Product Control in BowenType Spray Dryer PARTICLE SIZE AND PARTICLE DISTRIBUTION H. WA4LLMANAND H. A. BLYTH' American Cyanamid Co., Stamford, Conn.
c
T h e importance of control of the particle size and size OSTROL of particle the atomizer. Alternate hot distribution in spray drying is reflected in both the actual size and distribution in air inlets were provided for drying process and the end product itself. The heat input tangential addition of hot a spray dryer is important must be adjusted to dry the largest of the droplets, resultgases a t four points near the not only to the product but ing not only in poor thermal efficiency, but, more importop of the chamber. These also to the drying operation tant in the case of heat-sensitive materials, in decomposiwere normally closed. itself. Particle size and distion or curing of the smallest particles. Therefore, a Tangential louvers in the tribution of the product affect minimum deviation in particle size is desirable. In addiwall permitted addition of the density, ease of solubility, tion, particle size affects the density, solubility, flowability, atmospheric air. These were Bowability, and caking and and caking tendency of the product. normally open. Atmospheric. dusting tendencies of the All the available operational variables in a Bowen dryer air was also discharged from product. Often it is the were utilized to determine their relative effect on the the trailing edge Gf the floor relative importance of these particle size distribution of spray-dried sodium silicate. sweeper. The product and properties that dictates the The average particle size varied between extremes of 27 and drying medium left t h e choice of spray drying in 112 microns and the standard deviation between 11 and 40 chamber through an annular preference to other methods microns. Data were also obtained on the variation in bulk space in the chamber floo~, of drying. As required drying dcnsi t y thence through duct W O T ~to times vary approximately a pair of cyclones where the with the square of the particle product was separated from diameter, a wide particle size the drying medium. The latter was exhausted through a condistribution can result in failure to dry the larger particles on the stant volume fan which served as the prime mover for the drying one hand, or in decomposition or overcuring of the smaller medium. Residence time in the drying air is calculated to be in particles and poor thermal efficiency on the other. The ability the range of 10 to 15 seconds. to obtain rapid, uniform drying of heat-sensitive materials is The atomizing wheels used were of two types: A vane wheel one of the chief advantages of Spray drying. designated by Bowen as a V type is shown on the left in Figure 2 Although many qualitative generalizations have been published on the control of particle size and distribution, little or no quanti( 2 ) . With this wheel, the feed travels from the bottom plate Q U ~ , through the vanes which are shaped so that the outer edges are intative data have been presented on the relative magnitude of the clined in the direction of rotation. The second type, designated change in particle size that can be obtained with some of the as a C wheel, is an inverted dish shown on the right in Figure 2 available operating variables of a spray dryer. It should be emphasized a t the outset that the data presented herewith were ( 3 ) . Here the feed is supplied to the small center plate and is thrown onto the under side of the main dish from where it is fiobtained in one Bowen-type dryer using a rotary atomizer and nally atomized. one feed material. They are therefore of somewhat limited applicability. Marshall and Seltzer ( 5 ) , in a recent paper, disFEED MATERIAL cussed the characteristics of various types of spray dryers and the A feed material, which would have the following properties, was particles produced in them. sought for this study:
.
EQUIPMENT
The work reported here was done in a Bowen dryer, 7'/9 feet in diameter, As shoxn in Figure 1. this consisted of a chamber with a variable frequency atomizing motor mounted in the center of the ceiling. Products of combustion plus excess air were fed to the annular space around the atomizer and admitted through vanes which gave the gas a swirl countercurrent t o the rotation of 1
1. Fast drying so that a wide range of conditions could be used 2. Solubility in ~ a t e r 3. Stability a t high temperatures 4. High impact strength to prevent degradation in the duct work and collection syatem a. Xonhygroscopicity 6. Reasonable density to minimize cyclone losses
Only one material of reasonable cost, sodium silicate, appeared
Present addreas. American Cyanamid Co.. Karners, Linden S . J
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INDUSTRIAL AND ENGINEEqING CHEMISTRY
June 1951
this curve for 10-micron increments and the arithmetic average particle size, D,,in microns was calculated as follows:
R
(4)
CHAMBER
FAN
.
FURNACE
1481
where n = the weight fraction of each increment, and d = the diameter of the midpoint of each increment. Using the same data, the standard deviation, u, in microns was calculated as follows:
/ COLD AIR SWEEPER
Figure 1. Bowen 71/&00t
Spray Dryer
to fulfill these requirements; it was purchased as a 41% solution for use in these tests. Many materials give quite different particle characteristics under similar drying conditions-for example, solid particles, air-filled spheres, exploded particles, or fragments. This represents a further limitation of these data. Here again Marshall and Seltzer ( 5 ) give excellent treatment of this subject.
