Flow Patterns and Transitions of a Novel Annular Spouted Bed with

Ind. Eng. Chem. Res. 2008, 47, 9759–9766

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Flow Patterns and Transitions of a Novel Annular Spouted Bed with Multiple Air Nozzles Guoxin Hu,* Xiwu Gong, Bingnan Wei, and Yanhong Li School of Mechanical and Power Engineering, Shanghai Jiaotong UniVersity, Shanghai 200240, China

Experimental study on the flow patterns and transitions in a novel annular spouted bed with multiple air nozzles was carried out. Three distinct stable flow patterns, i.e. internal jet, jet-spouting, and fully developed spouting were identified. Two transitional flow patterns and flow instabilities, single internal jet, single jetspouting, and bubbling or slugging were found. Schematic diagrams and typical flow pattern images obtained by a digital charge-coupled display (CCD) camera were presented for classifying these flow patterns. Experimental results show that the internal jet or jet-spouting is easier to occur for forward nozzles than for angle nozzles with the increasing of spouting gas velocity. Due to the interaction of spouting gases from different nozzles, the turbulent exchange or mixing of particles among the nozzles in the fully developed spouting state can be observed distinctly. The total bed pressure drop varied with the spouting gas velocity increases, as a result of the transition of flow pattern. Typical flow pattern map at various static bed heights and spouting gas velocity were plotted for describing the transitions between flow patterns. 1. Introduction As a kind of high-performance reactor for gas-solid or fluid-solid particle reactions, the spouted bed technology is applied to a variety of chemical processes such as dryness, prilling, coating, gasification, combustion, and pyrolysis, etc. During the past decades, numerous modified spouted bed designs have been developed to overcome some of the limitations of the conventional spouted beds, to accommodate the diverse properties of the materials handled and enhance the operability, heat and mass transfer characteristics, and gas-solid or fluid-solid contacting efficiency,1 for example, multiple spouted beds,2-4 spouted fluidized beds,5,6 conical spouted beds,7-9 draft tube spouted beds,10-14 pulsed spouted beds,15 and rotating spouted beds1,16,17 et al. In addition, many experimental and theoretical studies, which aimed at grasping the more useful flow characteristics of the spouted beds, have also been performed.18-21 Recently, based on the conventional cylindrical (or square) multiair nozzles spouted bed, a novel annular spouted bed with multiair nozzles has been proposed by Shanghai Jiao Tong University.22,23 It is expected to offer good circumstances for coal cleaning combustion. The spouted bed, which combines spouting and coal dryness, pyrolysis, and gasification, can overcome some shortcomings that may be occurred in the spouted fluidized beds, such as gas distribution between spouting gas and fluidizing gas and blocked orifices for the guide plates etc., while its construction is relatively simple. On the basis of previous research, some hydrodynamic characteristics of the novel annular spouted bed were obtained.24 Flow patterns and transition are one of the important hydrodynamic characteristics of the spouted or fluidized bed, which plays an important role in operating reactors. Dogan et al.25 and Freitas et al.26 observed and described flow patterns and transitions in spouted beds. Sutanto et al.27 plotted a spoutfluid bed flow pattern map including the fixed bed, bubbling (JF II), jet in the fluidized bed (JF I), spout-fluidization, and spout with aeration. He et al.28 studied the flow pattern and transition in spout-fluid bed of 0.91 m ID and indicated that * To whom correspondence should be addressed. Tel./Fax: +86 21 34206569. E-mail address: [email protected].

four fairly distinct flow patterns were observed: spouting with aeration; spout-fluidization; submerged jets, slugs, and bubbles in a fluidized bed; and packed bed. Up to now, the characteristics of flow patterns and transitions for the novel annular spouted bed with multiple air nozzles proposed by us are still not completely unveiled. In this paper, experiments were carried out on the flow patterns and transitions in the novel annular spouted bed.

Figure 1. Schematic diagram of the overall experimental setup.

Figure 2. Schematic diagram of the nozzle and V-shaped guide plates.

