Flow Behaviors in a Circulating Fluidized Bed with Various Bubble

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Ind. Eng. Chem. Res. 2004, 43, 1756-1764

Flow Behaviors in a Circulating Fluidized Bed with Various Bubble Cap Distributors Qingjie Guo*,† and Joachim Werther‡ State Key Laboratory of Heavy Oil Processing, College of Chemistry and Chemical Engineering, University of Petroleum, Dongying, Shandong Province 250761, P. R. China, and Chemical Engineering I, Technical University HamburgsHarburg, Denickestrasse 15, D-21071 Hamburg, Germany

Solids volume concentration was determined in an 8.5-m-high circulating fluidized bed (CFB) riser with two types of bubble cap distributors by using capacitance probes. In the bottom region of the CFB, the lateral solids volume concentration in the center region is low with slight change, whereas lateral solids volume concentration increases significantly toward the wall with the highest solids concentration at wall. Furthermore, lateral solids volume concentration at highpressure drop of a bubble cap is lower than that at low-pressure drop of a bubble cap. Horizontal solids volume concentration in the center region remains flat and rises toward the wall. However, horizontal solids volume concentration at entrance region ranges from 0.43 to 0.5 at low superficial gas velocity. The pressure drop of a distributor has little influence on lateral profiles of solids volume concentrations in CFB upper dilute region. It has been found that the pressure drop of the distributor has little influence on the axial apparent solids concentration in upper dilute region. The bubble cap-type design with a duct where gas is flowing downward to the gas outlet is the recommended structure to avoid solids backflow. Introduction Circulating fluidized beds have been widely used in various technical processes,1,2 such as fluid catalytic cracking (FCC), Fischer-Tropsch synthesis, oxidation of butane, circulation fluidized bed combustion (CFBC), and alumina calcinations. These processes are grouped into two categories: gas-phase catalytic reaction processes and gas-solids reaction processes. The former is characterized by low reaction rate, which operates at low superficial gas velocity and low solids circulation rates. The latter requires high gas velocity in the riser to promote plug flow minimizing gas back-mixing because the short contact time between the gas and solids is preferable. In recent years, CFBC is the largest and fastest growing applications of circulating fluidized bed technology. Knowledge about the solids distribution is of importance to the design and operation of circulating fluidized beds. Bai et al.3 investigated the flow structure in a 0.093-m-i.d. circulating fluidized bed with FCC used as bed material. In the upper dilute region, the solids flow upward and downward at the wall region. In the bottom region, the solids are in a violent turbulent state. Rhodes et al.4 found an S-shaped profile of the solids volume fraction calculated from the pressure profile. The research by Berruti et al.5 showed that the core-annulus flow structure, a dilute core and a dense annulus, in the upper dilute region extended to the bottom region. The suspension density was fairly low and solids flowed upward over the entire riser height. Near the wall, the suspension density was high. Werther et al.6 performed a series of measurements using a capacitance probe in the bottom region of the Chalmers CFB boiler, confirm* To whom correspondence should be addressed. Tel.: 0086635-8396753. E-mail: [email protected]. † University of Petroleum. ‡ Technical University HamburgsHarburg.

