Literature Cited Davis' H'
G'' Farre'''
T' J'' lnd' Eng'
Process Des'
17'
(1973). Froment, G, F , , pijcke, H,, ~
~ G,, (-hem, ~
~ sei,, 13, h 173, 180 ~
11961)
Snow, R. H., Schutt, H. C., Chem. Eng. Prog., 53, 133 (1957). Sundaram, M., Froment, G. F., to be published, 1976. Van Damme, P.,Narayanan. S., Froment, G. F., A./.Ch.E. J., 21,1065(1975). ~Zdonik, S. ~ G., Green, , E. J., Hallee, L. P., "Manufacturing Ethylene", The Petroleum Publishing Company, Tulsa, Okla., 1970.
Mey& P S., Watson, K. M., Nat. Pet. News, R 388 (1946). Shah, M. J., lnd. Eng. Chem., 59 (5), 70 (1967). Snow, R. H., Peck, R. E., Von Fredersdorff, C. G., A.l.Ch E. J., 5, 304 (1959)
Receiued for reuiew June 30, 1975 Accepted April 6,1976
Granulation of Ammonium Sulfate Fertilizer in a Spouted Bed 0. Uemaki Department of Applied Chemistry, Hokkaido University, Sapporo, Japan
K. B. Mathur" Department of Chemical Engineering, University of British Columbia, Vancouver, British Columbia, Canada V6T 1 W5
Ammonium sulfate was granulated continuously in a 15 cm diameter X 46 cm deep spouted bed by atomizing a 40% solution of the salt into a bed of seed particles spouted with hot (170-200 ' C ) air. The granules produced, ranging in size from 1 to 4 mm, had a uniform layered structure. Growth rate data are interpreted in terms of a simple theory based on mass and number balance which takes into account particle growth by layering, generation of fresh nuclei by fracture, and demolition of bed particles by attrition. The fracture mechanism was found to dominate at high solution feed rates, causing the mean particle size to decrease rather than increase during the run. Evaporation rates varied between 0.025 and 0.049 kg of water/kg of air, bulk bed temperatures ranged from 70 to 95 OC,and fine dust elutriated out of the bed amounted to 10-30% of feed ammonium sulfate.
Introduction There are two main features of a spouted bed which make it attractive for granulation of fertilizers and other such materials-orderly cyclic movement of particles and intimate gas-solids contact. The production of granular solids is achieved by starting with a bed of seed granules spouted with hot air and building up these granules by spraying into the bed a solution, melt, or suspension of the product material. Compared to fluidized bed granulation, the spouted bed enables granules of a larger size to be made and yields a product with a more homogeneous layered structure, free of agglomerates. While the feasibility of spouted bed granulation has been amply demonstrated (Berquin, 1961, 1966; Nichols, 1966; Romankov and Rashkovskaya, 1968; Mathur and Epstein, 1974), and industrial units are in operation (Romankov and Rashkovskaya, 1968; Berquin, 1973; Mathur and Epstein, 1974), no investigation into the performance of a spouted bed granulator has so far been reported. The work described here was carried out with the objective of gaining some insight into the kinetics of particle growth in spouted bed granulation by studying the effect on growth rate of some of the operating variables. Experimental Section Apparatus. The equipment used is shown schematically in Figure la. The granulator consisted of a 15.2 cm diameter X 1 m high stainless steel column with a 60" included angle conical base. Spouting air entered the bed through a 1.9-cm diameter orifice, after passing through a rotameter and an electrical heater. The atomizer used for injecting the feed 504
Ind. Eng. Chem., Process Des. Dev., Vol. 15,No. 4,1976
solution into the bed is shown in Figure lb. It was installed a t the center of the orifice and was operated with an independent unheated air supply. Ammonium sulfate solution (-40% (NH4)2S04) was stored in a 15.2 cm diameter x 1m high graduated Pyrex glass column. A Chromolax nickel-plated immersion heater was placed a t the bottom of the column, and the temperature of the solution was regulated with a thermostat. The solution was pumped to the atomizer which sprayed it as a fine mist into the base of the bed. Thermocouples were used to measure air temperatures at the bed inlet and exit and a t several other locations along the height of the bed. Procedure. A known quantity (-6 kg) of closely sized particles of commercial ammonium sulfate fertilizer was placed in the column to serve as seed granules. The bed of seeds (46 cm deep) was spouted with hot air a t approximately 200 "C, and the supply of (NH&S04 solution (maintained a t -70 "C) was started after allowing sufficient time (10-15 min) for preheating the bed to its expected temperature during the run. Solids were withdrawn from the bed periodically (at 5-10-min intervals) through an outlet in the cone (shown in Figure l a ) so as to maintain an approximately constant bed height (between 45.5 and 46.5 cm), and samples of the material withdrawn were screened to determine their size distribution. No solids were recycled back to the granulator. The duration of the runs was between 5 and 9.5 h. In order to maintain a constant intensity of the spouting action, it was necessary to gradually increase the spouting air flow rate as the bed particles grew larger, the total increase in a typical run being of the order of 40%. Thus, granulation occurred in the unsteady state but as a continuous process with steady feed and withdrawal of (NH4)$304.
