FLUIDIZED BED DRYING OF SODIUM SULFATE SOLUTIONS N .
N .
B A K H S H I
A N D
C .
Y .
C H A I '
Department of Chemistry and Chemical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada Drying of sodium sulfate solutions was studied in a 2-inch diameter batch fluidized bed dryer a t 14OoC. The solution was fed through a hypodermic needle into the hot fluidized bed. The operating conditions were: bed height 2, 4, and 6 inches; air flow rate 2 0 , 27.5, and 3 5 SCFH; and feed concentration 10, 20, and 3 0 weight Y o . The bulk density and the particle size distribution of the dried product were measured. The product bulk density increased with the increase in feed concentration. A correlation between the bulk density and the operating variables is reported.
SODIUM sulfate is an important raw material in chemical industry. I t is primarily used in the manufacture of kraft pulp to add strength and toughness to paper and also in the manufacture of glass, detergents, and mineral feed supplements. Saskatchewan possesses the largest known sodium sulfate deposits in Canada. Bartley (1965) estimated that the exploitable deposits are of the order of 200,000,000 tons. In 1966, sodium sulfate production in Saskatchewan was of the order of 400,000 tons. I t is estimated that the annual production in 1968 ranged between 700,000 and 800,000 tons. Present Methods of Recovery and Dehydration
Almost all the production of sodium sulfate from Saskatchewan lakes is now being obtained by the brining process. The concentrated lake brine is pumped into large reservoirs in late summer. Glauber's salt (Ka2SOi.1 0 H 2 0 ) is crystallized during the cold weather and the remaining liquor is drained back to the lake. The decahydrate crystals are stockpiled near the plant site. Glauber's salt is dehydrated by first melting the crystals and removing a portion of the water by a submerged combustion unit or a Holland evaporator. The concentrated slurry is then transferred to a rotary kiln for final drying. The dried product leaving the kiln is ground and sieved to give the required particle size distribution. Purpose of Study
The present drying method is essentially a two-step process. Partial moisture removal takes place in the melter and the final drying takes place in a rotary kiln. An alternative approach would be to use a fluidized bed for drying the sodium sulfate slurries or solutions. The following advantages may be envisaged for this alternative approach: One-step drying process. Particle size of the product is fairly uniform and can be controlled easily. Present rotary kilns are usually made of stainless steel to withstand corrosive effects of sodium sulfate slurries. I n fluidized bed dryers, it may be possible to use carbon steel for the equipment. I t may be possible to reduce the equipment size considerably in comparison with rotary kilns.
' Present address, Brown Forest Industries (Pulp and Paper), Espanola, Ontario, Canada.
The bed temperature is uniform and the drying takes place a t relatively low temperatures. The high heat transfer rate in the fluidized bed results in a high thermal efficiency. If desired, a substantial part of the water in the feed solution may be recovered by using external air-cooled heat exchangers. The fluidized bed dryer has the potential to be developed into a portable mobile unit. These units can be moved from lake to lake for the recovery of sodium sulfate. Large reservoirs for the crystallization of Glauber's salt will not be needed.
The literature indicates that considerable work has been done on the fluidized bed drying of inorganic salt solutions in Europe (Markvart et al., 1962; Vanecek et al., 1966). However, it has received scant attention in North America. Some of the work pertinent to fluidized bed drying has been done in the nuclear energy field (Jackson et al., 1960; Jonke et al., 1957; Philoon et al., 1960). In the present investigation using fluidized bed techniques, the drying was studied under batch conditions on account of the simplicity of equipment and operation. No attempt was made to evaluate the process economically. Particularly, an attempt was made t o study the effect of bed height, air velocity, and feed concentration on the bulk density and particle size distribution of the dried product. Procedure
A simple flowsheet of the equipment is shown in Figure 1. Essentially, the actual dryer consisted of a 2-inch diameter, 2-foot high QVF glass column. A 12iji~-inchdiameter steel plate consisting of 70 randomly drilled holes ( > 3 2 -inch diameter) served 1s the gas distributor. The dryer was initially charged with a suitable amount of 100- to 150-mesh anhydrous sodium sulfate to serve as nuclei for the deposition and drying of the feed solution droplets. The initial charge was fluidized by hot air until the bed temperature became steady a t 140'C. The sodium sulfate solution of the desired concentration was allowed to drop into the hot fluidized bed through a No. 25 hypodermic needle. The temperature of the feed solution was maintained a t 50" C. After 30 minutes of operation, the feeding was discontinued. Hot air was allowed to flow through the fluidized bed for another 10 minutes to dry the last few drops of the feed solution. The dried product was removed from the dryer and separated from 100to 150-mesh seed charge by sieving. Tests were carried out to determine the moisture content, the bulk density, and the particle size distribution in the dried product.
