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Solids concentration in the fully developed region of circulating fluidized bed downers
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Powder Technology 183 (2008) 417 – 425 www.elsevier.com/locate/powtec
Solids concentration in the fully developed region of circulating fluidized bed downers Xiao-Bo Qi, Hui Zhang, Jesse Zhu ⁎ Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario, Canada N6A 5B9 Available online 8 February 2008
Abstract To investigate solids concentration in the fully developed region of co-current downward gas–solid flow, actual solids concentrations were measured in a circulating fluidized bed (CFB) downer with 9.3 m in height and 0.1 m in diameter using a fiber optical probe. The results obtained from this work and in the literature show that the average solids concentration in the fully developed region of the CFB downers is not only a function of the corresponding terminal solids concentration, but the operating conditions and particle properties also have influences on the average solids concentration in the fully developed region of the CFB downers. Particle diameter and density affect the solids concentrations differently under different operating conditions. Downer diameters almost have no influence on the solids concentrations. By taking into account the effects of operating conditions, particle properties and downer diameters, an empirical correlation to predict the solids concentrations in the fully developed region of CFB downers is proposed. The predictions of the correlation are in good agreement with the experimental data of this work and in the literature. © 2008 Elsevier B.V. All rights reserved. Keywords: Circulating fluidized beds; Solids concentration; Downer, Fully developed region, Gas–solid two-phase flow
1. Introduction Circulating fluidized bed (CFB) reactors are applied in various types of processes involving gas–solid contact because of their excellent mixing and transport characteristics [25,13]. As a novel gas–solid reactor, co-current downflow circulating fluidized bed (downer) has been drawing more and more attention due to its advantages over CFB riser. Compared to CFB risers, CFB downers exhibit many advantages [47]. As the gas and solids in CFB downers have the same direction with gravity, the amount of axial back-mixing is reduced greatly and the flow almost approaches plug flow conditions [47]. The radial profiles of particle velocity and solids concentration are also much more uniform across the downer cross-section, in comparison with the CFB riser where significant radial variations in particle velocity and solids concentration are
⁎ Corresponding author. Tel.: +1 519 661 3807; fax: +1 519 850 2441. E-mail address: [email protected] (J. Zhu). 0032-5910/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.01.018
present [47,45,20]. These advantages are particularly beneficial to those processes where extremely short but uniform contact time between gas and solids are required [47,20], such as solid wastes pyrolysis [19], high-selectivity fluidized catalytic cracking (FCC) [36,10], fast pyrolysis of coal and biomass [23,4] and fast drying of heat sensitive materials [1]. It is well known that, solids distribution in CFB downer reactors directly affect the gas–solid contact performance and reaction rate [47,9], the mass and heat transfer [28,10], and the erosion within CFB downers. Due to its importance, solids concentration distribution in CFB downers has been the subject of a number of studies as reported in some literature reviews [47,20]. But, few studies have been carried out to investigate the solids concentrations in the fully developed region of CFB downers and there is still no model or correlation to predict the solids concentrations in the fully developed zone of CFB downers. Consequently, for most gas–solid flow models for CFB downers, “terminal solids concentration”, ɛ′s (=Gs/[ρp(Ug + Ut)]), is used as the solids concentration in the fully developed region of CFB downers instead of the actual solids concentration [11]. However, given the fact that a uniform dispersion of particles in a gas flow
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is unstable [12], the above assumption is not reasonable. In fact, numerous experimental results have shown that there still exists particle clustering phenomenon even in the fully developed region of the downer [24,27], especially under the high-density operating conditions [7]. For the purpose of modeling, design and operation of CFB downer reactors, it is necessary to quantitatively predict the solids concentrations in the fully developed region of CFB downers. Therefore, the current work focuses on the solids concentrations in the fully developed region of CFB downers by measuring actual solids concentrations over a wide range of operating conditions. Based on experimental data from this work and in the literature, the influences of operating conditions, particle properties and downer diameters on the solids concentrations in the fully developed region of CFB downers have been extensively studied in this work and a comprehensive correlation for predicting the actual solids concentrations in the fully developed zone of CFB downers is proposed. 2. Experimental apparatus 2.1. Experimental setup and parameters measurement All experiments were carried out in a cold model CFB downer. The downer and its accompanying riser system are illustrated schematically in Fig. 1. The downer is 9.3 m in height and 100 mm in diameter. During the operation, solids coming from the storage tank were fluidized by the auxiliary air at the
Fig. 1. Schematic diagrams of the CFB riser/downer system.
riser bottom and then carried upwards by the main air along the riser column (15.1 m in height and 0.1 m i.d.). At the riser top the solids passed a smooth elbow into the primary cyclone for gas–solid separation, and some escaped solids entered into the secondary and tertiary cyclones for further separation, whereafter the final gas–solid separation was carried out in a bag filter. At the downer top, the solids were redistributed by a gas– solid distributor located below the dipleg of the riser primary cyclone. The solids distributor had a small fluidized bed (held at minimum fluidization) from which particles fell down into the downer through 31 vertically positioned brass tubes. The gas distributor was a plate located below the solids distributor fluidized bed. From the downer entrance, the co-current downflow gas–solid suspension traveled downward through the downer. After that, the solids were first separated from the air in a quick inertial separator with an efficiency of more than 99% and then drained to the storage tank. The air was further stripped of the entrained particles by two cyclones before it finally passed through the baghouse. Finally, the solids were eventually recycled to the riser bottom from the storage tank, through a butterfly valve located in the inclined feeding pipe. The solids circulation rate was regulated by the butterfly valve and was measured by the measuring pipe. In order to minimize the electrostatics found in both the riser and downer columns, a small stream of steam was introduced into the main air pipeline to humidify the de-oiled fluidization air to a relative humidity of 70–80%. According to Park et al. [32], at a relative humidity value between 50 and 60%, the electrostatic effects and the capillary forces can be controlled in an acceptable level to avoid misleading results. The fluidization gas used in the study was air at ambient temperature and pressure, supplied by a Roots-type blower. An orifice plate was employed to measure the gas flowrates. The particulate materials were spent FCC (Sauter mean diameter dp = 67 μm, particle density ρp = 1500 kg/m3). Differential pressure measurements have usually been used to estimate cross-sectional average solids concentration in CFB downers, assuming that the pressure drops due to gas-wall and solids-wall friction and acceleration of gas and solids are negligible [38]. This method has been accepted by many researchers, since it is non-intrusive, inexpensive and simple [38]. However, many experimental results showed that the contribution of particle acceleration to the total pressure drop cannot be neglected [47]. Comparing actual solids concentrations with apparent ones inferred from pressure gradients, Qi et al. [34] found that the apparent solids concentrations are significantly lower than the actual ones even in the fully developed zone under certain operating conditions and thus it should be very careful to use the differential pressure measurement result to evaluate the solids concentrations in the fully developed region of CFB downers under higher superficial gas velocities and solids circulation rates given the lower solids concentration and the relatively higher suspensionto-wall friction in CFB downers [47]. As a result, in this work, the differential pressure measurement method was not adapted to measure the solids concentrations in the fully developed region of CFB downers.
