Ind. Eng. Chem. Res. 2006, 45, 1805-1810
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Fluidization Quality Improvement for Cohesive Particles by Fine Powder Coating Huie Liu, Yan Li, and Qingjie Guo* State Key Laboratory of HeaVy Oil Processing, College of Chemistry and Chemical Engineering, China UniVersity of Petroleum, Dongying 257061, Shandong, P. R. China
A dry coating method via fine powder is used to improve the fluidization quality for cornstarch particles, belonging to the Geldart C group, which cannot fluidize normally. Two kinds of SiO2 fine powder are used to coat cornstarch particles. Both a conventional fluidized bed and a magnetic fluidized bed (MFB) are employed for the coating of cornstarch particles. The coating time ranges from 10 to 15 min in this study. The coated particles are observed via the scanning electron microscope (SEM) images. Furthermore, the fluidization behaviors of the coated particles are investigated. The results show that coating with fine SiO2 powder is an effective method to improve the fluidization quality of cohesive cornstarch particles. However, no significant difference in fluidization quality is observed between particles coated in a conventional fluidized bed and those coated in a MFB. 1. Introduction Cohesive particles, most belonging to the Geldart C group, are difficult to fluidize under normal conditions.1 Slugs, channels, and agglomerates are marked characteristics of their fluidization behavior.2 These characteristics bring the problem of weak contact between gas and solids and limit the processing of cohesive particles in fluidized reactors. Many researchers have developed methods to solve the above problems, all of which can be classified into two groups:2 external method and intrinsic method. The external method employs external forces to overcome the adhesion force between particles. For example, Chirone et al.,3,4 Nowak et al.,5 Levy et al.,6 and Herrera et al.7 introduced an acoustic field into the bed. With the presence of an acoustic field, the size of agglomerates, the minimum fluidization velocity, and the entrainment of fine powder decreased. Mori et al.,8 Dutaa et al.,9 Jaraiz et al.,10 Tang et al.,11 and Nam et al.12 found that a vibrating field promoted the fluidization of fine powder. Also, a magnetic field can significantly influence the fluidization behavior of particles.13,14 Recently, magnetic particles and a magnetic field were also applied to improve fluidization quality of Geldart C powder.15,16 The intrinsic method is altering the intrinsic properties of particles. Kusakabe et al.17 used nitrogen as the fluid and found that fine particles could be fluidized stably at reduced pressure. A lot of experiments have demonstrated that the fluidization quality of fine powder can improve significantly as another kind of particle is added into the bed.2,9,18-21 A great advantage of this method is that no additional device is needed; thus, the investment for equipment is low. Wang19 investigated the fluidization behavior of four kinds of Geldart C powders, CaCO3, Ni, R-FeO(OH), and γ-Fe2O3, when adding group A, B, C, and D particles, respectively. It is shown that each kind of fine powder has an optimal match particle and an optimal proportion for additive particles. Cornstarch particle, a kind of Geldart C cohesive particle, is employed in this investigation. It can be seen from the following * To whom correspondence should be addressed. Mailing address: College of Chemistry and Chemical Engineering, China University of Petroleum, Beier Road 271#, Dongying 257061, Shandong, P. R. China. Tel.: 86-546-8396753. Fax: 86-546-8391971. E-mail: qjguo@ mail.hdpu.edu.cn.
