Ind. Eng. Chem. Res. 1988,27, 1493-1496 Howat, C. S.; Swift, G . W. Fluid Phase Equilib. 1983,14,289-301. Howat, C. S.; Swift, G. W. J. Chem. Eng. Data 1985,30, 281-285. Hull, H. S.; Reid, A. F.; Turnbull, A. G. Aust. J. Chem. 1965, 18,
1493
Schmid, H.; Kubassa, F.; Herdy, R. Monat. Chem. (Wien) 1948, 79, 430-438.
Tsuboka, T.; Katayama, T. J. Chem. Eng. Jpn. 1975,8(3), 181-187.
549-552.
Received for review December 16, 1985 Revised manuscript received March 18, 1988 Accepted April 19, 1988
Miller, D. G. Ind. Eng. Chem. 1964, 56(3), 46-57. Shanker, G.; Howat, C. S., 111; Torres, R. R.; Swift, G . W. Fluid Phase Equilib. 1981, 5, 305-321.
Minimum Fluidization Velocities of Wet Coal Particles Jaroslav Pata,* Milan h s k j l , Miloslav Hartman, and VBclav Veseljl Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 165 02 Prague, Czechoslovakia
Minimum fluidization velocities of wet lignite coal beds were measured. The experiments were carried out in a n 8.5 X m i.d. column with particles of average size ranging from 0.15 X t o 4.25 X m, using air as a fluidizing gas. The experimental results were compared to the values predicted by commonly used correlations. On the basis of data obtained, a simple correction factor was derived. T h e factor makes it possible t o calculate the minimum fluidization velocities of wet coal from equations, originally developed for dry beds if relative moisture content of the bed and average particle diameter are known. Good agreement with experimental data has been found in all cases. An experimental fluidized bed boiler of 20-MW heat output was usually supplied with wet lignite coal which contained sometimes as much as 35% (by weight) water. Owing to the fact that coal transportation to the boiler was based also on the fluidized bed principle, a necessity of knowledge of minimum fluidization arose. A number of correlations has been published to predict minimum fluidization velocities of dry materials (e.g., Ergun (1952), Leva (1959),Goroshko et al. (1958), Borodulya et al. (1982), Broadhurst and Becker (1975)). Unfortunately all these equations have shown discrepancy between the predicted and measured velocities if applied to beds of wet material. That is why the relation between the minimum fluidization velocity and relative moisture content of lignite coal bed has been investigated. At first the experimental minimum fluidization velocities of various wet monodisperse beds of “Chabafovice” lignite coal were determined. These experimental data were starting points for derivation of a simple formula which makes it possible to convert results of the published equations that can be used for prediction of “wet” minimum fluidization velocities of coal if average particle diameter and relative moisture content of the bed are known. The present paper is a part of our study on materials which are used at fluidized bed combustion and simultaneous desulfurization (Pata and Hartman, 1978, 1980; Svoboda and Hartman, 1981a,b; Svoboda et al., 1983, 1984).
