Sound-Assisted Fluidization of SiO2 Nanoparticles with

Ind. Eng. Chem. Res. 2007, 46, 1345-1349

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Sound-Assisted Fluidization of SiO2 Nanoparticles with Different Surface Properties Huie Liu, Qingjie Guo,* and Shuang Chen State Key Laboratory of HeaVy Oil Processing, College of Chemistry and Chemical Engineering, China UniVersity of Petroleum, Dongying 257061, Shandong ProVince, People’s Republic of China

Sound-assisted fluidization of two kinds of SiO2 nanoparticles (having primary sizes of 5-10 nm) is investigated in this study. The two kinds of nanoparticles are SiO2 without any surface modification (NSM-SiO2) and SiO2 modified with an organic compound (SM-SiO2). The introduction of a 99.8-103.4 dB and 50 Hz acoustic field reduces the superficial minimum fluidization gas velocity, Umf,super, significantly for the two kinds of nanoparticles, and when the sound pressure level increases, the values of Umf,super decrease gradually. With the agitation of a 100 dB acoustic field, the SM-SiO2 nanoparticles could fluidize smoothly similar to Geldart-A particles over the frequency range of 40-60 Hz. NSM-SiO2 failed to fluidize so smoothly, but significant improvement was observed over such a frequency range. Different fluidization behavior, different bed expansion, and agglomerating behavior were also observed for the two kinds of nanoparticles, which indicate that the surface properties of nanoparticles have significant influences on their fluidization behaviors. 1. Introduction Because of their interesting properties, nanoparticles are now increasingly used for the production of catalysts, drugs, cosmetics, foods, and so on. One important means for nanoparticle processing is fluidization. Nanoparticles, belonging to Geldart group C,1 are easy to agglomerate and difficult to fluidize due to the great interactive forces among particles. It has been observed that slugs, channels, and agglomerates are always involved in their fluidization process.2 However, these ultrafine particles can fluidize smoothly at superficial gas velocities much higher than the minimum fluidization velocity of the primary particles.3,4 The high gas velocities to fluidize nanoparticles bring about high powder elutriation, which is a serious problem encountered in nanoparticle processing. Many methods have been developed to improve the fluidization behavior of cohesive Geldart-C particles, for example, adding another kind of particles to the bed5-7 and introducing a magnetic field,8,9 acoustic field,10-16 or vibrating field.5,17-20 Among these methods, sound-assisted fluidization is particularly attractive because no internals are needed and there is no limitation for the particle type. Morse21 investigated the fluidization behaviors of fine particles in an acoustic field and found that in the sound frequency range of 50-400 Hz and at a sound pressure level (SPL) higher than 110 dB, the channeling and slugging for fine particle fluidization can be suppressed significantly. It was observed by Chirone et al.10 that a high-intensity sound could suppress the elutriation of fine particles significantly. Russo et al.13 draw the conclusion that, for a given SPL and bed weight, channeling-free fluidization for nonfluent fine particles can only occur within a certain range of sound frequency. The results of Levy et al.14 showed that, at the natural frequency of the fine particle bed, high-intensity sound waves could lead to reductions in both the minimum bubbling and minimum fluidization velocities, and an increase in SPL caused a decrease in bed expansion. Zhu et al.16 made a preliminary study on soundassisted fluidization of nanoparticle agglomerates. It demon* To whom correspondence should be addressed. Tel.: 86-5468396753. Fax: 86-546-8391971. E-mail: [email protected].

Figure 1. Schematic diagram of the experimental apparatus: 1, compressor; 2, surge tank and desiccator; 3, rotamer; 4, fluidized bed; 5, loudspeaker; 6, sound amplifier; 7, signal generator; 8, manometer. Table 1. Physical Properties of the Particles Used in the Experiments no.

name

size/ nm

particle density/ kg‚m-3

bulk density/ kg‚m-3

1 2

NSM-SiO2 SM-SiO2

5-10 5-10

2560 2560

116 70

strated that the minimum fluidization velocity was significantly reduced and elutriation of nanoparticle agglomerates was much weakened. What should be noted is that most studies on sound-assisted fluidization were concentrated on micrometer or submicrometer particles. Very little work has been done on fluidization of nanoparticles in an acoustic field. Sound-assisted fluidization of two kinds of SiO2 nanoparticles was investigated and compared in this work. Some interesting results will be demonstrated. 2. Experiment The apparatus used in this study is shown in Figure 1. The two main parts of the experimental setup are a 1.6 m high glass fluidized bed and a sound generation system. The inner diameter of the bed is 0.056 m. It is equipped with a porous steel plate gas distributor. Air was used as the fluidization gas, which was supplied with a compressor. Pressure drops across the bed were measured by a U-manometer, with H2O being the indicator. To minimize any effect of humidity on the nanoparticle fluidization, a desiccator filled with silica gel was used to dry the fluidization gas. A WY1603 signal generator was used to produce sine

