Characteristics of Choking Behavior in Circulating Fluidized Beds for

The experimental unit consists of a 0.1-m i.d. riser with a height of 6.32 m, .... Three different initial bed heights, i.e., H0 of 0.6, 0.82, and 1.2...
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Ind. Eng. Chem. Res. 2004, 43, 5507-5520

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Characteristics of Choking Behavior in Circulating Fluidized Beds for Group B Particles Bing Du and Liang-Shih Fan* Department of Chemical Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, Ohio 43210

The electrical capacitance tomography (ECT) based on the neural network multi-criteria optimization image reconstruction technique (NNMOIRT) developed by this research group is used to study the choking phenomenon for Geldart Group B particles in a circulating fluidized bed (CFB). The Group B particles employed are sand particles with a mean diameter of 240 µm and a particle density of 2200 kg/m2. The ECT examines the real-time, quasi-3D cross-sectional flow structure of a gas-solid circulating fluidized bed. The ECT with NNMOIRT reveals a double solids-ring flow structure at a low solids circulation rate and the presence of solids blobs at the center of the bed at a high solids circulation rate before choking in a CFB. This flow structure undergoes a distinct variation during the choking transition forming wall slugs/gas intervals when the gas velocity is below the transport velocity, Utr, or forming open slugs with blobs/ particles at the center when the gas velocity is above Utr. The standard deviation of the solids concentration fluctuation and the cross-sectional time-averaged solids concentration variation with increasing solids circulation rates exhibit a sharp change during the choking transition for a low gas velocity ( 0.35

(1)

The incipient fast fluidization boundary or Type A choking boundary was, thus, coincident with the Type C choking boundary, which was used by Yang6 to illustrate the misinterpretation of different types of choking behavior in the literature. Xu et al.8 studied Type A choking initiated by the collapse of the dilute suspension for both Group A and B particles based on the pressure drop measurement. The differential pressure drop and the corresponding solids concentration at the choking point were reported to be independent of the gas velocity and the riser diameter, but varied with particle properties such as particle diameter and density. However, little distinction is made of the choking phenomenon between Group A and B particles. The choking transition for Group B particles in circulating fluidized beds has been analyzed primarily on the basis of pure-empirical or semiempirical relationship. Progress on the comprehensive mechanistic modeling of the choking transition has been, however, hampered by the lack of the fundamental understanding of the choking phenomenon. Capes and Nakamura9 noted that the slip velocity at the onset of choking could be greater than the terminal velocity of a single particle

10.1021/ie0499613 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/20/2004

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due to the particle-wall friction and particle recirculation effects. They reported that choking for Group B particles occurs when the particles begin to recirculate. Yousfi and Gau10 studied the choking phenomenon in gas-solid suspension systems for both Group A and B particles. They indicated that for large particles with the particle Froude number equal to or larger than 140, choking occurred when slugs were formed. An empirical correlation, as given below, from the stability analysis was proposed to predict the gas velocity at choking with slug formation:

(Ug)c

Re0.06 t 0.5

(gdp)

(

) 32

Gs,c

Fg(Ug)c

)

0.28

,

U2t (Fr)dp ) g 140 gdp (2)

Yang11 studied the choking phenomenon in vertical pneumatic conveying systems for Group B particles. For the choking transition with or without slug formation, he developed the following equation to describe the gas velocity and voidage at choking by introducing a correlation for the solids friction factor:

- 1) 2g(-4.7 c [(Ug)c - Ut]

2

) 6.81 × 105

() Fg Fp

2.20

(3)

