Ind. Eng. Chem. Res. 1993,32,1041-1045
1041
Consistency of Circulating Fluidized Bed Experimental Data S. Ouyang and 0. E. Potter* Department of Chemical Engineering, Monash University, Clayton, Victoria 3168, Australia
CFB experimental data from the literature are compared in terms of the ideal no-slip volume fraction of solids, ds. The lean phase is evaluated as c*Jds. It is found that E * , / E ’ ~ = 2.6 with a SD of 0.9. The average value of €d,s in the dense phase is 0.18, and the standard deviation is 0.05. Also evaluated are Z*d, the fraction of the riser occupied by the dense phase, and Zt, the height of the transition zone between the dense and lean phases. It is found that Zt = 2.25 m with a SD of 1.3 m. With the pressure differential available between the bottom and top of the riser for the given flow conditions (Uand G,), the average solids holdup as well as fractions of the riser occupied by the dense and dilute phases could be estimated. This compilation of data has been made to guide the authors in modeling but may be of interest to others, also. A notable omission from the available data is the pressure drop across the riser which determines the relative proportions of dense phase and lean phase (in the vertical dimension).
Introduction The concept of the circulating fluidized bed (CFB) is not a new one. The commercial interest in CFB technology can be traced back to the 1940s (Squires, 1986). CFB technology has found an increasing number of practical applications in recent years. During the 1980s research has led to significant progress in the understanding of circulating fluidized beds. Investigations of solidsdistribution and flow within CFB risers have demonstrated a nonuniform flow structure, both axially and radially. In general, the CFB can be divided in two zones, a dense zone at the bottom and a dilute zone at the top of the riser. The riser cross section may also be divided into a core region where solids rise in dilute suspension and a annular wall region with a dense phase flowing downward. Some of the early studies in CFB (Yerushalmi et al., 1976; Li and Kwauk, 1980) suggested that the solids holdup is only a function of the superficial gas velocity and the solids circulation rate. Weinstein et al. (1984)and Li et al. (1988)demonstrated that the solids holdup could be different under the same gas velocity and solids rate but for a different solid inventory. Schnitzlein and Weinstein (1988)and Brereton et al. (1988)further showed that the solids holdup is also strongly influenced in a complex fashion by system design. This has quite dramatic implications for the comparison of studies from different workers. Before a comprehensive model based on the accumulation of experimental knowledge on the flow structure of CFB, which can eventually be applied for reactor modeling, is developed it is worthwhile to examine the consistency of CFB experimental data.