10.1021/ie0608859 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/17/2007

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Figure 2. Bed collapse curves of NSM-SiO2 and SM-SiO2 nanoparticles.

Figure 3. Pressure drops for gas fluidization.

Figure 4. Microscope images of agglomerates before fluidization: (a) NSM-SiO2; (b) SM-SiO2.

Figure 6. SEM images of agglomerates before fluidization: (a) NSMSiO2; (b) SM-SiO2.

Figure 7. SEM images of agglomerates after fluidization: (a) NSM-SiO2; (b) SM-SiO2.

and 1 dB precision. A reflecting microscope is used to observe the agglomerating state of the nanoparticles. Two kinds of nanoparticles were used in the experiment. Both of them have the same size but have some different properties. The first kind is SiO2 nanoparticles without any surface modification (named “NSM-SiO2”), while the second kind is SiO2 modified with an organic compound (named “SM-SiO2”). The physical properties of these two kinds of nanoparticles are listed in Table 1. It can be found that a great difference exists between their bulk densities, 116 kg‚m-3 for NSM-SiO2 and 70 kg‚m-3 for SM-SiO2. Figure 2 demonstrates typical bed collapse curves of the two kinds of nanoparticles, which were obtained using 30 g nanoparticles. A comparison of the two curves shows that the settling velocities of NSM-SiO2 and SMSiO2 have a great difference. The bed height of NSM-SiO2 approaches the settled one in 2 s, while it takes 18 s for SMSiO2. This phenomenon demonstrates the different particle interactions for these two kinds of nanoparticles, which will lead to different fluidization behaviors. 3. Results and Discussion

Figure 5. Microscope images of agglomerates after fluidization: (a) NSMSiO2; (b) SM-SiO2.

waveforms, whose frequency ranged from 0.001 Hz to 3 MHz. The electric signal was then amplified with a sound amplifier and sent to a 160 mm loudspeaker installed on the top of the fluidized bed, which can generate sound up to 110 dB. An AZ 8925 type sound level meter was used to measure the SPL in the fluidized bed, with a 130 dB maximum SPL to be measured

3.1. Gas Fluidization. The fluidization behaviors of the two kinds of nanoparticles were first investigated. A 40 g sample of NSM-SiO2 nanoparticles was used in the experiment, while 30 g of SM-SiO2 nanoparticles was used to avoid a too high bed height. The fluidization behaviors of the two kinds of particles were similar, undergoing plugs, channels, and full fluidization with increasing gas velocity. Corresponding to the fluidization behavior, the curves of pressure drop across the bed went through three stages, a sharp increase, sometimes higher than the theoretical value, indicating the fixed bed and plug state, a quick decline to a lowest value and then a gradual

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Figure 8. Influence of the sound frequency on pressure drops: (a) NSM-SiO2 (W ) 40 g); (b) SM-SiO2 (W ) 30 g).

increase, caused by channeling and collapse, and a plateau attributed to a complete fluidization regime, as shown in Figure 3. The lowest superficial gas velocity at which the pressure drop reaches a plateau is defined as the superficial minimum fluidization velocity, Umf,super, in this work. The gas fluidization behavior is somewhat different with increasing gas velocity for the two kinds of nanoparticles. SMSiO2 underwent the bubbling fluidization stage with an increase of the gas velocity. However, the bubbling stage was not observed for NSM-SiO2, which arrived at turbulent fluidization directly from the channeling state at a relatively high gas

velocity. At the same time, a significant difference was observed between the values of Umf,super for the two kinds of nanoparticles, 0.170 m‚s-1 for NSM-SiO2 and 0.090 m‚s-1 for SM-SiO2. Spherical agglomerates were observed by eye in the fluidization process for the two kinds of nanoparticles. The particles are discharged from the bed after an about 10 min fluidization process, and the agglomerate states before and after fluidization are observed using a reflecting microscope. It is found that, before fluidization, the two kinds of nanoparticles both coalesce to form untidy agglomerates naturally (as shown in Figure 4) and they exist in spherical or ellipsoidal agglomerates after

