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Characterization of a high-density downer reactor
Powder Technology 146 (2004) 84 – 92 www.elsevier.com/locate/powtec
Characterization of a high-density downer reactor Hengzhi Chen*, Hongzhong Li1 Multiphase Reaction Laboratory, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100080, China Received 22 December 2003; received in revised form 20 July 2004; accepted 26 July 2004 Available online 21 September 2004
Abstract Experiments were carried out in a specially designed circulating fluidized bed with a 5.6-m tall, 0.08 m ID downer, in which the maximal the solids flux 600 kg m2 s1 and the maximal solids holdup 0.14 have been achieved. The results show that single peak exists in the PDD curves of the solids concentration for low-density operation, and a higher platform is present in the PDD curves for the high-density downer. Solids flux is a key factor that affects the solids holdup while particle properties are other important parameter. The heavier/better fluidity particle could achieve the higher solids flux and the smaller/lighter particle were ease to achieve the higher solids holdup. The radial nonuniform core/annulus structure still exists in the high-density downer, but radial solids distribution gradually becomes more uniform with downward distance and with the increase of solids flux. D 2004 Elsevier B.V. All rights reserved. Keywords: High-density downer; Solids flux; Transient solids concentration; Solids concentration distribution
1. Introduction Circulating fluidized beds (CFB) have been widely used in many fields such as chemical industry, energy, materials, which include the gas–solid cocurrent upflow system (riser) and the gas–solid cocurrent downflow system (downer). Due to such advantages as shorter residence time, more uniform distribution of particles, and less gas and solids backmixing, the downer has drawn much attention in the past two decades [1]. A series of hydrodynamic experiments have been carried out by several research groups [7–12]. Many industrial processes, such as fluid catalytic cracking [2,3], coal pyrolysis [4,5],biomass pyrolysis [6], may be benefited from this new type of reactor. However, shortcomings of the downer have been pointed out by the investigators [11]. One of the key weaknesses is that the solids holdup is so low that results in * Corresponding author. Tel.: +86 10 6255 8002; fax: +86 10 6256 1822. E-mail addresses: [email protected] (H. Chen)8 [email protected] (H. Li). 1 Fax: 86 10 62536108. 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.07.008
the lower heat transfer coefficient in the downer [12] and the lower bed-to-wall heat transfer coefficient [13]. The results of computer simulation showed that the gas oil conversion approximately increase 4% (wt) when the catalyst/oil ratio increased from 8 to 10 in the fluid catalysis and cracking process [14]. Though a large number of the papers about the low-density downer have been published, there is only one article in which hydrodynamics about high-density downflow were reported [15,16]. There is, therefore, an urgent need to study the characters of the downer operating at high solids holdup/high solids flux for understanding the flow mechanics and speeding up its industrial application. The high density circulating fluidized beds (HDCFB) has been proposed by Zhu and Bi [17] , and Grace et al. [18] has defined the HDCFB riser as operations with G sN200 kg m2 s1 and e sN0.1 throughout the entire riser. In high-density downer, the start point of the constant mean particle velocity has been suggested to demarcate low-density operation and high-density operation by the former researchers [16]. The solids concentration is an important parameter to affect the efficiency of the downer. We prefer the operation with e avN5% in the
H. Chen, H. Li / Powder Technology 146 (2004) 84–92
developed region as the criterion of HDCFB downer in this work.
2. Experimental equipment and measuring system Fig. 1 shows the circulating fluidized-bed system, consisting, on the left-hand side, of a downer (5.6 m high by 80 mm ID) followed by a bottom separator and a moving bed seal (3 m high by 120 mm ID) which feeds solid particles into a riser (12 m high by 120 mm ID), located on the righthand side. Air was used as the fluidizing gas and four kinds
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Table 1 Properties of the particle Glass beads FCC Silica gel A Silica gel B
d p (Am)
q p (kg m3)
w (8)
U t (m s1)
131 82 572 128
2480 992 750 750
23 34 25 26
0.76 0.2 1.73 0.31
of particles were used in this work, of which properties are shown in Table 1. The air exiting the riser at the top first goes to an inertia separator to remove the major portion of the particles, and then to a cyclone, the remainder portion.
Fig. 1. Schematic diagram of the setup.
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H. Chen, H. Li / Powder Technology 146 (2004) 84–92
Fig. 2. Block diagram for optical fiber measurement.
To increase the solids density in the downer three measures were taken as follows: !
! !
High solids flux at the entrance of the downer was achieved by throttling the air leaving the inertia separator to force a portion of the air flow downward and transport the solids via a pre-acceleration pipe into the downer. A relatively low gas velocity was used in the downer. A riser with larger diameter (120 mm ID) was adopted in order to increase the transportation ability of the riser.
0.0025 to 0.07 and the cross-section averaged solids holdup is about 0.017. The instantaneous solids concentration is very low in the most cases at six positions, but the higher solids concentration (e tN0.05) still exists, which was recognized as the trance of the cluster in the downer by the former investigator [19]. Under the high-density operation condition ( G s=258 kg m2 s1), the transient solids concentration varies from 0.005 to 0.26 and the crosssection averaged solids concentration is about 0.135. The ratio of the maximum value of the instantaneous solids concentration to the time-averaged concentration is about 4 at low-density operation, while it is about 2 at high-density operation. The previous research showed that the averaged solids concentration of clusters is about three to six times that of overall time-averaged solids concentration [19]. In this work, so outstanding value of instantaneous solids fraction exists in low-density operation, but it is not present in high-density operation. The previous studies on probability density distribution (PDD) of the local transient solids concentration in downers showed that one peak existed in the PDD curves and the peak position varied a little with the change of the radial position, and there existed particle aggregates near the wall due to the high solids concentration ring [19,20]. The similar result has been observed in this work under lowdensity operating condition as shown in Fig. 5. From Fig. 5,
An optical fiber probe was used to sample the transient signals of local solids fraction, e t ,in the bed, as shown in Fig. 2. Two beams of light, a reference light and a measuring light, were used to reduce environmental disturbances. The frontal tip area of the probe was only 4 mm2, small enough to minimize its effect on flow. The cross-section averaged solids fraction e av was obtained from measuring the pressure gradient along the bed height as follows: eav ¼
dp=dh qp g
ð1Þ
3. Results and discussion 3.1. The transient signals of local solids fraction The transient signals of the local solids concentration contain a lot of useful information and analyzing these signals is an important method to study the phase structure in the gas–solid fluidized bed. Figs. 3 and 4 show the transient signals of local solids concentration at six radial positions (r/R=0, 0.4, 0.6, 0.7, 0.8, 0.95) at the level of 2.3 m distance from the entrance of the downer for the solids flux 26 and 258 kg m2 s1, respectively. Under the low-density operation condition ( G s=26 kg m2 s1), the instantaneous solids concentration varies from
Fig. 3. Instantaneous solids concentration at low-density operation. Silica gel B, H=2.3m, Gs=26kg.m2.s1, Ug=0.2m.s1.
