Ind. Eng. Chem. Process Des. Dev. 1981, 20, 319-326
919
Influence of Temperature on Incipient Fluidization of Limestone, Lime, Coal Ash, and Corundum Karel Svoboda and Mlloslav Hartman' Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 165 02 Prague, Czechoslovakia
At temperatures ranging from 20 to 890 O C , experimental measurements were made of the minimum fluidization velocities of particles of corundum, lime, brown coal ash, and limestone. Average particle diameters of four narrow sized fractions ranged from 0.565 to 1.125 mm in the experiments done with air in an 85-mm i.d. reactor. There are systematic differences between the experimental onset velocltles and the predictions of correlations from the literature. Possible explanations are suggested and modified equations are proposed for the estimation of minlmum fluidization velocities at different temperatures.
Introduction Due to the excellent properties of fluidized beds, such as high thermal efficiency and intimate gas-solid contact, the fluidized bed technology has proved very useful in solving many industrial problems. Much attention has been paid in recent years to the study of combustion of low-grade coal in fluidized beds. If the bed of ash and burning coal also contains lime, sulfur dioxide is simultaneously removed in the combustion process. Although the majority of fluidized processes are conducted at elevated or high temperature, almost all research on the basic physical properties of fluidized systems has been done at room temperature. The incipient fluidized state is an important aspect of a fluidized bed, and the minimum fluidization velocity is the basic information required for the design and development of various contactors. This work is a continuation of our study of the basic physical characteristics of the fluidized bed of limestone and lime particles for removal of sulfur dioxide from flue gas. Experimental measurements of the minimum and terminal velocities of particles performed at ambient temperature have been reported recently (Pata and Hartman, 1978,1980). When the particle properties are assumed to be independent of temperature, an effect of temperature on the minimum fluidization velocity can be predicted by a number of available equations (Leva, 1959; Kunii and Levenspiel, 1969; Pata and Hartman, 1978). As found in the literature (Mii et al., 1973; Avedesian and Davidson, 1973; Singh et al., 1973; Saxena and Vogel, 1977; Doheim and Collinge, 1978) there are differences between the theoretical predictions and the experimental values of umfmeasured at elevated temperatures. At particles smaller than 1mm, a definite decrease of umtwith increasing temperature has been found most often in experiments. However, smoothed graphs of minimum fluidization velocity vs. temperature show considerable differences in their slopes. In contrast to other authors, Mii et al. (1973) report a larger decrease of umfwith temperature than predicted by the Ergun equation (Ergun, 1952). The aim of our work was to investigate and describe in a wide range the effect of temperature on the minimum fluidization velocity of beds of different materials, such as limestone, lime, coal ash, and corundum. Experimental Section Apparatus. The experimental setup is shown in Figure 1. The fluidized bed reactor was heated by two inde0196-4305181I1 120-0319$01.2510
pendent electrical heating coils (7) and (9) made of Kanthal. The lower part (10) was used as a preheat section with a maximum power input of 2 kW. Temperature in this section was measured by the thermocouple Pt-FthlOPt (11). The upper part of the reactor, divided into two sections (5) and (81, was heated by another Kanthal coil designed for a maximum power input of 4 kW. Temperature control acting on the thermocouples Hoskins and Pt-RhlOPt (12) made it possible to adjust and maintain a desired temperature in the range 100-1000 "C. The fluidized bed reactor (6) of inside diameter DK = 0.085 m and height HK= 0.5 m was equipped with a gas plate distributor (4) of free area (c = 2% and orifice diameter do = 8 X lo4 m. All parts of the reactor were constructed of a heat-resistant alloy. The flow rate of dried air (1) was controlled by the valve (2) and measured by rotameter (3). Pressure drop across the bed was measured by means of a probe (13). The dead end of the probe tube of ED = 8 X m, immersed in the particle bed, was provided with five symmetrically drilled orifices (do = 0.4 mm). The orifices were located 1 mm above the bottom of the probe. Pressure differences were read on a manometer filled with distilled water. Materials Used. Measurements of the minimum fluidization velocities were conducted with particles of the following materials. The fractions of a commercial limestone CI were prepared by crushing and careful sieving of the hand-picked stones. This rock contains a considerable amount of silica and aluminum oxide and it is classified as a calcareous marl. The crushed and sieved particles were of irregular shapes and mostly without sharp edges. Detailed description of the chemical composition and properties of this limestone can be found in our recent kinetic study (Hartman et al., 1979). The lime was prepared by thermal decomposition of the screened fractions of the limestone maintained for 5 h at 900 "C. The lime particles were sieved again and stored in air-tight containers. The ash was obtained by combustion of the lignite particles less than 3 mm in a fluidized bed combustor. The brown coal contained 7% water and 35.3% ash by weight and its heating value was 17.4 MJ/kg. Chemically, the ash can be viewed as a complex mixture of oxides such as Si02, A1203,CaO, Fe203, and silicates. From the above materials four narrow fractions were separated by careful sieving. The mean particle size of the fractions was expressed as an arithmetic mean of mesh size of the two sieves between which the particles were col0 1981 American Chemical Society
Ind. Eng. Chem.
