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

Mar 20, 2013 - For each mixture, three different mass ratios were prepared so that the mixing and segregation processes were examined. Thus, nine bina...
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Mixing and Segregation Behavior in a Spout-Fluid Bed: Effect of the Particle Density 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 effect of the particle size on the mixing and segregation in a spout-fluid bed has been investigated (Zhang, Y.; Zhong, W. Q.; Jin, B. S.; Xiao, R. Mixing and segregation behavior in a spout-fluid bed: effect of particle size. Ind. Eng. Chem. Res. 2012, 51, 14247−14257). This paper presents the effect of the particle density on the mixing and segregation behavior in a spout fluid. Four kinds of solid particles having the same size of 2.8 mm and different densities of 900, 1230, 1640, and 2540 kg/m3 were used. Three kinds of binary mixtures were obtained, indicated as PB−SP, PB−MB, and PB−GB. For each mixture, three different mass ratios were prepared so that the mixing and segregation processes were examined. Thus, nine binary mixtures were made by mixing the bed material and tracer in different mass ratios. The initial bed stayed in the state of well-mixed and complete segregation, respectively. A wide range of flow regimes were covered by changing the ratio of spouting-to-fluidizing gas flow. The mixing and segregation were analyzed in terms of the concentration profile, mixing index, and flow regimes. The results show that, in the flow regime of IJ, local segregation occurs in the initial well-mixed case, whereas bed inversion takes place in the initially complete segregation bed. In the case of JFB, the entire bed is divided into three sections due to segregation: a pure layer of light particle, one jet region occupied nearly by a heavy particle, and one annulus remaining at the initial distribution at rest. In the flow regime of F, the segregation pattern is dependent on the mass ratio and density difference between the light and heavy particles. On the mixing/segregation pattern map, even though the bed is operated at the same flow regime, the mixing degree varies with the gas velocity. Also, a monotonic increase in the spouting or fluidizing gas velocity is not necessary to promote mixing. There lies one major difference on the mixing/segregation pattern map; that is, the location of the transition region for a binary mixture with different densities (DD mixture) is different from one for a binary mixture with different sizes (DS mixture).

1. INTRODUCTION The spout-fluid bed is an alternative gas−solid contact system in which, in addition to the injection of spouting gas through the central orifice, fluidizing gas is also introduced through a perforated distributor surrounding the central orifice.2 It reduces some limitations of spouting and fluidization by providing a higher rate of mixing, better solid−fluid contact, even fluid flow distribution, and improved mass- and heattransfer characteristics. This technique was accepted traditionally as a solid−fluid contact method for physical operations such as drying, coating, and granulation of granular solids.3−9 In recent years, the application of spout-fluid bed has also been studied for catalytic reactors and combustion and gasification of coal, biomass, and solid waste.10−15 Mixing is a complex process to obtain a uniform mixture of ingredients distributed among each other as uniformly as possible, whereas segregation or demixing is the opposite of the mixing process to separate one component from the mixture.16 Both of them play a critical role in the performance of the particle processing operation and chemical treatment. In most applications, good mixing is required for a uniform product or in order to avoid defluidization of a certain component and decrease the heat- and mass-transfer rates. In some applications, a segregation pattern might be desirable where the tendency for segregation is utilized for the continuous removal of product or the classification of powder. In fact, mixing and segregation tend to take place simultaneously and compete with each other.16 As for their relative function to the final result, it depends on the particle properties and operating conditions. If © 2013 American Chemical Society