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3.2. Solids flux and the cross-section solids concentration The solids concentration is an important parameter which is affected by both gas velocity and solids flux. In the developed region, the cross-section averaged solids concentration can be calculated by the Eq. (1), which is also called the mean solids holdup by Lehner et al. [8]. The deviation of the evaluated solids holdup by pressure gradient from the actual solids holdup in the developed region of the downer is about 15% [16]. Figs. 7 and 8 show the relations between the mean solids concentration of silica gel, FCC particles, and glass beads, and the solids flux in the developed region (Dh=2.3–3.3 m). From the two figures, it can be seen that solids holdup markedly increases with the increase of solids flux for all four kinds of particles, which means that solids flux is the key factor affecting solids concentration in the downer. In other words, only when the higher solids flux is achieved, the higher solid holdup may be present. With the addition of the configuration of the overall circulating fluidized beds system, particle properties can affect the solids flux. First, particle density is an important parameter and the solids flux of glass beads can achieve the largest value of 600 kg m2 s1 due to its highest density. Second, particle fluidity is another parameter affecting solids flux. From Table 1, it can
Fig. 4. Instantaneous solids concentration at high-density operation. Silica gel B, H=2.3m, Gs=258kg.m2.s1, Ug=1.54m.s1.
it can been seen that the solids concentration at the peak position of PDD curves is about 0.015 and the fraction of the higher instantaneous solids concentration (e tN0.05) is less than 10%, which implies that gas phase is more dominant in the downer under low-density operation conditions. However, the trend of PDD curves under high-density operating conditions, as shown in Fig. 6, is different from both the previous studies [19,20] and this work under lowdensity operating conditions. From Fig. 6, it can be seen that no peak exits in the PDD curves and a higher platform is present in the PDD curves at high-density operation. The probability of the solids concentration approximates with variation of solids fraction from 0.05 to 0.2, which means that, from a statistical perspective, the solids concentration varies continuously in the range from 0.05 to 0.2. The instantaneous solids concentration at the radial position of r/ R=0.95 are all over 0.05, and even at the center (r/R=0), the solids fraction over 0.05 takes up the major time during the test time. These results show that particle phase is more dominant in the downer at high-density operation. The hydrodynamics of gas–solid flow in the high-density downer are some different from those in the low-density downer. So, the more work, especially with high-density operation, needs to be carried out to understand the phase structure in the downer.
Fig. 5. Probability density distribution of local instantaneous solids concentration at low-density operation. Silica gel B, H=2.3m, Gs=26kg.m2.s1, Ug=0.2m.s1.
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Fig. 8. Relation between solids flux and solids flux.
Fig. 6. Probability density distribution of local instantaneous solids concentration at high-density operation. Silica gel B, H=2.3m, Gs=258kg.m2.s1, Ug=1.54m.s1.
be seen that silica gel (A: w=258, B: w=268) and glass bead (w=238) have better fluidity than that of FCC particles (w=348). As a result, the maximum solids flux of FCC particles becomes the smallest one among the four kinds of particles, though the density of FCC particles is larger than that of silica gel. Third, particle size is the other parameter to affect solids flux, and the larger particles can achieve the
higher solids flux because the larger particles have the higher terminal velocity when particle density is equivalent. For instance, the solids flux of silica gel A (d p=572 Am) can achieve as high as 350 kg/m2 s, while that of silica gel B (d p=128 Am) is 258 kg m2 s1 only, as shown in Fig. 7. Particle properties have an important influence on solids concentration. The following relation, e s=G s/(q pU p), shows that particle density is one of important factor to affect solids concentration and the lighter particle can achieve the higher solids concentration when solids flux is fixed. The solids concentration of silica gel B, FCC and glass beads is about 13%, 10%, and 5%, respectively, when the solids flux is 240 kg m2 s1 as shown in Figs. 7 and 8. Particle size has an important influence on solids holdup, and the larger particle leads to the lower solids holdup due to the higher particle velocity in the developed region under the similar operation conditions. Comparing the results obtained in this work with that obtained by Liu et al. [16], the maximal solids flux in this work is much lower than that from the latter due to the different configuration of their downers. The larger diameter of downer is used in this work, which means that the more material need be provided to achieve the same solids flux. The cross-section averaged solids concentration in this work is slight greater than that obtained by Liu et al. [16] under the similar operation conditions, which attributes to the restriction of the conical structure at the exit of the downer in this work. 3.3. Axial profiles of pressure gradient and solids concentration Usually, the pressure gradient of the downer can be expressed by following equation:
Fig. 7. Relation between solids flux and solids flux.
dUp dp dPf ¼ qp ges Gs ð2Þ dh dh dh Figs. 9 and 10 show the axial profiles of the pressure gradient for silica gel B and glass beads with different solids
H. Chen, H. Li / Powder Technology 146 (2004) 84–92
Fig. 9. Axial pressure gradient for silica gel B.
fluxes. From the two figures, it can be seen that the pressure gradient increases with increasing solids flux because the higher solids flux leads to the higher solids holdup, which needs the larger pressure gradient to suspend the particle. The length of the particle acceleration section is about 1.5 m for silica gel B and about 2 m for glass beads, and does not vary much with the increase of solids flux. Many researchers have studied the axial pressure variation at low-density operation [9,21,22], and their results showed that the pressure gradient was negative in the top region of the downer due to the acceleration of particles, which leads to a large pressure loss. According to the pressure gradient profiles in the axial direction, the downer can be divided into three regions: the first particle acceleration region, the second particle acceleration region, and the fully developed region. However, the pressure gradient is positive in the overall downer in this work because a pre-accelerated pipe is employed in this CFB system, in which particles has been accelerated before feeding into the downer, and particle velocity at the entrance
Fig. 10. Axial pressure gradient for glass beads.
