Mixing and Segregation Behavior in a Spout-Fluid Bed: Effect of

Article pubs.acs.org/IECR

Mixing and Segregation Behavior in a Spout-Fluid Bed: Effect of Particle Size Yong Zhang, Wenqi Zhong,* Baosheng Jin, and Rui Xiao Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, 210096, China ABSTRACT: The mixing and segregation behavior of a binary mixture have been investigated experimentally in a spout-fluid bed. Three types of binary mixtures were used by mixing glass beads with equal density and dissimilar size. The spouting and fluidizing gas flow rate were adjusted to cover a range of flow regimes, typically including internal jet (IJ), jet in fluidized bed with bubbling (JFB), spouting (S) and spout-fluid (SF). The mixing and segregation behavior were analyzed in terms of flow regimes, concentration profile, and mixing index. The results show that in IJ, the particle circulation is combined with the local segregation where smaller particles migrate to the interface between the jet and stagnant region, even into the latter. In S, the distribution of particle depends greatly on the fountain structure. In JFB, the axial segregation takes place where the smaller particles prevail in the upper part of the bed. Segregation becomes more pronounced with increasing the particle size difference. In addition, a mixing/segregation pattern map is constructed. Three regions including mixing region, segregation region, and intermediate region, are identified by the criterion of mixing index. introduced the words flotsam and jetsam to describe the solids which occupy the top or bottom of the bed, respectively. Based on this, the following studies18−20 confirmed that the main mixing mechanism is due to the rising bubble carrying the particles in the wake from the bottom to top of bed, and the same bubbles cause segregation when the denser particles tend to fall through the disturbed region behind each bubble. Some authors such as Rice and Brainovich,21 Rasul et al.,22 Marzocchella et al.,23Wirsum et al.,24 Lu et al.,25 Sahoo and Roy,26 and Umekawa et al.27 investigated the mixing and segregation of binary particles having a narrow size distribution. They found that larger particles behave as jetsam and migrate to the bottom of the fluidized bed, whereas smaller particles act as flotsam and congregate at the top of the bed. The other authors such as Hoffmann and Romp,28 Wu and Baeyens,29 Wormsbecker et al.,30 and Dahl and Hrenya31 focused on the segregation behavior of a binary mixture with a wide size distribution. The results show that because of the presence of more size species, concentration maxima of particles of different sizes at different heights were observed along the bed. It was also found that species segregation continues to exist for velocities significantly higher than the minimum fluidization velocity. However, despite the amount of work carried out with fluidized beds, the results of mixing and segregation cannot be considered adaptable to spouted beds in which the particle movement is much more regular and well-defined than in fluidized beds. In the area of a spouted bed, segregation phenomena have been the subject of several experimental studies reported in the literature. The segregation phenomenon in a spouted bed was

1. INTRODUCTION The spout-fluid bed is a very successful modification to the conventional spouted bed. It reduces some limitations of spouting and fluidization by combining the features of the spouted and fluidized beds. This technique has been accepted traditionally as a solid−fluid contacting method for physical operations such as drying, coating, and granulation of granular solids.1−6 In recent years, the application of spout-fluid bed has also been studied for catalytic reactors and combustion and gasification of coal and biomass.7−10 In most industrial applications of fluidized bed and spouted bed, good mixing is required for a uniform product or in order to avoid partial defluidization of the bed and decrease of the heat and mass transfer rates. However, the mixing tends to be incomplete when particles with different physical properties are present in the bed. Differences in size, density, shape, roughness, and resilience may give rise to segregation, with size being the most important in a single phase system11 and density being the dominant driving force in the multiphase system.12 In some applications, on the other hand, a segregation pattern might be desirable where the tendency for segregation is utilized for the continuous removal of product. A specific example of this application is the case of carbon black separation in the pyrolysis of tires in a conical spouted bed.13,14 Therefore, mixing and segregation of particles have captured the interest of researchers across the world. A remarkable research effort to achieve a clear picture of mixing/ segregation behavior of mixture has been made by many groups in the past several decades. In the field of fluidized bed, mixing and segregation phenomena have been the subject of several experimental studies reported in the literature. Most studies on particle mixing have been carried out with particle tracers that satisfy the requirement of being typical of the bed material in terms of size, shape, and density. Initial investigations on segregation concentrated on density differences where Nienow et al.15−17 © 2012 American Chemical Society

Received: Revised: Accepted: Published: 14247

April 17, 2012 September 22, 2012 October 4, 2012 October 4, 2012 dx.doi.org/10.1021/ie301005n | Ind. Eng. Chem. Res. 2012, 51, 14247−14257

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic diagram of experimental setup. (1) compressor; (2) pressure gauge; (3) bypass valve; (4) desiccator; (5) ball valve; (6) control valve; (7) flow meter; (8) spout nozzle; (9) fluidizing gas inlet; (10) fluidizing gas chamber; (11) gas distributor; (12) spout-fluid bed body; (13) halogen lamp; (14) digital CCD; (15) pressure taps; (16) computer; (17) differential pressure sensor.

