CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 4 687-690
Articles Impact of Ultrasonic Energy on the Flow Crystallization of Dextrose Monohydrate Surya Devarakonda,†,# James M. B. Evans,§,‡ and Allan S. Myerson*,§ Department of Chemical Engineering, Polytechnic University, 6 Metrotech Center, Brooklyn, New York 11201, Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois 60616, and Glaxo Wellcome Manufacturing Pte Ltd., 1 Pioneer Sector 1, Singapore, 628413 Received September 19, 2003;
Revised Manuscript Received April 20, 2004
ABSTRACT: In this paper, we investigate the potential of ultrasonic energy in a flow crystallization situation, which was designed to mimic the industrial process, in which dextrose monohydrate is in a flow environment before it finally passes into the batch crystallizer where it is slow cooled. In this study, we were specifically interested in the impact of ultrasound on the seed size, lump breakage, and the induced nucleation of the dextrose monohydrate. Experimental results show that in a flow situation exposing the system to ultrasound energy will affect the seed size, and that for this system the optimum exposure/residence time is approximately 60 s. The results also show that ultrasound can be used to break up dextrose lumps that form in the process pipes (mean product size < 100 µm). Previous sonochemistry studies have shown that ultrasonic energy can be used to control various aspects of the crystallization processes of a number of systems including palm oil,1 potash alum,2 a number of APIs,3 and the batch crystallization of dextrose monohydrate.4 These studies have shown that sonochemistry can potentially control both the nucleation and crystal growth. Ultrasonic energy is thought to affect both the nucleation and the crystal growth phenomenon. In terms of nucleation, the application of ultrasonic energy is thought to increase the rate of nucleation by increasing the number of potential nucleation sites. Nucleation sites are formed by either a cavitation effect or the creation of surface imperfections (see Figure 1), which are known to act as heterogeneous nucleation sites.5,6 The cavitation effect,7 Figure 2, is where the application of ultrasound of an appropriate energy and frequency induces the formation and collapse of cavities.8-11 The formation and collapse of these cavities (bubbles) produces intense local heating (∼5000 K), high pressures (∼1000 atm) and enormous heating and cooling rates (>109 K/s).8 It has been postulated that this rapid * To whom correspondence should be addressed. Tel: +1 312 567 3163. Fax: +1 312 567 5205. E-mail: [email protected]
† Polytechnic University. § Illinois Institute of Technology. ‡ Glaxo Wellcome Manufacturing Pte Ltd. # Current address: Dr. Reddy’s Labs, Bulk Actives Unit II, Bollaram, Hyderabad 502 325, India.
Figure 1. Scanning electron micrograph of cavitation damage on a glass surface after sonication for 5 min.6
cooling produces high localized supersaturation which then triggers the nucleation process. In terms of crystal growth, previous research has shown that ultrasound is capable of inducing crystal breakage.4 One of the primary effects of this is to increase the total surface area of the system, which in turn gives rise to an increase in the overall mass growth rate of a system:9,12
dm ) Akg∆C dt
10.1021/cg034173m CCC: $27.50 © 2004 American Chemical Society Published on Web 06/18/2004
688 Crystal Growth & Design, Vol. 4, No. 4, 2004
Devarakonda et al. 2.1 Flow Crystallization. Flow crystallization experiments were carried out using a scaled version of an industrial flow system. A saturated (or supersaturated) solution, in this case dextrose, is pumped from a reservoir (reservoir 2) through a flow pipe, where the ultrasonic energy could be applied, before it enters the crystallization vessel (reservoir 1) (see Figure 3). Ultrasonic energy is introduced into the system via an ultrasonic horn, which is mounted on the flow pipe, approximately 0.5 in. in diameter, and it is powered by a variable power supply (0-100%), and operates at 20 kHz. The intensity13 (I) and the power of the ultrasound applied to the system can be calculated from
where A is the area of the vessel and Poutput is given by
Poutput ) Figure 2. Scheme of the growth and destruction of a cavitation bubble.6
Another effect of the crystal breakage is to increase the overall rate of the nucleation via secondary nucleation5,9,12 as the crystal breakage and crystal/ultrasound collisions give rise to fragments which are considered to act as nuclei. Previous studies on the impact of ultrasonic energy on the crystallization of dextrose monohydrate employed batch crystallization. While this system showed that ultrasonic energy impacted both nucleation and crystal growth it also demonstrated the major drawback of heating due to the use of ultrasound. In addition, the use of ultrasound in large tanks is difficult because of the rapid dissipation of the ultrasonic energy making the scale-up of batch processes difficult. A more realistic approach for the use of ultrasonic energy involves its use in a transfer line or box prior to the batch crystallizer where the advantages of small volume and short residence time can be employed. The purpose of this paper is to evaluate the use of ultrasonic energy on the crystallization of dextrose monohydrate in a flow system and evaluate its impact on nucleation, crystal growth, and crystal breakage. 2. Materials and Apparatus Dextrose monohydrate, with a purity of 99.2% D-glucose, was provided by Corn Products Company for all of the experiments.