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of the downer is often equal to, even higher than, the superficial gas velocity, which results in the positive pressure gradient in the overall downer, and the disappearance of the first acceleration region. Fig. 11 shows the axial distribution of solids concentration at G s=26 kg m2 s1 and G s=258 kg m2 s1. It can be seen that the axial solids distribution is quite uniform in the whole high-density downer, which was different from that observed by former researchers in low-density downer. The previous work showed that the cross-section averaged solids concentration in the entrance region was higher than that in the developed region [10,11]. This difference should be contributed to the pre-acceleration section employed in this CFB system. The integrated solids concentration obtained from the optical fiber probe does not vary much from the top to the bottom of the downer. The apparent solids concentration calculated from the pressure gradient [(dp/dh)/q pg] for G s=258 kg m2 s1 decreases significantly in the entrance region of the downer due to the influence of particles acceleration. Moreover, the apparent solids concentration calculated from the pressure gradient [(dp/dh)/q pg] obtained by Cao et al. [22] showed negative values in the entrance region of the low-density downer. The above results show that the solids concentration calculated from the pressure gradient [(dp/dh)/q pg] is not accurate, even is wrong at all in the particle acceleration region of the downer. The integrated solids concentration is approximate to the apparent solids concentration calculated from the pressure gradient [(dp/dh)/q pg] for G s=26 kg m2 s1, but larger than the apparent solids concentration for G s=258 kg m2 s1. Comparison between the high-density operation and the low-density operation, the difference between the integrated solids concentration and the apparent solids concentration at high-density operation is much greater than that at low-density operation. The reason may be that, one is the pressure loss due to wall friction increases with the increase of solids flux, and the other is that the error due
Fig. 11. Axial distribution of solids concentration for silica gel B.
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Fig. 12. Radial relative solids concentration profiles for silica gel B at H=0.2 m.
Fig. 14. Radial relative solids concentration profiles for silica gel B at H=5.3 m.
to optical fiber measuring system itself is enlarged when solids flux increases. The solids concentration obtained by both methods increases near the exit of the downer under high-density operation condition ( G s=258 kg m2 s1), which is caused by the restriction of the conical structure at the exit of the downer.
Among the many researchers who have studied the radial distribution in the downer [8,9,20], almost all investigators agreed that the radial distribution was lower and uniform in the center region and the solids concentration was higher near the wall. In this work, the core/annulus flow pattern is also clear in both particle acceleration region (H=0.2 m) and developed region (H=2.3 m), as shown in Figs. 12 and 13. In order to reduce disturbance of solids concentration, the relative solids concentration (e*=e s/e av) is employed in this work. In Fig. 12 for the particle acceleration region, the
solids concentration distribution shows a high peak value in the annular region, which is as four times as that of the averaged solids concentration in the central region. In the Fig. 13 for the developed region, the curves become more flat and the ratio of the peak value in the annular region to the averaged value of solids concentration in the central region decreases to 1.8, which means the solids concentration distribution leads to uniformity. Fig. 14 shows that the radial distributions of solids concentration are more uniform in the exit region, which is different from that of the other two regions showing in Figs. 12 and 13. The maximum value moves toward the center and the ratio of the maximum value in the annular region to the averaged value of solids concentration in the central region is 1.1 for the high-density operation ( G s=258 kg m2 s1, U g=1.54 m/s), which implies that the core/annulus flow pattern in the exit region is not as clear as that in the other regions. The similar results given by Lehner et al. [8] showed that this ratio decreased significantly with downward distance in a
Fig. 13. Radial relative solids concentration profiles for silica gel B at H=2.3 m.
Fig. 15. Relation between standard deviation and solids flux for silica gel B.
3.4. Radial solids concentration distribution
H. Chen, H. Li / Powder Technology 146 (2004) 84–92
low-density downer, in which the ratio was about 6 at H=1.3 m, and became 2 at H=6.1 m. From these three figures, it can be seen that the trend of radial solids distribution does not vary much with the change of solids flux in the high-density downer. Results obtained by Lehner et al. [8] and Zhang et al. [11] showed that the shape of solids distribution may be varied with the change of the superficial gas velocity, but does not be varied significantly with the increase of solids flux in low-density downer. From the above results, it can conclude that the superficial gas velocity has an important influence on the trend of solids distribution, while the solids flux has a little influence. From Figs. 12–14, it can been seen that the radial distribution of solids concentration become more uniform with increasing the solids flux at the same axial position, which can be indicated clearly with their standard deviation of the local averaged solids fraction data as plotted in Figs. 12–14. The relation between the standard deviation of the local averaged solids concentration (e s) and the solids flux is shown in Fig. 15. The standard deviation varies random in the particle acceleration region (H=0.2 m) and decreases linearly in both the developed region (H=2.3 m) and the particle deceleration region (H=5.3 m) with the increasing of solids flux. The standard deviation decreases gradually from top to bottom, which means that the radial solids distribution become more and more uniform when the gas–solid move downward. Comparing the standard deviation of relative solids concentration in the annular region with that in the central region, Fig. 16 shows that the standard deviation in the annular region is much higher than that in the central region, the former varying from 0.225 to 0.15, while the latter, only from 0.046 to 0.025, with the increase of G s from 56 to 258 kg m2s. It implies that the distribution of solids concentration in the central region is much more uniform than that in the annular region.
