Imaging the Choking Transition in GasSolid Risers Using Electrical

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Imaging the Choking Transition in Gas-Solid Risers Using Electrical Capacitance Tomography Bing Du, W. Warsito, and Liang-Shih Fan* Department of Chemical Engineering, The Ohio State UniVersity, 140 West 19th AVenue, Columbus, Ohio 43210

Electrical capacitance tomography, based on the neural network multicriteria optimization image reconstruction technique, developed by this research group, is used to examine the column size effect on the real time, cross-sectional flow variation during the choking transition in a circulating fluidized bed. Two sizes of columns, 0.05 and 0.1 m i.d., are employed in this study for two types of particles, sand particles (group B) and fluid catalytic cracking catalysts (group A). For group B particles, the formation of the square-nosed slugs (0.05 m i.d. column) or the wall slugs (0.1 m i.d. column) is observed to occur in the regime transition when the gas velocity Ug is below the transport gas velocity Utr or when the solids circulation rate Gs is below the transport solids velocity Gs,tr. When Ug > Utr or Gs > Gs,tr, the formation of open slugs for both 0.05 and 0.1 m i.d. columns is observed in the regime transition. Clearly, for group B particles, these regime transitions accompanied by a distinct bed structure change mark the choking transition. For group A particles, when Ug < Utr or Gs < Gs,tr, a distinct change in the bed structure in the columns is also observed as the flow transits from the dilute regime to the turbulent regime, marking the choking transition. When Ug > Utr or Gs > Gs,tr, however, the transition from the dilute regime to the dense regime is fuzzy. Even though this transition is noted in the literature as the choking transition, it involves little change in the bed structure. Introduction The term “choking” was first utilized in 1949 by Zenz1 to describe a sharp transition from a dilute transport to a slugging flow in the vertical pneumatic conveying of two kinds of Geldart group D particles (rape seed and sand particles) and two kinds of Geldart group B particles (glass beads and salt particles) in a 0.044 m i.d. column. The slugging transition has, therefore, been associated with the occurrence of choking, and the existence of the slugging transition is used to determine if a particular system is to be operated under choking or nonchoking conditions.2-6 The term “choking” was later more generally used to describe the gas-solid fluidization phenomenon in which a small change in the flow rates of a gas or solid prompts a sharp change in the hydrodynamic properties such as pressure drop and solids concentration during the gas-solid flows. Bi et al.7 summarized and categorized (types A, B, and C) various choking conditions reported in the literature. These conditions are closely associated with flow regime transitions or blower performance and cover a wide range of gas and solid flow rates. The present study is intended to examine the bed density variation and hence bed structure variation during the regime transition utilizing electric capacitance tomography (ECT) based on a novel image reconstruction technique (neural network multicriteria optimization image reconstruction technique, NNMOIRT) developed by the authors and to discern the “choking” phenomenon and the fluidization regime transition from the perspective of bed structure variations. Choking is a complex phenomenon. The gas and solid flow rates marking the point of choking for either vertical pneumatic conveying or circulating fluidized bed systems vary with the bed diameter and the particle property.8 Choking can be experimentally determined from the pressure gradient (∆p/∆z) variations with the gas velocity Ug at a constant solids circulation

rate Gs (Figure 1a) or the pressure gradient variations with the solids circulation rate at a constant gas velocity (Figure 1b). As shown in Figure 1a, at a given solids circulation rate, Gs, the pressure gradient decreases gradually to a minimum point (A to B) and then increases rapidly (B to C) as the gas velocity decreases. Curve AB represents the dilute transport regime where the concentration of the solid is low and the pressure drop is dominated by the wall friction. On Curve BC, as the gas velocity decreases, the flow undergoes transition eventually to the fast fluidization regime, where the concentration of the solid is relatively high and the pressure drop is dominated by the solid’s concentration. At point C, the pressure drop fluctuates highly and the flow behavior becomes extremely unsteady. Point C was noted as the choking transition point accompanied by the onset of slug flow,1 which was later examined by many other researchers2,3,5,6,9-18. Several examples of the criteria for predicting the occurrence of choking based on the terminal velocity of a particle (Ut), particle diameter (dp), bed diameter (D), and bed voidage () were given as follows. Criteria of Yousfi and Gau2

Ut2/(gdp) > 140

(1)

Ut2/(gD) > 0.12

(2)

Utn-1n(1 - )/(gD)1/2 > 0.41

(3)

Criteria of Yang3

Criteria of Smith4

Criteria of Leung5

Utn{[(n - 1)/n]n-1 - [(n - 1)/n]n} (gD)1/2

* To whom correspondence should be addressed: Tel: 614-6883262. Fax: 614-292-3769. Email: [email protected].

> 0.41

(4)

where, n is the exponent in the Richardson-Zaki equation.

10.1021/ie051401w CCC: $33.50 © 2006 American Chemical Society Published on Web 06/16/2006

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Figure 1. Choking transition based on the pressure gradient (∆p/∆z) variations with the gas velocity at a constant solid circulation rate (a) and with the solids circulation rate at given gas velocities (b).

