Impact of Ultrasonic Energy on the Crystallization of Dextrose


ABSTRACT: In this paper, we investigate the potential of ultrasonic energy in assisting the crystallization of dextrose monohydrate, which is primaril...
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CRYSTAL GROWTH & DESIGN

Impact of Ultrasonic Energy on the Crystallization of Dextrose Monohydrate Surya

Devarakonda,#,†

James M. B.

Evans,⊥,‡

and Allan S.

2003 VOL. 3, NO. 5 741-746

Myerson*,⊥

Department of Chemical Engineering, Polytechnic University, 6 Metrotech Center, Brooklyn, New York 11201, and Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois 60616 Received April 9, 2003;

Revised Manuscript Received June 25, 2003

ABSTRACT: In this paper, we investigate the potential of ultrasonic energy in assisting the crystallization of dextrose monohydrate, which is primarily manufactured via slow cool batch, lasting 48 h (0.5 °C/h), seeded crystallization; this cooling curve is designed to optimize the crystal growth and give rise to relatively large dextrose crystals. This study was interested in the impact of ultrasound on the nucleation, crystal breakage/size distribution, and rate of growth of the dextrose, while producing a product of the desired crystal size distribution. Experimental results show that ultrasonic energy can be used to induce nucleation and increase the overall mass rate of crystal growth while producing product with the desired crystal size distribution. 1. Introduction Dextrose monohydrate is primarily crystallized using a slow cool batch seeded approach. While the process yield is often acceptable (∼75%), it is a relatively slow process (48-h cooling cycle). The crystallization of the dextrose monohydrate is a slow process because a slow cooling cycle is required to prevent excessive nucleation, and in addition to this, the mass transfer and hence the growth rates within the system are limited because of the slow agitation rates. Increasing the rate of agitation within this system was deemed to be impracticable due to the high viscosity of the system (9.1 × 1015 c.p. at ambient temperatures) and the need for variable power agitators. The use of ultrasonic energy as a means of altering a crystallization process has been reported for a number of systems1-3 and might be suitable for enhancing the crystallization of dextrose monohydrate. Ultrasonic energy is thought to affect the crystallization process in two ways. Firstly, where ultrasound is creating heterogeneous nucleation sites, these nucleation sites are formed by cavitation where the application of ultrasound of an appropriate energy and frequency induces the formation and collapse of small bubbles.4,5 The formation and destruction of these bubbles is thought to create extremely high levels of localized supersaturation, which trigger the nucleation. Ultrasound is also thought to impact the nucleation by the creation of container imperfections, which have been identified as a common source of heterogeneous sites. Support for ultrasound creating surface imperfections and hence heterogeneous nucleation sites comes from the fact that any strong interaction such as scraping or hitting will result in heterogeneous sites,6,7 and if a corrosion token is placed into the system and the ultrasound is applied holes will be punched through the token (see for example Figure 1). * 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. † Current address: Wyeth Research, 401 N. Middletown Road, Pearl River, NY 10965, USA. ‡ Current address: GlaxoSmithKline, Cobden Street, Montrose, Angus, UK, DD10 8EA.

Figure 1. Cavitation damage on metal token (picture taken from European Society of Sonochemistry website8).

Secondly, ultrasound is capable of inducing crystal breakage.1 A primary effect of the crystal breakage is to increase the total surface area of the system, and from the mass growth rate equation (eq 1), it can be seen that this equates to an increase in the overall mass growth rate:6

dm ) Akg∆C dt

(1)

Another effect of the crystal breakage is to increase the overall rate of the nucleation via secondary nucleation,6,7 as the crystal breakage and crystal/ultrasound collisions give rise to fragments that are considered to act as nuclei. The purpose of this study was to investigate the impact of ultrasound on the crystallization of dextrose monohydrate in a batch-seeded vessel. With regard to the crystallization process, the study was specifically interested in the impact of ultrasound on the nucleation,

10.1021/cg034056r CCC: $25.00 © 2003 American Chemical Society Published on Web 07/12/2003

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crystal breakage/size distribution, and rate of growth of the dextrose, while producing a product in which there was no adverse affect to the quality.