first observed by Berquin.32 Mathur and Epstein33 performed experiments on systems consisting of particles of equal density, but different sizes. Piccinini et al.34 studied the segregation behavior of particles having equal size, but different densities. Cook and Bridgwater35 investigated the segregation in the continuous spouted bed consisting of binary mixtures for both different particle sizes and densities. Giorgio36 reported experimental results of baffle-induced segregation of different binary mixtures in two spouted beds. San Jose et al.37,38 quantified segregation by relating a global bed mixing index to the contactor design factors and to the properties of the binary and ternary mixture of solids differing in size at various average compositions. Robinson and Waldie39 studied the behavior of segregation of a binary mixture differing in size and allowed for possible segregation in the spout region. In addition to injecting the spouting gas through a central nozzle, the fluidizing gas is introduced to the bed via a gas distributor surrounding the central nozzle in a spout-fluid bed, which can produce more complicated flow regime than either spouted bed or fluidized bed.40 Therefore, it is inappropriate to apply the results of mixing and segregation studies about spouted beds or fluidized beds to spout-fluid bed. There is an urgent need to perform fundamental work to identify the mixing behavior and primary mechanism by which segregation occurs. However, only very limited research has been done to investigate the mixing/segregation characteristic in such a system.40−42 Furthermore, previous literatures predominantly concentrated on the mixing and segregation behavior induced by the change of gas velocity, not taking into account the effect of a single parameter, for instance, particle size. For this purpose, the investigation on the mixing and segregation behavior has been performed in a spout-fluid bed. Emphasis is laid on the influence of the particle size on the mixing and segregation pattern. In particular, the spouting and fluidizing gas flow rate were adjusted to cover a range of flow regimes, typically including IJ, JFB, and SF. The mixing and segregation behavior were analyzed in terms of flow regimes, particle concentration profile, and mixing index. To establish a connection between the operating condition and mixing behavior, the mixing/segregation pattern map was created on the basis of the flow regime map and the corresponding

snapshots. Besides, differences and similarities between thr current and the literature system were compared and interpreted in the light of the basic mixing/segregation pattern.

2. EXPERIMENTAL SECTION 2.1. Spout-Fluid Bed. The flat-bottom spout-fluid bed used is illustrated schematically in Figure 1. It is made of 5 mm thick plexiglas and has a cross-section of 100 mm × 30 mm and a height of 500 mm. There are three pressure taps located at the central line on the rear wall of the bed, one in the spout nozzle and the other two on the distributor with heights of 110 mm and 160 mm. The spout nozzle is located at the center of the bottom plate and its area is 10 mm × 30 mm. The gas distributor arranged symmetrically at the base of the bed is a perforated-plate with 1 mm holes spaced 8 mm apart, which results in an open area of 1.16%. The gas flow rates were measured by a set of rotameters connected to two control valves, respectively. The mixing and segregation process was captured with high speed digital CCD camera (Nikon Coolpix 5000). The dataacquisition unit consists of a multichannel differential pressure sensor and a PC equipped with one 8 A/D data acquisition board. The bed pressure drops were measured and then converted into voltage signals by a multichannel differential pressure sensor with a scale of 0−10 kPa. The voltage signals were sent to a computer through an A/D converter for processing. 2.2. Particle Characteristics. Four different glass beads having the same density of 2540 kg/m3 and different diameters of 3.2, 2.8, 2.1, and 1.1 mm were used, in which the biggest one was labeled as the bed material and the other ones as the tracer. Properties of the particles are presented in Table 1. Three types of binary mixtures were obtained, indicated as GB3.2−GB2.8, GB3.2−GB2.1, and GB3.2−GB1.1. For each binary mixture, three different average compositions were prepared, corresponding to the averaged volume fraction of small particles xt,v of 0.1, 0.3, and 0.5 in the bed. Thus, the hydrodynamic behavior of nine binary systems in all was studied. The relevant data for different binary mixtures are given in Table 2. All experiments were carried out with the static bed height of 0.1 m. 14248

dx.doi.org/10.1021/ie301005n | Ind. Eng. Chem. Res. 2012, 51, 14247−14257


Article

a key step in which the tracer particles were collected from the sample to calculate the particle concentration in each tracer region. 2.4. Sampling Method. The final state of mixing and segregation was commonly expressed in terms of the relationship between composition and height, usually obtained by the “bed-frozen” method. This technique was used by many research groups43−46 and widely accepted in the fluidization fields. For quantitative analysis, this method was developed and adopted here. To prevent the rearrangement of particles when the bed is sinking, the flashboard-box was used in this method, which acts just like a drawer (as shown in Figure 2b). The whole sampling process is described below. When the mixing and segregation process was finished, the ball valve was instantaneously closed and simultaneously the flashboard box was inserted from the top of the vessel. Thus, the “frozen” bed was separated into nine sections along the lateral direction. Then, the vessel was turned to horizon level and the flashboardbox was gently taken out from the top of the vessel. By means of a specially designed grid board, the mixture was isolated along the axial direction and divided into several samplers. Subsequently, the particle concentration was determined from the knowledge of the weight of the bed particle and tracer by sieving and weighting. Thus, each sampling region but the central one had a volume of 10 × 10 × 30 mm3 (shown in Figure 2a). Here, it should be emphasized that the sampling region in the bed is positioned by the dimensionless coordinate. So the sampling region labeled capital letter A (presented in Figure 2a) was located at distances of y/h0 above the distributor and x/w from the centerline. 2.5. Mixing Index. To exhibit the spatial distribution of tracer in the bed, the proportion of the tracer mass in the sampling region to the total mass of tracer in the whole system is used and expressed by:

Table 1. Properties of Solids Used

bed material tracer 1 tracer 2 tracer 3

material

ρp (kg/m3)

dp (mm)

ε

geldart classification

glass beads A

2540

3.2

0.429

D

glass beads B glass beads C glass beads D

2540 2540 2540

2.8 2.1 1.1

0.421 0.412 0.387

D D D

Table 2. Properties of Binary Solid Mixtures Investigated

system

particle size ratio dt/db(-)

GB3.2GB2.8

0.88

GB3.2GB2.1

0.66

GB3.2GB1.1

0.34

mass ratio xt, m(%)

volume ratio xt, v(%)

sauter mean diameter dm (mm)

10.13 30.29 50.35 10.27 30.62 50.73 10.66 31.51 51.77

10 30 50 10 30 50 10 30 50

3.15 3.07 2.99 3.04 2.76 2.53 2.66 2 1.61

2.3. Procedure. The experiments were basically performed using the following procedure. As the first step, particles were initially packed in the form of completely segregated or well mixed. For an initially fully segregated bed, the bed particles were first poured into the bed and then tracer particles were put on the top of it. For an initially perfect mixed bed, the bed first was fluidized at comparatively high gas velocity for about 15 min to ensure that the mixture was representatively mixed. And then the fluidized gas was suddenly stopped, so that the bed collapsed and formed a well-mixed bed. As the second step, the gas control valves for startup of the bed were turned on, and then the gas flow was maintained at the desired operating condition. After the mixing extent reached the preconceived value, the sampling method (described in the next section) was used to obtain the sample in the tracer regions. A third step was

p=

m mt

(1)

Figure 2. Schematic diagram of the sampling: (a) the sampling region; (b) the digital CCD and flashboard-box. 14249



Article

Figure 3. Schematic representation of mixing conditions. (a) fully segregated; (b) random mixing; (c) well mixed.