The details of the design of the equipment and operaVOL. 8 N O . 2 APRIL 1 9 6 9
275
Air flow rate, SCFH
100 90-
>
20
/u
27,5
35
'
A
~
80-
70-
s 60+
2
Figure 1. Flow diagram of fluidized bed drying equipment 1.
2. 3. 4.
5. 6. 7. 8.
Air inlet Air filter Pressure regulator Rotameter Heater
Grid
Rubber stopper .. Pocking 9. Fluidized bed 10. Air outlet
11. Hypodermic needle 12. Solution heater 13. Feed tube 14. Hot plate 15. Sodium sulfate solution 16. Stirrer 17. Overflow receiver V i , V2. Needle valves PI. Pressure indicator
tional procedure are given by Chai (1967). The operating conditions are shown in Table I. The minimum fluidization velocity for 100- to 150-mesh particles was calculated by the relationship given by Leva (1959). The actual superficial velocities used were 0.36, 0.49, and 0.63 foot per second, approximately 7, 9, and 12 times the minimum fluidization value. The corresponding air flow rates were 20, 27.5, and 35 SCFH. The salt solution feed rates are also shown in Table I. No attempt was made to operate the dryer a t any other solution feed rates. Table 1. Range of Variables and Operating Conditions 1. Feed solution concentration, wt. 7 c 2. Initial bed heights, inches 3. Air flow rate, SCFH Solution concentration, wt. Lro Solution temperature, O C. Solution feed rate, ml./min." Solution drop size, cm. Particle size of sodium sulfate in fluidized bed, mesh Bed temperature, approx. O C.
10, 20, and 30 2, 4, and 6 20, 27.5, and 35 10
50 0.565 0.248
20 50 0.425 0.248
30 50 0.366 0.257
100-150 140
100-150 140
100-150 140
"Solution feed rate varied with feed concentration because same hypodermic needle was used for all runs.
276
I&EC PROCESS DESIGN A N D DEVELOPMENT
4
6 2 4 6 2 Bed height, inches
4
6
Figure 2. Particle size distribution of dried sodium sulfate 10 weight Yo feed concentration
Particle size:m 8 -100 mesh
4 - 8 mesh 7 4 mesh
Results and Discussion
The dried product was obtained in the form of small pellets. A substantial portion was fairly spherical. The product was divided into three fractions: greater than 4-mesh, between 4- and 8-mesh, and between 8-and 100mesh. In all the tests, the largest fraction fell between 4- and 8-mesh (65 to 89 weight 70 of the dried product). Moisture Content of Dried Product. I n all, 27 test runs were made for the present investigation. The moisture content of the dried product never exceeded 0.13 weight % and the product can be assumed to be completely dry. Particle Size Distribution. Particle size distribution in the dried product is important from the industrial point of view. If the product particles are too big, they may have to be crushed and resized; too small, there may be carry-over problems during operation. The particle size distributions for 10, 20, and 30 weight % solutions are shown in Figures 2 , 3, and 4. It is desirable to have as large a fraction of the product as possible of uniform particle size. T o a large extent, this will depend on the droplet size of the atomized feed solution. In the present investigation, the droplet size was approximately 2.5 mm., which gave a dried particle size between 4- and 8-mesh. The largest fraction of the dried product, falling between 4-and 8-mesh, was obtained when the initial bed height was 6 inches and the air flow rate 35 SCFH. Both values were the maximum for this investigation. The results indicated that a high initial
Air flow rate, SCFH
A i r flow rate, SCFH
27,5 _h-
27
20
35 A
l 90 oor
35
F
801
+
I
6ot
~
301
~
201
'"i
4
2 4 6
2 2d height, inches
4
4 1 2 3ed height, inches
6
L
I 4
Figure 3. Particle size distribution of dried sodium sulfate
Figure 4. Particle size distribution of dried sodium sulfate
20 weight Yo feed concentration
30 weight % feed concentration
Particle size: El 8 - 100 mesh 0 4 - 8 mesh I > 4mesh
Particle size:B 8 - 100 mesh 4 - 8 mesh H > 4 mesh
bed height and a large air flow rate were equally effective in achieving the largest fraction of the dried product within the range 4- to 8-mesh. Bulk Density of Dried Product. An attempt was made to measure the bulk density of the dried product as obtained from the batch dryer without separating the product into various size fractions. As expected, the results were inconclusive because the particle size range was too wide. T o obtain meaningful results, the bulk density of the largest fraction (between 4- and 8-mesh) was used as the representative bulk density of the product. The values of the bulk densities are shown in Figures 5, 6, and 7 . The results indicated that the air velocity, bed height, and feed concentration all affected the bulk density of the dried product. The bulk density increased with the increase in any of these three variables. However, the effect of air velocity and bed height was comparatively smaller compared to the increase in bulk density produced by the increase in feed concentration. This would indicate that if a high bulk density product is desired, the feed solution concentration should be as high as possible. Though we have made no tests with a sodium sulfate slurry, it may be highly desirable to use a slurry. This will have the additional beneficial effect of increasing the throughput of the dryer. Crosby and Marshall (1958) report that the bulk density of spray-dried sodium sulfate solutions decreases as the feed concentration increases, directly opposite to what
