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Bubble characteristics in a two-dimensional vertically vibro-fluidized bed
CHINA PARTICUOLOGY Vol. 3, No. 4, 224-228, 2005
BUBBLE CHARACTERISTICS IN A TWO-DIMENSIONAL VERTICALLY VIBRO-FLUIDIZED BED Tao Zhou1,*, Hiroyuki Kage2 and Hongzhong Li3 1
College of Chemistry & Chemical Engineering, Central South University, Changsha 410083, P. R. China 2 Department of Applied Chemistry, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan 3 Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, P. R. China *Author to whom correspondence should be addressed. E-mail: [email protected]
Abstract Measurement of bubble size and local average bubble rise velocity was carried out in a vertically sinusoidal vibro-fluidized bed. Glass beads of Geldart group B particles were fluidized at different gas velocities, while the bed was vibrated at different frequencies and amplitudes to study their effects on the bubble behavior. This is compared with the case of no vibration in a two-dimensional bed and it is concluded that with vibration the local average bubble size, dbav, decreases significantly, especially at minimum bubbling velocity. The average bubble size increases slightly with increasing vibration frequency and amplitude. The local average bubble rise velocity is higher than that with no vibration, though with increasing vibration frequency and amplitude, it does not change significantly. Keywords
bubble size, vibro-fluidized bed, bubble size distribution, measurement of bubble
1. Introduction Vibro-fluidized beds are widely used in industry, such as coating of particles, granulation, drying, etc. If sufficient gas is fed into a fluidized bed, bubbles will form, which determine to a great extent the behavior of the bed. However, most of the gas in a bubble does not take part in any reaction between gas and solids because this gas is not in close contact with the particles. Thus, it is hoped that there exist only small bubbles, or no bubbles at all in a vibro-fluidized bed. There have been numerous investigations on bubble size in gas-solid fluidized beds (Geldart, 1967; Mori & Wen, 1975; Horio & Nonaka, 1987; Koksal & Vural, 1998; Davidson & Harrison, 1963). A few studies of the effect of vibration on fluidization quality, for example, bed expansion and pressure drop, etc., can also be found in the literature (Akiyama et al., 1996; Erdesz & Mujumdar, 1986; Marring et al., 1994; Zhou et al., 2001). Eccles and Mujumdar (1997) studied the effect of vibration frequency at low amplitudes (< 4 mm) on bubble rise velocity. Zhou et al. (2002; 2004) investigated bubble size and rise velocity in a horizontally vibro-fluidized bed. Kage et al. (1997; 1999) studied the effect of the vibration direction and frequency on powder coating in a vibro-fluidized bed. The objective of this paper is to study bubble behavior in a high-amplitude (> 5 mm) vertically vibrating fluidized bed. The influence of experimental operating parameters, such as amplitude and frequency of vibration, and superficial gas velocity, on bubble behavior is investigated.
2. Experimental Figure 1 shows schematically the experimental system consisting of a rectangular fluidized bed made of Perspex, 300 mm×10 mm×600 mm (width×thickness×height) in size. Two vibro-motors on a steel plate were attached directly onto the fluidized bed to impart to it vertical sinusoidal vi-
bration. The frequency and amplitude of vibration could be adjusted by means of an inverter and a couple of vibro-motors, respectively. Compressed air was used as the fluidizing gas after passing through a filter and an oil eliminator. The glass beads fluidized had an average diameter of 198 μm and a density of 2 520 kg⋅m−3. The static bed height was 205 mm in all experiments.
Fig. 1
Schematic diagram of experimental apparatus: 1. compressor; 2. air filter; 3. oil eliminator; 4. air dryer; 5. valve; 6. rotameter; 7. fluidized bed; 8. vibro-motor; 9. digital camera; 10. computer image analysis system.
The motion of bubbles and fluidization behavior were recorded by a digital camera (FC-1300, Takenaka System Co.). Image frames were captured at random and stored in the computer memory for subsequent analysis with a software (IMAGE-ProPLUS) developed by Media Cybernetics Company, to identify the size and location of the bubbles. The bubbles in a fluidized bed appear as brighter areas in the picture. The bubble size was expressed as its mean diameter, db, which was defined as the average value of all diameters measured at 2-degree intervals, with all chords passing through the bubble centroid, as shown in Fig. 2. In order to determine the location of a bubble, the origin of coordinate was taken as lowest left wall of a fluidized bed, as shown in Fig. 3.