Figure 4. Typical instantaneous snapshots of particle segregation at various spouting gas flow rates: (a) Qs* = 0 and Qf* = 0, (b) Qs* = 0.35 and Qf* = 0.41, (c) Qs* = 0.78 and Qf* = 0.41, (d) Qs* = 0.96 and Qf* = 0.41, (e) Qs* = 1.15 and Qf* = 0.41, (f) Qs* = 1.31 and Qf* = 0.41.

where m and mt are the tracer mass in a given sampling region and the total tracer mass contained in the bed, respectively. The concentration of tracer is defined as m c= mlt (2)

illustration of a binary mixture in three mixing conditions, in which the particles of one solid are represented by white spheres and those of the other by red ones. When the mixing index calculated by eq 3 has a zero value, it can be deduced that the state is completely segregated, as shown in Figure 3a. Then, the degree of mixing gradually increases from left to right and there is global progress from segregated state toward more homogeneous state. On the contrary, if the value of mixing index is 1, it could be inferred to be well mixed state (see Figure 3c), which means the concentration at any position is equal to the overall concentration. In practice, however, a well mixed mixture is only an ideal mixture and is difficult to produce naturally. A random mixing state, as an intermediate state between them, is the aim of most industries that deal with the solids mixing (as shown in Figure 3b).

in which mlt represents the total mass in a sampling region. On the basis of statistical analyses, various mixing indices are employed to describe the solid mixing in many different industrial processes.47 Herein, we use the well-known Lacey mixing index,48 defined as

σ0 2 − σ 2

M=

σ0 2 − σm 2

(3)

where σ indicates the concentration variance for the random mixture; σ02 is the value of σ2 when the mixture is in a fully segregated state; σm2 is the value of σ2 when the mixture is in a perfectly mixed state. Figure 3 displays a diagrammatic 2

14250



Article

Figure 5. Proportion of GB2.1 particles at different spouting gas flow rates along (a) axial direction and (b) lateral direction for GB3.2-GB2.1 mixture.

finer particles more readily than larger ones. These phenomena become particularly pronounced at Qs* = 0.96, as shown in Figure 4d. As Qs* is further increased to 1.15 and 1.31, the fountain is overdeveloped and the range of cycle extends. This causes more particles to cycle, especially for the ones at the corner. Under this condition, it can be observed that the concentration of GB1.1 particles at the bed base decreases, as shown in Figure 4e,f. Figure 5 shows the axial and lateral distribution of GB2.1 particle proportion at various spouting gas flow rates for the GB3.2−GB2.1 mixture. At Qs* = 0 and Qf* = 0.31, particle proportion is the same, implying that the bed stays at the wellmixed state. With the increase of spouting gas flow rate, the curve profile begins to change. In the axial direction, the proportion gradually increases until it passes through a maximum value, corresponding to an intermediate position in the annular, and then decreases with the increase of bed level. In the lateral direction, the proportion profiles have a peak located at the position of x = 15 mm, which is close to the location of the jet-annulus boundary, and then significantly decreases to the averaged value close to the wall. According to the flow regime and visual observation, it is reasonable to conclude that particle segregation occurs at the jet region and adjacent region at low gas velocity. This can be explained by the fact that GB2.1 particles are dragged by the jet gas to move toward the external region through the void among GB3.2 particles until it approaches a high concentration. This phenomenon is more pronounced at Qs* = 0.67 because an increase in the gas velocity results in an increase of void and triggers more GB2.1 particles to move, both enhancing subsequent segregation. But when Qs* is increased to 0.91, there exists an obvious difference for the curve profile. In the axial direction, the proportion reaches a maximum value at the bottom and then slowly decreases along the bed height. This suggests that the axial segregation occurs where smaller GB2.1 particles congregate at the bottom. Such segregation is attributed to the percolation mechanism where the smaller particles can easily percolate down to fill the void created behind the larger particles. In the lateral direction, the proportion at the jet is lower than the averaged value of 0.01 and then it increases at the annulus with increasing lateral distance. This indicates that in this case, the lateral segregation takes place mainly at the

3. RESULTS AND DISCUSSION 3.1. Mixing at Different Spouting Gas Velocities. According to Gyenis,49 there are two principal methods to examine the process of mixing and segregation and identify the way of mixing and segregation: (1) direct methods, by observing the movement of single particle, or a group of particles, or all particles during operation, and/or (2) indirect methods, by analyzing the spatial distribution of constituents after given mixing time at preset operational condition. In our study, both of them have been applied. To assist discussion, the spouting and fluidizing gas flow rate are expressed as dimensionless variables, given by Q s* = Q s/Q mf

(4)

