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Mixing and Packing of Fine Particles of Different Sizes Huiju Liu, Qun Yu,† Robert Pfeffer,*,‡ and Costas Gogos Otto York Department of Chemical, Biological and Pharmaceutical Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102
Two different approaches for mixing different sizes of fine particles without segregation and simultaneously increasing the packing density of the mixture are presented. The first application requires uniform mixing of 300 and 30 µm aluminum oxide particles acting as surrogates for the explosive RDX. The packing density needs to be as high as possible and the particles need to be well-mixed to prevent local hot spots. This was accomplished by adding moisture to uniformly mix the particles and then removing the moisture to increase the packing density. The second application requires mixing coarse (2.3 mm) and fine (65 µm) silica aerogel particles; the aerogels are very porous and light and have a very low thermal conductivity. Here we want to fill the voids between the coarse particles with fine particles so that the bulk (or packing) density of the mixture is increased, thereby further increasing their insulating properties. This was done by combining a negative pressure (vacuum) with an external force field, such as sound waves or vibration. 1. Introduction Mixing and blending of two or more fine particles (powders) of different size or different density is of considerable importance to the pharmaceutical, chemical, agricultural, mining, building materials, explosives, and food industries, which process huge quantities of granular materials. In most applications, a uniform mixing and blending of the different species of particles is desired since a nonuniform mixture showing segregation can severely degrade the quality of the final product and, in many situations, have serious safety implications. For example, in the pharmaceutical industry, it is essential that the same amount of the active ingredient be present in each individual tablet or capsule; and when loading energetic end items with explosive powder mixtures, a fixed ratio of coarse to fine particles as well as maximum packing density are both highly desirable and often necessary to avoid product sensitivity problems and to increase the product’s range and power. Mixing of dry fine particles of different physical and/or chemical properties is usually conducted in industrial devices such as tumbling blenders, which include double cone blenders and V-blenders1,2 and tote or bin blenders.3 Flow induced mixing by applying vertical vibrations to particles of nonuniform size has been studied extensively both experimentally and theoretically.4 Gas fluidized beds5-7 have also been used to uniformly mix different species and sizes of particles. All of these devices, however, suffer to some extent from segregation which can create regions of fine and coarse particles which behave very differently from one another.8 For example, the coarse particles of a binary mixture of coarse and fine particles may be freeflowing, while the fines may be cohesive, with a tendency to form ratholes in a storage bin or chemical reactor. Also, higher concentrations of fines can cause particles to stick together, promoting agglomeration and caking during downstream processing or storage. The packing density of a mixture of fine particles, or a related parameter, such as the bulk density or porosity is also a very * To whom correspondence should be addressed. Tel.: 1-(480) 9650362. Fax: 1-(480) 965-0037. E-mail:
[email protected]. † Current address: Mannkind Corp., 1 Casper Street, Danbury, CT 06810. ‡ Current address: Department of Chemical Engineering, Arizona State University, Tempe, AZ 85287.
important parameter in many industrial applications. Usually it is desirable to have the packing density of the particle mixture to be as high as possible so that it will take up less volume per unit weight. The packing density is dependent on many factors including particle size distribution, particle shape, interparticle forces (van der Waals, electrostatic, and capillary forces), surface chemistry, and agglomeration. It is possible to adjust these parameters to obtain high packing densities; for example, the use of bimodal particle size mixtures can greatly increase the packing density by filling the voids between the larger particles with the smaller ones.8 Theoretical and experimental studies of the packing densities of monosized spheres, monosized nonspherical particles, bimodal mixtures, multimodal and continuous particle distributions, and particles of different shapes are well documented in the literature and summarized in German’s textbook Particle Packing Characteristics.9 The effect of van der Waals and electrostatic forces10,11 and that of liquid addition (capillary force) on the packing density of monosized spheres has also been reported.12 More recently, work has been done to determine the effect of liquid addition on binary particle mixing13 and on the packing density of binary mixtures of coarse spheres.14 Li and McCarthy15 developed a new theoretical approach to the mixing and segregation of fine particles, taking into account differences in size, density, and liquid (moisture) addition. They generated phase diagrams based on size ratio, density ratio, and wetting characteristics. These diagrams can be used to predict a mixed or segregated state for binary particles when adding moisture and also can show how the location of the phase boundaries may be manipulated via modifying the mechanical and surface properties of the particles. Their theory is very useful for mitigating segregation in certain binary mixtures. In this paper, we examine two different applications of fine particle mixing which require uniform mixing of binary particles (limited or no segregation) as well as a mixture of high packing density. To achieve these goals, two completely different approaches for mixing the particles together are presented; we will show that neither method works for both applications. The first application requires uniform mixing of two sizes (roughly 300 and 30 µm) of fairly dense explosive particles such as RDX which are then packed in burster tubes, grenades, and artillery shells, followed by infusion of polymerizable liquid(s) which
10.1021/ie1008844 2011 American Chemical Society Published on Web 08/03/2010
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Figure 1. Photographs after tumbler mixing: (a) without water incorporation and (b) with 0.5 weight % water.