ing that a core-annulus-like flow structure exists with high solids concentration at the wall, 0.4, and low concentration, 0.15. Johnston et al.7 studied solids holdup and particle velocity at the entrance region of the downer with three types of gas-solids distributor designs. Previous investigations concentrated on a circulating fluidized bed at a given distributor; there is a lack of study of flow behaviors in a riser with various bubble cap distributors. Because of CFB complex hydrodynamics and the difficulties in measuring local fluid mechanics properties, the fluid mechanics of a large circulating fluidized bed are not very well understood. In this paper, lateral profile, horizontal profile, and axial profile of solids volume concentration were studied in a plant CFB with different bubble caps using a capacitance probe. Further, backflow of solids and erosion regarding the distributor were investigated. Experiments CFB System. Figure 1 illustrates the cold circulating fluidized bed. The riser has a cross section of 0.3 m × 1 m and a height of 8.5 m. Ambient air from a 200-kW roots blower was fed to the windbox and distributed by bubble caps. The off-gas was cleaned by a two-stage cyclone and a filter, and the separated solids were returned to the riser at a height of 1 m above the distributor at an angle of 45°. A weighing section located in the downcomer pipe measured the external solids circulation rate. The siphon is a bubbling fluidized bed in nature, which aids solids to circulate stably in the CFB system. The riser of CFB is equipped with 16 pressure sensors to measure the profiles of pressure drop along the riser. The downcomer has five pressure sensors. A data acquisition system was used for sampling operation parameters, including pressure, superficial gas velocity, sand weight in the hopper, external circulation rate, etc. Quartz sand with a particle density of 2600 kg/m3 and

10.1021/ie030629l CCC: $27.50 © 2004 American Chemical Society Published on Web 03/02/2004

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1757 Table 1. Parameters of Bubble Caps

Figure 1. Pilot plant CFB system.

a surface mean diameter of 140 µm is used as bed material. Figure 4 illustrates the particle size distribution of quartz sand. Bubble Cap Distributor. The design characteristics of the distributors are presented in Table 1. Two types of distributor applied in this investigation: one has 14 bubble caps and the other has 33 bubble caps. For convenience, the 14-bubble cap distributor is named distributor A and the 33-bubble cap distributor named distributor B. The 82.5-mm-i.d. bubble caps were employed in distributor A and the 54.5-mm-i.d. bubble caps in distributor B. For distributor A, two decreased caps with 58- and 35-mm i.d. were used to screw into the bubble cap to change its flow resistance. Distributor B employed 23- and 38.5-mm-i.d. decreased caps for varying its flow resistance. Figure 2 exhibits the crosssectional plots of bubble caps. Six bubble caps were utilized to detect the pressure drop across different bubble caps under various experimental conditions. More details of the bubble cap layout are depicted in Figure 3. For distributor A, bubble caps 1, 2, 4, 8, 10, and 14 were utilized for measurement of the pressure drops across bubble caps. Bubble caps 1,

parameter

unit

distributor A

distributor B

total volume number of bubble cap number of orifices outlet inside diameter, D1 orifice diameter, D2 inlet inside diameter, D3 decreased cap diameter, D4 inlet area orifice area annulus area

m3/s

1.2 14.0 4.0 126.0 42.0 82.5 58, 35 0.00535 0.00554 0.00626

1.2 33.0 4.0 84.0 28.0 54.5 38.5, 23 0.00233 0.00246 0.00269

mm mm mm mm m2 m2 m2

2, 4, 10, and 14 have two pressure probes. Note that bubble cap 8 has five pressure pipes with four lowpressure measuring pipes welded along the annulus wall and the high-pressure measuring pipe fixed in the low-pressure measuring pipes, as shown in Figure 2. The pressure drop of bubble cap 8 is the average of four individual pressure drops. For distributor B, bubble caps 12, 13, 15, 17, 20, and 22 located in the middle row determined the pressure drop across bubble caps. There were five measurement pipes for bubble cap 17. The pressure drop across an individual bubble cap can be expressed as

∆Pbc ) Cd(Fo/2)uin2

(1)

where Cd is the drag coefficient, uin the gas velocity at the inlet of the bubble cap, and Fo the gas density under the operation conditions at the inlet of a bubble cap. For all diameters of bubble caps, the drag coefficient is independent of Reynolds number as Reynolds number varies in a given range, which is reported in another paper.10 The flow rate through a bubble cap can be calculated by means of eq 1. To investigate the influence of positions of measurement bubble caps on the flow rate distribution, six bubble caps in distributor B were replicated from the middle row to the border row, where their new positions were 1, 2, 4, 6, 9, and 11. For CFB distributor pressure drop measurement, the high-pressure port is located at 0.18 m above the distributor, while the low-pressure port is connected with the windbox. In this test, the inlet areas of 82.5-, 58-, 35-, 54.5-, 38.5-, and 23-mm-i.d. caps are denoted by Aa1, Aa2, Aa3, Ab1, Ab2, and Ab3, respectively. In Table 2, the ratio of Aa2 to Aa1 (distributor A) is the same as that of Ab2 to

Figure 2. Cross-sectional plots of bubble in distributor A. Five measuring pipes (left); two measuring pipes (middle).