Obviously, the standard ,deviation is a measure of the particle size distribution-the smaller the deviation, the more uniform the particle size. The coefficient of variation, C , in per cent was calculated as follows:
c=&x
100
Several spot checks by microscopic examination indicated good agreement with this method of particle size determination.
PROCEDURE AND T E S T METHODS
Dryer runs were of 15-minute duration. The product from the first 10 minutes of each run was discarded. T h a t of the last 5 minutes was thoroughly blended and used for evaluation. Particle size determinations were made by a combination of sifting and sedimentation tests developed by the Cyanamid Laboratories for use on fluid cracking catalysts (1). This determination utilizes 100 (149 microns) and 200 (74 microns) U. S. standard sieves. A weighed sample of the -200 mesh fraction was then suspended in isopropyl alcohol, and the settling rate was determined by measuring the specific gravity of the mixture with a hydrometer at regular intervals. From each hydrometer reading, the particle diameter, D,in microns was calculated from the following equation based on Stokes’ law:
v = terminal velocity (cm./second) = h/t h = a function of the hydrometer reading (1) = settling time (seconds)
t
viscosity of isopropyl alcohol at 25 O C. = 0.0195 poise = particle density = 1.36 g./ml. for sodium silicate = density of isopropyl alcohol a t 25’ C. = 0.785 = acceleration due t o gravity = 980 cm./square second
P
=
PP
PI
o
Substituting the constants, Equation 1simplifies to
D = 250
47
(2)
The fraction finer than this diameter (or the fraction of material still in suspension) is given by: (3) where pt
hydrometer reading at time t density of isopropyl alcohol = initial density of sample-alcohol mixture (hydrometer reading a t xero time)
=
p1 = po
Spray Wheels
Apparent densities were determined by weighing the contents of a leveled 100-ml. container filled by pouring without tapping. A “dry apparent density” was calculated by deducting the moisture content. SOURCES OF ERROR
where 1.
Figure 2.
Further detail8 of this pr‘ocedure are given in reference ( I ) . From the combination of screen analysis and sedimentation data, a distribution curve (particle diameter versus per cent finer) was plotted on log probability paper. Readings were taken from
Two source8 of error should be mentioned: First, the recovery of the product from the spray dryer was in the range of 90%. The major part of the loss was presumed t o be 10 micrqns and smaller. The small samples collected in the 15-minute runs resulted in some variation in the recovery figures and therefore are not reported. Hollow spheres produced under certain conditions do not give true results in the sedimentation test. However, this error is minimized somewhat because the larger hollow spheres are for the most part collected in the screen fractions. An additional source of error in the sedimentation results lies in the dependence of the less-than-20-micron fraction determination on the reading of the hydrometer to four significant figures. This fraction is reported only to the nearest whole number. Four runs were made under identical operating conditions as a check on reproducibility. Except for the recovery cited, good agreement was obtained. These data are shown in runs 1 to 4, Table I. DATA AND RESULTS
Variations in feed, atomizing, and drying medium conditions were studied to determine their effect on the particle size, deviation, and apparent density. The major operating variables of the spray dryer were maintained at the following values except, of course, the variable under study:
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INDUSTRIAL AND ENGINEERING CHEMISTRY
IO0
90 80 70 c
260 +
50
+401074 microns
g40
30 20 10
0 '
Feed kats (Lb./Min.)
Figure 3.
4
Effect of Feed Rate
Feed rate, lb./niin. Feed solids, % Atomizing wheel (V type) diam., inches Atomizing \\-heel speed, r.p.m. Inlet gas temp., F.
2 25 41/2
10,000 500
Feed Rate. An increase in the feed rate from 1t o 4 poundsper minute caused an increase in the average particle size and apparent density with little change in the deviation (runs 5 to 7, Table I). The particle size distribution is shown as a bar chart in Figure 3. The increase in particle size obtained with increased feed rate does not necessarily indicate t h a t larger droplets were formed as they left the atomizing wheel. The increase could be due t o t h e greater probability of collision and subsequent coalescence of the
Figure 4.
Spray Dried Sodium Silicate
Vol. 43, No. 6
June 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
1483
TABLE 11. EFFECT OF FEED SOLIDS Run No. 9
8 Feed solids % 9 7 Feed viscolity cp. 12 Calculated pdrtiole size (based on increased solids), microns Actual average particle size, microns 44
.
