I
TRIPURANENI GOPICHAND,l Department of Chemical Technology, Andhra University, Waltair, India
K. J. R. SARMA and M. NARASINGA RAO, Institute of Technology, Karagphur, India
Pneumatic Conveyance and Continuous Fluidization of Solids Two new correlations are derived for calculating pressure drop and dispersed solids density
1,
C O N T i S U O U S FLUIDIZATION processes, solids are frequently added to and removed from the fluidizer by pneumatic transport. T o design such transfer lines, prior calculations are needed for pressure drop and velocities required to keep the suspension flowing. When part of the reaction occurs in the riser itself. dispersed solids density must be predicted for any given velocity and solids feed rate. I n the work reported here, pressure drop and dispersed solids density measurements for pneumatic conveying of five materials (Table I) were made at different carrier velocities. Tables referred to hereafter and Figure 4, A and B, are available from the American Documentation Institute.
sure drop in the fluidizer is measured. Valves V3 and Vq admit air into the storage pipe to keep the solids freeflowing in cases of caking. During nor-
Sand Silica gel I
Experimental In the experimental setup (Figures 1 and 2). compressed air from a blower enters the expansion chamber E and then to a special nozzle, through a n orificemeter 0. S is the storage pipe for solids and L'2 is the valve used to control flow of solids into the gas stream a t the nozzle. T h e gas-solid suspension passes through the vertical riser having a n inside diameter of '/2 inch, and enters the fluidizer section. From the fluidizer the suspension enters the cyclone separator, C. Most of the solids get separated here and the air finally escapes through a bag filter B. A rotating Greek cross valve GV allows the solids to fall into the storage pipe and maintains a steady pressure difyerence between the two parts of the apparatus. Pressure taps PI and P, record the pressure drop over a length of 5 feet. Vg and V Sare two quick-operating valves, having a ','2 inch cylindrical opening throughout. These are fitted a t a distance of 25 inches and are used in determining the dispersed solids density. I n Figure 2, F1 and F Zare two wooden flanges in each of which provision is made to operate copperplate shutter S. T h e shutters facilitate the trapping of solids in the fluidizer to determine the bed density a t any time. P3 and P4 are pressure taps across which the pres-
Silica gel I1
Crushed wheat
Present address, Indian Institute of Technology, Madras, India
Table I.
Material Silica-alumina gel cracking catalyst
mal operation they are kept closed. After the storage pipe is charged with the material, air is admitted and the flow rate is controlled with valve VI.
Pneumatic Conveyance for Five Materials W a s Studied Gas Veloc., U, Ft./Sec. PP L.'t Pneumatic Fluidizing D,, @ D,, Ft. Lb./Cu. Ft. Ft./Sec. sect. sect. 26
0.0000875
55.793
126
0.000427
196
0.000663
97.13
3.28
392
0.001233
97.13
6.809
750
0.002222
90.168
194.7
0.063
3,232
11.47
20.1 23.73 26.13 28.26 81.504
1.256 1.482 1.632 1.765 5.094
61.968 81.568
3.873 5.098
76.48 92.77 102.2 116.8 133.1
4.78 5.798 6.389 7.3 8.32
Figure 1 . Apparatus for simultaneously studying pneumatic conveyance and continuous fluidization VOL. 51, NO. 12
DECEMBER 1959
1449
Results
For comparing operating variables, materials studied by Vogt and White and Hariu and Molstad also are given in Tables I1 and 111. T h e pressure drop ( A P ) measured over the 5 foot length of the pipe is plotted us. the solids feed rate in pounds per hour, and solids trapped (M,) are measured in grams. T h e data is smoothed by reading the solids feed rate and solids trapped for any value of pressure drop (Tables I V through V I ) . Using the Vogt and White method (75) plots are made of
TOP FLAWOL
A
1
(CY
Figure 2.
-r(
The fluidizer
Pressure drop across points P I and P z is noted, and then by adjusting valve V Z ,the solids are allowed to enter the gas stream. After half an hour of steady operation, the pressure drop across the orificemeter, and across PI, Pz,Pa, and P4 are noted. T h e solids circulation rate is measured, by diverting the solids flow, with valves V,, and VF2. Valves V S , V s , and the bottom and top shutters are simultaneously closed and the blower is stopped. T h e union U is opened and the solids trapped in the riser and fluidizer are collected and weighed.