1758 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004

Figure 3. Layout of bubble caps on distributors A and B. Figure 5. Profile of flow rate in distributor B at u ) 3.0 m/s.

Figure 4. Cumulative mass distribution of particle sizes for quartz sand. Table 2. Inlet Area Parameters of Bubble Caps distributor (mm i.d.) distributor A 82.5 58 35 distributor B 54.5 38.5 23

symbol of inlet area of a bubble cap

inlet area ratio Figure 6. Profile of flow rate in distributor B at u ) 4.5 m/s.

Aa1 Aa2 Aa3

Aa2/Aa1 ) 0.494 Aa3/Aa1 ) 0.179

Ab1 Ab2 Ab3

Ab2/Ab1 ) 0.499 Ab3/Ab1 ) 0.178

Ab1 (distributor B). Also, the ratio of Aa3 to Aa1 (distributor A) is nearly equal to that of Ab3 to Ab1 (distributor B). Pressure Measurement System for the Bubble Caps. A pressure signal system for the bubble caps is composed of 26 differential pressure transducers, an A/D, and a computer. Nine SenSym 142SC01D pressure differential transducers and nine SenSym 143PC5D differential pressure transducers were used in lowpressure measurement and in high-pressure drop measurement, respectively. During all experiments, nine pressure signals were simultaneously recorded at a frequency of 100 Hz for 40 s. Capacitance Measurement System. A needle capacitance sensor can generate an electric field for measuring variations in the dielectric constants corresponding to changes in solids volume concentration at and around the needle. The capacitance probe is able to detect rapid fluctuations for solids volume concentration near its tip. Solids velocity is determined using the cross-correlation technique. The detailed theory and the configuration of such capacitance probe were reported elsewhere.8-9 A two-channel version of the guarded probe measures solids volume concentration and solids

velocity. One probe has an outer diameter of 25 mm with the center-to-center distance between two needles 9 mm. Two needles have a length of 10 mm and a diameter of 1.6 mm. The other has a diameter of 10 mm with one needle. For each sampling process, 131 072 points were collected at a sampling rate of 5 kHz. The sampling time for the evaluation of one solids velocity was 0.124 s, which allows velocity measurements up to 45 m/s with a resolution of (0.088 m/s. Results and Discussion Lateral Flow Rate Profiles of Bubble Caps. Lateral flow rate profiles measured in distributors A and B were illustrated in Figures 5 and 6. To describe the flow rate distribution through different bubble caps, Vmax/Vmin is defined as the ratio of the maximum flow rate through a bubble cap to the minimum flow rate through a bubble cap. Vmax/Vav refers to the ratio of the maximum flow rate through a bubble cap to average flow rate through all bubble caps, which characterizes flow rate distribution for different inlet diameters of bubble caps. For the 54.5-mm-i.d. bubble cap, Figure 5, the flow rate profiles show uneven distribution with Vmax/Vmin and Vmax/Vav being 2.6 and 1.7 at the riser pressure drop of 5500 Pa and superficial gas velocity of 3.0 m/s, respectively. With bubble cap diameters decreasing to 38.5 and 23.0 mm, the values of Vmax/Vmin decrease to 1.42 and 1.06 and the values of Vmax/Vav decrease to 1.34 and 1.0. The pressure drop of a bubble cap is increased with decreasing cap inlet diameter

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1759

Figure 8. Solids volume concentration of distributors A and B at u ) 4.5 m/s and z ) 0.345 m.

