Holsen, J. N., Struck, M. R., h d . €ng. Chem., Fundam., 3, 143 (1964). Hsu, H. W., Bird, R. B., A./.Ch.E J., 6, 516 (1960). Ivakin, E. A., Suetin, P. E.. Soviet Phys.-Tech. Phys. (Engl. Trans/.), 8, 748 (1964). Lee, C. Y., Wilke, C. R., lnd. Eng. Chem., 46, 2381 (1954). Lewis, W. K., Gilliland, E. R., McBride, G. T., Jr., h d . Eng. Chem., 41, 1213 (1949). Mathur, G. P., Thodos, G., A./.Ch.E. J., 11, 164(1965). Nagle, J.. Strickland-Constable, R. F.. Proc. Fifth Carbon Conf., 1, 154 (1962). Olander, D.R., lnd. Eng. Chem., Fundam., 6, 178 (1967). Reid, R. C., Sherwood, T. K., "The Properties of Gases and Liquids," 2nd ed, p 543, McGraw-Hill, New York, N.Y., 1966. Rosner, D.E., Allendorf, H. D.,A.I.A.A. J., 3, 1522 (1965); 6, 650 (1968). Rosner. D.E., Ann. Rev. Mater. Sci., 2, 573 (1972). Schweitzer, D.G., Gurinsky, D. H., Hrabak, G. C., Singer, R. M., Proc. Fiffh Carbon Conf., 1, 215, 223, 228 (1962). Shelef, M., Walker, P. L., Jr., Carbon, 5 , 93 (1967). Sherwood, T. K., Pigford, R. L., "Absorption and Extraction." 2nd ed,McGraw-Hill, New York, N.Y., 1952.
von Fredersdorff,C. G., Elliott, M. A.. in "Chemistry of Coal Utilization," Suppl. Vol., H. H. Lowry, Ed., Chapter 20, Wiley, New York, N.Y., 1963. Walker, P. L., Jr.. Rusinko, F.. Jr., Austin, L. G., Adv. Cataal., 11, 133 (1959). Walker. R. E., Westenberg. A. A.. J. Chem. Phys., 29, 1139(1958a); 29, 1147 (1958b); 32, 436 (1960). Walls, J. R., Strickland-Constable, R. F.. Carbon, 1, 333 (1964). Walsh, P. N., Quetz, J. M., Graff, R. A,, J. Chem. Phys., 46, 1144 (1967). Westenberg, A. A,, Walker, R. E., J. Chem. Phys., 26, 1953 (1957). Wilke, C. R., Chem. Eng. Prog., 46, 46, 95 (1950). Yang, R. T., Steinberg, M.. J. Phys. Chem., 80, 965 (1976).
R e c e i ~ ~ feodr reuieu March 8, 1976 Accepted October 1,1976
This work was performed under the auspices of the Office of Molecu l a r Sciences, Division of Physical Research, U.S. Energy Research and Development Administration, Washington, D.C.
Slugging in Fluidized Beds William J. Thiel and Owen E. Potter' Department of Chemical Engineering, Monash University, Clayton 3 768, Vlctoria, Australia
Experiments conducted with a variety of powders, in high aspect ratio slugging fluidized beds, indicate that raining (or square-nosed) slugs are formed by materials which in lower aspect ratio vessels form bubbling or smoothly slugging beds. The largest of the three beds investigatedwas of small industrial size having dimensions of 0.21 m diameter and 6.9 m high. Measurements of rise velocity, slug length, slug frequency, and inter-slug spacing are reported. The rise velocities are in agreement with: Usn = U - Umt 0.35(gD))112. The inter-slug spacing, which is related to frequency, has assumed values up to 8 bed diameters. The angle of internal friction of a particulate solid is shown to be a significant factor for predicting which slug flow regime a solid will form when fluidized. Some experimental techniques are reported which attempt to identify and measure the forces which operate in a raining slug bed.