Matsen,12 however, stated that the onset of slug flow could not be recognized as the definitive criterion, since the dense phase flow could also be bubbling or turbulent fluidization. Choking is, thus, more appropriately defined as the sharp transition from the dilute phase flow to the dense phase flow. Matsen12 observed that the actual particle slip velocity is larger than the terminal velocity of a single particle and attributed the initiation of choking to the formation of clusters. Cape and Nakamura11 attributed the increased slip velocity over the terminal velocity to the particle-wall friction and particle recirculation effects. Choking was treated by them as instability in a vertical riser with various forms that occurred as particles begin to recirculate in the riser system. Figure 1b illustrates the choking transition based on the variation of the pressure gradient with the solids circulation rate at a given gas velocity, i.e., (∆p/∆z)Ug ∼ Gs. When the gas velocity is lower than Utr, the pressure gradient follows the curve ABCD as the solids circulation rate increases. Along curve AB, the pressure gradient is low and particles accumulate gradually at the bottom of the riser. At point B, a slight increase in the solids circulation rate leads to an abrupt increase in the pressure gradient, as shown by curve BC in Figure 1b. Curve BC marks the choking transition.19-21 Bi et al.7 defined this transition as type C choking or classical choking. As the solids circulation rate further increases, the pressure gradient remains almost unchanged and the bed transits to dense-phase fluidization. Yerushalmi and Cankurt19 attributed the sharp increase in the

pressure gradient to the collapse of the bed suspension. The dense-phase fluidization after the choking transition can be slugging, turbulent, or bubbling fluidization, depending on the relative velocity between the gas and the solids, Ug - Us, and the onset velocity to turbulent fluidization, Uk. Du et al.20 studied the choking phenomenon and the bed density variation in a 0.1 m i.d. circulating fluidized bed with group A particles using the ECT technique. Choking occurred as the blobs of solids at the center of the bed break up. The bed suspension collapsed to form a turbulent fluidized bed. For group B particles, the ECT results indicated that the choking transition was initiated due to the formation of wall slugs or half-slugs.21 The bed transits to the slugging fluidization after the choking transition. When the gas velocity is equal to or higher than Utr, the pressure gradient increases gradually as the solids circulation rate increases and no sharp change is observed (curve EFGH in Figure 1b). Curve EFGH has been applied to determine the boundaries between the different fluidization regimes in the circulating fluidized beds, including the dilute transport, fast fluidization, and dense-phase fluidization regimes.19-22 Curve EF represents the dilute transport regime with almost unvaried low-pressure gradient and low solids concentration. After point F, the pressure gradient increases gradually with an increasing solids circulation rate and the bed transits to the fast fluidization regime (curve FG in Figure 1b). As Gs further increases, the variation of the pressure gradient begins to level off and the dense-phase fluidization regime is reached. Bi et al.7 defined the boundary between the dilute transport regime and the fast fluidization as type A choking or accumulative choking. The boundary between the fast fluidization regime and the slugging fluidization regime was noted as type C choking or classical choking. The transition to the nonslugging dense-phase flow was gradual, which was also referred to as the fuzzy transition. Several correlations for predicting the gas velocity or the solids circulation rate at the transition from the dilute transport to fast fluidization regimes (type A choking) have been developed by Bi et al.18 and Xu et al.23 for both group A and B particles. Although there is no sharp increase in the pressure gradient when the gas velocity is higher than Utr, Yerushalmi and Cankurt19 noted that choking still occurred when the solids circulation rate was so high that the relative velocity, Ug - Us, was smaller than Uk. Du et al.20 and Du and Fan21 studied the choking phenomenon in circulating fluidized beds for both group A and B particles using the ECT technique. When the gas velocity is greater than Utr, the flow structure variation during the transition from the dilute transport to the fast fluidization regimes varied for group A and B particles. The ECT results revealed that, for group B particles, open slugs were formed as the fast fluidization regime transits to the dense-phase fluidization regime.21 Mok et al.22 observed a similar flow structure and called it “diffused slugs”. For group A particles, the transition from the fast fluidization to the dense-phase fluidization regimes was also gradual, which is noted as the choking transition without flow structure variation.20 The knowledge concerning the mechanism governing the choking transition in high-velocity gas-solid flows is far from complete. Various models based on assumed mechanisms have been proposed to account for the observed choking point. It is crucial, however, to understand the transient internal flow structure variation, particularly in the vicinity of the choking transition, in order to discern the mechanism of choking. The common pressure drop measurements can only provide the overall phase holdup information, while the common probe measurements such as the optical fiber probe or the capacitance

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Figure 2. Schematic diagram of the 0.05 m i.d. vertical pneumatic system.