Figure 7. Lateral solids volume concentration of distributors A and B at u ) 3.0 m/s and z ) 0.345 m.

because the decreasing cap inlet diameter causes high flow resistance at the same superficial gas velocity. As the superficial gas velocity reaches 4.5 m/s, the values of Vmax/Vmin equal 3.43, 1.20, and 1.03, the values of Vmax/Vav range from 1.38 to 1.25 and to 1.03, suggesting that increasing gas velocity does not improve significantly flow rate distribution in the distributor although increasing superficial gas velocity leads to high-pressure drop of a bubble cap. However, great flow resistance results from decreasing inlet diameter of the bubble cap can considerably improve flow rate distribution at the same superficial gas velocity. Bubble cap inlet diameter is a predominant parameter to determine flow rate distribution through various bubble caps because the resistance characteristics of a bubble cap depend strongly on its inlet diameter. Furthermore, Figures 5 and 6 indicate that the bubble caps at the center region have large flow rates, while bubble caps at the border region have small flow rates. The relationship between the ratio of riser pressure drop to distributor pressure drop and Vmax/Vav will be reported in another paper.10 Lateral Solids Volume Concentration Distribution at CFB Bottom Region. To study flow behaviors in the bottom region, the capacitance probe was inserted into the opposite side of the air entrance at an axial height (z) of 0.345 m above the distributor. Figure 7 plots the lateral profiles of solids volume concentration using three sizes of caps at the superficial gas velocity of 3.0 and riser pressure drop of 5500 Pa, respectively. Such profiles can be divided into two categories: one is for low-pressure drop of the bubble cap, i.e., 82.5- and 54.5-mm-i.d. bubble caps, and the other is for highpressure drop of the bubble cap, i.e., 35-, 38.5-, and 23mm-i.d. bubble caps. Under low-pressure drop of the bubble cap, there is an uneven distribution in the profiles of solids volume concentration. The solids volume concentration in the center region is low, while solids volume concentration increases significantly toward the wall. The highest solids volume concentration (Cv) at the wall approaches the Cv value at the packed bed. Under high-pressure drop of the bubble cap, solids volume concentration in the center region is in the range from 0.087 to 0.25, whereas solids volume concentration in the wall region is increased from 0.25 to 0.45. Apparently, the lateral profiles reveal that solids volume

concentration varies sharply in the wall region and is nearly uniform in the center region. In particular, the solids volume concentration at high-pressure drop of a bubble cap is lower than at the low-pressure drop at all lateral positions. At the high-pressure drop of a bubble cap, the CFB bottom region fluidizes very well due to the fact that fluidization gas passes uniformly through the distributor. Whereas, at the low-pressure drop of a bubble cap, the bubble caps at the entrance and the zone opposite the entrance have low flow rates, which were in a defluidized state. As the superficial gas velocity is increased to 4.5 m/s, the profiles of lateral solids volume concentration for different sizes of bubble caps are depicted in Figure 8. All curves of solids volume concentration are parabolic distributions. The solids volume concentration at the low-pressure drop of the bubble cap is higher than that for the high-pressure drop. Further, Figure 8 also exhibits that the solids concentration at the entrance region is lower than that at the opposite wall region. In particular, the flow rate of the bubble cap at the entrance differs from that at the side opposite the entrance because the low-pressure drop of a bubble cap leads to maldistribution of flow rate through different bubble caps. Accordingly, lateral profiles of solids volume concentration in Figures 7 and 8 are asymmetrical. Horizontal Solids Volume Concentration Distribution at CFB Bottom Region. Figure 9 shows the horizontal profiles of solids volume concentration for three sizes of bubble caps at x ) 0.05 m and z ) 0.345 m. The solids volume concentration (Cv) keeps constant, 0.42, in the center region, and then Cv approaches 0.48 toward the wall nearly equal to that in the fixed bed, Figure 9a. This implies that solid particles are in defluidization conditions due to the fact that bubble caps at the gas entrance have small flow rate, as illustrated in Figure 5, resulting in solids accumulation at the gas entrance keeping thick. For the 38.5-mm-i.d. bubble cap, Cv ranges from 0.125 to 0.23 in the center region, while Cv increases sharply to 0.40 at the wall region. For the 23.0-mm-i. d. bubble cap, the solids volume concentration curve is also characterized by the same trend as the 38.5-mm-i.d. solids volume concentration profile. At x ) 0.5 m and z ) 0.345 m, Figure 9b, the horizontal profile of the solids volume concentration becomes flattened with Cv varying from 0.2 to 0.4 by using the 54.5-mm-i.d. bubble cap. Solids volume concentration using 38.5- and 23-mm-i.d. bubble caps is lower than that using the 54.5-mm-i.d. bubble cap. The