Comments and Discussions
No model is currently capable of predicting solids holdup in the riser at a selected solids circulation rate, G,, and gas velocity, U. Such a physical model is of vital important for CFB reactor modeling. For gas-particle flow in a vertical pipe, there are two ideal characteristic cases: (1)First, the solids and gas move uniformly with no slip between gas and solids; the ideal solids holdup (which is also the minimum possible holdup) in the riser is then
(2) Second, for the ideal case of uniform flow with slip velocity equal to the individual particle terminal velocity, the solids holdup is = 1-
(AK+ A + 1)*
2AK
where A = Up$Gs and K = UTIU. For most conditions, the solids holdups for the above two ideal cases, dS and e”,, are expected to differ only slightly,because the particle terminal velocity, UT, is very small with respect to the superficialgas velocity. However, as shown in Table I and 11, the solids holdup in a CFB riser could be many times higher than that for the ideal cases. For gas-solid cocurrent up-flow in a vertical tube, Grace and Tuot (1979)concluded that a uniform dispersion of particles in a gas is unstable and can lead to formation of “clusters” or “packets” of particles in a continuum of gas. Similar suggestions and observations are made by others (Li and Kwauk, 1980;Yerushalmi et al., 1976;Horio
Table I. Summary of Experimental Results on Solids Holdup ~~
investigators Ouyang et al. (1993)
apparatus 0.254 m i.d. Z-10m AP= 23kPa APll8kPa
AP=16kPa
AP=16kPa
solids property FCC d, = 65 p p = 1380
U (mls)
G, (kg/(m28 ) )
2.08 2.08 2.27 3.88 3.83 3.88 3.95 5.71 5.73 5.71 5.68 7.92 7.66 7.47
10.15 24 54.1 15.2 44.6 75 117.9 17.5 60 157.1 183.3 18.6 103.1 206.3
€’e
0.008 0.017 0.1177 0.0058 0.0138 0.0318 0.0654 0.0067 0.0248 0.0702 0.0789 0.0055 0.0376 0.087
0.0035 0.0083 0.017 0.0028 0.0084 0.0138 0.0212 0.0022 0.0075 0.0196 0.0229 0.0017 0.0097 0.0196
d d e 2.28 2.05 6.93 2.03 1.67 2.3 3.09 3.0 3.29 3.59 3.45 3.25 3.89 4.43
€*d€’e 2.28 2.05 2.25 2.03 1.67 2.3 2.25 2.88 2.77 1.96 1.99 2.51 2.26 2.48
0888-5885/93/2632-1041$04.00/00 1993 American Chemical Society
Sda/€’s
z*d
~
zt (m)
13.5
0.4
0.7
7.2
0.15
0.5
11.2 11.1
0.13 0.13
1.2 1.2
14.4 13.2
0.13 0.17
1.0 0.8
1042 Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993 Table 11. Some Experimental Data Reported in the Literature. investigators Li and Kwauk (1980)
Li et al. (1988)
Yerushalmi et al. (1976)
solids apparatus property 0.09 m i.d. iron ore 2=8m d,=105 p, = 4510 alumina d, = 81 p, = 3090 FCC pp = 1780 Pyr Cin d, = 56 p, = 3050 0.09 m i.d. FCC Z = 10m p,=930 I = 30 kg I = 35 kg I=40kg I = 15 kg I = 20 kg I = 22 kg I = 25 kg I = 35 kg I = 40 kg I = 25 kg I = 30 kg I = 35 kg I = 40 kg 0.076 m i.d. FCC Z = 7.2m d, = 6 0 pp = 881