work that this kind of particle is difficult to fluidize. Nevertheless, the fluidization of cornstarch particles is of important significance for the pharmacy industry. In the present study, a dry coating method via fine powder is developed to improve the fluidization quality for cornstarch particle. Two different kinds of coating methods were explored. One is coating in a conventional fluidized bed, and the other is coating in a magnetic fluidized bed (MFB). Moreover, the proportion of the fine powder is changed to investigate the best matching. 2. Experiment The experimental setup is shown in Figure 1. The fluidized bed is made of glass with a height of 1.6 m and an inner diameter of 0.056 m. A porous steel plate is used as a gas distributor. A U-manometer is installed at the bed bottom to measure the pressure drop across the bed. The fluidization gas, air, is supplied with a compressor, and the gas flowrate is measured with a rotameter. A 0.61 m high coil encircling the bed is used to generate an axial magnetic filed through the bed. The coil has an inner diameter of 0.14 m and an outer diameter of 0.16 m. A HT102 type gaussmeter is used to measure the magnetic flux density. Four kinds of particles are used in this work. The physical properties of the materials used in the experiment are listed in Table 1. Two kinds of particle size for the cornstarch and SiO2 are listed in the table, in which dp1 is obtained from a Coulter LS-230 laser diffraction particle size analyzer and dp2 is obtained by measuring from scanning electron microscope (SEM) images. The particle sizes obtained from the SEM images are given here because they will be used for comparison with the coated particles in the following parts. The particle size of magnetic particle is obtained through screening. Figure 2 shows the particle size distributions of cornstarch and the two kinds of SiO2. The fluidization behaviors of pure cornstarch particles are first investigated. The total weight of cornstarch particles used is 140 g. In theory, the pressure drop can be calculated from the weight of the particles above the manometer, i.e., Mg/A, equaling 462.55 Pa. Under the same conditions, two curves of pressure drop versus increasing gas velocity (exps 1 and 2) are drawn, as shown in Figure 3. The line of theoretical pressure drop is also shown in the figure. As expected, the pressure drop first undergoes a sharp increase, to a highest value even much higher than the theoretical pressure drop, and then a quick
10.1021/ie050083y CCC: $33.50 © 2006 American Chemical Society Published on Web 02/03/2006
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Figure 1. Schematic diagram of the experimental apparatus: (1) compressor; (2) surge tank and desiccator; (3) rotameter; (4) coil; (5) fluidized bed; (6) filter bag; (7) manometer. Table 1. Physical Properties of the Experimental Materials material
dp1, µm
dp2, µm
Fb, kg‚m-3
Ft, kg‚m-3
cornstarch 1# SiO2 2# SiO2 Fe3O4
13.47 2.97 8.56
10.7 2.5 10.0 752.26
444 500
564 625
2000
2000
decline, to a lowest value near zero where an open valley forms. Finally, the pressure drop continues to increase and becomes relatively stable, and it is observed that the particles suffer plugs, channels, and full fluidization states in turn. Moreover, the two fluidization curves differ greatly in their configurations. These are typical characteristics of cohesive particles. To improve the fluidization quality, a dry particle coating method, i.e., coating the cohesive cornstarch particles with another kind of fine powder (SiO2), is used in this work. It is expected that the surface property of the cohesive particles can be modified when coated by fine powder, and thus, the fluidization quality can be improved. The coating is performed in a conventional bubbling fluidized bed and a preliminary study for coating in a magnetic fluidized bed is also carried out. All the work is aimed at improving the coating effect, thereby enhancing the fluidization behavior of the cohesive particles. 3. Results and Discussion 3.1. Coating Cornstarch Particles with 1# SiO2 Powders and Fluidization of the Coated Particles. 3.1.1. Selection of Coating Time. Cornstarch particles, mixed with the 1# SiO2 powder (dp1 ) 2.97 µm) in the proportion of 0.95:0.05 (wt), are fed into the fluidized bed and fluidized for a certain period of time to coat the cornstarch particles with the SiO2 powder. The gas velocity used in the coating process is 0.18 m‚s-1. A 140 g portion of coated particles discharged from the bed is used to observe the fluidization behaviors in a conventional fluidized bed. Cornstarch particles coated for 5, 10, and 15 min are investigated, respectively, and curves of the pressure drop across the bed with increasing gas velocity are presented in Figure 4. It is shown that, with the increase of gas velocity, the fluctuation of the pressure drop for the coated particles is much less than that of pure cornstarch particles, which indicates that the coated particles can fluidize more smoothly. As expected, the longer coating time used, the better the fluidization behavior of the coated particles. However, no marked difference can be observed when the coating time is longer than 10 min. Accordingly, a coating time of 10-15 min is used in the following experiments. 3.1.2. Coating in a Conventional Fluidized Bed and Fluidization of the Coated Particles. Different proportions of 1# SiO2 powders (dp1 ) 2.97µm), 1% (wt), 2% (wt), 3% (wt), 5% (wt), 8% (wt), 10% (wt), and 15% (wt) are used to blend with the cornstarch particles. The coating is carried out in a conventional fluidized bed without any magnetic particles and
Figure 2. Particle size distributions from Coulter LS-230 laser diffraction particle size analyzer: (a) cornstarch; (b) 1# SiO2; (c) 2# SiO2.
