Investigation on Flow Patterns and Transitions in a Multiple-Spouted

Feb 19, 2010 - Typical flow regime maps at three static bed heights were plotted to describe the transitions of flow patterns with central and auxilia...
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Energy Fuels 2010, 24, 1941–1947 Published on Web 02/19/2010

: DOI:10.1021/ef901449m

Investigation on Flow Patterns and Transitions in a Multiple-Spouted Bed Bing Ren, Wenqi Zhong,* Yong Zhang, Baosheng Jin,* Xiaofang Wang, He Tao, and Rui Xiao School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China Received December 1, 2009. Revised Manuscript Received February 7, 2010

Experimental studies on flow patterns and transitions were carried out in a visible multiple-spouted bed. The bed combines three spouted bed cells, each with a cross-section of 100  30 mm, and each cell has an independent spout nozzle of 10 mm in width. Polypropylene beads with a density of 900 kg/m3 and mean diameter of 2.8 mm were used as bed materials. Six distinct flow patterns, i.e., fixed bed (FB), internal jet (IJ), internal jet with bubble (IJB), single spouting (SS), multi-spouting (MS), and internal jet with slugging (IJS), were determined on the basis of criteria as well as schematic diagrams and typical flow pattern images obtained from a high-resolution digital charge coupled device (CCD) camera. Typical flow regime maps at three static bed heights were plotted to describe the transitions of flow patterns with central and auxiliary spouting gases. Besides, some important flow characteristics associated with flow patterns and transitions, i.e., minimum spouted velocity and pressure drop, were studied. The results showed that the kind of flow pattern was dependent upon the static bed height; in particular, the flow pattern of IJS was only found at a high static bed height. The central minimum spouting velocity increased with an increasing static bed height, decreased with a low auxiliary spouting gas flow rate, but increased with a high auxiliary spouting gas flow rate. The total pressure drop increased first and then decreased gradually with the auxiliary spouting gas at a certain central spouting gas flow rate, while it increased first and then remarkably decreased with the central spouting gas at a given auxiliary spouting gas flow rate.

include multiple-spouted bed,12-14 spout-fluid bed,15,16 spouted bed with draft tube,17-19 rotating spouted bed,20-22 etc. These modifications have been proven to be quite effective for gas/ solid contacting in the laboratory scale. However, the spouted bed technique has seldom been applied in large-scale industrial processes because of scale-up difficulties, such as the inability to achieve good quality spouting in large vessels, and difficulties in predicting the performance of spouted beds larger than about 0.3 m in diameter.23 Thus, the multiple-spouted bed seems to be a valuable modification in industry application.

1. Introduction Spouted beds have been widely used in energy transform, fuel processing, pharmaceutical industry, etc. for drying,1,2 coating,3,4 pyrolyzing,5,6 combustion,7,8 and gasification,9,10 especially for the handling of solids that are sticky, of irregular texture, or with a large range of particle size distribution.11 To achieve the optimum performance of the spouted bed, modifications have been carried out in engineering applications. Examples *To whom correspondence should be addressed. Telephone: þ86-2583794744. Fax: þ86-25-83795508. E-mail: [email protected] (W.Z.); [email protected] (B.J.). (1) Jenkins, S. A.; Waszkiewicz, S.; Quarini, G. L.; Tierney, M. J. Drying saturated zeolite pellets to assess fluidised bed performance. Appl. Therm. Eng. 2002, 22, 861–971. (2) Benali, M.; Amazous, M. Drying of vegetable starch solutions on inert particles: Quality and energy aspects. J. Food Eng. 2006, 74, 484– 489. (3) Jono, K.; Ichikawa, H.; Miyamoto, M.; Fukumori, Y. A review of particulate design for pharmaceutical powders and their production by spouted bed coating. Powder Technol. 2000, 113, 269–277. (4) Ichikawa, H.; Arimoto, M.; Fukumori, Y. Design of microcapsules with hydrogel as a membrane component and their preparation by a spouted bed. Power Technol. 2003, 130, 189–192. (5) Aguado, R.; Olazar, M.; Gaisan, B.; Prieto, R.; Bilbao, J. Kinetics of polystyrene pyrolysis in a conical spouted bed reactor. Chem. Eng. J. 2003, 92, 91–99. (6) Scott, D. S.; Czernik, S. R.; Piskorz, J.; Radlein, A. G. Fast pyrolysis of plastic wastes. Energy Fuels 1990, 4, 407–411. (7) Lim, C. J.; Watkinson, A. P.; Khoe, G. K.; Low, S.; Epstein, N.; Grace, J. R. Spouted, fluidized and spout-fluid bed combustion of bituminous coals. Fuel 1988, 67, 1211–1217. (8) Rasul, M. G. Spouted bed combustion of wood charcoal: Performance comparison of three different designs. Fuel 2001, 80, 2189–2191. (9) Sue-A-Quan, T. A.; Cheng, G.; Watkinson, A. P. Coal gasification in a pressurized spouted bed. Fuel 1995, 74, 159–164. (10) Kato, S.; Fukuda, H. Spouted-bed coal gasifier. Fuel Energy 1997, 38 (1), 93–93. (11) Olazar, M.; Alvarez, S.; Aguado, R.; San Jose, M. J. Spouted bed reactors. Chem. Eng. Technol. 2003, 26, 845–852. r 2010 American Chemical Society