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4. Conclusions A special designed circulating fluidized bed system has been constructed to study the hydrodynamics of gas–solid downflow operating at high solids concentration and high solids flux. The characters of the transient solids fraction, the radial solids concentration distribution and axial profiles of pressure gradient under the condition of high solids concentration (e avN10%) have been investigated. !
!
!
Characterization of instantaneous solids fraction in the high-density downer is different from that in the lowdensity downer. A single peak with lower solids concentration about 0.015 exists in the PDD curves, which means that the dilute phase is more dominant in the low-density downer, while a higher platform is present in the PDD curves, which means that dense phase is more dominant in the high-density downer. Solids flux is the key factor to affect the solids holdup in the developed region. The heavier and the better fluidity particles could achieve the higher solids flux, while the smaller and lighter particles are easy to achieve the higher solids holdup. The radial nonuniform core/annulus structure still exists in the high-density downer. However, the radial solids distribution gradually become uniform with downward distance, and the radial distribution is near uniform in the exit region.
Notation dp particle diameter, Am Gs solids flux, kg m2 s1 H distance from the entrance of the downer, m r radial distance from the center of the downer, mm, R downer radius, mm Ug superficial gas velocity, m s1 Ut particle terminal velocity, m s1 Up particle velocity, m s1 dh height of the measured section, m dP pressure drop, Pa dP f pressure drop for wall friction, Pa dU p difference of particle velocity, m/s w rest angle of material, 8 e av the cross-section averaged solids fraction es local averaged solids holdup et local transient solids holdup e* the relative solids holdup, (=e s/e av) qp particle density, kg m3 rs standard deviation of the relative solids holdup
Acknowledgement
Fig. 16. Comparison of the deviation between the annular region and the central region for the silica gel B at H=4.3 m.
The authors are grateful to the National Natural Science Foundation of China for financial support with contract no. 20221603 to this work.
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References [1] 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. [2] J.A. Talman, R. Geier, L. Rel, Development of a downer reactor for fluid catalytic cracking, Chem. Eng. Sci. 54 (1999) 2123 – 2130. [3] R. Deng, F. Wei, Y. Jin, Q. Zhang, Y. Jin, Experimental study of deep catalytic cracking process in a downer reactor, Ind. Eng. Chem. Res. 41 (2002) 6015 – 6019. [4] Y.J. Kim, S.D. Kim, S.H. Lee, Coal gasification characteristics in a downer reactor, Fuel 80 (13) (2001) 1915 – 1922. [5] E. Arenas, F. Chejne, J. Espinel, C. Londono, J. Aguirre, J.D. Perez, An experimental study on gasification of Colombian coal in fluidised bed, Fuel 82 (2) (2003) 161 – 164. [6] D.A. Berg, C.L. Briens, M.A. Bergougnou, Reactor development for the ultrapyrolysis reactor, Can. J. Chem. Eng. 67 (1989) 96 – 101. [7] P. Lehner, K.E. Wirth, Effects of the gas/solids distributor on the local and overall solids distribution in a downer reactor, Can. J. Chem. Eng. 77 (1999) 199 – 206. [8] P. Lehner, K.E. Wirth, Characterization of the flow pattern in a downer reactor, Chem. Eng. Sci. 54 (1999) 5471 – 5483. [9] H. Zhang, J.X. Zhu, M.A. Bergougnou, Hydrodynamics in downflow fluidized (1): solids concentration profiles and pressure gradient distribution, Chem. Eng. Sci. 54 (1999) 5461 – 5470. [10] Y. Bolkan, F. Berruti, J. Zhu, B. Milne, Modeling circulating fluidized bed downer, Powder Technol. 132 (2003) 85 – 100. [11] H. Zhang, W.X. Huang, J.X. Zhu, Gas–solids flow behavior: CFB riser vs. Downer, AIChE J. 47 (9) (2001) 2000 – 2011.
[12] Y. Ma, J.X. Zhu, Experimental study of heat transfer in a co-current downflow fluidized bed (downer), Chem. Eng. Sci. 54 (1999) 41 – 50. [13] Y.J. Kim, J.H. Bang, S.D. Kim, Bed-to-wall heat transfer in a downer reactor, Can. J. Chem. Eng. 77 (2) (1999) 207 – 212. [14] Y.G. Bolkan-Kenny, T.S. Pugsley, F. Berruti, Computer simulation of the performance of fluid catalytic cracking risers and downers, Ind. Eng. Chem. Res. 33 (12) (1994) 3043 – 3052. [15] H.Z. Chen, H.Z. Li, Research review and prospects of high-density circulating fluidized beds, Chin. J. Process Eng. 2 (2) (2002) 186 – 192 (in Chinese). [16] W. Liu, K.B. Luo, J.X. Zhu, Characterization of high-density gas– solids downward fluidized flow, Powder Technol. 115 (2001) 27 – 35. [17] H.T. Bi, J.X. Zhu, Static instability analysis of circulating fluidized beds and concept of high-density risers, AIChE J. 39 (1993) 1272 – 1280. [18] J.R. Grace, A.S. Issangya, D. Bai, Situating the high-density circulating fluidized bed, AIChE J. 45 (10) (1999) 2108 – 2116. [19] K. Tuzla, A.K. Sharma, J.C. Chen, Transient dynamics of solid concentration in downer fluidized bed, Powder Technol. 100 (1998) 166 – 172. [20] M. Zhang, Z. Qian, H. Yu, F. Wei, The solids flow structure in a circulating fluidized bed riser/downer of 0.42-m diameter, Powder Technol. 129 (2003) 46 – 52. [21] P.M. Herbert, T.A. Gauthier, C.L. Briens, M.A. Bergougnou, Flow study of a 0.05m diameter downflow circulating fluidized bed, Powder Technol. 96 (1998) 255 – 261. [22] C.S. Cao, H. Weinstin, Characterization of downflow high velocity fluidized beds, AIChE J. 46 (3) (2000) 515 – 522.