become and act as a binder. The packing density needs to be as high as possible so as to pack more explosive particles into the armaments; yet the particles need to be well-mixed to prevent local hot spots. The second application requires mixing coarse silica aerogel particles (about 2.3 mm in size) with much finer silica aerogel particles (about 65 µm in size). The aerogel particles, unlike the RDX (or RDX surrogate) particles, are very porous and extremely light, with a particle density of about 125 kg/m3. Here we want the fine particles to fill the void spaces between the coarse particles so that the packing density of the mixture is much larger than the packing density of either the coarse or fine particles. 2. Mixing and Packing of Surrogate Explosive Particles As already stated above, the objective of this part of our research is to intimately mix two different size particles having physical properties similar to RDX (surrogate particles) without segregation and to increase the packing density of the mixture as compared to the packing density of the individual unmixed particles. If we fill a given volume, e.g., a tube, with particles, we define the packing density, φ, as φ)
Vp Wp ) Vt FpVt
(1)
where Vp is the volume of the particles and Vt is the volume of the particle filled tube, respectively. The volume of the particles is calculated knowing the particle density, Fp and the weight of particles inside the tube, Wp. The volume of the particle filled tube is calculated by measuring the height of the particles in the tube and knowing its diameter. The bulk density of the mixture is related to the packing density by the equation Fb ) φFp )
Wp Vt
(2)
2.1. Experimental Methods. Since our laboratory is not equipped to handle explosive particles such as RDX, we chose aluminum oxide particles as surrogates; the physical properties of aluminum oxide are similar to those of RDX, although alumina has a higher density. We originally used KCL particles which have a density closer to that of RDX; however, unlike the alumina particles, the fine KCL particles were the same color
as the coarse KCL particles and it was difficult to see visually how well they became mixed. That is why we switched to the alumina. We do not think that the difference in density will have much of an effect on the mixing and packing properties since only binary particles of the same density are used in the experiments. On the basis of the theory and phase diagrams presented by Li and McCarthy,15 we should be able to mitigate segregation when mixing two different sizes of the aluminum oxide surrogate particles by incorporating water (RDX has a low solubility in water so that the moisture addition is justifiable). We therefore placed relatively coarse (300 µm) aluminum oxide (Al2O3) white colored particles into a container (i.d. ) 62 mm, height ) 60 mm) and injected a small amount of water (0.5 to 1.0% by weight) onto the coarse particles. We used a tumbler mixer (JRM 2 in. × 24 in., Paul Abbe Inc.) slowly rotated at 150 rpm or a mechanical stirrer for about 5 min to obtain a thin film of water coated onto the coarse particles. We then added fine (30 µm) Al2O3 brown colored particles (using different fine to coarse weight ratios) into the container and mixed them with the moisture coated coarse particles for about 15 min in the slowly rotating tumbler mixer. The 300 and 30 µm particle sizes are mean sizes; the standard deviation of the coarse and fine particle size are 95 and 16 µm, respectively (characterized by a Beckman Coulter LS 230). We did the mixing with water in two separate steps to minimize agglomeration of the fine particles. With the use of 0.5 wt % of water, this procedure results in very uniformly mixed brown colored particles without segregation as seen in Figure 1b, and the fine particles are coated onto the coarser particles through liquid (water) bridges. We repeated the experiment under the exact same conditions (as a control) except we do not add any water. As expected, severe segregation of the coarse and fine particle was observed as seen in Figure 1a. In both of these experiments, the weight ratio of the coarse white Al2O3 particles to the fine brown Al2O3 particles was 80:20. We then placed the mixed particles into a polycarbonate tube, (i.d. ) 9.525 mm, length ) 150 mm), fastened the tube on the packing stand (Figure 2), and vibrated it at 60 psig (Cleveland ACM 3/4) for less than 1 min and measured the height of the particles to determine the packing density. However, at this point, we find no increase in the packing density, on the contrary the packing density actually goes down (see Table 1). We then