(12) Xu, X. P.; Zhu, J. G.; Zhang, B. Z.; Pan, J. S. Fuel particle coating in a multiple gas inlet spouted bed. J. Tsinghua Univ. 2000, 40, 63–66 (in Chinese). (13) Albina, D. O. Combustion of rice husk in a multiple-spouted fluidized bed. Energy Sources 2003, 25, 893–904. (14) Saidutta, M. B.; Murthy, D. V. R. Mixing behaviour of solids in multiple spouted beds. Can. J. Chem. Eng. 2000, 78, 382–385. (15) Chaterjee, A. Spout-fluid bed technology. Ind. Eng. Chem. Process Des. Dev. 1970, 9, 340–341. (16) Sutanto, W.; Epstein, N.; Grace, J. R. Hydrodynamics of spoutfluid beds. Powder Technol. 1985, 44, 205–212. (17) Ishikura, T.; Nagashima, H.; Ide, M. Hydrodynamics of a spouted bed with a porous draft tube containing a small amount of finer particles. Powder Technol. 2003, 131, 56–65. (18) Zhao, X. L.; Yao, Q.; Li, S. Q. Effects of draft tubes on particle velocity profiles in spouted beds. Chem. Eng. Technol. 2006, 29, 875–881. (19) Subramanian, G.; Turton, R.; Sheluker, S.; Flemmer, L. Effect of tablet deflectors in the draft tube of fluidized/spouted bed coaters. Ind. Eng. Chem. Res. 2003, 42, 2470–2478. (20) Jumah, R. Y.; Mujumdar, A. S.; Raghavan, G. S. V. Batch drying kinetics of corn in a novel rotating jet spouted bed. Can. J. Chem. 2009, 74, 479–486. (21) Devahastin, S.; Mujumdar, A. S.; Raghavan, G. S. V. Hydrodynamic characteristics of a rotating jet annular spouted bed. Powder Technol. 1999, 103, 169–174. (22) Jumah, R. Y. Flow and drying characteristics of a rotating jet spouted bed. Drying Technol. 1995, 13, 2243–2250. (23) Chen, Z. W. Hydrodynamics, stability and scale-up of slotrectangular spouted beds. Ph.D. Thesis, The University of British Columbia, Vancouver, British Columbia, Canada, 2008.

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: DOI:10.1021/ef901449m

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Many valuable experimental and theoretical studies on the conventional spouted beds have been carried out, and detailed information can be found in the work by Mathur and Epstein24 and other publications over the last 30 years.25-28 However, only few studies have been focused on the multiple-spouted beds. Fakhimi et al.29 studied the flow behavior of multi-orifice distributors in a gas-solid fluidized bed and, particularly, focused on the height of the entrance effect and the mechanics of gas-solid flow in the region immediately above the distributor plate. Murthy et al.30,31 studied the minimum spouting velocities in three rectangular columns with two, three, and four spout cells. Zhang et al.32,33 investigated the minimum spouted velocity, maximum spouted pressure drop, and maximum spoutable height in a double-nozzle rectangular draft tube spouted bed. Hu et al.34 carried out experimental studies on the flow patterns in a spouted bed with two air nozzles. However, applications of the multiple-spouted bed is still restricted in industry application, because of some limitations, in particular, the lack of full knowledge on the flow characteristics, e.g., minimum spouted velocity, spouted pressure drop, maximum spoutable bed height, flow pattern and transition, gas mixing, particle mixing, etc. As one of the most important hydrodynamic characteristics, the flow pattern and transition is of special interest and plays an essential role in designing and operating multiple-spouted bed reactors. However, it has not yet been completely revealed. In the study by Hu et al.,35 three distinct stable flow patterns, i.e., internal jet, jet spouting, and fully developed spouting, were identified. However, a detailed understanding of flow pattern transitions is still needed for this study and other investigations. Many questions, e.g., how many flow patterns on earth are there in a multiple-spouted bed, how do they transit with operating conditions (i.e., static bed height and central/annular spouting gas velocity), what are the significant differences of these flow patterns and their transitions between a multiple-spouted bed and a conventional spouted bed, etc., remain. To answer some of these interesting questions, experiments have been carried out in a spouted bed with three spout nozzles in the present work. It focuses on classifying the