E = (newp
+ S+
01, kg/hr
t
Product
W = NbwP,kg
L , kg/hr
solulton
LxL, k g / h r
("&SO,
Figure 2. Ammonium sulfate mass balance.
rnrn ITm
being the number of particles in the bed a t any instant. Solving eq 3 for n p and substituting the result in eq 2, we get
Nb
dNb L X L= ( n - ne)wp- - w p d0
+ newp + S + D
(4)
+ + D) in the above equation, we
Substituting E for (newp S get
( L x I-~ E ) d8 = ( n - ne)ulpd0 - (dNb)w,
An expression for dNb in terms of d, can be developed as follows
LiY NH, ? S O , S 3 l u t i s n
(6)
F i g u r e 1. a. Schematic diagram of apparatus used: 1,granulators; 2, rotameters; 3, cyclone; 4, air heater; 5 , gear pump; 6, water tank; 7 , solution tank; 8, solids discharge. b. Atomizer.
Theory The objective of the following analysis is to relate the growth rate of particles during granulation to the main variables involved. The approach taken was suggested by the work of Harada and associates on fluidized bed granulation (Harada and Fujita, 1967; Harada, 1972). Referring to Figure 2, mass balance for (NH4)zSOd for constant bed weight gives LxL=P+E
(1)
which, in terms of number of particles per unit time, can be written as
+
L X L= nPwp (newp+ S
+D)
(2)
where S represents undeposited spray droplets, D is the fine dust arising from surface abrasion of bed particles, and ne is the number of bed particles demolished to dust by severe attrition and elutriated out of the bed. Since solids in a spouted bed are well mixed, it has been assumed that the product particles (n,) and bed particles ( N b ) , including those which subsequently undergo attrition (ne),have the same mean unit weight (w,). The overall number balance for the time interval d0 may be written as n,,.d0
product
+ ned8 bed particles demolished to dust (elutriated out of bed) = -dNb
(5)
+ n d0 fresh nuclei generated by fracture of bed particles (remaining in bed) (3)
WP
where W is the bed weight which is invariant with time, d, is the mean diameter of the growing particles (assumed spherical), and ps is the particle density. Differentiation of eq 6 gives
(7) Substituting the above expression to eliminate dNb from eq 5 and putting w p = rdp3ps/6in the last term yields
( L x I ,- E ) d8 = ( n - ne)wpd8 + 3W
d(d,)
(8)
d,
The difference ( n - ne)represents the net rate of appearance of fresh nuclei in the bed. Replacing ( n - n e )with n' and w p with w/Nh in eq 8 gives n' 3W d(dp) ( L X L- E ) d8 = - W d8 + (9) lvh dP Integration of eq 9 with the initial condition 8 = 0, d, = d,,,, and with the assumptions that the elutriation rate E and the ratio n'/Nh do not change with time 0, yields the following final equation
Experimental data, when plotted as In dp/d,,, vs. 8, showed a substantially linear relationship (Figure 3),although in several runs the growth rate tended to increase somewhat with time as it does in Figure 3. This nonlinearity suggests that the assumption of constant total attrition rate made in integrating eq 9 is probably not strictly valid. Most of the data obtained, however, could be fitted to straight lines within &15%, and therefore the above simplifying assumption would seem to be a reasonable approximation. The assumption is likely to Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 4, 1976
505
0.4
1
I
I
I
I
I
C
1
I
8,hr 0
0 3-
2
3 5 -
0
2
4
6
8, hrs
Figure 3. Typical growth rate data (run no. 5 ) .
z
3
01 06
I I 08 1.0 12 Dimensionless particle diameter, dp/dpm
14
Figure 5. Data of Figure 4 plotted in dimensionless form.