Figure 3. Fluidization curves of pure cornstarch particles.
magnetic field, with the gas velocity being 0.18 m‚s-1 and the coating time being 15 min. A 140 g portion of coated particles is used to investigate the fluidization behaviors. The curves of pressure drop versus increasing superficial gas velocity for the
Ind. Eng. Chem. Res., Vol. 45, No. 5, 2006 1807 Table 2. Superficial Minimum Fluidization Velocity for Figure 5 plot
a
b
c
d
e
f
g
proportion of SiO2, 1 2 3 5 8 10 15 % (wt) Umf, super, m‚s-1 0.210 0.125 0.125 0.136 0.147 0.147 0.170
Figure 4. Influence of coating time on fluidization behavior of coated cornstarch particles.
seven groups of coated cornstarch particles are shown in Figure 5. The theoretical pressure drops are also illustrated in dashed lines in Figure 5. The pressure drops go through an increase and a decline and eventually arrive at a plateau. The plateau values of the pressure drops are somewhat smaller than the theoretical values. Probably, some elutriation of very fine particles took place, and there were some errors in the pressure drop measurement. It is obvious that the pressure drop curves exhibit smaller fluctuation for the coated cornstarch particles compared with that of the pure ones (as shown in Figure 3) when the proportion of SiO2 is higher than 1% (wt). The fluidization behavior of cornstarch particles coated with 1% (wt) SiO2 is somewhat like that of pure ones, and the pressure drop curve undergoes a sharp increase, a quick decline to a lowest value near zero where an open valley forms, and finally a plateau. This indicates that 1% (wt) is too low a proportion for SiO2 to improve the fluidization quality of cornstarch particles. Under the conditions of 2-15% (wt) SiO2 used for coating, no significant slugs and channels are observed, suggesting that the coating method does improve the fluidization quality for the cornstarch particles. In this study, a superficial minimum fluidization velocity, Umf,super, is defined as the lowest superficial gas velocity at which the pressure drop across the bed reaches a plateau. The values of Umf,super for the seven groups of particles
are measured in this study, as shown in Table 2, which reveals that the Umf,super has a lowest value. The pressure drop curves tend to fluctuate sharply when the content of SiO2 powder is too low (such as 1% (wt)) or too high (such as 15% (wt)), which means that the fluidization behavior of the cohesive particles is undesirable when too little or too much fine powder exists. It is anticipated that the state of the cornstarch particles changes little when too little fine powder is blended and that the strong adhesion force among the fine SiO2 particles becomes dominant when too much SiO2 exists in the particle mixtures. With respect to this study, 2% (wt) SiO2 fine powder is an appropriate proportion to coat the cohesive particles for approaching stable fluidization. To depict the coating effect, samples of pure cornstarch particles and some coated ones (2% (wt), 5% (wt), 10% (wt), and 15% (wt) SiO2 powder is used, respectively) were observed by a scanning electron microscope (SEM), and the images are shown in Figures 6 and 7. It is clearly seen from Figure 7 that some fine particles adhere to the surfaces of cornstarch particles, and these fine particles should be SiO2. However, it also can be seen that some SiO2 powder exists among the cornstarch particles, instead of adhering to the surfaces of cornstarch particles. The more SiO2 powder used, the more of it fails to coat the surfaces of cornstarch particles. The average diameters of the coated particles in Figure 7a-d are obtained by measuring the particle size in the SEM images, being 11.6, 11.6, 11.4, and 10.4 µm, respectively. The particle sizes in Figure 7a-c are larger than the average diameter of pure cornstarch particles (i.e., 10.7 µm from the SEM image). This indicates that some SiO2 powder does coat on the cornstarch particles, whereas, for the size increase that is much smaller than 5 µm (i.e., double dp2 of SiO2), most of the SiO2 powder coating on the cornstarch particles should be very fine, which indicates that a finer SiO2 powder may give a good coating effect. Thus, further research
Figure 5. Fluidization curves of cornstarch particles coated with different proportions of 1# SiO2, without magnetic particles and a magnetic field: (a) 2% (wt) 1# SiO2; (b) 3% (wt) 1# SiO2; (c) 5% (wt) 1# SiO2; (d) 8% (wt) 1# SiO2; (e) 10% (wt) 1# SiO2; (f) 15% (wt) 1# SiO2.