observed flow patterns by means of certain criteria as well as schematic diagrams and typical flow pattern images and plotting typical flow regime maps at different static bed heights. In addition, two important flow characteristics related to this topic of minimum spouted velocity and total bed pressure drop were also studied. 2. Experimental Section The visible multiple-spouted bed experimental system is schematically shown in Figure 1, which includes a gas supply system, a rectangular multiple-spout vessel, and a charge coupled device (CCD) imaging system. The vessel has a cross-section of 300  30 mm and a height of 1200 mm. The vessel was made of 8 mm thick transparent Plexiglas, which can be regarded as the combination of three spouted bed cells with a cross-section of 100  30 mm. Each cell has an independent spout nozzle with 10 mm in width. The central and two auxiliary spouting gases were supplied by an air compressor. A bypass ball valve was installed to achieve a steady gas flow and avoid pressure and flux oscillations. The gas flow rates were controlled by three independent float flowmeters. The flow rates through both left and right spout nozzles were denoted as Qa, and the flow rate through the central spout nozzle was denoted as Qc. The total flow rates introduced into the bed is the sum of the flux via these spout nozzles, i.e., 2Qa þ Qc. A multichannel differential pressure signal acquisition system was applied to measure the pressure fluctuations. There were 12 pressure-measuring holes located on the back wall of the vessel, at heights of 0, 40, 50, 80, 120, 250, 270, 350, and 550 mm, respectively. The differential pressures were measured and then converted into voltage signals by a multichannel differential pressure signal transmitter with a scale of 0-10 kPa. Then, the voltage signals were sent to a computer through an A/D converter. During the experiment, the total bed pressure drop was obtained between the two pressure taps at the height of 0 and 550 mm. A digital camera (Nikon 5000) and a digital video recorder (Sony DCR-PC330E) were employed to photograph the flow patterns and transitions in the experiment. The high-speed digital camera includes a high-speed consecutive shooting mode of up to 3 frames/s in JPEG mode, which can meet the need of capturing a series of the gas-solid flow structure. To obtain photographs as vivid as possible, the vessel was illuminated by two 2000 W floodlights, one on each side for uniform lighting. Polypropylene beads were used as bed material, and detailed properties are listed in Table 1.

(24) Mathur, K. B.; Epstein, N. Spouted Beds; Academic Press: New York, 1974. (25) He, Y. L.; Lim, C. J.; Grace, J. R. Spouted and spout-fluidized beds behavior in a column of diameter 0.91 m. Can. J. Chem. Eng. 1992, 70, 848–851. (26) Kalwar, M. I.; Raghavan, G. S. V.; Mujumdar, A. S. Spouting of two-dimensional beds with draft plates. Can. J. Chem. Eng. 1992, 70, 887–894. (27) Passos, M. L.; Mujumdar, A. S.; Raghavan, V. S. G. Prediction of the maximum spoutable bed height in two-dimensional spouted beds. Power Technol. 1993, 74, 97–105. (28) Wu, S. W. M.; Lim, C. J.; Epstein, N. Hydrodynamics of spouted beds at elevated temperatures. Chem. Eng. Commun. 1987, 63, 251–268. (29) Fakhimi, S.; Sohrabi, S.; Harrison, D. Entrance effects at a multiorifice distributor in gas-fluidised beds. Can. J. Chem. Eng. 1983, 61, 364–369. (30) Murthy, D. V. R.; Singh, P. N. Minimum spouting velocity in multiple spouted beds. Can. J. Chem. Eng. 1994, 72, 235–239. (31) Saidutta, M. B.; Murthy, D. V. R. Mixing behaviour of solids in multiple spouted beds. Can. J. Chem. Eng. 2000, 78, 382–385. (32) Zhang, S. F.; Wang, S. H.; Zhao, J. B. Experimental study on hydrodynamics characteristics of dobble-nozzle rectangular spouted bed. Chem. Eng. 2006, 34, 33–39. (33) Zhang, S. F.; Zhao, B.; Liu, Y. Hydrodynamics and desulphurization effectiveness of double-nozzle rectangular spouted bed. Environ. Pollut. Control 2006, 28, 419–422. (34) Hu, G. X.; Gong, X. W.; Wei, B. N.; Li, Y. H. Flow patterns and transitions of a novel annular spouted bed with multiple air nozzles. Ind. Eng. Chem. Res. 2008, 47, 9759–9766. (35) Mathur, K. B.; Gishler, P. E. A technology for contacting gases with coarse solid particles. AIChE J. 1955, 1, 129–144.