Yerushalmi and Avidan (1985) 0.152 m i.d. FCC Z=8.5m d,=49 p, = 1070 HFZ-20 d, = 49 pp = 1450 Rhodes (1986)
0.152 m i.d. 9G 2=6m d,=64 p, = 1800
U (m/s) G. (kg/(m28 ) )
(1988) Louge and Chang (1990)
73 73 73 16 16 129 129
0.1112 0.0435 0.0375 0.1194 0.0129 0.0983 0.0209
0.0106 10.5 0.0059 7.4 0.0042 8.9 0.0111 10.75 0.0045 2.89 0.0274 3.58 0.0139 1.5
1.5
14.3 14.3 15.4 26.6 48.2 24.1 24.1 24.1 32.0 64.2 96.3 42.8 64.2 96.3 192.7 19.5 50 50 120 212 63 112 120 113 137 153 173 8.5 27 41 13.5 67 80 24 65 107 70 90 100 160 16 26 49 72 75
2.9 3.6 2.0
64 65 118 7 49 40 40
0.067 0.095 0.11 0.174 0.190 0.044 0.0608 0.080 0.1107 0.1575 0.188 0.0975 0.1313 0.1575 0.195 0.0227 0.1364 0.0256 0.1136 0.1818 0.1786 0.1394 0.0799 0.2059 0.1786 0.1661 0.1453 0.009 0.0313 0.162 0.008 0.0672 0.1175 0.008 0.0277 0.1012 0.078 0.1819 0.026 0.0681 0.026 0.106 0.1425 0.0690 0.0310 0.1032 0.069 0.0358 0.0805 0.0669 0.0393 0.0388 0.123
0.0100 6.7 2.59 18.1 0.0100 9.5 2.59 18.1 0.0108 10.22 2.4 16.8 0.0185 9.41 9.41 0.0330 5.77 5.77 0.0122 3.60 1.844 0.0122 4.98 1.844 12.3 0.0122 6.55 1.844 12.3 0.0161 6.87 2.33 9.32 0.0318 4.95 4.95 0.0470 4.00 4.00 0.0174 5.60 3.44 0.0259 5.07 4.33 0.0383 4.11 3.92 0.0738 2.64 2.54 0.009 2.52 2.52 0.0227 6.0 3.25 6.5 0.0124 2.06 2.06 0.0293 3.88 3.1 4.84 0.0507 3.59 3.08 4.15 0.0468 3.82 0.0435 3.2 0.0215 3.71 0.0394 5.22 1.52 5.9 0.0364 4.9 1.46 5.5 0.031 5.36 1.72 6.24 0.0283 5.14 1.41 6.13 0.0019 4.24 4.24 0.006 5.23 3.69 O.Oo90 17.9 3.32 26.7 0.0021 3.8 3.8 0.0105 6.4 2.86 16.2 0.0125 9.4 2.79 17.55 0.003 2.7 2.7 0.0080 3.47 3.01 0.0130 7.76 2.61 15.32 0.0045 17.3 5.5 35.8 0.0057 31.7 6.2 45.6 0.0048 5.43 2.09 9.6 0.0076 8.92 1.85 13.9 0.0028 9.28 4.76 0.0046 23.04 4.34 57.9 0.0111 12.84 3.6 30 0.0081 8.50 1.6 18.5 0.0075 4.13 1.66 13.3 0.0086 12.0 2.58 22.39 0.0058 11.9 2.55 34.48 0.005 7.19 3.2 16 0.0092 8.77 2.2 16.3 0.0045 14.87 1.84 44.8 0.0128 3.07 1.38 7.81 0.0085 4.56 1.74 11.8 0.0152 8.1 2.8 18.4
4.3 9.1
147 147
0.1575 0.0196 0.0258 0.0093
8.1
3.7
42.6 84.4 42.1 93.9 133.8
0.0445 0.15 0.0232 0.0625 0.111
2.77 4.79 2.36 2.88 3.63
2.1
2.6
2.44 4.51 1.2 2.3 5.1 1.9 2.5 3.3 4.1 2.5
Bader et al. (1988) Bi et al. (1989)
6.0
0.15 m i.d. Z=10m
FCC
3.8
0.05 m i.d. Z = 3.3 m 0.4 m i.d. Z = 7.8 m
quartz sand p, = 2600
0.4m i.d.
FCC
0.203 m i.d. FCC 2=7m d,=72 p, = 1300 0.305 m i.d. FCC 2 = 12.2 m d, = 76 p, = 1714 0.186 m i.d. silica gel 2=8m d, = 280 pp = 706
Z*d Zt (m) 16.4 0.78 1.44 18.65 0.27 1.84
€d,d€’.
2.2 4.0 5.6 0.8 2.0 1.5 3.0
sand d, = 270 pp = 2600
dp = 56
€*$€fa
0.1087 0.0074 14.6 3.19 0.043 0.0054 7.95 3.29
4.5
Hartge et al. (1986)
id€’.