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Reflective-type optical fiber probes are effective tools for measuring the local solids concentration in fluidized bed reactors, and were widely used by many investigators [15,14,30,38]. They yield high signal-to-noise ratios and, if properly designed, they create a minimum disturbance to the overall flow structure [22]. More importantly, they are nearly free of interference by temperature, humidity, electrostatics and electromagnetic fields [44,40,38]. Therefore, a multi-fiber optical probe, PC-4, developed by the Institute of Process Engineering, Chinese Academy of Sciences, was chosen to measure local solids concentration in this study. The active area of the probe tip with 3.8 mm o.d. is approximately 2 mm × 2 mm, containing approximately 8000 emitting and detecting quartz fibers, each 15 μm in diameter. The precise calibration procedure of the probe and other details can be found in Zhang et al. [44]. Local solids concentrations, ɛs, under 11 operating conditions were measured at 11 radial positions (r/R = 0.0, 0.158, 0.382, 0.498, 0.590, 0.670, 0.741, 0.806, 0.866, 0.922, 0.975) on 8 axial levels (z = 0.246, 0.835, 1.634, 2.548, 3.691, 5.063, 6.435, 8.036 m). Actual crosssectional average solids concentration, ɛ¯s, was obtained by integrating the local values, ɛs, at 10 different radial positions excluding the center since the 10 radial sampling positions had been determined using an area-weighted method. In order to ensure the accuracy of solids concentration measurements, preliminary measurements and statistical error analyses were taken for 2 conditions (Ug = 7.2 m/s, Gs = 101 kg/m2·s; Ug = 10.0 m/s, Gs =202 kg/m2·s) at several axial levels. For each level, 10 measurements were taken for every one of 11 radial positions. The relative standard deviation was found to be within 5%. 2.2. Determination of fully developed region and average solids concentration Under a given operating condition, cross-sectional average solids concentration, ɛ¯s, decreases gradually down the downer column and eventually approaches a constant value in the fully developed region. In theory, the point where the solids concentration reaches a constant is the starting point of the fully developed region. But, because there is fluctuation due to experimental errors, the obtained curve (ɛ¯s vs. z) is not smooth and therefore it is difficult to determine the exact starting point. In this work, the practical method proposed by Huang et al. [17] was adopted to determine the start point of the fully developed region and the average solids concentration in the fully developed region of the downer. That is, the starting point of the fully developed region is considered to have been reached if the maximum relative deviation between the value of (ɛ¯s)i at the test point n and the average (ɛ¯s)m for all points above the test point n is less than 10%, i.e.:
j
P P ɛs i ɛs m ðɛPs Þm
j
V 10k i ¼ n; n þ 1; N ; N
ð1Þ
max
where N is the number of cross-sectional average solids concentrations (for this work, N = 8). Then, (ɛ¯s)m is the average
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solids concentration in the fully developed region of the downer. 3. Results and discussion 3.1. Effect of operating conditions Fig. 2(a), (b) and (c) presents the solids concentrations in the fully developed region of a low-density downer and a highdensity downer as a function of solids circulating rates under different superficial gas velocities for three kinds of particles, respectively. The three figures show that for a given superficial gas velocity, the solids concentration in the fully developed region increases linearly with solids circulation rate for all three co-current downward gas–solid flow systems and the solids concentration decrease with superficial gas velocity when solids circulation rate remains constant, as observed in the previous studies. This is reasonable since increasing gas velocity increases the particle velocity, which in turn results in lower solids concentration under fixed solids flux. Furthermore, the solids concentration in the fully developed region increases linearly with solids circulation rate and the slope decreases with the increase of superficial gas velocity, as shown in Fig. 2. In the past, most correlations [25,31,3,18] to predict the solids concentration in the fully developed zone of CFB risers are mainly based on a parameter “terminal solids concentration”, ɛs′ (= Gs / [ρp(Ug + Ut)]), based on the assumption that there is no particle agglomeration. To this end, Fig. 3(a), (b) and (c) plots the solids concentrations in the fully developed region, ɛs⁎, as a function of the corresponding terminal solids concentration under different superficial gas velocities for three different particles, respectively. Obviously, for the three co-current downward gas–solid flow systems, the slopes of ɛs⁎ against ɛs′ differ for different superficial gas velocities. This means that ɛs⁎ is not only a simple function of ɛs′. This seems to be reasonable since for a given terminal solids concentration ɛs′ (= Gs / [ρp(Ug + Ut)]), there should be many combinations of superficial gas velocity and solids circulation rate. In view of the different influences of superficial gas velocity and solids circulation rate on the solids concentration in the fully developed region of CFB downers, as shown in Fig. 2, it would be not enough to attribute the effect of operating conditions on ɛs⁎ with a simple factor such as the terminal solids concentration. Consequently, it should be more reasonable to include the parameters of superficial gas velocity and solids circulation rate when correlating the solids concentrations in the fully developed region of CFB downers. Furthermore, as shown in Fig. 3, all the actual solids concentrations in the fully developed region of the downers are not greater than the corresponding terminal solids concentration. This is expected since the terminal solids concentration denotes theoretically the highest solids concentration of particulate gas–solid flow with slip velocity equal to the terminal velocity of a single particle under certain operating condition. This clearly indicates that the particles in CFB downers under most operating conditions would, in different degrees, aggregate into clusters, consistent with theoretical
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the difference between ɛs⁎ and ɛs′ increases with the decreasing of superficial gas velocity. This is reasonable because decreasing superficial gas velocity and/or increasing solids circulating rate, the particles in downers are more prone to aggregate into clusters [24,27,7] and therefore the particle velocity becomes
Fig. 2. Variation of solids holdup in the fully developed region with solids circulation rate under different superficial gas velocities.
analysis [12] and many experimental results [24,27,7]. Due to aggregation of particles, particle velocity would increase and thus the solids concentration would be lower than the terminal solids concentration. Further examining Fig. 3, one can see that
Fig. 3. Variation of solids concentration in the fully developed region with terminal solids concentration under different superficial gas velocities.
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faster and the solids concentration tends to be much lower than the corresponding terminal solids concentration. Extensively examining Fig. 3, one can also find that with increasing superficial gas velocity, the slope of ɛs⁎ against ɛs′ gradually approaches 1 and that the slopes under higher superficial gas velocities differ only slightly. This suggests that with increase in superficial gas velocity, the gas–solid flow in the downers tends to become particulate flow and thus the solids concentration approaches the terminal solids concentration. 3.2. Effect of particle properties Many studies have shown that many aspects of CFB hydrodynamics change with particle properties [2,29]. Thereby, particle properties would accordingly affect the solids concentration in the fully developed region of CFB downers. Fig. 4(a), (b) and (c) compares the solids concentrations in the fully developed section of a high-density CFB downer obtained by Liu et al. [26] under different operating conditions for three kinds of particles, respectively. As shown in Fig. 4(a), under the same operating condition with lower superficial gas velocity (i.e. Ug = 1.02 m/s), increasing particle density and/or particle size leads to a lower solids concentration in the fully developed region of the high-density downer, consistent with the experimental results of Bai et al. [2]. This is expected since larger particles and/or denser particles would lead to less effective drag force on the particles, which results in higher particle velocity and therefore lower solids concentration. However, with increase of superficial gas velocities, the influence trend of the particle diameter and density on the solids concentration in the fully developed region of the downer is different than that under lower superficial gas velocity. That is, under higher superficial gas velocities (N 3.4 m/s), the solids concentration changes little with particle diameter but only changes with particle density, as shown in Fig. 4 (b) and (c). This may be explained as follows: since smaller particles are more prone to agglomerate than coarser particles, the tendency of particle aggregation under lower superficial gas velocities is more significant than that under higher gas velocities so that with increasing in gas velocity, the effect of particle diameter tends to be negligible. To further examine quantitatively the effect of particle diameter and density on the particle aggregation in the fully developed region, the ratios of the solids concentration in the fully developed region of the high-density downer to the corresponding terminal solids concentration under different superficial gas velocities are plotted in Fig. 5. Generally, the ratio, ɛs⁎/ɛs′, reflects the extent of particle aggregation. That is, the higher the ratio, the more particles aggregate into clusters. From Fig. 5, it can be clearly seen that ɛs⁎/ɛ′s increases with superficial gas velocity and gradually approaches 1.0. This suggests that the higher the superficial gas velocity, the closer to particulate flow the gas–solid flow in the downer. This is expected since the drag force exerted on particles increases with superficial gas velocity and clusters are more prone to become discrete particles under higher gas velocity [12]. Meantime, it can also be seen from Fig. 5 that with increasing superficial gas velocity, the ratios change non-
Fig. 4. Effect of particle diameter on the solids concentration in the fully developed region of the CFB downer (data obtained from Liu et al. [26]).
linearly. This means that with decreasing of superficial gas velocity, the extent of particle aggregation under lower superficial gas velocities is much more significant than that under higher superficial gas velocities. On the other hand, particle diameter and density also have significant influence on the particle aggregation in the gas–solid
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Consequently, with increase in downer diameters, the influence of downer wall effect tends to become negligible. To investigate the effect of downer diameters on the solids concentrations in the fully developed region, Fig. 6(a) and (b) compares the solids concentrations in the fully developed region of the CFB downers with different diameters for FCC and glass bead particles, respectively. Under the same operating conditions, the solids concentrations in the downers with different diameters are almost the same. For example, under the same operating condition, the solids concentration almost keeps constant when the downer diameter increases from 0.025 m to 0.127 m. Within the range of this study, the solids concentration in the fully
Fig. 5. Effect of particle diameter and density on the ratios of solids concentrations in the fully developed region of the high-density downer to the terminal solids concentrations (data obtained from Liu et al. [26]).
flow in the downer, as shown in Fig. 5. Increasing particle density and/or particle size leads to lower ɛs⁎/ɛs′, which means that the coarser and/or denser particles are prone to move in discrete particle. For example, under Ug = 0.34 m/s, ɛs⁎/ɛs′ of large glass bead, small glass bead and FCC particles are 0.56, 0.31 and 0.21, respectively. At Ug = 7.82 m/s, ɛs⁎/ɛs′ of large glass bead, small glass bead and FCC particles are 0.98, 0.90 and 0.80, respectively. However, all ɛs⁎/ɛs′ are lower than 1.0. This suggests that at lower superficial gas velocity (Ug = 0.34 m/s), most FCC particles in the downer move downward in the form of clusters and considerable portion of glass bead particles aggregate into clusters in the downer. Even at higher superficial gas velocity (Ug = 7.82 m/s), there are still some clusters in the gas–solid flow. These conclusions have been verified by the experimental results of Krol et al. [24], Lu et al. [27] and Chen et al. [7]. 3.3. Effect of downer diameters Yan and Zhu [41,42] found that riser diameters have significant influences on the axial and radial solids concentration and particle velocity profiles and flow development in the riser. The cross-sectional average solids concentration increases with riser diameters but the cross-sectional average particle velocity somewhat decreases with increasing riser diameters. In other words, the scale of the riser does influence significantly the particle distribution in the riser reactor. However, for CFB downers, the simulation results of Cheng et al. [8] in the downers with different diameters have shown that for larger diameter downers, the wall effect can be considered negligible. The above conclusion of Cheng et al. [8] has been partly validated by the experimental results of Zhang et al. [46]. That is, although the relative radial distance from the peak position of dense ring to the downer wall varies with different diameter, the absolute radial distance from the peak position of dense ring to the downer wall is a relative constant value. The magnitude of this absolute distance is about several millimeters and remains relatively unchanged in the downers with different scale.