Q f * = Q f /Q mf

(5)

in which Qs and Qf are, respectively, the actual spouting and fluidizing gas flow rate; Qmf is the minimum fluidizing gas flow rate. The solid flow regimes are examined first to generate some visual understanding of the mixing/segregation process. Figure 4 shows an illustrative example of the segregation process for the GB3.2-GB1.1 mixture at variant spouting gas flow rates. Starting from perfect mixed packing, bed expansion is observed once the gas is introduced. Then, the internal jet occurs and is accompanied by the local segregation where a majority of GB1.1 particles move toward the external jet region and finally concentrate at the interface between jet and stagnant region until a dynamical equilibrium state is achieved. At Qs* = 0.78 and Qf* = 0.41, the jet breaks the packed bed upper surface and external spouting is reached. This behavior is similar to that reported for a conventional spouted bed, the central core a spout, the surrounding annular region the annulus, and the particles above the bed surface entrained by the spout and then raining down on the annulus are designated as the fountain.50 In this case, one interesting phenomena can be seen that some of GB1.1 congregate at two regions. One region is located at the corner of the lower bed where the bed is at rest. During operation, it was observed that smaller particles travel slowly toward the distributor through the gap between the larger particles until the gap is filled. The other region exists in the upper inner part of the bed around the junction of three regions (spout, annulus, and fountain) where a short cycle takes place.51 The short cycle acts just likes a vortex which entrains 14251



Article

through the void of particles and leads to the bed expansion, especially at the upper part of the bed. As a result, the bed voidage is increased. Thus, gravitational forces acting on the particles lead to a net downward motion of the smaller GB2.1 particles which can readily pass through the voids in the expanded bed. This phenomenon is different from our previous observation where under the same condition, the top tracer particles remain static52 where binary mixture consists of particles of equal density, but different sizes. After the formation of an internal jet, an increase in the fluidizing gas flow rate causes partial fluidization of the upper part of the bed. This is a typical flow regime of JFB. In this case, bubbles emerge and lift off from the top of the jet, leading to bubbling in the upper part of the bed. Some of GB3.2 particles are carried by the bubbles to the top layer and mixed with the GB2.1 particle. Since bubbles appear occasionally in the central part of bed, GB2.1 particles are not uniformly distributed in the bubbling layer. When Qf* is further increased to 0.85 and 1.07, the bed is fluidized and the particle mixing improves significantly. Even that, nonuniformity of the bed structure can still be found where GB2.1 particles are distributed at the top of the bed and the middle and lower part of the bed, as illustrated in Figure 7d. The cause of such a result can be better explained from the forces on the particles. At the top of the bed, the fluid drag force is large enough to lift the GB2.1 particle but not the GB3.2 particle. This allows the GB2.1 particles to move up more easily than the GB3.2 particles. Meanwhile, at the middle and lower part of the bed, the introduction of an upward-flowing fluid stream (even at low gas velocity) into the bed induces a short cycle for the fluid, which is similar to a vortex. Vortices contain a lot of energy in the circular motion of the fluid, which entrains particles to the core (here means the spout). Once the particle is entered into the vortex, it tends to cycle repeatedly there, with a small possibility of dropping out from the core. A similar phenomenon was observed by other researchers where this vortex was defined as the entrainment zone.53 Compared with the coarse particles, the finer particles are more likely to be entrained by the spouting gas. Therefore, it can be observed that the finer particles have the tendency to segregate to the spout. When the fluidizing gas velocity is further increased from the flow regime of F, larger bubbles form in the middle and upper part of the bed. When the diameter of the bubbles increases to near the diameter of the column, periodic slugging takes place in the middle and upper part of the bed, which is similar to the conventional fluidized bed operated in the slugging flow regime. At this condition, the flat slugging takes place primarily. The presence of flat slugging is due to the fact that the increase of fluidizing gas velocity causes more small bubbles released from the annulus and large bubbles easily form in the upper region of the bed. When the flat slugging emerges, the particles are carried by the bubble to move upward. At the same time, the only way the particles at the slug top can pass through the gas slug is by raining down through the slugs as particles stream. From Figure 7f, it can be seen that the mixing degree seems to be improved greatly. Figure 8 shows the axial and lateral distribution of GB1.1 particles at various fluidizing gas flow rates for GB3.2−GB1.1. The initial bed is in a completely segregated state and arranged with GB1.1 particles in the upper par of the bed. In the beginning, the bed is in the flow regime of IJ and there is no fluidizing gas to inject into the bed. In the axial direction, it can be found that the proportion at the bottom is very low and it is

annulus with small GB2.1 particles preferentially going to the outside part of the annulus. This has also been observed by Piccinini et al. through the transparent wall of a conicalcylindrical spouted bed.34 The explanation for this type of segregation probably lies in the fact that GB2.1 particles rise up higher than GB3.2 particles in the fountain and therefore fall back onto the annulus surface closer to the wall according to trajectories. Then, the further increase of spouting gas flow rate, the proportion profile in the middle part of the bed seems to become a little smooth and the proportion at the lower part of the bed decreases. Also, in the lateral direction, the proportion decreases at the outer annulus. This reveals that the segregation is attenuated as a result of the increase of spouting gas. In other words, in this case, increasing the spouting gas flow rate promotes the mixing. This phenomenon is approximately attributed to the following two reasons: (1) the increase of spouting gas induces more particles at the dead zone to cycle, promoting the axial mixing; (2) the overdeveloped fountain causes particles to bounce inward from the outer wall, improving the lateral mixing. The influence of spouting gas on mixing is studied by increasing Qs* step by step while keeping Qf* at 0.25 and 0.9, respectively. Figure 6 shows the results plotted as the steady-

Figure 6. Steady-state mixing index versus the dimensionless spouting gas flow rate for GB3.2−GB1.1 mixture.