the particle properties are regarded as internal factors that affect the mixing and segregation behavior, the operation conditions can be deemed as an external factors. In the gas−solid system, the gas flow rate is considered to be a crucial factor influencing the competition between mixing and segregation.17 For the spout-fluid beds, changing the spouting and fluidizing gas flow rate will generate different flow regimes, which is one of the important hydrodynamic characteristics of the spout-fluid bed. Therefore, it is very necessary to establish the connection between the mixing/segregation pattern and the flow regime. There have been a lot of studies of the flow regime in a spout-fluid bed. Nagarkatti and Chatterjee18 reported the first spout-fluid bed flow-pattern map, which was based on visual observation. Using a similar method, Heil and Tels19 found four flow regimes of a packed bed, a bubbling bed, a fluctuating spouted bed, and a stable spouted bed, while Vukovic et al.20 defined three different flow patterns, i.e., the spout-fluid bed (H>Hmsf), the spout-fluid bed (H 0.65, while in the lateral direction, every proportion value is the same and zero. Similar evolution is also observed when the spouting gas velocity is increased to 0.25. With an increase of the spouting gas flow rate to 0.62, considerable change happens for the curve profile. In the axial direction, the proportion gradually increases to a maximum and then decreases to zero when the bed level increases from y/h0 = 0.15 to 0.65. In the lateral direction, every proportion value remains zero. In view of the flow regime and visual observation, it is reasonable to conclude that in this case bed inversion occurs where mung beans at the upper layer run into the jet region, whereas polypropylene beans in the upper section of the annulus fill the space left by polypropylene beans. The cause of such a flow pattern can be better explained by the fact that, with an increase of the spouting gas velocity, the jet region extends upward and eventually a path is opened for the downward motion of heavy mung beans. Therefore, the jet gas velocity promoting bed inversion occurring depends on the height of the upper layer and the density difference of the binary mixture. When heavier glass beans are used, these phenomena become more obvious. In addition, it is interesting to note that, once the top mung beans run into the jet region, the jet penetration height decreases significantly. It is understood that compared with polypropylene beans, mung beans aggregating together in the internal jet region dissipate more momentum of jet gas because of the greater particle density. Meanwhile, the current bed-inversion phenomenon is remarkably different from that observed in the liquid fluidized bed where the upper particles run into the lower region but the lower particles move into the upper region.36 As the spouting gas velocity is increased to 0.88, the curve seems to become a little flat. In the axial direction, the proportion in the middle part is higher than that in the upper and lower sections of the bed. 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 decrease to a very low value near the wall side. This indicates that, with further spread of the jet region, mung beans gradually begin to mix with polypropylene beans, irrespective of the axial or lateral direction. However, mixing is

σ0 2 − σ 2 σ02 − σm 2

(3)

where σ indicates the concentration variance for a random mixture, σ02 is the value of σ2 when the mixture is fully segregated, and σm2 is the value of σ2 when the mixture is in a perfectly mixed state. The variance is determined by calculating the average of the squared difference from the mean of the particle concentration. The influence of the spouting gas upon mixing is studied by a step-by-step increase of the spouting gas flow rate while the fluidizing gas flow rates are kept at 0.78 and 1.18, respectively. Figure 3 shows the results plotted as the steady-state mixing 2

Figure 3. Steady-state mixing index for the PB−GB mixture.

index versus the dimensionless spouting gas flow rate for the PB−GB mixture. The initial bed is in a well-mixed state. Clearly, it can be found that different gas velocities produce different degrees of mixing at stable states. In the case of Qf* = 0.78, the mixing index decreases quickly and increases slowly with an increase of the spouting gas velocity. It is clear that the mixing index first decreases with a slight increase of the gas velocity as a result of strong segregation. Then, with a further increase of the gas velocity, it increases as a result of intensive agitation, while in the case of Qf* = 1.18, the mixing index increases quickly in the initial stage and then increases gradually from 0.089 to 0.92 with increasing spouting gas velocity. It can be found that even when no spouting gas is introduced to the bed, the bed is in a state of segregation. Then, the bed exhibits a distinct mixing tendency. Therefore, it is reasonable to conclude that the development of a mixing index with the spouting gas velocity is dependent on the fluidizing gas velocity. 5492

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Figure 4. Typical instantaneous snapshots of particle segregation at various fluidizing gas flow rates: (a) Qs* = 0 and Qf* = 0; (b) Qs* = 0.79 and Qf* = 0.4; (c) Qs* = 0.79 and Qf* = 0.79; (d) Qs* = 0.79 and Qf* = 0.95; (e) Qs* = 0.79 and Qf* = 1.11; (f) Qs* = 0.79 and Qf* = 1.35.