1)os.
(gy [(:) ($31
on logarithmic coordinates for the materials, silica-alumina gel cracking catalyst, silica gel I, and silica gel 11, for which the data was taken a t different carrier gas velocities (Figure 3). A single line should result for each material but each carrier-gas velocity gives a different straight line. Thus, the Vogt and White correlations are not applicable for very low values of ( a - 1) and r used in the present investigation. The study of the continuous fluidization in situ restricts the range of ( a - 1) and r . A friction factor similar to that of Fanning, which is used for the flow of single phase fluids and calculated by the equation,
- -& 6'
-
A plot of 2f LIS. r . e , . V ' . Dc gives U , DGJ (Figure 4, A and B ) a single line for all the data of the present investigation and that of Hariu and Molstad (Figure 4 and Table V I I ) . T h e major deviation U
from the correlation occurs when 2
UQ
>
0.5. The calculation of for a given solidfeed rate and gas velocity is important from the standpoint of its prior calculation to obtain the pressure drop by the above correlation which takes place in the riser leg. Values of E , measurcd in the present work and those of Hariu and Molstad are correlated bv d o t t i n c es Z'S. r
(2)
3.l
I
I
on logarithmic coordi:
nates and a straight line is drawn through all the points (Figure 4, B ) . This correlation will also be useful in calculating the gas velocities required to obtain a dispersed solids density bvhen the solids feed rate is known. T h e range of variables covered by this correlation are given in Table V I I I .
Continuous Fluidization
is used to correlate the observed pressure drop with the other operating variables. T h e major variables that contribute to the variation of pressure drop are r and E,. T h e value of f for any particular material and fluid sy-stem in a riser depends upon r ,
and
U -'-.
=
The
'Q
'5
Silico-alumina gel
Silica gel I Silica gel I1
Ft./Sec. 20.10 23.73 26.13 28.26 61.968 81.568 76.48 92.77
102.2 116.8
Figure 3. The Vogt and White correlation i s not applicable at low values of andr
a-1
1450
values of 2 f are plotted against r'Es'-Ut LJQ for all the materials of the present investigation and the data of Hariu and Molstad in a 0.267-inch tube. A single smooth curve was obtained for each D, and the velocity merger in each material was good. A factor, Dt/D,, is incorporated to obtain a single line correlation for all the variables encountered in pneumatic conveying.
INDUSTRIAL AND ENGINEERING CHEMISTRY
When any mass of particles is kept in a suspended state in a continuous fluid medium, it represents a fluidized system. If such a system is maintained without the addition of any fresh supply of particles it is called batch fluidization. On the other hand in continuous fluidization, the feeding and removal of fluid and solid particles is continuous. T h e minimum velocity of fluid required to maintain batch fluidization conditions, in general is lower than the terminal free-falling velocity of a single particle, whereas in continuous fluidization it is higher. Most of the fluidized bed units used in industry are of the continuous type. There are two kinds of continuous fluidized bed unitsnamely, the up-flow and down-flow types. In continuous fluidization of solids, different bed densities can be maintained for a single carrier gas velocity, with different solid feed rates. T h e extreme limits are the lowest bed density that could be obtained with a very lean stream of gas with solids moving in cocurrent direction approximating pneumatic conveying, and the highest bed density that could be obtained, iust below the point beyond which any increase in
F L U I D I Z A T I O N OF S O L I D S solids feed rate will apparently plug the gas stream. Lewis, Gilliland, and Bauer (72) correlated their data by using a slip factor based on fictitious velocity of solids. Their plots of solids feed rate versus bed density give a picture of the steady and unsteady regions of fluidization. No attempt was made by them to correlate the pressure drop which occurs in fluidization. Results and Correlations. T h e experimental d a t a is given in Table IX. T h e pressure drop AP is plotted versus log pbd as the abscissa on semilog coordinates with velocity as parameter. Figure 5 clearly indicates the change of pressure d r o p with bed density as taking place in three