Crystal Shape Control by Manipulating Supersaturation in Batch Cooling Crystallization Yang,*,†
Guangyu Jinfu Wang†
Noriaki
Kubota,‡
Zuoliang
Sha,#
Marjatta
Louhi-Kultanen,§
and
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 12 2799-2803
Department of Chemical Engineering, Tsinghua UniVersity, 100084, Beijing, P. R. China, Department of Applied Chemistry, Iwate UniVersity, 3-5, Ueda 4, Morioka-shi, Iwate, 020-8551, Japan, Tianjin UniVersity of Science and Technology, College of Marine Science of Technology, Street 13, No. 29, 300457, TEDA, Tianjin, P. R. China, and Department of Chemical Technology, Lappeenranta UniVersity of Technology, P.O. Box 20, FI-53851, Lappeenranta, Finland ReceiVed June 23, 2006; ReVised Manuscript ReceiVed October 7, 2006
ABSTRACT: This paper describes a study on the shape control of potassium dihydrogen phosphate (KDP) in batch cooling suspension crystallization by selecting the supersaturation as a media control variable. The effects of different cooling modes on the supersaturation level and hence on the crystal shape were investigated. The results clearly show that different shapes of crystals were obtained at the various supersaturation courses. When a specific shape of crystal is desired from suspension batch cooling crystallization, a suitable cooling mode should be selected and furthermore the optimal one can be found. 1. Introduction It is well-known that the morphology of crystals influences downstream processes, such as filtration, washing, drying, packaging, etc. Much work has been published on controlling the shape of crystal; however, most studies focus on modification of the crystal shape using additives or different solvent systems.1,2 Furthermore, different polymorphs of organic compounds in partiular may have distinct crystal habits.3,4 However, there is little discussion in the literature on how to control the shape of multidimensional crystals in suspension crystallization by only adjusting the operating conditions, e.g., cooling mode. For multidimensional crystals, crystal shape is determined by the different growth rates of the different crystallographic faces. It is well known that supersaturation is a key parameter to determine the crystal growth kinetics, and usually the effect of supersaturation on growth kinetics is different for each crystallographic face. Therefore, it is possible to modify the crystal shape by controlling the supersaturation level during crystal growth. In this work, to investigate a method to control the shape of crystals in batch cooling suspension crystallization, potassium dihydrogen phosphate (KDP) was selected as a model compound. The modification of KDP crystal shape has been widely studied by changing pH value or using additives.5,6 However, this work demonstrates the dependence of the crystal shape of KDP on the supersaturation level in suspension crystallization. The results show that the different shapes of KDP particles can be obtained by adjusting the cooling profile, seed loading, and batch time, and hence a suitable operating condition can be selected for a desired crystal shape. 2. Experimental Investigation The batch cooling experiments were performed in a 4 L glass crystallizer with a U-shaped bottom, which was equipped with a fourblade impeller and four baffles. The impeller was rotated at a speed of * To whom correspondence should be addressed. Phone: + 861062796109. Fax: +861062772051. E-mail:
[email protected]. Web: http:// info.tsinghua.edu.cn. † Tsinghua University. ‡ Iwate University. # Tianjin University of Science and Technology. § Lappeenranta University of Technology.
350 rpm during the batch to ensure that all the particles were well suspended. The cooling range was selected from the initial temperature (see Figure 1) to 20 °C. The mass of water and KDP to obtain a 4 L saturated solution at 35 °C was calculated on the basis of the solubility data published by Mullin.7 The suspension was first heated to 40 °C, 5 °C above the saturation temperature, to ensure the complete dissolution of the solute, and then was linearly cooled to the initial temperature to start-up a batch. A specific cooling profile was followed during the batch after a certain amount of seeds was fed into the crystallizer. The seed crystals were obtained by sieving with two sieves of close successive sizes, and the average size of seeds was calculated to be 190 µm based on the sizes of the two sieves. The shape of seed crystals is shown in Figure 2. During the batch, the temperature of the slurry was recorded by a computer. At a certain cooling temperature, a microfilter and a sampling tube, preheated to this cooling temperature, were used to withdraw the sample of the mother liquor from suspension. Duplicate samples were taken at the almost same time to ensure the reliability of the measurement. The liquid sample was weighed before and after being dried overnight in an oven at 80 °C, and hence the concentration of the mother liquor and supersaturation were calculated. At the end of a batch, the wet solid sample was washed with acetone to prevent sticking or growing of particles during drying. The shapes of dried crystals were observed by microscope and recorded with a digital camera attached to the microscope. Two types of cooling modes, i.e., natural cooling mode and controlled cooling mode, were used in the present study. The method to predict the controlled cooling mode can be referenced to previous work.8 The natural cooling was implemented by circulating water at a constant temperature of 20 °C, where the different seed loadings were used to induce crystallization. For the controlled cooling mode, the different cooling profiles were predicted at the different operating conditions (seed loading and batch time). The kinetic models used in the prediction of cooling profiles were presented in Table 1, which were obtained from the separate batch experiments for the same crystallizer. The cooling modes used in this work are shown in Figure 1 and Table 1. The selected seed loadings and batch times are presented in Table 1.