probe can only provide the local point holdup information; both measurements contribute very limited insights into the flow structure variation and hence the mechanism for the choking transition. ECT coupled with NNMOIRT has provided accurate, quantitative, real-time measurements of the cross-sectional phase holdups for gas-solid flows. The measurements also provide the microscopic and macroscopic bed density distribution in the cross-section of the flow during the choking transition.20,21,24 Du et al.20 and Du and Fan21 have reported the quasi-3D ECT results on choking phenomena for group A and B particles in a 0.1 m i.d. circulating fluidized bed (CFB). The present study examines the column size effects on the phenomena of the choking transition for both group A and B particles. Specifically, the column of 0.05 m i.d. is used, and its results are compared with those from the 0.1 m i.d. column. Furthermore, a realtime, three-dimensional image reconstruction technique using the 3D ECT technique newly developed by this research group25 is used to substantiate the quasi-3D images obtained in this study. In the light of the findings, comments with respect to regime transition and choking phenomenon in the context of the bed structure variation are presented. Experimental Studies Figure 2 shows the schematic diagram of a 0.05 m i.d. vertical pneumatic system used in this study. The configuration of the system is a duplication of that employed by Zenz1 with the exception of the column size of 0.05 m i.d. instead of 0.044 m i.d. used by Zenz.1 The 0.05 m i.d. column is made of Plexiglas and consists of a 2-m-high riser, a cyclone system, a 0.15 m i.d. solids feed tank, and an L-valve. The experiments are conducted batchwise. Particles are carried upward in the riser and exit at the top through a long-radius bend into a horizontal tube connected to a cyclone, where the particles are separated from the gas. Subsequently, the particles flow downward through a dipleg to a container. The particles are then transported to the solids feed tank and fed back to the bottom of the riser through the nonmechanical L-valve. The solids circulation rate, which is controlled by adjusting the air aeration rate at the injection

points of the L-valve, is measured by timing the falling distance of tracer particles in the standpipe. The compressed air is introduced into the riser through an oil filter, humidifier, pressure manometer, and flow meter. The superficial gas velocity is measured by the flow meter adjusted by the temperature and pressure of the airflow. A 0.1 m i.d. circulating fluidized bed is also 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. The particles are then fed back to the bottom of the riser through a nonmechanical L-valve. Details of the 0.1 m i.d. circulating fluidized bed are described elsewhere.20,21 The electrical capacitance tomography is applied to measure the dynamic flow behavior in the gas-solid high-velocity flows. The ECT 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 each electrode is 0.05 m. The two measuring planes (planes 1 and 2) are located at 1.0 and 1.05 m above the distributor in the 0.05 m i.d. column. For the 0.1 m i.d. CFB, the measurement locations, L, are 0.5 and 0.55 m above the distributor. The data acquisition board manufactured by Process Tomography Limited is employed for capturing the ECT images at 100 frames per second. A novel reconstruction algorithm based on an analogue NNMOIRT is applied to perform the image reconstruction. The details of the 2-D or quasi-3D ECT technique and the image reconstruction algorithm are described elsewhere.24 For the 3D ECT imaging, a modified Hopfield neural network is invoked to solve the optimization problem by minimizing the four objective functions: negative entropy function, leastsquares errors, smoothness and small peakedness function, and 3D-2D matching function. The 3D ECT image reconstruction is accomplished by introducing the 3D sensitivity matrix into the NNMOIRT algorithm. The image is reconstructed into a three-dimensional image consisting of image voxels in a number of frames instead of a single frame as in two-dimensional image reconstruction. The NNMOIRT algorithm reconstructs simultaneously the volume image into 20 × 20 × 20 voxels from 276 capacitance data based on 12-electrode twin-plane sensor for simulation and 66 capacitance data obtained from sixelectrode twin plane sensor for actual measurement. The capacitance sensor array is a twin-plane sensor using six electrodes for each plane. The length of each electrode is 8 cm; thus, the total interrogation volume is 16 cm in length. The details on the algorithm and the 3D ECT technique are described in Warsito and Fan.25 A differential pressure transducer and a U-shape manometer are installed to measure the pressure drop and the overall voidage of the fluidized bed. The two pressure taps for the U-shape monometer measurement in the 0.05 m i.d. column are located at 0.5 and 1.62 m above the distributor, respectively. The tap locations and the measurement method are identical to those used in the study of Zenz.1 For the 0.1 m i.d. column, the two pressure taps are located at 0.15 and 0.95 m above the distributor, respectively. The optical fiber probe is applied to measure the local solids concentration distribution.26 The group A particles employed in this study are fluid catalytic cracking (FCC) catalysts with the mean diameter of 60 µm and the


Figure 3. Comparison between quasi-3D flow structure and real 3D flow structure in a 0.1 m i.d. circulating fluidized bed with group B particles.