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Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007 Table 2. Umf,super (m‚s-1) under Different SPLs (50 Hz) Umf,super/m‚s-1 particle

no acoustic field

99.8 dB

102.5 dB

103.4 dB

NSM-SiO2 SM-SiO2

0.170 0.090

0.101 0.022

0.080 0.022

0.090 0.018

Table 3. Average Sizes of the Biggest Agglomerates under Different SPLs (50 Hz) diameter/mm

Figure 9. Influence of the sound frequency on Umf,super.

fluidization (as shown in Figure 5). However, the surfaces of the SM-SiO2 agglomerates are much tidier than those of the other kind of nanoparticles. This means that the surface property of the nanoparticles influences the roughness of the agglomerates formed during fluidization. The structure of the agglomerate is also observed via scanning electron microscopy (SEM). SEM images are shown in Figures 6 and 7. Figure 6 indicates that both NSM-SiO2 and SM-SiO2 nanoparticles exist as loose agglomerates before fluidization, while they form complex agglomerates, which are joined by many small ones, after fluidization, as shown in Figure 7. 3.2. Sound-Assisted Fluidization. The effects of the sound pressure level and sound frequency are thoroughly discussed in this work. The two kinds of nanoparticles have different fluidization behaviors in a conventional fluidized bed. It is expected they will show different fluidization behaviors in an acoustic fluidized bed. 3.2.1. Influences of the Sound Frequency. To investigate the effects of the sound frequency on the fluidization of SiO2 nanoparticles, experiments were carried out with the sound frequency varying from 30 to 300 Hz and SPL fixed at 100 dB. The plots of pressure drops vs increasing gas velocity for the two kinds of nanoparticles under 100 dB at different sound frequencies are shown in Figure 8. Three stages can be clearly distinguished in Figure 3. However, we cannot discriminate between them in Figure 8 when the sound frequency is in the range of 30-70 Hz. This means that, under 100dB, the plugs and channels can be significantly suppressed over the frequency range of 30-70 Hz for the two kinds of nanoparticles. For SMSiO2 nanoparticles, especially, fluctuations of pressure drop curves hardly exist in the sound frequency range from 40 to 60 Hz. The curves increase with gas velocity gradually and reach a plateau finally. Corresponding to the pressure drop curves, the SM-SiO2 nanoparticles fluidize smoothly like Geldart-A particles with the assistance of an acoustic field ranging from 40 to 60 Hz, with no obvious plugs and channels being observed. This is a very attractive phenomenon. However, more or less channels still exist over the whole frequency range for NSMSiO2. The results of Russo et al.13 on sound-assisted fluidization of 0.5-45 µm particles showed that, for a given SPL and bed weight, channeling-free fluidization can only occur within a certain range of sound frequency. Their conclusion on micrometer or submicrometer particles is similar to the results obtained for nanoparticles in this work. They theoretically analyzed the forces around the particles (0.5-45 µm) and agglomerates and found that when SPL was fixed, with increasing sound frequency, the force yield by the acoustic field increased first, followed by a decrease, while the sizes of the clusters decreased and then increased, reaching a minimum at some frequency.

particle

no acoustic field

99.8 dB

102.5 dB

103.4 dB

NSM-SiO2 SM-SiO2

2.49 2.90

2.17 2.15

1.4 1.78

1.3 1.52

Also the particles could fluidize channeling-free only in a certain frequency range. It is expected that the sizes of the agglomerates also have a minimum value in this work. From the Ergun equation for a low Reynolds number22