Dextrose monohydrate was provided by CPC, Celerose 2001 batch number XD3835, with a purity of 99.2% D-glucose. 2.1 Batch Crystallization. Crystallization experiments were carried out in a Paar pressure reactor. The reactor was equipped with a variable steel drive, set at 100 rpm and cooling coil, and the temperature was controlled by employing a Neslab temperature circulator and programmer. The Paar reactor had a volume of 1100 cm3, which provided an operational volume of 400-1000 cm3. Ultrasonic energy was introduced into the system via an ultrasonic horn, which was mounted at the base of the vessel, approximately 0.5 in. in diameter; it is powered by a variable power supply and operates at 20 kHz. Particle size analysis was carried out using a M100 Lasentec probe; the Lasentec probe measures a lengthbased distribution, which is calculated via chord lengths, which are a function of the particle size, shape, and number.9 The ultrasonic intensity (I) can be calculated from a power output relationship:10

Poutput A

(2)

Mdm+wCp(T2 - T1) t2 - t1

(3)

For example, based on a 500-mL working volume and a saturated dextrose solution at 47.2 °C, with the power setting on the probe power supply at 100%, Poutput ) 4.93 W and intensity ) 3.71 w/cm3. The concentration of the dextrose monohydrate in solution (see for example Table 2), concentration %, was monitored via the refractive index11 using an Abbe refractometer.

Mwda Mdm Mwdm concentration % ) Mdm + Mw

(4)

The concentration was then used in the calculation of the solid-phase yield (SPY) which was used as an indicator of the progression of the crystallization process, with the degree of completion rising as the SPY increases.

( ) Mwda Mwdm

(

)

Mwda Mdm Mwdm Mdm + Mw (Mdm + Ms + Mw) SPY ) Mwda Mwda (Mdm + Ms) Mdm Mwdm Mwdm 1 - 1.1 Mdm + Mw (Mdm + Ms + Mw) (Mdm + Ms)

( )

[ (

)]

( )

0 20 40 60

(5)

3. Experimental Procedures The procedures used in this study will be presented in two sections: Firstly, experiments that were designed to investigate the impact of ultrasound on the crystal breakage and growth of the dextrose and secondly, experiments that looked at the impact of ultrasound on the nucleation of the dextrose monohydrate. 3.1 Crystal Breakage/Growth. The effect of ultrasound on the crystallization process of the dextrose monohydrate was investigated in several stages: Firstly, determining the impact of ultrasound on seeds, at three different volumes (400, 500, and 1000 mL) and four different power levels (0-60%). In each case, a seeded saturated solution was prepared at 25 °C; for

207 165 133 121

207 168 139 125

207 179 153 141

Table 2. Crystallization of Dextrose Using 500 mL Working Volumea time (h)

temp °C

refract index

conc %

SPY %

0.0 1.0 3.0 4.0 15.0 17.0 23.0 26.0 27.0 40.0 44.0 48.0 mean size of final product

47.2 47.2 47.2 47.2 43.6 41.9 39.2 36.7 36.7 24.2 23.3 22.5 201.5 µm

1.4740 1.4710 1.4673 1.4656 1.4515 1.4488 1.4431 1.4395 1.4392 1.4258 1.4228 1.4218

74.76 73.54 71.91 71.07 64.90 63.85 61.48 59.73 56.68 52.95 51.61 51.07

9.18 16.94 25.75 29.72 51.05 53.71 59.02 62.42 62.51 72.64 74.24 74.85

a

where A is the area of the vessel and Poutput is given by

Poutput )

mean seed diameter (µm) after 1 h exposure to ultrasound 400 mL 500 mL 1000 mL

ultrasonic power (%)

2. Materials and Apparatus

I)

Table 1. Mean Diameter of the Dextrose Seeds after 1-h Sonication

No ultrasound.