zones-namely, pneumatic, fluidization, and plugging regions. T h e transition is gradual ; also as the velocity increases the first two zones merge, with the former having more dominance. I n some cases only two regions could be observed, because with the solidsfeeding device that was used, solids feed rates could not be increased to obtain the plugging zone a t the higher velocities. T h e trend for fluctuations of pressure d r o p a t the end of the second zone and also the fact that there will be a maximum limit for the solid carrying capacity of a fluid a t constant velocity support the visualization of three zones which were established experimentally in some cases. For a single material, transition from the first region to the second occurred approximately a t the same pressure d r o p for different velocities studied. This pressure drop, called the critical pressure drop, apparently depends on the physical properties of the solid and gas and the dimensions of the fluidizer. This critical pressure drop along with the di-
ameters of the particles and the different velocities studied is given in Table X. T h e bed density in continuous fluidization is a function of the solids feed rate a t any constant velocity of the fluidizing gas. T h e solids feed rate in pounds per hour is plotted LIS. the bed density in pounds per cubic feet on rectangular coordinates T o correlate the pressure drop encountered in fluidization, it is compared with the pressure drop that wculd have been observed had there been pneumatic conveyance under the same conditions. T h e pressure drop in pneumatic conveyance is calculated by
where f is
T h e pressure drop APc calculated by this method is fictitious, because a t the loiv gas velocities maintained in the fluidizer, pneumatic Conveyance is impossible. T h e pressure drop APACT measured in the fluidizer is always greater than the pressure drop calculated. T h e variation of this index APC:IAPACT with es a t a single carrier gas velocity for all the materials presents a n interesting feature (Figure 6 ) . This relationship also indicates the transition a t the same as that of a A p ~8s.log pad curve a t the same gas velocity, indicating that the dominance of pneumatic conveying shifts to the fluidization dominance with increasing bed densities. In initial stages, the ratio decreases (although
0.8
I
fluidizing conditions increase gradually until they become significant in the fluidization dominant region), because the pressure drop contribution caused by increasing fluidizing conditions is less than the increase of pressure drop in pure pneumatic conveying caused by the change in E,. However, in the fluidization-dominant region, contribution of pressure drop due to fluidization dominance is more than the increase of pressure drop due to pure pneumatic conveying Lvith change in E,. H a d the process been completely pneumatic, the ratio would have been unity for all values of E , . As conditions approach fluidization. the ratio increases away from unity and vice versa. Accordingly with the increase in velocity, the curves shift toward unity in the graphs. T h e deviation from this behavior for silica gel I1 a t the velocity of 4.78 feet per second is caused by the low velocity which is not suficient for fluidization and also due to the plugging observed visually even a t loiv bed densities. These observations only help in choosing a bed density which should be maintained in the fluidizer to achieve a definite purpose. T o predict the bed density for a known solids feed rate or the gas velocity. a n empirical relationship is developed. \$'hen E, is plotted us.
r
LT*Z
.
Po
on logarithmic coordinates
(Figure 7), the points can be well represented by a straight line. T h e errors in measurement of lo\v bed densities
+
-t
I
i
0.6 U
> -I
U
0.4
w 3 u)
w
0.2
2
I 0.4
0.2
06
0.0
1.0
2.0
3.0
4.0
Figure 5. Change in pressure drop with bed density occurs in three zones (silicaahmina gel) 1.256;
X
1.632;
0
'5
I'0 € 5 2
G
1.765
Ft./Sec.
6
Figure 6. Actual pressure drop in the fluidizer i s always greater than that calculated (silica gel II)
A 4.780; 1.482;
4
5.0 6.0
BED DENSITY (LB.ICFT.)
A
ij 5.798;
+
6.389;
7.300
Ft./Sec.
VOL. 51,
NO.
12
DECEMBER 1959
1451
A
Particle Dio Ft Silica-Alumcno Gel 0.000875
. .
Ft./Sec. I
W
I256 1.482 1.652 1.765
x
3.873 5 098
e
+
Silica Gel I $& 0 . 0 0 0 6 6 3 Silica Gel Ii C 0.001233
+A‘ .t * *
.
d
4.78 5.798 6.389 7.3
4
/*
+A.