Fig. 6. Influences of downer diameters on the solids concentrations in the fully developed region of CFB downers.
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under different superficial gas velocities for FCC and glass bead particles, respectively. As stated above, the ratio, ɛs⁎/ɛs′, generally illustrates the extent of particle agglomeration and the relative position between aggregate flow and particulate flow. It is clearly seen from Fig. 7 that for the FCC and glass bead particles, ɛs⁎/ɛs′ remains relatively unchanged in the downers with diameter ranging from 0.025 m to 0.15 m, suggesting that the extents of particle agglomeration are almost the same in the downers with different diameters when the operating condition remains constant. As a result, when correlating the solids concentrations in the fully developed region of the CFB downers, the parameter of downer diameter can be excluded from the equation but the superficial gas velocity should be included. 3.4. Correlation of the solids concentrations in the fully developed zone of CFB downers As discussed above, the slope of the solids concentrations in the fully developed zone against the terminal solids concentration changes with superficial gas velocity, so that the influence of superficial gas velocity should be taken into consideration when correlating the solids concentration. And, particle diameter and density also have effects on the solids concentrations in the fully developed region of the CFB downers. However, the solids concentrations in the fully developed region of the CFB downers with different diameters almost remain the same under the same operating condition. Based on the above conclusions, a general correlation is proposed to correlate the experimental data of the actual solids concentrations in the fully developed region of the CFB downers: G s ɛ⁎s ¼ 0:125 qp Ug þ Ut
!
Ug pffiffiffiffiffiffiffi gdp
!0:25 Ar0:15
ð2Þ
A comparison between the predicted values of Eq. (2) and the experimental data is shown in Fig. 8. It is clearly shown that predictions of the proposed correlation fit well with the
Fig. 7. Effect of downer diameters on the ratio of solids concentration in the fully developed region to terminal solids concentration under different superficial gas velocities.
developed region of the CFB downers has no significant scale-up effect in downer scale, consistent with simulation results of Cheng et al. [8] and experimental results of Zhang et al. [46]. To further study the extent of particle aggregation in the downers with different diameters, Fig. 7(a) and (b) plots the ratios of the solids concentration in the downers with different diameters to the corresponding terminal solids concentration
Fig. 8. Comparison between the predicted solids concentrations in the fully developed region by Eq. (2) and experimental values of this work and in the literature (the keys are indicated in Table 1).
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Table 1 Experimental conditions for the data in the literature Investigator(s)
H (m)
D (m)
Particles
dp (μm)
ρp (kg/m3)
Ug (m/s)
Gs (kg/m2·s)
Number of data points
Legend in Fig. 8
This work [6]
9.3 5.6
0.1 0.08
[26]
5.0
0.025
5.8 5.8 4.6 8.6 4.6 9.3 8.6 5.8
0.14 0.14 0.05 0.15 0.127 0.1 0.15 0.14
67 572 128 82 131 70 123 332 59 59 75 125 82 67 125 59
1500 750 750 992 2480 1300 2500 2500 1545 1545 1630 2480 1480 1500 2500 1545
3.7 ∼ 10.2 1.65 1.05 0.8 1.1 1.02 ∼ 7.82 0.17 ∼ 7.82 1.02 ∼ 7.82 4.33 ∼ 7.94 4.33 ∼ 6.14 0.4 ∼ 6.1 3.6 ∼ 6.6 2.9 ∼ 3.7 5.2 ∼ 9.5 1.0 ∼ 6.0 4.33 ∼ 6.14
49 ∼ 205 45 ∼ 345 26 ∼ 258 45 ∼ 240 70 ∼ 552 16 ∼ 387 21 ∼ 1397 66 ∼ 1340 67 ∼ 165 100 92 50 51 ∼ 236 45 ∼ 180 51 ∼ 89 65 ∼ 138
11 7 6 6 7 27 31 28 7 2 5 2 5 6 2 5
□ ▲ △ ▼ ▽ ■
[39] [33] [16] [35] [5] [21] [37] [43]
FCC Silica gel A Silica gel B FCC Glass beads FCC Glass beads Glass beads FCC FCC FCC Glass beads FCC FCC Glass beads FCC
experimental data obtained from this work and in the literature. The average relative deviation is ± 12.6% (157 points). Considering the inevitable divergence among the data in the literature, the error is acceptable. It should be noted that all the solids concentrations used for correlation are the actual solids concentrations but not the apparent solids concentrations inferred from pressure gradients obtained by differential pressure measurements. Therefore, the friction between the gas–solid suspension and downer wall has been excluded. The equation can be reliably used to predict the solids concentrations in the fully developed region of CFB downers when modeling and designing downer reactors. Although the influence factors such as operating conditions, particle properties and downer diameters have been intensively studied in this work, the effect of downer height on the solids concentrations in the fully developed region has not been taken into account. Since any empirical correlation cannot be extrapolated outside the range of the used experimental data, further experimental investigation is needed for a more general correlation. 4. Conclusions The solids concentrations in the fully developed region of co-current downward gas–solid flow were experimentally investigated in a 9.3 m high CFB downer by measuring the actual solids concentrations with a fiber optical probe. The experimental results obtained from this work and in the literature show that the average solids concentration in the fully developed region of the CFB downers is not only a function of the corresponding terminal solids concentration. The particles in the CFB downers under most operating conditions of this work would aggregate into clusters. Under lower superficial gas velocities, increasing particle density and/ or particle size leads to a lower solids concentration in the fully developed region, while under higher superficial gas velocities, only particle density has influence on the solids concentration in the fully developed region of the downers. Downer diameters
◆ ◇
● ○ ★ ☆ ⊗ ⊙ × ◄
almost have no influence on the solids concentrations in the fully developed region of the CFB downers. By taking into account the effects of operating conditions, particle properties and downer diameters, an empirical correlation for predicting the actual solids concentrations in the fully developed region of the CFB downers has been proposed. The predictions of the correlation are in good agreement with the experimental data obtained from this work and in the literature. List of Symbol Ar Archimedes Number ¼ dp3 qg g qp qg =A2g , (–) D riser internal diameter, (m) Sauter mean diameter of particles, (μm) dp Gs solids circulation rate, (kg/m2·s) g acceleration due to gravity, (m/s2) H downer height, (m) r radial coordinate, (m) r/R normalized radial distance from the riser center, (–) R riser radius, (m) Ug superficial gas velocity, (m/s) Ut terminal particle velocity, (m/s) z axial position from the riser gas distributor, (m) Greek letters local solids concentration, (–) ɛs ɛ¯s actual cross-sectional average solids concentration, (–) ɛs⁎ average solids concentration in the fully developed region, (–) terminal solids concentration (= Gs / (ρp(Ug + Ut))), (–) ɛs′ μg gas viscosity, (Pa·s) ρg gas density, (kg/m3) ρp particle density, (kg/m3)
Acknowledgement The authors are grateful to the Natural Science and Engineering Research Council of Canada for financial support.