Figure 3. Schematic of flow apparatus (not to scale).
Mdm+wCp(T2 - T1) t2 - t 1
For example, with the power setting on the probe power supply at 100% based, on a 24.54 mL3 working volume and a 50.44% saturated dextrose solution at 24.4 °C the Poutput ) 4.0978 W and I ) 0.005 w/cm3. Particle size analysis was carried out using a Lasentec M100 probe; the Lasentec probe measures a length-based distribution, which is calculated via chord lengths, which are a function of the particle size, shape, and number.14
3. Experimental Procedures The procedures used in this study will be presented in three sections: Firstly, experiments were designed to investigate the impact of ultrasound on seed size. The second series of experiments examined the impact of ultrasonic energy on lump breakage. The final set of experiments examined the impact of ultrasound on dextrose nucleation and its subsequent crystallization. All experiments in this work demonstrated that ultrasonic energy in a flow system produced only a negligible heat effect with a temperature rise of less than 1 °C. After each experiment, the flow system was flushed with water, thus ensuring that any dextrose monohydrate crystals present were removed from the system and thus any nucleation/crystallization observed during the experiments could be attributed to the impact of the ultrasonic energy. 3.1 Effect of Ultrasound on Seed Size. The effect of ultrasound on the seed size of dextrose monohydrate in a flow process was investigated by pumping a saturated solution, 50.44% dextrose by weight at 24.4 °C with seeds through the system (Figure 1) at a fixed flow rate. Ultrasonic energy (power set at 80% of maximum) was applied until all of the solution had been pumped from the reservoir to the crystallization vessel. Once the solution is in the crystallization vessel, a
Flow Crystallization of Dextrose Monohydrate
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Table 1. Effect of Ultrasound on the Mean Size of the Dextrose Seeds experiment no.
residence time (s)
1 2 3 4 5
145.0 81.7 61.7 56.4 39.4
mean size µm before USa after US 128.98 128.98 128.98 110.69 106.67
70.29 90.23 96.13 89.75 99.31
change % 45.50 30.04 25.47 18.92 6.90
portion of the sample is withdrawn and its particle size is measured via the Lasentec. 3.2 Effect of Ultrasonic Energy on Lump Breakage. Dextrose monohydrate lumps of approximately 12.7 mm in diameter were prepared and then dried for at least 24 h. An unseeded saturated dextrose solution, 50.44% dextrose by weight at 24.4 °C was then prepared and the lumps were added. The solution was then pumped through the system, and the flow rate was set to provide two different residence times (63 and 32 s), while applying ultrasonic energy (power set at 60%). After application of the ultrasound, the solution was collected in the crystallizer and the sample was checked visibly for the presence of any lumps; if any lumps are detected their approximate size is noted. The solution is then filtered to remove any lumps > 1 mm in size and analyzed via the Lasentec. 3.3 Effect of Ultrasound on Inducing Nucleation and subsequent Crystallization. The effect of ultrasonic energy on the nucleation and subsequent crystallization of dextrose was investigated using two different experimental methods. In the first experiment, ultrasonic energy (power set at 80% of maximum) was applied to a supersaturated solution at room temperature as it passed through the process piping. After the solution entered the crystallizing vessel, it was studied for the presence of any nuclei. In the second set of experiments, a saturated solution at 47.2 °C was prepared and allowed to cool naturally to room temperature, it was then pumped through the system, while being exposed to ultrasonic energy. Samples were collected at the end of the pipe to check for induced nucleation. In this second set of experiments, two identical samples were pumped through the flow system with only one of the samples exposed to the ultrasonic energy. The two samples were collected and then allowed to stand until they crystallized without any external influence, i.e., agitation, and the resulting product was then analyzed with the Lasentec system.
