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Investigation of the segregation of a binary particle mixture in a square circulating fluidized bed with air staging
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Investigation of the segregation of a binary particle mixture in a square circulating fluidized bed with air staging Mingkun Du a,∗ , Shuai Wang b,∗ a b
Guangdong Institute of Special Equipment Inspection and Research, Zhuhai Inspection Institute, Zhuhai 519002, China School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 14 March 2017 Received in revised form 7 September 2017 Accepted 8 July 2019 Available online xxx Keywords: Circulating fluidized bed Binary mixture Segregation Air staging
a b s t r a c t To further understand the segregation characteristics of a binary particle mixture in a riser, the flow behaviors of particles with different sizes were experimentally and numerically investigated in a cold square circulating fluidized bed. The impact of the gas velocity, solid circulation rate, initial coarse particle fraction, and air staging on the coarse particle fraction and axial pressure drop were examined. The experimental results show that air staging can significantly promote the pressure drop of a binary mixture at the bottom of the bed compared with that of monosized particles. Meanwhile, the segregation of the binary mixture at the bottom is hindered. It was also found that there was clear radial segregation owing to a higher coarse particle fraction near the wall compared with the bed’s center and its corner. © 2019 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction During coal combustion and gasification processes in a circulating fluidized bed (CFB), particles generally have a broad size distribution. The mixing and segregation performance of particles plays a crucial role on bed expansion, the chemical reaction, and the heat and mass transfer rates (Bi, Jiang, Jean, & Fan, 1992; Feng & Yu, 2007). Owing to the complex interactions between coarse and fine particles, it is challenging to obtain a deep insight into the flow behaviors of binary mixtures that are essential for designing and scaling-up fluidized systems (Chew et al., 2011). In a fluidized system with a binary mixture, the fluidizing gas preferably elutriates lighter and finer particles, while coarser and denser particles tend to sink and remain at lower levels. The difference in size or density will lead to particle segregation. A large number of researchers have investigated the segregation behavior of binary mixtures using various methods (Chao, Wang, Jakobsen, Fernandino, & Jakobsen, 2012; MShoushtari, Hosseini, & Soleimani, 2013; Wang, Yin, Liu, & Song, 2019; Kiani, Rahimi, Hosseini, & Ahmadi, 2017). The terms “flotsam” and “jetsam” were introduced to describe the solid components that float to the top and those
∗ Corresponding authors. E-mail addresses: dumk [email protected] (M. Du), [email protected], [email protected] (S. Wang).
that ultimately sink, respectively (Nienow, Rowe, & Cheung, 1978). Segregation intensity was proposed as a property to characterize the normalized difference between the local and mean volume fractions of coarse particles (Nakagawa et al., 1994). Jang, Park, and Cha (2010) analyzed the residence time in a fluidized bed for particles with a binary density and pointed out that the degree of mixing in this system with its different solid densities depended on the residence time distribution. Babu, Sai, and Krishnaiah (2017) developed the empirical correlation for predicting the separation factor and product quality by means of conducting continuous segregation of a binary heterogeneous solids mixture. Segregation behaviors include axial segregation and lateral segregation. Axial segregation reduces the mean particle size along the riser height, while lateral segregation results in larger particles, which are preferentially found in the down-flow near the wall of the fluidized bed. Das, Banerjee, and Saha (2007) examined the influence of the operating conditions on axial segregation and evaluated the degree of segregation. They concluded that increasing the superficial gas velocity can decrease segregation effects. A year later, Das, Meikap, and Saha (2008) found that there was a significant difference in segregation between the radial and axial flow directions: axial segregation was more obvious than lateral segregation. Köhler, Rasch, Pallarès, and Johnsson (2017) studied axial fuel mixing and segregation in fluidized beds on the basis of a magnetic particle tracking system and identified three fuel segregation
https://doi.org/10.1016/j.partic.2019.07.002 1674-2001/© 2019 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Du, M., & Wang, S. Investigation of the segregation of a binary particle mixture in a square circulating fluidized bed with air staging. Particuology (2019), https://doi.org/10.1016/j.partic.2019.07.002
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2 Table 1 Properties of the bed materials. Material
dp (m)
(kg/m3 )
Umf (m/s)
Ut (m/s)
Geldart type
Fine Coarse
257 2432
2550 2550
0.054 1.241