243
15
10 408 125
60
71
60
88
1
droplets. The latter explanation is supported by photomicrographs of the product, Figure 4. Some further evidence of this coalescence has been obtained from photographs taken with a microsecond light source of water atomized from rotary atomizing wheels. Further work, however, will be needed to overcome the inherent difficulties of this means of obtaining particle size data in the dryer. Feed Solids. Increasing the feed solids concentration from 10 t o 40% increased the average particle size from 44 t o 88 microns as shown in runs 8 t o 10 and Figures 5 and 6. The particle size increase is not entirely due t o the larger volume occupied by the higher solids concentration. This was determined by calculating the increase in particle size t o be expected by utilizing the fact that the diameter is proportional t o the cube root of the volume. From the results shown in Table 11, the actual particle size is apparently influenced by the physical properties of the feed, mainly viscosity. Microscopic examination of the larger product indicated a greater proportion of hollow spheres which explains the drop in apparent density from 67 t o 58 grams per 100 ml. The moisture content of the products illustrates a n interesting point: As the solids concentration is increased, the reduced drying load tends to produce a drier product; a t the same time the larger particle size tends t o give a wetter product The combined effects gave a maximum moisture content with the 25y0solids feed. Feed Temperature. Increasing the feed temperature from 68 to 140 F. showed only a slight increase in particle size and decrease in density when using a 25% solids feed (runs 11 t o 13, Table I). However, with a 40% solids feed where the viscosity variation with temperature was greater, the average particle size increased from 68 to 83 microns (runs 14 t o 16). A decrease in particle size was expected because of improved atomization a t a lower viscosity. Since the increased particle size occurred in the larger particle size range, it was believed to be due to a skinning over or case hardening mechanism entrapping the vapor which subsequently expanded t o form large hollow spheres. The case hardening is further evidenced by the increase in moisture content of the product from 12.9 t o 14.5y0. It should be emphasized again that the case hardening encountered is charO
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1484
7060 -
50c
v)
g 40-
-40
E 30-
-30
20-
- 20
*d
10
10 I
10
I
20
I 40
I
30
Yo Solids Figure 5 .
70c
600
Atomizing Wheel Speed and Wheel Type. Runs were made a t 10,000, 15,000, and 20,000 r.p.m. using both the V and C type wheels. Runs 22 t o 27 show the expected inverse relationship with the average particle size decreasing from 65 t o 41 microns for the V tSpe and from 65 to 44 for the C type. At 25% solids the average particle size from the V- type wheel appears t o be approaching a minimum at 20,000 r.p.m. When the effect of speed on the particle size from the V wheel was rerun a t 40% solids (runs 28 t o 30), the minimum was not apparent. This is shown in Figure 7. Limitations of the atomizer motor speed prevented the determination of the existence of a similar minimum for t h e 40% solids feed a t a higher speed. Figure 8 shows two differences between the V and C wheels: The V wheel gives a higher apparent density, and the particle size distribution, as measured by the standard deviation, decreases n ith increasing wheel speed with the V Ivheel whereas it remains practically constant with the C wheel. An additional difference lies in the increased power load on the V type wheel. At 20,000 r.p.m. and 2 pounds of feed per minute the 41/2-in~hV wheel requires 1000 watts as compared t o 675 watts for the C type wheel of similar size a t the samr spwd
Effect of Feed Solids
acteristic of certain types of feed materials. Wholly different results could be obtained with other materials. Surface Tension. The surface tension of the feed solution was reduced from 52 to 31 dynes per em. by the addition of 1%Berosol OT (runs 17 and 18, Table I), Contrary to expectations, the average particle size increased from 57 to 67 microns, and, a t the same time, the apparent density dropped from 69 to -16 grams per 100 ml. Apparently, the addition of a wetting agent caused the formation of a high proportion of large hollow spheres as the liquid was sprayed. There was also an increase in the finer material causing a wider distribution. However, even though the product was less uniform it was drier, indicating that this combination of large, holloly spheres and small spheres gives an increased surface area resulting in a shorter drying time. The moisture content of l2.O'% obtained with the reduced surface tension was lomrr than any of seven comparable runs without Aerosol OT 'ii hich averaged 13.4%. Viscosity. To determine the effect of viscosity independent of solids, 0.4 and 1% carboxymethylcellulose were added to a 10% feed solution (runs 19 to 21). The increase in viscosity from 11 t o 32 cp. was lvithout significant effect on the particle size or distribution.
l90 o0l
Vol. 43, No. 6
1"
->74
microns
TABLE 111.