state mixing index vs the dimensionless spouting gas flow rate for the GB3.2−GB1.1 mixture. Clearly, it can be found that different gas velocities produce different degrees of mixing at stable states. In the case of Qs* = 0.9, the mixing index increases with the increase of spouting gas flow rate. While for Qs* = 0.25, the V-shaped curve is found. It is clear that the mixing index first decreases with the increase of gas flow rate as a result of increased segregation. Then, with the further increase of gas flow rate, it increases as a result of strong fluidization. 3.2. Mixing at Different Fluidizing Gas Velocities. Figure 7 displays an illustrative example of the mixing process for GB3.2−GB2.1 mixture at different fluidizing gas flow rates. In the beginning, the GB3.2−GB2.1 mixture takes on a completely segregated state and the bed remains a fixed bed. It can be observed during the experiment that once the dimensionless fluidizing gas flow rate is increased to 0.41, the internal jet forms where only GB3.2 particles begin to circulate. In this case, although the jet region does not cover the top GB2.1 layer (shown in Figure 7b), local mixing occurs primarily in the interface of two layers. This type of local mixing can be explained as follows. The introduced fluidizing gas diffuses 14252



Article

Figure 7. Typical instantaneous snapshots of particle mixing at various fluidizing gas flow rates: (a) Qs* = 0.29 and Qf* = 0.23, (b) Qs* = 0.29 and Qf* = 0.41, (c) Qs* = 0.29 and Qf* = 0.68, (d) Qs* = 0.29 and Qf* = 0.85, (e) Qs* = 0.29 and Qf* = 1.07, (f) Qs* = 0.29 and Qf* = 1.39.

Figure 8. Proportion of GB1.1 particle at different fluidizing gas flow rates along (a) axial direction and (b) lateral direction for the GB3.2−GB1.1 mixture.

Then, with the further increase of fluidizing gas, not only the axial profile but the lateral profile becomes more and more flatter as well. This suggests that the increase of fluidizing gas promotes the axial and lateral mixing. The variation of the mixing index under various fluidizing gas flow rates when Qs* = 0.71 and 1.22 is further illustrated in Figure 9. Similar to the result in Figure 6, various combinations of spouting and fluidizing gas flow rate lead to different mixing and segregation behaviors, which gives rise to diverse degrees of mixing at stable states. In the case of Qs* = 0.71, the mixing index first decreases and then grows with the progressive rise of fluidizing gas flow rate. This means that at the initial stage, the increase of fluidizing gas promotes segregation and then again enhances mixing. In the case of Qs* = 1.22, the evolution of mixing index is characterized by three-stage pattern, which corresponds to two types of flow structures. At first, the mixing index rises up slowly when Qf* is increased from 0 to 0.41, corresponding to the flow regime of JFB. After that, the mixing index increases rapidly when Qf* is increased to 0.71, corresponding to the flow regime of S. Eventually, the mixing index does not nearly vary with the fluidizing gas. On this condition, the flow regime has transited from S to SF.

high at the top of the bed, being equal to an averaged value of 0.02. While in the middle part, it increases along the bed height. In the lateral direction, the proportion increases from the central part of the bed to the wall. This indicates that in this case mixing occurs mainly in the lower and inner regions of the bed. With the increase of fluidizing gas velocity, the curve profile becomes flat in the axial direction. That is, the proportion increases in the lower part of the bed and reduces in the upper section of the bed. This tendency is particularly noticeable for the case of Qf* = 0.56. At this gas flow rate, it can be noticed that the proportion is still equal to the averaged value of 0.02 at the top of the bed. This does not mean that the particles at the top do not move. In fact, the axial segregation takes place under this condition. During the operation, a nearly pure layer of GB1.1 particles is seen in the top layer and remains in the fluidized state. This is similar to the particle segregation in the fluidized bed reported in other works.25,28,29 In the lateral direction, the increase of fluidizing gas leads to the decrease of the proportion at the outer annulus and the increase at the inner annulus. This means that the range of lateral mixing enlarges as a result of the extension of the external jet region. 14253



Article

and GB3.2−GB1.1 mixture with the same Xt,v = 30%. It can be noted that three cases follows the same trend. That is, the mixing index decreases with the increase of Qf*. Then, the following growth of Qf* leads to the growth of mixing index. But at the same operation condition, the value of the mixing index for the GB3.2−GB1.1 mixture is lower compared with that of the GB3.2−GB2.8 and GB3.2−GB2.1 mixtures, while it is much higher for the GB3.2-GB2.8 mixture than for the GB3.2−GB2.1 mixture in the sharp decreasing stage and slightly higher in the increasing process. This suggests that the size difference in the binary mixture has influence on the mixing degree. The bigger the size difference is, the poorer the mixing becomes. 3.3. Flow Regime and Mixing Pattern Map. In fact, mixing and segregation tend to take place simultaneously and compete for each other. As for their relative function to the global result, it depends on the operating condition to a large extent. If the particle size is regarded as an internal factor that affects the mixing and segregation behavior, the operation condition can be deemed as an external factor. Under current conditions, the gas flow rate is considered as a crucial factor influencing the competition between mixing and segregation. While for the present spout−fluid beds, changing the spouting and fluidizing gas flow rate can generate different flow regimes. Therefore, the connection between the mixing/segregation pattern and the operating condition (Qs* and Qf*) could be established by means of the flow regime map. For current binary mixture differing in particle sizes with the same density, flow regimes were determined by adjusting Qs* for a given static bed height while holding Qf* constant, or by changing Qf* stepwise while Qs* remain invariable. Besides, the changes in pressure drop and photographs recorded by a highresolution digital CCD camera were used as accessory tools to determine the flow regimes. For detailed process on how to plot the flow regime diagram, the interested reader can refer to the relevant literature.10

Figure 9. Steady-state mixing index versus the dimensionless spouting gas flow rate for the GB3.2−GB1.1 mixture.

Figure 10 shows the evolution of the steady-state mixing index with the fluidizing gas for GB3.2−GB2.8, GB3.2−GB2.1

Figure 10. Effect of particle size on the steady-state mixing index.