10.1021/ie800733n CCC: $40.75  2008 American Chemical Society Published on Web 10/31/2008

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Figure 3. Schematic of the pressure port arrangement. Table 1. Physical Properties of Bed Particles Used in the Experiments material corn polystyrene

Fb Fs L (mm) W (mm) H (mm) (kg m-3) (kg m-3) ε (-) 7.935 2.483

6.071 1.768

5.923 3.066

1352.94 1083.22

813.72 640.37

0.399 0.331

Table 2. Experimental Conditions Ug (m/s)

Hs (mm)

f (Hz)

t (s)

17-33

150, 180, 200, 210, 230

1000

60

Different flow patterns will be identified. Schematic diagrams and typical flow pattern images obtained by a digital chargecoupled device (CCD) camera will be provided for distinguishing different flow patterns. The operating conditions for flow pattern transitions will be presented. 2. Experiments The schematic diagram of the novel annular spouted bed experimental setup is shown in Figure 1, which consists of a gas supply system, a feeding system, an annular spouted bed, a multichannel differential pressure signal acquisition system, and a digital camera. The annular spouted bed is made up of two homocentric upright pellucid methyl-methacrylate cylinders, 300 mm and 400 mm in diameters, respectively. The height is 750 mm for the inner cylinder, 1080 mm for the outer cylinder. Eight nozzles are mounted uniformly along circumference on the bottom of annular space between the inner and outer cylinders. The cross-sectional size of the nozzle is 30 mm × 30 mm; its length is 60 mm. Different nozzle structures, forward or angle nozzles, are adopted in the study. The inclination angle for the angle nozzle is 60°. It is a well-documented fact that the multinozzle spouted beds are not easy to operate properly because certain nozzles may collapse, and consequently, dead zones are formed in the bed. In order to avoid this problem, an independent inlet for each orifice has been proposed in the

Figure 4. Schematic representation of various stable flow patterns.

literature.4,29 In the design of the novel annular spouted bed, an air room is connected under the bed. The spouting gas is sent to the air room through an inlet, 100 mm in diameter, which is located at the center of the room base. Then, the gas was divided equivalently into eight nozzles, which is helpful to develop uniform spouting without partial choking of the nozzle. Compared to the bed with an independent inlet for each nozzle, the novel spouted bed has simpler construction. On the nozzle, V-shaped guide plates are arranged to prevent dead zone formation, 120 mm in its height, 60° in inclination angle. The schematic diagram of the nozzle and V-shaped guide plates are shown in Figure 2. The spouting gas is supplied by an air blower, regulated by a gate valve. The gas flow rates were measured by a rotameter. The experiments are run at room temperature and atmospheric pressure. After the air blower is powered, the spouting gas is blown into the air room first. Then, it spouted into the annular space of spouted bed through eight nozzles. Subsequently, the oscillating feeder machine begins to offer the rotating cone with test materials. With the help of the centrifugal action produced by the rotating cone driven by the electromotor, the test materials move upward along the wall of the cone, escape the cone, and then fall into annular space between the inner and outer cylinders finally. These particles are spouted by the airflow in the annular space. The airflow will be discharged through the exhaust tube on the top of the spouted bed. A multichannel differential pressure signal acquisition system, which is made up of some pressure sensors, a serial port, and a personal computer, is used to record the pressure fluctuations in the spouted bed. The pressure signal measurements are taken at different locations, the arrangement of the pressure taps is shown in Figure 3. All probes are arranged at the center between the inner and outer cylinder. Every differential pressure sensor has two ports: one connected with the pressure measuring hole in the bed wall and the other connected to measuring point 1 located in the air room. The multichannel differential pressure

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Figure 5. Typical flow pattern images of internal jet (IJ) with angle nozzles: (a) corn, Hs ) 210 mm, Us/Ums ) 0.95; (b) polystyrene beads, Hs ) 210 mm, Us/Ums ) 0.93.

Figure 7. Typical flow pattern images of fully developed spouting (FDS) with angle nozzles: (a) corn, Hs ) 210 mm, Us/Ums ) 1.28; (b) polystyrene beads, Hs ) 210 mm, Us/Ums ) 1.23.

3. Results and Discussion

Figure 6. Typical flow pattern images of jet-spouting (JS) with angle nozzles: (a) corn, Hs ) 210 mm, Us/Ums ) 1.18; (b) polystyrene beads, Hs ) 210 mm, Us/Ums ) 1.15.

signal transducer, with a pressure range of 0-5 KPa and an accuracy of (0.01 KPa, was used to measure pressure signals and then convert them into voltage signals. The voltage signals are sent to a computer through an analog to digital (A/D) converter. The pressure fluctuations and pressure drop were used as accessory tools to determine the flow patterns and transition. A digital camera was employed to photograph the flow regimes through the transparent wall during the experiments. A summary of particle properties tested in this work and experimental and sampling conditions are listed in Tables 1 and 2, respectively.