(“)
n’ LXL-3m Nb Knowing L X L , and E , the values of the ratio n‘/Nb were calculated from eq 12, and these are also listed in Table I. It is seen that with the exception of run no. 9,10, and 14, the net hourly rate of generation of fresh nuclei remains less than 7% of the number of bed particles. For some of the runs, the ratio n’/Nb has even a small negative value, indicating that fracture of particles into fragments (nuclei) was slightly slower than more severe attrition to fine dust. The fracture mechanism, however, becomes dominant in run no. 9, 10, and 14, which all show high values of n’/Nb. As a consequence, the mean particle size decreased instead of increasing in run no. 9 and 10 and showed a very slow increase in run no. 14. The solution feed rates in these runs were higher than in all other runs. I t would therefore appear that the friability of ammonium sulfate granules under spouted bed conditions increases sharply above a certain critical evaporation rate, though it is likely that factors such as thermal stresses within a granule also exert an influence on its physical structure and attrition behavior. The above inference concerning the effect of evaporation rate on friability of the granules is supported by a series of attrition tests carried out on the same apparatus, either without any liquid feed or with injection of water rather than ammonium sulfate solution. The temperature of the spouting air was kept at -200 “ C and samples of the bed solids were screen-analyzed periodically. The data were interpreted according to eq 10 and are reported in Table 11. A substantial increase in attrition rate clearly occurs with increasing water injection rate, the values of the attrition term [(EIW) (n’ /Nb)]for the runs with water injection being similar in magnitude to those for the high feed rate granulation runs (no. 9, 10, and 14). A quantitative agreeqent cannot be expected since the attrition tests were carried out batchwise while the granulation runs were continuous; nevertheless, the data in Table I1 provide strong qualitative support for the suggestion that high attrition rates in the granulation runs were caused by high liquid feed rates. Heat Balance. A heat balance over the granulator can be written as -=
w,
Figure 4. Size distribution of growing particles (run no. 5 ) .
breakdown after a long time period since otherwise eq 10 would imply that if particles grew at all, they will continue to grow indefinitely with time, which is intuitively unreasonable. In the present series of runs, however, the limiting time period was not reached, and particles did in fact continue to grow in the longest runs made (no. 6 , 9 h; no. 13,9.5 h).
Results and Discussion Size Distribution. Typical sieve analysis data obtained at hourly intervals during the course of a run are shown in Figure 4. The different curves are approximately parallel and collapse on a single curve (Figure 5 ) when the abscissa is converted to d,, being the a dimensionless particle diameter, d,/d,,, median diameter (Le., corresponding to 50% cumulative weight). The product size distribution for most runs in which net growth occurred showed similar “self-preserving” behavior. This type of behavior is encountered in wet pelletization but is not considered to be indicative of any particular mechanism of size enlargement (Linkson et al., 1973). The size spread of product samples was usually such that 80% of the material (between 10 and 90% cumulative weight) fell within f 1 0 % to f 2 5 % of the median diameter. Growth Rate. The growth rate data have been-analyzed using the mass mean diameter
to characterize the size of the particles, since in granulation the important quantity is the increase in mass of the growing particles. The Sauter mean diameter (l/,Xxl/dpl) was also computed for some of the data and was found to agree closely (within -2%) with the mass mean values. The data from all the granulation runs are summarized in Table I. Values of the slope m listed in the table were obtained from a least-squares straight line fit of growth rate data plotted as In (dp/dpo)vs. 0 for each run. From eq 10
or 506
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 4, 1976
+
- Te) + Gacp,(T,‘
Gs~p,(Ti
+ LCp,(TI
- Te)
- Tb) = L ( 1 - X L ) ( X
+h)
(13)
where G , and G, are the flow rates of spouting air and atomizing air, ‘?respectively, T , and T,’ (= ambient temperature) are their temperatures, X is the latent heat of vaporization of water, and h is the heat of solution of (NH4)2S04.The values of the second and third terms on the left-hand side of eq 13 are, however, negligible in comparison with the first term so
Table I. Data from Granulation Runsa ~
m,
Run no.