+
Introduction Stewart and Davidson (1967) describe the two main regimes which can be observed in a fluidized bed when the gas pockets are in slug-flow. In the smoothly slugging bed, slugs are round-nosed and solids flow past the slug in an annular region on the wall. This regime occurs with materials which fluidize easily and is contrasted with that in which square-nosed slugs are observed. The square-nosed slugs fill the complete cross-section, solids raining through the slug. The pressure drop across a raining slugging bed is higher and more irregular than across a smoothly slugging bed because of the "locking" of solids on to the bed wall. In addition to axisymmetric slugs asymmetric- or half-slugs, which run up the wall, have been described by Stewart and Davidson (1967) and Kehoe and Davidson (1970). In this paper experiments are reported on deep fluidized beds using a number of different materials. Slug frequency, inter-slug spacing, slug length, and slug rise velocity data are reported. General observations on the slugging behavior are made and it is shown that the angle of internal friction of a particulate solid largely controls which slugging regime will form in a bed of high aspect ratio. Previous Studies Slug Rise Velocity, Round-Nosed Slugs. For axisymmetric slugs, Stewart and Davidson (1967) write
USA= k l ( U - U,f)
+ k2(gD)'/2
(1)
and show that if kz = 0.36 there is agreement between fluid242
Ind. Eng. Chem., Fundam., Vol. 16, No. 2, 1977
ized beds and gas-liquid systems, while k l = 1.0 for fluidized beds compared with a value k l = 1.2 reported by Nicklin et al. (1962) for gas-liquid systems. These statements apply when coalescence is not a major factor. Kehoe and Davidson (1970) find that where coalescence is a factor k l 3 1.0. Stewart and Davidson (1967) and Kehoe and Davidson (1970) showed that for round-nosed asymmetric slugs
U S A= l.O(U - U,f)
+ 0.35(2gD)'/2
(2)
The slug behaves as if it were in a bed twice the diameter. Slug Rise Velocity, Square-Nosed Slugs. Gel'perin et al. (1970a) report experimental values of rise velocity of square-nosed slugs and discuss the break-down to "normal" fluidization. These workers have also investigated an annular bed, Gel'perin et al. (19704 Slug Length and Frequency, Round-Nosed Slugs. Kehoe and Davidson (1970) derive the following expressions for slug length, L,, in terms of the inter-slug spacing T
+ 0.061 - ( T - 0.061)2]:i;g;0:[
=O
(3)
Except, for small values of ( U - U,f)/0.35(gD)1/2, Ls/D can be related quite accurately by the approximation
L,
= JD(U -
Umf)/0.35(gD)1/2
where the values of J are as follows
(4)
Table I. Physical Properties of Solids
T
I 2 3 4 5 6 7 J 2.44 3.83 5.41 6.41 7.63 8.84 10.0 The corresponding frequency of slugs, that is the number passing a given position per unit time, is given by (5) As a simple approximation the slugs may be considered to be can-shaped and then
L , = T D ( l J- Umf)/O.35(gD)1’‘
(6)
and f = 01.35(g)~/~/TD~/~
(7)
The extent of the approximation may be ascertained by comparing values of T and J . Slug Length and Frequency, Square-Nosed Slugs. Gel’perin et al. (1970a) present experimental values of slug length a t different air flow rates in 0.05 and 0.16-m diameter beds, for raining slugs. L‘alues of initial slug length as well as slug lengths at various beights in the bed are given. The frequency of raining slugs is reported by Gel’perin et al. (1970a,c). Inter-Slug Spacing, Smoothly Slugging Beds. Stewart and Davidson (1967) found a value of inter-slug spacing (T) equal to 2, while Kehoe and Davidson (1970) report values ranging from 2 to 5 in a 0.102-m diameter bed, values of T increasing with increasing height above the distributor. In a 0.46-m diameter bed, Hovmand et al. (1971) found T ranged from 1to 2 while in a 0.14-m diameter bed, Matsen and Tarmy (1970) report a value equal to 2.4. Comparison between Round-Nosed and Square-Nosed Slugs. Typically the slug frequency in a smoothly slugging bed of diameter 0.05-0.20 m is of the order 1 slug/s, whereas in a raining bed it is lower, say 0.3 slug/s. The slug rise velocity in a smoothly slugging bed is approximately twice that in a raining slugging bed. It follows that in a raining slugging bed the slugs carry a smaller fraction of the gas passing through the bed, the remainder passing through in packed bed flow. This accounts for the higher and more irregular pressure drop across a raining slug bed. According to Gel’perin et al. (1970a) for 263-pm sand in a 0.16-m diameter bed and bed heights up to 1.6 m, the fraction of inlet gas passing through the bed as slugs is about 0.2-0.3 when U = 2 1 cm/s. The Onset of Slug Flow. Using eq 1 for slug rise velocity with hl = 1.0 and h2 = 0.35 and a value of inter-slug spacing ( T )equal to 2. Stewart and Davidson (1967) developed the following criterion for the onset of smooth slug flow