CORRELA4TIONO F PARTICLE 8IZB WITFI
CENTRIFUGAL FORCE Run No. 38 10.2 5 12,000 280 262 52
37 Gravitv factor. C X 10-8 Wheel "diameter, inches Wheel rotational speed. r.p.tn Power load, watts Peripheral speed, ft.isec. Average particle size, microns
TARLE I\;.
a.5
6 10,000 280 262 fit
39 11.3 4 '/? 13,300
280 262 +6
S~XIIARY OF EFFECT ow VARIABLES Particle Sizea Direct (6) Direct ( 2 ) Direct (8) Inverse ( 5 ) None Inverse (1) Inverse (3) Direct (4) Inverse (7)
Variable Feed rate Feed solids Feed temperature Feed surface tension Feed viscosity Atomizer rotational speed Atomizer wheel diam. Drying gas temp. Drying gas velocity
Apparent Densitya Direct (4) Inverse (6) Inverse (7) Direct (3) Direct (5) Inverse (2) None Inverse (1) None
a Figures in parentheses indicate relative magnitude of effert of each variable on properties.
1
I O 0I
microns
I IO
2:
40
Effect of Feed Solids
I
15,000
I
20,000
Wheel Speed R.F?M.
S
Figure 6.
I
10,000
Figure 7 .
Particle Size vs. Speed-V
Wheel
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1951
90-
-60
80-
-50
g
70 -
-40
8
60-
-30 Size
50c 0 240-
E eEn
u)
.-e
I
10,000 Figure 8.
X
I
I
15,000
I
20pOO
Atomizing Wheels--C Type
RPM.
Type us. V
1485
gas by closing the cold air louvers in the side of the chambe and by opening the four side hot gas inlets. Entering Gas Velocity. The velocity of the drying gas entering the annulus around the atomizing wheel was increased from 24 t o 35 feet per second by increasing the exhaust fan speed (runs 43 and 44, Table I). No appreciable effect was noted. A further increase in the incoming gas velocity was accomplished by closing the cold air louvers (runs 45 to 47). At 41 feet per second inlet velocity a slight decrease in particle size was noted. SUMMARY O F EFFECT OF VARIABLES
The effect of the several variables on the particle size and apparent density is summarized in Table IV. Here the relationship between the variable and the property is shown as direct, inverse, or indeterminate. For most of the operating variables studied, the standard deviation varied directly with the arithmetic average particle size. For this reason the deviation should not be considered alone and has been omitted from Table IV. Several conclusions can be summarized: 1. At higher atomizer speeds a V-type wheel gave a smaller deviation than a C-type wheel 2. Reducing the solids gave a smaller deviation than increasing the atomizer speed 3. Reduced surface tension gave an increased deviation
Wheel Diameter. Variations in wheel diameter of 4 l / 2 , 5, and 6 inches using both the C and V type wheels showed similar Two runs were made with the object of obtaining maximum and results (runs 31 to 36). The particle size and distributions were minimum particle sizes from sodium silicate (runs 48 and 49, inversely proportional t o wheel diameter whereas the density held Table I). Table V shows that a maximum average particle size fairly constant. In order to determine the relative effects of wheel speed and wheel diameter, the average particle size was plotted against the peripheral speed (Figure 9). The average particle size was also TABLE lr. CONDITIONS FOR MAXIMUM AND MINIMUM plotted against a centrifugal factor-that is, number of times PARTICLE SIZE gravity. The correlation obtained in the latter case indicates Maximum Minimum that the particle size is an inverse function of the gravity factor Run No. 48 49 4 1 Feed rate, lb./min. and not the peripheral speed. The series of runs shown in Table 40 10 Feed solids, % I (runs 37 t o 39) and Table 111were made to confirm this conclu50 25 Feed temp.. C. Atomizer-wheel type 6V 4'/P c sion. Atomizer wheel speed, r . p . m 20,000 10,000 Drying gas inlet tem F 680 400 These data show that although constant peripheral speed was 26 31 H o t gas velocity, ft.,kc. used the particle size is not constant but decreases with increasing 10.2 10.5 Product moisture, Product apparent E n s i t y , g./lOO i d . 30 57 centrifugal force, This is contrary t o the accepted belief that Particle size, yo finer than 149 microns 79.3'L 9 8 .7 peripheral speed is controlling. A possible explanation for this 20.4 74 microns 98.5 discrepancy lies in the mechanism of atomization-that is, disin40 microns 1 89 20 microns 38 tegration by direct-drop formation, ligament formation, or film