Matsen4 attributed the initiation of choking in vertical pneumatic systems to the formation of clusters and it depended only on the dilute phase slip-voidage behavior. Satija et al.12 studied the choking phenomenon in vertical pneumatic conveying systems for both Group A and B particles on the basis of characteristics of pressure fluctuations and bed voidage. Four kinds of particles are classified into choking and nonchoking types of particles. The sand particles (Group B) and the FCC particles (Group A), so-called choking types of particles, exhibited the choking transition with the sharp change in the bed voidage and the formation of slugging. However, no slugging was observed up to the transition to the solids-batch mode operation for the glass beads (Group A). Mok et al.13 observed two dense phase regimes in a gas-solid transport system with Group B particles based on the average solids concentration and velocity measurements. The transition from the fast fluidization regime to the dense-phase fluidization regime was characterized by the occurrence of the “diffused slugs” and extensive internal solids recirculation, corresponding to the initiation of choking. By matching five similarity dimensionless groups with different gas mixtures and solid particles of various sizes and densities, Chang and Louge14 studied the choking phenomenon for Group B particles for CFB risers up to 1 m in diameter, based on the experiments conducted in a 0.2-m i.d. riser. They reported that the beds with large-diameter risers experienced a gradual collapse of suspension. The onset of choking took place initially at the bottom of the riser and eventually filled the entire riser.14 Bi and Fan15 developed a correlation to describe the boundary between the fast fluidization regime and the dense-phase fluidization regime for both Group A and B particles as given by

Utf

xgdp

) 39.8

( ) Gs FgUtf

0.311

Re-0.078 t

(4)

For Group B particles, the initiation of choking in circulating fluidized beds is attributed to the formation of slugs by most of the reported results. Some criteria were given to identify the choking or slugging systems based on the maximum stable bubble size concept.6,7,16 Experimentally, the slugging behavior is identified through visualization from the outside of the bed. Although initiation of the choking can be reflected on the pressure-drop profile or the related analysis on the time-series pressure-drop signals, little is known regarding the mechanics of the choking transition for Group B particles based on internal flow structure variations. Recent work by Du et al.,17 based on the electrical capacitance tomography (ECT) measurements with the neural network multi-criteria optimization image reconstruction technique (NNMOIRT) developed by Warsito and Fan,18,19 obtained the time-averaged and real-time field images of the gas-solid fluidized bed and revealed the flow structure variation during the choking transition for Group A particles in circulating fluidized bed systems. They reported that the disintegration of blobs and the collapse of the solids suspension characterize the mechanism of the initiation of the choking transition due to the instability of solids suspension. However, the fundamental nature of the occurrence of choking and its underlying mechanism for Group B particles still remain unclear. The objective of this work is, thus, to characterize the dynamic behavior of a gassolid circulating fluidized bed with Group B particles using electrical capacitance tomography (ECT), thereby, the internal flow behavior and the mechanism underlying the choking behavior for Group B particles can be revealed. Experimental Studies Figure 1 shows the schematic diagram of the circulating fluidized bed together with the ECT system used in this study. The experimental unit consists of a 0.1-m i.d. riser with a height of 6.32 m, a separator and secondary cyclone system, a large-volume particle storage hopper, a 0.15-m i.d. downcomer, and an L-valve. Particles are carried upward in the riser and exit at the top through a right-angled bend into a horizontal tube connected to the separator and secondary cyclone where the particles are separated from the gas. Subsequently, the particles are fed back to the bottom of the riser through the nonmechanical L-valve. The sand particles with a mean diameter of 240 µm and particle density of 2200 kg/m3 are employed as the solids materials in the bed. The particle size distribution is shown in Table 1. The solids circulation rate, which is controlled by adjusting the aeration rate at the injection points of the L-valve, is measured by timing the falling distance of tracer particles in the standpipe. The air humidity is controlled by the water level and water temperature in the humidifier to minimize the electrostatic effect. Differential pressure transducers are used to measure the pressure drop and the overall voidage of the fluidized bed. The local solids holdup is measured by the optical fiber probe.20 The electrical capacitance tomography (ECT) system consists of a capacitance sensor, data acquisition system, and computer system for image reconstruction, interpretation, and display. The capacitance sensor array consists of a twin-plane sensor (planes 1 and 2) and two guard sensors using 12 electrodes for each plane attached to the outside of the column wall. The length of