Gas Flow Regimes in Slug Flow Hovmand and Davidson (1971) suggest that raining or square-nosed slugs are very common in tubes up to 0.05 m diameter and uncommon in larger tubes. As will be seen later, the work reported here suggests that square-nosed slugs can occur readily in beds of 0.22-m diameter when the beds have a large aspect ratio. Kehoe and Davidson (1970) describe the breakdown of slugs to a so-called “turbulent” regime in 0.05 and 0.10-m diameter beds of fluid cracking catalyst at gas velocities between 16 and 35 cm/s. In the work reported here a bed in which round-nosed slugs existed broke down into the turbulent regime a t superficial gas velocities of 20 cm/s in a 0.10-m diameter bed and 2.5 cm/s in a 0.22-m diameter bed. Zenz (1957) suggests that the formation of a raining slugging bed can be used as a method of measuring tan /3 and further
Solid
Label
Fluid cracking catalyst (Katoleum LA651 Aluminum powder Mixture of FCC and aluminum (71.5%w t aluminum) Copper powder Glass spheroids Glass spheroids Glass spheroids closely sized Equilibrium FCC (carbonized) broad size range Sand
urd,
cm/s
PS>
g/cm3 tan p
FCC
0.18 0.93
7.1
AI FCC-AI
0.62 0.25
2.65 1.73
6.0
2.2 8.92 0.25 2.45 2.1 2.45 14.0 2.45
5.9 3.9
cu MS/XL MS/H 359p
ballotini EFCC (broad) Sand
4.1 4.4 4.3
0.27
1.31
5.8
2.6
2.65
3.4
that the value of tan 6 is significant in determining the onset of the raining regime. However, Gel’perin et al. (1970a) point out the tendency for a raining slug bed to form increases with decreasing bed diameter, so they conclude that the value of tan /3 alone cannot be used to determine the structure of a raining bed. This conclusion is supported by the results of this work, but it is apparent that the value of tan p is very significant in determining which slug flow regime can be expected for a particular solid.
Experimental Work Three different diameter rigs were built to measure the solids mixing in slugging fluidized beds by heating the solids at the bottom of the bed and cooling at the top. It was desired to investigate the axial temperature gradient and axial heat flux over a section of bed where coalescence was small. This requirement resulted in beds of comparatively large aspect ratio. Simultaneously with the solids mixing measurements, the slugging bed was characterized by observation and measuring the slug rise velocity, slug length, inter-slug spacing, and slug frequency. The solids mixing results are reported in detail by Thiel (1972). The beds were (a) 0.051 m diameter by 3.6 m high, (b) 0.102 m diameter by 4.7 m high, and ( c ) 0.218 m diameter by 6.9 m high. The sections in which slug properties were measured were, in terms of height above the distributor: (a) from 0.60 to 1.8 In; (b) 1.05 to 2.84 m; and (c) 1.8 to 3.6 m. Sintered bronze plate distributors were used in the smaller beds and a large single bubble-cap in the large bed. The two smaller beds could be vibrated by an electromagnetic vibrator attached to the framework of the rig. Kehoe and Davidson (1970) showed that vibration reduced the tendency for solids to lock together forming a raining slug bed. The presence of gas slugs in the bed was detected by the change in electrical capacitance between two semicircular plates attached round the outside of the glass working section. The angle of internal friction /3 of the different solids was measured by the method quoted by Zabrodsky (1966) and by Zenz (1957). The apparatus, shown in Figure 4A, consisted of a 0.05-m diameter tube of glass in which a closely fitting piston slid. Solids were poured into the vertical tube, the tube was tapped, and an attempt was made to push the piston upward. Solids were added until the piston could not be moved. The length of the settled plug of solids (hcrit)was then measured and p was evaluated from hcrit= D tan /3
(9)
Values of tan /3 for the solids employed are presented in Table I, Ind. Eng. Chem., Fundam., Vol. 16, No. 2, 1977
243
Table 11. Size Distribution of Solids (Numbers Are Weight Percent) Size, pm Solid
250
... ...