1.57 12.34
B D
regimes. It has also been pointed out that the bed height and the density of tracer particles determines the transition velocities in each of those regimes. Salatino and Solimene (2017) analyzed the axial and lateral segregation of fuel particles during devolatilization in fluidized bed reactors. It was emphasized that the lateral dispersion of solid particles depends on scale effects; the dispersion coefficient in a large-scale system was much greater than it was in a small-scale system. However, there are few reports on the segregation of binary mixtures under the condition of air staging. Based on a study by Koksal, Golriz, and Hamdullahpur (2008), air staging has a significant impact on heat transfer in a CFB. The present study aims to investigate the segregation behaviors of mixtures of solids that differ significantly in size in a square CFB system with air staging. The effects of the operating conditions including the operating gas velocity, the circulation rates of the solids, the initial coarse particle fraction, and air staging on the segregation characteristics of a binary mixture were investigated; our results are beneficial for understanding the interaction mechanics of binary mixtures and for optimizing CFB systems. Experimental This work was conducted on the basis of an experimental setup consisting of a circulating fluidized bed made of poly(methyl methacrylate). It consisted of a square riser, two stage cyclones, a down comer, and a loop seal, as shown in Fig. 1. The riser had a cross-sectional area of 0.25 m × 0.25 m and a height of 6.07 m. The air from a root blower was divided into primary air, secondary air, and air for the U-type loop seal. The primary air entered the riser section of the CFB through a multi-hole distributor plate. The secondary air was injected into the riser via nozzles (0.025 m in diameter), which were located at a height of 0.832 m. A smooth exit was connected with two stage cyclones to collect particles. The circulating particles were transferred back to the riser from the downer through a U-type loop seal. The solid circulation rate Gs was adjusted with the use of a stop-watch and by shutting the flapper valve quickly in the return line; Gs was also changed by adjusting the loosing air and solid inventory. The axial gas pressure was measured using pressure taps connected to water manometers. Samples of the solids were withdrawn by sampling probes that were each made of a stainless steel tube that was bent at the end in the vertical direction, which allowed sampling either vertically upward near the wall or vertically downward at other locations. The samples of the solids were transported to sampling vessels via vacuum pump suction for detailed analysis. The flow of the particles was independent of the air sampling rate as a result of particle inertia. To eliminate the electrostatic effect, plastic films were employed inside the fluidized bed reactors. Additionally, the relative humidity and the temperature of the fluidizing gas were monitored during the experiments and adjusted using a packed water column and a dryer. The experiments were conducted with two kinds of particles composed of quartz sand that had different sizes. The mean particle diameters were 257 and 2432 m. Both had solid densities of 2550 kg/m3 . Their minimum fluidizing velocities and terminal velocities are listed in Table 1. The particle compositions for the binary mixture are shown in terms of their mass fraction of the
Fig. 1. Schematic diagram of the experimental setup.
coarse component, which was determined through measuring the sampled particle density. By adding coarse particles into fine ones, the initial coarse particle mass fraction Xc0 was varied from 0 to 0.5. The superficial gas velocity U0 changed from 3.4 to 4.6 m/s and Gs varied between 8 and 21 kg/(m2 s). The secondary air ratio varied from 0 to 0.3. Results and discussion The overall flow behavior can be studied by measuring the axial pressure profiles. Fig. 2 shows the effect of the solid circulation rate Gs on the pressure drop along the riser. At a constant gas velocity U0 and initial coarse particle fraction Xc0 , the pressure drop at the bottom increases along with Gs . This is because there is a higher solid concentration at a higher solid flux. Additionally, the increased collisions between particles will lead to greater accumulation of particles at the bottom. When air is supplied by staging, a clear increase of the pressure drop at the bottom is observed, as shown in Fig. 2, which is attributed to the fact that the amount of air introduced from the bottom of the riser decreases with the injection of secondary air
Please cite this article in press as: Du, M., & Wang, S. Investigation of the segregation of a binary particle mixture in a square circulating fluidized bed with air staging. Particuology (2019), https://doi.org/10.1016/j.partic.2019.07.002
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Fig. 2. Profiles of the pressure drop along the riser with an initial coarse particle fraction Xc0 , solid circulation rate Gs , and air staging SAR.