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Figure 2. Schematic of the packing experimental apparatus: 1, polycarbonate tube; 2, vibrator; 3, packing stand. Table 1. Comparison of the Packing Density before and after Removing the Added Water
sample coarse/fine 100% coarse 100% fine 80/20 70/30 65/35 60/40
packing density after dry mixing
packing density after mixing with 0.5% water
packing density after mixing with 0.5% water and removing water
packing density of coarse particles ) 0.512 ( 0.002 packing density of fine particles ) 0.471 ( 0.009 0.589 ( 0.011 0.407 ( 0.013 0.637 ( 0.005 0.607 ( 0.007 0.409 ( 0.004 0.679 ( 0.004 0.615 ( 0.01 0.432 ( 0.009 0.683 ( 0.004 0.612 ( 0.013 0.411 ( 0.01 0.682 ( 0.004
removed the tube and either placed it in a microwave oven or subjected it to dry hot air to remove the water from the mixture; RDX melts at about 205 °C, so removing the moisture by dry hot gas should not be a problem. During heating in the microwave oven (total time less than 15 min), we removed the tube about every 2 min and tapped the tube several times before placing it back in the microwave. Finally, the fine particles fall away from the coarse particles and occupy the void space between the coarse particles so the packing density is increased significantly without sacrificing the uniformity of the mixture. After mixing the two different-sized explosive particles, the RDX mixture is packed into a burster tube, a grenade, or an artillery shell and a monomer is infused into the tube. The monomer is then polymerized in situ to form the final explosive material. We followed a similar procedure using our surrogate aluminum oxide particle mixture after removing the moisture in a microwave oven. We infused a mixture of monomer (SR 205, triethylene glycol dimethacrylate, viscosity ) 11 cP, Mw ) 286), catalyst (copper(I) bromide), complexing ligand (2-2dipyridyl)), and initiator (ethyl R-bromoisobutyrate) into the polycarbonate tube containing the Al2O3 mixture using compressed air at 10 psig at room temperature. The polymerization was completed in about 12 h. The polymerization reaction tended to make the polycarbonate tube brittle, and the tube was easily cut away with a hand saw. The hardened polymerized powder mixture was sliced vertically, and vertical samples were examined under the scanning electron microscope (SEM, LEO 982 FESEM). The sample was coated with carbon before SEM pictures were taken. 2.2. Results and Discussion. Parts a and b of Figure 3 are images of the pure Al2O3 materials under the optical microscope (Carl Zeiss Universal Research Microscope). Figure 3c is an optical microscope image of the coarse alumina particles coated
by fine alumina particles (80:20 mixture) by adding 0.5% water, and it shows that after tumbling, almost all of the fine particles have been coated on the surface of coarse particles. Figure 3d is a SEM image of the same sample as Figure 3c. This image shows that most of the fine particles are no longer sticking to the surface of the coarse particles but have fallen into the voids between the coarse particles. This occurs because of the disappearance of the liquid (water) bridges that hold the fine particles onto the surface of the coarse particles; that is, the added water, which produced a uniform mixture without segregation, has been evaporated during carbon sputtering and the very high vacuum present in the SEM chamber. This important observation that the fine particles fall away from the coarse particles after removing the moisture in the SEM led us to conclude that the fine particles would fill the interstices (voids) among the coarse particles if the water was removed and thus the packing density would increase. As seen in Table 1, when the two differently sized particles are mixed dry, the packing density of the mixture is higher than that of either the coarse or the fine particles even though the mixture is severely segregated. Adding 0.5% water results in a fairly large decrease in packing density, but after the water is removed, the packing density increases and is significantly higher than after dry mixing. The highest packing density (0.686) is obtained using a weight ratio of fine to coarse particles of about 35 wt %, and the packing density has increased by 34% and 45.6% as compared to the packing densities of the unmixed coarse and fine particles of 0.512 