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References [1] P.I. Alvarez, R. Vega, R. Blasco, Cocurrent downflow fluidized bed dryer: experimental equipment and modeling, Drying Technol. 23 (2005) 1435–1449. [2] D.-R. Bai, Y. Jin, Z.-Q. Yu, J.-X. Zhu, The axial distribution of the crosssectionally averaged voidage in fast fluidized beds, Powder Technol. 71 (1992) 51–58. [3] D.-R. Bai, K. Kato, Quantitative estimation of solids holdups at dense and dilute regions of circulating fluidized beds, Powder Technol. 101 (1999) 183–190. [4] D.A. Berg, C.L. Briens, M.A. Bergougnou, Reactor development for the ultra fast pyrolysis reactor, Can. J. Chem. Eng. 67 (1989) 96–101. [5] C.-S. Cao, H. Weinstein, Characterization of downflowing high velocity fluidized beds, AIChE J. 46 (3) (2000) 515–523. [6] H.-Z. Chen, H.-Z. Li, Characterization of a high-density downer reactor, Powder Technol. 146 (2004) 84–92. [7] H.-Z. Chen, H.Z. Li, M. Kwauk, Two-phase structure in a high-density downer, Powder Technology 158 (1–3) (2005) 115–123. [8] Y. Cheng, Y.C. Guo, F. Wei, Y. Jin, W.Y. Lin, Modeling the hydrodynamics of downer reactors based on kinetic theory, Chem. Eng. Sci. 54 (1999) 2019–2027. [9] R.-S. Deng, F. Wei, Q. Zhang, Y. Jin, Experimental study of deep catalytic cracking process in a downer reactor, Ind. Eng. Chem. Res. 41 (2002) 6015–6019. [10] R.-S. Deng, F. Wei, T.-F. Liu, Y. Jin, Radial behavior in riser and downer during the FCC process, Chem. Eng. and Proc. 41 (2002) 259–266. [11] R.-S. Deng, H.E. Liu, F. Wei, Y. Jin, Axial flow structure at the varying superficial gas velocity in a downer reactor, Chem. Eng. J. 99 (2004) 5–14. [12] J.R. Grace, J. Tuot, A theory for cluster formation in vertically conveyed suspension of intermediate density, Trans. IChemE 57 (1979) 49–54. [13] J.R. Grace, H.T. Bi, Introduction to circulating fluidized beds, in: J.R. Grace, A.A. Avidan, T.M. Knowlton (Eds.), Circulating Fluidized Beds, Chapman & Hall, London, 1997, pp. 1–18. [14] E.-U. Hartge, D. Rensner, J. Werther, Solids concentration and velocity patterns in circulating fluidized beds, in: P. Basu, J.F. Large (Eds.), Circulating Fluidized Bed Technology, Pergamon Press, Oxford, 1988, pp. 165–180. [15] H. Hatano, M. Ishida, The entrainment of solid particles from a gas–solid fluidized bed, J. Chem. Eng. Jpn. 14 (1981) 306–311. [16] P.M. Herbert, T.A. Gauthier, C.L. Briens, M.A. Bergougnou, Flow study of a 0.05 m diameter downflow circulating fluidized bed, Powder Technol. 96 (1998) 255–261. [17] W.-X. Huang, J.-X. Zhu, J.H. Pärssinen, Comprehensive study on solids acceleration length in a long CFB riser, Chem. Eng. & Technol. 29 (2006) 1197–1204. [18] A.S. Issangya, J.R. Grace, J.-X. Zhu, in: K. Cen (Ed.), Bottom and Exit Region Solids Hold-ups in Circulating Fluidized Bed Risers, Circulating Fluidized Bed Technology, Vol. VIII, International Academic Publishers, Hangzhou, 2005, pp. 209–215. [19] S.-Y. Jiao, J.-X. Zhu, M.A. Bergougnou, M. Ikura, M. Stanciulescu, Investigation and modelling of the thermal cracking of waste plastics derived oil in a downer reactor, Trans IChemE. 76 (B) (1998) 319–331. [20] Y. Jin, Y. Zheng, F. Wei, in: J.R. Grace, J.-X. Zhu, H.I. de Lasa (Eds.), State-of-the-art Review of Downer Reactors, Circulating Fluidized Bed Technology, Vol. VII, Canadian Society of Chemical Engineering, Ottawa, 2002, pp. 40–60. [21] P.M. Johnston, H.I. de Lasa, J.-X. Zhu, Axial flow structure in the entrance region of a downer fluidized bed: effects of the distributor design, Chem. Eng. Sci. 54 (1999) 2161–2173. [22] H. Johnsson, F. Johnsson, Measurements of local solids volume-fraction in fluidized bed boilers, Powder Technology 115 (2001) 13–26. [23] Y.J. Kim, S.H. Lee, S.D. Kim, Coal gasification characteristics in a downer reactor, Fuel 80 (13) (2001) 1915–1922.
425
[24] S. Krol, A. Pekediz, H. de Lasa, Particle clustering in down flow reactors, Powder Technol. 108 (1) (2000) 6–20. [25] D. Kunii, O. Levenspiel, in: P. Basu, M. Horio, M. Hasatani (Eds.), Flow Modeling of Fast Fluidized Beds, Circulating Fluidized Bed Technology, Vol. III, Pergamon Press, Oxford, 1991, pp. 91–98. [26] W.-D. Liu, K.-B. Luo, J.-X. Zhu, J.M. Beeckmans, Characterization of high-density gas–solids downward fluidized flow, Powder Technol. 115 (2001) 27–35. [27] X.-S. Lu, S.-G. Li, L. Du, J.-Z. Yao, W.-G. Lin, H.-Z. Li, Flow structures in the downer circulating fluidized bed, Chem. Eng. J. 112 (1–3) (2005) 23–31. [28] Y.-L. Ma, J.-X. Zhu, Experimental study of heat transfer in a co-current downflow fluidized bed (downer), Chem. Eng. Sci. 54 (1) (1999) 41–50. [29] M.L. Mastellone, U. Arena, The effect of particle size and density on solids distribution along the riser of a circulating fluidized bed, Chem. Eng. Sci. 54 (1999) 5383–5391. [30] J.J. Nieuwland, R. Meijer, J.A.M. Kuipers, W.P.M. van Swaaij, Measurements of solids concentration and axial solids velocity in gas– solids two-phase flows, Powder Technol. 87 (1996) 127–139. [31] S. Ouyang, O.E. Potter, Consistency of circulating fluidized bed experimental data, Ind. Eng. Chem. Res. 32 (1993) 1041–1045. [32] A. Park, H.T. Bi, J.R. Grace, Reduction of electrostatic charges in fluidized beds, Chem. Eng. Sci. 57 (2002) 153–162. [33] C.-M. Qi, Z.-Q. Yu, Y. Jin, D.-R. Bai, W.-H. Yao, Study on the gas–solids cocurrently downflow fluidization (I), J. Chem. Ind. Eng. China 3 (1990) 273–280. [34] X.-B. Qi, H. Zhang, J.-X. Zhu, in: H.-T. Bi, F. Berruti, T. Pugsley (Eds.), Friction Between Gas–Solid Suspension and Circulating Fluidized Bed Downers, Fluidization, Vol. XII, Engineering Foundation, Vancouver, 2007, pp. 95–102. [35] T. Schiewe, K.E. Wirth, O. Molerus, K. Tuzla, A.K. Sharma, J.C. Chen, Measurements of solid concentration in a downward vertical gas–solid flow, AIChE J. 45 (5) (1999) 949–955. [36] J.A. Talman, R. Geier, L. Rel, Development of a downer reactor for fluid catalytic cracking, Chem. Eng. Sci. 54 (1999) 2123–2130. [37] K. Tuzla, A.K. Sharma, J.C. Chen, T. Schiewe, K.E. Wirth, O. Molerus, Transient dynamics of solid concentration in downer fluidized bed, Powder Technol. 100 (2–3) (1998) 166–172. [38] J.R. Van Ommen, R.F. Mudde, in: H.-T. Bi, F. Berruti, T. Pugsley (Eds.), Measuring the Gas–Solids Distribution in Fluidized Beds—A Review, Fluidization, Vol. XII, Engineering Foundation, Vancouver, 2007, pp. 31–45. [39] Z.-W. Wang, D.-R. Bai, Y. Jin, Hydrodynamics of cocurrent downflow circulating fluidized bed (CDCFB), Powder Technol. 70 (1992) 271–275. [40] J. Werther, Measurement techniques in fluidized beds, Powder Technol. 102 (1999) 15–36. [41] A.-J. Yan, J.-X. Zhu, Scale-up effect of riser reactors (1)—axial and radial solids concentration distribution and flow development, Ind. Eng. Chem. Res. 43 (19) (2004) 5810–5819. [42] A.-J. Yan, J.-X. Zhu, Scale-up effect of riser reactors—particle velocity and flow development, AIChE J. 51 (11) (2005) 2956–2964. [43] Y.-L. Yang, Z.-W. Wang, Y. Jin, Z.-Q. Yu, Theoretical studies on hydrodynamics in co-current downward circulating fluidized beds, Chem. React. Eng. and Technol. 11 (3) (1995) 217–226. [44] H. Zhang, P.M. Johnston, J.-X. Zhu, H.I.D.e. Lasa, M.A. Bergougnou, A novel calibration procedure for a fiber optic solids concentration probe, Powder Technol. 100 (1998) 260–272. [45] H. Zhang, W.-X. Huang, J.-X. Zhu, Gas–solid flow behavior: CFB riser vs. downer, AIChE J. 47 (9) (2001) 2000–2011. [46] M.-H. Zhang, Z. Qian, H. Yu, F. Wei, The near wall dense ring in a large-scale down-flow circulating fluidized bed, Chem. Eng. J. 92 (2003) 161–167. [47] J.-X. Zhu, Z.-Q. Yu, Y. Jin, J.R. Grace, A. Issangya, Cocurrent downflow circulating fluidized bed (downer) reactors—a state of the art review, Can. J. Chem. Eng. 73 (1995) 662–677.