when the total amount of air is kept constant. This will result in a decrease of the solid carrying capacity in the primary region. Hence, more solid inventories in the primary region are required to maintain a constant Gs . Koksal and Hamdullahpur (2004) analyzed the effect of secondary air injection on the solid distribution in a circulating fluidized bed using monosized bed material. It was found that secondary air injection can considerably increase the solid carrying capacity, which was inconsistent with our observation. Compared with the results for monosized particles, the bed pressure drop is greatly affected when adding coarse particles into fine ones. When Xc0 is changed from 0.0 to 0.25 under the air staging condition, the
increases in the pressure drop at the bottom of the riser become significant. Bi et al. (1992) and Bai, Nakagawa, Shibuya, Kinoshita, and Kato (1994) pointed out that fine particles move faster owing to the increasing interstitial gas velocity resulting from the decrease in the effective cross-sectional area of flow. However, fine particles slow down in the presence of coarse particles owing to the promotion of interactions between the fine and coarse particles; this results in considerable back-mixing and down-flow in the wall region. In this work, there is a clear difference in size between the coarse and fine particles, which leads to increased interactions between the particles, thereby causing the fine particles to slow down.
Fig. 3. Profiles of the coarse particle fraction along the riser with gas velocity U0 and initial coarse particle fraction Xc0 .
Please cite this article in press as: Du, M., & Wang, S. Investigation of the segregation of a binary particle mixture in a square circulating fluidized bed with air staging. Particuology (2019), https://doi.org/10.1016/j.partic.2019.07.002
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Fig. 4. Profiles of the coarse particle fraction along the riser with air staging SAR and solid circulation rate Gs .
A study by Wang et al. (2009) showed that increasing the original coarse particle fraction causes a higher fraction of coarse particles to be present in the external circulating particles. Here, the variation of the fraction of coarse particles along the height of the riser was also investigated to understand the axial segregation behaviors of the particles, as show in Fig. 3. A partial segregation pattern was found under all operating conditions owing to the gradual decrease of the coarse particle fraction with increasing height. Coarse particles can be entrained by the stream of gas and the fine particles, while the movement of the fine particles is hampered by coarse particles
because of collisions and aggregation, although they are preferably elutriated by the gas. The exact value of the coarse particle fraction depends on the equilibrium state reached for the above-mentioned effects. The effect of U0 is apparent from Fig. 3, which shows data from separate experiments that were conducted for a gas velocity from 3.4 to 4.6 m/s at a constant Gs . A larger coarse particle fraction can be observed at higher gas velocities. To maintain a constant solid circulation rate, a lower solid inventory is required, which will result in a weakening of collisions between coarse and fine parti-
Fig. 5. Profiles of the coarse particle fraction in the center, near the rear wall, and in the corner.
Please cite this article in press as: Du, M., & Wang, S. Investigation of the segregation of a binary particle mixture in a square circulating fluidized bed with air staging. Particuology (2019), https://doi.org/10.1016/j.partic.2019.07.002
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Fig. 6. Comparison of the simulated prediction and the experimental data.
cles. Meanwhile, the carrying capacity of the gas will be improved. Fine particles are more easily separated from the mixture and flow out of the riser, which results in an increase of the coarse particle fraction. Fig. 3 also demonstrates the effect of the initial coarse particle fraction Xc0 on the coarse particle fraction. As Xc0 is increased, the coarse particle fraction along the riser increases, with a greater fraction at the bottom. Coarse particles tend to aggregate in the bottom section as Xc0 increases owing to the weak entrainment of coarse particles by the gas. In contrast, fine particles are more easily entrained by the gas. Simultaneously, coarse particles in the upper section fall down owing to their higher terminal velocity. Therefore, it can be observed that particle segregation becomes more severe when Xc0 is increased. The profiles of the coarse particle fraction along the riser with and without air staging are plotted in Fig. 4. Without air staging, the fine particles tend to be more easily elutriated by a high primary gas velocity than the coarse particles. After air staging, the coarse particle fraction decreases obviously at the bottom owing to the fine particle flow being hindered by a low primary gas velocity and the interactions between the coarse and fine particles. Fig. 4 also shows the effect of Gs on the coarse particle fraction. At a lower solid circulation rate, there is a significant discrepancy between the coarse particles in the bottom and the upper section. As the solid circulation rate increases, the difference decreases. This indicates that a good mixing of the coarse and fine particles can be achieved by increasing the solid circulation rate. The coarse particle fractions in the center, near the rear wall and in the corner at three different heights are displayed in Fig. 5. We found that there was a high coarse particle fraction at the wall. This is attributed to the core–annuls structure in a CFB. Coarse particles in the core are segregated from the up-flow to the down-flow in the annulus, while fine particles in the annulus are preferentially stripped from the down-flow to the up-flow in the core, which results in a low coarse particle fraction in the center. In contrast, the coarse particle fraction in the corner is lower than that at the wall. This is attributed to the lower gas velocity in the corner restricting the fine particles that are stripped from the down-flow.