Zhou, Kage & Li: Bubble Characteristics in a Two-Dimensional Vertically Vibro-fluidized Bed
225
3. Results and discussion 3.1 Bubble size distribution and transverse distribution of bubbles Fig. 2
Schematic diagram for measurement. H Area A2
Area A1
Fig. 3
0 x Location of coordinate axis of bubble size.
Free bubbling beds were observed with and without vibration. Data on the size and centroid location of the bubbles were acquired. Five different superficial gas velocities were used: 0.069 4 (minimum bubbling velocity, ≈Umf), 0.080 6, 0.090 7, 0.101 4 and 0.122 2 m⋅s−1.
Fig. 4
It was observed in experiments that bubbles form just above the distributor plate and at intermediate heights some bubbles may coalesce to form larger bubbles while others may completely collapse and vanish or split into smaller bubbles further up in the bed. Figure 4(a) shows that bubbles first form at a superficial gas velocity of 0.069 4 m⋅s−1 in the no-vibration mode, whereas hardly any bubble appears at this same gas velocity but with vibration. With increasing superficial gas velocity, Figures 4(b) and (c) show that there are fewer large bubbles at intermediate heights with vibration, possibly because the upward and downward movements of particles brought about by vibration tend to break the bubbles, although there are much more large bubbles at these heights at higher gas velocities (Fig. 4(d)), at which bubble movement becomes violent thus leading to their easier coalescence.
Bubble size distributions at different gas velocities.
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CHINA PARTICUOLOGY Vol. 3, No. 4, 2005
In order to confirm the reproducibility of above analysis, the standard deviation S of bubble size distribution with and without vibration at intermediate heights (bed height: 50–150 mm) in different gas velocities is calculated, as shown in Table 1, for the mean of 60 individual measurements. The standard deviation in Table 1 of bubble size distribution with vibration is smaller than that without vibration, which is in agreement with the above analysis for the intermediate heights. Table 1
decreases with increasing vibration amplitude a, e.g. form no vibration (a=0) to a=5 mm, and then increases significantly with increasing vibration amplitude. This means that increasing vibration amplitude is disadvantageous to gas-solid reaction. On the other hand, it is worth noticing that no bubbles can be observed at u=0.069 4 m⋅s−1 for a=15 mm.
Standard deviation of the bubble size distribution at intermediate heights
u/(m⋅s−1) 0.0806 0.0907 0.1222
S (without vibration)
S (with vibration)
8.273 8.692 15.291
4.802 4.924 14.750
Figure 5 shows some examples of transverse distributions of bubbles with and without vibration, indicating that at the same superficial gas velocity, transverse distribution of bubbles is hardly influenced by vibration. That is, vertical vibration does not appear to improve homogeneity of the transverse distribution of bubbles, as might be expected. (a)
(b)
(c)
(d)
Fig. 6
Fig. 5
Transverse distribution of bubbles at different gas velocities with and without vibration: (a) u=0.086 3 m⋅s−1, without vibration; (b) u=0.194 m⋅s−1, without vibration; (c) u=0.086 3 m⋅s−1, with vibration; (d) u=0.194 m⋅s−1, with vibration.
3.2 Effect of vibration amplitude on local average bubble size The local average bubble size, dbav, was measured in areas at two elevations: from 9 to 11 cm above the bottom of the bed for area A1 and from 15 to 17 cm for area A2, as shown in Fig. 3. All bubbles whose centroids fell within area A1 or A2 were included in measurement of their local average bubble sizes, which are the means of 60 individual measurements. The effect of vibration amplitude on the local average bubble size is shown in Fig. 6 for vibration frequency of 15 Hz, indicating that the local average bubble size first
Relationship between local average bubble size dbav and vibration amplitude a.