Figure 11. Flow regime and mixing/segregation pattern map. 14254



Article

the light of the flow regime and mixing index. The following conclusions can be drawn from this work: (1) In the flow regime of IJ, the occurrence of particle circulation is combined with local segregation where smaller particles migrate to the interface between jet and stagnant region, even into the latter. In this case, the increase of spouting gas velocity promotes primarily the axial segregation, whereas enhancing the fluidizing gas velocity contributes principally to the lateral segregation. The small particles carried by the gas have a tendency to fill the void between large particles, which is principally responsible for the local segregation. (2) In the flow regime of S, the distribution of particles depends greatly on the fountain structure. On the condition of an under-developed fountain, particle segregation is extremely pronounced where small particles concentrate at the outer annulus and in the bottom corner. The former is induced by the fact that the smaller particles are prone to land at larger lateral position when falling from the fountain to the surface of annulus, whereas the latter is attributed to the percolation mechanism where the smaller particles can easily percolate down to fill the void created behind the larger particles. Conversely, the mixing is significantly improved as the fountain is developed. (3) In the case of JFB, the axial segregation takes place where the smaller particles prevail in the upper part of the bed, whereas the lateral segregation occurs in which the smaller particles migrate to the lower and central part of the bed. The axial segregation can be explained by the fact that the fluid drag force acting on smaller particles is large enough to not only balance its gravity force but also break through the suppression of the surrounding larger particles. The lateral segregation may be due to the entrainment of jet. An increase in fluidizing gas flow rate attenuates the percolate effect by lifting up the small particle through the interstices between larger particles. (4) The size of the particle exerts profound influence on the mixing and segregation behavior. Segregation becomes more pronounced with an increase in the difference in particle size. (5) On the basis of the flow regime diagram, a mixing/ segregation pattern map is established, which makes a connection between the flow regime and the mixing degree and describes the transitions between the mixing/segregation pattern and operating conditions. On the mixing/segregation pattern map, three regions including the mixing region, segregation region, and intermediate region, are identified by the criterion of the mixing index. Each mixing/segregation region covers several flow regimes. Each flow regime, on the contrary, also covers several mixing/segregation regions.

A typical flow regime diagram for the GB3.2−GB1.1 mixture with Xt,v = 30% is plotted in Figure 11, with the fluidizing gas flow rate Qf* plotted on the ordinate axis and the spouting gas flow rate Qs* plotted on the abscissa. There are six different flow regimes identified and they are internal jet (IJ), spouting (S), fluidizing (F), jet in fluidized bed with bubbling (JFB), jet in fluidized bed with slugging (JFS) and spout−fluidizing (SF). It can be observed that the transitions of flow regimes with the spouting and fluidizing gas flow rates are visible. For example, in the case of low spouting gas flow rate Qs* = 0.24, with increasing Qf*, the flow regime transits from IJ to JFB and then to JFS. In the case of high spouting gas flow rate Qs* = 1.22, the flow regime changes from JFB to S and then to SF with the increase of Qf*. At Qf* = 0.25, the flow regime transits from IJ to JFB and then to S with the increase of Qs*. At a given fluidizing gas flow rate (Qf* = 0.9), the flow regime transits from F to JFS and then to SF with increasing Qs*. Steady-state mixing index corresponding to the operation parameter is presented on the flow regime map in order to establish a connection between flow regime and mixing/ segregation behavior in spout-fluid bed. Typical flow regime images are also shown in Figure 11 to illustrate the distribution of the mixture components. As shown in Figure 11, three distinct zones exist with different mixing indices where the blue lines show the transition thresholds between the regimes. A mixing region where a high mixing degree can be achieved (here identified by the criterion Mequ > 0.7) is found to exist in the flow regime of JFS and SF. At the same time, a part of the mixing region is covered by the flow regime of S. A segregation region where the binary mixture tends clearly to undergo segregation (here identified by the criterion Mequ < 0.3) is shown to cover partial flow regimes of F and JFB. Finally, an intermediate zone is found between the mentioned two regions, where the system exhibits no clear tendency to either state. So, by changing the spouting and fluidizing gas flow rate, we can obtain the desired mixing degree. For example, when the spout-fluid bed is operated at the Qs* = 0.08 and Qf* = 0.76 (in the flow regime of F), a increase in Qf* can give rise to better mixing. When the operation condition is Qs* = 0.71 and Qf* = 0.65 (in the flow regime of JFB), mixing can be improved by enhancing the spouting and/or fluidizing gas velocity or decreasing fluidizing gas velocity. When the bed is operating at the flow regime of S with low spouting gas velocity, better mixing can be reached, provided that the spouting and/or fluidizing gas velocity is enhanced. It is vice verse for desired segregation. In fact, the flow structure has a great influence on the solid transport between annulus and spout and particle trajectories in the fountain, which was also considered to be closely related to mixing and segregation.54,55

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-25-83794744. Fax: +86-25-83795508. E-mail: [email protected].

4. CONCLUSIONS Experimental investigation on the effect of particle size on mixing and segregation behavior has been conducted in a spout-fluid bed. Nine binary mixtures with different proportions were obtained by mixing glass beans with equal density and dissimilar sizes. An initial packing condition of complete segregation was adopted to study the mixing process, whereas an initial perfect mixed bed was used to study the segregation process. The spouting and fluidizing gas flow rate were adjusted to cover a range of flow regimes, typically including IJ, JFB, and SF. Besides, the mixing and segregation pattern were mapped in

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51106028), the National Key Program of Basic Research in China (2012CB215306, 2011CB201505-05), and the National High Technology Research and Development Program of China (863 Program) (No. 2012AA051801). 14255