and 0.471, respectively. The maximum theoretical packing density for binary particles, at a size ratio of 10:1 and a volume fraction of 70% coarse particles, is about 0.66 (Zheng et al.16), which is in good agreement with the experimental results. We measured the effect of the amount of water added on the packing density of the mixture before and after removing the water in the microwave oven. Figure 4 shows that the packing density after mixing with added water decreases as the water percentage increases, whereas the packing density after mixing with water, and then removing the water in the microwave oven, increases with increasing water percentage. The first observation is explained by the fact that the powder flowability decreases with an increase in water content, i.e., the angle of repose increases sharply with the volume fraction of added water,17 so that the packing density decreases due to the poor flowability or increased cohesiveness of the particles. The second observation can be explained by assuming that the more water bridges formed, the more fine particles are coated on the surface of coarse particles and these fill up the voids after the moisture is removed; therefore, both the flowability of the fine particles and the packing density increases. After infusion of the mixture with a monomer and the polymerization reaction carried out in the polycarbonate tube, the hardened polymerized powder mixture was sliced vertically and samples were examined under the SEM (LEO 982 FESEM). Figure 5 is a SEM picture of a cut away sample (80:20 mixture; 0.5 wt % water) in which the incorporated water was removed by heating in the microwave oven and monomer infusion and polymerization was performed. The SEM image confirms that even after polymerization, nearly all of the fine particles occupy the space among the coarse particles rather than sticking to the surface of the coarse particles. The image also shows that both particle sizes are uniformly distributed in the cross-section. Although mixture uniformity at the cross-section shown in Figure 5 appears very good, it is necessary to measure the composition uniformity at different column heights to make sure
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Figure 3. Optical and SEM images of (a) coarse Al2O3; (b) fine Al2O3; (c) optical microscope image of the 80:20 mixture; and (d) SEM image of the 80:20 mixture.
Figure 4. The effect of water content on the packing density for the sample (9) after mixing with water and removing the water and (0) after mixing with water.
that the packing density of the mixture remains uniform throughout the column after polymerization. In order to examine the uniformity at different vertical cross sections, samples were cut by a hand saw into pieces every 5 mm and the density of each individual sample was calculated by weighing and measuring the volume of the sample (Archimedes method). The different samples are numbered from the bottom to the top. A control experiment (using dry mixing without adding moisture) was also performed. Figure 6 shows a comparison of the sample density between dry mixing and adding moisture with microwave drying along the length of the tube. The numbering of samples is from the bottom to the top. The figure shows that the densities, after adding and removing moisture, are much more uniform than the densities using dry mixing because segregation occurs in
Figure 5. SEM image after polymerization (80:20 mixture).
dry mixing, whereas it is significantly mitigated when mixing with added water. In addition, during the infusion process into an 80/20 mixture treated with water and dried in a microwave, although the monomer is infused by 10 psig compressed air, the height of the sample remains constant and the color of the powder remains very uniform (brown). That is, no fine particles are flushed down to the bottom of tube by the infusion process. Hence we conclude that mixture uniformity both at the same cross-section and throughout the length of the column is achieved. 2.3. Conclusions. As mentioned in the Introduction, the major problem when mixing particles of different size using methods such as mechanical mixing,1-3 vibration,4 or fluidized
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Figure 6. Density comparison along the tube after polymerization: ([) 80/ 20 mixture by adding moisture and then removing the moisture in a microwave oven and (9) 80/20 mixture using dry mixing. The numbering of the samples is from the bottom to the top.