4. Results/Discussion 4.1 Impact on Dextrose Seed Size. The results in Table 1 show that the residence time has a significant impact on the mean size of the dextrose seeds. The results show that an increase in the residence time results in a decrease in the mean particle size. The results also show that a residence time of approximately 60 s will lead to a significantly larger change in the particle size than either shorter or longer residence times. Plotting the data reinforces the idea that a 60 s residence time has the largest impact on the particle size as Figure 4 clearly shows that at 60 s there is a change in the slope. A change in slope on a graph is often indicative of either a change in the driving force or of a key zone/point in a process. With this set of experiments, we see that above 60 s the efficiency of the particle size reduction is reduced, as we see there is only an additional 20.03% change for an additional 83.3 s of ultrasonic energy being applied. As with previous work carried out in a batch crystallizer,1 the results in Table 1 show that the application
Figure 4. Percentage change in dextrose seed size as a function of residence time. Table 2. Effect of Ultrasound on Lumps of Dextrose in a 250 mL Saturated Solutionsa exposure time (min)
approximate weight change %
5 4 3 2 1
80 70 65 58 50
Mean size of filtered product was 130 µm for all runs.
of ultrasound has a significant impact on the seed size. The results from this and the previous study show that the seed size and hence the final product size can be controlled by varying the total ultrasonic energy input into the system in terms of either the total power or the exposure time. 4.2 Lump Breakage. The second stage of this study was to determine the impact of ultrasonic energy on lump breakage; for both residence times the ultrasonic energy was found to be very effective in breaking the lumps. In fact, when investigating the crystallizer no lumps where detected. Lasentec analysis of the solution gives a mean size between 96 and 100 µm. Additional evidence of the effectiveness of applying ultrasonic energy to break dextrose lumps comes from a series of batch experiments (Table 2), in which spherical lumps of dextrose monohydrate of approximately 12.7 mm in diameter were prepared by hand; after drying for 24 h they were weighed and then exposed for between 1 and 5 min to 60% power ultrasound. After applying the ultrasound, the solution was filtered and the lumps were collected and then reweighed. Reweighing the lumps provides a rough estimate of the impact of the ultrasound on the dextrose monohydrate. 4.3 Nucleation and Associated Growth. In terms of the first set of nucleation experiments in which ultrasonic energy was applied to a supersaturated solution at room temperature as it passed through the process piping, no discernible effect was noticed either visually or via the Lasentec probe. The probable reason for this is that the experimental time scale was much smaller than the dextrose induction time. In fact, based
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on previous research,1 it would perhaps have been more realistic to allow at least 1 h to elapse before studying the system for the presence of any nuclei. The results for the second set experiments, in which two samples, one prior and one post the application of ultrasonic energy were collected and then allowed to stand until they crystallized without any external influence, were more interesting. As they showed that while dextrose monohydrate will eventually crystallize out spontaneously the application of ultrasonic energy can be used to significantly reduce the induction time of a crystallization process; in these experiments, spontaneous nucleation was seen to take in excess of 14 h, whereas the system that was exposed to the ultrasonic energy was seen to crystallize out in approximately 