To gain a better insight into the radial distribution of the binary mixture, a three dimensional simulation was conducted by means of a computational fluid dynamic approach. A multi-fluid model was employed with the kinetic theory of granular mixture (Wang, Jin, Wang, & Hu, 2015). To validate the feasibility of the model, a comparison of the predicted results and the experimental data was conducted as shown in Fig. 6. It can be observed that the model provides a fair prediction of the measured data. Fig. 7 demonstrates the contour plots of the simulated solid concentration at different heights. We found that there was an obvious zone with a high particle density near the wall, especially at the bottom. Additionally, the concentration of large particles was higher in the bottom zone. When the height was increased and the secondary air was injected, both solid concentrations decreased, although there was still a lateral discrepancy of the solid concentrations. At the upper section of the riser, the solid concentration was very low. Meanwhile, the lateral discrepancy was significantly weaker.
Conclusions The segregation behavior of a binary mixture of different particles in a square circulating fluidized bed was investigated experimentally. The influence of the operating parameters including the gas velocity, solid circulation rate, initial coarse particle fraction, and air staging on the pressure drop and coarse particle fraction was evaluated. It was found that there was an obvious increase of the coarse particle fraction in the bottom section when the initial coarse particle fraction was increased. Compared with mono-sized particles, air staging can greatly promote the pressure drop of a binary mixture at the bottom. The air staging and the reduction of the gas velocity hinder the segregation of fine particles from the mixture. For a square cross-section riser, radial segregation may not be ignored for mixtures with a significant size difference as there are more coarse particles near the wall and less in the center, while an intermediate particle density is found at the corner.
Please cite this article in press as: Du, M., & Wang, S. Investigation of the segregation of a binary particle mixture in a square circulating fluidized bed with air staging. Particuology (2019), https://doi.org/10.1016/j.partic.2019.07.002
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Fig. 7. Contour plots of the simulated solid concentration at different heights.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51606053) and the China Postdoctoral Science Foundation Funded Project (2016T90285). References Babu, M. P., Sai, P. S. T., & Krishnaiah, K. (2017). Continuous segregation of binary heterogeneous solids in fluidized beds. Particuology, 35, 93–100. Bai, D., Nakagawa, N., Shibuya, E., Kinoshita, H., & Kato, K. (1994). Axial distribution of solids holdups in binary solids circulating fluidized beds. Journal of Chemical Engineering of Japan, 27, 271–275. Bi, H., Jiang, P., Jean, R. H., & Fan, L. S. (1992). Coarse-particle effects in a multisolid circulating fluidized bed for catalytic reactions. Chemical Engineering Science, 47(12), 3113–3124. Chao, Z., Wang, Y., Jakobsen, J. P., Fernandino, M., & Jakobsen, H. A. (2012). Multi-fluid modeling of density segregation in a dense binary fluidized bed. Particuology, 10(1), 62–71. Chew, J. W., Hays, R., Findlay, J. G., Karri, S. R., Knowlton, T. M., Cocco, R. A., et al. (2011). Species segregation of binary mixtures and a continuous size distribution of group B particles in riser flow. Chemical Engineering Science, 66(20), 4595–4604. Das, M., Banerjee, M., & Saha, R. K. (2007). Segregation and mixing effects in the riser of a circulating fluidized bed. Powder Technology, 178(3), 179–186. Das, M., Meikap, B. C., & Saha, R. K. (2008). Characteristics of axial and radial segregation of single and mixed particle system based on terminal settling velocity
in the riser of a circulating fluidized bed. Chemical Engineering Journal, 145(1), 32–43. Feng, Y. Q., & Yu, A. B. (2007). Microdynamic modelling and analysis of the mixing and segregation of binary mixtures of particles in gas fluidization. Chemical Engineering Science, 62(1-2), 256–268. Jang, H. T., Park, T. S., & Cha, W. S. (2010). Mixing–segregation phenomena of binary system in a fluidized bed. Journal of Industrial and Engineering Chemistry, 16(3), 390–394. Kiani, M., Rahimi, M. R., Hosseini, S. H., & Ahmadi, G. (2017). Mixing and segregation of solid particles in a conical spouted bed: Effect of particle size and density. Particuology, 32, 132–140. Köhler, A., Rasch, A., Pallarès, D., & Johnsson, F. (2017). Experimental characterization of axial fuel mixing in fluidized beds by magnetic particle tracking. Powder Technology, 316, 492–499. Koksal, M., & Hamdullahpur, F. (2004). Gas mixing in circulating