bed mixing5-7 is segregation which results in both a nonuniform composition and a relatively low packing density. The incorporation of water solves the segregation issue, and the subsequent removal of the water by microwave heating increases the packing density. Although our method is based on the theory and phase diagrams presented by Li and McCarthy,15 the idea of removing the water incorporated after the first mixing step is novel. Adding water and mixing can mitigate segregation and improve the homogeneity of the mixture but actually decreases the packing density. Removing the water, in the second step, keeps the homogeneity of the mixture and simultaneously increases the packing density significantly because fine particles coated onto coarse particles due to liquid (water) bridges fall away and fill the interstices or void space between the coarse particles. It is also worth noting that we place the coarse particles and water into a container and tumble mix to coat the coarse particles with a film of water before we add the fine particles into the container and tumble mix again, whereas Li and McCarthy15 place the coarse and fine particles and water into the container and mix them together at the same time. Our split feeding method has an advantage over their method because particle agglomeration of the fine particles is substantially reduced. This is because all of the water spreads onto the surface of the coarse particles in the first mixing step, and then the fine particles are coated onto the coarse particles through water bridges in the second step so that the formation of agglomerates of fine particles is minimized. 3. Mixing and Packing of Hydrophobic Nanostructured Silica Aerogels Aerogels are very low density, high porosity, and high surface area solid-state materials derived from a gel in which the liquid component of the gel has been replaced with air. Aerogels have the lowest density, highest thermal insulation, lowest refractive index, and highest surface area per unit volume of any solid and are sometimes referred to as “frozen smoke.” They consist of tangled, fractal-like chains of spherical clusters of molecules each about 3-4 nm in diameter that form a solid structure surrounding air-filled pores that average about 20 nm. Nanogel
is Cabot Corporation’s trade name for its family of hydrophobic silica aerogels which have particle sizes ranging from 5 µm to 3.5 mm, particle densities of 40-125 kg/m3, and surface areas of 600-800 m2/g, in both opaque and translucent forms. Since aerogels have an extremely low thermal conductivity (even lower than that of air), they are being used as insulators in architectural daylighting structures, building and pipeline insulation, and outdoor gear and apparel. Because aerogels are much better insulators than air, it is advantageous to mix coarse and fine aerogel particles to produce a mixture that has a packing density higher than that of either the coarse or the fine particles by themselves. For example, a mixture of coarse (2.3 mm) and fine (65 µm) Nanogel will produce a better insulator since the mixture will have fine aerogels filling the voids between the large aerogels, and the fine aerogels have a lower thermal conductivity than air. However, to produce such a mixture without segregation is not an easy task. Since the Nanogel particles are free-flowing, a number of well-known mixing methods were considered including single rain, double rain, fluidization, and the use of a variety of commercial mixing equipment. Single rain refers to letting the smaller particles fall by gravity (rain) on top of the larger particles and thus fill in the spaces between the larger particles; this method was tried but gave poor results (see Table 2). Double rain refers to letting both the coarse and fine particles fall by gravity (settle together) but is both complicated to set up and slow. Fluidization gives very good internal mixing but also leads to segregation. Commercial mixing equipment is unsuitable for mixing Nanogel because of the delicate nature of these particles, which will rapidly attrit in a high shear environment. Because the Nanogel particles are hydrophobic and highly porous, the addition of a liquid to promote uniform mixing as descibed above for mixing aluminum oxide particles will not work. If water is used, the hydrophobicity of the Nanogel surface will repel the water, and if an organic solvent is used, the solvent will simply enter the pores of the Nanogel. 3.1. Experimental Methods. The fluidization literature contains many references on the use of sound waves18-20 and vibration21-24 as a means of improving the fluidization quality of micrometer-sized particles and nanoparticles in the form of micrometer-sized nanoagglomerates. We devised a simple experiment to test whether the use of such an external force field could be used to mix Nanogel particles supplied by Cabot Corporation without segregation. We used a combination of a sonic field (sound waves) supplied by a loudspeaker resting on top of a cylindrical or square column filled with two layers of Nanogel, coarse Nanogel on the bottom and fine Nanogel on the top. The coarse Nanogel particles were about 2.3 mm in size with a bulk density of 0.073 g/cm3, and the fine Nanogel particles were 65 µm in size with a bulk density of 0.055 g/cm3. Since the particle density of different size Nanogel is not known very accurately, we measure the bulk density, see eq 2, rather than the packing density in these experiments. The bulk density was measured in situ, i.e., the final level of particles was measured in the column, and knowing the cross sectional area and the amount of particles loaded into the column, the bulk
Table 2. Evaluation of Different Mixing Methods Using 75 wt % Coarse Nanogel With 25 wt % Fines methods
hand-mixing + tapping
single rain + tapping
fluidization at low gas velocity
sonic field alone
sonic field + fluidization
sonic field + vacuum
Fb mixture (g/cm3) uniformity operation time (s)
0.0869 poor 300
0.0816 poor 600
0.0621 poor 300
0.0816 poor 300
0.0621 poor 300
0.0925 excellent 30
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Figure 7. Experimental setup with a sound field (a) and with mechanical vibration (b).