has been observed in the present investigation. From the industrial point of view, a dried product of higher bulk density is preferable. Since a higher bulk density can be obtained from higher feed concentrations in fluidized bed drying, it would appear that fluidized bed drying, in this respect, serves better than spray drying. If the effects of the bed height and air velocity are neglected-that is, if it is assumed that the bulk density is a function of the feed concentration only-the results of this investigation can be correlated by the following simple equation: PB
= 0.582
+0.44~
(1)
where p B = bulk density of the dried product, grams per milliliter, and y = weight fraction of the solute in the feed solution, dimensionless. The predicted bulk densities from Equation 1 were compared with the measured bulk densities (column 6, Table 11). The largest deviation of the experimental values from the predicted values was about &8%. Thus, this equation can be used as a first approximation for the prediction of the bulk density of the dried product. On the other hand, if the effects of air velocity and bed height on the bulk density are also taken into account, Equation 1 can be modified to give PB
= 0.582
+ 0 . 4 4 ~+ 0.034 ( L / 4 - 1) + 0.094 ( V a / 2 7 . 5- 1) VOL. 8 NO. 2 APRIL 1969
(2) 277
A i r flow r a t e
0,781 B e d heiqht
0.76 -
=
-
35 SCFH 6 in. 4 in.
A 0
0.740.72-0.70 i h0.68r
0.581
1
0.561
0
IO
20
30
0 1
0.58
Feed concentration,wt.%
Figure 5 . Effect of feed concentration on bulk density of dried sodium sulfate
0.54
0
IO
20
30
Feed concentration, wt. %
0.74 0.72
. 0.70
A i r flow r a t e = 27.5 SCFH
c
A
6
Figure 7. Effect of feed concentration on bulk density of dried sodium sulfate
in.
$ 0,681 01
- 0.66 0.64 U
-
y 0.62r
m'
0.56 0.54
0
IO
20
30
Feed concentration, wt. %
Figure 6. Effect of feed concentration on bulk density of dried sodium sulfate
where L = bed height, inches, and V , = air flow rate, SCFH. Equation 2 shows a better correlation of the results. The predicted bulk densities by this equation are also shown in Table I1 (column 8). The largest deviation of the experimental values from the predicted values was I n most cases, the deviation was less than about k2.87~. 1%. Equations 1 and 2 are applicable only for a batch fluidized bed operation and the particular feeding system used in this investigation. .Moisture Balance. An attempt was made to measure the dry- and wet-bulb temperatures a t the inlet and outlet of the fluidized bed dryer. However, because of faulty instrumentation, correct measurements could not be made for the wet-bulb temperatures. Hence the results obtained were rather inconclusive and are not reported. 278
I & E C PROCESS D E S I G N A N D DEVELOPMENT
However, the lack of these results does not affect the basic conclusions obtained from the present study. Difficulties during Operation. Two main difficulties were encountered during this investigation. The sodium sulfate solution was fed through a hypodermic needle into the hot fluidized bed. When 30 weight % solution and 35 SCFH air flow rate were used, the mouth of the hypodermic needle got clogged many times, resulting in the discarding of the run. The whole test run had to be started all over again. Many times, the solution droplets did not fall vertically into the bed but fell on the column walls. Great care had to be exercised to have the droplets fall into the fluidized bed. In addition, during the test run, the product particles being larger in size than the initial charge, settled down a t the bottom of the bed while the bed consisting of the finer particles (100- to 150-mesh) remained fluidized. If the dryer must be operated for long intervals, the product will have to be removed and some fine (100to 150-mesh) dried sodium sulfate particles added periodically. This is not a desirable situation and is a limitation for the batch fluidized-bed dryer. Conclusions
A more uniform particle size distribution was obtained in the dried product a t higher air flow rates and bed heights than a t lower air flow rates and bed heights. The bulk density of the dried product increased with the increase in feed concentration. The effects of air velocity and bed height were small. Acknowledgment
The authors express their sincere thanks to the National Research Council of Canada for financial support of this project. Thanks are also extended to Saskatchewan Min-
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Table II. Effect of Drying Conditions on Bulk Density of Dried Products
Run No
Feed Concn., Wt. %
Bed Height, In.