Figure 10. Local solids volume concentrations fluctuation curves for three radial locations at u ) 3.0 m/s and ∆Pr ) 5500 Pa. Figure 9. Horizontal profiles of solids volume concentration in distributor B at u ) 3.0 m/s and z ) 0.345 m.

reason is that there is a serious maldistribution of flow rate across the 54.5-mm-i.d. bubble cap distributor compared with 38.5- and 23-mm-i.d. bubble cap distributors. In summary, all three curves show a lean core phase surrounded by an annular region with higher solids concentration. At x ) 0.95 m and z ) 0.345 m, Figure 8c, solids volume concentrations are increased from 0.15 to 0.45 for 38.5- and 23.0-mm-i.d. bubble caps. For the 54.5mm-i.d. bubble cap, solids volume concentration rises gradually from 0.08 to 0.38 at the wall. It is strange to observe that solids volume concentration measured at x ) 0.95 m and y ) 0 m by a 10-mm-i.d. capacitance

probe is much lower than that measured by a 25-mmi.d. capacitance probe in the lateral direction. The reason is that pressure fluctuates with negative amplitude, causing much gas sucked into the windbox, which forces the 10-mm capacitance probe to vibrate strongly. The measurements by small-capacitance probe have relatively small value with large errors. To reach a better understanding of local flow behaviors, it is necessary to study the instantaneous behavior. Figure 10 shows 26.2-s traces of instantaneous solids volume concentration at three positions. At x ) 0.05 m and y ) 0 m, 0.145 m, the solids volume concentration is high with low-amplitude fluctuation, demonstrating that solid particles are in a defluidized state. It is worth noticing that Cv by a 10-mm capacitance probe is slightly less than that with 25-mm-i.d. capacitance



Figure 11. Horizontal profiles of solids volume concentration in distributor B at u ) 4.5 m/s and z ) 0.345 m.

because the 10-mm capacitance probe is subjected to strong vibration caused by gas aspirated into the windbox. At x ) 0.5 m and y ) 0 m, solids volume concentration is low with large-amplitude fluctuation, indicating that the flow is a dense suspension with Cv ) 0.2. Variations in solids volume concentration are expressed in the signal pattern result from the movements of solids or gas pockets. The rapid movement of a gas pocket through the sensor volume exhibits steep slopes of signals, which are easily distinguished from the slower movement of the bulk suspension phase. Figure 11 presents the horizontal profiles of solids volume concentrations at x ) 0.05 m and u ) 4.5 m/s. Three profiles were merged into one curve with Cv ranging from 0.05 to 0.15 as y in the range 0-0.145 m. It can be seen from this figure that solids volume concentration rises sharply toward the wall. At x ) 0.5