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Figure 1. Schematic diagram of the circulating fluidized bed and the ECT system. Table 1. Size Distribution of the Bed Materials (Sand Particles) size (µm)

percentage

151.3 166.8 183.9 202.8 223.6 246.6 271.9 299.8 330.6

5.86 7.36 8.88 10.35 11.71 12.92 13.84 14.43 14.66

each electrode is 0.05 m. The two guard sensor planes, which adjust the electrical field within the sensoring area, are located above and below the measuring sensor planes. The experiments with ECT measurements are conducted at both the lower portion and the upper portion of the riser. As shown in Figure 1, the two measuring planes (planes 1 and 2) are located at 0.45 and 0.5 m above the distributor at the lower part of the riser. For the upper part, the locations are 4.45 and 4.5 m above the distributor. There are 66 combinations of independent capacitance measurements between electrode pairs from 12 electrodes. The data acquisition system manufactured by Process Tomography Limited (UK) is employed for capturing the ECT images at 100 frames per second. A novel reconstruction algorithm based on an analogue neural network multi-criteria optimization image reconstruction technique (NNM-

OIRT) developed earlier by the authors19 is applied to achieve the image reconstruction. The technique transforms capacitance data into cross-sectional images of the gas-solid two-phase flow at 32 × 32 pixels per image. The technique has been verified and applied to the study of the flow dynamics of multiphase flow systems including gas-solid fluidized beds,17,20,21 gas-liquid bubble columns, and three-phase fluidized beds.18,22,23 The details of this technique, the image reconstruction algorithm, its comparisons with other algorithms, and the solids concentration measurements in a gas-solid fluidized bed are described elsewhere.19,20 Results and Discussion The study on the choking phenomenon in circulating fluidized beds for Group A particles, reported earlier by Du et al.,17 revealed that the flow behavior during the choking transition is different at gas velocities smaller than the transport velocity to the fast fluidization regime, Utr, compared to that at the gas velocities larger than Utr. Experiments for Group B particles conducted in this study are, therefore, also carried out at gas velocities smaller than Utr and larger than Utr. The transport velocity, Utr, is predicted to be 2.7 m/s for the Group B particles employed in this work based on the empirical equation proposed by Bi and Fan.24

Retr ) 2.28Ar0.419

(5)

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Figure 2. Quasi-3D flow structures at the lower part of a CFB at Ug ) 2.4 m/s.

Dynamic Flow Behavior of a CFB with Group B Particles. Figure 2 shows the quasi-3-dimensional solids concentration distribution by stacking 200 tomographic images obtained in 4 s in plane 1 of the ECT at the lower part of the riser under different solids circulation rates at a gas velocity of 2.4 m/s (< Utr). The socalled quasi-3-dimensional flow structure refers to the reconstructed results obtained by the ECT technique based on NNMOIRT, which is averaged over the 0.05m-high sensor plane at one time. The images of the two sliced sections for each solids circulation rate represent the solids concentration distributions in the X-Z and Y-Z directions. The Z direction refers to the variation of time. A color bar, varying from blue to red, under the two images in the X-Z and Y-Z directions for each solids circulation rate represents the variation of solids concentration from low (empty bed) to high. The value of the high solids concentration in the color bar varies with the solids circulation rate to clearly reflect the flow structure in a riser. Figure 2 (a) depicts the flow structure of a CFB for Group B particles at a low solids circulation rate of 8.57 kg/m2s. As shown in the figure, especially in the Y-Z plane, the bed exhibits three regions over the riser cross-section. A solids ring with the highest solids concentration across the bed section is formed in the wall region. These particles move downward from visual observation during the experiments. In the central region, there exists a dilute small gas core represented by the blue color in Figure 2 (a), surrounded by another solids ring. The solids concentration in this solids ring is much lower than that in the wall region. This flow structure is, thus, also called the double solids-ring flow structure. Between the central region and the wall region, there is a gas ring region with a very low solids concentration. It is also observed that particles interchange continuously between the central and the wall regions, which make the boundaries of the gas ring region obscure. With an increase in the solids circulation rate to 11.43 kg/m2s, the solids concentration in the column especially near the wall region, as shown in Figure 2 (b), increases