particle density of 1400 kg/m3. The group B particles employed are sand particles with the mean diameter of 240 µm and the particle density of 2200 kg/m3. The particle size distribution is identical to that in the study of Du and Fan.21 Results and Discussion For a gas-solid circulation system, the fluidization regimes (dilute transport regime, fast fluidization regime, and densephase fluidization regime) are typically confined by both the superficial gas velocity and the solids circulation rate. The transport gas velocity, Utr, and the transport solids circulation rate, Gs,tr, can be calculated by the correlation of Bi and Fan.27 For FCC catalysts, Utr is 1.5 m/s and Gs,tr is 6.87 kg/m2 s. For sand particles, Utr is 2.7 m/s and Gs,tr is 17.1 kg/m2 s. This study systematically examines the choking phenomenon in the vertical gas-solid high-velocity flows by considering four cases. Cases 1 and 2 represent the experiments operated with decreasing gas velocity at solids circulation rates below Gs,tr and above Gs,tr, respectively. Cases 3 and 4 represent the experiments operated with increasing solids circulation rate at gas velocities below Utr and above Utr, respectively. Comparison between Quasi-3D Flow Structure and Real 3D Flow Structure. Figure 3 compares the quasi-3D flow structure obtained by the 2D ECT and the real 3D flow structure obtained by the 3D ECT in a 0.1 m i.d. circulating fluidized bed with group B particles. The 2D ECT provides the twodimensional cross-sectional images obtained from averaging over an axial distance of 5 cm representing the thickness of the electrodes. The quasi-3D diagram in Figure 3 is generated by stacking 150 two-dimensional tomographic images in plane 1 of the ECT (0.5 m above the distributor). A color bar from blue to red represents the variation of the solids concentration from 0 to 0.3. The 3D flow structure or the volume image is constructed from permittivity voxel values in three-dimensional matrix components, i.e., three space components with spatial resolution of 5 × 5 × 8 mm3. Under the operating condition at Ug ) 2.4 m/s and Gs ) 14.86 kg/m2 s for group B particles, the bed undergoes the choking transition from the three-region structure with blobs of solids at the central region to the

formation of wall slugs as shown in the quasi-3D diagram in Figure 3.21 The similar bed density variation during the choking transition is observed on the basis of the real 3D flow structure with the existence of a blob of solids at the center of the column before the choking transition and the formation of wall slug after the choking transition. The similarity between the quasi3D and the real 3D flow structure indicates that the major characteristics of the bed behavior in the gas-solid circulating fluidized beds can be readily captured and obtained by the quasi3D diagram. Therefore, in this study, the flow structure variation during the choking transition in gas-solid fluidization is illustrated on the basis of the quasi-3D diagram. Regime Transition and Choking Phenomenon for Group B Particles. Figure 4 shows the variations of the pressure gradient with the gas velocity for the 0.05 m i.d. column for group B particles at a given solids circulation rate of 11.43 kg/ m2 s (Gs,tr) is shown in Figure 5 (case 2 operating condition). Compared to Gs of 11.43 kg/m2 s, as shown in Figure 4, the pressure gradient variation exhibits a similar trend to that at a higher solids circulation rate of 91.44 kg/m2 s. However, the pressure gradients under the case 2 operating condition are higher than those under the case 1 operating condition at the same gas

velocity. The minimum point value of the pressure drop corresponds to the transition from the dilute transport (double solids ring) to the fast fluidization (three-region structure). The pressure gradient increases sharply after the minimum point with decreasing gas velocity up to Uch. At Uch, the open slugs21 with the blobs of solids inside are formed in the bed, which is characterized as the choking transition to the dense-phase fluidization regime. The gas velocity at choking is 3.0 m/s (>Utr) for the solids circulation rate of 91.44 kg/m2 s. When the gas velocity decreases to that below Utr, there is a slight increase in the pressure gradient. The solids concentration at the center


of the bed increases largely, resulting in the appearance of the solids plug or the square-nosed gas slug. As shown in Figure 5, a dense solids core region comprising the blobs of solids with a high solids concentration formed between two continuous solids plugs. Under this operating condition, particles still circulate in the system, although the gas velocity is below the transport velocity, Utr. With further decrease in the gas velocity below the particles terminal velocity, Ut, of 1.52 m/s, particles no longer circulate in the system and the bed undergoes slugging fluidization. Compared to the operating condition above Ut, the length of the solid plug increases and the dense core region disappears. Consequently, based on the variation of the bed flow structure obtained by the ECT, the dense-phase fluidization regime after the choking transition can be described in terms of three different regions, i.e., open slugs at Utr < Ug < Uch, square-nosed slugs with blobs of solids inside at Ut < Ug < Utr, and square-nosed slugs at Ug < Ut. The variations of the pressure gradient with the solids circulation rate at a given gas velocity for group B particles in the 0.05 m i.d. column are similar to those in the 0.1 m i.d. column.21 Under the gas velocity lower than Utr (case 3 operating condition), there is a sharp increase in the pressure gradient with an increasing solids circulation rate. The bed density obtained by the ECT signifies a sharp change of the pressure gradient with the solids circulation rate. At low solids circulation rates, the bed exhibits the double solids ring flow structure. As the solids circulation rate increases, the bed flow behavior transits to the three-region flow structure. As the solids circulation rate further increases to Gs,ch, a remarkable flow structure variation (formation of solids plug) occurs accompanied by a sharp change of the pressure gradient. A square-nosed gas slug or gas interval21 is present in the bed following the solids plug, which is characterized as the choking transition.19,21 The choking transition due to the formation of solids plugs or square-nosed gas slugs under the case 3 operating condition is the same as that under the case 1 operating condition. Under the case 4 operating condition at a given gas velocity above Utr, there is no sharp increase in the pressure gradient variation with increasing solids circulation rate. At the low solids circulation rates in the dilute transport regime, the bed exhibits the double solids ring flow structure. As the solids circulation rate increases to the fast fluidization regime, the flow behavior of the pneumatic system exhibits the three-region flow structure. As the solids circulation rate further increases up to Gs,ch, the open slugs with the blob of solids inside are formed, which signifies the choking transition to the dense-phase fluidization regime. This type of the choking definition is consistent with that based on the pressure gradient plot against the gas velocity at Gs > Gs,tr (shown in Figure 5) in terms of the flow structure variation (formation of open slugs). On the basis of the ECT imaging, both the time averaged and the transient cross-sectional solids concentration distribution can be obtained. Figure 6a shows the variation of the timeaveraged cross-sectional solids concentration with the gas velocity for the solids circulation rates of 11.43 and 91.44 kg/ m2 s. For both solids circulation rates, in the dilute transport regime, the solids concentration remains almost unvaried with decreasing gas velocity. As the gas velocity further decreases, the solids concentration begins to increase significantly. This point corresponds to the boundary between the dilute transport regime and the transition regime (Gs < Gs,tr) or the fast fluidization regime (Gs > Gs,tr). The solids concentration then levels off as the gas velocity decreases below Uch in the densephase regime. The ECT images indicate different flow structures