Characterization of a high-density downer reactor Hengzhi Chen*, Hongzhong Li1 Multiphase Reaction Laboratory, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100080, China Received 22 December 2003; received in revised form 20 July 2004; accepted 26 July 2004 Available online 21 September 2004
Abstract Experiments were carried out in a specially designed circulating fluidized bed with a 5.6-m tall, 0.08 m ID downer, in which the maximal the solids flux 600 kg m2 s1 and the maximal solids holdup 0.14 have been achieved. The results show that single peak exists in the PDD curves of the solids concentration for low-density operation, and a higher platform is present in the PDD curves for the high-density downer. Solids flux is a key factor that affects the solids holdup while particle properties are other important parameter. The heavier/better fluidity particle could achieve the higher solids flux and the smaller/lighter particle were ease to achieve the higher solids holdup. The radial nonuniform core/annulus structure still exists in the high-density downer, but radial solids distribution gradually becomes more uniform with downward distance and with the increase of solids flux. D 2004 Elsevier B.V. All rights reserved. Keywords: High-density downer; Solids flux; Transient solids concentration; Solids concentration distribution
1. Introduction Circulating fluidized beds (CFB) have been widely used in many fields such as chemical industry, energy, materials, which include the gas–solid cocurrent upflow system (riser) and the gas–solid cocurrent downflow system (downer). Due to such advantages as shorter residence time, more uniform distribution of particles, and less gas and solids backmixing, the downer has drawn much attention in the past two decades [1]. A series of hydrodynamic experiments have been carried out by several research groups [7–12]. Many industrial processes, such as fluid catalytic cracking [2,3], coal pyrolysis [4,5],biomass pyrolysis [6], may be benefited from this new type of reactor. However, shortcomings of the downer have been pointed out by the investigators [11]. One of the key weaknesses is that the solids holdup is so low that results in * Corresponding author. Tel.: +86 10 6255 8002; fax: +86 10 6256 1822. E-mail addresses: [email protected] (H. Chen)8 [email protected] (H. Li). 1 Fax: 86 10 62536108. 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.07.008
the lower heat transfer coefficient in the downer [12] and the lower bed-to-wall heat transfer coefficient [13]. The results of computer simulation showed that the gas oil conversion approximately increase 4% (wt) when the catalyst/oil ratio increased from 8 to 10 in the fluid catalysis and cracking process [14]. Though a large number of the papers about the low-density downer have been published, there is only one article in which hydrodynamics about high-density downflow were reported [15,16]. There is, therefore, an urgent need to study the characters of the downer operating at high solids holdup/high solids flux for understanding the flow mechanics and speeding up its industrial application. The high density circulating fluidized beds (HDCFB) has been proposed by Zhu and Bi [17] , and Grace et al. [18] has defined the HDCFB riser as operations with G sN200 kg m2 s1 and e sN0.1 throughout the entire riser. In high-density downer, the start point of the constant mean particle velocity has been suggested to demarcate low-density operation and high-density operation by the former researchers [16]. The solids concentration is an important parameter to affect the efficiency of the downer. We prefer the operation with e avN5% in the
H. Chen, H. Li / Powder Technology 146 (2004) 84–92
developed region as the criterion of HDCFB downer in this work.
2. Experimental equipment and measuring system Fig. 1 shows the circulating fluidized-bed system, consisting, on the left-hand side, of a downer (5.6 m high by 80 mm ID) followed by a bottom separator and a moving bed seal (3 m high by 120 mm ID) which feeds solid particles into a riser (12 m high by 120 mm ID), located on the righthand side. Air was used as the fluidizing gas and four kinds
85
Table 1 Properties of the particle Glass beads FCC Silica gel A Silica gel B
d p (Am)
q p (kg m3)
w (8)
U t (m s1)
131 82 572 128
2480 992 750 750
23 34 25 26
0.76 0.2 1.73 0.31
of particles were used in this work, of which properties are shown in Table 1. The air exiting the riser at the top first goes to an inertia separator to remove the major portion of the particles, and then to a cyclone, the remainder portion.
Fig. 1. Schematic diagram of the setup.
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H. Chen, H. Li / Powder Technology 146 (2004) 84–92
Fig. 2. Block diagram for optical fiber measurement.
To increase the solids density in the downer three measures were taken as follows: !
! !
High solids flux at the entrance of the downer was achieved by throttling the air leaving the inertia separator to force a portion of the air flow downward and transport the solids via a pre-acceleration pipe into the downer. A relatively low gas velocity was used in the downer. A riser with larger diameter (120 mm ID) was adopted in order to increase the transportation ability of the riser.
0.0025 to 0.07 and the cross-section averaged solids holdup is about 0.017. The instantaneous solids concentration is very low in the most cases at six positions, but the higher solids concentration (e tN0.05) still exists, which was recognized as the trance of the cluster in the downer by the former investigator [19]. Under the high-density operation condition ( G s=258 kg m2 s1), the transient solids concentration varies from 0.005 to 0.26 and the crosssection averaged solids concentration is about 0.135. The ratio of the maximum value of the instantaneous solids concentration to the time-averaged concentration is about 4 at low-density operation, while it is about 2 at high-density operation. The previous research showed that the averaged solids concentration of clusters is about three to six times that of overall time-averaged solids concentration [19]. In this work, so outstanding value of instantaneous solids fraction exists in low-density operation, but it is not present in high-density operation. The previous studies on probability density distribution (PDD) of the local transient solids concentration in downers showed that one peak existed in the PDD curves and the peak position varied a little with the change of the radial position, and there existed particle aggregates near the wall due to the high solids concentration ring [19,20]. The similar result has been observed in this work under lowdensity operating condition as shown in Fig. 5. From Fig. 5,
An optical fiber probe was used to sample the transient signals of local solids fraction, e t ,in the bed, as shown in Fig. 2. Two beams of light, a reference light and a measuring light, were used to reduce environmental disturbances. The frontal tip area of the probe was only 4 mm2, small enough to minimize its effect on flow. The cross-section averaged solids fraction e av was obtained from measuring the pressure gradient along the bed height as follows: eav ¼
dp=dh qp g
ð1Þ
3. Results and discussion 3.1. The transient signals of local solids fraction The transient signals of the local solids concentration contain a lot of useful information and analyzing these signals is an important method to study the phase structure in the gas–solid fluidized bed. Figs. 3 and 4 show the transient signals of local solids concentration at six radial positions (r/R=0, 0.4, 0.6, 0.7, 0.8, 0.95) at the level of 2.3 m distance from the entrance of the downer for the solids flux 26 and 258 kg m2 s1, respectively. Under the low-density operation condition ( G s=26 kg m2 s1), the instantaneous solids concentration varies from
Fig. 3. Instantaneous solids concentration at low-density operation. Silica gel B, H=2.3m, Gs=26kg.m2.s1, Ug=0.2m.s1.