L,
Seed size, mm
G s, kg/h
kg/h
Air temp, "C Inlet Exit
LXL,
defined by eq 11
kglh
W,
E, kglh
kg
n'/Nb, defined by eq 12
0.78 0.13 6.48 -0.059 2.02 47.12 170.0 79.5 0.053 1.17-1.68 0.97 0.11 6.34 -0.039 48.28 189.6 82.9 0.058 2.68 1.17-1.68 5.98 -0.038 1.05 0.20 189.0 76.2 0.060 6 1.17-1.68 2.88 49.51 1.20 0.27 6.26 0.035 188.6 80.4 0.038 7 1.68-2.00 3.43 58.62 1.22 0.37 6.32 -0.001 3.55 47.41 198.9 76.6 0.045 8 1.17-1.68 -0.022 1.42 0.28 6.50 0.241 196.9 79.7 3.91 59.92 9 2.00-2.38 10 2.38-2.83 4.13 61.76 195.5 78.5 -0.056 1.39 0.36 6.28 0.333 11 0.99-3.36 3.59 53.94 188.3 70.3 0.057 1.36 0.38 6.40 -0.018 12 2.00-2.38 3.10 76.69 171.8 94.5 0.038 1.15 0.14 5.94 0.066 13 0.7-4.70 3.02 59.16 184.9 84.8 0.043 1.06 0.10 6.28 0.024 14 2.38-2.83 4.42 66.83 192.2 77.1 0.006 1.55 0.28 6.26 0.185 Note: flow rates and temperatures reported in the above table are average values of observations made about once every hour during the course of a run. 4 5
Table 11. Data from Attrition Runs
Run no.
Particle size, mm
1 2
2.38-4.70 1.68-2.00 2.38-2.83 2.38-2.83 2.38-4.70
3 4 5
Water injection rate, kglh 0 0
0.66 1.23 2.40
m,
E n' -+w Nh
defined by eq 11
(= -3rn)
-0.012 -0.018 -0.040 -0.058 -0.051
0.036 0.054 0.120 0.174 0.153 7
that an approximate heat balance may be written as
(Ti- T,) =
L ( l - XL)(A
+h)
0.02
0.03 LiI
(14)
0
-
0.04
7 0.05
xL)/G,
Figure 6. Heat balance data plotted according to eq 14.
GSCP,
Figure 6 shows a plot of the data in Table I, according to eq 14. Most of the data points fall on a straight line (within f10%) with a slope approximately equal to (A + h)/c,, = 2300 "C (A = 2420 kJ1kg @BO "C, h = 58 kJ/kg, and cp, = 1.05 kJ/kg "C). Temperature profiles within the bed (Figure 7) measured with a bare thermocouple show that evaporation largely occurs in the lower half of the bed, the temperature in the upper half being relatively constant and equal to the air temperature immediately above the bed ( T,). This implies that granulation capacity in the experimental runs was probably not limited by bed depth but by temperature and flow rate of the spouting air. However, the effect of these two variables was not specifically studied. Granule Structure. A number of product granules were sectioned, examined under a microscope and also photographed. The sectioning technique consisted in setting the granules in a mould of transparent polymeric material and sand-papering the surface of the hardened mould until the cross section of the granule close it its equator could be seen. The original nucleus could be clearly distinguished from the surrounding shell deposited during the granulation experiment (see Figure 8a). The shell had a homogeneous structure and did not contain any clusters or agglomerates of fine particles, which is hardly surprising since any fines would be swept out of the bed by the spouting air as soon as they are formed. The product consisted of spherical granules, had a bulk density of 0.9-1.0 Mg/m3, and was substantially free from lumps or granules of irregular shape (Figure 8b).