compared to that in Figure 2 (a). The bed exhibits the three-region flow structure over the cross section, similar to the low solids circulation rate situation. However, in the central region of the bed, the gas core disappears and particles tend to aggregate to form blobs under the operating condition with a solids circulation rate of 11.43 kg/m2s. It is observed that the area of the central region shrinks compared to that at a low solids circulation rate as shown in Figure 2 (a) due to the effect of particle aggregation. The size, shape, and concentration of the blobs vary significantly with time. It is observed that the blobs do not move upward straight but spiral from one side to another in the bed. The solids concentration in the wall region is much higher than that in the central region. Solids exchange occurs between the central and the wall regions. Sometimes, the solids ring in the wall region is connected with the solids blobs in the central region of the bed. The blobs may connect with each other forming blob jets in the column; the behavior is transient between formation and breakage for the blob. When the solids circulation rate further increases to 14.86 kg/m2s, the flow behavior is shown in Figure 3 together with the real-time topview flow structure over the bed cross section. The flow structure at the top of Figure 3 shows a flow behavior similar to that in Figure 2 (b). It is observed, however, that, after a short period, the continuous solids blob flow disappears and is replaced by the intermittent solids blobs with high solids concentrations. Sometimes the solids ring in the wall region extends to the inner part of the bed and connects to the blobs in the central region. During this process, the particles/clusters in the wall region are combined with the blobs in one side of the central region, which blocks the upward moving gas in this side and pushes the gas to the other side of the central region with a relatively low solids concentration. The formation and the breakage of the particles/clusters and the blobs due to the violent movement of the gas phase are in a dynamic process. Ultimately, particles/ clusters are connected to the blobs to form solids plugs of a high solids concentration, as shown in the X-Z

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Figure 3. Flow structure variation during choking transition at Ug ) 2.4 m/s.

plane of Figure 3. Following the solids plug, a large wall slug with an irregular shape, i.e., “diffused slug” as noted by Mok et al.,13 is observed. The whole bed shakes violently during the formation of wall slugs, based on the visual observation of the column. This operating condition is noted as the choking transition to densephase fluidization and corresponds to Type C choking as defined by Bi et al.2 The transient process lasts for around 2 s until the slugging fluidization regime is reached. After the transition, the bed behaves as slugging fluidization and the wall slugs or half slugs, surrounded by the particles/clusters, are present. The bed varies from the three-region flow structure at low solids circulation rates noted above to the typical tworegion flow structure, the central region and the wall region, in the dense-phase fluidization regime at high solids circulation rates. As the solids circulation rate further increases to 25.72 kg/m2s as shown in Figure 2 (c), which corresponds to the dense-phase fluidization or slugging fluidization regime in the gas-solid vertical conveying systems after the choking transition, the flow pattern of the bed contains not only the wall slugs surrounded by particles/clusters but also the “gas intervals” as shown in Figure 2 (c). The gas interval, covering almost the bed diameter, is more like a gas plug with a small amount of particles even in the wall region. The gas interval usually follows a particle plug with a high solids concentration. It is observed that the flow behavior in the gas interval exhibits the double solids-ring structure, similar to that at a lower solids circulation rate as shown in Figure 2 (a). Under this operating condition, the gas interval, as shown in Figure 2 (c), can last for about 0.6 s. Mok et al.13 reported a similar flow structure in the dense-phase fluidization regime based on visual observation from the outside of the column. The flow structure does not change much with further increase in the solids circulation rate to 35.72 kg/m2s as shown in Figure 2 (d). It is observed that some slugs are broken up into smaller ones with more particles in the system. The gas interval is less distinctive than that at a lower solids circulation rate