3.3 Effect of vibration frequency on local average bubble size Figure 7 presents, for both areas A1 and A2, the effect of vibration frequency on local average bubble size at vibration amplitude of a=5 mm, showing that at lower vibration frequencies local average bubble size decreases with increasing frequency up to f =15 Hz, after which bubble size increases slightly with vibration frequency. Table 2 presents measured values of local average bubble sizes in a free bubbling bed without vibration and with vibration at different amplitudes and frequencies, showing that the local average bubble size increases slightly with vibration frequency at vibration amplitudes of a=10 and 15 mm. Moreover, in the case of no vibration, the local average bubble size at u=0.069 4 m⋅s−1 (minimum bubbling velocity) is larger than that at all other velocities, possibly because of difference in performance of the different holes of the distributor.
Zhou, Kage & Li: Bubble Characteristics in a Two-Dimensional Vertically Vibro-fluidized Bed
Fig. 7
Relationship between local average bubble size dbav and vibration frequency f.
Table 2
Local average bubble size in a free bubbling bed with and without vibration −1
u/(m⋅s ) No vibration a=5 mm, f=15 Hz a=5 mm, f=20 Hz a=5 mm, f=30 Hz a=10 mm, f=15 Hz a=10 mm, f=20 Hz a=10 mm, f=30 Hz a=15 mm, f=15 Hz a=15 mm, f=20 Hz a=15 mm, f=30 Hz
227
dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm
0.0694
0.0806
0.0907
0.1014
0.1222
20.50 24.67 11.90 16.05 12.06 17.30 14.48 18.48 14.50 19.30 15.51 20.85 13.57 14.31 − − − − − −
16.56 18.53 15.99 16.25 15.09 17.09 16.20 17.87 18.36 21.12 21.20 23.37 21.01 23.80 18.33 21.85 19.71 24.21 22.58 25.10
18.60 19.84 17.52 18.52 17.44 18.89 17.92 19.62 23.76 25.46 23.54 24.78 23.14 24.55 24.93 26.59 19.55 25.15 20.89 24.91
17.57 23.58 16.56 19.82 16.81 20.04 18.91 21.11 24.33 26.96 25.45 28.04 24.42 26.35 26.79 29.29 17.14 26.69 25.01 27.48
25.31 28.63 22.65 25.80 23.98 27.98 24.67 28.06 27.69 31.90 24.48 27.25 25.10 28.81 31.75 32.64 24.81 28.64 27.97 33.82
The local average bubble rise velocity was measured experimentally for different vibration amplitudes (5, 10,
15 mm), frequencies (15, 20, 30 Hz) and superficial gas velocities, as shown in Figs. 8 and 9. It can be seen that all local average bubble rise velocities are enhanced by vibration, though not significantly with vibration amplitude and frequency.
Fig. 8
Fig. 9
3.4 Effect of vibration on local average bubble rise velocity
Relationship between local average bubble rise velocity and vibration frequency.
Relationship between local average bubble rise velocity and vibration amplitude
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CHINA PARTICUOLOGY Vol. 3, No. 4, 2005
4. Conclusions Findings on bubble characteristics obtained from direct measurements in a two-dimensional vibrating fluidized bed revealed that average bubble size decreases significantly with vibration especially at the minimum bubbling velocity, increases with vibration amplitude, but remains almost constant with vibration frequency. Vibration also results in increase of local average bubble rise velocity, although its response to vibration frequency and amplitude is not significant.
Acknowledgement The authors are grateful to Japan Society for the Promotion of Science for providing the financial support.
Nomenclature a db dbav dbav1 dbav2 f H S u Ub Umf x
amplitude of vibration, mm mean diameter of a bubble, mm local average bubble size, mm local average bubble size at area A1, mm local average bubble size at area A2, mm frequency of vibration, Hz bed height, mm standard deviation of the bubble size distribution 1 superficial gas velocity, m⋅s− 1 local average bubble rise velocity, cm⋅s− 1 minimum fluidization velocity, m⋅s− lateral position, mm
References Akiyama, T., Kimura, N. & Iguchi, T. (1996). Effects of reduced air pressure on the particle circulation rate in two-dimensional vibrating particle beds. Powder Technol., 89, 133-138. Davidson, J. F. & Harrison, D. (1963). Fluidized Particles. London: Cambridge University Press.