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3.2. Solids flux and the cross-section solids concentration The solids concentration is an important parameter which is affected by both gas velocity and solids flux. In the developed region, the cross-section averaged solids concentration can be calculated by the Eq. (1), which is also called the mean solids holdup by Lehner et al. [8]. The deviation of the evaluated solids holdup by pressure gradient from the actual solids holdup in the developed region of the downer is about 15% [16]. Figs. 7 and 8 show the relations between the mean solids concentration of silica gel, FCC particles, and glass beads, and the solids flux in the developed region (Dh=2.3–3.3 m). From the two figures, it can be seen that solids holdup markedly increases with the increase of solids flux for all four kinds of particles, which means that solids flux is the key factor affecting solids concentration in the downer. In other words, only when the higher solids flux is achieved, the higher solid holdup may be present. With the addition of the configuration of the overall circulating fluidized beds system, particle properties can affect the solids flux. First, particle density is an important parameter and the solids flux of glass beads can achieve the largest value of 600 kg m2 s1 due to its highest density. Second, particle fluidity is another parameter affecting solids flux. From Table 1, it can
Fig. 4. Instantaneous solids concentration at high-density operation. Silica gel B, H=2.3m, Gs=258kg.m2.s1, Ug=1.54m.s1.
it can been seen that the solids concentration at the peak position of PDD curves is about 0.015 and the fraction of the higher instantaneous solids concentration (e tN0.05) is less than 10%, which implies that gas phase is more dominant in the downer under low-density operation conditions. However, the trend of PDD curves under high-density operating conditions, as shown in Fig. 6, is different from both the previous studies [19,20] and this work under lowdensity operating conditions. From Fig. 6, it can be seen that no peak exits in the PDD curves and a higher platform is present in the PDD curves at high-density operation. The probability of the solids concentration approximates with variation of solids fraction from 0.05 to 0.2, which means that, from a statistical perspective, the solids concentration varies continuously in the range from 0.05 to 0.2. The instantaneous solids concentration at the radial position of r/ R=0.95 are all over 0.05, and even at the center (r/R=0), the solids fraction over 0.05 takes up the major time during the test time. These results show that particle phase is more dominant in the downer at high-density operation. The hydrodynamics of gas–solid flow in the high-density downer are some different from those in the low-density downer. So, the more work, especially with high-density operation, needs to be carried out to understand the phase structure in the downer.
Fig. 5. Probability density distribution of local instantaneous solids concentration at low-density operation. Silica gel B, H=2.3m, Gs=26kg.m2.s1, Ug=0.2m.s1.
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Fig. 8. Relation between solids flux and solids flux.
Fig. 6. Probability density distribution of local instantaneous solids concentration at high-density operation. Silica gel B, H=2.3m, Gs=258kg.m2.s1, Ug=1.54m.s1.
be seen that silica gel (A: w=258, B: w=268) and glass bead (w=238) have better fluidity than that of FCC particles (w=348). As a result, the maximum solids flux of FCC particles becomes the smallest one among the four kinds of particles, though the density of FCC particles is larger than that of silica gel. Third, particle size is the other parameter to affect solids flux, and the larger particles can achieve the
higher solids flux because the larger particles have the higher terminal velocity when particle density is equivalent. For instance, the solids flux of silica gel A (d p=572 Am) can achieve as high as 350 kg/m2 s, while that of silica gel B (d p=128 Am) is 258 kg m2 s1 only, as shown in Fig. 7. Particle properties have an important influence on solids concentration. The following relation, e s=G s/(q pU p), shows that particle density is one of important factor to affect solids concentration and the lighter particle can achieve the higher solids concentration when solids flux is fixed. The solids concentration of silica gel B, FCC and glass beads is about 13%, 10%, and 5%, respectively, when the solids flux is 240 kg m2 s1 as shown in Figs. 7 and 8. Particle size has an important influence on solids holdup, and the larger particle leads to the lower solids holdup due to the higher particle velocity in the developed region under the similar operation conditions. Comparing the results obtained in this work with that obtained by Liu et al. [16], the maximal solids flux in this work is much lower than that from the latter due to the different configuration of their downers. The larger diameter of downer is used in this work, which means that the more material need be provided to achieve the same solids flux. The cross-section averaged solids concentration in this work is slight greater than that obtained by Liu et al. [16] under the similar operation conditions, which attributes to the restriction of the conical structure at the exit of the downer in this work. 3.3. Axial profiles of pressure gradient and solids concentration Usually, the pressure gradient of the downer can be expressed by following equation:
Fig. 7. Relation between solids flux and solids flux.
dUp dp dPf ¼ qp ges Gs ð2Þ dh dh dh Figs. 9 and 10 show the axial profiles of the pressure gradient for silica gel B and glass beads with different solids
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Fig. 9. Axial pressure gradient for silica gel B.
fluxes. From the two figures, it can be seen that the pressure gradient increases with increasing solids flux because the higher solids flux leads to the higher solids holdup, which needs the larger pressure gradient to suspend the particle. The length of the particle acceleration section is about 1.5 m for silica gel B and about 2 m for glass beads, and does not vary much with the increase of solids flux. Many researchers have studied the axial pressure variation at low-density operation [9,21,22], and their results showed that the pressure gradient was negative in the top region of the downer due to the acceleration of particles, which leads to a large pressure loss. According to the pressure gradient profiles in the axial direction, the downer can be divided into three regions: the first particle acceleration region, the second particle acceleration region, and the fully developed region. However, the pressure gradient is positive in the overall downer in this work because a pre-accelerated pipe is employed in this CFB system, in which particles has been accelerated before feeding into the downer, and particle velocity at the entrance
Fig. 10. Axial pressure gradient for glass beads.