... ... ...
... ... ... ...
11.3
1.4
1.7 0.1 0.6
... ... ...
...
1.0 18.3
... ...
21.1
...
...
...
...
...
Table 111. Observed Slug Flow Regimes ~
Solid FCC
tan
0
7.1
D = 0.051 m
Bed diameter (bed not vibrated) D = 0.102 m
a U < 4 1 cm/s d U > 4 1 cm/s a U Q 20 cm/s a + c U > 20 cm/s b+c
a U < 20 cmls d U > 20 cm/s a U < 1 5 cm/s a + c U > 1 5 cm/s b+c
D = 0.218 m a
U Q 2.5 cm/s
d U
> 2.5 cm/s -
FCC-A1
6.0
MS/XL
5.9
EFCC (broad) Copper 359 1-1 glass Aluminum
5.8
a
a
a
4.4 4.3 4.1
b b b+c
b+c b+c
-
< 9 cm/s b + c U > 9cm/s
a U
D
= 0.051
m
U Q 23 cmls d U > 2 3 cmls a U Q 20 cm/s a+cU>20cm/s a U Q 5 cmls a+cU>5cm/s a
a
a+b+c b+c a + c U < 10cm/s b + c U > 10cmls b+c b+c
U < 1 5 cm/s U > 1 5 cm/s MS/H 3.9 b b+c b+c Sand 3.4 b b+c b+c Legend: QSmoothlyslugging bed. bRaining slugging bed. CAsymmetric slugs. d“Turbu1ent” bed. a +c b +c
Table IV. The Superficial Gas Velocity a t Which the Transition between the Smoothly Slugging and the Turbulent Gas Flow Regimes Occurs in a Bed of FCC
Bed diameter, m 0.051 0.051 0.102 0.102 0.218
Utrsns, Bed cm/s vibrated 41 23 20 11 2.5
No
Yes No Yes No
Height above the distributor at which the observation was made, m
Ratio of the height to the bed diameter
2.07 2.07 2.82 2.82 4.20
40.5 40.5 27.6 27.6 19.3
In the Appendix, experiments are described in which attempts were made to measure the frictional forces between inter-slug particles and the bed wall in a raining slug bed. Properties of t h e Solids. The properties of the solid particles employed are presented in Table I and the size distribution in Table 11. Experimental Results In a general sense, the first experimental finding was that in the relatively high aspect ratio beds employed, squarenosed slugs tended to form rather readily in beds of diameter greater than 0.05 m. Indeed, considerable difficulty arose in finding solids which fluidized in the smoothly slugging regime in each of the three different diameter beds. Regimes of Slug Flow. Table I11 lists the gas flow regimes formed by the different solids. As can be expected, fluid cracking catalyst was not conducive to square-nosed slug formation but the slugging bed broke down to the “turbulent” regime. This may be akin to the breakdown of gas bubbles to smaller ones in a turbulent liquid. Table IV lists the gas ve244
Ind. Eng. Chem., Fundam., Vol. 16, No. 2, 1977
~~~~
~
~~~
Bed diameter (bed vibrated) D = 0.102 m
< 11 cm/s d U > 11 cm/s a U
a
< 6 cm/s b + c U > 6cm/s
a U
a
a+b+c b+c a U Q 15cm/s b + c U > 15cmls b+c b+c
locities at which the transition between the smoothly slugging and the “turbulent” flow regime occurs. The transition velocity (Utrm) is very dependent on bed diameter and whether the bed is vibrated or not. What is most striking in Table I11 is that the formation of square-nosed slugs was the common feature for many of the materials, even in the 0.218-m diameter bed. Only the fluid cracking catalyst or mixtures of cracking catalyst with aluminum powder or aluminum at U < 15 cm/s, or glass spheroids (MS/XL) formed smoothly slugging beds. With the latter two solids (A1 and MSKL) vibration of the bed was required to form a smoothly slugging bed. All the other materials formed raining slugging beds containing varying numbers of asymmetric slugs. Referring to Table 111,it is apparent that there is a significant relationship between the slug flow regime formed and the value of tan p for the solid. Those solids which fluidize to form a smoothly slugging bed have the larger values of tan @. Solids with the smaller values of tan @ tended to form beds containing asymmetric and raining slugs. Table I11 also indicates the effect of bed diameter on the slug flow regime. As discussed in the survey of previous work, smaller diameter beds have a greater tendency to operate id the raining regime. Rise Velocity a n d Interslug Spacing for Round-Nosed Slugs. Figure 1presents the data on rise velocity, each point being the average value for 25 slugs. Equation 1 for roundnosed slugs with k l = 1.0 and k 2 = 0.35 becomes