135 135
3.5
Herb et al. (1989)
€‘a
4.0 5.5
8.0
2.9 3.4 3.8 4.0 4.2 5.0 4.9 1.2
6.0
90
0.0161 0.0313 0.0098 0.0217 0.0306
2.46 2.66 2.76 2.42 2.89 1.4 1.5
1.99 2.76 2.08 2.49 2.87 2.13 2.3 2.29
11.62 15.08 17.34 11.53
0.84 0.2 0.06 0.8
0.8 2.0 6.4 1.2
4.5
0.6
2.4
0.15 0.3 0.4 1.0 1.0
2.5 4 4
0.2 0.3 0.4 1.0 1.0 7 3 2 2 0.65
1.66
0.2 0.2
5.04 5.04
0.7 0.7 0.7 0.7
2.13 2.13 2.13 2.13
0.4
2 2.0
0.2 0.3
0.8 2
0.2 0.3 0.4 0.33 0.28
1.2 3.6 2.0 2.0 1.98 2.7
0.3 0.3 0.3 0.15 0.3 0.15 0.15 0.3 0.25 0.25 0.25 0.28
9.28 0.7
5.75 0.36 4.61 0.15 6.5 0.15
1 1 1
1.0 0.4 3.5 2.8 2.5 3.5 1.5 1.0 1.0 1.0 2.07 2.44 2.8 3.6 2.8 1.6 5.2
Ind. Eng. Chem. Res., Vol. 32, No. 6,1993 1043 Table 11. (Continued)
Chesonis et al. (1990)
0.1m i.d. 2 = 6.2 m
Yang et al. (1984)
0.115 m i.d. 2=8m
alumina dp = 120 pp = 3460 silica gel
3.6 3.5 4.5 5.3
dp = 220 pp = 794 Arena et al. (1991)
0.12 m i.d. 2 = 5.75 m 0.4 m i.d. 2 = 10.5 m Weinstein et al. (1984) 0.152 m i.d. 2 = 8.5 m In = 2.5 m In = 4.1 m
Ballotini d, = 90 pp sc 2543 HFZ-20 dp = 49 pp = 1450
a
2.9
3.4
In = 4.1 m 0.05 m i.d. Z = 2.79 m
5.0
2.9
In = 2.5 m
Horio et al. (1988)
5.0
3.4 FCC d, = 60 pp = lo00
1.17 1.20 1.29
0.097 0.121 0.084 0.02 0.025 0.106 0.129 0.0991 0.1553 0.0262 0.1011 0.0846 0.1531 0.1661 0.1105 0.1588 0.20 0.0642 0.1255 0.1749 0.0938 0.1533 0.2152 0.0921 0.1074 0.0694
11 9 7 43.5 57 132 160 92 115 114 251 75 108 118 74 106 118 71
106 140 79 112 129 11.7 13.6 11.3
109.9 0.0007 162.9 0.0005 186.9 1.96 0.0102 0.0134 1.87 0.0304 3.48 3.53 0.0366 0.0072 13.8 17.3 0.009 2.94 0.0089 5.22 0.0194 4.83 0.0175 6.11 0.025 6.08 0.0273 6.39 0.0173 6.46 0.0246 7.33 0.0273 4.52 0.0142 0.0211 5.96 0.0276 6.33 0.0158 5.95 0.0222 6.9 8.44 0.0255 9.3 0.0099 0.0112 9.59 7.98 0.0087 O.OOO9
4.53 5.4 8.9 1.54 1.55 1.64 1.55 3.27 2.62 1.99 1.37 1.52 1.6 1.71 1.93 2.17
110 163 187
2.71 1.83 2.09 2.41 2.49 2.52 2.23 2.87
1.0 1.0 1.0
6.58 6.83 27.8 23.6
0.25 0.25 0.33 0.51
12.15 8.83 8.25 7.57 9.64 7.32 7.33 11.74 9.5 8.36 9.4 8.38 8.44 15.78 13.39 14.37
0.28 0.38 0.5 0.66 0.52 0.76 1.0 0.15 0.38 0.5 0.35 0.55 1.0 0.2 0.4 0.15
1 1 2 2 1.61 1.38 3.15 3.15 1.87 2.55 1.7 1.63 1.19 1.7 2.13 2.72 2.46 2.98 1.95 1.17 1.95
In, solids inventory, meter height in downcomer;I, solids inventory, kg.