of 25.72 kg/m2s. Compared to the solids concentration distribution at a lower solids circulation rate as shown in Figure 2 (c), the solids concentration in the wall region as shown in Figure 2 (d) increases largely but that in the central region does not change much. To compare the flow behavior in the dense-phase fluidization regime of a gas-solid circulating fluidized bed with that of a solids-batch dense-phase fluidized bed, a set of experiments was conducted using the same CFB system with different initial bed heights under varied gas velocities up to 2.3 m/s. Beyond this gas velocity, all particles were blown out of the bed in a short time. Figure 4 shows the quasi-3D solids concentration distributions in a solids-batch dense-phase fluidized bed at a gas velocity of 2.1 m/s; the results are compared with those in a solids circulation system at a gas velocity of 2.4 m/s. Three different initial bed heights, i.e., H0 of 0.6, 0.82, and 1.2 m, are chosen for comparisons as shown in Figure 4 (a), (b), and (c), respectively. It can be seen that the effect of the initial bed height on the flow structure of a solids-batch densephase fluidized bed is insignificant. The bed contains the slugs and/or the gas intervals surrounded by the solids plugs for all three conditions. With an increase in the initial bed height, the flow structure, including slugs and gas intervals, becomes more distinctive. The solids-batch dense-phase fluidized bed exhibits a tworegion (the central and the wall regions) flow structure. It is similar to the flow structure of the solids circulation system shown in Figure 2 (c) and (d), especially under the situations with higher initial bed heights shown in Figure 4 (c). Compared to the solids circulation system, more particles exist at the central region of the bed and the size of slugs becomes smaller in the solids-batch dense-phase fluidized bed. It is observed that the solids concentration at the center of the gas intervals increases, resembling the solids blobs shown in Figure 2 (b). In the noncirculation system, most of the particles are recirculated in the column in both the large scale and the small scale. This recirculation generates more turbulence for particles to transport from side to side

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Figure 4. Quasi-3D flow structures of a solids-batch dense-phase fluidized bed at Ug ) 2.1 m/s.

Figure 5. Quasi-3D flow structures at the low part of a CFB at Ug ) 3.4 m/s.

across the bed, which results in the presence of more particles in the central region of the bed and the breakage of large slugs into small ones. Figure 5 illustrates the quasi-3D solids concentration distribution at the lower part of the riser under different solids circulation rates at a higher gas velocity of 3.4 m/s (>Utr). At a lower solids circulation rate of 28.58 kg/m2s, the bed exhibits the double solids-ring flow structure as shown in Figure 5 (a). The solids concentrations of the two solids rings in the central region and the wall region of the bed are comparable. The bound-

aries of the gas ring between the central and the wall regions are indistinct, which indicates that the exchange of particles between the central region and the wall region is intensive. With an increase in the solids circulation rate to 45.72 kg/m2s as shown in Figure 5 (b), the solids concentration near the wall increases greatly. The gas core disappears and the blobs with different sizes and shapes are formed in the central region, which is similar to that under the operating condition with a gas velocity of 2.4 m/s and a solids circulation rate of 11.43 kg/m2s as shown in Figure 2