3.1. Identification of Flow Patterns. 3.1.1. Stable Flow Pattern. Flow patterns were determined by changing the spouting gas flow rate stepwise for a given static bed height. Besides, the changes in pressure drop and photographs recorded by a digital CCD camera were used as accessory tools to determine the flow patterns. According to our experimental observation, three different stable flow patterns were identified. Figure 4 presents a schematic representation of three stable flow patterns. They are internal jet (IJ), jet-spouting (JS), and fully developed spouting (FDS). The typical pictures for the description of every flow pattern obtained by the digital CCD camera are presented as follows: 1. Internal Jet (IJ). As observed from Figure 5, in this case, the submerged cavity or jet forms at all nozzle outlets, while the rest of the bed still remains a fixed bed. However, due to the structure failure of air nozzles, the jet height for each nozzle would be different. As a flow pattern, the internal jet in this paper is in accordance with the definition for the spouted or spout-fluid bed reported in the literature.25,26,28 2. Jet-Spouting (JS). With the spouting gas flow rate increase further, the spouting gas for all air nozzles would penetrate the bed. The spouting forms at all air nozzles. As shown in Figure 6, the overall appearance of the bed in this flow pattern is similar to that of a conventional spouted bed operated at, or just above, the maximum spoutable bed height, as a result of the interaction of spouting gas at different nozzles. The spout region, or jet, can be distinctly observed. At the spouting gas outlets, the path of spout jets may change, and the spouting gas may swing occasionally, where particles near the spout or jet region are spouted. The results would be attributed to the following: (a) For the multinozzle spouted bed, the spouting state for each nozzle is influenced by other nozzles. So, during the course of spouting, the pressure of spout gas can not be kept steady. It makes the direction of spout jet change and the spouting gas swing. (b) In jet-spouting, some visible bubbles form in the bed. When the spouting gas penetrates the bed, the spout jet tends to divert to where the bubbles or cavities

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Figure 8. Typical flow pattern images of single internal jet (SIJ) with angle nozzles: (a) schematic representation; (b) corn, Hs ) 210 mm, Us/Ums ) 0.91; (c) polystyrene beads, Hs ) 210 mm, Us/Ums ) 0.90.

Figure 9. Typical flow pattern images of single jet-spouting (SJS) with angle nozzles: (a) schematic representation; (b) corn, Hs ) 210 mm, Us/Ums ) 1.12; (c) polystyrene beads, Hs ) 210 mm, Us/Ums ) 1.09.

Figure 10. Typical flow pattern images of bubbling or slugging (BS) with angle nozzles: (a) schematic representation; (b) corn, Hs ) 230 mm; (c) polystyrene beads, Hs ) 240 mm.

form, due to less resistance. It also changes the spouting direction for the spouting gas. 3. Fully Developed Spouting (FDS). Figure 7 presents typical flow pattern images of fully developed spouting in the annular spouted bed. The interaction of spout jets coming from different air nozzles causes the direction of the spout jets to continually change during developed spouting. The coalitions of spout jets were also observed where particles were turbulently mixed and spouted in the annular bed. 3.1.2. Flow Transition and Flow Instabilities. On account of the unavoidable nonuniformity of the nozzle structure, two

transitional flow patterns, single internal jet, and single jetspouting were found in our experiments with the spouting gas velocity increases. In addition, when the spouted bed operated at deep static bed height (higher than maximum spoutable bed height), flow instabilities, bubbling, or slugging was also observed. 1. Single Internal Jet (SIJ). Figure 8 shows typical flow pattern images of single internal jet recorded by digital CCD camera during experiments. In this flow pattern, a submerged cavity or jet forms at certain gas nozzle outlet, while no cavity or jet can be observed at the others. The rest of the bed remains

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Figure 11. Typical flow pattern images at various spouting gas velocities with forward nozzles, corn, and Hs ) 180 mm: Us/Ums ) (a) 0.79, (b) 0.92, (c) 1.04, (d) 1.13, (e) 1.24.