Figure 5. Proportion of GB particles with the fluidizing gas velocity along (a) the axial and (b) lateral directions for the PB−GB mixture.

3.2. Mixing at Different Fluidizing Gas Velocities. Figure 4 presents an illustrative example of the segregation process for the PB−GB mixture at different fluidizing gas flow rates. In the beginning, the PB−GB mixture takes on a wellmixed state and the bed remains a fixed bed. Because the operation parameters are Qs* = 0.79 and Qf* = 0.4, an internal jet forms and local circulation takes place where some particles run upward in the central cavity and then move downward in the vicinity of the cavity. At the same time, this process is accompanied by local segregation where polypropylene beans concentrate at the top of the jet. Clearly, this kind of local segregation is different from that observed in the DS mixture, where fine particles assemble in the external jet region.35 The main reason for this difference is that the introduced jet gas penetrates through the surrounding dense region. For the DS mixture, fine particles can be carried by the gas through the interspace of coarse particles, whereas for the DD mixture, light particles can only go through the loose jet region to reach the top of the jet. After formation of an internal jet, an increase in the dimensionless fluidizing gas flow rate to 0.79 leads to partial bubbling of the upper bed. In this case, single small bubble forms at the top of the jet region and moves up slowly until it

erupts at the surface. During the operation, it can be witnessed that sometimes small bubbles can reach the surface of the bed, but sometimes they disappear in the rising process. It is the movement of bubbles that makes the upper bed on the roof of the jet segregate completely. This is similar to the binary fluidized bed operating at low fluidizing gas velocity, where obvious particle segregation takes place.39 As shown in Figure 4c, the entire bed is divided into three sections: a pure layer of polypropylene beans, one jet region occupied nearly by glass beans, and one annulus remaining at the initial distribution at rest. This phenomenon becomes more obvious when the operating conditions are Qs* = 0.79 and Qf* = 0.95. In this case, successive bubbles generate and merge into one large bubble in the process of rising. Eventually, it erupts at the center of the bed surface, ejecting particles to the freeboard, while the internal jet extends in not only the axial but also the lateral direction, causing more glass beans to assemble in the jet region. When the dimensionless fluidizing gas flow rate is further increased to 1.11, the bed operates at the flow regime of a spout-fluid bed. In this case, stable spouting is commonly difficult to achieve, and instead the spouting region sways from side to side periodically. It is difficult to observe a fountain like 5493

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that observed here for stable spouting, which is very often flat. It can be observed from Figure 4e that the particles seem to distribute uniformly with the exception of the top bed region, where one thin layer of polypropylene beans remains. A higher value of the fluidizing gas velocity gives a more uniform bed distribution, as shown in Figure 4f. Figure 5 shows the axial and lateral distributions of the proportion of glass beans at various fluidizing gas velocities for PB−GB. The initial bed is in a well-mixed state. At the beginning of the operation, the bed is in the flow regime of the internal jet and no fluidizing gas flow is introduced through the bed. In the axial direction, it can be found that the proportion first decreases slowly and then increases gradually along the bed height. At the bottom and upper sections, it still has an average value of 0.01, implying that the bed remains at initial uniform distribution, while in the middle zone, it is comparatively lower, suggesting that the jet covers this section and local segregation occurs. This tendency is more obvious in the lateral direction. In the lateral direction, the proportion decreases and then increases from the central line to the wall. This indicates that the glass beans in the middle region converge toward the jet region because of segregation, whereas in the wall region that the jet does not cover, the particles still remain at the original distribution according to the average proportion of 0.01. As the fluidizing gas is introduced and kept at 0.32, the overall bed undergoes segregation where polypropylene beans are segregated to the upper part bed and glass beans concentrate at the jet region. This phenomenon gives rise to the following bed structure. In the axial direction, the proportion at the bottom continues with the same value and then increases until it reaches the maximum value at y/h0 = 0.55. After that, it reduces quickly to zero at the upper part of the bed. In the lateral direction, it is relatively high at the central region and lower at the wall region. During the operation, a pure layer of polypropylene beans is seen in the bed. This is similar to the particle segregation in the fluidized bed reported in other work.40−42 This tendency is particularly noticeable for Qf* = 0.68. In this condition, there are two kinds of segregation tendencies. On the one hand, an increase of the fluidizing gas velocity leads to an increase of the bubble size and bubbling frequency, promoting segregation. On the other hand, an increase of the fluidizing gas velocity causes the jet to extend and more particles are activated especially in the annulus. As a result, more particles are separated. Compared with the lateral direction, it is more obvious in the axial direction, where the proportions at the bottom and middle parts increase. At Qf* = 1.04, more bubbles are produced in the bed and the interactions and coalescence between bubbles are intensified. In this case, bubbles have a greater capacity to carry particles. Hence, partial glass beans are also carried to the upper part of the bed. Thus, it can be noticed that the proportion decreases with an increase of the bed level. Then, with a further increase of the fluidizing gas to 1.35, not only the axial but also the lateral profile becomes more and more flat as well. This suggests that when the fluidizing gas velocity is increased to a certain higher value, it has a positive effect on the axial and lateral mixing. Variation of the mixing index under various fluidizing gas velocities when Qs* = 0.24 and 1.06 is further illustrated in Figure 6. Similar to the results in Figure 3, different combinations of spouting and fluidizing gas velocities contribute to different mixing and segregation behaviors, which gives rise to diverse degrees of mixing at stable states.

