PRESSURE DROP AND SPOUTABLE BED
HEIGHT IN SPOUTED BEDS M. A.
M A L E K ’ A N D
B E N J A M I N C.-Y.
L U
Department of Chemical Engineering, University o/ Ottawa, Ottawa 2, Ontario, Canada
Maximum and spouting pressure drops and maximum spoutable bed height in spouted beds have been 6-, and 9-inch columns, using wheat, millet, timothy seeds, polyethylene, polystyrene, Ottawa studied in 4-,, sand, and brlucite as b e d materials and air as the spouting medium. When the ratio of column diameter to bed height is greater than unity, maximum pressure drop is nearly equal to the b e d weight. The spouting pressure drop is independent of the column and the orifice diameters but dependent on air density, solid, and bed properties. At the maximum spoutable b e d height, it is about 8OY0 of the b e d weight. A simple correlation proposed for calculating maximum bed height relates it with column, orifice, and particle diameters, density and shape factor of particles, and air density.
spouted bed technique for bringing fluids in contact solid particles too coarse to fluidize has been developed b> hlathur and Gishler (74). who described the mechanism of spouting and presented the following empirical correlation for predicting the minimum air required for spouting. HE
Twith
determined by the methods reported previously (72). Experimental conditions of this investigation are listed in Table 11. Data collected from the 6-inch column for wheat-1 a t all bed heights are listed in Table I11 and some data for maximum bed weights in different columns are listed in Table IV. Pressure Drop
T h e relationship between pressure drop and air flow rate shown in Figure 1 is for spouting wheat-1 in the 6-inch column a t a bed height of 13 inches. Pressure drops were recorded Since particle shape, size distribution, and particularly surface roughness were not considered in the correlation, Equation 1 is valid only for uniform-sized particles with smooth surfaces. T h e spouted bed serves the same purpose for coarse particles as fluidization for fine particles. The technique has been used for drying wheat (2, 75) and wood chips (4, 5) and for low temperature carbonization ( 3 ) . Recently Peterson (76) installed the first commercial spouted bed dryers in Canada for drying peas. lentils, and flax. Although heat transfer and flow characteristics of a spouted bed have been studied and empirical correlations have been proposed ( 7 , 6, 8, 77--74, 7 7 ) , no generalized correlation is available for estimations of pressure drop, maximum spoutable bed height, and bed turnover, which are needed for the scale-up and design of commercial spouted bed reactors.
Table 1.
Materials Wheat-1 Wheat-2 Wheat-3 Wheat-4 Polyethylene Polystyrene Millet Timothy seeds Sand Brucite
Physical Properties of Solid Materials Absolute Particle Density of Shape Void Diameter, Materials, Factor, Fraction, Inch Lb./Cu. F t . X 6 0.145 88.5 1.16 0.417 0.079 87.5 1.18 0.500 0,060 88.0 1.o 0.495 0.047 87.3 1. o 0.492 0.133 57.6 1.18 0.358 0.109 66.0 1.19 0.444 0.078 73,4 1.o 0,398 0,035 70.9 1. o 0.450 0.031 166.0 1.o 0.460 0.039 166.5 1.10 0.455
Table II. Experimental
Data were obtained from three columns : two Plexiglas columns, 4-inch i.d. X: 4 feet and 6-inch i.d. X 6 feet, and one steel column, 9-inch i.d. X 7 feet with 1- X 4-inch groove fitted with Plexiglas for observing particles movement. All columns were fitted a t the bottom with a 60’ cone. In each column an o:rifice plate was placed between the cone and the air inlet pipe. A fine screen was placed just under the orifice plate for surrounding the bed materials. Compressed air used for spouting was filtered and dried before passing through the columns. A rotameter was used for measuring the flow rate through the 4-inch and 6-inch columns, and a calibrated orifice for the 9-inch column. Physical properties of t.he solids are listed in Table I and were
Column Dtameter, Inches 4
3i4,
3/4,
1, 11/+ 1 1 / 2
1, iii2,iii,
a/&
3/4,
1, l ’ / r
3/8,
1/2,
5/8
‘/n, ’/4, 1, I’/Z, 2, 21/2 1, 1’/4, I1/z, l a / , 3 / 8 , a/a, 1, 1 1 / 4 , 1 ‘ / 2 13/4, 2 I/%
9
Present address, Fuels Division, Mines and Technical Surveys Department, Government of Canada, Ottawa, Ontario, Canada. 1
Orifice Diameter, Inches 3/n, 3 / r , 1, 11/, 3/5, l / 2 ,
6
Experimental Conditions
3/r
3 / 6 , 1/2, I/%
3/4,
1, 1,
11i2,
2
11/4,
11/2
VOL. 4
NO. 1
Material Used Wheat-1 Wheat-2 Wheat-3 Millet Timothy Wheats-1 and -2 Wheats-3 and -4 Millet, polyethylene, and polystyrene Timothy, sand, and brucite Wheat-1 hlillet
JANUARY
1965
123
Table 111.
