Ind. Eng. Chem. Res. 1987, 26, 1936-1939
1936
COMMUNICATIONS A Possibility of Crystal Size Distribution Control Using Additives in Batch-Cooling Crystallization An experiment was done on batch-cooling crystallization of ammonium sulfate from an aqueous solution contaminated with a small amount of chromium ion added as sulfates (Cr3+ ion in 0-5 g/m3 (NH4)$304solution). T h e results demonstrate that the time a t which the impurity is added is important. Larger product crystals were obtained when chromium ion was added after a large number of nuclei had been produced; smaller crystals were obtained when it was initially added to the nucleus-free solution. Although chromium ion retards both nucleation and growth rates, it can make the product crystals larger only if it is added to the solution a t an appropriate stage. Batch-cooling crystallizers are widely used in the chemical industry. They have many advantages, such as flexibility in operation and less investment due to simplicity, etc. One of the major difficulties in batch crystallization is to control the size distribution of product crystals (CSD). Mullin and Njvlt (1971), Jones and Mullin (1974), and Jones (1974) showed that larger product crystals could be obtained by adopting an appropriate cooling policy: controlled cooling (Mullin and Njrvlt, 19711, constant nucleation cooling (Jones, 1974;Jones and Mullin, 1974),and size-optimal cooling (Jones, 1974). Cooling curves in these policies roughly have the reverse shape of natural cooling where the temperature decreases rapidly at the early stage of cooling and slowly toward the end. Besides the different cooling policies, other measures such as addition of a small amount of additives or impurities, changing agitation speed or impeller types, etc., can influence the final CSD. The aim of the present paper is to show a possible control of CSD in a batch-cooling crystallization operation by adding a small amount of additives a t an appropriate stage. Although a number of studies have been done on the impurity effect on the crystallization kinetics itself (Mullin, 1972a), there is no report discussing the importance of the additive addition time in relation to CSD control as in this study. The working system was an ammonium sulfate-water system. The additive was a small amount of Cr3+ ion, which retards both secondary nucleation and growth rates of ammonium sulfate (Kubota et al., 1986; Larson and Mullin, 1973). Although this system and additive may not be the best example for practical use, our results show clearly the possibility of CSD control through additives in batch-cooling crystallization. Experimental Section Crystallizer, Solution, Additive, and Seed Crystals. Figure 1 shows the crystallizer, which is similar to that described by Jones and Mullin (1974). It is a glass vessel with a conical baffle on the bottom, four equally spaced vertical baffles, and a draft tube. The vertical baffles and the draft tube are made from Perspex. A three-bladed pitched Perspex propeller was used for agitating the solution. It made the solution circulate upward in the annulus. The solution volume was 1.2 x m3. The whole crystallizer was immersed in a water bath, and the solution temperature was controlled as desired. The temperature OSSS-5SS5/87/2626-1936$01.50/0
was measured by a thermometer immersed in the crystal suspension. An ammonium sulfate solution, saturated at 35.0 "C, was filtered through a sintered glass filter (20 30 pm) and was stored before use at 38.0 "C in a Kjeldahl flask with a rubber stopper. Ammonium sulfate (Kanto Chemical Co., first grade) was used without further purification. Distilled, deionized water was used. The additive, Cr3+ ion, was added as the sulfate, Cr,(S04)2-4H20 (Kanto Chemical Co., first grade), and it was used without further purification. We added Cr3+ion in two different ways (methods A and B) as described in the following section. Concentrations of additives, C,, were 0, 1,3, and 5 g/m3 solution in method A and 0 and 5 g/m3 solution in method B. Crystals, nucleated and grown in our laboratory in seeded pure solutions, were used as seeds after drying and sieving. Some of the prepared crystals (840- 1000 pm) are shown in Figure 2. As only 20 seed crystals were used in each run, most of our product crystals came from nuclei rather than seeds. This is a different situation from that described in the literature (Jones, 1974; Jones and Mullin, 1974; and Mullin and N*lt, 1971) in which the product came from seed crystals only. Crystallization Procedure. Method A. Figure 3 shows the cooling curve, the seeding time, and the time at which the addition of Cr3+ion occurred. The solution, stored at 38.0 "C, was poured into the crystallizer in the water bath held at 36.0 "C. The solution cooled to the bath temperature, 36.0 "C, while