Figure 6. Steady-state mixing index for the PB−GB mixture.

In the case of Qs* = 0.24, a V-shaped curve is found. That is to say, the mixing index first decreases rapidly to an extremely low value of 0.076 when the fluidizing gas velocity is increased from 0.33 to 0.62. Then, it grows slowly from 0.076 to 0.749 with a progressive increase of the fluidizing gas velocity. This means that an increase of the fluidizing gas promotes segregation at the initial stage and after that again enhances mixing. Evolution of the mixing index at Qs* = 1.06 is different from that at Qs* = 0.24. The lowest value achieved by the latter is higher than that achieved by the former. This shows that, in the case of low spouting gas velocity, one lower mixing index can be achieved by a change in the fluidizing gas velocity. These phenomena can be explained by the following fact. In the low spouting gas velocity conditions, the fluidizing gas plays a dominant role in affecting the mixing and segregation behavior, whereas the spouting gas only plays a supporting role. Therefore, adjusting the flow of fluidizing gas can reach poorer mixing. In the case of high spouting gas velocity, the role of the spouting gas cannot be ignored, especially in the low fluidizing gas conditions, where the central jet forms and enters the particles in the annulus, promoting mixing. When the fluidizing gas velocity is increased to a higher value, although the role of the spouting gas is weakened, the spouting and fluidizing gases together improve the quality of particle mixing. Figure 7 shows evolution of the steady-state mixing index with the spouting gas flow rate for the PB−GB, PB−MB, and PB−SP mixtures with the same Xt,v = 30%. The initial bed holds in the well-mixed state. Clearly, it could be found that three cases follow the same trend. In the initial stage, a slight increase in the spouting gas velocity leads to a sharp decrease of the mixing index. This