60-90 min. While the data in this study does not provide any significant new developments, it does however suggest that as with the batch study1 the nucleation is a combination of both primary (heterogeneous) and secondary nucleation. Earlier studies identified two potential mechanisms for inducing heterogeneous nucleation: firstly, a cavitational effect3 and secondly, the formation of surface imperfections, which act as heterogeneous nucleation sites.5,6 In terms of primary nucleation, the design of the flow system used in this study suggests that the heterogeneous nucleation is more likely to be controlled by the cavitational effect,3 rather than interactions with surface imperfections. Secondary nucleation5,6 is thought to play an important role in this crystallization process because the breaking (Tables 1 and 2) of the dextrose crystals gives rise to fragments/ seeds that act as a trigger for further nucleation. With regard to crystal growth, the results in Tables 1 and 2 show that the mean seed size decreases when a system is exposed to ultrasound, and as a result of this the system is left with a larger number of smaller seeds. This increase in the number of particles gives rise to a significant increase in the surface area of the system. If the surface area increase is considered in conjunction with the overall mass growth rate (see eq 1), then it is readily shown that an increase in the surface area will result in an increase in the mass transfer and hence the overall growth rate.5 5. Conclusions The results of this study have demonstrated that ultrasonic energy can be employed to control the seed
Devarakonda et al.
size, break up lumps of the solute, and induce nucleation and associated growth in a supersaturated dextrose solution. One or all of these results can be used to improve the overall efficiency of the process by reducing the crystallization time needed or by reducing the seed used. In addition, it has been shown that ultrasonics could also be used to produce the dextrose seed, of a controlled size, required for the process. It is likely that ultrasonic energy also improves the mass transfer in dextrose solution and thus increases crystal growth. While in a batch system the level of power input required causes significant heating thus making the cooling of the solution a problem, the approach used in this paper suggests that for a continuous system this would not be a problem. On the basis of our experiments, it is clear that ultrasound has a positive impact on both the nucleation and crystal growth phases of a process, and it should be feasible and valuable to scale-up the ultrasound process for industrial applications. References (1) Patrick, M.; Blindt, R.; Janssen, J. Ultrason. Sonochem. 2004, 11, 251-255. (2) Amara, N.; Ratsimba, B.; Wilhelm, A.-M.; Delmas, H. Ultrason. Sonochem. 2001, 8, 265-270. (3) Li, H.; Wang, J.; Bao, Y.; Guo, Z.; Zhang, M. J. Cryst. Growth 2003, 247, 192-198. (4) Devarakonda, S.; Evans, J. M. B.; Myerson, A. S. Cryst. Growth Des. 2003, 3, 741-746. (5) Liu, X. Y. Langmuir, 2000, 16, 7337-7345. (6) URL: http://www.fb-chemie.uni-rostock.de/ess/sonochem_image.htm. (7) McCausland, L. J.; Cains, P. W.; Martin, P. D., CEP, July 2001, 56-61. (8) Suslick, K. S. Science 1990, 247, 1439-1445. (9) Myerson, A. S.; Ginde, R. In Handbook of Industrial Crystallization; Myerson, A. S., Ed., 2nd ed.; ButterworthHeinemann: London, 2002; pp 43-50. (10) Abramov, O. V. High-Intensity Ultrasonics, Theory and Industrial Applications; Gordon and Breach Science Publishers: Overseas Publishers Association, New York, 1998. (11) Hem, S. L. Ultrasonics 1967, 5, 202-207. (12) Mullin, J. W. Crystallization, 3rd ed.; Butterworth-Heinemann: London, 1997; pp 172-189. (13) Hagenson, L. C.; Doraiswamy, L. K. Chem. Eng. Sci. 1998, 53, 131-148. (14) Lasentec M100 Hardware Manual, Laser Sensor Technology Inc., USA, 1998.