it was agitated at 500 rpm. Although the temperature was still 1 "C higher than the saturation temperature, 20 seeds were introduced quickly into the solution. Three minutes later the solution was quenched by lowering the bath temperature quickly to 33.0 O C by introducing cold water into the bath. The seed surface was expected to dissolve slightly during the 10-min period, while the solution temperature still remained higher than the saturation point (35.0 "C). This technique was used to avoid initial breeding (detachment of small crystals from the seed surfaces), which would have given an unexpected and uncontrolled number of nuclei. The solution temperature reached 33.0 "C in 30 min, during which the agitation was kept at 1000 rpm to produce as many secondary nuclei as possible by vigorous contacting between the impeller (or the wall) and the seed crystals or among the seeds. Then, the agitation speed was returned to 500 rpm, and a chromium sulfate aqueous so-
-
0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987 1937
1 IC)
0 O
D
0 "
1 N 0 I
-
SO@
-
_
.
\ .
1 1 o+
0 1.2x10-3 m3
glass vessel
0 Pew% draft tube
8
@ Perspex
vertical baffle @ Perspex 3-Maded pitched prqxller @ Glass conical baffle
Figure 1. Crystallizer.
Figure 2. Photograph of seed crystals.
1
Ming
-
Y Y
8
lminl
Figure 3. Cooling curve in method A. The seeding and C$+ ion addition points are indicated.
lution (3 X lo4 m3), the concentration of which was adjusted to give a desired level of Cr3+concentration, was added to the solution. (If no Cr3+ was needed, 3 X lo4 m3 pure water was added.) By this addition, the solution was diluted and the saturation temperature became 34.0 "C, which is 1.0 "C lower than that of the original solution. A t the time of C P addition, the water bath temperature began to be reduced a t a constant rate of 0.01 "C/min. The solution temperature also decreased linearly. (The time delay in the temperature response was negligible because of the slow cooling.) The run was stopped when the solution temperature reached a chosen termination temperature, t , = 30.0 "C. The cooling time, e,, from the start of constant cooling was
Cminl
Figure 4. Cooling curve in method B. The seeding and C9+ ion addition points are indicated.
300 min. The whole suspension was quickly filtered by vacuum a t room temperature. The crystals on the filter were washed with ethanol and dried a t room temperature. Grown nuclei, which were easily distinguished by size from the grown seeds, were sieved for a CSD analysis and for obtaining the crystal yield. Photographs of the product crystals were also taken. Method B. Figure 4 shows the cooling curve in method B. The solution stored originally at 38.0 "C was transferred to the crystallizer in the water bath (36.0 "C). When it had reached 36.0 "C by natural cooling, the solution temperature began to be lowered linearly a t 0.04 "C/min. Since linear cooling started from a temperature 3 "C higher than that in method A, a greater cooling rate was employed to save time. The termination temperature was either 20.0 and 30.0 "C, corresponding to cooling times from saturation of 375 and 125 min, respectively. These two termination temperatures were employed to examine the effects of the termination temperature on the CSD and crystal yield. Agitation speed was 500 rpm throughout the run. Twenty seeds were introduced to the solution 10 min before the solution temperature passed the saturation point. This prevented the initial breeding. Measurement of Concentration Change. Since supersaturation affects both nucleation and growth rates, which are directly related to the product CSD, we measured the changing solution concentrations in both experiments. A simple technique was used for determining the concentrations. A sample solution (15 X 10" m3) was taken from the crystallizer through an injection needle, the top of which was covered with cotton to prevent crystals being introduced with the sample. The sample was immediately transferred into a glass ampule, and i t was sealed with a vinyl chloride cap. The sample was quenched a t once to 0 "C to make a lot of small crystals form. Then the crystallized sample was put into a warm-water bath, held a t a constant temperature, where the sample solution was agitated for 30 min by a small magnetic stirrer. If crystals did not dissolve during this period, the temperature was raised by 0.2 "C, agitation was repeated for another 30 min, etc. Thus, the temperature a t which all crystals dissolved was determined. This temperature was compared with solubility data (Mullin, 1972b) to give the concentration of the sample. The measurements were done for solutions
1938 Ind. Eng. Chem. Res., Vol. 26,
No. 9, 1987
1
-s
0
s
Y
Y
a
a
"0
L
L
Figure 5. Crystal size distributions of the product crystals obtained by method A.