Figure 7. Effect of the particle density on the steady-state mixing index. 5494

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rate, the flow regime transits from F to JFB and then to S and finally to SF. At Qf* = 1.18, the flow regime transits from F to JFS and then to SF with an increase of the spouting gas flow rate. Steady-state mixing index corresponding to the operation parameter is presented on the flow-regime map in order to establish a connection between the flow-regime and mixing/ segregation behavior in the spout-fluid bed. Typical flow-regime images are also shown in Figure 8 to illustrate the distribution of the mixture components. As shown in Figure 8, there exist five distinct zones with different mixing indices, where four curves 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 regimes of JFS and SF. Meanwhile, part of the flow regime of S covers also the mixing region. Because the initial bed is in a well-mixed state, the whole flow region of FB is covered by the mixing region. A segregation region where the binary mixture tends to clearly undergo segregation (Mequ < 0.3) is shown to cover the partial flow regimes of F and JFB. Finally, a transition region (here identified by the criterion 0.3 ⩽ Mequ ⩽ 0.7) is found in two zones. One is between curves 1 and 2, and the other is located between curves 3 and 4. In the intermediate zone, the bed exhibits a tendency to transfer to the mixing or segregation region. So, by changing the spouting and fluidizing gas flow rates, we can obtain the desired mixing or segregation degree. For example, when the spout-fluid bed is operated at Qs* = 0.15 and Qf* = 0.78 (in the flow regime of F and in the segregation region), an increase in the fluidizing gas flow rate can give rise to better mixing. When the operation conditions are Qs* = 0.89 and Qf* = 0.68 (in the flow regime of JFB and in the segregation region), mixing can be improved by enhancing the spouting and/or fluidizing gas velocity. Because the gas flow rate is at Qs* = 0.71 and Qf* = 0.33 (in the flow regime of IJ and in the transition region), mixing may be improved by increasing the spouting gas velocity or segregation may be enhanced by increasing the 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 rises. It can also found that, even though the bed is operated at the same flow regime, the degree of mixing varies with the gas velocity. For example, in the case of the flow regime of F with low fluidizing gas velocity, poor mixing can be achieved as a result of segregation. However, the mixing improves significantly at high fluidizing gas velocity because of strong fluidization. Also, the same mixing degree can be achieved at different flow regimes. For example, poor mixing can be achieved in the flow regime of F or JFB. Good mixing may be obtained in the flow regime of S or SF. In the case of minimum spouting, although the bed is operated in the same flow regime, the mixing degree increases with an increase of the fluidizing gas velocity. A monotonic increase in the spouting or fluidizing gas velocity does not necessarily promote mixing. For example, at Qs* = 0.52, segregation first takes place and then mixing improves with an increase of the fluidizing gas velocity. By comparing the mixing pattern maps between DS and DD mixtures, we can find that, for the former, the transition region is located chiefly in the flow regimes of F and JFB, whereas for the latter, it is located mainly in the flow regimes of IJ and JFB.35 For both of them, the segregation region is in the flow regimes of F and JFB with low fluidizing gas velocity. So, for the

phenomenon was not observed in our previous work.35 For example, when the spouting gas velocity is increased from 0 to 0.15, the mixing index immediately decreases from 1 to 0.274 for the PB−MB mixture. However, under the same conditions, the value of the mixing index for the PB−GB mixture is the lowest, while that for the PB−SP mixture is a lot higher than that for the PB−MB mixture. This suggests that an initial increase in the spouting gas velocity induces particle segregation and segregation becomes more pronounced with an increase in the difference in the particle density. In the later stage, the mixing index increases gradually with the following growth of the spouting gas flow rate. It is quite obvious that the difference in the adjacent mixing index decreases when the spouting gas velocity is increased. This implies that the influence of the density difference on the segregation trend weakens. 3.3. Flow-Regime and Mixing Pattern Map. A typical flow-regime diagram for the PB−GB mixture with Xt,v = 30% is plotted in Figure 8, with the fluidizing gas flow rate Qf* plotted

Figure 8. Flow-regime and mixing/segregation pattern map.

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 a fluidized bed with bubbling (JFB), jet in a fluidized bed with slugging (JFS), and spout fluid (SF). It can be observed that the transitions of the flow regimes with the spouting and fluidizing gas flow rates are visible. For example, in the case of the low spouting gas flow rate Qs* = 0.24, with increasing fluidizing gas flow rate, the flow regime transits from JFB to F and then to JFS. In the case of high spouting gas flow rate Qs* = 0.52, the flow regime changes from IJ to JFB and then to JFS with an increase of the fluidizing gas flow rate. At Qf* = 1.06, the flow regime transits from IJ to JFB and then to S and finally to SF with an increase in the fluidizing gas flow rate. When the fluidizing gas flow rate is kept the same and the spouting gas flow rate is increased gradually, the bed exhibits different flow regimes. For example, at a given fluidizing gas flow rate (Qf* = 0.33), the flow regime transits from IJ to S with an increase in the spouting gas flow rate. In the case of the low fluidizing gas flow rate Qs* = 0.78, with an increase of the fluidizing gas flow 5495

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promote mixing. There lies a major difference on mixing/ segregation pattern map that the location of the transition region for a binary mixture only differing in the particle density is different from that for a binary mixture only differing in the particle size.

purpose of achieving segregation, the best operating condition should be adjusted to the flow regimes of F and JFB with low fluidizing gas velocity.