Data Collected from 6-Inch Column for Wheat-1 at Different Bed Heights
Orifice
Mass Veloczty, Lb.,f(Hr.) (Sq. F t . )
Gm
G,
323 443 582 706 900
287 359 51 5 700 1020
479 567 658 754 790 790 527 637 706 754 719 790 778 814 575 682 718 718 754 778 766 575 634 689 694 706 718 754
380 460 661 814 990 1220
Lb./Sq. Ft. ______ AP, APa 2.7 0.8 11.6 2.6 7.7 19.9 23.4 44.8 95.9 126.5 1.9 3.4 3.4 7.8 20.7 12.0 28.0 44.4 48.0 67.5 71.4 96.0 1.7 2.8 4.5 9.5 12.2 20,8 2 1.1 32.8 41 . O 56.6 51.6 69.0 62.5 81.2 67.5 88.4 2.6 3.4 6.9 9.9 15.1 21.5 24.7 33.6 34.4 46.0 45.5 58.5 55.6 70.4 3.2 4.3 7.8 9.9 16.8 23.4 25.8 35.2 36.6 46.4 46.5 59.5 51.6 69.5
508 623 730 804 898 910 958 982 647 754 814 862 886 898 910 670 785 833 886 886 946 934
IO
c L
P
Bed Weight . W: Lb., Sq. Ft. 5.8 11.7 23.4 46.9 122.0 5.9 11.7 23.4 46.9 70.3 93.8 5.9 11.7 23.5 35.2 58.7 70.4 82.1 87,9 5.9 11.7 23,4 35.2 46.9 58.6 70.3 5.9 11.7 22.4 35.2 46,9 58.6 64.5
Pressure Drop,
t-
Diameter, Di, Inches
Bed Height , L , Inches 1.7 2.9 5.5 10.9 2 8 . 0 Max. 1.7 2.9 5.5 10.9 16.5 2 1 . 9 Max. 1.7 2.9 5.5 8.3 13.9 16.5 19.3 2 0 . 5 Max. 1.7 2.9 5.5 8.3 11 . o 13.9 1 6 . 5 Max. 1.7 2.9 5.6 8.3 I1 .o 13.9 1 5 . 0 Max.
"5
1
l'/z
2
simultaneously a t 1 -, 5-, and 9-inch bed levels from the orifice. As the air flow rate was progressively increased, the initially packed bed passed through a transition region and ultimatelyachieved a fully developed spouted condition. Observations of the three stages may be described as follo\vs.
1. Packed Bed. Initially a t low air flow rates, the air passed through the bed without disturbing the solid particles. The pressure drop observed was nearly a linear function of the rate of air flow. 2. Transition Bed. When the air rate \vas increased to a certain value, a cavity (as observed in the semicircular column, with Plexiglas front) was formed a t the bottom of the bed, lifting the solid particles upward from the air entrance; when the rate of air was further increased, some of the particles started whirling inside the cavity and reoriented themselves. surrounding the internal spout in such a manner as to present the greatest resistance to the air flow and an arch \vas formed. The thickness of the arch was not uniform. T h e pressure drop was increased with increasing air f l o i ~rate and reached a maximum ,(A%Figure 1) on the verge of the bed expansion. With increasing air flo\v rate, the height of the internal spout increased, the bed expanded. and the pressure drop a t any section of the bed above the internal spout also ncreased. 3. Spouted Bed. When the air rate increased to a certain point, the particles moved LIP the bed through the spout and fell through the annulus like a fountain. T h e pressure drop decreased suddenly and remained constant during the spouting. nearly independent of further increase of the air flow rate, as shown in Figure 1. The particles folloived a regular pattern during spouting (Figure 2). moved u p in the spout and down in the annulus, and there was cross flow of solids from the annulus into the spout along the bed height. From visual observation a t the column Mall it appears that with the exception of both ends: the void fraction of the bed during spouting is nearly uniform along the bed height. However, the same condition may not prevail across the annulus. At the moment when maximum pressure drop occurs. the void fraction of the bed is not uniform all through the bed and the flow pattern of the fluid is also different because of the formation of the internal spout and the surrounding arch. .4s a result, difficulties were encountered in correlating the maxim u m and the spouting pressure drop data.