320
Process Des. Dev., Vol.
20, No. 2, 1981
12 -
-\ 15
Table 11. Determined Values of Bed Voidages and Mean Sphericities of Particles
.s;,r- -~
material corundum limestone
lime
brown coal ash
ZP
9
" 0.85 0.565 0.715 0.900 1.125 0.565 0.715 0.900 1.125 0.565 0.715
EpJnin
Epmmax
Ep
0.375 0.396
0.454 0.471
0.381 0.470
0.474 0.521
0.463 0.569
0.512 0.620
0.578
0.627
0.430 0.460 0.456 0.460 0.462 0.515 0.510 0.505 0.501 0.596 0.590 0.600 0.582
0.900 1.125
Figure 1. Experimental setup: 1,dryer of air; 2, valve; 3, rotameter; 4, gas plate distributor; 5, fluidized bed; 6, reactor; 7, electric heating; 8, T i e d bed of ceramic spheres; 9, electric heating; 10, preheater fded with ceramic spheres; 11,12, thermocouples; 13, presaure probe; 14, manometer; 15, thermal insulation. Table I. Physical Properties of Particles
material corundum limestone
line
brown coal ash
-
Results and Discussion Minimum Fluidization Voidage and Particle Sphericity, As suggested by Leva (1959), a close approximation of the minimum fluidization porosity can be obtained by substituting the value emf for the value tp obtained by pouring the particles from one container into another. Since the buoyancy is usually negligible at gas fluidization, we can write
o!
size range, mm
dp, mm
os, kg/m3
0.80-0.90 0.50-0.63 0.63-0.80 0.80-1.00 1.00-1.25 0.50-0.63 0.63-0.80 0.80-1.00 1.00-1.25 0.50-0.63 0.63-0.80 0.80-1.00 1.00-1.25
0.85 0.565 0.715 0.900 1.125 0.565 0.715 0,900 1.125 0.565 0.715 0.900 1.125
3330 2220
5.0 X 3.0 X
1340
4.0 X
1680
7.0
(estimate), deg-'
low6 lo-'
x
lected. The particles were of irregular shape; in the case of ash they were almost flake-like. Values of the thermal cubic expansion of the employed materials found in the literature (D'Ans-Lax, 1967) are on the order of magnitude to deg-'. For comparison the particles of sintered corundum were also selected for our study. In contrast to the above materials, the sintered corundum particles were of regular, ellipsoidal shape and exhibited the minimum thermal expansion. Densities of the particles were determined by mercury displacement. From the standpoint of density, the materials were homogeneous, except the coal ash. The ash was composed of particles, the density of which varied from 1200 to 2100 kg/m3. The characteristics of the materials and fractions used are collected in Table I. Air, the density and viscosity of which differ only slightly from flue gas, was used as a fluidization fluid. The density and viscosity of air at different temperatures were evaluated from the eq 1 and 2. 293 PF = 1.2(1) T 1.46 x 104.~1.504 ( 2) lrF = (T + 120) These equations predict pF and pF with a maximum error < 1% compared with the measured values (Gas of 6, Encyclopaedia, 1976).