fluidized beds with secondary air injection. Chemical Engineering Research and Design, 82(8), 979–992. Koksal, M., Golriz, M. R., & Hamdullahpur, F. (2008). Effect of staged air on heat transfer in circulating fluidized beds. Applied Thermal Engineering, 28(8–9), 1008–1014. Nakagawa, N., Bai, D., Shibuya, E., Kinoshita, H., Takarada, T., & Kato, K. (1994). Segregation of particles in binary solids circulating fluidized beds. Journal of Chemical Engineering of Japan, 27(2), 194–198. Nienow, A. W., Rowe, P. N., & Cheung, L. L. (1978). A quantitative analysis of the mixing of two segregating powders of different density in a gas-fluidised bed. Powder Technology, 20(1), 89–97. Salatino, P., & Solimene, R. (2017). Mixing and segregation in fluidized bed thermochemical conversion of biomass. Powder Technology, 316, 29–40.
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Shoushtari, N. A., Hosseini, S. A., & Soleimani, R. (2013). Investigation of segregation of large particles in a pressurized fluidized bed with a high velocity gas: A discrete particle simulation. Powder Technology, 246, 398–412. Wang, X., Jin, B., Wang, Y., & Hu, C. (2015). Three-dimensional multi-phase simulation of the mixing and segregation of binary particle mixtures in a two-jet spout fluidized bed. Particuology, 22, 185–193.
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Wang, Z., Sun, S., Chen, H., Deng, Q., Zhao, G., & Wu, S. (2009). Experimental investigation on flow asymmetry in solid entrance region of a square circulating fluidized bed. Particuology, 7(6), 483–490. Wang, S., Yin, W., Liu, S., & Song, X. (2019). Numerical studies of mass transfer performance in fluidized beds of binary mixture. Applied Thermal Engineering, 158, 113465.
Please cite this article in press as: Du, M., & Wang, S. Investigation of the segregation of a binary particle mixture in a square circulating fluidized bed with air staging. Particuology (2019), https://doi.org/10.1016/j.partic.2019.07.002
ARTICLE IN PRESS
PARTIC-1273; No. of Pages 7
Particuology xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Investigation of the segregation of a binary particle mixture in a square circulating fluidized bed with air staging Mingkun Du a,∗ , Shuai Wang b,∗ a b
Guangdong Institute of Special Equipment Inspection and Research, Zhuhai Inspection Institute, Zhuhai 519002, China School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 14 March 2017 Received in revised form 7 September 2017 Accepted 8 July 2019 Available online xxx Keywords: Circulating fluidized bed Binary mixture Segregation Air staging
a b s t r a c t To further understand the segregation characteristics of a binary particle mixture in a riser, the flow behaviors of particles with different sizes were experimentally and numerically investigated in a cold square circulating fluidized bed. The impact of the gas velocity, solid circulation rate, initial coarse particle fraction, and air staging on the coarse particle fraction and axial pressure drop were examined. The experimental results show that air staging can significantly promote the pressure drop of a binary mixture at the bottom of the bed compared with that of monosized particles. Meanwhile, the segregation of the binary mixture at the bottom is hindered. It was also found that there was clear radial segregation owing to a higher coarse particle fraction near the wall compared with the bed’s center and its corner. © 2019 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction During coal combustion and gasification processes in a circulating fluidized bed (CFB), particles generally have a broad size distribution. The mixing and segregation performance of particles plays a crucial role on bed expansion, the chemical reaction, and the heat and mass transfer rates (Bi, Jiang, Jean, & Fan, 1992; Feng & Yu, 2007). Owing to the complex interactions between coarse and fine particles, it is challenging to obtain a deep insight into the flow behaviors of binary mixtures that are essential for designing and scaling-up fluidized systems (Chew et al., 2011). In a fluidized system with a binary mixture, the fluidizing gas preferably elutriates lighter and finer particles, while coarser and denser particles tend to sink and remain at lower levels. The difference in size or density will lead to particle segregation. A large number of researchers have investigated the segregation behavior of binary mixtures using various methods (Chao, Wang, Jakobsen, Fernandino, & Jakobsen, 2012; MShoushtari, Hosseini, & Soleimani, 2013; Wang, Yin, Liu, & Song, 2019; Kiani, Rahimi, Hosseini, & Ahmadi, 2017). The terms “flotsam” and “jetsam” were introduced to describe the solid components that float to the top and those
∗ Corresponding authors. E-mail addresses: dumk [email protected] (M. Du), [email protected], [email protected] (S. Wang).