Powder Technology 183 (2008) 417 – 425 www.elsevier.com/locate/powtec
Solids concentration in the fully developed region of circulating fluidized bed downers Xiao-Bo Qi, Hui Zhang, Jesse Zhu ⁎ Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario, Canada N6A 5B9 Available online 8 February 2008
Abstract To investigate solids concentration in the fully developed region of co-current downward gas–solid flow, actual solids concentrations were measured in a circulating fluidized bed (CFB) downer with 9.3 m in height and 0.1 m in diameter using a fiber optical probe. The results obtained from this work and in the literature show that the average solids concentration in the fully developed region of the CFB downers is not only a function of the corresponding terminal solids concentration, but the operating conditions and particle properties also have influences on the average solids concentration in the fully developed region of the CFB downers. Particle diameter and density affect the solids concentrations differently under different operating conditions. Downer diameters almost have no influence on the solids concentrations. By taking into account the effects of operating conditions, particle properties and downer diameters, an empirical correlation to predict the solids concentrations in the fully developed region of CFB downers is proposed. The predictions of the correlation are in good agreement with the experimental data of this work and in the literature. © 2008 Elsevier B.V. All rights reserved. Keywords: Circulating fluidized beds; Solids concentration; Downer, Fully developed region, Gas–solid two-phase flow
1. Introduction Circulating fluidized bed (CFB) reactors are applied in various types of processes involving gas–solid contact because of their excellent mixing and transport characteristics [25,13]. As a novel gas–solid reactor, co-current downflow circulating fluidized bed (downer) has been drawing more and more attention due to its advantages over CFB riser. Compared to CFB risers, CFB downers exhibit many advantages [47]. As the gas and solids in CFB downers have the same direction with gravity, the amount of axial back-mixing is reduced greatly and the flow almost approaches plug flow conditions [47]. The radial profiles of particle velocity and solids concentration are also much more uniform across the downer cross-section, in comparison with the CFB riser where significant radial variations in particle velocity and solids concentration are
⁎ Corresponding author. Tel.: +1 519 661 3807; fax: +1 519 850 2441. E-mail address: [email protected] (J. Zhu). 0032-5910/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.01.018
present [47,45,20]. These advantages are particularly beneficial to those processes where extremely short but uniform contact time between gas and solids are required [47,20], such as solid wastes pyrolysis [19], high-selectivity fluidized catalytic cracking (FCC) [36,10], fast pyrolysis of coal and biomass [23,4] and fast drying of heat sensitive materials [1]. It is well known that, solids distribution in CFB downer reactors directly affect the gas–solid contact performance and reaction rate [47,9], the mass and heat transfer [28,10], and the erosion within CFB downers. Due to its importance, solids concentration distribution in CFB downers has been the subject of a number of studies as reported in some literature reviews [47,20]. But, few studies have been carried out to investigate the solids concentrations in the fully developed region of CFB downers and there is still no model or correlation to predict the solids concentrations in the fully developed zone of CFB downers. Consequently, for most gas–solid flow models for CFB downers, “terminal solids concentration”, ɛ′s (=Gs/[ρp(Ug + Ut)]), is used as the solids concentration in the fully developed region of CFB downers instead of the actual solids concentration [11]. However, given the fact that a uniform dispersion of particles in a gas flow
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is unstable [12], the above assumption is not reasonable. In fact, numerous experimental results have shown that there still exists particle clustering phenomenon even in the fully developed region of the downer [24,27], especially under the high-density operating conditions [7]. For the purpose of modeling, design and operation of CFB downer reactors, it is necessary to quantitatively predict the solids concentrations in the fully developed region of CFB downers. Therefore, the current work focuses on the solids concentrations in the fully developed region of CFB downers by measuring actual solids concentrations over a wide range of operating conditions. Based on experimental data from this work and in the literature, the influences of operating conditions, particle properties and downer diameters on the solids concentrations in the fully developed region of CFB downers have been extensively studied in this work and a comprehensive correlation for predicting the actual solids concentrations in the fully developed zone of CFB downers is proposed. 2. Experimental apparatus 2.1. Experimental setup and parameters measurement All experiments were carried out in a cold model CFB downer. The downer and its accompanying riser system are illustrated schematically in Fig. 1. The downer is 9.3 m in height and 100 mm in diameter. During the operation, solids coming from the storage tank were fluidized by the auxiliary air at the
Fig. 1. Schematic diagrams of the CFB riser/downer system.
riser bottom and then carried upwards by the main air along the riser column (15.1 m in height and 0.1 m i.d.). At the riser top the solids passed a smooth elbow into the primary cyclone for gas–solid separation, and some escaped solids entered into the secondary and tertiary cyclones for further separation, whereafter the final gas–solid separation was carried out in a bag filter. At the downer top, the solids were redistributed by a gas– solid distributor located below the dipleg of the riser primary cyclone. The solids distributor had a small fluidized bed (held at minimum fluidization) from which particles fell down into the downer through 31 vertically positioned brass tubes. The gas distributor was a plate located below the solids distributor fluidized bed. From the downer entrance, the co-current downflow gas–solid suspension traveled downward through the downer. After that, the solids were first separated from the air in a quick inertial separator with an efficiency of more than 99% and then drained to the storage tank. The air was further stripped of the entrained particles by two cyclones before it finally passed through the baghouse. Finally, the solids were eventually recycled to the riser bottom from the storage tank, through a butterfly valve located in the inclined feeding pipe. The solids circulation rate was regulated by the butterfly valve and was measured by the measuring pipe. In order to minimize the electrostatics found in both the riser and downer columns, a small stream of steam was introduced into the main air pipeline to humidify the de-oiled fluidization air to a relative humidity of 70–80%. According to Park et al. [32], at a relative humidity value between 50 and 60%, the electrostatic effects and the capillary forces can be controlled in an acceptable level to avoid misleading results. The fluidization gas used in the study was air at ambient temperature and pressure, supplied by a Roots-type blower. An orifice plate was employed to measure the gas flowrates. The particulate materials were spent FCC (Sauter mean diameter dp = 67 μm, particle density ρp = 1500 kg/m3). Differential pressure measurements have usually been used to estimate cross-sectional average solids concentration in CFB downers, assuming that the pressure drops due to gas-wall and solids-wall friction and acceleration of gas and solids are negligible [38]. This method has been accepted by many researchers, since it is non-intrusive, inexpensive and simple [38]. However, many experimental results showed that the contribution of particle acceleration to the total pressure drop cannot be neglected [47]. Comparing actual solids concentrations with apparent ones inferred from pressure gradients, Qi et al. [34] found that the apparent solids concentrations are significantly lower than the actual ones even in the fully developed zone under certain operating conditions and thus it should be very careful to use the differential pressure measurement result to evaluate the solids concentrations in the fully developed region of CFB downers under higher superficial gas velocities and solids circulation rates given the lower solids concentration and the relatively higher suspensionto-wall friction in CFB downers [47]. As a result, in this work, the differential pressure measurement method was not adapted to measure the solids concentrations in the fully developed region of CFB downers.
