Ind. Eng. Chem. Process Des. Dev. 1986, 25, 504-507
504
Spiral Distributor for Fluidized Beds Fan Ouyang' and Octave Levenspiel Department of Chemical Engineering, Oregon State University, Corvallis, Oregon 9733 1
Performance characteristics of a new type of distributor, the spiral, are evaluated as a function of the number of slits used in the spiral and the fraction of open area. The quality of fluidization, pressure drop characteristics, and heat-transfer coefficients between the distributor and bed are measured and correlated with the geometry of the unit. Comparison with the sintered-plate distributor shows that depending on the fluidizing condffions, sometimes one, sometimes the other is preferred.
The selection of an appropriate distributor plate is an important consideration in the design of fluidized beds. First of all, the quality of fluidization and the amount of gas bypassing can be strongly influenced by the type of distributor used. Second of all, the pressure drop through the distributor may significantly increase the power consumption of the blower, often a major cost factor for operations. These two considerations are well-known (see: Kunii and Levenspiel, 1978). Certain processing operations also require having a cool distributor plate to support a hot fluidized bed. To prevent the sintering of solids during the reduction of iron ore is one example of such an operation. A second example is in the production of very pure silicon by the decomposition of silane SiH,(g)
heat
Si(s) + 2Hz
Here, cold silane enters a bed of hot silicon particles. The gas heats up and decomposes, and the silicon which is released then deposits and fuses onto the bed particles which grow in size. In this operation, it is imperative that the silane does not decompose while passing through the distributor plate, to deposit silicon therein and plug it. So, in the design of reactors of this type, it is important to know the heattransfer coefficient between the bed and its distributor plate. The purpose of this research is to evaluate a new type of bed support, called the spiral distributor, and compare its characteristics, such as the pressure drop, quality of fluidization, and heat-transfer coefficient, with that of a sintered-plate distributor. Experimental Section The apparatus consisted of a 152 mm i.d. fluidized bed resting on either a spiral or sintered-plate distributor, room-temperature air supply, heating lamps, and dust collection system. The spiral distributor was made of N overlapping plates shaped as sectors of a circle with a gap between the plates, as shown in Figure 1. This gap was largest at the bed wall and decreased to zero a t the center such that the fraction of open area is the same for all concentric circles. A t the center, the plates were welded together. Three versions of the spiral distributor of geometry given in Table I were tested and compared with a 1.6 mm thick sintered stainless steel plate with 5 - ~ m pores obtained from Mott Metallurgical Corp. *On leave, Institute of Chemical Metallurgy, Academia Sinica, Beijing, China.
Table I. Geometry of Spiral Distributors Tested distributor A B C 24 24 32 no. of blades 0.243 0.214 av gap, mm 0.813 1.43 4.1 70 open area 1.22 Table 11. Properties of Particles Fluidizedn
US standard mesh no. sand silicon zirconia
-40+50 -50+70 -70+80 -40+200 -50+70
mean diameter, d,, Mm 296 204 158 161 204
density, kp/m3 2610 2610 2610 2330 5900
heat capacity at 5a o c , J/(kp.K)
aoo aoo aoo ai5 445
For nonspherical particles with no particular short or long dimension, Levenspiel (1984) gives d, = $dscleen. (I
Three types of bed particles were used in this study with properties given in Table 11. Silicon, obtained from the Union Carbide Corp., had a wide size distribution; the other materials had narrow size distributions. The pressure drops across the bed and across the distributor plate were measured with PIN connections and recorded with a Hewlett-Packard 7402A recorder, and the bed was heated radiantly, by spotlights, from above. Finally, the temperatures of the incoming air, distributor plate, and bed were measured by 0.254-mm chromel-alumel thermocouples and recorded by using an Esterline Angus PD 2064 data logger. The location of the thermocouples is also shown in Figure 1.
Results and Discussion 1. Pressure Drop vs. u o for the Spiral and the Porous-Plate Distributors. Figure 2 shows that the pressure drop across the spiral distributor is from 1 to 2 orders of magnitude smaller than for the sintered plate tested; however, this pressure drop increases more rapidly with an increase in gas velocity than for the sintered plate. 2. Quality of Fluidization with Spiral Distributors. Gas enters the bed tangentially through the many slots of the spiral distributor, and in shallow bed operations, this imparts a swirling motion to the solids. In deep beds (height > bed diameter), this action is restricted to the lower portion of the bed. Normal bubbling fluidization is seen in the main portion of the bed, above. 3. Comparison of the Quality of Fluidization with Beds Using Sintered Plates. The pressure fluctuation across a fluidized bed is a convenient, if rough, indication
0196-4305/86/1125-0504$01.50/00 1986 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2, 1986 u.
thermocouple locations ,&.,
u,/u,f
With Sintered Plate Distributor
(cm/secl I85
177-
273 220360 290
440 362
5 6 4 455
Figure 1. Spiral distributor showing the location of the test thermocouples.