Eccles, E. R. A. & Mujumdar, A. S. (1997). Bubble phenomena in aerated vibrated beds of small particles. Drying Technol., 15(1), 95-116. Erdesz, K. & Mujumdar, A. S. (1986). Hydrodynamic aspects of conventional and vibrofluidized beds ⎯ a comparative evaluation. Powder Technol., 46, 167-172. Geldart, D. (1967). The expansion of bubble fluidized beds. Powder Technol., 1, 355-368. Horio, M. & Nonaka, A. (1987). A generalized bubble diameter correlation for gas-solid fluidized beds. AIChE J., 33, 18651872. Kage, H., Oba, M., Ishmatsu, H., Ogura, H. & Matsuno, Y. (1997). The effects of frequency and amplitude on powder coating of fluidizing particles in vibro-fluidized bed. Proceedings of Regional Symposium on Chemical Engineering 1997, Vol. 2, Session C1-8, Johor, Malaysia. Kage, H., Shibata, H., Ishmatsu, H., Ogura, H. & Matsuno, Y. (1999). The effects of direction and frequency of vibration on powder coating of fluidizing particles in vibro-fluidized bed. Adv. Powder Technol., 10, 77-87. Koksal, M. & Vural, H. (1998). Bubble size control in a two-dimensional fluidized bed using a moving double plate distributor. Powder Technol., 95, 205-213. Marring, E., Hoffmann, A. C. & Janssen, L. P. B. M. (1994). The effect of vibration on the fluidization behaviour of some cohesive powders. Powder Technol., 79, 1-10. Mori, S. & Wen, C. Y. (1975). Estimation of bubble diameter in gaseous fluidized beds. AIChE J., 21, 109-115. Zhou, T., Kage, H., Funaoka, S., Ogura, H. & Matsuno, Y. (2001). Fluidization behavior of glass beads under different vibration module. Adv. Powder Technol., 12(4), 559-575. Zhou, T., Kage, H., Funaoka, S. & Ogura, H. (2002). The bubble behavior in a new-type horizontal vibro-fluidized bed. J. Chem. Eng. Jpn., 35(8), 737-743. Zhou, T., Ogura, H., Yamamura, M. & Kage, H. (2004). Bubble motion pattern and rise velocity in two-dimensional horizontal and vertical vibro-fluidized beds. Can. J. Chem. Eng., 82(2), 236-242. Manuscript received October 21, 2004 and accepted March 29, 2005.
BUBBLE CHARACTERISTICS IN A TWO-DIMENSIONAL VERTICALLY VIBRO-FLUIDIZED BED Tao Zhou1,*, Hiroyuki Kage2 and Hongzhong Li3 1
College of Chemistry & Chemical Engineering, Central South University, Changsha 410083, P. R. China 2 Department of Applied Chemistry, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan 3 Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, P. R. China *Author to whom correspondence should be addressed. E-mail: [email protected]
Abstract Measurement of bubble size and local average bubble rise velocity was carried out in a vertically sinusoidal vibro-fluidized bed. Glass beads of Geldart group B particles were fluidized at different gas velocities, while the bed was vibrated at different frequencies and amplitudes to study their effects on the bubble behavior. This is compared with the case of no vibration in a two-dimensional bed and it is concluded that with vibration the local average bubble size, dbav, decreases significantly, especially at minimum bubbling velocity. The average bubble size increases slightly with increasing vibration frequency and amplitude. The local average bubble rise velocity is higher than that with no vibration, though with increasing vibration frequency and amplitude, it does not change significantly. Keywords
bubble size, vibro-fluidized bed, bubble size distribution, measurement of bubble
1. Introduction Vibro-fluidized beds are widely used in industry, such as coating of particles, granulation, drying, etc. If sufficient gas is fed into a fluidized bed, bubbles will form, which determine to a great extent the behavior of the bed. However, most of the gas in a bubble does not take part in any reaction between gas and solids because this gas is not in close contact with the particles. Thus, it is hoped that there exist only small bubbles, or no bubbles at all in a vibro-fluidized bed. There have been numerous investigations on bubble size in gas-solid fluidized beds (Geldart, 1967; Mori & Wen, 1975; Horio & Nonaka, 1987; Koksal & Vural, 1998; Davidson & Harrison, 1963). A few studies of the effect of vibration on fluidization quality, for example, bed expansion and pressure drop, etc., can also be found in the literature (Akiyama et al., 1996; Erdesz & Mujumdar, 1986; Marring et al., 1994; Zhou et al., 2001). Eccles and Mujumdar (1997) studied the effect of vibration frequency at low amplitudes (< 4 mm) on bubble rise velocity. Zhou et al. (2002; 2004) investigated bubble size and rise velocity in a horizontally vibro-fluidized bed. Kage et al. (1997; 1999) studied the effect of the vibration direction and frequency on powder coating in a vibro-fluidized bed. The objective of this paper is to study bubble behavior in a high-amplitude (> 5 mm) vertically vibrating fluidized bed. The influence of experimental operating parameters, such as amplitude and frequency of vibration, and superficial gas velocity, on bubble behavior is investigated.