Air Flou; Rate, SCFH
Measured Bulk Density , G. ,MI.
Calcd. Bulk Density by Eg. 1, G./M1.
Deuiat io n, (Col. 5 - Col. 6 ) (Col. 6)j x 100,
5;
Calcd. Bulk Density by Eq. 2, G.jM1.
25 21 24 19 18 20 22 17 23 26 27 28 29 30 31 32 34 46 43 45 42 41 40 39 38 37 36
10 10 10 10 10 10 10 10 10 20 20 20 20 20 20 20 20 20 30 30 30 30 30 30 30 30 30
2 2 2 4 4 4 6 6 6 2 2 2 4 4 4 6 6 6 2 2 2 4 4 4 6 6 6
20 27.5 35 20 27.5 35 20 27.5 35 20 27.5 35 20 27.5 35 20 27.5 35 20 27.5 35 20 27.5 35 20 27.5 35
0.573 0.594 0.628 0.585 0.620 0.652 0.610 0.626 0.672 0.630 0.646 0.679 0.633 0.651 0.690 0.657 0.688 0.715 0.677 0.690 0.712 0.704 0.715 0.740 0.710 0.729 0.750
0.626 0.626 0.626 0.626 0.626 0.626 0.626 0.626 0.626 0.670 0.670 0.670 0.670 0.670 0.670 0.670 0.670 0.670 0.714 0.714 0.714 0.714 0.714 0.714 0.714 0.714 0.714
-8.46 -5.11 +0.32 -6.65 -0.96 +4.15 -2.55 0.00 +7.35 -5.97 -3.58 +1.34 -5.52 -2.84 +2.98 -1.94 +2.69 +6.72 -5.18 -3.36 -0.28 -1.40 +0.14 +3.64 -0.56 +2.10 +5.05
0.573 0.608 0.633 0.601 0.626 0.651 0.619 0.644 0.669 0.627 0.652 0.677 0.645 0.670 0.695 0.663 0.688 0.713 0.671 0.696 0.721 0.689 0.714 0.739 0.707 0.732 0.757
erals, Sodium Sulphate Division, Chaplin, Saskatchewan, for financial assistance in the form of an equipment grant for this project. Literature Cited
Bartley, C. M., “Canadian Minerals Yearbook,” Mineral Report 14, pp. 395-400, Mineral Resources Division, Department of Energy, Mines and Resources, Ottawa, 1965. Chai, C. Y., M. Sc. thesis, Department of Chemistry and Chemical Engineering, University of Saskatchewan, 1967. Crosby, E . J., Marshall, W. R., Jr., Chem. Eng. Progr. 54,56-63 (1958).
Dei rat ion. [ K O / .5 - Col 8) (Col. 8 ) x 100,
50 0.00 -2.30 -0.78 -2.66 -0.96 -0.15 -1.45 -2.80 +0.43 +0.48 -0.92 +0.30 -1.86 -2.84 -0.72 -0.91 0.00 +0.28 +0.89 -0.86 -1.25 +2.18 +0.14 +0.14 +0.42 -0.41 -0.92
Jackson, J. D., Sorgenti, H. A . , Wilcox, G. A , , Brodkey, R. S., Ind. Eng. Chem. 52, 795-8 (1960). Jonke, A. A., Petkus, E. J., Loeding, J. W., Lawroski, S., Nuclear Sci. Eng. 2, 303-19 (1957). Leva, Max, “Fluidization,” p. 64, McGraw-Hill, New York, 1959. Markvart, M., Vanecek, V., Drbohlav, R., Brit. Chem. Eng. 7,503-7 (1962). Philoon, W. C., Sanders, E. F., Trask, W . T., Chem. Eng. Progr. 56, 106,108,110, 112 (1960). Vanecek, V., Markvart, M., Drbohlav, R., “Fluidized Bed Drying,” Leonard Hill, London, 1966. RECEIVED for review July 12, 1968 ACCEPTED January 7,1969
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