density was calculated. The loudspeaker was powered by a sound excitation system with a signal generator, so as to be able to adjust the frequency of the sound waves; both the frequency and the sound pressure level were recorded for each experiment. After the loudspeaker was powered up and the sound intensity and frequency were set, a vacuum cleaner attached to the bottom of the column through flexible tubing was turned on. The entire experiment was recorded using a digital video camera. The experimental apparatus is shown in Figure 7a. It consists of a polycarbonate tube 25.4 mm i.d., 120 mm high, a perforated plate distributor made of foam metal to hold the particles in place, a loudspeaker (JBL EON10 G2) rated at 125 W and 117 dB with an audible range frequency of 20 Hz to 16 kHz located at the top of the column, an amplifier, a sound signal generation system (BK Precision 4017A) and a simple vacuum cleaner rated at 500W to generate vacuum. The sound waveform used in the experiments was sinusoidal. For experiments using vibration instead of a sonic field, the loudspeaker was replaced by a mechanical linear vibrator (Cleveland ACM 3/4) installed
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behind the base of the column (Figure 7b) and was driven by compressed air. The vibration frequency is 60 Hz when the compressed air is set to 80 psig. Experiments were also conducted to investigate the effects of a positive pressure differential and sound waves (sinusoidal, 500 Hz, 87.0 dB) on the mixing and packing of the Nanogel particles using a high-pressure tank of nitrogen, rather than the vacuum cleaner. Nitrogen gas flow was regulated by a flowmeter adjusted by a needle valve. Since the high-pressure nitrogen entered the top of the column, the loudspeaker was mounted at the bottom of the column which was open to the atmosphere so that the positive pressure differential was relative to 1 atm. 3.2. Results and Discussion. Table 2 shows some preliminary results using different methods of mixing 75 wt % of coarse Nanogel with 25 wt % of fine Nanogel. The table shows that all of the methods used except the combination of sound waves and vacuum resulted in a mixture of poor uniformity and large segregation (by visual observation). Not only did the combination of sound waves and vacuum produce a uniform mixture and the highest bulk density, but the entire process took less than 30 s. Parts a and b of Figure 8 show before and after mixing photographs of the Nanogel particles. In Figure 8a, 25 wt % of fine particles (opaque) were pored above 75 wt % of coarse particles (translucent). First the loudspeaker and then the vacuum were turned on for about 30 s. Figure 8b shows that all of the fine particles have entered the voids between the coarse particles with a layer of coarse particle still present at the top. This indicated that the 75:25 ratio of coarse to fine particles was too small. Table 3 shows the results of mixing different weight % ratios of coarse to fine particles; the optimum ratio is 68.5:31.5 which gives a bulk density of 0.097 g/cm3, 33% higher than that of the coarse particles alone. This ratio also produced the most uniform mixture as seen in Figure 8c. The maximum theoretical packing density for binary particles, at a size ratio greater than 10:1 and a volume fraction of 70% coarse particles, is about 0.76 (Zheng et al.16). Assuming the density of the Nanogel particles is about 0.125 g/cm3, the bulk
Figure 8. Mixing of coarse and fine Nanogel particles; frequency at 500 Hz, intensity at 80.0 dB. Table 3. Bulk Density as a Function of wt % Ratio (Frequency at 500 Hz, Intensity at 80.0 dB) combination (coarse + fines ) 4.0 g)
75 wt % coarse +25 wt % fines
70 wt % coarse +30 w% fines
65 wt % coarse +35 wt % fines
68.5 wt % coarse +31.5 wt % fines
final height (cm) final volume (cm3) final bulk density (g/cm3) empty space among coarse particles additional fines amount
5.3 43.22 0.0925 large
5.2 42.44 0.0943 small
5.1 41.65 0.0960
5.05 41.24 0.0970
large
negligible
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Figure 9. Bulk density of a 75:25 (weight ratio) mixture at different sound intensity; frequency at 500 Hz, vacuum power 500W, operation time 30 s.