A
8 -
3 +
M
6 -
.c
.-c
-
1
a 4 -
e
n ?!
2 2
2 -
L1
, 0 M-. I
'
.
.
'
'
'
'
-
Air in
Figure 2. 124
l & E C PROCESS D E S I G N A N D D E V E L O P M E N T
Particles flow pattern in a spouted b e d
Madonna and Lama (77) correlated their pressure drop data using Leva's equation for fixed bed (70) AP
- =
L
K'
);,(
D,G
pz
Dp3
(1 - 6)63
Table IV.
(2) .\!aterials
and reported a n average K ' value of 1120 for the maximum pressure drop data. However, data obtained in this investigation and those available in the literature indicate that the I;' value varies for different column sizes. For example, for semicircular columns of 4- and 6-inch diameters and for full columns of 6- and 24-inch diameters, using wheat as the bed material, the I;' values for the maximum pressure drop data are approximatel). 750, 900, 1150, and 2500, respectively. Furthermore, in a given column, the K ' value decreases as the particles become smaller than 8 inch. An attempt was made in this investigation to correlate the approximate K' values as a function of the column diameter and the following equation \vas obtained
K'
=
470 D,O.j
(3)
However, \\.hen Equation 3 \vas substituted into Equation 2, the modified equation \vas still unsatisfactory. I t was therefore decided to see whether a relationship could be established between the maximum pressure drop and the \\eight of the bed. Pressure drop data were collected by increasing the bed hright with regular increment. T h e weight of the bed material was determined every time before it \vas poured into the column. In order to determine the pressure drop caused b) solids alone, pressure drop data were also collected for empty columns at different air flow rates. ;2 simple but satisfactor) relationship was obtained : the maxim u m pressure drop is equal to the bveight of the bed, as shown in Figure 3. ?'his relationship is valid in 4-, 6-:9-: and 12inch columns Lvhen the ratio of bed height to column diameter is greater than unity. In some cases, particularly at a high bed height, the maximum pressure drop may be about 10 to 15% greater than the b8:d weight, depending on the type and strength of arch formed surrounding the internal spout formed by the solid materials. ~l'he spouting pressure drop has been suggested to be t\vo thirds of the pressure drop due to the \\eight of the bed (77). Data obtained in this investigation and those available in the literature (9:77) indicate that the relation between the bed \\eight and the spouting pressure drop is rather complicated. A plot of the ratio of the spouting pressure drop to the bed weight against the bed height is shoivn in Figure 4 with orifice diameter as the parame1:er. I t is observed that at a low bed height (2 inches) with a small orifice ( 3 / ~ inch) the spouting pressure drop is only 18% of the bed \\eight, whereas for a 2-inch orifice the spouting pressure drop is about 55Yc of the bed byeight. I t is undtmtandable that with the increase of orifice diameter the spouting pressure drop will increase, and for the limiting condition-i.e., when the orifice diameter becomes too large to cause a spout, the bed changes to a fluidized state-it would be nearly equal to the weight of the bed. Lt'ith the increase of bed height the ratio of spouting pressur; drop to bed wcight increases and a t the maximum spoutable bed height it is about SOTc of the weight of the bed. From a plot of P , ' L LS. G in Figure 5 a linear relationship is obtained as observed in .I fixed or a moving bed. Vt'ithin the experimental range the spouting pressure drop is independent of the column and the orifice diameters, but dependent on particle, bed? and air properties. Happel (7) studied the flolv of gases through moving beds of particles. In his study, the entire bed moved with respect to the walls of the container, but the moving particles remained
\Theat-1