Figure 6. Variation of the time-averaged cross-sectional solids concentration and the standard deviation of the cross-sectional solids concentration fluctuation with the gas velocity in a 0.05 m i.d. CFB for group B particles.

even within the dense-phase fluidization regime after the choking transition, particularly at a higher solids circulation rate, as shown in Figure 5. The variation of the cross-sectional solids concentration with the gas velocity, however, does not reflect the structure difference. Figure 6b shows the variation of the standard deviation of the cross-sectional solids concentration fluctuation with the gas velocity under the same operating conditions as shown in Figure 6a. The standard deviation exhibits a similar trend for the gas velocity for both solids circulation rates. As the gas velocity decreases to Uch, the choking transition occurs, which corresponds to the inflection point in Figure 6b. It is observed that the standard deviation during the choking transition at lower Gs is larger than that at higher Gs. It can be attributed to the different flow structure formed during the choking transition, i.e., square-nosed slugs for a lower Gs and open slugs for a higher Gs. The standard deviation levels off with decreasing gas velocity in the densephase fluidization regime after the choking transition when the solids circulation rate is lower than Gs,tr. However, for the solids concentration rate above Gs,tr, the standard deviation increases at a relatively moderate rate compared to that in the fast fluidization regime with decreasing gas velocity to Utr. It corresponds to the bed flow behavior in the dense-phase fluidization regime with open slugs, as shown in Figure 5. When the gas velocity is below Utr, the standard deviation begins to level off gradually, indicating the bed flow structure variation from open slugs to square-nosed slugs. The standard deviation after the choking transition at Gs < Gs,tr is compared well with that when the gas velocity is below Utr at Gs > Gs,tr, which

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Figure 7. Pressure gradient and flow structure in a 0.1 m i.d. CFB for group B particles at Gs ) 12.86 kg/m2 s (case 1).

further substantiates a comparable square-nosed flow structure under these two operating conditions. Compared to the variation of the solids concentration, as shown in Figure 6a, the variation of the standard deviation reflects the different regions in the dense-phase fluidization regime after the choking transition at Gs > Gs,tr. Figure 7 shows the variation of the pressure gradient and the flow structure with the gas velocity at a given solids circulation rate of 12.86 kg/m2 s in a 0.1 m i.d. CFB (case 1 operating condition). It is observed that there is no minimum pressure gradient before choking, which is different from that in the 0.05 m i.d. column, as shown in Figure 4. This observation can be attributed to the insignificant effect of the wall friction in a larger CFB. The flow structure variation before Uch in the 0.1 m i.d. columns is the same as that in the 0.05 m i.d. column, as shown in Figure 4. At the gas velocity of Uch, the wall slugs or half slugs are formed in the bed, which is signified as the choking transition. The variations of the pressure gradient and the flow structure with the gas velocity at a given solids circulation rate of 50 kg/m2 s in a 0.1 m i.d. CFB (case 2 operating condition) are shown in Figure 8. The pressure gradient variation shows a trend similar to that under the case 1 operating condition as shown in Figure 7. The flow structure variation before choking in the 0.1 m i.d. column is similar to that in the 0.05 m i.d. column, as shown in Figure 5. The choking transition is characterized by the formation of the open slugs with blobs of solids inside. As the gas velocity further decreases below Utr, the pressure gradient does not change much. The flow behavior in the bed, however, varies from the open slugs to the wall slugs. When the gas velocity is below Utr, the top and bottom boundaries of the open slugs close up, resulting in the formation of wall slugs. It is noted that the gas velocity and solid circulating rate at choking, i.e., Uch and Gs,ch, are different for the 0.05 m i.d. pneumatic system and the 0.1 m i.d. circulating fluidized bed. The difference can be attributed to the effect of the bed diameter and the specific ECT measurement or pressure measurement location.