+ 0.819 0.780 0.801 0.780 0.724 0.704 0.694 0.708 0.670 0.541 0.54 1 0.520 0.524
(3)
Our experience shows that the values of tP are dependent on the rate of pouring and also on such circumstances as whether a gas passes through the poured bed or the container is shaken. Minimum values of the voidage, ep,min, were obtained by pouring the solids into a glass cylinder as slowly as possible. Maximum values of the bed porosity, tp,", were found after thorough shaking of the particles in the cylinder. The values of cp obtained at a low but practical rate of pouring (Leva, 1959) are a few percent lower than the values cPmmaxas shown in Table 11. Particle sphericity can be determined by a photographic method (Broadhurst and Becker, 1975) or through c (Foust et al., 1965). More accurate results are obtainec! by a procedure based on pressure drop measurements of the fixed bed. According to this method the particle sphericity can be expressed from the Ergun equation Ap 1.75(1 - E ~ ) ~ F U F ~ 150(1 *$ - C,)~~F*UF - $2 = 0 (4)
H
€$ap
tp3.ap2
Both voidage, cp, and sphericity, $, as used in the Ergun equations are concepts which may be only imperfectly realized in a given real system. In practice, both factors are not mutually independent and are influenced by the experimental techniques. As follows from eq 4 an error in the voidage results in a twofold error in the particle sphericity. Systematic evaluations of the particle sphericity with the aid eq 4 in the velocity range 0 < U F < u d suggested an apparent slight increase of $ with increasing gas velocity for a measured porosity of the fixed bed ep' To have values of $ unaffected by the gas velocity it requires a definite small increase of tp with increasing gas flow rate. The evaluations of $ at different temperatures for uF= 0.7 u d showed a similar effect of temperature on the sphericity as can be deduced from Figure 2. However, one can hardly reconcile with a concept of varying sphericity of the particles. It seems to be more likely that rather the bed porosity or the constants in the Ergyn equation can change. The mean values of sphericity Ji evaluated in the temperature range 20-900 "C and the corresponding values of tp used for their computation are summarized in Table
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 321
,
60
I
i
0161
1
1
I
,
0
200
100
600
! 800 t
I
!
I
I
1 I
I
I
I 1000
PCI
Figure 2. Computed variation of bed voidage of corundum with temperature to maintain sphericity of particles unaffected by temperature, = 0.8; U F = 0 . 7 ~ ~
+
t
(TI
Figure 3. Comparison of measured and computed values of minimum fluidization velocities of corundum: curve 1, eq 5; curve 2, eq 11; curve 3, eq 12.
11. As it follows from this table, the mean values of sphericity decrease from corundum to coal ash in accordance with increasing irregularities in the shape of the particles. Somewhat surprising values of E and 4 were found for limestone particles. The values of$ are higher and those of t,.lower than the corresponding sphericities and bed porosities of the particles of lime. A definitive answer to the question of a possible effect of temperature or gas flow rate on the fixed or incipient bed voidage can be obtained by direct and accurate measurements of the bed height. Exact determination of the bed voidage as a function of temperature requires accurate data on the thermal cubic expansion of the bed material. Unfortunately, our experience suggests that the experimental data on the thermal expansion at high temperatures are not available in the literature. Minimum Fluidization Velocity Minimum fluidization velocity was measured in the reactor shown in Figure 1at temperatures ranging from 20 to 890 "C. After the fluid bed was heated to a desired, steady-state temperature, the air velocity was gradually reduced from a well-fluidized state to a static bed and the pressure drop Ap was measured. The minimum fluidization velocity was then determined from the plot of pressure drop vs. velocity of air in the same way as in previous work (Pata and Hartman, 1978). All measurements were made a t HIDK = 1. Repeated experiments on three levels of temperature 20, 300, and 700 " C showed good reproducibility of the measurements of the minimum fluidization velocity. The mean relative deviations of umffound at these conditions were less than 3 % . From potential sources of systematic error one should consider a radiation error of the thermocouple, variations of the atmospheric pressure, and possible changes in the particle size in the course of experiments due to abrasion. The radiation error of the thermocouple was not too significant since measured temperature differences between the reactor wall and the core of the particle bed did not exceed 35 " C in the work. When the bed was well fluidized the temperature was uniform throughout the whole bed. At uF = umf the temperature was almost uniform in the radial direction and differences between the top and bottom of the bed were less than 15 "C. Variations of the atmospheric pressure affected the rotameter readings by less than 2.5%. The particle abrasion was detected at the experiments with lime and limestone. During a complete set of measurements of umfin the whole temperature range, weight loss of the bed amounted to 2-3.5% and a slight shift in particle size was also detected. Rather less abrasion was found at the coal ash particles. These ash particles tended to agglomerate slightly at
I
'I
18
10
200
LOO
600
t ("c)
Figure 5. Comparison of measured and computed values of minimum fluidization velocities of limestone: curve 1, eq 14; curve 2, eq 5; curve 3, eq 10; d, = 1.125 mm.