example, the 500-mL experiments consisted of 523 g of dextrose monohydrate, 114 g of water, and 58.1 g of dextrose seed. Initial size distribution measurements were made using the Lasentec probe. The ultrasound power was then turned to the desired level for 1 h; after exposing the systems to the ultrasound the Lasentec probe was then used to remeasure the size distributions. The second stage of this investigation was to study the impact of ultrasound (60% power ∼ 1.7 w/cm2) on batch seeded crystallization using a 500-mL working volume. The final stage of this study was to investigate the impact of varying the ultrasonic power (10-60%) on the crystallization process in a batch seeded system. 3.2 Nucleation. The effect of ultrasound on the nucleation of dextrose was investigated by preparing two identical aqueous solutions of 20% dextrose. The solutions were then cooled to 21.7 °C, which resulted in a supersaturated solution. Both solutions were then left to age for an hour, after which one of the samples was exposed to ultrasonic energy for a period of 1 h, while the other sample was left as a unsonicated control. In both cases, the concentration, which will be used to monitor the systems for the onset of nucleation, was measured via refractive index measurement.10

4. Results/Discussion 4.1 Crystal Growth. The effect of ultrasound energy on the mean diameter of the seeds (Table 1) shows that the seed diameter decreases as the power increases. The results in Table 1 also show that the volume of the solution will have an impact on the diameter of the seeds, with an increase in the working volume of the system corresponding to an increase in the mean diameter of the seeds. From these results, we can see that even relatively low power ultrasonic energy can influence the size of the dextrose seeds. The second stage of this study was to investigate the impact of ultrasound on the crystal growth/breakage in the batch seeded cooling vessel. The results in Table 2 are baseline results that were obtained from a control dextrose system, i.e., no exposure to ultrasound. The results are also plotted in Figure 2 and show that

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Figure 2. Plot of the concentration of dextrose monohydrate in solution as a function of time. Table 3. Crystallization of Dextrose Using 500 mL Working Volumea time (h)

temp °C

refract index

conc %

SPY %

0.0 0.5 1.0 2.0 3.0 4.0 5.0 17.0 21.0 24.0 41.0 45.0 48.0 mean size of final product

47.2 47.2 47.2 47.2 47.2 47.2 47.2 41.9 39.2 39.2 23.3 22.5 22.5 171.1 µm

1.4731 1.4715 1.4702 1.4660 1.4662 1.4590 1.4587 1.4496 1.4447 1.4440 1.4214 1.4220 1.4208

74.59 73.75 73.24 71.40 69.56 68.56 68.05 64.11 61.85 61.58 52.33 51.37 50.64

10.34 15.68 18.68 28.20 36.08 39.82 41.60 53.07 58.25 58.81 73.39 74.51 75.33

a

Ultrasound 60% power.

initially the system is moderately supersaturated (∼10%) and that after about 15 h the concentration of the dextrose monohydrate in solution decreases and is equivalent to the saturation point, i.e., system is just saturated, which implies that nucleation and growth have occurred. After the crystallization has occurred the system runs much closer to the saturation point until the process ends. The results also show that over the 48-h period that the SPY increases from 9.18 to 74.85% and that the dextrose concentration declines from 74.76 to 51.07%. This experiment was repeated, but this time the ultrasound was turned on for a period of 1 h (power 60%). Comparing the results of both systems in Figure 2 and Tables 2 and 3 it is immediately apparent that the use of ultrasound had a significant impact on the crystallization of the dextrose, with the ultrasound essentially causing the system to desupersaturate after only 6 h, i.e., concentration of dextrose remaining in solution is equal to the saturation point for that respective temperature, instead of 15 h. Further proof of the impact of the ultrasound was obtained by comparing the