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Figure 6. SEM image for pure cornstarch particles.
Figure 7. SEM images for cornstarch particles coated in a conventional fluidized bed: (a) with 2% (wt) 1# SiO2 powder; (b) with 5% (wt) 1# SiO2 powder; (c) with 10% (wt) 1# SiO2 powder; (d) with 15% (wt) 1# SiO2 powder.
in this regard is needed. In Figure 7d, the average particle size is slightly smaller than that of pure cornstarch particles. It is expected that when 15% (wt) SiO2 powder is used, much of it fails to coat the surfaces of cornstarch particles. However, it is difficult to distinguish the cornstarch particles and the small SiO2 particles in our measurement. The average particle size we obtained is that of the particle mixture. Thus, the average particle size decreases slightly. 3.1.3. Coating in a Magnetic Field and Fluidization of the Coated Particles. Much work has been done on magnetic fluidization of Geldart C particles.15,16 It was reported that magnetic particles form a chain structure in a magnetic field,
Figure 8. SEM image for cornstarch particles coated with 5% (wt) 1# SiO2 powder in a magnetic fluidized bed (B ) 12.27 mT).
which splits the bubbles and eliminates the channels effectively. Accordingly, a magnetic field improves the fluidization quality. In the following work, magnetic particles and a magnetic field are used to improve the fluidization quality of the particle mixture of cornstarch and SiO2 and thereby to improve the coating effect. The cornstarch particles mixed with various proportions of 1# SiO2 powder (3%, 5%, 8%, 10%, and 15% (wt), respectively), together with 10% (wt) magnetic particles, Fe3O4, are fluidized in an axial uniform magnetic field for about 15 min, thereby to coat the SiO2 powder on the surfaces of cornstarch particles. The magnetic flux density in the bed, measured by a guassmeter, is 12.27 mT, and the gas velocity is 0.18 m‚s-1. The particle mixture is discharged afterward, separating the magnetic particles from the coated cornstarch particles using a magnet. A SEM image for a sample of the coated particles (with 5% (wt) SiO2) was shown in Figure 8. As has been pointed out, Figure 7b gives the SEM image for cornstarch particles coated in a conventional fluidized bed with the same mass fraction of SiO2. It can be seen that more very fine SiO2 particles exist among those large cornstarch particles in Figure 7b when compared with Figure 8. This may indicate that more SiO2 particles coat on the cornstarch particles in Figure 8, because the proportion of SiO2 used is the same in these two figures. The fluidization behaviors of these coated cornstarch particles are also investigated. The curves for pressure drop across the bed versus increasing gas velocity are presented in Figure 9, with theoretical pressure drop curves in dashed lines. The Umf,super values are summarized in Table 3. A comparison of Umf,super values in Tables 2 and 3 reveals that the Umf,super values are very close under the same mass fraction of SiO2 no matter if the magnetic fields exist or not. This indicates that the fluidization behavior of the cornstarch particles coated in a magnetic fluidized bed is similar to that of those coated in a conventional fluidized bed. Maybe, the size of the magnetic particles used in the present work is not the appropriate one. This calls for further efforts to investigate the fit size of magnetic particles. 3.2. Adding 2# SiO2 Powder and Fluidization of the Particle Mixture. To clarify the advantage of the coating method on improvement of fluidization quality for Geldart C particles, 2# SiO2 powder (dp1 ) 8.56 µm) is also used in this study. It is expected that 2# SiO2 powder may not coat cornstarch particles for its size is too large and it may just act as a kind of additive particle. Particle mixtures of cornstarch and 2# SiO2 are first blended in a conventional fluidized bed