620
500
505
With Spirol Distributor
Figure 4. Pressure fluctuations across fluidized bed of zirconia particles. center of
1 fluidized bed L (cm)
V Thermocouple being lowered A Thermocouple being raised
Figure 5. Bed temperature directly above the weld point of the spiral distributor.
pressure fluctuations, hence no consistent advantage of more slots or less open area. uo uu /,, With Sintered Plate Distributor With Spiral Distributor Visual observations just above the spiral distributor (cm/secl show a horizontal convective movement of particles as 496 165 opposed to essentially stationary particles above the sin735 245 tered plate. I04 347 Finally, the plates of the spiral are welded together a t 136 453 v the center of the distributor, and this may result in particles heaping up a t this point. To measure the magnitude 190 633 7 of this effect, a thermocouple was affixed to the weld point, 234 780while a second, movable thermocouple was located directly 336 I12 above along the axis of the bed. Typical temperature measurements for these two thermocouples are shown in 460 153 Figure 5. The fact that the bed temperature stays constant to within 5 mm of the weld point indicates that very 510 I70 little solids, if any, are piled up a t the weld point. Figure 3. Pressure fluctuation across fluidized bed of silicon par4. Mechanism of Heat Transfer between Bed and ticles. Distributor Plates. Heat transfer between fluidized beds of the quality of fluidization; small fluctuations represent and immersed tubes or bed walls has been explained in small bubbles and less gas bypassing, while large fluctuaterms of various mechanisms: conduction through gas tions represent poorer contacting of the gas with solids. films, direct contact of bed particles with the surface, packets of emulsion resting on the surface, etc. Figures 3 and 4 show the recorded pressure fluctuations Heat transfer between fluidized beds and their distriacross the bed when supported by either the spiral or the bution plates has not been discussed until recently (Lesintered-plate distributor. A comparison of these traces venspiel et al., 1983). Here, the mechanism is somewhat shows that for low density solids (silicon, in Figure 31, the different in that gas flows normal to the surface, rather sintered plate gives better fluidization at low uo,but at uo than sliding along the surface. Thus, no gas film is present > loud, the advantage shifts to the spiral distributor. On at the heat-transfer surface, and transfer is only by direct the other hand, for high density solids (zirconia, in Figure impingement of particles with the surface. 4), the spiral distributor seems to give a better quality of The behavior of the spiral distributor is somewhat as fluidization a t all gas velocities. shown in Figure 6. A t the mouth of the slot, gas flows Similar pressure trace measurements for different spiral horizontally along the surface, entraining and dragging geometries (24 and 32 slots, 1.4% and 4.1% open area) and solids along with it; then the gas turns upward. Thus, heat using sand (narrow size distribution) and then using silicon transfer represents a complex situation somewhere be(wide size distribution) show roughly no difference in Figure 2. Pressure drop across distributor plates vs. air velocity.
w
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2, 1986
506
,Sintered
/
0
q , 50
"0
0
1
,
,
,
60
70
80
90
plate distributor tibed = m c m u. :9 7 cm/s
, . 100
0 e d Temperoture ( T I
Figure 8. Heat-transfer coefficient independent of temperaturedriving force. Figure 6. Bed behavior at the spiral distributor.
2Ocm
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m:mj bed nth sprd dstnbutor
coil heater
l5Zmm roundbed with sintered plate distributor
d
I
8 ; 10cy c
$
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zoo: 0
/
,
,
,
20
30
40
u. ( c d s e c )
Figure 7. Heat-transfer coefficient as a function of gas velocity for different distributors and different bed heights.
tween the above two cases. Also, we may expect that the number of particles dragged along the surface and giving up heat is proportional to the flow rate or velocity of the gas issuing from the slots. If the heat given up by each particle is constant, then we would expect h to be directly related to uo. Assuming an isothermal distributor plate, a heat balance about the plate gives Qtransferred
=
kgCp,g(Tg,out
-
Tg,in)
= hA(Tbed
T
O
IO
20
30
40
uo (cm/sec)
Figure 9. Heat-transfer coefficient increasing with a decrease in particle size.
i
-0
500-
a c
2 2c
B
400-
300-
- Tdistributor)
Assuming, in addition, that the gas leaves the distributor plate a t the temperature of the distributor plate itself, or Tg,out = Tdistributor, we obtain
I
- 6 0
?
10
20
u,
30
40
(cm/sec)
Figure 10. Heat-transfer coefficient very different for different kinds of solids of the same size.