m, solids volume concentration also rises along the horizontal direction. Also, solids volume concentration with a 54.5-mm-i.d. bubble cap is slightly higher than that with the 38.5- and 23-mm-i.d. bubble caps, which agrees with the conclusions drawn from lateral solids volume concentration profiles, Figure 8. As illustrated in Figure 6, bubble caps at the air entrance region and in the center region have high a flow rate entering the bed, which makes bed be under good fluidization conditions. At x ) 0.95 m, three profiles with different inlet diameters of caps reveal that solids volume concentration increases toward the wall. It is obvious to observe that the solids volume concentration with the 54.5-mmi.d. bubble cap is higher than that with 38.5- and 23mm-i.d. bubble caps because the 54.5-mm-i.d. bubble caps have a much lower flow rate entering the bed in this region, Figure 6. Such findings fit into conclusions obtained from Figure 9. On the other hand, bubble caps at the side opposite gas entrance have low flow rates, implying that solids particles were in the defluidization conditions. Compared with Figures 8 and 11c, it is found that Cv measured by the 10-mm capacitance probe is slightly smaller than that measured by the 25-mm capacitance probe at high superficial gas velocity. Lateral Solids Volume Concentration in the Dilute Region. At Z ) 3.82 m above the distributor, the solids volume concentration both in the center region and in the wall region ranges from 0.37 to 2.8 vol % at a superficial gas velocity of 3.0 m/s, as shown in Figure 12. No notable differences can be observed in the lateral profiles of solids volume concentration of bubble caps with different diameters. Such a flow pattern can be characterized by the core-annulus structure where low solids volume concentration keeps flat in the core region and solids volume concentration rises in the wall region. At u ) 4.5 m/s, Figure 13, profiles of solids volume concentration in the center reveal a flat distribution with solids volume concentration in the range of 0.84.0 vol %. There are a dense annular region near the wall and a relatively dilute region in the core. Axial Apparent Solids Concentration. To investigate the effect of the pressure drop of a bubble cap on the axial apparent solids concentration, the axial profiles of apparent solids concentration were determined using the measured axial pressure gradient evaluated by

Cv,app ) ∆p/∆zgFs

(2)



Figure 14. Axial profiles of apparent solids concentration for six inlet diameters of bubble caps.

where g and Fs are the gravitational acceleration and the solids density, respectively. Figure 14 reveals axial apparent solids concentration for distributors A and B with various inlet diameters of caps at u ) 4.22 m/s and ∆Pr ) 5500 Pa. It can be seen from this figure that the apparent solids concentration in the CFB bottom zone is highest for the 54.5mm-i.d. bubble cap, distributor B. Solids volume concentration is high in the CFB bottom region in the case of low-pressure drop of bubble caps. This conclusion is consistent with the experimental findings obtained in Figures 7, 8, 12, and 13 as well as previous investigations,11-13 which is characterized by a dense bottom zone and an upper dilute region. All axial profiles of apparent solids concentration are very similar at an axial height greater than 0.481 m. Consequently, the pressure drop of a bubble cap has little effect on axial apparent solids concentration in the upper dilute zone.

Figure 15. Pressure fluctuation curves for three local positions.

Backflow of Solids. One of the undesirable problems encountered in circulating fluidized bed boiler operation is solids backflow into the windbox resulting from pressure fluctuations in the fluidized bed. The pressure fluctuation curves were presented in Figure 15. Observing the signals from the bottom region in Figure 15a and b, the fluctuation curves differ greatly as the negative component and amplitude of the fluctuation increase rapidly as x increases from 0.093 to 0.982 m. This suggests that the pressure at the outlet of a bubble cap is higher than that at a bubble cap inlet. In this case, the gas flow reverses which solids were probably aspirated into the nozzles. Whereas, pressure fluctuation in the center region bubble caps exhibits positive amplitude, as shown in Figure 15c. The investigation by Hartge14 demonstrated that a bubble cap design with a duct where gas is flowing downward to the gas outlet could avoid solids backflow and erosion. With duct structure, the solids flow upward against gravity is


Figure 16. Erosion pictures in distributors A and B.