1200
800
400
tpml
1600
1200
Ipml
Figure 8. Crystal-size distributions of the product crystals obtained by-method B.
1
6 LOO
I
0
I
I
1
I
2
4
6
8
ca tg/d-sotution~ Figure 6. Plot of median size of the produce crystals (massbasis) vs. concentration of C?+ ion.
I
100 i -o-1
h
80
2 U
Figure 9. Photograph of sieved product crystals obtained by method B at C, = 5 g/m3 solution. '
I
I
I
J1
:k q=O.O4 l 5Wmin o1 qZO.04
A
601
Saturatioi; curve Saturatioi;'
76t 0
0
2
4
I
6
8
ca Cg/m3-sOlution3
Figure 7. Plot of crystal yield (mass basis) vs. concentration of (21.9' ion.
of Ca = 0 and 5 g/m3 solution, both in methods A and B. Results Product Crystals: Size and Yield. Figure 5 shows product CSDs obtained by method A. They are expressed on a mass basis. The distribution curve moves to the right, and the crystal sizes become larger when the concentration of Cr3+ increases. In Figure 6, the median size of the product crystals (mass basis) is plotted against the Cr3+ concentration. The increasing crystal size is clearly shown. The median size at C, = 5 g/m3 solution became roughly twice that for the chromium-free solution. It may be surprising that, although Cr3+ion is a growth-inhibiting additive (Kubota et al., 1986; Larson and Mullin, 1973), it made the product crystals larger. The crystal yield (mass percent of the crystals produced, on the basis of the maximum quantity calculated from the solubility) became smaller, however, as the Cr?+concentration increased. Figure 7 shows the decreasing trend of the crysal yield with an increase of the additive. Unlike method A, method B, in which Cr3+ ion was initially added to the solvent water and in which the tem-
0
300 0
'I 600 Cminl
i
900
Figure 10. Concentration changes of the solution in method A. Table I. Median Crystal Sizes and Yields of the Products Obtained by Method B concn of C P ion, median size crystal termination C,, g/m3 of crystals, yield, Y, run temp, t,, "C soh L,, Pm % 0 770 103.2 1 30 5 590 63.9 2 30 0 lo00 95.2 3 20 5 750 95.7 4 20
perature fell a t a constant rate throughout the run, gave smaller product crystals than were obtained without the impurity. The CSDs are shown in Figure 8. Median sizes of the product crystals and the yields are listed in Table I. The yield decreased upon addition of Cr"+ ion when the termination temperature, t,, was 30.0 "C but did not when t , was 20.0 "C. Figure 9 is a photograph of sieved product crystals obtained by method B a t Ca = 5 g/m3 solution. Similar products were obtained by method A from solutions containing Cr3+. Concentration Change. Figure 10 shows concentration changes of the solutions used in method A. Although the
Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987 1939 method needs virtually no additional investment. Therefore, if the mechanism becomes clear, it can become a dependable technique for operation of batch crystallizers.