et al., 1988). These clusters can fall relative to the gas at velocities many times the individual particle terminal velocity. On the other hand, many investigators have observed the nonhomogeneity in solids flow and distribution, with the downward flowing (or stagnant) wall region having a solids concentration significantly higher than in the rapid rising core region of the bed (Rhodes, 1986; Hartge et al., 1986, 1988; Bader et al., 1988). Therefore, the local heterogeneity (aggregationof particles) and the global heterogeneity (radial and axial profiles of solids flow and distribution) may contribute to the high slip velocity between gas and solids which results in a much higher solids holdup than might otherwisehave been anticipated. Arnold (1987) suggests that for pneumatic conveying the pressure gradient in vertical upward flow is doubled over that for the horizontal flow. This suggests a tentative "rule-of-thumb":
-
2 in dilute zone (3) In trying to verify this tentative rule-of-thumb, values of e*B/d8 were taken from some experimental results at low solids circulation rates or high enough risers such that solids holdups leveled off in the upper section of the riser. Table I shows the authors' experimental results on solids holdup which were obtained in a 0.254 m i.d. circulating fluidized bed with a height of 10.85 m. The facility has been described by Ouyang et al. (1992). Values of E*$C'~ in our experimental datalie in the range 2.26 f 0.32. Table I1 summarizes some experimental data reported in the literature. The average value of e*$cf8 in Table I1 is 2.6, and the standard deviation is 0.9. Those values are in line with the tentative rule-of-thumb illustrated by eq 3. Equation 3 is also consistent with the slip factor criterion for the hydrodynamicallyfully developed flow zone of risers used by Patience et al. (1992). It should be noted that other authors, e.g., Kunii andLevenspiel(1991)and Kwauk and co-workers (Li and Kwauk, 1980; Li et al., 19881, assume that the lean phase will approach et8, or a parameter equivalent in value. Given the instability of the upward flow situation, it is perhaps not surprising that measureE*~/E'~
0.5
0.4
-
0.3
-
Li&Kwauk Yerushalmi Annaetal. Herb et al.
+
Weinsteinad. h u g e & Chang Hangeetal. Bi et d.
0
R~&S
0
-0 Y
0.2
'
0.1
'
e :
n"." n 0
2
4
6
8
10
u (ds) Figure 1. Solids holdup in the dense phase at differentgas velocities.
menta of various authors indicate that in their experiments the lean phase approaches a value at least twice eta. There is an important point here to be resolved. It could be that the limit approaches only in small-diameterequipment, say, less than 9 cm, and that in larger diameters the wall effect and/or instability serve to drive the limit to 2d8 or even higher. It could also be that heights employed have not been sufficient to reach the limiting condition. The solids holdups in the dense phase, taken from experimental data, shown in Tables I and 11,are plotted against the superficial gas velocity in Figure 1. As can be seen from the figure, the gas velocity does not seem to appreciably affect the solids holdup in the dense phase of the riser. Tables I and I1 and Figure 1show that particle characteristics do not seem to affect the values of and Zhang et al. (1991) also suggest that particle property does not affect the radial voidage profile at the same operating condition. The average value of €d,s in Figure 1is 0.18 with a standard deviation of 0.05. Figure 1may be compared with Figure 8b of Kunii and Levenspiel
1044 Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993
(1991), which indicates that Ed,, varies little with U from 2 to 5 m/s. These authors’ Figure 8a indicates Ed,, also to
be substantially independent of G,, up to 200 kg/(m2 s). A value for the dense phase of 0.22-0.16 is obtained by them. For design purposes, Kunii and Levenspiel (1991) recommend e*, = G*a/p,U (- e’,) and Ed,, taken from their Figure 8. It is a matter of great interest to understand how the dense-phase density could be independent of U and G,. Elucidating this problem is not part of this paper. The height of the transition phase, Zt, in Tables I and I1 varies over a wide range. In some cases, most of the riser was occupied by the transition phase, while in others negligible transition phase is evident. The average value of Zt is 2.25 m with a SD of 1.3 m. Kunii and Levenspiel (1991) indicated that the height of the transition phase decreases with increasing particle size and with decreasing riser diameter and gas velocity. They provide a model for its estimation. To the extent that riser heights reach 20 mor