Conclusions 1. Particle growth in spouted bed granulation of ammonium sulfate occurred by a layering mechanism, yielding
spherical granules of uniform structure. 2. The size distribution of the growing particles was "selfpreserving". 3. The net growth rate was independent of seed size and was determined by the difference in the feed rate of ammonium sulfate as solution and the rate of particle breakdown, whether by fracture into fragments which served as nuclei or by more severe attrition to dust which was elutriated out of the bed. The fracture mechanism became dominant a t high solution feed rates, causing the growth rate to decline to very small and even negative values. 4. The amount of fine dust elutriated out of the bed varied between 0.10 and 0.38 kglh, which corresponds to 1.6% and 6% of bed weight and 10% and 30% of feed ammonium sulfate. High dusting rates were associated with high solution feed rates. 5. Heat balance calculations confirmed that the drop in air temperature during its passage through the bed was determined primarily by the amount of water evaporated per unit mass of spouting air, which varied between 0.025 and 0.049.
Acknowledgments We would like to express our thanks to Dr. Norman Epstein, who read the draft of this paper and offered many useful comments. We are also grateful to the National Research Council of Canada for financial support which enabled 0. Uemaki to spend a year on this work a t the University of British Columbia. Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 4, 1976
507
0
*
E = total dust elutriated out of the bed, k g h G , = mass flow rate of atomizing air, kg/h G , = mass flow rate of spouting air, kg/h
SPOUT ANNULUS
$100
e c w
50
0
05 BED LEVEL, J H
1.0
Figure 7. A typical bed temperatures profile during granulation (run no. 14).
H = spouted bed height to annulus surface, em L = mass flow rate of feed solution, k g h m = defined by eq 11 n = rate of generation of fresh nuclei by fracture of bed particles, number/h (remaining in bed) Nb = number of particles in the bed a t any instant ne = rate of demolition of bed particles to dust, n u m b e r h (elutriating out of bed) n, = rate of product withdrawal from the bed, n u m b e r h n' = n - ne, net rate of appearance of fresh nuclei in the bed, number/h , P = product withdrawal rate, kg/h S = undeposited spray droplets elutriated out of the bed, kg of (NH4)zSOdh h = heat of solution of (NH4)2S04,kJ/kg of water evaporated from a 40% (NH4)&304 solution Tb = bed temperature, "C T , = air temperature a t bed exit, "C T , = spouting air temperature a t bed inlet, OC Ti' = atomizing air temperature, OC T I = feed solution temperature a t bed inlet, " C W = bed weight, kg w,, = mean particle weight, kg xI = mass fraction of particles of sized,, X L = mass fraction (NH4)2S04 in feed solution z = bedleve1,cm R = time, h X = latent heat of vaporization of water, kJ/kg ps = particle density, Mg/m3 Literature Cited
a
b
Figure 8. a. Granule structure. b. Granule shape.
Nomenclature c , ~ = specific heat of air, kJ/kg " C cp, = specific heat of feed solution, kJ/kg "C D = dust generated by surface abrasion of bed particles, kgh d, = particle diameter, mm d,, = particle diameter a t R = 0, mm dDi = particle diameter of mass fraction x i , mm dPm= median particle diameter, mm
508
Ind. Eng. Chem., ProcessDes. Dev., Vol. 15, No. 4, 1976
Berquin. Y . F.. Gen. Chim, 86, 45 (1961). Berquin, Y . F., US. Patent 3 231 413 issued to Potasse et Engrais Chimiques, Paris, 1966. Berquin. Y . F., Paper NO. 38e presented at the 4th Joint A.1.Ch.E.-C.S.Ch.E. Conference in Vancouver. B.C.. 1973. Harada, K.. Fujila. J., KagakuKogaku. 31, 790 (1967). Harada, K.. KagakuKogaku, 36, 79 (19721. Linkson, P. B.. Glastonbury,J. R., Dufty. G. J., Trans. lnst. Chern. En& 51, 251
Receiued for reuieu, August 14, 1975 Accepted May 14,1976