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Reflective-type optical fiber probes are effective tools for measuring the local solids concentration in fluidized bed reactors, and were widely used by many investigators [15,14,30,38]. They yield high signal-to-noise ratios and, if properly designed, they create a minimum disturbance to the overall flow structure [22]. More importantly, they are nearly free of interference by temperature, humidity, electrostatics and electromagnetic fields [44,40,38]. Therefore, a multi-fiber optical probe, PC-4, developed by the Institute of Process Engineering, Chinese Academy of Sciences, was chosen to measure local solids concentration in this study. The active area of the probe tip with 3.8 mm o.d. is approximately 2 mm × 2 mm, containing approximately 8000 emitting and detecting quartz fibers, each 15 μm in diameter. The precise calibration procedure of the probe and other details can be found in Zhang et al. [44]. Local solids concentrations, ɛs, under 11 operating conditions were measured at 11 radial positions (r/R = 0.0, 0.158, 0.382, 0.498, 0.590, 0.670, 0.741, 0.806, 0.866, 0.922, 0.975) on 8 axial levels (z = 0.246, 0.835, 1.634, 2.548, 3.691, 5.063, 6.435, 8.036 m). Actual crosssectional average solids concentration, ɛ¯s, was obtained by integrating the local values, ɛs, at 10 different radial positions excluding the center since the 10 radial sampling positions had been determined using an area-weighted method. In order to ensure the accuracy of solids concentration measurements, preliminary measurements and statistical error analyses were taken for 2 conditions (Ug = 7.2 m/s, Gs = 101 kg/m2·s; Ug = 10.0 m/s, Gs =202 kg/m2·s) at several axial levels. For each level, 10 measurements were taken for every one of 11 radial positions. The relative standard deviation was found to be within 5%. 2.2. Determination of fully developed region and average solids concentration Under a given operating condition, cross-sectional average solids concentration, ɛ¯s, decreases gradually down the downer column and eventually approaches a constant value in the fully developed region. In theory, the point where the solids concentration reaches a constant is the starting point of the fully developed region. But, because there is fluctuation due to experimental errors, the obtained curve (ɛ¯s vs. z) is not smooth and therefore it is difficult to determine the exact starting point. In this work, the practical method proposed by Huang et al. [17] was adopted to determine the start point of the fully developed region and the average solids concentration in the fully developed region of the downer. That is, the starting point of the fully developed region is considered to have been reached if the maximum relative deviation between the value of (ɛ¯s)i at the test point n and the average (ɛ¯s)m for all points above the test point n is less than 10%, i.e.:
j
P P ɛs i ɛs m ðɛPs Þm
j
V 10k i ¼ n; n þ 1; N ; N
ð1Þ
max
where N is the number of cross-sectional average solids concentrations (for this work, N = 8). Then, (ɛ¯s)m is the average
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solids concentration in the fully developed region of the downer. 3. Results and discussion 3.1. Effect of operating conditions Fig. 2(a), (b) and (c) presents the solids concentrations in the fully developed region of a low-density downer and a highdensity downer as a function of solids circulating rates under different superficial gas velocities for three kinds of particles, respectively. The three figures show that for a given superficial gas velocity, the solids concentration in the fully developed region increases linearly with solids circulation rate for all three co-current downward gas–solid flow systems and the solids concentration decrease with superficial gas velocity when solids circulation rate remains constant, as observed in the previous studies. This is reasonable since increasing gas velocity increases the particle velocity, which in turn results in lower solids concentration under fixed solids flux. Furthermore, the solids concentration in the fully developed region increases linearly with solids circulation rate and the slope decreases with the increase of superficial gas velocity, as shown in Fig. 2. In the past, most correlations [25,31,3,18] to predict the solids concentration in the fully developed zone of CFB risers are mainly based on a parameter “terminal solids concentration”, ɛs′ (= Gs / [ρp(Ug + Ut)]), based on the assumption that there is no particle agglomeration. To this end, Fig. 3(a), (b) and (c) plots the solids concentrations in the fully developed region, ɛs⁎, as a function of the corresponding terminal solids concentration under different superficial gas velocities for three different particles, respectively. Obviously, for the three co-current downward gas–solid flow systems, the slopes of ɛs⁎ against ɛs′ differ for different superficial gas velocities. This means that ɛs⁎ is not only a simple function of ɛs′. This seems to be reasonable since for a given terminal solids concentration ɛs′ (= Gs / [ρp(Ug + Ut)]), there should be many combinations of superficial gas velocity and solids circulation rate. In view of the different influences of superficial gas velocity and solids circulation rate on the solids concentration in the fully developed region of CFB downers, as shown in Fig. 2, it would be not enough to attribute the effect of operating conditions on ɛs⁎ with a simple factor such as the terminal solids concentration. Consequently, it should be more reasonable to include the parameters of superficial gas velocity and solids circulation rate when correlating the solids concentrations in the fully developed region of CFB downers. Furthermore, as shown in Fig. 3, all the actual solids concentrations in the fully developed region of the downers are not greater than the corresponding terminal solids concentration. This is expected since the terminal solids concentration denotes theoretically the highest solids concentration of particulate gas–solid flow with slip velocity equal to the terminal velocity of a single particle under certain operating condition. This clearly indicates that the particles in CFB downers under most operating conditions would, in different degrees, aggregate into clusters, consistent with theoretical
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the difference between ɛs⁎ and ɛs′ increases with the decreasing of superficial gas velocity. This is reasonable because decreasing superficial gas velocity and/or increasing solids circulating rate, the particles in downers are more prone to aggregate into clusters [24,27,7] and therefore the particle velocity becomes
Fig. 2. Variation of solids holdup in the fully developed region with solids circulation rate under different superficial gas velocities.
analysis [12] and many experimental results [24,27,7]. Due to aggregation of particles, particle velocity would increase and thus the solids concentration would be lower than the terminal solids concentration. Further examining Fig. 3, one can see that
Fig. 3. Variation of solids concentration in the fully developed region with terminal solids concentration under different superficial gas velocities.
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faster and the solids concentration tends to be much lower than the corresponding terminal solids concentration. Extensively examining Fig. 3, one can also find that with increasing superficial gas velocity, the slope of ɛs⁎ against ɛs′ gradually approaches 1 and that the slopes under higher superficial gas velocities differ only slightly. This suggests that with increase in superficial gas velocity, the gas–solid flow in the downers tends to become particulate flow and thus the solids concentration approaches the terminal solids concentration. 3.2. Effect of particle properties Many studies have shown that many aspects of CFB hydrodynamics change with particle properties [2,29]. Thereby, particle properties would accordingly affect the solids concentration in the fully developed region of CFB downers. Fig. 4(a), (b) and (c) compares the solids concentrations in the fully developed section of a high-density CFB downer obtained by Liu et al. [26] under different operating conditions for three kinds of particles, respectively. As shown in Fig. 4(a), under the same operating condition with lower superficial gas velocity (i.e. Ug = 1.02 m/s), increasing particle density and/or particle size leads to a lower solids concentration in the fully developed region of the high-density downer, consistent with the experimental results of Bai et al. [2]. This is expected since larger particles and/or denser particles would lead to less effective drag force on the particles, which results in higher particle velocity and therefore lower solids concentration. However, with increase of superficial gas velocities, the influence trend of the particle diameter and density on the solids concentration in the fully developed region of the downer is different than that under lower superficial gas velocity. That is, under higher superficial gas velocities (N 3.4 m/s), the solids concentration changes little with particle diameter but only changes with particle density, as shown in Fig. 4 (b) and (c). This may be explained as follows: since smaller particles are more prone to agglomerate than coarser particles, the tendency of particle aggregation under lower superficial gas velocities is more significant than that under higher gas velocities so that with increasing in gas velocity, the effect of particle diameter tends to be negligible. To further examine quantitatively the effect of particle diameter and density on the particle aggregation in the fully developed region, the ratios of the solids concentration in the fully developed region of the high-density downer to the corresponding terminal solids concentration under different superficial gas velocities are plotted in Fig. 5. Generally, the ratio, ɛs⁎/ɛs′, reflects the extent of particle aggregation. That is, the higher the ratio, the more particles aggregate into clusters. From Fig. 5, it can be clearly seen that ɛs⁎/ɛ′s increases with superficial gas velocity and gradually approaches 1.0. This suggests that the higher the superficial gas velocity, the closer to particulate flow the gas–solid flow in the downer. This is expected since the drag force exerted on particles increases with superficial gas velocity and clusters are more prone to become discrete particles under higher gas velocity [12]. Meantime, it can also be seen from Fig. 5 that with increasing superficial gas velocity, the ratios change non-
Fig. 4. Effect of particle diameter on the solids concentration in the fully developed region of the CFB downer (data obtained from Liu et al. [26]).
linearly. This means that with decreasing of superficial gas velocity, the extent of particle aggregation under lower superficial gas velocities is much more significant than that under higher superficial gas velocities. On the other hand, particle diameter and density also have significant influence on the particle aggregation in the gas–solid
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Consequently, with increase in downer diameters, the influence of downer wall effect tends to become negligible. To investigate the effect of downer diameters on the solids concentrations in the fully developed region, Fig. 6(a) and (b) compares the solids concentrations in the fully developed region of the CFB downers with different diameters for FCC and glass bead particles, respectively. Under the same operating conditions, the solids concentrations in the downers with different diameters are almost the same. For example, under the same operating condition, the solids concentration almost keeps constant when the downer diameter increases from 0.025 m to 0.127 m. Within the range of this study, the solids concentration in the fully
Fig. 5. Effect of particle diameter and density on the ratios of solids concentrations in the fully developed region of the high-density downer to the terminal solids concentrations (data obtained from Liu et al. [26]).
flow in the downer, as shown in Fig. 5. Increasing particle density and/or particle size leads to lower ɛs⁎/ɛs′, which means that the coarser and/or denser particles are prone to move in discrete particle. For example, under Ug = 0.34 m/s, ɛs⁎/ɛs′ of large glass bead, small glass bead and FCC particles are 0.56, 0.31 and 0.21, respectively. At Ug = 7.82 m/s, ɛs⁎/ɛs′ of large glass bead, small glass bead and FCC particles are 0.98, 0.90 and 0.80, respectively. However, all ɛs⁎/ɛs′ are lower than 1.0. This suggests that at lower superficial gas velocity (Ug = 0.34 m/s), most FCC particles in the downer move downward in the form of clusters and considerable portion of glass bead particles aggregate into clusters in the downer. Even at higher superficial gas velocity (Ug = 7.82 m/s), there are still some clusters in the gas–solid flow. These conclusions have been verified by the experimental results of Krol et al. [24], Lu et al. [27] and Chen et al. [7]. 3.3. Effect of downer diameters Yan and Zhu [41,42] found that riser diameters have significant influences on the axial and radial solids concentration and particle velocity profiles and flow development in the riser. The cross-sectional average solids concentration increases with riser diameters but the cross-sectional average particle velocity somewhat decreases with increasing riser diameters. In other words, the scale of the riser does influence significantly the particle distribution in the riser reactor. However, for CFB downers, the simulation results of Cheng et al. [8] in the downers with different diameters have shown that for larger diameter downers, the wall effect can be considered negligible. The above conclusion of Cheng et al. [8] has been partly validated by the experimental results of Zhang et al. [46]. That is, although the relative radial distance from the peak position of dense ring to the downer wall varies with different diameter, the absolute radial distance from the peak position of dense ring to the downer wall is a relative constant value. The magnitude of this absolute distance is about several millimeters and remains relatively unchanged in the downers with different scale.