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Figure 9. Fluidization curves of cornstarch particles coated with different proportions of 1# SiO2, with 10% (wt) magnetic particles and the magnetic flux density being 12.27 mT. Table 3. Superficial Minimum Fluidization Velocity for Figure 9 plot
a
b
c
d
e
proportion of SiO2, % (wt) Umf, super, m‚s-1
3 0.136
5 0.136
8 0.136
10 0.159
15 0.170
or in a magnetic fluidized bed to realize full mixing. Two kinds of mass fractions of 2# SiO2, 3% (wt) and 5% (wt), are investigated in this study. The fluidization behaviors of the mixed particles are studied, and the curves of pressure drop across the bed versus gas velocity are shown in Figure 10, in which the theoretical pressure drops are also marked in dashed lines. The mixed particles can fluidize more stably than the pure cornstarch particles, as shown in Figure 3. No significant channeling was observed, and the mixed particles could eventually fluidize smoothly. These results show that adding 2# SiO2 powder can improve the fluidization behavior for cornstarch particles. However, with an increase of the gas velocity, the pressure drops across the bed first increase sharply to a maximum value much higher than the theoretical one, as shown in Figure 10. This phenomenon, corresponding to an obvious plug state, is similar to that of the pure cornstarch particles. Thus, the fluidization quality for cornstarch particles mixed with 2# SiO2 powder is not as good as that for those coated with 1# SiO2 particles. The above evidence confirms that coating cornstarch particles with 1# SiO2 powder is more effective for fluidization quality improvement than adding the larger one, 2# SiO2 powder.
Figure 10. Fluidization curves of cornstarch particles mixed with different proportions of 2# SiO2: (a) without magnetic particles and a magnetic field; (b) with the presence of 10% (wt) Fe3O4 magnetic particles and the magnetic flux density being 12.14 mT.
4. Conclusions The cornstarch particle used in this study is a typical Geldart C cohesive particle, which cannot fluidize normally. Coating with fine SiO2 powder is an effective method to improve the fluidization quality of the cohesive cornstarch particles. In the present study, the following conclusions can be drawn: (1) The longer coating time used, the better the fluidization quality for the coated particles. However, no marked difference can be observed when the coating time is longer than 10 min. (2) When coating is carried out in a conventional fluidized bed, 2% (wt) 1# SiO2 powder (dp1 ) 2.97 µm) gives the best results for fluidization quality improvement of cornstarch particles. (3) No significant difference exists between the fluidization behavior of particles coated in a magnetic fluidized bed and that of those coated in a conventional fluidized bed. (4) Coating cornstarch particles with 1# SiO2 (dp1 ) 2.97 µm) powder is more effective for fluidization quality improvement than adding 2# SiO2 (dp1 ) 8.56 µm) particles. Acknowledgment This research is supported by the Natural Science Foundation of Shandong Province under Contract No. Z2003B01 and by Project, sponsored by SRF for ROCS and SEM, 2004527. Nomenclature A ) cross sectional area of the fluidized bed, m2 B ) magnetic flux density, T dp1 ) average particle diameter obtained from Coulter LS-230 laser diffraction particle size analyzer, µm dp2 ) average particle diameter measured from SEM images, µm g ) acceleration of gravity, 9.81 m‚s-2 M ) the total particle weight above the manometer in smooth fluidization, kg Umf, super ) superficial minimum fluidization velocity, m‚s-1 Fb ) bulk density of particle, kg‚m-3 Ft ) tap density of particle, kg‚m-3 Literature Cited (1) Geldart, D. Types of gas fluidization. Powder Technol. 1973, 7, 285292.