Experimental Section The experimental setup was described in detail previously (Pata and Hartman, 1978). The apparatus consisted of a glass column of inside diameter of 8.5 X m and height of 0.9 m. The column was equipped with a plate distributor of 1% free area. The pressure drop across the bed was measured by a U manometer, and the velocity of air was checked by a rotameter. Experiments were carried out using air as a fluidizing gas a t a temperature of 295 f 3 K. The lignite coal of ps = 1.61 X lo3 kg m-3 a t 16.6-37.5% w t moisture content from the Chabafovice open pit mine was crushed and sieved. Eight differently sized fractions of coal, 0.05-0.25, 0.25-0.50, 0.50-0.63, 0.63-0.80,0.80-1.0, 1.0-1.6, 1.6-2.8, 3.5-5.0 mm, were used 0888-5885/88/262~-1493$01.50/0
Table I. Physical Properties of Lignite Coal Particles fraction 1 2 3 4 5 6 7 8 103d,, m 0.15 0.38 0.57 0.72 0.90 1.30 2.20 4.25 0.65 0.64 0.63 0.62 0.61 0.60 0.58 0.57 0.23 0.33 0.37 0.37 0.37 0.35 0.32 0.21
7
in these experiments. The properties of coal particles are summarized in Table I. The same quantity of each coal fraction was put into a glass-stoppered bottle, and a measured quantity of water was gradually added to each bottle after the previous measurement of Umfhad been concluded. Then the bottles were closed tightly, and their contents were regularly mixed for 2-day intervals. After 14-20 days, an equilibrium state at room temperature was achieved, and the next minimum fluidization velocity was measured. The moisture content of the fractions was computed from the known quantity of coal and water addition. A t moisture contents of 16.6%, 27.5%, 37.5% by weight, the calculated concentrations were checked experimentally by distillation of wet coal samples with toluene. The computed and experimental results agreed within f5%. The minimum fluidization velocities were determined from the plots of superficial velocity vs pressure drop on log-log paper. The velocity corresponding to the point of intersection of the two differently sloping portions of the plot was taken as the minimum fluidization velocity. Measurement started at a well fluidized state; afterwards air velocity was gradually reduced to zero. The ratio h l D for the fixed bed was within 1-2. For each minimum fluidization velocity, the height of the bed hmfwas simultaneously recorded. The part of the pressure drop vs velocity plot under minimum fluidization velocity (providing R e < 10) was used for calculation of particle sphericity.
Results and Discussion Density of Coal Particles. Particle densities were measured by the methanol displacement method. The true density of coal was calculated by Ps = w,
W,PMe
+ w,- wb
0 1988 American Chemical Society
(1)
1494
Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988
The densities of the wetted fractions were calculated according to the relation
Minimum Fluidization Voidage. The minimum fluidization voidage was determined from the known weight w, and height hmfof the bed which were measured under the minimum fluidization velocity conditions. The values of emf were calculated by 4u;, (3)
I
:
0 16
I
I
24
32
__1
P,
Oh
by w t
4C
Figure 1. Comparison of measured and calculated minimum fluidm. ization velocities: original equations, d, = 0.38 X
It
=I
1 5 0 p ~ h (-l t)'UG
0'5
(4)
Apd,2gt3
40 16
24
32
P,% bY w t
40
Figure 2. Comparison of measured and calculated minimum fluidm. ization velocities: original equations, d, = 1.3 X Table 111. Maximum Relative Deviations of Uncorrected Equations max re1 dev of eq, 70 io3&, m eq 5" eq 6b eq 7c eq ad 0.38 -67 to +40 -62 to +60 -31 to +180 -54 to +80 0.57 -38 to +10 -48 to 0 -40 to +13 -9 t o +60 -30 to +12 0.72 -31 to +7 -40 to 0 -3 to +60 0.90 -20 to +3 -30 to +3 +8 to +50 -21 to +10 1.30 -2 to +6 -5 to +1 +28 to +42 +5 to +15 2.20 -2 to +8 +7 to +15 +29 to +55 +11 t o +18 4.25 -8 to +4 +40 to +60 +54 to +65 +32 to +42 OErgun, 1952. bGoroshko et al., 1958. 'Borodulya et al., 1982. dBroadhurst and Becker, 1975.
is affected, to some extent, by the fact that the values of $ were evaluated with the aid of Ergun's equation. In1 - emf Ar = 150-Remf
+1
Re,?