temperatures above 800 "C. The corundum particles were stable and resistant against abrasion and agglomeration. The overall error in the determination of umfestimated by summing up the respective inaccuracies is as large as about 6% at low temperatures and about 9% at high temperatures. Measured minimum fluidization velocities for the tested materials are shown in Figures 3-9. As it follows from these figures, the values of umfdecrease with increasing temperature, particularly in the range 20-600 "C. In order to compare the experimental values with theoretical predictions, some equations available in the literature were employed. The equations and correlations used for computation of ulllf can be divided into two groups: (a) the equations requiring estimates of the minimum fluidization voidage and particle sphericity; these equations are presented in Table 111; (b) the relations and correlations in which neither the minimum fluidization voidage nor particle sphericity appear; such simplified equations are given in Table IV.
322 Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 Table 111. Equations Employed to Estimate the Minimum Fluidization Velocity range of no. of equation applicability eq
I 200
‘0
1
I
LOO
600
not limited
5
Ergun (1952)
n o t limited
6
Wen and Yu (1966)
R e m f < 20
7
Anderson (1961)
R e m f < 20
8
Kunii and Levenspiel (1969)
Remf < 10
9
Leva (1959)
I
BOO
author
‘Coo
0-
I
203
I
600 t
Figure 6. Comparison of measured and computed values of minimum fluidization velocities of lime: curve 1, eq 5; curve 2, eq 11; curve 3, eq 9; d, = 0.565 mm.
20
’
0
I
I
MO
iW
I 600
I
I
LOO
t i‘C)
800
1000
t (‘CI
8W
1000
(‘c1
Figure 8. Comparison of measured and computed values of minimum fluidization velocities of brown coal ash curve 1, eq 5; curve 2, eq 9; curve 3, eq 11; d, = 0.565 mm.
L-
0
I
1
200
400
003
603
lOm
t (“CI
Figure 7. Comparison of measured and computed values of minimum flui4ization velocities of lime: curve 1,eq 5;curve 2, eq 6; curve 3, eq 11;d, = 1.125 mm.
Figure 9. Comparison of measured and computed values of minimum fluidization velociiies of brown coal ash curve 1, eq 6; curve 2, eq 14;curve 3, eq 5; d, = 1.125 mm.
The results of comparison of the experimental minimum fluidization velocities and the predictions of the best six equations are also shown in Figures 3-9. It can be seen that the mean relative deviation 6 calculated for the whole temperature region exceeds 30% only in a few cases. From the general equations with emf and J/,the best agreement (6 13%) was found with the Ergun eq 5 at all materials employed. However, one has to realize that this result is affected to some extent by the fact that the values of $ were evaluated with the aid of the Ergun equation. The Anderson eq 7 was found inaccurate in predicting the magnitude of umfas well as its dependence on temperature. The Kunii eq 8 represents a special case of the Ergun eq 5 for application in the laminar region. In a
region of Red C 5 the predictions of eq 8 are, therefore, almost equal to those provided by the Ergun eq 5. The Leva eq 9 gives in the laminar region a similar dependence of umfon temperature, but the values are systematically lower than the predictions obtained from the Kunni eq 8 because of a different numerical constant. At low temperatures the Wen-Yu eq 6 predicts the values close to those provided by the Ergun equation. Higher estimates of umfare obtained, however, in a hightemperature region. From the simplified relations without emf and J/, the most reliable, particularly for “more spherical particles”, proved the recent correlation of Broadhurst and Becker (11). Except for ash, the equations of Todes (12), Wen-Yu (10)
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 323
Table IV. Simplified Equations Employed to Estimate the Minimum Fluidization Velocity range of no. equation applicability of eq
+
Ga = 1650Re,f
24.5Remfz
= 2.42
g ( P S - PF’P
x 105
Remf =
pF2
pF(pS -
Fpmf2
Remf =
[
1400
+
I]
0.85
-
PF)gdp
E]
+
not limited
10
Wen and Yu (1966)
not limited
11
Broadhurst and Becker (1975)
not limited
1 2 Todes (1957)
0.13
+37.7
Ga 5.22(Ga)”’
Ga
