high bed expansions, can be obtained in correspondence to an optimal frequency range ( Hz) and to sound intensities at the top of the bed higher than 135 db; under the effect of suitable acoustic fields a homogeneous fluidization regime like Geldart's group A powders, without bubbles, with high bed expansion ratio, characteristic of APF fluidization, has been observed; fluidized aggregate sizes of 100 μm and 450μm have been obtained for Al 2 O 3 and CuO, respectively, using three methods based on the Richardson Zaki equation. They are significantly larger than both the elementary particles and the so called natural forming aggregates. A preliminary investigation on the efficiency of sound assisted aeration to promote the mixing of two nano-sized powders has been carried out. On the basis of visual observation of the bed and of SEM analysis of captured samples it is possible to make the following considerations: Fig. 10. Cumulative distribution of aggregates at different times. A) A mixing test; B) B mixing test. sound assisted aeration is able to promote a complete mixing of powders on a global scale, whereas, at a local scale aggregates of particles are characterized by a maximum mixing degree of about 50%, whatever the initial order of placement of powders in the column; the time required to reach the global mixing of the two powders is of few minutes, smaller than those required to obtain a rather stationary condition in mixing inside aggregates, which is also active, but requires min depending on the initial order of placement of powders. Acknowledgments Roberto Sisto and Giovanni Gaggiano are gratefully acknowledged for their assistance during experiments. Authors are also grateful to Sabato Russo for performing the SEM/EDS analysis. References Fig. 11. Time dependence of aggregates mixing degree for A and B mixing tests. [1] C. Zhu, Q. Yu, R.N. Dave, R. Pfeffer, Gas fluidization characteristics of nanoparticle agglomerates, AIChE J. 51 (2005) 426. [2] S. Matsuda, H. Hatano, T. Muramoto, A. Tsutsumi, Modeling for size reduction of agglomerates in nanoparticle fluidization, AIChE J. 50 (2004) [3] Q. Guo, Y. Li, M. Wang, W. Shen, C. Yang, Fluidization characteristics of SiO 2 nanoparticles in an acoustic fluidized, Chem. Eng. Technol. 29 (2006) 78. [4] C.H. Nam, R. Pfeffer, R.N. Dave, S. Sundaresan, Aerated vibrofluidization of silica nanoparticles, AIChE J. 50 (2004) [5] X.S. Wang, V. Palero, J. Soria, M.J. Rhodes, Laser-based planar imaging of nanoparticle fluidization: Part I Determination of aggregate size and shape, Chem. Eng. Sci. 61 (2006) [6] Y. Wang, G. Gu, W. Fei, J. Wu, Fluidization and agglomerate structure of SiO 2 nanoparticles, Powder Technol. 124 (2002) 152. [7] L.F. Hakim, J.L. Portman, M.D. Casper, A.W. Weimer, Aggregation behavior of nanoparticles in fluidized beds, Powder Technol. 160 (2005) 149. [8] J. Jung, D. Gidaspow, Fluidization ofnano-size particles, J. Nanopart Res. 4 (2002) 483. [9] Q. Guo, H. Liu, W. Shen, X. Yan, R. Jia, Influence of sound wave characteristics on fluidization behaviors of ultrafine particles, Chem. Eng. J. 119 (2006) 1. [10] J. Quevedo, R. Pfeffer, Y. Shen, R. Dave, H. Nakamura, S. Watano, Fluidization of nanoagglomerates in a rotating fluidized bed, AIChE J. 52 (2006) 2401.

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