that ultimately sink, respectively (Nienow, Rowe, & Cheung, 1978). Segregation intensity was proposed as a property to characterize the normalized difference between the local and mean volume fractions of coarse particles (Nakagawa et al., 1994). Jang, Park, and Cha (2010) analyzed the residence time in a fluidized bed for particles with a binary density and pointed out that the degree of mixing in this system with its different solid densities depended on the residence time distribution. Babu, Sai, and Krishnaiah (2017) developed the empirical correlation for predicting the separation factor and product quality by means of conducting continuous segregation of a binary heterogeneous solids mixture. Segregation behaviors include axial segregation and lateral segregation. Axial segregation reduces the mean particle size along the riser height, while lateral segregation results in larger particles, which are preferentially found in the down-flow near the wall of the fluidized bed. Das, Banerjee, and Saha (2007) examined the influence of the operating conditions on axial segregation and evaluated the degree of segregation. They concluded that increasing the superficial gas velocity can decrease segregation effects. A year later, Das, Meikap, and Saha (2008) found that there was a significant difference in segregation between the radial and axial flow directions: axial segregation was more obvious than lateral segregation. Köhler, Rasch, Pallarès, and Johnsson (2017) studied axial fuel mixing and segregation in fluidized beds on the basis of a magnetic particle tracking system and identified three fuel segregation
https://doi.org/10.1016/j.partic.2019.07.002 1674-2001/© 2019 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Du, M., & Wang, S. Investigation of the segregation of a binary particle mixture in a square circulating fluidized bed with air staging. Particuology (2019), https://doi.org/10.1016/j.partic.2019.07.002
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2 Table 1 Properties of the bed materials. Material
dp (m)
(kg/m3 )
Umf (m/s)
Ut (m/s)
Geldart type
Fine Coarse
257 2432
2550 2550
0.054 1.241
1.57 12.34
B D
regimes. It has also been pointed out that the bed height and the density of tracer particles determines the transition velocities in each of those regimes. Salatino and Solimene (2017) analyzed the axial and lateral segregation of fuel particles during devolatilization in fluidized bed reactors. It was emphasized that the lateral dispersion of solid particles depends on scale effects; the dispersion coefficient in a large-scale system was much greater than it was in a small-scale system. However, there are few reports on the segregation of binary mixtures under the condition of air staging. Based on a study by Koksal, Golriz, and Hamdullahpur (2008), air staging has a significant impact on heat transfer in a CFB. The present study aims to investigate the segregation behaviors of mixtures of solids that differ significantly in size in a square CFB system with air staging. The effects of the operating conditions including the operating gas velocity, the circulation rates of the solids, the initial coarse particle fraction, and air staging on the segregation characteristics of a binary mixture were investigated; our results are beneficial for understanding the interaction mechanics of binary mixtures and for optimizing CFB systems. Experimental This work was conducted on the basis of an experimental setup consisting of a circulating fluidized bed made of poly(methyl methacrylate). It consisted of a square riser, two stage cyclones, a down comer, and a loop seal, as shown in Fig. 1. The riser had a cross-sectional area of 0.25 m × 0.25 m and a height of 6.07 m. The air from a root blower was divided into primary air, secondary air, and air for the U-type loop seal. The primary air entered the riser section of the CFB through a multi-hole distributor plate. The secondary air was injected into the riser via nozzles (0.025 m in diameter), which were located at a height of 0.832 m. A smooth exit was connected with two stage cyclones to collect particles. The circulating particles were transferred back to the riser from the downer through a U-type loop seal. The solid circulation rate Gs was adjusted with the use of a stop-watch and by shutting the flapper valve quickly in the return line; Gs was also changed by adjusting the loosing air and solid inventory. The axial gas pressure was measured using pressure taps connected to water manometers. Samples of the solids were withdrawn by sampling probes that were each made of a stainless steel tube that was bent at the end in the vertical direction, which allowed sampling either vertically upward near the wall or vertically downward at other locations. The samples of the solids were transported to sampling vessels via vacuum pump suction for detailed analysis. The flow of the particles was independent of the air sampling rate as a result of particle inertia. To eliminate the electrostatic effect, plastic films were employed inside the fluidized bed reactors. Additionally, the relative humidity and the temperature of the fluidizing gas were monitored during the experiments and adjusted using a packed water column and a dryer. The experiments were conducted with two kinds of particles composed of quartz sand that had different sizes. The mean particle diameters were 257 and 2432 m. Both had solid densities of 2550 kg/m3 . Their minimum fluidizing velocities and terminal velocities are listed in Table 1. The particle compositions for the binary mixture are shown in terms of their mass fraction of the
Fig. 1. Schematic diagram of the experimental setup.