2. Experimental Figure 1 shows schematically the experimental system consisting of a rectangular fluidized bed made of Perspex, 300 mm×10 mm×600 mm (width×thickness×height) in size. Two vibro-motors on a steel plate were attached directly onto the fluidized bed to impart to it vertical sinusoidal vi-
bration. The frequency and amplitude of vibration could be adjusted by means of an inverter and a couple of vibro-motors, respectively. Compressed air was used as the fluidizing gas after passing through a filter and an oil eliminator. The glass beads fluidized had an average diameter of 198 μm and a density of 2 520 kg⋅m−3. The static bed height was 205 mm in all experiments.
Fig. 1
Schematic diagram of experimental apparatus: 1. compressor; 2. air filter; 3. oil eliminator; 4. air dryer; 5. valve; 6. rotameter; 7. fluidized bed; 8. vibro-motor; 9. digital camera; 10. computer image analysis system.
The motion of bubbles and fluidization behavior were recorded by a digital camera (FC-1300, Takenaka System Co.). Image frames were captured at random and stored in the computer memory for subsequent analysis with a software (IMAGE-ProPLUS) developed by Media Cybernetics Company, to identify the size and location of the bubbles. The bubbles in a fluidized bed appear as brighter areas in the picture. The bubble size was expressed as its mean diameter, db, which was defined as the average value of all diameters measured at 2-degree intervals, with all chords passing through the bubble centroid, as shown in Fig. 2. In order to determine the location of a bubble, the origin of coordinate was taken as lowest left wall of a fluidized bed, as shown in Fig. 3.
Zhou, Kage & Li: Bubble Characteristics in a Two-Dimensional Vertically Vibro-fluidized Bed
225
3. Results and discussion 3.1 Bubble size distribution and transverse distribution of bubbles Fig. 2
Schematic diagram for measurement. H Area A2
Area A1
Fig. 3
0 x Location of coordinate axis of bubble size.
Free bubbling beds were observed with and without vibration. Data on the size and centroid location of the bubbles were acquired. Five different superficial gas velocities were used: 0.069 4 (minimum bubbling velocity, ≈Umf), 0.080 6, 0.090 7, 0.101 4 and 0.122 2 m⋅s−1.
Fig. 4
It was observed in experiments that bubbles form just above the distributor plate and at intermediate heights some bubbles may coalesce to form larger bubbles while others may completely collapse and vanish or split into smaller bubbles further up in the bed. Figure 4(a) shows that bubbles first form at a superficial gas velocity of 0.069 4 m⋅s−1 in the no-vibration mode, whereas hardly any bubble appears at this same gas velocity but with vibration. With increasing superficial gas velocity, Figures 4(b) and (c) show that there are fewer large bubbles at intermediate heights with vibration, possibly because the upward and downward movements of particles brought about by vibration tend to break the bubbles, although there are much more large bubbles at these heights at higher gas velocities (Fig. 4(d)), at which bubble movement becomes violent thus leading to their easier coalescence.
Bubble size distributions at different gas velocities.
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CHINA PARTICUOLOGY Vol. 3, No. 4, 2005
In order to confirm the reproducibility of above analysis, the standard deviation S of bubble size distribution with and without vibration at intermediate heights (bed height: 50–150 mm) in different gas velocities is calculated, as shown in Table 1, for the mean of 60 individual measurements. The standard deviation in Table 1 of bubble size distribution with vibration is smaller than that without vibration, which is in agreement with the above analysis for the intermediate heights. Table 1
decreases with increasing vibration amplitude a, e.g. form no vibration (a=0) to a=5 mm, and then increases significantly with increasing vibration amplitude. This means that increasing vibration amplitude is disadvantageous to gas-solid reaction. On the other hand, it is worth noticing that no bubbles can be observed at u=0.069 4 m⋅s−1 for a=15 mm.