3. Results and Discussion 3.1. Identification of Flow Patterns. The patterns were determined by adjusting the central spouting gas flow rate for a given static bed height while holding both auxiliary spouting gas flow rates constant or by changing the auxiliary spouting gas flow rate stepwise, while the central spouting gas flow rate remains invariable. Six representative flow patterns were observed in the experiment, they are fixed bed (FB), internal jet (IJ), internal jet with bubble (IJB), single spouting (SS), multi-spouting (MS), and internal jet with slugging (IJS). A schematic representation of these flow patterns is illustrated in Figure 2, and typical pictures obtained by the high-resolution digital CCD camera are presented in Figure 3. 3.1.1. FB. When the central and auxiliary spouting gas flow rates are low, the bed is immobile. In this case, small internal jet cavities can be seen at the air inlet orifices. These jet cavities are quite stable and do not easily collapse. 3.1.2. IJ. With the increase of the central or auxiliary spouting gas flow rate, obvious jets will occur inside the bed; small bubbles release from the top of the jet and erupt at the 1942

Energy Fuels 2010, 24, 1941–1947

: DOI:10.1021/ef901449m

Ren et al.

Figure 1. Schematic diagram of the multiple-spouted bed experimental setup: (1) compressor, (2) pressure gauge, (3) desiccator, (4) ball valve, (5) safety relief valve, (6) control valve, (7) flow meter, (8) spout nozzle, (9) pressure port, (10) multiple-spouted bed column, (11) digital CCD, (12) floodlight, (13) differential pressure sensor, (14) multichannel differential pressure signal transmitter, (15) A/D converter, and (16) computer.

Figure 2. Schematic representation of various flow patterns: (a) FB, (b) IJ, (c) IJB, (d) SS, (e) MS, and (f) IJS.

bed surface. Here, it is worth pointing out that the jet penetration depth will increase until it reaches a peak value. This flow pattern is similar to those definitions of IJ for conventional spouted beds or spout-fluid beds in the literature.36,37 3.1.3. IJB. Bubbles lift off the top of the jets, then coalesce into a relatively big bubble, and break up in the bed surface. This phenomenon is similar to that in a jetting fluidized bed.

Table 1. Particle Properties particles

dp (mm)

Fp (kg/m3)

εp

umf (m/s)

polypropylene

2.8

900

0.42

0.82

It was also observed that, when the central spouting gas flow rate is higher than the auxiliary spouting gas flow rate, the two auxiliary spouting jets always incline toward the central jet region at most times and sometimes swing occasionally. This flow pattern has a potential advantage for specific application of coal and biomass gasification, which can make it difficult for much of the gasification agent (steam and air) to pass through the bed and become wasted because

(36) He, Y. L.; Lim, C. J.; Grace, J. R. Spouted bed and spout-fluid bed behaviors in a column of diameter 0.91 m. Can. J. Chem. Eng. 1992, 70, 848–857. (37) Sutanto, W.; Epstein, N.; Grace, J. R. Hydrodynamics of spoutfluid beds. Powder Technol. 1985, 44, 205–212.

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Energy Fuels 2010, 24, 1941–1947

: DOI:10.1021/ef901449m

Ren et al.