more, the transition zone is of decreasing importance. In design or reactor modeling for CFB, there is required the knowledge of solids holdup as well as fractions of riser occupied by dense phase and dilute phase for the given flow conditions. The general agreement between eq 3 and the experimental results suggests that, as a first approximation, eq 3 and the average value of solids holdup in the dense phase could apply to the regions above the acceleration zone at the entrance and below the exit zone where the solids holdups are affected by solids acceleration and reflection. The analysis of the pressure loop around the CFB system indicates that the pressure a t the bottom of the riser cannot exceed that at the bottom of the downcomer. If the pressure differential available between the bottom and top of the riser is known, the average solids holdup in the riser should be calculated. Then the fractions of the riser occupied by the dense and dilute phases can be estimated. To some extent, entrance and exit effects will affect the solids holdup in the riser. However, if there are two risers, one 10 times taller than the other, it would be expected that the influence of riser geometry on solids holdup for the short riser would be much stronger than for the tallriser. The pressure drop contribution by solids acceleration and reflection in the tall riser would be insignificant. Therefore, for a given CFB system, the fractions of riser occupied by dense phase and dilute phase are very much dependent on the pressure differential available between the bottom and top of the riser. The higher the pressure differential across the riser, the larger the fraction of riser occupied by dense phase. If a sufficiently large pressure differential were available for a given riser height, the dense phase would be supported over the entire height of the riser. One of the important issues in designing and operating CFB reactors is classification of the optimum operating conditions. In terms of bed-to-wall heat transfer, higher bed density will generally lead to a higher heat-transfer coefficient. From a gas-solid reaction point of view, an increase in the solids holdup and solids circulation rate in the riser will certainly achieve high capacity of the CFB reactor. However, the particle surface efficiencydecreases due to aggregation of particles; such aggregation effects would be expected to strengthen as solids holdup increased (Dry et al., 1987). Therefore, the contact efficiency between the gas and solids may decrease with increasing solids holdup. Similar suggestions were made by Ouyang et al. (1992), but more information is needed before any conclusion with regard to the optimum operating conditions of a CFB can be made.
Conclusions The ratio of average solids holdup to solids holdup at ideal case, e.g., eq 3, is used to approximate the solids holdup in the dilute phase of the riser. Comparison with some of the experimental data in literature is made. The average e*$da in the dilute phase for the experimental data is 2.6, and the standard deviation is 0.9. The average value of Ed,, in the dense phase is 0.18, and the standard deviation is 0.05. The average height of the transition zone between phases is about 2.25 m and the SD is 1.3 m. The height of the dense region is variable and should be determined by the pressure differential available between the bottom and top of the riser. With the pressure differential available across the riser for the giving flow conditions, the average solids holdup as well as fractions of the riser occupied by the dense and dilute phases could be estimated. It is important to decide whether the best operating condition occurs when the riser is wholly occupied by lean phase or wholly occupied by dense phase or when there is an as yet undetermined mixture of the two.
Nomenclature A: constant in eq 2,Up$G, d,: mean particle diameter, pm G,: solids circulation rate, kg/(m2s) K: constant in eq 2,uT/U hp: maximum pressure differential available across the downcomer, kPa U: superficial gas velocity, m/s UT: particle terminal velocity, m/s z*d:dimensionless height of dense zone Zt: height of transition zone, m 2 height of riser, m Greek Symbols
pp: particle density, kg/m3 e,:
average solids holdup
da: solids holdup at no slip case
solids holdup at uniform flow with slip velocity equal to particle terminal velocity e*,: average solids holdup at dilute region q a : average solids holdup at dense region e”,:
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Received for review June 19, 1992 Revised manuscript received December 15, 1992 Accepted March 6,1993