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of the downer is often equal to, even higher than, the superficial gas velocity, which results in the positive pressure gradient in the overall downer, and the disappearance of the first acceleration region. Fig. 11 shows the axial distribution of solids concentration at G s=26 kg m2 s1 and G s=258 kg m2 s1. It can be seen that the axial solids distribution is quite uniform in the whole high-density downer, which was different from that observed by former researchers in low-density downer. The previous work showed that the cross-section averaged solids concentration in the entrance region was higher than that in the developed region [10,11]. This difference should be contributed to the pre-acceleration section employed in this CFB system. The integrated solids concentration obtained from the optical fiber probe does not vary much from the top to the bottom of the downer. The apparent solids concentration calculated from the pressure gradient [(dp/dh)/q pg] for G s=258 kg m2 s1 decreases significantly in the entrance region of the downer due to the influence of particles acceleration. Moreover, the apparent solids concentration calculated from the pressure gradient [(dp/dh)/q pg] obtained by Cao et al. [22] showed negative values in the entrance region of the low-density downer. The above results show that the solids concentration calculated from the pressure gradient [(dp/dh)/q pg] is not accurate, even is wrong at all in the particle acceleration region of the downer. The integrated solids concentration is approximate to the apparent solids concentration calculated from the pressure gradient [(dp/dh)/q pg] for G s=26 kg m2 s1, but larger than the apparent solids concentration for G s=258 kg m2 s1. Comparison between the high-density operation and the low-density operation, the difference between the integrated solids concentration and the apparent solids concentration at high-density operation is much greater than that at low-density operation. The reason may be that, one is the pressure loss due to wall friction increases with the increase of solids flux, and the other is that the error due
Fig. 11. Axial distribution of solids concentration for silica gel B.
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Fig. 12. Radial relative solids concentration profiles for silica gel B at H=0.2 m.
Fig. 14. Radial relative solids concentration profiles for silica gel B at H=5.3 m.
to optical fiber measuring system itself is enlarged when solids flux increases. The solids concentration obtained by both methods increases near the exit of the downer under high-density operation condition ( G s=258 kg m2 s1), which is caused by the restriction of the conical structure at the exit of the downer.
Among the many researchers who have studied the radial distribution in the downer [8,9,20], almost all investigators agreed that the radial distribution was lower and uniform in the center region and the solids concentration was higher near the wall. In this work, the core/annulus flow pattern is also clear in both particle acceleration region (H=0.2 m) and developed region (H=2.3 m), as shown in Figs. 12 and 13. In order to reduce disturbance of solids concentration, the relative solids concentration (e*=e s/e av) is employed in this work. In Fig. 12 for the particle acceleration region, the
solids concentration distribution shows a high peak value in the annular region, which is as four times as that of the averaged solids concentration in the central region. In the Fig. 13 for the developed region, the curves become more flat and the ratio of the peak value in the annular region to the averaged value of solids concentration in the central region decreases to 1.8, which means the solids concentration distribution leads to uniformity. Fig. 14 shows that the radial distributions of solids concentration are more uniform in the exit region, which is different from that of the other two regions showing in Figs. 12 and 13. The maximum value moves toward the center and the ratio of the maximum value in the annular region to the averaged value of solids concentration in the central region is 1.1 for the high-density operation ( G s=258 kg m2 s1, U g=1.54 m/s), which implies that the core/annulus flow pattern in the exit region is not as clear as that in the other regions. The similar results given by Lehner et al. [8] showed that this ratio decreased significantly with downward distance in a
Fig. 13. Radial relative solids concentration profiles for silica gel B at H=2.3 m.
Fig. 15. Relation between standard deviation and solids flux for silica gel B.
3.4. Radial solids concentration distribution
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low-density downer, in which the ratio was about 6 at H=1.3 m, and became 2 at H=6.1 m. From these three figures, it can be seen that the trend of radial solids distribution does not vary much with the change of solids flux in the high-density downer. Results obtained by Lehner et al. [8] and Zhang et al. [11] showed that the shape of solids distribution may be varied with the change of the superficial gas velocity, but does not be varied significantly with the increase of solids flux in low-density downer. From the above results, it can conclude that the superficial gas velocity has an important influence on the trend of solids distribution, while the solids flux has a little influence. From Figs. 12–14, it can been seen that the radial distribution of solids concentration become more uniform with increasing the solids flux at the same axial position, which can be indicated clearly with their standard deviation of the local averaged solids fraction data as plotted in Figs. 12–14. The relation between the standard deviation of the local averaged solids concentration (e s) and the solids flux is shown in Fig. 15. The standard deviation varies random in the particle acceleration region (H=0.2 m) and decreases linearly in both the developed region (H=2.3 m) and the particle deceleration region (H=5.3 m) with the increasing of solids flux. The standard deviation decreases gradually from top to bottom, which means that the radial solids distribution become more and more uniform when the gas–solid move downward. Comparing the standard deviation of relative solids concentration in the annular region with that in the central region, Fig. 16 shows that the standard deviation in the annular region is much higher than that in the central region, the former varying from 0.225 to 0.15, while the latter, only from 0.046 to 0.025, with the increase of G s from 56 to 258 kg m2s. It implies that the distribution of solids concentration in the central region is much more uniform than that in the annular region.