(φsds)2 Fs - Ff mf3 umf ) g 150 µf 1 - mf

(1)

which means the minimum fluidization velocity is proportional to ds2. Therefore, the minimum fluidization velocity should also have a lowest value. Values of Umf,super were analyzed under different sound frequencies. The relation between Umf,super and sound frequency, f, is shown in Figure 9. Minimum values do exist at 50 Hz, which is consistent with the foregoing deduction. 3.2.2. Effects of SPL. Form the above results, a 50 Hz frequency acoustic field is ideal. Thus, the sound frequency was fixed at 50 Hz in the following experiments. Three SPLs, 99.8, 102.5, and 103.4 dB, were used to observe the influence of SPL on the fluidization of SiO2 nanoparticles. The relation between SPL and Umf,super is shown in Table 2. It can be clearly seen that the agitation of the acoustic field can decrease Umf,super. A 99.8 dB SPL can reduce its value from 0.170 to 0.101 m‚s-1 for NSM-SiO2 and from 0.090 to 0.022 m‚s-1 for SM-SiO2, and with an increase of SPL, the values of Umf,super decrease gradually. The increase of SPL means increasing energy intensity introduced to the bed. Thus, the external force yielded by the acoustic field on the nanoparticles or agglomerates is enhanced. It is supposed that the agglomerates should be broken into smaller ones with increasing external force, which brings about a decrease of Umf,super and improvement of the fluidization behavior. Thus, after the experiments, the particles are discharged and the agglomerates are observed using the microscope and the sizes are measured each time. Because the visual field of the microscope is too limited, we cannot obtain an exact average diameter of the agglomerates. Typically, the 30 biggest agglomerates are chosen and their sizes are measured each time. With the increase of SPL, no significant changes in the shape of the agglomerates are found, but changes of the agglomerate sizes are obvious. The average sizes of the biggest agglomerates are listed in Table 3. It can be clearly seen that the diameters of the biggest agglomerates decrease with increasing SPL. These values validate our supposition. Although the two kinds of nanoparticles used in this work have similar sizes, the different surface properties make them show different fluidization properties. Differences exist between their bulk densities and between their collapse behaviors, which directly demonstrate the different particle interactions. The two kinds of particles showed different fluidization behaviors when

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agitated by an acoustic field. SM-SiO2 can fluidize very smoothly like Gledart-A particles with the assistance of a 4060 Hz acoustic field. However, NSM-SiO2 fails to fluidize so smoothly under all of the conditions in this work. The agglomerates formed in fluidization with and without an acoustic field showed distinct sizes and shapes for each kind of nanoparticles. At the same time, the values of Umf,super are much smaller for SM-SiO2 than NSM-SiO2 under the same conditions. These phenomena may all relate closely to the different surface properties of the two kinds of nanoparticles. An experimental and theoretical study is now in progress to explain these interesting phenomena. 4. Conclusions Sound-assisted fluidization of two kinds of SiO2 nanoparticles was investigated in this work. With the agitation of the sine sound wave, the fluidization behaviors of the nanoparticles are different from those without an acoustic field. From the present investigation, the following conclusions can be drawn. (1) When SPL was fixed at 100 dB, the SM-SiO2 nanoparticles could fluidize smoothly like Geldart-A particles over the frequency range of 40-60 Hz, while NSM-SiO2 failed to fluidize so smoothly. However, significant improvements were also observed for NSM-SiO2 over such a frequency range. (2) Under the agitation of acoustic fields of 99.8-103.4 dB and 50 Hz, the superficial minimum fluidization velocities, Umf,super, and the diameter of the biggest agglomerates decrease significantly with an increase of SPL for the two kinds of nanoparticles. (3) Surface properties of the nanoparticles have significant influences on their agglomerate sizes, shapes, and fluidization behaviors in acoustic fields. Acknowledgment This research is supported by the Natural Science Foundation of Shandong Province (Contract Nos. Z2003B01 and Q2006B08), the Projectssponsored by SRF for ROCS, SEM (Contract No. 2004527), the Key Project of Chinese Ministry of Education (Contract No. 105106), and the National Natural Science Foundation of China (Contract No. 20490200). Nomenclature ds ) diameter of a particle or agglomerate, m f ) sound frequency, Hz g ) acceleration of gravity, m‚s-2 NSM-SiO2 ) SiO2 nanoparticles without any surface modification SM-SiO2 ) SiO2 nanoparticles modified with an organic compound SPL ) sound pressure level, dB Ug ) superficial gas velocity, m‚s-1 Umf ) minimum fluidization velocity, m‚s-1 Umf,super ) superficial minimum fluidization velocity, m‚s-1 W ) total weight of particles in the bed, g ∆p ) pressure drop across the bed, Pa mf ) total voidage at minimum fluidization φs ) sphericity of the particles or agglomerates µf ) viscosity of the fluidizing gas, Pa‚s

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ReceiVed for reView July 10, 2006 ReVised manuscript receiVed December 6, 2006 Accepted December 12, 2006 IE0608859