Data Collected from Different Columns at Maximum Bed Height Column Orijce .liar. Bed Pressure Diameter, Diameter, Height, Drop, AP?. D,, Inches D,. Inches L,, Inchei Lb.,Sq. b't. 48.7 8.8 7.8 6.6 20.0 17.8 16.5 9.9 6.7
46.8 31.4 26.7 21.5 5;. 6 54.9 51.2 29.2 20.8
\Theat-3
21 . 0 19.5 9.7
59.6 59.5 29.2
Millet
11.4 8.7
34.8 25.8
Timothy
28.0 26.0
-77
28.0 21.9 20.5 16.5 15.0
85.9 71.4 67.5 55.6 51.6
38.0 30.2 23.6 18.0 9.6 4.0
106.5 93.0 '1.3 53.6 29.1 10.6 128.0 93.1
4
"8 3: 4
1 l'/, LVheat-2
Sand \Theat-1
Millet
6
3
8
3/ 4
1
LVheat-2
1 1' 1' 13
4
2 4
\Vheat-3
\\-heat-4
6
1 11, 4 1' 2
Polyethylene
6
3~ 3'1
I
11/2 2
Polystyrene
6
"8 3! 4
1 l',? 2 Sand-1 Brucite Gravel-1 LVheat-1
6
s
31 3
6 6 9
3
14 !a
11
i 2
&/g
1'1 2
Millet
9
2
1', 2
35.0 29.5 6.4 6.0 40.5 34 5 7.5 6 3 56.0 14 0 8.0 23.0 16.0 14.5 11.4 6.5 25 5 25.0 22.0 18.0 8.0
:3.3
14.6 16.3 140,3 120.3 22.1 18.' 16.5 3.3.9 19.8 40 0 33. i 31 2 23.3 14.6 44.6 43,9 45.5 38 1- 2
26.0 20.0 27.1 -0.0 64 3 53.3 48.0 42,5 45.0
181 .O 176.3 148.2 151 . 0
in a fixed position relative to each other. l-arious sphcrical materials as \vel1 as granular materials of mised size \vcre investigated, ranging in average particle diameter from 9 . 1 5 to 0.182 inch. LVith n o movement of bed particle relative to each other, Happel's study is therefore similar to an invcsrigation of static fixed-bed pressure drops by espr&ng the su11erficial fluid velocity relative to the bed. T h e annulus in a spouted bed is a do\\n\vard moving bed of solids. Should there be no cross f l o ~ v , Happcl's rquation VOL. 4
NO. 1
JANUARY
1965
125
l.8
8 40 30
-
p!
h
20-
*h
(PI (L)
Present data Literature data
10
" _
6 -
5
AJ
'. 20 30 40 60 80 100 Bed Weight, Lb. /Sq.ft.
8 D
200
.5
4
.5
Figure 3. Relation between maximum pressure drop and bed weight Figure 6.
.6
.7
.a
.9
I
ad level = Bedheight L Effect of bed height on particle velocity
would be applicable for correlating the spouting pressure drop data. I n this investigation, a plot of
-
APS 2L p,v,z
gD, us. DPVmPf (1 - 6 ) (1 - 6)s P ~
was made but the correlation obtained was not satisfactory, indicating that the cross flow in a spouted bed is not negligible. Maximum Spoutable Bed Heighl 0.1 1
1
I
3
8 IO 20 304050 Bed Height L (in) Figure 4. Effect of orifice diameter on ratio of spouted pressure drop to bed weight
2
4 5 6
Maximum spoutable bed height, L,, is an important factor in spouted bed studies, for it is closely associated with bed turnover and with the gas-solid contact. Observations on the effect of bed height on bed turnover indicate that particle velocity a t the column wall increases with the increase of bed height (Figure 6), and for a fixed bed height, the particle velocity a t a given point at the wall increases with the decrease of the orifice diameter (Figure 7). To obtain the bed turnover (defined here as the volumetric flow of the solids from the particle velocity a t the wall), the cross-sectional area of the annulus and therefore the diameter of the spout must be known. Spout diameter may be estimated by Equation 4 (73)
D , = (0.115 log D ,
4 3
loo
200 xx) Moss Velocity G
500
8001000
2000
Lb./(M (sq. ft.)
Figure 5. Effect of mass velocity on spouted pressure drop See Figure 3 for legend
126
I & E C PROCESS D E S I G N A N D D E V E L O P M E N 1
- 0.031)"J
(4)
assuming that the particle velocity a t the wall and the downward movement of the bed a t the annulus were the same. Results of a bed turnover study will be reported later. Sufficient data were collected on the variables which may affect the maximum spoutable bed height: mass velocity of air, ratio of column diameter to particle diameter, ratio of orifice diameter to particle diameter, and particle shape and ,size. T h e effect of the ratio of orifice to particle diameter on the spoutability of wheat was investigated in a 6-inch column. As the ratio of orifice to particle diameter increases, the spoutable bed height decreases. When D f / D , becomes larger than 20, no spouting takes place. The whole bed is lifted upward,
u
E 60
la-
50
g40
3
1.6
'
-
B #