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(2) Wang, Z.; Kwauk, M.; Li, H. Fluidization of fine particles. Chem. Eng. Sci. 1998, 53 (3), 377-395. (3) Chirone, R.; Massimilla, L.; Russo, S. Bubbling fluidization of a cohesive powder in an acoustic field. In Fluidization VII; Potter, O. E., Nicklin, D. J., Eds.; New York, 1992; pp 545-553. (4) Chirone, R.; Massimilla, L.; Russo, S. Bubble-free fluidization of a cohesive powder in an acoustic field. Chem. Eng. Sci. 1993, 48 (1), 4152. (5) Nowak, W.; Hasatani, M.; Derczynski, M. Fluidization and heat transfer of fine particles in an acoustic field. AIChE Symp. Ser. 1993, 89 (296), 137-149. (6) Levy, E. K.; Shnitzer, I.; Masaki, T.; et al. Effect of an acoustic field on bubbling in a gas in a gas fluidized bed. Powder Technol. 1997, 90, 53-57. (7) Herrera, C. A.; Levy, E. K. Bubbling characteristics of sound-assisted fluidized beds. Powder Technol. 2001, 119, 229-240. (8) Mori, S.; Yamamoto, A.; Haruta, T.; Yamada, T.; Mizutani, E. Vibrofluidization of very fine particles. In Fluidization ‘88-Science and Technology; Kwauk, M., Kunii, D., Eds.; Academic Press: Beijing, 1988; pp 7581. (9) Dutta, A.; Dullea, L. V. Effects of external vibration and the addition of fibers on the fluidization of a fine powder. AIChE Symp. Ser. 1991, 87 (281), 38-46. (10) Jaraiz, E.; Kimura, S.; Levenspiel, O. Vibrating beds of fine particles: estimation of interparticle forces from expansion and pressure drop experiments. Powder Technol. 1991, 72, 23-30. (11) Tang, H.; Zhao, J. The agglomerating behavior of fine particles in a Vibro-fluidized bed. Chem. Ind. Eng. (Tianjin, China) 1996, 13 (3), 6-20. (12) Nam, C. H.; Pfeffer, R.; Dave, R. N.; Sundaresan, S. Aerated vibrofluidization of silica Nanoparticles. AIChE J. 2004, 50 (8), 17761785.
(13) Rosensweig, R. E. Fluidization: Hydrodynamic stabilization with a magnetic field. Science. 1979, 204, 57-60. (14) Jaraiz, E.; Levenspiel, O.; Fitzgerald, T. J. The use of magnetic fields in the processing of solids. Chem. Eng. Sci. 1983, 38 (1), 107-114. (15) Zhu Q.; Li H. Study on magnetic fluidization of group C powders, Powder Technol. 1996, 86, 179-185. (16) Lu, X.; Li, H. Fluidization of CaCO3 and Fe2O3 particle mixtures in a transverse rotating magnetic field. Powder Technol. 2000, 107, 6678. (17) Kusakabe, K.; Kuriyama, T.; Morooka, S. Fluidization of fine particles at reduced pressure. Powder Technol. 1989, 58, 125-130. (18) Liu, Y.; Kimura, S. Fluidization and entrainment of difficult-tofluidize fine powder mixed with easy-to-fluidize large particles. Powder Technol. 1993, 75, 189-196. (19) Wang, Z. Fluidization of Fine particles and effects of additive particles. Ph.D. Dissertation, Institute of Chemical Metallurgy, Academia Sinica, Beijing, 1995 (in Chinese). (20) Zhou, T.; Li, H. Effect of adding different size particles on fluidization of cohesive particles. Powder Technol. 1999, 102, 215220. (21) Zhou, T.; Ye, H.; Dave, R.; Pfeffer, R. Behavior of cohesive particles coated with nanoparticles in a fluidized bed. Chin. J. Process Eng. 2004, 4 (Suppl.), 567-572.
ReceiVed for reView January 20, 2005 ReVised manuscript receiVed October 16, 2005 Accepted January 9, 2006 IE050083Y