caused by reversed gas, which stops solids from feeding into a bubble cap annulus. After every experiment, no sand was observed to backflow into the windbox. Erosion. Erosion is a serious operation problem occurring in the CFB gas distributor. According to Hartge et al.,14 erosions in circulating fluidized bed boiler operation occur at the distributor, at the nozzle, and at its neighboring nozzles or walls. Erosion at the bottom plate exists where nozzles generate jets having a significant velocity component in the downward direction. Erosion results from the horizontal component of gas jets at the gas outlet being responsible for erosion at neighboring nozzles and walls. Generally, a sufficient distance between the nozzle and other objects is dominant parameter to prevent such erosion. Erosion at the nozzle is always found at the outlet of a nozzle cap, which can be attributed to two mechanisms. The first is the pressure-induced mechanism of flow reversal, where solid particles entered the nozzles impinging on the edges of the outlet holes causing erosion. The second is that gas jet entrains solid particles into the hole. For bubble caps, erosion is ascribed to the upflowing gassolids suspension. The gas jets emitted from bubble caps have a larger horizontal velocity component, causing many solid particles to move upward at the CFB bottom region. These jets tend to generate bubbles whose wakes entrain a lot of solids as jets penetrate the bed in the vertical direction. To check erosion situations, all bubbles caps were painted with black paint. After a 100-h operation, the surfaces of the bubble caps became bright without any black paint, as shown in Figure 16. No erosion exists in the distributor. Interestingly, Figure 16 illustrates that the surface in the low section of a bubble cap corrodes more seriously than the upper section, indicating that the upward flow has a large effect on corrosion. Conclusions In the CFB bottom region, for low-pressure drop of a bubble cap, there is an obvious nonuniform distribution in the profile of solids volume concentration. The solids volume concentration in the center region is low, while solids volume concentration increases significantly toward the wall. The highest solids volume concentration

at the wall approaches the value at the packed bed. Under high-pressure drop of a bubble cap, the lateral profile shows that solids volume concentration varies significantly in the wall region and is nearly uniform in the core region where the solids volume concentration is in the range from 0.18 to 0.25. Furthermore, the solids volume concentration at high-pressure drop of a bubble cap is lower than that at low-pressure drop. Horizontal solids volume concentration in the center region remains flat and rises toward the wall. By contrast, horizontal solids volume concentration ranges from 0.43 to 0.5 at low superficial gas velocity and entrance region. Horizontal solids volume concentration with the 54.5-mm-i.d. bubble cap is slightly higher than that with the 38.5- and 23-mm-i.d. bubble caps. In the CFB upper dilute region, lateral profiles of solids volume concentration exhibit similar distributions at a given riser pressure drop and a given superficial gas velocity with various inlet diameters of caps. Such profiles confirm that a core-annulus flow structure exists in a circulating fluidized bed with a square cross section. It follows that the pressure drop of the distributor has little influence on the profiles of solids volume concentrations and particle velocities in the dilute region. It has been demonstrated that the pressure drop of the distributor has little influence on the axial apparent solids concentration. Bubble cap-type design with a duct where gas is flowing downward to the gas outlet is the recommended structure to avoid solids backflow. Acknowledgment Q.G. gratefully acknowledges a research fellowship awarded by the Alexander von Humboldt Foundation. The partial financial support from Natural Science Foundation of Shandong Province (Contract No. Z2003B01) and State Key Laboratory of Heavy Oil Processing is appreciated. Nomenclature A ) entrance area of a bubble cap, m2 Cd ) drag coefficient Cv,app ) axial profiles of axial apparent solids concentration Cv ) solids volume concentration g ) gravitational acceleration, m/s2 t ) time, s uin ) gas velocity at the entrance of the bubble cap, m/s u ) superficial gas velocity, m/s Vbc ) flow rate through a bubble cap, m3/h Vmax ) maximum flow rate through a bubble cap, m3/h Vmin ) minimum flow rate through a bubble cap, m3/h Vav ) average flow rate through a bubble cap, m3/h X ) lateral distance, m Y ) horizontal distance, m Z ) axial height, m ∆P ) axial pressure gradient, Pa ∆Pbc ) pressure drop of a bubble cap, Pa ∆Pr ) riser pressure drop, Pa ∆Pd ) distributor pressure gradient, Pa Fs ) solids density, kg/m3 Fo ) gas density, kg/m3