■

Article

feeding into a conical spouted bed reactor. Ind. Eng. Chem. Res. 2010, 49, 8990−8997. (15) Nienow, A. W.; Rowe, P. N.; Cheung, L. Y. L. A quantitative analysis of the mixing of two segregated powders of different density in a gas fluidized bed. Powder Technol. 1978, 20, 89−97. (16) Nienow, A. W.; Cheesman, D. J.; Grace, J. R.; Matsen, J. M. Fluidization; Plenum: New York, 1980, 373. (17) Nienow, A. W.; Chiba, T. Fluidization, 2nd ed.; Academic Press: 1985; p 9. (18) Baeyens, J.; Geldart, D. Solids mixing. In Gas Fluidization Technology; Gelart, G., Ed.; Wiley: New York, 1986; p 97. (19) Formisani, B.; Girimonte, R.; Longo, T. The fluidization pattern of density-segregating binary mixtures. Chem. Eng. Res. Des. 2008, 86, 344−348. (20) Yang, S. C. Density effect on mixing and segregation processes in a vibrated binary granular mixture. Powder Technol. 2006, 164, 65− 74. (21) Rice, R. W.; Brainovich, J. F. Mixing/segregation in two- and three-dimensional fluidized beds: Binary systems of equidensity spherical particles. AIChE J. 1986, 32, 7−16. (22) Rasul, M. G.; Rudolph, V.; Carsky, M. Segregation potential in binary gas fluidized beds. Powder Technol. 1999, 103, 175−181. (23) Marzocchella, A.; Salatino, P.; Pastena, V. D.; Lirer, L. Transient fluidization and segregation of binary mixtures of particles. AIChE J. 2000, 46, 2175−2182. (24) Wirsum, M.; Fett, F.; Iwanowa, N.; Lukjanow, G. Particle mixing in bubbling fluidized beds of binary particle systems. Powder Technol. 2001, 120, 63−69. (25) Lu, H. L.; He, Y. R.; Gidaspow, D.; Yang, L. D.; Qin, Y. K. Size segregation of binary mixture of solids in bubbling fluidized beds. Powder Technol. 2003, 134, 86−97. (26) Sahoo, A.; Roy, G. K. Mixing characteristic of homogeneous binary mixture of regular particles in a gas-solid fluidized bed. Powder Technol. 2005, 159, 150−154. (27) Umekawa, H.; Furuia, S.; Oshima, Y.; Okura, M.; Ozawa, M.; Takenak, N. Quantitative measurement of segregation phenomena in a binary-mixture fluidized bed by neutron radiography. Nucl. Instrum. Methods Phys. Res. Sect. 2005, 542, 219−225. (28) Hoffmann, A. C.; Romp, E. J. Segregation in a fluidized powder of continuous size distribution. Powder Technol. 1991, 66, 119−126. (29) Wu, S. Y.; Baeyens, J. Segregation by size difference in gas fluidized beds. Powder Technol. 1998, 98, 139−150. (30) Wormsbecker, M.; Adams, A.; Pugsley, T.; Winters, C. Segregation by size difference in a conical fluidized bed of pharmaceutical granulate. Powder Technol. 2005, 153, 72−80. (31) Dahl, S. R.; Hrenya, C. M. Size segregation in gas-solid fluidized beds with continuous size distributions. Chem. Eng. Sci. 2005, 60, 6658−6673. (32) Mathur, K. B.; Epstein, N. Spouted Beds; Academic Press: New York, 1974. (33) Berquin Y. F. Method and apparatus for granulating melted solid and hardenable fluid products. U. S. Patent No. 3.231.413 to PEC, 1966. (34) Piccinini, N.; Bernhard, A.; Campagna, P.; Vallana, F. Segregation phenomenon in spouted beds. Can. J. Chem. Eng. 1977, 55, 122−125. (35) Cook, H. H.; Bridgwater, J. Segregation in spouted beds. Can. J. Chem. Eng. 1978, 56, 636−638. (36) Giorgio, R. Baffle-induced segregation in spouted beds. Can. J. Chem. Eng. 1983, 61, 325−330. (37) San Jose, M. J.; Olazar, M.; Penas, F. J.; Bilbao, J. Segregation in conical spouted beds with binary and ternary mixtures of equidensity spherical particles. Ind. Eng. Chem. Res. 1994, 33, 1838−1844. (38) Olazar, M.; San José, M. J.; Peñas, F. J.; Aguayo, A. T.; Bilbao, J. The stability and hydrodynamics of conical spouted beds with binary mixtures. Ind. Eng. Chem. Res. 1993, 32, 2826−2834. (39) Robinson, T; Waldie, B. Particle cycle times in a spouted bed of polydisperse particles. Can. J. Chem. Eng. 1978, 56, 632−635.

NOMENCLATURE c = concentration of the tracer particles (−) dm = sauter mean diameter (mm) dp = particle diameter (mm) m = mass of the tracer particles in a given sampling region (kg) mt = total mass of the tracer particles in the bed (kg) p = proportion of the mass of tracer particles (−) mlt = total mass of the particles in a sampling region (kg) xt,m = mass ratio (%) xt,v = volume ratio (%) Qmf = minimum fluidizing gas flow rate (m3/s) Qf = fluidizing gas flow rate (m3/s) Qf* = dimensionless fluidizing gas flow rate (−) Qs = spouting gas flow rate (m3/s) Qs* = dimensionless spouting gas flow rate (−) σ2 = variance of the concentration for the random mixture (−) σ02 = variance of the concentration for the fully segregated mixture (−) σm2 = variance of the concentration for the perfectly mixed mixture (−) ε = voidage (−) ρp = particle density (kg/m3)