Figure 3. Typical flow pattern images of various flow patterns: (a) FB, H0/Dt0 = 1.0, Qc/Qmf = 0.47, Qa/Qmf = 0.70; (b) IJ, H0/Dt0 = 1.0, Qc/ Qmf = 1.32, Qa/Qmf = 0.51; (c) IJB, H0/Dt0 = 3.0, Qc/Qmf = 3.06, Qa/Qmf = 1.83; (d) SS, H0/Dt0 = 1.5, Qc/Qmf = 3.6, Qa/Qmf = 1.65; (e) MS, H0/Dt0 = 1.0, Qc/Qmf = 1.2, Qa/Qmf = 3.03; and (f) IJS, H0/Dt0 = 3.0, Qc/Qmf = 1.26, Qa/Qmf = 1.98.

of no spout formation in the bed, which can prolong the resident time of steam and air in the bed and enhance the gas-solid two-phase interaction between the spout and annular regions, improving gasification efficiency. 3.1.4. SS. When the gas flow rate is beyond the minimum spouting velocity, the packing of particles is loosened and then spouting takes place. Under this condition, the bed has essential characteristics to a conventional spouted bed in the flow pattern of spouting. It can be observed that the spout is stable and non-pulsating, and the movement of particles is smooth. The flow area can be obviously divided into three regions: a central spout region, where the gas and particles rise at high velocity and the particle concentration is low; a fountain zone, where particles rise to their highest positions and then rain back onto the surface of the annulus; and an annulus zone between the spout and the column wall, where particles move slowly downward as a dense phase, with counter-current percolation of the fluid. However, spouting in a multiple-spouted bed is somewhat different to that in a conventional spouted bed. The main difference is that the auxiliary jets swing toward the central jet because of the entrainment of the central jet at a relatively high auxiliary gas flow rate. This flow pattern, on the one hand, greatly improves the particle circulation in the annulus and, on the other hand, significantly reduces the dead zone. 3.1.5. MS. In this case, two or three distinct fountains can be observed. When the central spouting gas is relatively larger than the auxiliary spouting gas, the central fountain expands through the surface of the bed and, therefore, restricts the growth of the two auxiliary spouting gases. On the contrary, when the auxiliary gas flow rate is somewhat larger than the central gas flow rate, the development of the central spout will be suppressed by the auxiliary spouts, leading to a lower central fountain than the other two. It is noticed that both of the two situations present instability forms. Three spouts may continually change their moving directions or even merge with each other because of the turbulent interactions between them. This flow pattern has a potential advantage for the specific application of drying, fast pyrolysis, and combustion because of intensive interactions of gas and particles, while large spouting gas velocity in gasification might result in spouting and much of the steam and air passing through the bed and becoming wasted.

Figure 4. Typical flow regime map at various central and auxiliary spouting gas flow rates: H0 = 100 mm (H0/Dt0 = 1.0).

3.1.6. IJS. This is an unstable gas-solid flow pattern that generally occurs at high central and annular spouting gas flow rates. In this flow pattern, small bubbles lift off from the jets because of the acute momentum dissipation and then combine into a larger bubble at the middle and lower parts of the bed. When the bubble size grows to the entire bed section, periodic slugging occurs, causing particles of the whole cross-section to move up and down. This flow pattern can often be observed when the bed depth is relatively high. In the aspect of the operation on a multiple-spouted coal gasifier, a low static bed height easily brings a spout, resulting in much of the gasification agent passing through the bed and becoming wasted, while a high static bed height will lead to slugging. The uniform distribution of pressure, gas, and temperature will be destroyed when slugging occurs. Hence, a proper static bed height is expected for gasification. 3.2. Flow Regime Map and Flow Pattern Transition. A typical flow regime map at static bed height H0 = 100 mm is plotted in Figure 4, with the central spouting gas flow rate Qc plotted on the abscissa axis and the auxiliary spouting gas flow rate Qa plotted on the ordinate, both normalized with respect to the gas flow rate at the minimum fluidized condition Qmf for a cell. It can be seen that there are five different flow patterns in this case, i.e., FB, IJ, IJB, SS, and MS. In this flow regime map, the flow pattern of IJS could not be observed. Because small bubbles that lift off from the jets 1944

Energy Fuels 2010, 24, 1941–1947

: DOI:10.1021/ef901449m

Ren et al.

Figure 5. Typical flow regime map at various central and auxiliary spouting gas flow rates: H0 = 150 mm (H0/Dt0 = 1.5).

Figure 7. Minimum central spouting velocity as a function of the static bed height.