3. Experimental Results The shape of KDP crystal is a tetragonal prism combined with a tetragonal bipyramid, and the angle between the prism sides and pyramid faces is 45°. Therefore, two dimensions, i.e., width, rw, and length, rL, can be used to describe the basic shape of a KDP crystal.9 The natural habit of KDP crystals in diagrammatic form is shown in Figure 3. The aspect ratio of length to width is defined as R ) rL/rw.
10.1021/cg0603873 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/18/2006
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Figure 1. The cooling profiles used in batch crystallization.
Figure 3. Natural habit of KDP crystals.
Figure 2. The seed crystals with an average size of 190 µm. Table 1. Operating Conditions in Batch Cooling Crystallizationa
operating condition
cooling profile
seed loading cs (%)
batch time τ (h)
1 2 3 4 5 6
A A B C D E
6 12 6 8 12 12
2 2 1 2 4 1
a c : ratio of mass of seed crystals to product yield. ∆c: supersaturation, s kg/m3, Mt: suspension density, kg/m3. A: natural cooling; B, C, D, and E: controlled cooling. Growth rate model: G ) 5.3 × 10-11∆c2.678 (m/s). Nucleation rate model: B ) 1.01 × 103∆c1.513Mt0.943 (#/m4).
The photographs of crystals obtained at the different cooling modes are shown in Figures 4-7 and the aspect ratios, R, are presented in Table 2. The results clearly show that the different shapes of crystals might be obtained when the different cooling modes are used to operate the batch. The supersaturation courses, corresponding to the various cooling modes (in Figure 1 and Table 1), are presented in Figure 8 and Table 2. In natural cooling, the supersaturation has a peak in the early cooling time, and then its level gradually decreases with time. The shapes of product crystals obtained with the natural cooling have relatively low aspect ratios. Furthermore, the shape of crystals has a small difference with natural cooling at the seed loadings of 6 and 12%. The reason might be that the corresponding supersaturation courses in these two cases only have a small difference, as shown in Figure 8. However, the relatively large variation in the shape of product crystals was observed in controlled cooling modes because the corresponding supersaturation courses have a relatively large difference, as shown in Figure 8. It can be seen that the aspect ratio of the product crystals was small at high supersaturation level, but it becomes large with decreasing
supersaturation level. If the growth rate GL and Gw correspond to length and width of a crystal shown in Figure 3, the results obtained in this work show that the ratio of GL to Gw is expected to increase with decreasing supersaturation level. To identify the relative impact of a single factor, the seed loading was fixed at 12%, but the cooling profile E (Figure 1) was employed to control batch. The obtained supersaturation course is presented in Figure 8, and the shape of crystals is shown in Figure 9. It can be seen that the shape of crystals presented in Figure 9 is similar to the one shown in Figure 5. The reason might be that the supersaturation courses in these two cases are very close, as shown in Figure 8. Therefore, it can be concluded that supersaturation plays a dominant role in determination of crystal shape. 4. Discussion The natural morphology of KDP crystal was studied by Xu and Xue9 with the intrinsic physicochemical properties of the crystal by calculation of bond strength of constituent chemical bonds formed between growth units. Both calculation and experimental results confirmed that growth rate along Z-axis (Figure 3) is the most rapid one, and hence the preferential growth direction of KDP is parallel to the Z-axis. In the single KDP crystal growth studies reported by Mullin and Amatavivadhana,10 it is observed that at low supersaturation level, growth occurs only along the Z-axis (Figure 3). When the supersaturation level increases to a critical value, growth then commences along the XY-axes. It is also reported that the growth rates of KDP crystals along both Z- and XY-axes are the second-order process. The single KDP crystal growth studies were also carried out by Joshi and Paul,11 who observed that the relatively longer and narrower crystals can be obtained at low supersaturation, whereas high supersaturation gives relatively broader crystals. Moreover, the ratio of length to width and the ratio of growth rates of pyramidal and prism faces decrease nonlinearly with increasing supersaturation level. The reason was explained that the growth rate of the prism faces is parabolic at low supersaturations until a critical value, and then it becomes linear. However, the growth rate of pyramidal linearly varies within the supersaturation range studied. Rudiyanto et al.12 determined
Crystal Shape Control by Manipulating Supersaturation
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Figure 6. The crystals obtained from the controlled cooling mode with a batch time of 2 h and a seed loading of 8%.