A
A 1
= 19.25
A
.
m
ut
+
e A
m A A
+
+ *
8
.
8
A
4
d
A
+
A
/
0
m
4
hrticle Dia. Ft. Ft,/Sec Glass Spheres(l2) A 0.0016in. 0.0040in. 0 0.01i2in. Crushed Wheat 6 0.000427 8.32 Sand 0.002222 5.09
++
/
+ * I
I
0.1
0.2
+
I
I
0.4
I
I
I
0.6
I
I
0.8
+ I
I
I
I
2.0
1.0
I
4.0
I
I
6.0
I
I
8.0
I
I
io
I
I
1
20
*,E Pt, u9&
Figure 7.
For predicting bed density for a known solids feed rate, or gas velocity, an empirical relationship i s developed
being significant, there is some dispersion. T h e d a t a of Lewis, Gilliland, and Bauer (72) are also plotted on the same coordinates, which has shown a different
L
M,
= solids trapped in pneumatic
U,
=
€8
=
line probably due to the 2 ratio of their Dt
apparatus being different.
L
=
U,
=
pad
=
$ AP,,,
=
AP,
=
L,
=
Acknowledgment
T h e authors wish to express their sincere thanks to J. L. Franklin for kindly supplying the silica-alumina gel microcracking catalyst, and to the government of India for kindly giving grant in aid for this proiect.
=
Nomenclature cr
DL D,
= ratio of pressure drop of sus-
DT
= = =
r
=
Re
=
pg pp
= =
g Ap
=
1452
=
4
pension to that of the carrier alone riserdiameter particle diameter fluidizer diameter solids loading ratio, lb. solid per lb. gas gas Reynolds number gas density particle density acceleration from gravity pressure drop, lb. per sq. ft.
1% g. gas velocity, based o n empty cross section of the tube percentage volume occupied by the solids length of pneumatic line terminal velocity of a freefalling particle; calculated from the standard drag coefficient relationship bed density, lb. solid per unit volume of fluidizer functional relation actual pressure drop measured across the fluidizer fictitious pneumatic conveying pressure drop for the same length across which APAcT is measured length of fluidizer
References (1) Belden, D. H., Kassel, L. S., IND. ENG.CHEM.41, 1175 (1949). (2) Chateley, H., En_eineering 149, 230
(1940). (3) Cramp, W., Chem. 3 2nd. 44, 207 (1925). (4) Dallavalle, J. M., Heating, Piping Air Conditioning 4, 639 (September 1932). (5) Farber, L., IND. ENG. CHEM. 41, 1175 (1949).
INDUSTRIAL AND ENGINEERINGCHEMISTRY
(6) Gasterstadt, J., “Particle and Hydromechanics,” Delaware University, 1950. (7) Hariu, 0. H., Molstad, M. C., IND. ENC.CHEM.41, 1160 (1949). (8) Hillyar, Russ, J . Imp. Coll. Chem. Eng. SOC.5 , 115 (1949). (9) Hudson, W. G., Chern. & Met. Eng. 51, 147 (1944). (IO) Jennings, M., Engineering 150, 361 ( 1940). ( l i ) Lapidus, L., Elgin, J. C., A.I.Ch.E. Journal 3, No. 1 63 (1957). (12) Lewis, W. K., Gilliland, E. R., Bauer, W. C., IND.ENG.CHEM.41, 1104 (1949). (13) Pinkus, Oscar, J . Appl. Mechanics 19, 4, 425 (1952). (14) Selger, “Untersuchungen an Korenergiblasen und grundlagen ihrem Berchung,” Mannheim Wiehold Co., 1934. (15) Vogt, E. G., White, R. R., IND. EKG.CHEM.40, 1731 (1948). (16) Zenz, I?. A.,Ibid., 41, 2804 (1949). RECEIVED for review June 26, 1957 ACCEPTEDJune 2, 1958 A more detailed form of this paper (or extended version or material supplementary to this article) has been deposited as Document No. 6082 with the AD1 Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. A copy may be secured by citing the document number and by remitting $3.75 for photoprints or $2.00 for 35-mm microfilm. Advance payment is required. Make checks or money orders payable to Chief, Photoduplication Service, Library of Congress.