Regime Transition and Choking Phenomenon for Group A Particles. Figure 9 illustrates the variation of the pressure gradient and the flow structure with the gas velocity at a given solids circulation rate of 5.94 kg/m2 s (Gs,tr) in a 0.05 m i.d. column for group A particles are shown in Figure 10 (case 2 operating condition). The pressure gradient variation with the gas velocity is similar to that under the case 1 operating condition, as shown in Figure 9. Compared to the lower solids circulation rate condition as shown in Figure 9, Udf becomes larger at a higher solids circulation rate. Uch is commonly noted as the choking transition based on the pressure gradient variation. However, the bed density pattern obtained by the ECT does not vary at Uch, even when the solids concentration is greatly increased. Du et al.20 noted it as the choking transition without the flow structure variation for group A particles in a 0.1 m i.d. circulating fluidized bed. Compared to the fast fluidization regime, the blobs of solids at the solids core region in the dense-phase fluidization regime are disconnected by the gas flow. The velocity at the transition to the dense-phase fluidization regime is 2.1 m/s at


Figure 8. Pressure gradient and flow structure in a 0.1 m i.d. CFB for group B particles at Gs ) 50 kg/m2 s (case 2).

Figure 9. Pressure gradient and flow structure in a 0.05 m i.d. CFB for group A particles at Gs ) 5.94 kg/m2 s (case 1).

Gs ) 59.44 kg/m2 s in a 0.05 m i.d. column. The pressure gradient increases slightly with decreasing gas velocity after Uch. When the gas velocity is below Utr, it is observed that the bed suspension or blobs of solids at the center of the bed disappear, which is similar to that under the case 1 operating condition after choking, as shown in Figure 9. In a 0.05 m i.d. column for group A particles, the pressure gradient variation with the solids circulation rate exhibits a trend similar to that for group B particles in the same column. Under case 3 operating condition, the choking transition is characterized by variation from the three-region flow structure to

turbulent fluidization. Under the case 4 operating condition, as the solids circulation rate increases up to Gs,ch, no distinct flow structure variation is observed. Such flow transition is consistent with the regime transition observed under the case 2 operating condition, as shown in Figure 10. Parts a and b of Figure 11 show the variation of the timeaveraged cross-sectional solids concentration and the standard deviation of the cross-sectional solids concentration fluctuation with the gas velocity at given solids circulation rates in a 0.05 m i.d. column for group A particles, respectively. No minimum point is observed for both the variations of the solids concentra-

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Figure 11. Variation of the time-averaged cross-sectional solids concentration and the standard deviation of the cross-sectional solids concentration fluctuation with the gas velocity in a 0.05 m i.d. CFB for group A particles.

tion and the standard deviation, as shown in Figure 11a,b. When the solids circulation rate is below Gs,tr, the choking transition occurs at the inflection point of the solids concentration or the

standard deviation plot as the gas velocity decreases to Uch. When the solids circulation rate is above Gs,tr, the gas velocity at the transition from the dilute transport to the fast fluidization regimes becomes greater at a higher solids circulation rate. As the gas velocity decreases to Uch, the solids concentration and the standard deviation reach the inflection point, which is noted as the transition to the dense-phase fluidization regime. After the transition, the solids concentration and the standard deviation increase at a relatively moderate rate compared to that in the fast fluidization regime as the gas velocity further decreases. When the gas velocity is below Utr, the solids concentration and the standard deviation begin to level off, which corresponds to the dense-phase fluidization regime. Figure 12 shows the variation of the pressure gradient and the bed density variation with the gas velocity at a given solids circulation rate of 3.71 kg/m2 s (Gs,tr) in a 0.1 m i.d. CFB for group A particles. At Uch, there is no distinct flow structure variation during the transition, as shown in Figure 13. The solids concentration near the wall in the dense-phase fluidization regime becomes higher compared to that in the fast fluidization regime. The blobs of solids or the solids core region in the fast fluidization regime become disconnected and smaller in the dense-phase fluidization regime. As the gas velocity further decreases to below Utr, the blobs of solids at the center of the bed break up and are transported to the wall region. The solids concentration near the wall region greatly increases compared to that at Ug > Utr. The bed flow behavior transits to the turbulent fluidization regime.