Figure 4. (a) The crystals obtained from the natural cooling with a batch time of 2 h and a seed loading of 6%. (b) The crystals obtained from the natural cooling with a batch time of 2 h and a seed loading of 12%.
Figure 5. The crystals obtained from the controlled cooling mode with a batch time of 1 h and a seed loading of 6%.
the two-dimensional (2D) growth rate model of KDP using batch cooling experiments initiated with a certain amount of seed crystals. The results also show that the growth rate along the Z-axis (Figure 3) is higher than the one along the XY-axis.
Figure 7. The crystals obtained from the controlled cooling mode with a batch time of 4 h and a seed loading of 12%. Table 2. Product Crystals and Corresponding Supersaturation Courses operating condition
aspect ratio of product crystals, R
supersaturation course
1 2 3 4 5 6
1.5-2.0 2.0-2.5 2.0-2.5 4.0-5.0 5.5-6.5 2.0-3.0
a a′ b c d e
The published work mentioned above supports the results obtained in this study. Because the working levels of supersaturation studied by Mullin and Amatavivadhana10 and Rudiyanto et al.12 were not within the supersaturation range in this work, the 2D growth rate models in their work cannot be used to predict the product size for the present study. However, the 2D sizes of product crystals were calculated using the growth rate models determined by fitting experimental data reported by Joshi and Paul,11 as shown in Table 3. The time-average supersaturation levels shown in Figure 8 were used in the calculation. Because the overall growth rate model (Table 1) was used in this work, the 2D product sizes cannot be predicted. However, the average 2D crystal sizes measured in the present study are shown in Table 3. It can be seen that the calculated width and length of product crystals can be compared to the measured
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Figure 8. The supersaturation courses obtained at different operating conditions.
Figure 9. The crystals obtained from the controlled cooling mode with a batch time of 1 h and a seed loading of 12%. Table 3. Comparison of Calculated and Measured Two-Dimensional Product Sizes
calculated measured calculated measured
saturation level, e.g., 12.87 kg/m3. The natural cooling has an advantage over the controlled cooling in that it is easy to operate. However, with the controlled cooling mode, the product crystals with the desired shape can be obtained with a short batch time, which might highlight the optimal way to operate the batch. For instance, for obtaining the cubic-like shape of KDP crystals, the favorable operating condition might be cooling profile B (Figure 1) and seed loading of 6% within the operating conditions listed in Table 1. It should be noticed that with the controlled cooling mode the supersaturations have the certain deviations around the timeaveraged values, as shown in Figure 8, and these deviations are larger than the experimental error of 4%. The reason is that the cooling profiles used to control the batch, as shown in Figure 1, were predicted based on an average shape factor of crystals and the overall growth rate model. Therefore, the supersaturation might be controlled more accurately when the size-dependent shape factor together with the multidimensional growth rate models are taken into account in the prediction of cooling profile. On the other hand, the feedback process control based on the in-line measurement of supersaturation has been presented recently,13,14 which might be a promising way to control crystal shape by accurately measuring supersaturation during the batch process. It is also worth mentioning that KDP was selected as a model compound for a case study, which implies that the proposed method can be applied to the suspension crystallization of other multidimensional crystal compounds. The developed method proposes the supersaturation as a key parameter to be controlled to modify the crystal shape. In this work, the supersaturation is generated by cooling; however, the principle of the proposed method can be used for other supersaturationgenerating systems, i.e., evaporating, salting-out, and precipitation. 5. Conclusions
supersaturation s
crystal width rw, µm
crystal length rL µm
size ratio rL/rw
0.021 0.021 0.046 0.046
205 200 203 200
921 1000 622 500
4.5 5.0 3.0 2.5
a Growth rate model used in calculation (Joshi and Paul11). G ) 2.05 w × 10 -6 s2. GL ) 3.48 × 10 -6 s1.09. Supersaturation: s ) (c - c*)/c*. c: concentration of mother liquor, c*: concentration of saturated solution.