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4. Conclusions A special designed circulating fluidized bed system has been constructed to study the hydrodynamics of gas–solid downflow operating at high solids concentration and high solids flux. The characters of the transient solids fraction, the radial solids concentration distribution and axial profiles of pressure gradient under the condition of high solids concentration (e avN10%) have been investigated. !
!
!
Characterization of instantaneous solids fraction in the high-density downer is different from that in the lowdensity downer. A single peak with lower solids concentration about 0.015 exists in the PDD curves, which means that the dilute phase is more dominant in the low-density downer, while a higher platform is present in the PDD curves, which means that dense phase is more dominant in the high-density downer. Solids flux is the key factor to affect the solids holdup in the developed region. The heavier and the better fluidity particles could achieve the higher solids flux, while the smaller and lighter particles are easy to achieve the higher solids holdup. The radial nonuniform core/annulus structure still exists in the high-density downer. However, the radial solids distribution gradually become uniform with downward distance, and the radial distribution is near uniform in the exit region.
Notation dp particle diameter, Am Gs solids flux, kg m2 s1 H distance from the entrance of the downer, m r radial distance from the center of the downer, mm, R downer radius, mm Ug superficial gas velocity, m s1 Ut particle terminal velocity, m s1 Up particle velocity, m s1 dh height of the measured section, m dP pressure drop, Pa dP f pressure drop for wall friction, Pa dU p difference of particle velocity, m/s w rest angle of material, 8 e av the cross-section averaged solids fraction es local averaged solids holdup et local transient solids holdup e* the relative solids holdup, (=e s/e av) qp particle density, kg m3 rs standard deviation of the relative solids holdup
Acknowledgement
Fig. 16. Comparison of the deviation between the annular region and the central region for the silica gel B at H=4.3 m.
The authors are grateful to the National Natural Science Foundation of China for financial support with contract no. 20221603 to this work.
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References [1] 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. [2] J.A. Talman, R. Geier, L. Rel, Development of a downer reactor for fluid catalytic cracking, Chem. Eng. Sci. 54 (1999) 2123 – 2130. [3] R. Deng, F. Wei, Y. Jin, Q. Zhang, Y. Jin, Experimental study of deep catalytic cracking process in a downer reactor, Ind. Eng. Chem. Res. 41 (2002) 6015 – 6019. [4] Y.J. Kim, S.D. Kim, S.H. Lee, Coal gasification characteristics in a downer reactor, Fuel 80 (13) (2001) 1915 – 1922. [5] E. Arenas, F. Chejne, J. Espinel, C. Londono, J. Aguirre, J.D. Perez, An experimental study on gasification of Colombian coal in fluidised bed, Fuel 82 (2) (2003) 161 – 164. [6] D.A. Berg, C.L. Briens, M.A. Bergougnou, Reactor development for the ultrapyrolysis reactor, Can. J. Chem. Eng. 67 (1989) 96 – 101. [7] P. Lehner, K.E. Wirth, Effects of the gas/solids distributor on the local and overall solids distribution in a downer reactor, Can. J. Chem. Eng. 77 (1999) 199 – 206. [8] P. Lehner, K.E. Wirth, Characterization of the flow pattern in a downer reactor, Chem. Eng. Sci. 54 (1999) 5471 – 5483. [9] H. Zhang, J.X. Zhu, M.A. Bergougnou, Hydrodynamics in downflow fluidized (1): solids concentration profiles and pressure gradient distribution, Chem. Eng. Sci. 54 (1999) 5461 – 5470. [10] Y. Bolkan, F. Berruti, J. Zhu, B. Milne, Modeling circulating fluidized bed downer, Powder Technol. 132 (2003) 85 – 100. [11] H. Zhang, W.X. Huang, J.X. Zhu, Gas–solids flow behavior: CFB riser vs. Downer, AIChE J. 47 (9) (2001) 2000 – 2011.
[12] Y. Ma, J.X. Zhu, Experimental study of heat transfer in a co-current downflow fluidized bed (downer), Chem. Eng. Sci. 54 (1999) 41 – 50. [13] Y.J. Kim, J.H. Bang, S.D. Kim, Bed-to-wall heat transfer in a downer reactor, Can. J. Chem. Eng. 77 (2) (1999) 207 – 212. [14] Y.G. Bolkan-Kenny, T.S. Pugsley, F. Berruti, Computer simulation of the performance of fluid catalytic cracking risers and downers, Ind. Eng. Chem. Res. 33 (12) (1994) 3043 – 3052. [15] H.Z. Chen, H.Z. Li, Research review and prospects of high-density circulating fluidized beds, Chin. J. Process Eng. 2 (2) (2002) 186 – 192 (in Chinese). [16] W. Liu, K.B. Luo, J.X. Zhu, Characterization of high-density gas– solids downward fluidized flow, Powder Technol. 115 (2001) 27 – 35. [17] H.T. Bi, J.X. Zhu, Static instability analysis of circulating fluidized beds and concept of high-density risers, AIChE J. 39 (1993) 1272 – 1280. [18] J.R. Grace, A.S. Issangya, D. Bai, Situating the high-density circulating fluidized bed, AIChE J. 45 (10) (1999) 2108 – 2116. [19] K. Tuzla, A.K. Sharma, J.C. Chen, Transient dynamics of solid concentration in downer fluidized bed, Powder Technol. 100 (1998) 166 – 172. [20] M. Zhang, Z. Qian, H. Yu, F. Wei, The solids flow structure in a circulating fluidized bed riser/downer of 0.42-m diameter, Powder Technol. 129 (2003) 46 – 52. [21] P.M. Herbert, T.A. Gauthier, C.L. Briens, M.A. Bergougnou, Flow study of a 0.05m diameter downflow circulating fluidized bed, Powder Technol. 96 (1998) 255 – 261. [22] C.S. Cao, H. Weinstin, Characterization of downflow high velocity fluidized beds, AIChE J. 46 (3) (2000) 515 – 522.