Figure 12. Typical flow pattern images at various spouting gas velocities with forward nozzles, polystyrene beads, and Hs ) 200 mm: Us/Ums ) (a) 0.87, (b) 0.92, (c) 1.06, (d) 1.12, (e) 1.20.

Figure 13. Fully developed spouting at different operating times with angle nozzles, corn, Hs ) 210 mm, and Us/Ums ) 1.28.

a fixed bed. When increasing spouting gas flow rate, the jet height increases until the flow pattern changes to internal jet (IJ). 2. Single Jet-Spouting (SJS). In this flow pattern shown in Figure 9, the single jet-spouting forms at certain air nozzle outlet, while other air nozzle outlets still remain submerged cavity or internal jet. The bed has the appearance of a conventional spouted bed in the flow pattern of spouting for the certain air nozzle outlet. Particles are transported individually by the spouting gas flowing upward, and spouted out of the upper surface of bed. Some particle clusters can be seen in the images, which present some typical flow pattern of fluidizing.

3. Bubbling or Slugging (BS). When the static bed height is higher than the maximum spoutable bed height, the flow instabilities, bobbling or slugging, were found in our experiment, as shown in Figure 10. In this case, even if the spouting gas velocity increases to a larger value, the spouting behavior cannot forms at all nozzle outlet. By analyzing these flow pattern images, the flow instabilities seem to be caused by the formation of large bubbles in the spouted bed. However, further experimental and theoretical study is required. For all flow patterns, it can be observed that the spouting gas velocity for polystyrene is lower than for corn. Due to results that the density of polystyrene is lower than that of corn,

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Figure 14. Fully developed spouting at different operating times with angle nozzles, polystyrene beads, Hs ) 210 mm, and Us/Ums ) 1.23.

Figure 15. Total bed pressure drop at different typical flow patterns (forward nozzles, corn, Hs ) 210 mm).

Figure 16. Typical flow pattern map at various static bed heights and spouting gas velocities with forward nozzles and corn: (1) fixed bed, (2) single internal jet, (3) internal jet, (4) single jet-spouting, (5) jet-spouting, (6) fully developed spouting, (7) bubbling or slugging.

polystyrene is easier to be spouted under the condition of the same static bed height. In addition, the voidage in the bed for corn is bigger, the spouting gas in the spout region is easier to

diffuse to the dense region, and exchanges of the mass and momentum between the spout region and dense region become more intensive. The spouting gas momentum is easily dissipated as the spouting gas ascends in the bed. So, to sustain a certain spouting state, it is necessary to increase the spouting gas velocity. 3.2. Flow Pattern for Forward Nozzle Structure. For the angle nozzle structure, flow patterns images can be observed in section 3.1. Figures 11 and 12 presents five flow pattern images, respectively, with the increasing of spouting gas flowrate for the forward nozzle structure. Observed from Figures 11 and 12, the internal jet or jet-spouting occurs at a low spouting gas velocity, and they are easier to identify compared with that for the angle nozzle structure. The spouting air has a higher angular velocity component at a higher nozzle spouting velocity for an angled nozzle structure, as a result of a deviation from the vertical spouting velocity. Since only the vertical velocity component sustains the spouting by offsetting the gravitational effect, the particles for the forward nozzle are spouted upward easily. 3.3. Particle Mixing and Transport. Figures 13 and 14 present some fully developed spouting images in the bed at a given spouting gas velocity and static bed height. The interaction of spout gases from different nozzles causes the jets to distort or coalesce where particles are spouted. The turbulent exchange of particles among the nozzles can be observed. Visible larger bubbles form and disintegrate intermittently at the upper bed, which would intensify the mixing and collision action of particles in the annular bed. In fact, intensive mixing and transport of particles are also found in the fully developed spouting state for forward nozzle structure of spouted bed, while it seems to be more distinct and turbulent for angle nozzle structure, which causes the spout-directed transport of particles in the direction of jet from nozzles. 3.4. Flow Patterns and Pressure Drop. Figure 15 shows the total bed pressure drop at different flow patterns with the increasing of spouting gas velocity. In this experiment, pressure measurements are carried out by changing the spouting gas flow rate stepwise, at which the differential pressure at a given spouting gas velocity are recorded every second for 3 min while keeping all operating conditions constant. As shown in Figure 15, for Us/Ums ) 0.68, the fluctuation of total bed pressure drop is comparatively stable, presenting a fixed bed (FB) state. With elevating the spouting gas velocity, the mean value of total bed pressure drop increases while the flow pattern transits from the fixed bed to single internal jet and then to entire internal jet.