Table 4. Crystallization of Dextrose Using 1000 mL Working Volume time (h)

temp °C

0 1.0 4.0 20.0 44.0

47.2 47.2 47.2 39.2 22.5

0.0 1.0 8.0 19.0 44.0

refract index

conc %

SPY %

60% Power 1.4744 1.4718 1.4626 1.4445 1.4265

74.82 73.83 69.88 61.75 53.37

8.77 15.20 34.81 58.46 72.11

47.2 47.2 45.8 39.2 22.5

40% Power 1.4739 1.4719 1.4576 1.4395 1.4206

74.70 73.87 67.63 62.07 53.57

9.59 14.95 43.01 57.78 71.86

0.0 1.0 7.0 20.0 44.0

47.2 47.2 45.8 39.2 22.5

20% Power 1.4749 1.4725 1.4588 1.4454 1.4276

74.81 73.80 68.12 62.15 53.81

8.84 15.38 41.36 57.61 71.55

0.0 1.0 7.0 20.0 44.0

47.2 47.2 45.8 39.2 22.5

10% Power 1.4732 1.4729 1.4613 1.4444 1.4271

74.63 74.23 69.32 62.18 53.87

10.07 12.69 37.01 57.54 71.47

SPY of the two systems, where it can be seen for example that after 4 h the system exposed to the ultrasound has a SPY of 39.82%, whereas the control system has as a SPY of 29.72%. The results in Table 3 also show that over the 48-h period that the SPY increases from 10.34 to 74.51% and that the dextrose concentration declines from 74.59 to 51.37%. The reason for the increase in the overall growth rate of dextrose monohydrate is that the application of ultrasound breaks the crystals (as shown by the change in the mean size of the product (see Tables 1-3)). As a result of this, batches exposed to ultrasound have a larger surface area. From the overall mass rate of crystallization (eq 1), it can be seen that

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Figure 3. Plot of the concentration of dextrose monohydrate in solution as a function of time.

Figure 4. Mean product size as a function of ultrasonic power.

an increase in the surface area will result in an increase in the mass transfer and hence the overall growth rate. The final part of this study was to investigate the impact of changing the ultrasonic power that the system was exposed to (60, 40, 20, and 10%). The results in Table 4 and Figure 3 show two things. Firstly, that exposure to even low power ultrasound will increase the overall rate of crystallization with all four systems initially desupersaturating somewhere between 6 and 10 h after they have been exposed to ultrasound; the desupersaturation of the solutions is indicative of crystallization having occurred, whereas from prior experiments (see Figure 1) it can be seen that in the absence of ultrasound it takes a similar dextrose monohydrate solution 15 h to desupersaturate. Secondly, the results clearly show that the increase in the overall rate of crystallization is a function of

ultrasonic power, as an increase in the power is seen to lead to a system desupersaturating more quickly. For example, after 4 h the concentration of dextrose in a solution that has been exposed to 60% power is 69.88%, whereas it takes somewhere between 6 and 7 h for a system that has been exposed to only 10% of the ultrasonic power to reach this level. The results of this phase of the study also indicate that as the ultrasonic power decreases then the supersaturation in the early stages of the process will increase. As a result of this, the average particle size increases from 178.6 µm at 60% power to 199.2 µm at 10% power (see Figure 4). 4.2 Nucleation. In terms of the control system (i.e., not exposed to ultrasonic energy), Figure 4 shows that during the first 2 h the concentration and hence the degree of supersaturation is constant and that no nucleation has occurred. Between hours two and four,

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Figure 5. Weight percent of dextrose in solution as a function of time for two samples, in which one sample is exposed to ultrasonic energy and one is not.

a very slight decline in the concentration was detected, which might indicate that a small amount of nucleation and subsequent crystal growth had occurred. In terms of the sonicated sample, the graph in Figure 5 clearly shows that after 1.5 h (0.5 h after initiating ultrasound) there is a decrease (2.16%) in the concentration of dextrose monohydrate in solution, which suggests that the nucleation process has been initiated. At this point the nucleation is believed to be primarily heterogeneous and is controlled by one of two mechanisms: The first mechanism is a cavitational effect4 where transient cavities are formed, grow, and then collapse. The collapse of these cavities produces intense local heating (∼5000 K), high pressures (∼1000 atm), and enormous heating and cooling rates (>109 K/s)12 and it is this rapid cooling that produces the very high localized supersaturation which then triggers the nucleation process. The second mechanism (as shown in Figure 1) is that the ultrasonic energy creates imperfections in the vessels and surface imperfections are known to give rise to heterogeneous nucleation sites.6,7 Between hours two and four, there is a further 8.5% decrease in the dextrose monohydrate concentration, as the system moves toward equilibrium, i.e., the concentration of dextrose monohydrate in solution is equal to the saturation point. The fact that the system is now desupersaturating more rapidly confirms that the nucleation process has been initiated. At this stage in the process, secondary nucleation is also thought to play an important factor. Support for a secondary nucleation mechanism6,7 playing a significant role in the overall process comes from the seed experiments (Table 1), which clearly show that ultrasound breaks up crystals. It is the crystal breakage and crystal/ultrasound collisions associated with the experiments that give rise to fragments of dextrose monohydrate which are considered to act as nuclei giving rise to further nucleation. Further evidence of the ability of ultrasound to assist nucleation comes from the batch experiments that were used to investigate the impact of ultrasound on the