Figure 11. Mixing of coarse and fine Nanogel particles (70:30) using positive pressure (15 psi), frequency at 500 Hz, intensity at 87.0 dB: (a) before mixing and (b) after mixing.
Figure 10. Bulk density of a 75 wt %/25 wt % mixture at different sound frequencies; sound intensity at 80.0 dB, vacuum power 500 W, operation time 30 s.
density of the Nanogel at a packing density of 0.76 is 0.095 which is in good agreement with the experimental results. Figure 9 shows the effect of varying the sound intensity on the bulk density, keeping the frequency constant at 500 Hz, while Figure 10 shows the effect of varying the sound frequency on the bulk density, keeping the intensity constant at 80.0 dB. The figures show that the higher the sound intensity, the better the mixing (higher bulk density) and that the bulk density stays relatively constant at lower frequencies and then drops off at high frequencies greater than 1000 Hz. We also investigated the relative importance of the sonic field and vacuum on the mixing properties of the two different sizes of Nanogel by first turning on the sonic field, then the vacuum, and visa versa. These experiments showed that vacuum plays the more important role in mixing the particles. However, the sonic field caused the fine Nanogel particles to vibrate and, when combined with vacuum, substantially improved the bulk (or packing) density when compared to using vacuum alone. The application of the sonic field followed by vacuum gave better results than that of vacuum followed by the sonic field. In the former case (speaker turned on and then vacuum), the packed column of fine particles will expand (voidage becomes higher) due to the sonic energy, and when the vacuum is turned on, the overall pressure drop will be smaller. In the latter case, turning on the vacuum first will impose a downward force before the expansion of the fine particles occurs due to the sonic energy, resulting in a somewhat higher pressure drop. Experiments using 70 wt % coarse and 30 wt % fine particles were also conducted using a high-pressure tank of nitrogen, rather than vacuum, to investigate the effects of combining a positive pressure differential with sound waves (500 Hz, 87.0 dB) on the mixing and packing of the Nanogel particles. A relatively high positive pressure differential was used in the range of 15-30 psi. Here the loudspeaker had to be placed at the bottom of the column just below the distributor, and the bottom of the column was kept open to the atmosphere. As seen
in Figure 11, a relatively high positive pressure differential (15 psi) led to severe segregation and poor mixing. After the pressure was released, the smaller particles appeared to bounce back to the top of the mixture. An experiment using a smaller, 3-4 psi, differential pressure also was conducted, showing no noticeable improvement in mixing and packing. Because of the consistently poor results obtained, probably because the sound waves had to travel upward through the coarse particles before reaching the fine particles, these experiments were discontinued. One experiment was also conducted to study the mixing and packing of Nanogel using vertical sinusoidal vibration with a negative pressure differential (vacuum) by mounting the cylindrical module on a vibrating plate (vibration intensity of 3 times gravity and frequency 500 Hz) with results similar to that using the sonic field combined with vacuum. However most of the experiments using vibration were conducted using the setup shown in Figure 7b. Figure 12 shows a typical (qualitative) result when mixing coarse and fine Nanogel using vertical vibration at 60 Hz. The results indicated that vertical vibration when combined with vacuum will also work well to improve mixing and increase the bulk density of a mixture of coarse and fine Nanogel particles. 3.3. Conclusions. Low-density hydrophobic silica aerogels (Cabot Nanogel) of two different sizes can be well-mixed without segregation with a substantial increase in bulk (or packing) density by combining either a sonic field or vertical vibration with vacuum in a vertical tube. The method is extremely simple to use, does not involve expensive equipment, takes very little time to run (10)