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However, for Us/Ums higher than 1, the mean value of total bed pressure drop decline to a low value and almost stays constant but with fluctuations around this mean value. The possible explanation is that the spouting gas penetrates the bed and forms jet-spouting when the spouting gas velocity beyond a value, Ums, which is defined as the minimum spouting gas velocity. The tendency of total bed pressure drop with increasing spouting gas velocity is the same as those observed in conventional spouted beds. When the spouting gas velocity increases further, the flow pattern transitions from the jet-spouting to the fully developed spouting state. In this case, for the spouting or jet region, the resistance of test particles to the spouting gas is similar to that of JS, due to the fact that spouting behavior had been formed. So, as shown in Figure 15, the bed pressure drop seems to have little differentiation between the JS and FDS regimes. In FDS, more spouting gas diffused to the dense region and intensified the fluidizing behavior of particles in the dense region. More severe spouting phenomenon can be observed with periodic discharge of bubbles near the bed surface in the experiment. 3.5. Bed Height and Flow Patterns. Generally, the spouting mechanism in which the bed of particles changes from a packed state to fully developed spouting is related to the static bed height and the superficial air velocity. To generalize this analysis, the static bed height and superficial air velocity are written in dimensionless form, Hs/D0 and Re. A typical flow pattern map for corn at various static bed heights and spouting gas velocities is plotted in Figure 16. For a given static bed height, with the increase of spouting gas velocity, the bed shows fixed bed, single internal jet, internal jet, single jet-spouting, and jet-spouting, successively, finally reaching fully developed spouting. In addition, when the static bed height is higher than the certain bed height (the maximum spoutable bed height), even if the spouting gas velocity increases to a larger value, the spouting behavior is mainly bobbling or slugging; other flow patterns are not distinct. As shown in Figure 16, the static bed heights and spouting gas velocity have significant effects on the flow patterns and transitions. Further experimental and theoretical work is required, in order to acquire the complex relationships between these parameters with flow patterns and transitions. 4. Conclusions In this paper, a multichannel differential pressure signal acquisition system was used to record the pressure fluctuations in a novel annular spouted bed under different operating conditions. The flow pattern and transition of particles in the novel spouted bed were studied. Three distinct stable flow patterns and two transitional flow patterns, i.e. internal jet, jetspouting, fully developed spouting, single internal jet, and single jet-spouting were identified. With increasing spouting gas velocity, the flow pattern transitions from the fixed bed to single internal jet and then to entire internal jet, and when Us/Ums is higher than 1, the flow pattern will transition from the jet-spouting to the fully developed spouting state, in which the mean value of total bed pressure drop statys constant but with fluctuations around the value. The interaction of spout gases from different nozzles causes the jets to distort or coalesce. Intensive mixing and transport of particles are found in the fully developed spouting state for forward nozzle structure of spouted bed, while it seems to be more distinct and turbulent for angle nozzle structure, which causes the spout-directed transport of particles in the direction of the jet from the nozzles.

Though the present results can provide an effective method to identify the flow pattern in the bed, however, the complexity of flow behavior in the novel spouted bed may require further experimental and theoretical study. Acknowledgment The authors gratefully acknowledge financial support by the ministry of science and technology of China (No. 2007AA05Z313, No. 2006EA105033). Nomenclature D0 ) equivalent cylindrical column diameter, mm Dp ) effective particle diameter, mm H ) height, mm Hb ) height of measuring point above the nozzle outlet, mm Hg ) distance of measuring point away from the inlet of nozzle, mm Hs ) static bed height, mm f ) sampling frequency, Hz L ) length, mm Us ) spouting gas velocity, m s-1 Ums ) minimum spouting gas velocity, m s-1 W ) width, mm Fb ) bulk density, kg m-3 Fg ) fluid density, kg m-3 Fs ) particle density, kg m-3 ug ) fluid viscosity, kg m-1 s-1 ε ) voidage, [1 -(Fb/Fs)] Re ) Reynolds number, (DpUsFg/ug)

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ReceiVed for reView May 6, 2008 ReVised manuscript receiVed September 5, 2008 Accepted September 18, 2008 IE800733N