Literature Cited (1) Nieuwland, J.; Meijer, R.; Kuipers, J. A. M.; van Swaaij, W. P. M. Measurements of Solids Concentration and Axial Solids Velocity in Gas-Solid Two-Phase Flows. Powder Technol. 1996, 87, 127-129.

1764 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 (2) Issangya, A. S.; Bai, D.; Bi, H. T.; Lim, K. S.; Zhu, J.; Grace J. R. Suspension Densities in a High-Density Circulating Fluidized Bed Riser. Chem. Eng. Sci. 1999, 54, 5451-5460. (3) Bai, D.; Shibuya, E.; Masuda, Y.; Nakagawa, N.; Kato, K. Flow Structure in a Fast Fluidized Bed. Chem. Eng. Sci. 1996, 51, 957-966. (4) Rhodes, M. J.; Sollaart, M.; Wnag, X. Flow Structure in a Fast Fluidized Bed. Powder Technol. 1998, 99, 201-209. (5) Berruti, F.; Kalogerakis, N. Modeling of the Internal Flow Structure of Circulating Fluidized Beds. Can. J. Chem. Eng. 1989, 67, 1010-1014. (6) Werther, J.; Hartge, E.-U.; Kruse, M. Radial gas mixing in the upper dilute core of a circulating fluidized bed. Powder Technol. 1992, 70, 293-301. (7) Johnston, P. M.; Zhu, J.; Lasa, I.; Zhang, H. Effect of Distributor Designs on the Flow Development in Downer Reactor. AIChE J. 1999, 45, 1587-1592. (8) Hage, B.; Werther, J. The Guarded Capacitance Probe. A Tool for the Measurement of Solids Flow Pattern in Laboratory and Industrial Fluidized Bed Combustors, Powder Technol. 1997, 93, 235-245. (9) Wiesendorf, V.; Werther, J. Capacitance Probes for Solids Volume Concentration and Velocity Measurements in Industrial Fluidized Bed Reactors. Powder Technol. 2000, 110, 143-157.

(10) Guo, Q.; Werther, J.; Aue-Klett, C.; Hartge, E.-U. Flow Maldistribution at Bubble Cap Distributor in a Plant-scale CFB Riser. Submitted to AIChE J. (11) Li, Y.; Kwark, M. The Dynamics of Fast Fluidization. In Fluidization; Grace, J. R., Matsen, J. M., Eds.; Plenum Press: New York, 1980; pp 537-544. (12) Arena, U.; Cammarota, A.; Pistone, L.; Tecchio, P. V. The High Velocity Fluidization Behavior of Solids in a Laboratory Scale Circulating Bed. In Circulating Fluidized Bed Technology; Basu, P., Ed.; Pergamon Press: New York, 1986; pp 119-125. (13) Bai, D.; Jin, Y.; Yu, Z.; Zhu, J. The Axial Distribution of the Cross-sectionally Averaged Voidage in Fast Fluidized Beds. Powder Technol. 1992, 71, 51-58. (14) Hartge, E.-U.; Werther, J. Gas Distributors for Circulating Fluidized Bed Combustors. In Fluidization IX,; Fan, L.-S., Thowlton, T. M., Eds.; Engineering Foundation: New York, 1998; pp 213-220.

Received for review July 29, 2003 Revised manuscript received December 3, 2003 Accepted January 26, 2004 IE030629L

Flow Behaviors in a Circulating Fluidized Bed with Various Bubble

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