(10) while for asymmetric slugs eq 2 applies. If U S Dis the rise velocity of an isolated slug which is given by
U S D= 0.35(gD)”* then eq 10 and 2 may be written
(11)
I
o
0.1
0.1
0.1
0.4
0.6
0.1
0.1
I 0.1 IU-U,,I
UlO
4
0.051n
0
O . l O 2 n dim. b a d
(I*.
Figure 2. Comparison of measured slug lengths with predictions of eq 3 for different values of inter-slug length ratio, T . Experimental values of inter-slug spacing ( T ) are shown beside each data point. Failure of lines to pass through origin is a minor defect of eq 3. Bed material as in Figure 1.
bad
O.211n do*. bad 0 005Im
ia.Lad
solid M&L 0
0.2
0.4
0.1
1.0
0.1
- ,u t -
(U
us0
Figure 1. Comparison of measured slug rise velocities with predict IS of eq 12--round-nosed synimetrical slugs, and eq 13-half- or wallslugs. Bed material is FCC and FCC-A1, as well as glass spheroids MS/XL.
air flow rates represent fully developed round-nosed slugs existing a t a value of (U - U m f ) / U s D less than 0.2. The lowest value a t which rise velocities were measured was ( U -Umf) / U S Dequal to approximately 0.05, a quarter of the gas flow rate predicted by the criterion of Stewart and Davidson (1967). Following their method
U - u,f = CbUSA
(14)
where U S Ais given by eq 10, then
u-U m f -
(12)
-US"-fi+-
U
USA
umf
cb
(15)
1-tb
(13)
The value of q, = I& at T = 2 used by Stewart and Davidson is the upper limit for the fraction of bed which is occupied by s o a graph of ( USA/U;~D) vs. [ ( U - u m f ) / u S D ] yields two slugs, since values of T are not less than 2 in a high aspect ratio straight lines of the same gradient but with different interbed. Values of T = 5 are commonly observed, which gives a cepts for axisymmetric and asymmetric slugs. value of 6b = v l 2 and a value of ( u- ~ , ~ ) / U S=D0.09 for the In Figure 1,the data for glass spheroids (MS/XL) fall below onset of slug flow. The largest value of T measured in this the theoretical, as the bled operated mainly in the raining slug study was T = 8, giving a value of tb = Ihs and - umf)/USD regime. For the more easily fluidizable materials which formed = 0.06 for the onset of slug flow. These values for the onset of smoothly slugging beds, the results at low gas velocities lie slug flow are in agreement with the value of ( u- u m f ) / u S D mainly between the axisymmetric and asymmetric slug values. = 0.05 determined from the experimental values in Figure At higher gas rates the slug velocities tend to the values for 1. asymmetric slugs. The conclusion is that the criterion represented by eq 8 Slug frequency measurements indicated that a moderate gives a conservative estimate of the gas flow rate a t which a amount of slug coalescence was occurring, despite the high bed will slug. If the air flow rate in a high aspect ratio bed is aspect ratio. This accounts for the observed slug rise velocities such that (U - u m f ) / U S D 0.2, the bed will certainly be in being higher than is predicted by eq 1 2 or 13. Slug frequency slug flow. However, the reverse should not be interpreted, that measurements corresponding to each value of slug rise velocity if (U - U m f ) / U s