77i
76
"
0
60
120 8
180 240 [minl
300
360
Figure 11. Concentration changes of the solution in method B.
concentration was virtually constant during quenching and vigourous agitation (1000 rpm), we could actually observe a large number of nuclei being produced in the crystallizer. Then, adding either 3 X lo* m3 of Cr3+solution or pure water caused the concentration to drop suddenly, though it still remained in the supersaturated region. The subsequent concentration change was delayed when Cr3+ion was added, compared with that in the Cr3+-freesolution. This is thought to be due to the retarding action of Cr3+ ion on the rates of both nucleation and growth. Concentration changes observed in method B are shown in Figure 11. The change was also delayed when Cr3+was present, compared to that in the pure system. The supersaturation (the difference between the existing concentration and that a t saturation) became greater than in method A, especially in the contaminated system. Discussion In method A, Cr3+ion was added to the solution after many nuclei had already been produced, while in method B it was initially added to the crystal-free solution. Since this is the main difference between the two methods, it must have been the cause of the result that larger crystals have been obtained by adding a small amount of an additive that can retard both nucleation and growth rates. But it is impossible here to explain clearly the experimental results, since we do not have any kinetic model which can describe the complicated phenomenon in an unsteady-state batch-cooling crystallization. There, temperature, concentration, crystal suspension density, nucleation rate, and growth rate, all of which are related in a complicated manner, are changing throughout the run. The decrease in crystal yield is explained simply by the retarding action of Cr3+on both the nucleation and growth rate processes. If sufficient crystallization time is given, the yield reaches its maximum, which is expected from the equilibrium as shown in Table I at t , = 20.0 " C . Jones et al. (1984) recently proposed an idea for making the product crystal size larger by adding a simple finesdissolving circuit to a batch crystallizer. This idea is not intended to suppress nucleation but simply to remove some freshly nucleated crystals. This plan might be easier for application in industry. Its mechanism is very clear; it requires an additional investment. On the other hand, our
Conclusions Larger product crystals of ammonium sulfate were obtained from its aqueous solution by adding Cr3+ion to the solution in a batch-cooling crystallizer provided the addition occurred after a large number of nuclei were already present. Smaller crystals were obtained when Cr3+ion was added before many nuclei had formed. This demonstrates a possibility for controlling product crystal size during batch crystallization operation by using small amounts of certain additives. Acknowledgment We express our thanks to Dr. Jones of the Department of Chemical and Biochemical Engineering, University College London, for his valuable comments. This paper was presented in part at the 49th Annual Meeting of The Society of Chemical Engineers, Japan, a t Nagoya, April 1984. Nomenclature C, = additive (Cr3+ion) concentration g/m3 solution c = concentration of solution, kg/kg of H,O L = crystal size, pm L , = median size of product crystals (mass basis), pm N , = agitation rate, rpm q = cooling rate, OC/min R = cumulative mass percent oversize, % t = temperature, O C t, = termination temperature, a temperature at which the run is stopped, O C Y = crystal yield (mass basis), % Greek Symbols 0 = time, min Oe = cooling period from the time at which saturation occurs until the termination temperature is reached, min Registry No. (NH&S04, 7783-20-2; Cr3+, 16065-83-1. Literature Cited Jones, A. G. Chem. Eng. Sci. 1974, 29, 1075. Jones, A. G.; Chianese, A.; Mullin, J. W. Industrial Crystallization 84; JanEiE, S. J., de Jong, E. J., Eds.; Elsevier: Amsterdam, 1984; p 191. Jones, A. G.; Mullin, J. W. Chem. Eng. Sci. 1974, 29, 109. Kubota, N.; Ito, K.; Shimizu, K. J . Crystal Growth 1986, 76, 272. Larson, M. A.; Mullin, J. W. J . Crystal Growth 1973, 20, 183. Mullin, J. W. Crystallization, 2nd ed.; Butterworths: London, 1972a, p184, 196; 1972b, p 420. Mullin, J. W.; Njvlt, J. Chem. Eng. Sci. 1971, 26, 369.
Noriaki Kubota,* Hiroshi Takahashi Department of Applied Chemistry Iwate University 4-3-5 Ueda, Morioka, 020 Japan Received for review July 26, 1985 Revised manuscript received February 13, 1987 Accepted May 11, 1987