crystal growth of dextrose monohydrate. These experiments also show that sonication of a sample leads to an increase in the rate of desupersaturation, and in terms of a batch cooling crystallization process the rate of desupersaturation is controlled by the degree of nucleation/crystal growth that has occurred. The results in these studies also show us that there is a clear correlation between the level of ultrasound applied and the rate of nucleation, because as Figure 2 shows, increasing the ultrasonic power level results in the system desupersaturating more quickly, i.e., increased rate of nucleation/crystal growth. 5. Conclusions The results of this study have demonstrated that ultrasonic energy can be employed to induce nucleation in supersaturated dextrose solutions and/or to break existing dextrose crystals. One or both of these results can be used to improve the overall rate of crystallization in existing seeded batch dextrose crystallizers by reducing the time needed per batch or by reducing the seed used. In addition to this the seed experiments suggest that ultrasonics could be used to produce dextrose seeds of a controlled size which would potentially enable greater control over the end product. It is likely that ultrasonic energy could also improve the mass transfer in dextrose solution and thus increase crystal growth; however, the level of power input required to allow this causes significant heating thus making the cooling of the solution 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. Nomenclature A ∆C

total surface area of the crystals, m2 supersaturation, kmol m-3

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Cp kg M Mw T1 T2 t1 t2

heat capacity of water and dextrose monohydrate, kg-1 K-1 growth rate constant, kg/[m2 s-1 (kg/kg)g] mass, g molecular weight, kg/kmol initial temperature °C final temperature °C initial times-1 final times-1

Subscript da dm s w

dextrose anhydrate dextrose monohydrate seed water

References (1) Amara, N.; Ratsimba, B.; Wilhelm, A.-M.; Delmas, H. Ultrason. Sonochem. 2001, 8, 267-270. (2) Li, H.; Wang, J.; Bao, Y.; Guo, Z.; Zhang, M. J. Cryst. Growth 2003, 247, 192-198.

Devarakonda et al. (3) Rozenberg, L. D. Physical Principles of Ultrasonic Technology: 001; Plenum Press: New York, 1973. (4) McCausland, L. J.; Cains, P. W.; Martin, P. D. CEP 2001, 56-61. (5) Suslick, S.; Didenko, Y.; Fang, M. M.; Hyeon, T.; Kolbeck, K. J.; McNamara, W. B., III; Mdleleni, M. M. Philos. Trans. R. Soc. London A 1999, 357, 335-353. (6) Myerson, A. S.; Ginde, R. In Handbook of Industrial Crystallization; Myerson, A. S., Ed., 2nd ed.; ButterworthHeinemann: London, 2002; pp 43-62. (7) Mullin, J. W. Crystallization, 3rd ed.; Butterworth-Heinemann: London, 1997; pp 172-189. (8) URL http://www.fb-chemie.uni-rostock.de/ess/sonochem_image.htm. (9) Lasentec M100 Hardware Manual, Laser Sensor Technology Inc., USA, 1998. (10) Hagenson, L. C.; Doraiswamy, L. K. Chem. Eng. Sci. 1998, 53 (1), 131-148. (11) Galleguillos, H. R.; Flores, E. K.; Aguirre, C. E. J. Chem. Eng. Data 2001, 46 (6), 1632-1634. (12) Suslick, K. S. Science 1990, 247, 1439-45.

CG034056R