Figure 6. Typical flow regime map at various central and auxiliary spouting gas flow rates: H0 = 200 mm (H0/Dt0 = 2.0).

Figure 8. Minimum central spouting velocity as a function of the auxiliary spouting gas velocity.

are easy to penetrate the bed material when the bed depth is low, this leads to little probability of formation of a larger bubble at the middle-lower part of the bed. A similar nature of the flow regime map has been found at H0 = 150 mm, as shown in Figure 5. However, the flow regime map at a relatively high static bed height is somewhat different from those at a low static bed height. As presented in Figure 6 for H0 = 200 mm, there are six different flow patterns at this static bed height. In addition to the above five flow patterns, the unstable flow pattern of IJS is often observed at high central and auxiliary spouting gas flow rates. In this case, small bubbles are easy to combine into a large bubble when they penetrate the bed materials. Once the large bubble forms at the middle-lower part of the bed, periodic and unstable slugging can be seen. The difference of flow pattern transitions can be clearly seen by comparing these flow region maps. For example, when Qa/Qmf = 1.8, the flow pattern remains MS at H0 = 100 mm (H0/Dt0 = 1.0; Dt0 is a bed width of a spouted cell) by increasing the central gas flow rate, while the gas-solid flow pattern transits from IJB to MS and then to SS when H0 increases to 150 mm (H0/Dt0 = 1.5); for the static bed depth at 200 mm (H0/Dt0 = 2.0), the flow pattern transits from IJ to IJB and then to flow instability of IJS and, finally, reaches SS. Besides, it is found that, when the flow pattern of IJS occurs, the boundary between MS and this flow instability is unclear, as shown in Figure 6. 3.3. Minimum Central Spouting Velocity. The central minimum spouting velocity ums in a multiple-spouted bed

is defined as the local spouted-nozzle-based gas velocity at the point where the jet is sufficient to penetrate the entire bed materials and a fountain appears at the surface of the bed. This definition is similar to a previous study by Zhang et al.32 Figure 7 presents the effect of the static bed height on the minimum central spouting velocity at different static bed heights. It can be seen that the central minimum spouting velocity increases with the increasing of the static bed height. The increasing of the bed depth leads to the larger resistance force. The larger spouting velocity is needed to supply enough air to overcome the resistance force and penetrate the bed.38 Besides, the annular spouting gas shows no effect on the change of the minimum central spouting velocity with static bed heights. It is found that the auxiliary spouting gas flow rate has a significant effect on the minimum central spouting velocity at a certain static bed height. Figure 8 plotted the minimum central spouting velocity varying with the auxiliary spouting gas flow rate at different bed heights. When the auxiliary spouting gas flow rate is low, the minimum central spouting velocity decreases with an increasing auxiliary spouting gas flow rate, while the velocity increases when further raising the auxiliary spouting gas at a high auxiliary spouting gas flow rate. For example, at the static bed depth of 150 mm, the minimum central spouting velocity keeps decreasing with the (38) Dogan, O. M.; Uysal, B. Z.; Grace, J. R. Hydrodynamic studies in a half slot-rectangular spouted bed column. Chem. Eng. Commun. 2004, 191, 566–579.

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Energy Fuels 2010, 24, 1941–1947

: DOI:10.1021/ef901449m

Ren et al.

Figure 11. Total bed pressure drop as a function of the central spouting velocity at the static bed height of H0/Dt0 = 1.0 and different auxiliary spouting gas velocities.

Figure 9. Total bed pressure drop as a function of the auxiliary spouting gas velocity at H0/Dt0 = 1.0 and different central spouting gas flow rates.

Figure 12. Total bed pressure drop as a function of the central spouting velocity at Ua = 12 m/s and the different static bed heights.

Figure 10. Total bed pressure drop as a function of the auxiliary spouting gas flow rate Uc = 12 m/s and different static bed heights.

transition point of auxiliary spouting velocity increases with the static bed height. The changes of the total bed pressure drop with different central spouting gas velocities at the static bed height of H0/ Dt0 = 1.0 and auxiliary spouting gas flow rates of Ua = 5.6 and 12 m/s are plotted in Figure 11. It is found that the total bed pressure drop increases and then remarkably decreases with the central spouting gas velocity. The transitions seems to be due to the trends of central spouting at Uc = 8.8 and 6.5 m/s, respectively. Figure 12 presents the total bed pressure drop as a function of the central spouting velocity at Ua = 12 m/s and the static bed heights of H0/Dt0 = 1.5 and 2.0. The changes of the total bed pressure drop with different central spouting gas velocities at the different static bed heights appear to have the same characteristics as those in Figure 11; i.e., the total bed pressure drop increases and then remarkably decreases with the central spouting gas velocity. The transition point of the central spouting gas velocity increases with the static bed height. This could be explained as more spouting gas momentum is needed for the jet to penetrate the bed with the increasing of the static bed depth, leading to the increase of the bed pressure drop.