5
Ergun, 1952
Ar Remf = 1400 + 5.22ArO.j
6
Goroshko et al., 1958
Remr = (266 + 0.037Ar)05 - 16
7
Borodulya et al., 1982
Remf2= .4r1 1 [2.72 X 1 0 5 ( p , / p c ) 0l3 +
8
Broadhurst and Becker, 1975
. 7 5 3
em1
.37 7 4
85
1
emf
creasing moisture content increases not only deviation of calculated and experimental values but predictions of individual equations start to be different. Conclusions may be drawn that for beds of mean particle diameter ranging from 1.3 X to 4.25 X m the results calculated from eq 5 show very good agreement with the experimental data. The maximum error of this equation is less than f 8 % within the measured total moisture content of 16.6-38.0% by weight. When simplified equations are employed (which avoid the problem of sphericity and porosity de-
Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988 1495 U,f.lOZ
m Is
36
t
&-02
I
i i
I
MOnr,.sg
.D VALUES
WnnCL"
0 €0.6 17-
'-1 6
Figure 3. Comparison of measured and calculated minimum fluidization velocities: original and corrected equations, d, = 0.38 X m.
termination), reasonable agreement is obtained with eq 6 and 8. Their maximum error varies from -5% to +60%. The second group, in which the presence of moisture causes cohesive and adhesive forces to be more pronounced, includes beds of particles from 0.15 X to 0.90 x m. The minimum fluidization velocity increases considerably with moisture content above 25 % by weight. This increase is more significant for higher moisture contents and smaller particles. The published equations are incapable of predicting Ud reliably under these conditions and cannot be, therefore, used directly, as is shown in Figure 1. All equations fit the real behavior better it the computed results are subsequently multiplied by the correction factor f,defined as f,= U m , (e@) / um, (calcd) (9) The factor was calculated based on the experimental velocities and velocities calculated by eq 5. The factor f, strongly depends on water content, particularly above 25% by weight. Increasing the solid particle diameter decreases the factor value and its effect may be neglected if d, I1.3 X m. The influences of water content and particle diameter are correlated by a simple relation fn
d, = 1.182dp - k b
-
~0.50
I
24
-16
!
0
I
CORRECTED VALUES
I I
I
24
32 pv% by wt.
40
Figure 4. Comparison of measured and calculated minimum fluidization velocities: original and corrected equations, d, = 0.57 X m. fU ,
.lo2
m/s
I
' I
1
0.'7
IV
16
24
32
40 PVo/oby w t
Figure 5. Comparison of measured and calculated minimum fluidization velocities: original and corrected equations, d, = 0.72 X m. Umf.
mis
where the constant k b represents the effect of moisture and can be correlated by PV
The numerical constants in eq 10 and 11 were determined by a least-squares procedure. Figures 3-6 show predictions multiplied by the factor f,. These corrected curves are much steeper than the original ones calculated from eq 5-8, and also agreement between experimental and calculated values is much better for water contents above 25% by weight. Only a bed of the finest particles of d, = 0.15 X m stopped fluidization at a water content of pv = 22.9% by weight. Corrected curves of the other particle sizes follow the trend of experimental values well, and their relative deviations are listed in Table IV. The errors are significantly less than those in Table 111. Equation 5 yields the best results again. Its maximum error varies from -16% to +E%.The maximum deviation
36t
1 //
/
P,
O/*
by w t
Figure 6. Comparison of measured and calculated minimum fluidization velocities: original and corrected equations, d, = 0.9 X m.
1496 Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988 Table IV. Maximum Relative Deviations of Corrected Eauations max re1 dev of eq, 70 i03d,, m ea 5” eq 6* eq 7 c eq 8d +8 to +15 -7 to +17 +65 to +lo0 +5 to +25 0.38 -5 to 0 -3 to 0 -12 to +17 +50 to +55 0.57 0.72 -7 to -2 -16 to -14 +37 to +44 -7 to 0 -23 to +4 -16 to -6 -15 to -12 +29 to +31 0.90 +3 to +14 -7 to +3 +29 to +43 1.30 -4 to +5 +2 to +15 -10 to +2 -1 to +11 +18 to +33 2.20 +19 to +28 -16 to -7 +28 to +28 +29 to +33 4.25 Ereun. 1952. * Goroshko et al.. 1958. dBroaihurst and Becker, 1975. I
Borodulva et al.. 1982. I
40
of the correction factor is recommended. The finest particles of diameter equal to 0.15 X m stick together and do not fluidize if the water content is above 22.9% by weight. Nomenclature D = inside column diameter, m d e = effective particle diameter, m d p = mean particle diameter, m f, = correction factor defined by eq 9 g = gravitational acceleration, m/s h = height of the bed, m hmf= height of the bed at minimum fluidization, m k b = quantity defined by eq 11 py = total moisture content, % by weight A, = pressure drop, Pa Umf= superficial gas velocity required for minimum fluidization, m/s UG = superficial gas velocity, m/s w, = weight of coal sample, kg w b = weight of pycnometer and coal sample filled out with methanol, kg w, = weight of pycnometer filled out with methanol, kg Dimensionless Groups
‘ U3
Re =
1
UGdpPG/wG PG/pG
Remf = umd Ar = gd,”PC&S
z 0
1
5
10 dp
10km
Figure 7. General behavior of bed of wet lignite coal particles.