Fig. 6. Influences of downer diameters on the solids concentrations in the fully developed region of CFB downers.
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under different superficial gas velocities for FCC and glass bead particles, respectively. As stated above, the ratio, ɛs⁎/ɛs′, generally illustrates the extent of particle agglomeration and the relative position between aggregate flow and particulate flow. It is clearly seen from Fig. 7 that for the FCC and glass bead particles, ɛs⁎/ɛs′ remains relatively unchanged in the downers with diameter ranging from 0.025 m to 0.15 m, suggesting that the extents of particle agglomeration are almost the same in the downers with different diameters when the operating condition remains constant. As a result, when correlating the solids concentrations in the fully developed region of the CFB downers, the parameter of downer diameter can be excluded from the equation but the superficial gas velocity should be included. 3.4. Correlation of the solids concentrations in the fully developed zone of CFB downers As discussed above, the slope of the solids concentrations in the fully developed zone against the terminal solids concentration changes with superficial gas velocity, so that the influence of superficial gas velocity should be taken into consideration when correlating the solids concentration. And, particle diameter and density also have effects on the solids concentrations in the fully developed region of the CFB downers. However, the solids concentrations in the fully developed region of the CFB downers with different diameters almost remain the same under the same operating condition. Based on the above conclusions, a general correlation is proposed to correlate the experimental data of the actual solids concentrations in the fully developed region of the CFB downers: G s ɛ⁎s ¼ 0:125 qp Ug þ Ut
!
Ug pffiffiffiffiffiffiffi gdp
!0:25 Ar0:15
ð2Þ
A comparison between the predicted values of Eq. (2) and the experimental data is shown in Fig. 8. It is clearly shown that predictions of the proposed correlation fit well with the
Fig. 7. Effect of downer diameters on the ratio of solids concentration in the fully developed region to terminal solids concentration under different superficial gas velocities.
developed region of the CFB downers has no significant scale-up effect in downer scale, consistent with simulation results of Cheng et al. [8] and experimental results of Zhang et al. [46]. To further study the extent of particle aggregation in the downers with different diameters, Fig. 7(a) and (b) plots the ratios of the solids concentration in the downers with different diameters to the corresponding terminal solids concentration
Fig. 8. Comparison between the predicted solids concentrations in the fully developed region by Eq. (2) and experimental values of this work and in the literature (the keys are indicated in Table 1).
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Table 1 Experimental conditions for the data in the literature Investigator(s)
H (m)
D (m)
Particles
dp (μm)
ρp (kg/m3)
Ug (m/s)
Gs (kg/m2·s)
Number of data points
Legend in Fig. 8
This work [6]
9.3 5.6
0.1 0.08
[26]
5.0
0.025
5.8 5.8 4.6 8.6 4.6 9.3 8.6 5.8
0.14 0.14 0.05 0.15 0.127 0.1 0.15 0.14
67 572 128 82 131 70 123 332 59 59 75 125 82 67 125 59
1500 750 750 992 2480 1300 2500 2500 1545 1545 1630 2480 1480 1500 2500 1545
3.7 ∼ 10.2 1.65 1.05 0.8 1.1 1.02 ∼ 7.82 0.17 ∼ 7.82 1.02 ∼ 7.82 4.33 ∼ 7.94 4.33 ∼ 6.14 0.4 ∼ 6.1 3.6 ∼ 6.6 2.9 ∼ 3.7 5.2 ∼ 9.5 1.0 ∼ 6.0 4.33 ∼ 6.14
49 ∼ 205 45 ∼ 345 26 ∼ 258 45 ∼ 240 70 ∼ 552 16 ∼ 387 21 ∼ 1397 66 ∼ 1340 67 ∼ 165 100 92 50 51 ∼ 236 45 ∼ 180 51 ∼ 89 65 ∼ 138
11 7 6 6 7 27 31 28 7 2 5 2 5 6 2 5
□ ▲ △ ▼ ▽ ■
[39] [33] [16] [35] [5] [21] [37] [43]
FCC Silica gel A Silica gel B FCC Glass beads FCC Glass beads Glass beads FCC FCC FCC Glass beads FCC FCC Glass beads FCC
experimental data obtained from this work and in the literature. The average relative deviation is ± 12.6% (157 points). Considering the inevitable divergence among the data in the literature, the error is acceptable. It should be noted that all the solids concentrations used for correlation are the actual solids concentrations but not the apparent solids concentrations inferred from pressure gradients obtained by differential pressure measurements. Therefore, the friction between the gas–solid suspension and downer wall has been excluded. The equation can be reliably used to predict the solids concentrations in the fully developed region of CFB downers when modeling and designing downer reactors. Although the influence factors such as operating conditions, particle properties and downer diameters have been intensively studied in this work, the effect of downer height on the solids concentrations in the fully developed region has not been taken into account. Since any empirical correlation cannot be extrapolated outside the range of the used experimental data, further experimental investigation is needed for a more general correlation. 4. Conclusions The solids concentrations in the fully developed region of co-current downward gas–solid flow were experimentally investigated in a 9.3 m high CFB downer by measuring the actual solids concentrations with a fiber optical probe. The experimental results obtained from this work and in the literature show that the average solids concentration in the fully developed region of the CFB downers is not only a function of the corresponding terminal solids concentration. The particles in the CFB downers under most operating conditions of this work would aggregate into clusters. Under lower superficial gas velocities, increasing particle density and/ or particle size leads to a lower solids concentration in the fully developed region, while under higher superficial gas velocities, only particle density has influence on the solids concentration in the fully developed region of the downers. Downer diameters
◆ ◇
● ○ ★ ☆ ⊗ ⊙ × ◄
almost have no influence on the solids concentrations in the fully developed region of the CFB downers. By taking into account the effects of operating conditions, particle properties and downer diameters, an empirical correlation for predicting the actual solids concentrations in the fully developed region of the CFB downers has been proposed. The predictions of the correlation are in good agreement with the experimental data obtained from this work and in the literature. List of Symbol Ar Archimedes Number ¼ dp3 qg g qp qg =A2g , (–) D riser internal diameter, (m) Sauter mean diameter of particles, (μm) dp Gs solids circulation rate, (kg/m2·s) g acceleration due to gravity, (m/s2) H downer height, (m) r radial coordinate, (m) r/R normalized radial distance from the riser center, (–) R riser radius, (m) Ug superficial gas velocity, (m/s) Ut terminal particle velocity, (m/s) z axial position from the riser gas distributor, (m) Greek letters local solids concentration, (–) ɛs ɛ¯s actual cross-sectional average solids concentration, (–) ɛs⁎ average solids concentration in the fully developed region, (–) terminal solids concentration (= Gs / (ρp(Ug + Ut))), (–) ɛs′ μg gas viscosity, (Pa·s) ρg gas density, (kg/m3) ρp particle density, (kg/m3)
Acknowledgement The authors are grateful to the Natural Science and Engineering Research Council of Canada for financial support.