For sintered-plate distributors, the latter assumption should be quite reasonable; however, for the spiral distributor plate with its few large open channels, this assumption is questionable because Tg,outis likely to be Thus, the values of h somewhat lower than Tdiatributor. calculated by eq 1 will be somewhat higher than the true value of h, giving a conservative (higher) estimate for the temperature of the spiral distributor plate. Nonetheless, adopting eq 1 while recognizing its limitation and measuring the flow rate of gas and the temperatures of incoming gas, distributor, and bed give h directly. These measurements were made for a variety of situations, and the values were compared to the findings with a sintered-plate distributor, as discussed below. 5. Effect of Air Velocity and Bed Height on h . The results of Figure 7 show that h is independent of the bed height and that h rises linearly with the air velocity uo,for uo > ump This result is quite different from the findings with the sintered-plate distributor, where h rises to a maximum at about 2umfand then decreases slowly.
Thus, at low multiples of umf,the spiral distributor has a lower h than has the sintered plate; the reverse is true a t high gas velocity. For the distributor of Figure 7 , the transition occurs a t uo = 6ump Considering the discussion directly following eq 1,the true h for the spiral distributor plate should be somewhat lower than that reported in Figure 7, giving it added advantage over the sintered-plate distributor. 6. Effect of Temperature Driving Force on h If the assumptions underlying eq 1 are reasonable and if h given by this equation is to be useful, then the measured h should be independent of A T between bed and distributor plate. Figure 8 shows the measured h values for both the sintered-plate and spiral distributor for different AT, all other variables kept constant. The h value is found to be independent of AT, as expected. 7. Effect of Particle Diameter. Figure 9 shows the plot of h vs. uo for narrow size cuts of sand. The results are straight lines similar to that for silicon. The h value is found to be very sensitive to the particle size, with beds of smaller particles giving higher h values.
.
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2, 1986 507
-5 - 5oo-
Nx
6oo-
.c
24 blade distributor (4 I % open area)
x
24 blade distributor (I 22% open area)
3
0 32 blade distributor (I 43% open area)
P k v1
400-
(2) Note that D and were not varied in our experiments. Figure 12 is a graphical representation of this correlation. Conclusion A new type of distributor for fluidized beds, the spiral distributor, is examined and its behavior is compared with the sintered-plate distributor. At high gas velocities, the spiral distributor gives a better quality of fluidization; a t low velocities, it gives lower heat-transfer coefficients between the bed and distributor. Equation 2 correlates the upper bound of the heat-transfer coefficient (see discussion after eq 1) with the properties of the fluidizing material and with the geometry of the distributor.
c
f
*
300-
+
d 5y
200-
c
8
V
$
100-
e G
I
I
O
I
I
I
1
20
IO
30
I
Distributor &
C
Silicon
X
I
"
'
7
Silicon and Sand
z 002-
L -
00102
05
10
(T) dP PP (uo-unlf) Figure 12. Dimensionless correlation for the heat-transfer coefficient between the bed and spiral distributors.
8. Effect of Particle Properties. Figure 10 shows the h vs. uocurves for narrow cuts of particles of the same size but having different properties and shows that the particle properties have a strong effect on the heat-transfer coefficient. 9. Effect of Open Area and Number of Slots in the Spiral Distributor. Figure 11 shows that for beds of sand, changing the fraction of open area from 1.2% to 4.1 % does not seem to affect the h value, while changing the number of blades in the spiral from 24 to 32 may have a slight effect on h. Unfortunately, since the fractional change in the number of blades is small, its effect on h is not certain. The same result is found with the other solids. 10. Empirical Correlation for Heat Transfer between the Bed and Spiral Distributor. Correlating the measurements with three kinds of solids, different particle sizes, and three spiral geometries and putting in dimensionless form gives
Acknowledgment We thank Riley Chan and Nick Wannenmacher for their technical assistance in constructing the experimental apparatus, to UNIDO for providing one of us (0.F.) the opportunity to study in Oregon, to JPL (Contract 956133) for the idea which led to this study, and to NSF (Grant CPE-8026799) for support (for 0. F.) while doing this project. Nomenclature A = cross-sectional area of distributor, m2 C, = specific heat, J/(kg.K) D = diameter of distributor, m d p = mean particle size for fluidization purposes, ni h = heat-transfer coefficient, W/(m2.K) k , = thermal conductivity of gas, W/(mK) m = mass flow rate, kg/s N = number of blades in the spiral distributor T = temperature, K umf = minimum fluidizing velocity, m/s uo = superficial velocity of fluidizing gas, m/s Greek Letters = density, kg/m3 = viscosity, kg/(m-s) 4 = sphericity of particles
p
Subscripts g = gas s = solid
Literature Cited Kunii, D.; Levenspiel, 0. "Fluidization Engineering": Krieger: Melbourne, FL,
-.
1978
Levenspiel, 0.; Larson, M.; Zhang, G. T.: Ouyang, F. Final Report to JPL, No.
956133,1983. Levenspiel, 0. "Engineering Flow and Heat Exchange"; Plenum Press: New York, 1984;p 122.
Received for review February 6, 1984 Revised manuscript received September 3, 1985 Accepted September 19, 1985