of eq 6 varies from -16% to +17% and that of eq 8 changes from -23% to +%YO. Equation 7 yields the maximum deviation between +29% and 100%. The computed U,, for particles larger than 1.3 X loT3 m may also be multiplied by the correction factor, but the improvement is not distinctive. We believe that the results obtained by the visual observation of fluidized beds can be useful, and that is why we present them in Figure 7. Conclusions Minimum fluidization velocities of coal particles vary with moisture content. The finer the particles in the bed, the bigger are the deviations of calculated and experimental values. Equations 5-8 can be used for direct calculations of minimum fluidization velocities (a) if total water content is less than 25% by weight for all investigated particle sizes and (b) if total water content is higher than 25% by weight for particles equal or bigger than 1.3 X m only. The best agreement of experimental and calculated values yields eq 5 with values of emf and $ which are summarized in Table I. The simplified equations, eq 6 and 8, provide satisfactory results. Equations 5-8 cannot be used directly for U , calculation if the total moisture content is higher than 25% by weight and the particle diameter is simultaneously equal or less than 1.3 X m. Then the use
- pG)/wG2
Greek Symbols t = bed voidage fraction tmf = bed voidage fraction at minimum fluidization 4 = particle sphericity p G = gas density, kg/m3 p s = solid density, kg/m3 p S l = solid density at moisture content 1, kg/m3 p s 2 = solid density at moisture content 2, kg/m3 p M e = methanol density, kg/m3 pG = gas viscosity, Pa-s
Literature Cited Borodulya, V. A.; Ganzha, V. L.; Kovensky, V. I. “Gidrodynamika i Teploobmen v Psevdoozhizhenom Sloye pod Davleniem”. (in Russian) Izd. Nauka Techn., Minsk 1982, 1. Broadhurst, T. E.; Becker, C. D. AIChE J. 1975,21, 238. Ergun, S. Chem. Eng. Prog. 1952, 48, 89. Goroshko, V. D.; Rozenbaum, P. B.; Todes, D. M. Izv. VUZOVNeft. Gas (in Russian) 1958, 1, 125. Leva, M. Fluidization; McGraw-Hill: New York, 1959. Pata, J.; Hartman, M. Ind. Eng. Chem. Process Des. Dev. 1978,17, 231.
Pata, J.;Hartman, M. Ind. Eng. Chem. Process Des. Dev. 1980,19, 98.
Svoboda, K.; Hartman, M. Ind. Eng. Chem. Process Des. Dev. 1981a, 20, 319.
Svoboda, K.; Hartman, M. AIChE J. 1981b, 27, 866. Svoboda, K.; Cermlk, J.; Hartman, M.; DrahoB, J.; Seluckjl, K. Ind. Eng. Chem.-Process Des. Dev. 1983, 22, 514. Svoboda, K.; Cermlk, J.; Hartman, M.; DrahoB, J.; Seluckjl, K. AIChE J. 1984, 30, 513. Received for review May 6, 1986 Revised manuscript received February 22, 1988 Accepted March 10, 1988