4. CONCLUSIONS An experimental investigation of the effect of the particle density on mixing and segregation behaviors has been conducted in a spout-fluid bed. Nine binary mixtures with different proportions were obtained by mixing particles with equal size and dissimilar density. An initial packing condition of complete segregation was adopted to study the mixing process, whereas an initial packing condition of a perfectly mixed state was used to study the segregation process. The spouting and fluidizing gas flow rates were adjusted to cover a range of flow regimes. Besides, the mixing and segregation patterns were mapped in light of the flow regime and mixing index. The following conclusions can be drawn from this work: (1) When the bed is operated in the flow regime of IJ, for the initial well-mixed bed, the occurrence of internal circulation is combined with local segregation where lighter particles migrate to the roof of the jet, which is different from that observed for a dissimilar size mixture. For the initial complete segregation bed, bed inversion occurs where heavier particles at the upper layer run into the jet region, whereas lighter particles in the upper section of the annulus fill the space left by heavier particles, causing the jet penetration height to decrease. Because of the effect of the jet, this phenomenon is distinctively different from that occurring in the fluidized bed. (2) In the case of JFB, the entire bed is divided into three sections due to segregation: a pure layer with light particles, one jet region occupied nearly by heavy particles and one annulus remaining at initial distribution at rest. In this case, an increase of the fluidizing gas velocity promotes primarily lateral segregation, which causes more heavy particles in the annulus to concentrate in the jet region. (3) In the flow regime of S, the distribution of the particle depends heavily on the fountain structure. Under the conditions of the underdeveloped fountain, particle segregation is extremely pronounced, where light particles concentrate at the outside part of the fountain and especially the annulus. This segregation is induced by the fact that the light particles rise higher than the heavy particles in the fountain, and they are prone to land at larger lateral positions when falling from the fountain to the surface of the annulus. Conversely, the segregation attenuates gradually with the development of the fountain. In the case of minimum spouting, although the bed is operated in the same flow regime, the mixing degree increases with an increase of the fluidizing gas velocity. (4) In the flow regime of F, the segregation pattern is dependent on the mass ratio and density difference between light and heavy particles. With changes in the operating parameters, nearly complete segregation takes place, where a pure fluidized layer is covered by light particles and a pure defluidized layer is covered by heavy particles. At the same operating conditions, the latter may not be obtained for a mixture with a lower mass ratio or smaller density difference because the central jetting can break up the defluidized layer and bring about local circulation. (5) On a mixing/segregation pattern map, even though the bed is operated at the same flow regime, the degree of mixing varies with the gas velocity. Also, the same mixing degree can be achieved at different flow regimes. A monotonic increase in the spouting or fluidizing gas velocity does not necessarily

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant 51106028) and the National Basic Research Program of China (Grants 2012CB215306 and 2011CB201505-05).

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NOMENCLATURE c = concentration of tracer particles dp = particle diameter (mm) m = weight of the tracer particles in a given sampling cell (kg) mt = total weight of the tracer particles in the bed (kg) p = proportion of the mass of tracer particles mlt = total weight of the tracer particles in a sampling cell (kg) xt,m = mass ratio (%) xt,v = volume ratio (%) Qf* = dimensionless fluidizing gas flow rate Qs*/** = dimensionless spouting gas flow rate σ2 = variance of the concentration in the bed ε = voidage ρp = particle density (kg/m3) REFERENCES

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dx.doi.org/10.1021/ie303577m | Ind. Eng. Chem. Res. 2013, 52, 5489−5497