Standard deviation of the bubble size distribution at intermediate heights
u/(m⋅s−1) 0.0806 0.0907 0.1222
S (without vibration)
S (with vibration)
8.273 8.692 15.291
4.802 4.924 14.750
Figure 5 shows some examples of transverse distributions of bubbles with and without vibration, indicating that at the same superficial gas velocity, transverse distribution of bubbles is hardly influenced by vibration. That is, vertical vibration does not appear to improve homogeneity of the transverse distribution of bubbles, as might be expected. (a)
(b)
(c)
(d)
Fig. 6
Fig. 5
Transverse distribution of bubbles at different gas velocities with and without vibration: (a) u=0.086 3 m⋅s−1, without vibration; (b) u=0.194 m⋅s−1, without vibration; (c) u=0.086 3 m⋅s−1, with vibration; (d) u=0.194 m⋅s−1, with vibration.
3.2 Effect of vibration amplitude on local average bubble size The local average bubble size, dbav, was measured in areas at two elevations: from 9 to 11 cm above the bottom of the bed for area A1 and from 15 to 17 cm for area A2, as shown in Fig. 3. All bubbles whose centroids fell within area A1 or A2 were included in measurement of their local average bubble sizes, which are the means of 60 individual measurements. The effect of vibration amplitude on the local average bubble size is shown in Fig. 6 for vibration frequency of 15 Hz, indicating that the local average bubble size first
Relationship between local average bubble size dbav and vibration amplitude a.
3.3 Effect of vibration frequency on local average bubble size Figure 7 presents, for both areas A1 and A2, the effect of vibration frequency on local average bubble size at vibration amplitude of a=5 mm, showing that at lower vibration frequencies local average bubble size decreases with increasing frequency up to f =15 Hz, after which bubble size increases slightly with vibration frequency. Table 2 presents measured values of local average bubble sizes in a free bubbling bed without vibration and with vibration at different amplitudes and frequencies, showing that the local average bubble size increases slightly with vibration frequency at vibration amplitudes of a=10 and 15 mm. Moreover, in the case of no vibration, the local average bubble size at u=0.069 4 m⋅s−1 (minimum bubbling velocity) is larger than that at all other velocities, possibly because of difference in performance of the different holes of the distributor.
Zhou, Kage & Li: Bubble Characteristics in a Two-Dimensional Vertically Vibro-fluidized Bed
Fig. 7
Relationship between local average bubble size dbav and vibration frequency f.
Table 2
Local average bubble size in a free bubbling bed with and without vibration −1
u/(m⋅s ) No vibration a=5 mm, f=15 Hz a=5 mm, f=20 Hz a=5 mm, f=30 Hz a=10 mm, f=15 Hz a=10 mm, f=20 Hz a=10 mm, f=30 Hz a=15 mm, f=15 Hz a=15 mm, f=20 Hz a=15 mm, f=30 Hz
227
dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm dbav1/mm dbav2/mm
0.0694
0.0806
0.0907
0.1014
0.1222
20.50 24.67 11.90 16.05 12.06 17.30 14.48 18.48 14.50 19.30 15.51 20.85 13.57 14.31 − − − − − −
16.56 18.53 15.99 16.25 15.09 17.09 16.20 17.87 18.36 21.12 21.20 23.37 21.01 23.80 18.33 21.85 19.71 24.21 22.58 25.10
18.60 19.84 17.52 18.52 17.44 18.89 17.92 19.62 23.76 25.46 23.54 24.78 23.14 24.55 24.93 26.59 19.55 25.15 20.89 24.91
17.57 23.58 16.56 19.82 16.81 20.04 18.91 21.11 24.33 26.96 25.45 28.04 24.42 26.35 26.79 29.29 17.14 26.69 25.01 27.48
25.31 28.63 22.65 25.80 23.98 27.98 24.67 28.06 27.69 31.90 24.48 27.25 25.10 28.81 31.75 32.64 24.81 28.64 27.97 33.82
The local average bubble rise velocity was measured experimentally for different vibration amplitudes (5, 10,
15 mm), frequencies (15, 20, 30 Hz) and superficial gas velocities, as shown in Figs. 8 and 9. It can be seen that all local average bubble rise velocities are enhanced by vibration, though not significantly with vibration amplitude and frequency.