low (10)
high (>2)
coarse glass beads coarse alumina walnut shell KCL particles coarse glass beads PP particles coarse aerogel coarse aerogel coarse glass beads coarse glass beads coarse glass beads KCL particles PS particles PS particles PS particles PP particles PP particles coarse Nanogel coarse corn starch PS particles
walnut shell fine alumina fine Nanogel fine Nanogel fine Nanogel coarse Nanogel fine glass beads fine alumina fine cornstarch silica coated fine cornstarch fine alumina silica coated fine cornstarch fine cornstarch silica coated fine cornstarch fine glass beads fine corn starch fine glass beads fine Nanogel fine Nanogel fine Nanogel
1.43 10 5.4 4.3 7.7 1.3 46 76.7 25 25 16.7 14 150 150 60 150 60 35 15.4 46
1.92 1 10.4 15.9 20 7.24 0.05 0.034 1.61 1.61 0.68 1.29 0.68 0.68 0.42 0.58 0.36 1 12.4 8.42
clear clear clear clear clear clear clear clear clear clear clear clear fuzzy no no fuzzy no no fuzzy no
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porous structure and low bulk density since porous materials can absorb more sonic or vibration energy, and low bulk density materials can be more easily expanded. When we combined free-flowing particles such as 50 µm glass beads with coarse PP particles, mixing occurred easily. However when we tried to mix fine cornstarch with either coarse PP or coarse PS particles, only a small amount of the fine cornstarch entered the voids between the PP or PS particles (see Table 5). This is due to the fact that fine cornstarch particles are very cohesive and will not flow freely. However, once the flowability of the fine corn starch particles is improved (cohesiveness decreased) by coating their surface with fumed nanosilica, the cornstarch particles can fill the voids between the PP or PS particles and good mixing is obtained. 4.2. Conclusions. The method used successfully for mixing two different size Nanogels using a combination of a sonic or vibration field with vacuum and does not work for a large variety of denser particles, cohesive particles, or for small particle size ratios. For example, the method did not work for mixing and increasing the packing density of 30 and 300 µm alumina particles, which were well-mixed using the method of adding moisture as described in section 1 above. To obtain good mixing and an improved packing density, different methods need to be applied depending on the individual properties of the particles to be mixed. It also should be noted that the experimental results obtained above are limited by the experimental operating conditions and equipment used; different vibration wave forms and much larger sound pressure levels, for example, may yield different results. Acknowledgment We would like to acknowledge support from the U.S. Army through Contract DAAE30-03-D-1015 DO No. 0020 [Advanced Cluster Energetics (ACE) Program] for the mixing and packing of Al2O3 surrogate particles for the explosive RDX. We also acknowledge support from Cabot Corporation, for supplying the Nanogel, and for the mixing and packing of aerogels. The authors also wish to thank the scientists and engineers from Picatinny Arsenal and Cabot Corporation who provided expertise and guidance during our research work, in particular, Mr. Peter Bonnett of Picatinny Arsenal and Dr. Steve Reznek of Cabot. Literature Cited (1) Moakher, M.; Shinbrot, T.; Muzzio, F. J. Experimentally Validated Computations of Flow, Mixing and Segregation of Non-Cohesive Grains in 3D Tumbling Blenders. Powder Technol. 2000, 109, 58. (2) Shinbrot, T.; Alexander, A.; Moakher, M.; Muzzio, F. J. Chaotic granular mixing. Chaos 1999, 9, 611. (3) Sudah, O. S.; Coffin-Beach, D.; Muzzio, F. J. Effects of blender rotational speed and discharge on the homogeneity of cohesive and freeflowing mixtures. Int. J. Pharm. 2002, 247, 57.
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ReceiVed for reView April 14, 2010 ReVised manuscript receiVed June 29, 2010 Accepted July 12, 2010 IE1008844