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(b). The solids concentration in the central region is much lower than that in the wall region. It is also observed that the blobs in the central region connect with each other to form the blob jets moving upward together in the riser. The boundaries of the gas ring region become relatively clearer, although the particle exchange still occurs frequently. Figure 5 (c) depicts the flow structure of a CFB as the solids circulation rate increases to 57.15 kg/m2s. At the beginning, the bed exhibits a three-region flow structure with blobs in the central region, solids ring in the wall region, and gas ring between them. With an increase in the solids circulation rate, the solids concentration in the wall region increases greatly and the solids ring in the wall region begins to periodically expand to the inner part of the bed. During the period when the wall region expands the most, the particles in the wall region are connected to the particles/blobs in the central region. This connection undergoes a dynamic variation between the formation and the breakage. After a short period as shown at the top of Figure 5 (c), the tendency of the connection becomes stronger and the solids concentration in the central region surrounded by the expanded solids ring in the wall region becomes low. A flow structure, similar to a slug but with open upper and lower boundaries, is observed between two periods when the solids ring in the wall region expands to the central region, as shown in Y-Z plane of Figure 5 (c). This sluglike flow structure is noted as open slug with particles/ blobs existing inside. Although the structure of the open slugs is different from that of the wall slugs formed at the lower gas velocity as shown in Figure 3, this transition to dense-phase fluidization can also be regarded as the choking transition due to the formation of the open slugs. After the choking transition, the bed exhibits the two-region flow structure over the cross section of the bed with open slugs in the central region and particles/clusters in the wall region. Compared to the wall slug at the lower gas velocity, the formation of the open slug can be attributed to the high momentum of the gas movement. Due to the high solids concentration in the wall region, the gas velocity in the central region is much higher. This violent movement of gas hampers the connection of the solids ring in the wall region with the blobs in the central region, and thus prevents the formation of solids plugs and the formation of the wall slugs. However, the difference of the flow behavior between open slugs and wall slugs cannot be identified on the basis of visual observation from the outside of the column due to the high solids concentration in the wall region. This finding explained why Mok et al.13 was still able to observe the “dispersed slugs” in the dense-phase fluidization regime for Group B particles at gas velocities higher than Utr. The probe measurement cannot provide the accurate radial solids concentration profiles due to much lower solids concentrations in the central region of the bed compared to that in the wall region. All of the reported radial solids concentration profiles in the fast fluidization and densephase fluidization regimes are low at the center and gradually increase to the maximum solids concentration near the wall. The probe, thus, cannot yield the real structure of the open slugs with particles/blobs present inside. This study using the ECT technique identifies the formation of open slugs and reveals the mechanism initiating the choking transition for Group B particles. With further increase in the solids circulation rate to

80.0 kg/m2s, the flow structure, as shown in Figure 5 (d), does not change much compared to that after the choking transition at a solids circulation rate of 57.15 kg/m2s. Under this operating condition, more open slugs with smaller sizes containing small blobs are formed. Particles in the wall region tend to move inward and connect with particles in the central region periodically. In some situations, the wall region expands inward and connects to the central region to form a solids plug, as shown at the bottom of Y-Z plane in Figure 5 (d). The solids concentration in the dense wall region increases greatly, especially at the time when the wall region expands to the inner part of the bed. To study the flow behavior of the gas-solids conveying system with Group B particles, experiments are also conducted at the upper part of the riser (4.5 m above the distributor) under different solids circulation rates for the gas velocities of 2.4 and 3.4 m/s, the same operating conditions as those applied at the lower part of the riser. It is observed that the solids concentration at the upper part of the riser is much lower than that at the lower part. For both gas velocities, no choking transition occurs and thus no dense-phase fluidization regime exists at the upper part of the riser under all the solids circulation rates employed in this study. The flow structures at the upper part of the riser for these two gas velocities are similar. The bed exhibits the double solids-ring flow structure under all the operating conditions in this work, which is similar to that in the dilute transport regime at the lower part of the riser as shown in Figures 2 (a) and 5 (a). However, at the upper part, the solids concentration in the solids ring in the central region is comparable to that in the solids ring in the wall region. At low solids circulation rates corresponding to the dilute transport regime at the lower part of the riser, the boundaries between these three regions along the radial direction are not distinct. With an increase in the solids circulation rate, the solids concentrations in all the three regions across the bed section increase gradually. The boundaries between the three regions become distinct, although particles still interchange violently between the central and the wall regions. When the solids circulation rate further increases, the lower part of the riser lies in the densephase fluidization regime after the choking transition. The flow structure of the upper part of the riser at the gas velocity of 2.4 m/s (