ones. The difference between the calculated and the measured results might be that the growth rate models determined in the work reported by Joshi and Paul11 were based on the singlecrystal growth study; however, the crystallizer operated at the suspension crystallization in this work. Moreover, the timeaveraged supersaturation values based on Figure 8 were used in the calculation; however, the real supersaturation courses fluctuate around these time-averaged values in the experiments, as shown in Figure 8. The results obtained in this study highlight the practical way to control crystal shape in batch suspension crystallization to meet the different product requirements. For example, the small aspect ratio R might facilitate the downstream process of product crystals and might produce good powder flow ability and high packing density. To obtain this cubic-like shape of KDP crystals in batch cooling crystallization, it is proposed to use either the natural cooling mode with a certain amount of seeds or the controlled cooling mode operated at a relatively high super-
The different shapes of KDP crystals were observed by adjusting the cooling mode, seed loading, and batch time in batch suspension crystallization. The results show that the supersaturation plays a dominant role in the determination of crystal shape. The experimental results were confirmed by the growth law and the kinetic model reported in the literature for KDP crystals. To meet the different product requirements for particle materials, e.g., cubic-like shape of KDP crystals, the suitable operating condition might be either natural cooling with a certain amount of seed loading or controlled cooling at relatively high supersaturation level. The proposed method can be applied in other supersaturation-generating systems or other multidimensional crystal compounds with the aim of modifying the crystal shape based on the supersaturation control. Acknowledgment. Tsinghua University and the Academy of Finland (Academy Research Fellow post No. 76440 and Project No. 204513) are gratefully acknowledged for financial support for the present work. References (1) Korlakunte, V. R. P.; Ristic, R. I.; Sheen, D. B.; Sherwood, J. N. Int. J. Pharm. 2001, 215, 29-44. (2) Nokhodchi, A.; Bolourtchian, N.; Dinarvand, R. Int. J. Pharm. 2003, 250, 85-97. (3) Bernstein, J. Polymorphism in Molecular Crystals; Oxford University Press: Oxford, 2002. (4) Brittain, H. G. Polymorphism in Pharmaceutical Solids; Marcel Dekker Inc.: New York, 1999.
Crystal Shape Control by Manipulating Supersaturation (5) Qu, H. Y.; Louhi-Kultanen, M.; Kallas, J. J. Cryst. Growth 2006, 289, 286-294. (6) Sharma, S. K.; Verma, S.; Shrivastava, B. B.; Wadhawan, V. K. J. Cryst. Growth 2002, 244, 342-348. (7) Mullin, J. W. Crystallization, 4th ed.; Butterworth-Heinemann: Oxford, UK, 2001. (8) Yang, G.; Louhi-Kultanen, M.; Sha, Z.; Kubota, N.; Kallas, J. J. Chem. Eng. Jpn. 2006, 39, 426-436. (9) Xu, D. L.; Xue, D. F. J. Cryst. Growth 2006, 286, 108-113. (10) Mullin, J. W.; Amatavivadhana, A. J. Appl. Chem. 1967, 17, 151-156.
Crystal Growth & Design, Vol. 6, No. 12, 2006 2803 (11) Joshi, M. S.; Paul, B. K. J. Cryst. Growth 1974, 22, 321-327. (12) Rudiyanto, G.; Ma, D. L.; Fujiwara, M.; Braatz, R. D. Int. J. Mod. Phys. B 2002, 16, 367-374. (13) Fujiwara, M.; Chow, P. S.; Ma, D. L.; Braatz, R. D. Cryst. Growth Des. 2002, 2, 363-370. (14) Gron, H.; Borissova, A.; Roberts, K. J. Ind. Eng. Chem. Res. 2003, 42, 198-206.
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