coarse component, which was determined through measuring the sampled particle density. By adding coarse particles into fine ones, the initial coarse particle mass fraction Xc0 was varied from 0 to 0.5. The superficial gas velocity U0 changed from 3.4 to 4.6 m/s and Gs varied between 8 and 21 kg/(m2 s). The secondary air ratio varied from 0 to 0.3. Results and discussion The overall flow behavior can be studied by measuring the axial pressure profiles. Fig. 2 shows the effect of the solid circulation rate Gs on the pressure drop along the riser. At a constant gas velocity U0 and initial coarse particle fraction Xc0 , the pressure drop at the bottom increases along with Gs . This is because there is a higher solid concentration at a higher solid flux. Additionally, the increased collisions between particles will lead to greater accumulation of particles at the bottom. When air is supplied by staging, a clear increase of the pressure drop at the bottom is observed, as shown in Fig. 2, which is attributed to the fact that the amount of air introduced from the bottom of the riser decreases with the injection of secondary air
Please cite this article in press as: Du, M., & Wang, S. Investigation of the segregation of a binary particle mixture in a square circulating fluidized bed with air staging. Particuology (2019), https://doi.org/10.1016/j.partic.2019.07.002
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Fig. 2. Profiles of the pressure drop along the riser with an initial coarse particle fraction Xc0 , solid circulation rate Gs , and air staging SAR.
when the total amount of air is kept constant. This will result in a decrease of the solid carrying capacity in the primary region. Hence, more solid inventories in the primary region are required to maintain a constant Gs . Koksal and Hamdullahpur (2004) analyzed the effect of secondary air injection on the solid distribution in a circulating fluidized bed using monosized bed material. It was found that secondary air injection can considerably increase the solid carrying capacity, which was inconsistent with our observation. Compared with the results for monosized particles, the bed pressure drop is greatly affected when adding coarse particles into fine ones. When Xc0 is changed from 0.0 to 0.25 under the air staging condition, the
increases in the pressure drop at the bottom of the riser become significant. Bi et al. (1992) and Bai, Nakagawa, Shibuya, Kinoshita, and Kato (1994) pointed out that fine particles move faster owing to the increasing interstitial gas velocity resulting from the decrease in the effective cross-sectional area of flow. However, fine particles slow down in the presence of coarse particles owing to the promotion of interactions between the fine and coarse particles; this results in considerable back-mixing and down-flow in the wall region. In this work, there is a clear difference in size between the coarse and fine particles, which leads to increased interactions between the particles, thereby causing the fine particles to slow down.
Fig. 3. Profiles of the coarse particle fraction along the riser with gas velocity U0 and initial coarse particle fraction Xc0 .
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Fig. 4. Profiles of the coarse particle fraction along the riser with air staging SAR and solid circulation rate Gs .
A study by Wang et al. (2009) showed that increasing the original coarse particle fraction causes a higher fraction of coarse particles to be present in the external circulating particles. Here, the variation of the fraction of coarse particles along the height of the riser was also investigated to understand the axial segregation behaviors of the particles, as show in Fig. 3. A partial segregation pattern was found under all operating conditions owing to the gradual decrease of the coarse particle fraction with increasing height. Coarse particles can be entrained by the stream of gas and the fine particles, while the movement of the fine particles is hampered by coarse particles
because of collisions and aggregation, although they are preferably elutriated by the gas. The exact value of the coarse particle fraction depends on the equilibrium state reached for the above-mentioned effects. The effect of U0 is apparent from Fig. 3, which shows data from separate experiments that were conducted for a gas velocity from 3.4 to 4.6 m/s at a constant Gs . A larger coarse particle fraction can be observed at higher gas velocities. To maintain a constant solid circulation rate, a lower solid inventory is required, which will result in a weakening of collisions between coarse and fine parti-
Fig. 5. Profiles of the coarse particle fraction in the center, near the rear wall, and in the corner.