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References [1] P.I. Alvarez, R. Vega, R. Blasco, Cocurrent downflow fluidized bed dryer: experimental equipment and modeling, Drying Technol. 23 (2005) 1435–1449. [2] D.-R. Bai, Y. Jin, Z.-Q. Yu, J.-X. Zhu, The axial distribution of the crosssectionally averaged voidage in fast fluidized beds, Powder Technol. 71 (1992) 51–58. [3] D.-R. Bai, K. Kato, Quantitative estimation of solids holdups at dense and dilute regions of circulating fluidized beds, Powder Technol. 101 (1999) 183–190. [4] D.A. Berg, C.L. Briens, M.A. Bergougnou, Reactor development for the ultra fast pyrolysis reactor, Can. J. Chem. Eng. 67 (1989) 96–101. [5] C.-S. Cao, H. Weinstein, Characterization of downflowing high velocity fluidized beds, AIChE J. 46 (3) (2000) 515–523. [6] H.-Z. Chen, H.-Z. Li, Characterization of a high-density downer reactor, Powder Technol. 146 (2004) 84–92. [7] H.-Z. Chen, H.Z. Li, M. Kwauk, Two-phase structure in a high-density downer, Powder Technology 158 (1–3) (2005) 115–123. [8] Y. Cheng, Y.C. Guo, F. Wei, Y. Jin, W.Y. Lin, Modeling the hydrodynamics of downer reactors based on kinetic theory, Chem. Eng. Sci. 54 (1999) 2019–2027. [9] R.-S. Deng, F. Wei, Q. Zhang, Y. Jin, Experimental study of deep catalytic cracking process in a downer reactor, Ind. Eng. Chem. Res. 41 (2002) 6015–6019. [10] R.-S. Deng, F. Wei, T.-F. Liu, Y. Jin, Radial behavior in riser and downer during the FCC process, Chem. Eng. and Proc. 41 (2002) 259–266. [11] R.-S. Deng, H.E. Liu, F. Wei, Y. Jin, Axial flow structure at the varying superficial gas velocity in a downer reactor, Chem. Eng. J. 99 (2004) 5–14. [12] J.R. Grace, J. Tuot, A theory for cluster formation in vertically conveyed suspension of intermediate density, Trans. IChemE 57 (1979) 49–54. [13] J.R. Grace, H.T. Bi, Introduction to circulating fluidized beds, in: J.R. Grace, A.A. Avidan, T.M. Knowlton (Eds.), Circulating Fluidized Beds, Chapman & Hall, London, 1997, pp. 1–18. [14] E.-U. Hartge, D. Rensner, J. Werther, Solids concentration and velocity patterns in circulating fluidized beds, in: P. Basu, J.F. Large (Eds.), Circulating Fluidized Bed Technology, Pergamon Press, Oxford, 1988, pp. 165–180. [15] H. Hatano, M. Ishida, The entrainment of solid particles from a gas–solid fluidized bed, J. Chem. Eng. Jpn. 14 (1981) 306–311. [16] P.M. Herbert, T.A. Gauthier, C.L. Briens, M.A. Bergougnou, Flow study of a 0.05 m diameter downflow circulating fluidized bed, Powder Technol. 96 (1998) 255–261. [17] W.-X. Huang, J.-X. Zhu, J.H. Pärssinen, Comprehensive study on solids acceleration length in a long CFB riser, Chem. Eng. & Technol. 29 (2006) 1197–1204. [18] A.S. Issangya, J.R. Grace, J.-X. Zhu, in: K. Cen (Ed.), Bottom and Exit Region Solids Hold-ups in Circulating Fluidized Bed Risers, Circulating Fluidized Bed Technology, Vol. VIII, International Academic Publishers, Hangzhou, 2005, pp. 209–215. [19] S.-Y. Jiao, J.-X. Zhu, M.A. Bergougnou, M. Ikura, M. Stanciulescu, Investigation and modelling of the thermal cracking of waste plastics derived oil in a downer reactor, Trans IChemE. 76 (B) (1998) 319–331. [20] Y. Jin, Y. Zheng, F. Wei, in: J.R. Grace, J.-X. Zhu, H.I. de Lasa (Eds.), State-of-the-art Review of Downer Reactors, Circulating Fluidized Bed Technology, Vol. VII, Canadian Society of Chemical Engineering, Ottawa, 2002, pp. 40–60. [21] P.M. Johnston, H.I. de Lasa, J.-X. Zhu, Axial flow structure in the entrance region of a downer fluidized bed: effects of the distributor design, Chem. Eng. Sci. 54 (1999) 2161–2173. [22] H. Johnsson, F. Johnsson, Measurements of local solids volume-fraction in fluidized bed boilers, Powder Technology 115 (2001) 13–26. [23] Y.J. Kim, S.H. Lee, S.D. Kim, Coal gasification characteristics in a downer reactor, Fuel 80 (13) (2001) 1915–1922.
425
[24] S. Krol, A. Pekediz, H. de Lasa, Particle clustering in down flow reactors, Powder Technol. 108 (1) (2000) 6–20. [25] D. Kunii, O. Levenspiel, in: P. Basu, M. Horio, M. Hasatani (Eds.), Flow Modeling of Fast Fluidized Beds, Circulating Fluidized Bed Technology, Vol. III, Pergamon Press, Oxford, 1991, pp. 91–98. [26] W.-D. Liu, K.-B. Luo, J.-X. Zhu, J.M. Beeckmans, Characterization of high-density gas–solids downward fluidized flow, Powder Technol. 115 (2001) 27–35. [27] X.-S. Lu, S.-G. Li, L. Du, J.-Z. Yao, W.-G. Lin, H.-Z. Li, Flow structures in the downer circulating fluidized bed, Chem. Eng. J. 112 (1–3) (2005) 23–31. [28] Y.-L. Ma, J.-X. Zhu, Experimental study of heat transfer in a co-current downflow fluidized bed (downer), Chem. Eng. Sci. 54 (1) (1999) 41–50. [29] M.L. Mastellone, U. Arena, The effect of particle size and density on solids distribution along the riser of a circulating fluidized bed, Chem. Eng. Sci. 54 (1999) 5383–5391. [30] J.J. Nieuwland, R. Meijer, J.A.M. Kuipers, W.P.M. van Swaaij, Measurements of solids concentration and axial solids velocity in gas– solids two-phase flows, Powder Technol. 87 (1996) 127–139. [31] S. Ouyang, O.E. Potter, Consistency of circulating fluidized bed experimental data, Ind. Eng. Chem. Res. 32 (1993) 1041–1045. [32] A. Park, H.T. Bi, J.R. Grace, Reduction of electrostatic charges in fluidized beds, Chem. Eng. Sci. 57 (2002) 153–162. [33] C.-M. Qi, Z.-Q. Yu, Y. Jin, D.-R. Bai, W.-H. Yao, Study on the gas–solids cocurrently downflow fluidization (I), J. Chem. Ind. Eng. China 3 (1990) 273–280. [34] X.-B. Qi, H. Zhang, J.-X. Zhu, in: H.-T. Bi, F. Berruti, T. Pugsley (Eds.), Friction Between Gas–Solid Suspension and Circulating Fluidized Bed Downers, Fluidization, Vol. XII, Engineering Foundation, Vancouver, 2007, pp. 95–102. [35] T. Schiewe, K.E. Wirth, O. Molerus, K. Tuzla, A.K. Sharma, J.C. Chen, Measurements of solid concentration in a downward vertical gas–solid flow, AIChE J. 45 (5) (1999) 949–955. [36] J.A. Talman, R. Geier, L. Rel, Development of a downer reactor for fluid catalytic cracking, Chem. Eng. Sci. 54 (1999) 2123–2130. [37] K. Tuzla, A.K. Sharma, J.C. Chen, T. Schiewe, K.E. Wirth, O. Molerus, Transient dynamics of solid concentration in downer fluidized bed, Powder Technol. 100 (2–3) (1998) 166–172. [38] J.R. Van Ommen, R.F. Mudde, in: H.-T. Bi, F. Berruti, T. Pugsley (Eds.), Measuring the Gas–Solids Distribution in Fluidized Beds—A Review, Fluidization, Vol. XII, Engineering Foundation, Vancouver, 2007, pp. 31–45. [39] Z.-W. Wang, D.-R. Bai, Y. Jin, Hydrodynamics of cocurrent downflow circulating fluidized bed (CDCFB), Powder Technol. 70 (1992) 271–275. [40] J. Werther, Measurement techniques in fluidized beds, Powder Technol. 102 (1999) 15–36. [41] A.-J. Yan, J.-X. Zhu, Scale-up effect of riser reactors (1)—axial and radial solids concentration distribution and flow development, Ind. Eng. Chem. Res. 43 (19) (2004) 5810–5819. [42] A.-J. Yan, J.-X. Zhu, Scale-up effect of riser reactors—particle velocity and flow development, AIChE J. 51 (11) (2005) 2956–2964. [43] Y.-L. Yang, Z.-W. Wang, Y. Jin, Z.-Q. Yu, Theoretical studies on hydrodynamics in co-current downward circulating fluidized beds, Chem. React. Eng. and Technol. 11 (3) (1995) 217–226. [44] H. Zhang, P.M. Johnston, J.-X. Zhu, H.I.D.e. Lasa, M.A. Bergougnou, A novel calibration procedure for a fiber optic solids concentration probe, Powder Technol. 100 (1998) 260–272. [45] H. Zhang, W.-X. Huang, J.-X. Zhu, Gas–solid flow behavior: CFB riser vs. downer, AIChE J. 47 (9) (2001) 2000–2011. [46] M.-H. Zhang, Z. Qian, H. Yu, F. Wei, The near wall dense ring in a large-scale down-flow circulating fluidized bed, Chem. Eng. J. 92 (2003) 161–167. [47] J.-X. Zhu, Z.-Q. Yu, Y. Jin, J.R. Grace, A. Issangya, Cocurrent downflow circulating fluidized bed (downer) reactors—a state of the art review, Can. J. Chem. Eng. 73 (1995) 662–677.