Fig. 8
Fig. 9
3.4 Effect of vibration on local average bubble rise velocity
Relationship between local average bubble rise velocity and vibration frequency.
Relationship between local average bubble rise velocity and vibration amplitude
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CHINA PARTICUOLOGY Vol. 3, No. 4, 2005
4. Conclusions Findings on bubble characteristics obtained from direct measurements in a two-dimensional vibrating fluidized bed revealed that average bubble size decreases significantly with vibration especially at the minimum bubbling velocity, increases with vibration amplitude, but remains almost constant with vibration frequency. Vibration also results in increase of local average bubble rise velocity, although its response to vibration frequency and amplitude is not significant.
Acknowledgement The authors are grateful to Japan Society for the Promotion of Science for providing the financial support.
Nomenclature a db dbav dbav1 dbav2 f H S u Ub Umf x
amplitude of vibration, mm mean diameter of a bubble, mm local average bubble size, mm local average bubble size at area A1, mm local average bubble size at area A2, mm frequency of vibration, Hz bed height, mm standard deviation of the bubble size distribution 1 superficial gas velocity, m⋅s− 1 local average bubble rise velocity, cm⋅s− 1 minimum fluidization velocity, m⋅s− lateral position, mm
References Akiyama, T., Kimura, N. & Iguchi, T. (1996). Effects of reduced air pressure on the particle circulation rate in two-dimensional vibrating particle beds. Powder Technol., 89, 133-138. Davidson, J. F. & Harrison, D. (1963). Fluidized Particles. London: Cambridge University Press.
Eccles, E. R. A. & Mujumdar, A. S. (1997). Bubble phenomena in aerated vibrated beds of small particles. Drying Technol., 15(1), 95-116. Erdesz, K. & Mujumdar, A. S. (1986). Hydrodynamic aspects of conventional and vibrofluidized beds ⎯ a comparative evaluation. Powder Technol., 46, 167-172. Geldart, D. (1967). The expansion of bubble fluidized beds. Powder Technol., 1, 355-368. Horio, M. & Nonaka, A. (1987). A generalized bubble diameter correlation for gas-solid fluidized beds. AIChE J., 33, 18651872. Kage, H., Oba, M., Ishmatsu, H., Ogura, H. & Matsuno, Y. (1997). The effects of frequency and amplitude on powder coating of fluidizing particles in vibro-fluidized bed. Proceedings of Regional Symposium on Chemical Engineering 1997, Vol. 2, Session C1-8, Johor, Malaysia. Kage, H., Shibata, H., Ishmatsu, H., Ogura, H. & Matsuno, Y. (1999). The effects of direction and frequency of vibration on powder coating of fluidizing particles in vibro-fluidized bed. Adv. Powder Technol., 10, 77-87. Koksal, M. & Vural, H. (1998). Bubble size control in a two-dimensional fluidized bed using a moving double plate distributor. Powder Technol., 95, 205-213. Marring, E., Hoffmann, A. C. & Janssen, L. P. B. M. (1994). The effect of vibration on the fluidization behaviour of some cohesive powders. Powder Technol., 79, 1-10. Mori, S. & Wen, C. Y. (1975). Estimation of bubble diameter in gaseous fluidized beds. AIChE J., 21, 109-115. Zhou, T., Kage, H., Funaoka, S., Ogura, H. & Matsuno, Y. (2001). Fluidization behavior of glass beads under different vibration module. Adv. Powder Technol., 12(4), 559-575. Zhou, T., Kage, H., Funaoka, S. & Ogura, H. (2002). The bubble behavior in a new-type horizontal vibro-fluidized bed. J. Chem. Eng. Jpn., 35(8), 737-743. Zhou, T., Ogura, H., Yamamura, M. & Kage, H. (2004). Bubble motion pattern and rise velocity in two-dimensional horizontal and vertical vibro-fluidized beds. Can. J. Chem. Eng., 82(2), 236-242. Manuscript received October 21, 2004 and accepted March 29, 2005.