increasing of the auxiliary spouting gas velocity until Ua reaches about 8 m/s, and then the development of the minimum central spouting velocity follows a steadily higher trajectory. For a low auxiliary spouting gas flow rate, the auxiliary spouting gas tends to be entrained in the central spout jet to add the central spouting gas. As a result, the central spouting velocity required for minimum spouting will decrease. However, when the auxiliary gas flow rate reaches a certain value, it would dissipate the momentum of central spouting. In this case, the development of central spouting is suppressed by the auxiliary fountain and a larger central spouting velocity is needed to overcome the restriction. 3.4. Total Bed Pressure Drop. Figure 9 shows the total pressure drop as a function of the auxiliary spouting gas velocity at the static bed height of H0/Dt0 = 1. It can be seen that, with the increasing of the auxiliary spouting gas velocity, the total bed pressure drop first increases and then decreases gradually after it reaches a maximum value at about Ua = 9.3 m/s. This transition might be due to the change of gas-solid flow behaviors, because the auxiliary cells tend to spout at this auxiliary spouting gas velocity. Figure 10 gives the changes of total bed pressure with the auxiliary spouting gas velocity at different static bed heights. The same trend can be found that the total pressure drop first increases and then decreases gradually with the auxiliary spouting velocity. The transition points are found to be about Ua = 11.5 m/s for H0/Dt0 = 1.0, Ua = 15.7 m/s for H0/Dt0 = 1.5, and Ua = 18.5 m/s for H0/Dt0 = 2.0. The

4. Conclusions Experiments were carried out in a visible multiple-spouted bed, which is a combination of three spouted bed cells each with a cross-section of 100  30 mm. Typical flow patterns by 1946

Energy Fuels 2010, 24, 1941–1947

: DOI:10.1021/ef901449m

Ren et al.

certain criteria as well as schematic diagrams and typical flow pattern images were determined. Flow regime maps at different static bed heights were studied. Besides, some important flow characteristics associated with this topic, i.e., minimum spouted velocity and bed pressure drop, were discussed. The notable findings are presented as follows: (1) Six distinct flow patterns, i.e., FB, IJ, IJB, SS, MS, and IJS, could be identified. (2) The kind of flow pattern is different at different static bed heights, and most obviously, the flow pattern of IJS only exists at a high static bed height. (3) The central minimum spouting velocity increases with an increasing bed height, and auxiliary spouting gas has no effect on this trend. However, auxiliary spouting gas appears to affect remarkably the central minimum spouting velocity; i.e., the minimum central spouting velocity decreases with a low auxiliary spouting gas flow rate but increases with a high auxiliary spouting gas flow rate. (4) The total bed pressure drop increases first and then decreases gradually with the auxiliary spouting gas at a certain central spouting gas flow rate, while the total pressure drop increases first and then remarkably decreases with the central spouting gas at a certain auxiliary spouting gas flow rate.

Program on Key Basic Research Project of China (973 Program) (2010CB732206), and the Foundation of Excellent Young Scholar of Southeast University (4003001039) was sincerely acknowledged.

Nomenclature Dt = bed width (m) Dt0 = bed width of each cell (m) H0 = static bed height (m) Qc = central spouting gas flow rate (m3/h) Qa = auxiliary spouting gas flow rate (m3/h3) Qmf = mimimum fluidizing gas flow rate for a cell (m3/h) Ua = spouted-nozzle-based auxiliary spouting velocity (m/ s) Uc = spouted-nozzle-based central spouting velocity (m/s) Umc = spouted-nozzle-based minimum central spouting velocity (m/s) ums = spouted-nozzle-based minimum spouting velocity (m/s) umf = minimum fluidizing velocity (m/s) εp = particle bulk voidage λi = spout nozzle width of each cell (mm) Fp = particle density (kg/m3) dp = particle diameter (m)

Acknowledgment. Financial support from the National Natural Science Foundation of China (50706007), the National

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