Figure 14 shows the radial profiles of the time-averaged solids concentration in a 0.1 m i.d. CFB for group A particles. The comparison between the ECT and the optical fiber probe is also presented. In the dilute transport regime with a double solids ring flow structure at Ug ) 0.97 m/s and Gs ) 1.32 kg/m2 s, the two solids rings are located at 0.2 < r/R < 0.45 and r/R > 0.75. In the transition regime before choking at Ug ) 0.97 m/s and Gs ) 2.30 kg/m2 s, the bed exhibits a three-region flow structure. The solids core region is located at r/R < 0.45. Two other regions including the solids ring near the wall and the gas ring are located at the same position as those for the dilute transport regime. Under the operating condition of Ug ) 0.97

m/s and Gs ) 4.95 kg/m2 s, the bed undergoes the turbulent fluidization regime after the choking transition. The solids concentration increases gradually along the radial direction. Compared to the operating conditions before choking, the solids concentration at the center of the bed in the turbulent fluidization regime is similar, while the solids concentration near the wall is greatly higher. The results obtained by the optical fiber probe for different flow regimes in a CFB further substantiate the accuracy of the bed density images obtained by the ECT. Some deviations, however, exist on the solids concentration at different radial positions, particularly in the gas ring area under the operating conditions before choking. This deviation can be

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Figure 14. Time-averaged radial solids concentration distributions in a 0.1 m i.d. CFB for group A particles.

attributed to the intrusive effect of the optical fiber probe on the flow field and the ill-defined measurement volume of the probe.26 Concluding Remarks Electrical capacitance tomography with the neural network multicriteria optimization image reconstruction technique can provide robust, real-time images of the cross-sectional dynamic behavior in gas-solid high-velocity flows. The ECT of this study measures the 2-D images obtained from averaging over an axial distance of 5 cm, representing the thickness of the electrodes. The quasi-3D bed density diagram is generated by stacking the 2-D images in one plane of the ECT, which has been verified by the real 3D images obtained by the 3D ECT newly developed by this research group. For group B particles, the flow structures for the 0.05 m i.d. pneumatic conveying system and the 0.1 m i.d. circulating fluidized bed are similar in the transition between the dilute transport and fast fluidization regimes, characterized by the change from the double solids ring to the three-region flow structures. When Ug > Utr or Gs > Gs,tr, the open slugs with blobs of solids inside the slugs are formed for both systems during the transition from the fast fluidization to the densephase fluidization regime. When Ug < Utr and Gs < Gs,tr, the square-nosed slugs or solids plugs in 0.05 m i.d. pneumatic conveying system and wall slugs or half-slugs in 0.1 m i.d. CFB are formed during the regime transition to dense-phase fluidization. These transitions with significant flow structure variation from the dilute transport regime to the dense-phase fluidization regime are characterized by the choking transition. The different flow behavior or different types of slugs observed after the choking transition can be attributed to the wall effect due to the different sizes of columns for gas-solid high-velocity flows. For group A particles, the 0.05 m i.d. pneumatic conveying system and the 0.1 m i.d. circulating fluidized bed exhibit similar flow structures in the dilute transport and the fast fluidization regimes. The transition between these two regimes is characterized also by the change from the double solids ring to the threeregion flow structures based on the ECT measurement. For both the 0.05 and the 0.1 m i.d. columns, when Ug < Utr and Gs < Gs,tr, the bed suspension or the blobs of solids at the center of the bed collapse during the choking transition from the dilute transport to the dense-phase fluidization regime. The bed transforms to the turbulent fluidization after the choking transition. When Ug > Utr or Gs > Gs,tr, however, the transition from the dilute regime to the dense regime is fuzzy. Even though this transition is noted in the literature as the choking transition, it involves little change in the bed structure.

The choking transition in gas-solid high-velocity flows based on the flow structure variation is summarized below. (1) At Ug < Utr and Gs < Gs,tr, the choking transition is characterized by the formation of square-nosed slugs (0.05 m i.d. column) or wall slugs (0.1 m i.d. column) for group B particles. (2) At Ug > Utr or Gs > Gs,tr, the choking transition is characterized by the formation of open slugs for group B particles. (3) At Ug < Utr and Gs < Gs,tr, the regime transition to the turbulent fluidization with collapse of bed suspension and breakage of blobs of solids at the center of the bed is signified as the choking transition for group A particles. (4) At Ug > Utr or Gs > Gs,tr, no significant flow structure variation exists for group A particles and the regime transition to dense-phase fluidization is fuzzy. Acknowledgment The helpful comments from Dr. Wen-Ching Yang on this work are gratefully acknowledged. Notation ∆p/∆z ) pressure drop gradient, kPa/m D ) bed diameter, m dp ) particle diameter, m dp/dz ) pressure drop gradient, kPa/m Gs ) solids circulation rate, kg/m2 s Gs,ch ) solids circulation rate at choking, kg/m2 s Gs,tr ) transport solids circulation rate, kg/m2 s L ) measurement position of ECT, m r ) radial position in the bed, m R ) radii of the bed, m Uch ) gas velocity at choking, m/s Udf ) gas velocity at the boundary between the dilute transport and the fast fluidization regimes, m/s Ug ) superficial gas velocity, m/s Uk ) onset velocity to the turbulent regime, m/s Us ) superficial solids velocity, m/s Ut ) terminal velocity of a particle, m/s Utr ) transport velocity, m/s Greek Letters ) overall voidage of the bed s ) solids concentration Fp ) particle density, kg/m3 Literature Cited (1) Zenz, F. A. Two-phase fluid-solid flow. Ind. Eng. Chem. 1949, 41, 2801. (2) Yousfi, Y.; Gau, G. Aerodynamicque de l’ecoulement vertical de suspensions concentrees gaz-solides: I. Regimes d’ecoulement et stabilite aerodynamique. Chem. Eng. Sci. 1974, 29, 1939. (3) Yang, W. C. A criterion for “fast fluidization”. Pneumotransport 3, Bath, England, 1976, E5, 49. (4) Smith, T. N. Limiting volume fractions in vertical pneumatic transport. Chem. Eng. Sci. 1978, 33, 745. (5) Leung, L. S. Vertical pneumatic conveying: A flow regime diagram and a review of choking versus non-choking systems. Powder Technol. 1980, 25, 185. (6) Satija, S.; Young, J. B.; Fan, L.-S. Pressure fluctuations and choking criterion for vertical pneumatic conveying of fine particles. Powder Technol. 1985, 43, 257. (7) Bi, H. T.; Grace, J. R.; Zhu, J. X. Types of choking in vertical pneumatic systems. Int. J. Multiphase Flow 1993, 19 (6), 1077. (8) Yang, W. C. “Choking” revisited. Ind. Eng. Chem. Res. 2004, 43, 5496.

Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006 5395 (9) Lewis, W. K.; Gilliland, E. R.; Bauer, W. C. Characteristics of fluidized particles. Ind. Eng. Chem. 1949, 41, 1104. (10) Capes, C. E. Dense phase vertical pneumatic conveying. Can. J. Chem. Eng. 1971, 49, 182. (11) Capes, C. E.; Nakamura, K. Vertical pneumatic conveying: An experimental study with particles in the intermediate and turbulent flow regimes. Can. J. Chem. Eng. 1973, 51, 31. (12) Matsen, J. M. Mechanisms of choking and entrainment. Powder Technol. 1982, 32, 21. (13) Chong, Y. O.; Leung, L. S. Comparison of choking velocity correlations in vertical pneumatic conveying. Powder Technol. 1986, 47, 43. (14) Biswas, J.; Leung, L. S. Application of choking correlations for fast-fluid bed operation. Powder Technol. 1987, 51, 179. (15) Takeuchi, H.; Hirama, T.; Chiba, T.; Biswas, J.; Leung, L. S. A quantitative defination and flow regime diagram for fast fluidization. Powder Technol. 1986, 47, 195. (16) Drahos, J.; Cermak, J.; Guardani, R.; Schugerl, K. Characterization of flow regime transition in a circulating fluidized bed. Powder Technol. 1988, 56, 41. (17) Bi, H. T.; Grace, J. R. Flow regime diagrams for gas-solid fluidization and upward transport. Int. J. Multiphase Flow 1995, 21 (6), 1229. (18) Bi, H. T.; Grace, J. R.; Zhu, J. X. Regime transitions affecting gas-solids suspensions and fluidized beds. Chem. Eng. Res. Des. 1995, 73 (A2), 154. (19) Yerushalmi, J.; Cankurt, N. T. Further studies of the regimes of fluidization. Powder Technol. 1979, 24, 187. (20) Du, B.; Warsito, W.; Fan, L.-S. ECT studies of the choking phenomenon in a gas-solid circulating fluidized bed. AIChE J. 2004, 50 (7), 1386.

(21) Du, B.; Fan, L.-S. Characteristics of choking behavior in circulating fluidized beds for group B particles. Ind. Eng. Chem. Res. 2004, 43 (18), 5507. (22) Mok, S. L. K.; Molodtsof, Y.; Large, J. F.; Bergougnou, M. A. Characterization of dilute and dense phase vertical upflow gas-solid transport based on average concentration and velocity data. Can. J. Chem. Eng. 1989, 67, 10. (23) Xu, G.; Nomura, K.; Gao, S.; Kato, K. More fundamentals of dilute suspension collapse and choking for vertical conveying systems. AIChE J. 2001, 47 (10), 2177. (24) Warsito, W.; Fan, L.-S. Neural network based on multi-criteria optimization image reconstruction technique for imaging two- and threephase flow systems using electrical capacitance tomography. Meas. Sci. Technol. 2001, 12, 2198. (25) Warsito, W.; Fan, L.-S. Dynamics of spiral bubble motion in the entrance region of bubble columns and three-plane fluidized beds using 3D ECT. Chem. Eng. Sci. 2005, 60, 6073. (26) Du, B.; Warsito, W.; Fan, L.-S. Bed non-homogeneity in turbulent gas-solid fluidization. AIChE J. 2003, 49 (5), 1109. (27) Bi, H. T.; Fan, L.-S. Regime transition in gas-solid circulating fluidized beds. AIChE Annual Meeting, Los Angeles, CA, Nov 17-22, 1991.

ReceiVed for reView December 15, 2005 ReVised manuscript receiVed May 3, 2006 Accepted May 11, 2006 IE051401W

Imaging the Choking Transition in GasSolid Risers Using Electrical

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