■

REFERENCES

(1) Link, J.; Zeilstra, C.; Deen, N.; Kuipers, H. Validation of a discrete particle model in a 2D spout-fluid bed using non-instrusive optical measuring techniQues. Can. J. Chem. Eng. 2004, 82, 30−36. (2) Zielinska, M.; Markowski, M. Drying behavior of carrots dried in a spout-fluidized bed dryer. Dry. Technol. 2007, 25, 261−270. (3) Białobrzewskia, I.; Zielińskaa, M.; Mujumdarb, A. S.; Markowskia, M. Heat and mass transfer during drying of a bed of shrinking particles-simulation for carrot cubes dried in a spout-fluidized-bed drier. Int. J. Heat Mass Trans. 2008, 51, 4704−4716. (4) Park, K. B.; Plawsky, J. L.; Littman, H.; Paccione, J. D. Mortar properties obtained by dry premixing of cementitious materials and sand in a spout-fluid bed mixer. Cem. Concr. Res. 2006, 36, 728−734. (5) Plawsky, J. L.; Littman, H. Design and simulation of a spout fluid bed coating system. Excerpt from the Proceedings of the COMSOL Users Conference, Boston, November 14, 2006. (6) Link, J. M.; Godlieb, W.; Deen, N. G.; Kuipers, J. A. M. Discrete element study of granulation in a spout-fluidized bed. Chem. Eng. Sci. 2007, 62, 195−207. (7) Arnold, M. S. J.; Laughlin, M. K. The British coal spout-fluid bed gasification process. Can. J. Chem. Eng. 1992, 70, 991−997. (8) Lim, C. J.; Watkinson, A. P.; Khoe, G. K.; Low, S.; Epstein, N.; Grace, J. R. Spout, fluidized, spout-fluid bed combustion of bituminous coal. Fuel 1988, 67, 1211−1217. (9) Xiao, R.; Zhang, M.; Jin, B. S.; Huang, Y. J.; Zhou, H. C. Hightemperature air/steamblown gasification of coal in a pressurized spoutfluid bed. Energy Fuels 2006, 20, 715−720. (10) Zhong, W. Q.; Zhang, M. Y.; Jin, B. S.; Chen, X. P. Flow regime and transition of rectangular spout-fluid bed. Chem. Eng. Process. 2006, 9, 734−746. (11) Williams, J. C. The mixing of dry powders. Powder Technol. 1968, 2, 13. (12) Hoffmann, A. C.; Janssen, L. P. B. M.; Prins, J. Particle segregation in fluidized binary mixtures. Chem. Eng. Sci. 1993, 48, 1583−1592. (13) López, G.; Olazar, M.; Amutio, M.; Aguado, R.; Bilbao, J. Influence of tire formulation on the products of continuous pyrolysis in a conical spouted bed reactor. Energy Fuel. 2009, 23, 5423−5431. (14) López, G.; Olazar, M.; Aguado, R.; Elordi, G.; Amutio, M.; Artetxe, M.; Bilbao, J. Vacuum pyrolysis of waste tyres by continuously 14256



Article

(40) Zhong, W. Q.; Zhang, M. Y.; Jin, B. S.; Zhang, Y.; Xiao, R.; Huang, Y. J. Experimental investigation of particle mixing behavior in a large spout−fluid bed. Chem. Eng. Process. 2007, 46, 990−995. (41) Zhang, Y.; Jin, B. S.; Zhong, W. Q. Experiment on particle mixing in flat-bottom spout-fluid bed. Chem. Eng. Process. 2009, 48, 126−134. (42) Zhang, Y.; Jin, B. S.; Zhong, W. Q. Experimental investigations on the effect of the trancer location on mixing in a spout-fluid bed. Int. J. Chem. React Eng. 2008, 6, A 46. (43) Marzocchella, A.; Salatino, P.; Pastena, V. D.; Lirer, L. Transient fluidization and segregation of binary mixtures of particle. AIChE J. 2000, 46, 2175−2182. (44) Rhodes, M. J.; Wang, X. S.; Nguyen, M.; Stewart, P.; Liffman, K. Study of mixing in gas-fluidized beds using a DEM model. Chem. Eng. Sci. 2001, 56, 2859−2866. (45) Lu, H. L.; He, Y. R.; Gidaspow, D.; Yang, L. D.; Qin, Y. K. Size segregation of binary mixture of solids in bubbling fluidized beds. Powder Technol. 2003, 134, 86−97. (46) Leaper, M. C.; Seville, J. P. K.; Hilal, N.; Kingman, S. W.; Burbidge, A. S. Investigating the dynamics of segregation of high jetsam binary batch fluidized bed systems. Chem. Eng. Process. 2004, 43, 187−192. (47) Fan, L. T.; Chen, S. J.; Watson, C. A. Solid mixing. Ind. Eng. Chem. 1970, 62, 53−69. (48) Lacey, P. M. C. Developments in the theory of particle mixing. J. Appl. Chem. 1954, 4, 257. (49) Gyenis, J. Assessment of mixing mechanism on the basis of concentration pattern. Chem. Eng. process. 1999, 38, 665−674. (50) Epstein, N.; Grace, J. R. Spouted and Spout-Fluid Beds Fundamentals and Applications; Cambridge University Press: Cambridge, UK, 2011, p 1. (51) Zhong, W. Q.; Zhang, Y.; Jin, B. S. Novel method to study the particle circulation in a flat-bottom spout-fluid bed. Energy Fuel. 2010, 24, 5131−5138. (52) Jin, B. S.; Zhang, Y.; Zhong, W. Q.; Xiao, R. Experimental study of the effect of particle density on mixing behavior in a spout-fluid bed. Ind. Eng. Chem. Res. 2009, 48, 10055−10064. (53) Maria Laura de, A. P. Flow characteristics of two-dimensional spouted and spout-fluidized beds of particles. Ph.D. Thesis, McGiIl University, Montreal, Canada, 1990. (54) San José, M. J.; Olazar, M.; Alvarez, S.; Izquierdo, M. A.; Bilbao, J. Solid cross-flow into the spout and particle trajectories in conical spouted beds. Chem. Eng. Sci. 1998, 53, 3561−3570. (55) San José, M. J.; Olazar, M.; Izquierdo, M. A.; Alvarez, S.; Bilbao, J. Solid trajectories and cycle-times in spouted beds. Ind. Eng. Chem. Res. 2004, 43, 3433−3438.

14257


Mixing and Segregation Behavior in a Spout-Fluid Bed: Effect of

Recommend Documents