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Fig. 6. Comparison of the simulated prediction and the experimental data.
cles. Meanwhile, the carrying capacity of the gas will be improved. Fine particles are more easily separated from the mixture and flow out of the riser, which results in an increase of the coarse particle fraction. Fig. 3 also demonstrates the effect of the initial coarse particle fraction Xc0 on the coarse particle fraction. As Xc0 is increased, the coarse particle fraction along the riser increases, with a greater fraction at the bottom. Coarse particles tend to aggregate in the bottom section as Xc0 increases owing to the weak entrainment of coarse particles by the gas. In contrast, fine particles are more easily entrained by the gas. Simultaneously, coarse particles in the upper section fall down owing to their higher terminal velocity. Therefore, it can be observed that particle segregation becomes more severe when Xc0 is increased. The profiles of the coarse particle fraction along the riser with and without air staging are plotted in Fig. 4. Without air staging, the fine particles tend to be more easily elutriated by a high primary gas velocity than the coarse particles. After air staging, the coarse particle fraction decreases obviously at the bottom owing to the fine particle flow being hindered by a low primary gas velocity and the interactions between the coarse and fine particles. Fig. 4 also shows the effect of Gs on the coarse particle fraction. At a lower solid circulation rate, there is a significant discrepancy between the coarse particles in the bottom and the upper section. As the solid circulation rate increases, the difference decreases. This indicates that a good mixing of the coarse and fine particles can be achieved by increasing the solid circulation rate. The coarse particle fractions in the center, near the rear wall and in the corner at three different heights are displayed in Fig. 5. We found that there was a high coarse particle fraction at the wall. This is attributed to the core–annuls structure in a CFB. Coarse particles in the core are segregated from the up-flow to the down-flow in the annulus, while fine particles in the annulus are preferentially stripped from the down-flow to the up-flow in the core, which results in a low coarse particle fraction in the center. In contrast, the coarse particle fraction in the corner is lower than that at the wall. This is attributed to the lower gas velocity in the corner restricting the fine particles that are stripped from the down-flow.
To gain a better insight into the radial distribution of the binary mixture, a three dimensional simulation was conducted by means of a computational fluid dynamic approach. A multi-fluid model was employed with the kinetic theory of granular mixture (Wang, Jin, Wang, & Hu, 2015). To validate the feasibility of the model, a comparison of the predicted results and the experimental data was conducted as shown in Fig. 6. It can be observed that the model provides a fair prediction of the measured data. Fig. 7 demonstrates the contour plots of the simulated solid concentration at different heights. We found that there was an obvious zone with a high particle density near the wall, especially at the bottom. Additionally, the concentration of large particles was higher in the bottom zone. When the height was increased and the secondary air was injected, both solid concentrations decreased, although there was still a lateral discrepancy of the solid concentrations. At the upper section of the riser, the solid concentration was very low. Meanwhile, the lateral discrepancy was significantly weaker.
Conclusions The segregation behavior of a binary mixture of different particles in a square circulating fluidized bed was investigated experimentally. The influence of the operating parameters including the gas velocity, solid circulation rate, initial coarse particle fraction, and air staging on the pressure drop and coarse particle fraction was evaluated. It was found that there was an obvious increase of the coarse particle fraction in the bottom section when the initial coarse particle fraction was increased. Compared with mono-sized particles, air staging can greatly promote the pressure drop of a binary mixture at the bottom. The air staging and the reduction of the gas velocity hinder the segregation of fine particles from the mixture. For a square cross-section riser, radial segregation may not be ignored for mixtures with a significant size difference as there are more coarse particles near the wall and less in the center, while an intermediate particle density is found at the corner.
Please cite this article in press as: Du, M., & Wang, S. Investigation of the segregation of a binary particle mixture in a square circulating fluidized bed with air staging. Particuology (2019), https://doi.org/10.1016/j.partic.2019.07.002
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Fig. 7. Contour plots of the simulated solid concentration at different heights.
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