Ind. Eng. Chem. Res. 2006, 45, 1631-1635
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MATERIALS AND INTERFACES Agglomeration Control of L-Ornithine Aspartate Crystals by Operating Variables in Drowning-out Crystallization Sang-Mok Chang,† Jong-Min Kim,† In-Ho Kim,‡ Dong-Myung Shin,§ and Woo-Sik Kim*,§ Department of Chemical Engineering, Dong-A UniVersity, Busan 604-714, Korea, Department of Chemical Engineering, Chungnam National UniVersity, Daejeon 305-764, Korea, and Department of Chemical Engineering, Kyunghee UniVersity, Kyungki-do 449-701, Korea
The agglomeration of L-ornithine aspartate crystallized by the drowning-out method was investigated in a turbulent agitated semi-batch reactor. To modify the agglomeration conditions, the crystallization operating factors, including the agitation, feed flow rate, feed concentration, and drowning ratio (volume ratio of antisolvent to feed solution), were all varied. The mean particle size of the L-ornithine aspartate in the product suspension increased with increasing feed concentration, drowning ratio, and feeding time, as crystal agglomeration was promoted via an increased crystal population and holding time in the agitated reactor. Conversely, it was sensitively reduced with increasing agitation speed because of the enhanced external force of the turbulent impact. Furthermore, the agglomeration behavior of the crystals was monitored during and after the crystallization to determine the role of supersaturation and turbulent fluid motion. Introduction Drowning-out crystallization is frequently used for the separation and purification of materials in the fine chemical, food, pharmaceutical, and biochemical industries. However, the benefits of this type of crystallization are invariably frustrated by agglomeration that entraps the impurities and mother liquor among the agglomerated particles. Thus, crystal agglomeration has drawn much attention in crystallization studies. Drowning-out crystallization is driven by decreasing the solubility of a solution via the addition of an antisolvent agent to inhibit a stable equilibrium of the solution. According to Plasari et al.’s1 study on the drowning-out crystallization of ethylcellulose, when water was used as the antisolvent agent against an ethanol solvent, the agitation speed, impellor type, water/ethanol ratio, and initial concentration of ethylcellulose were all found to be significant factors in determining the particle size and shape. In particular, the initial concentration of the solute had the most critical influence on the particle size and distribution of the product suspension. Meanwhile, in albumin precipitation,2 the agitation speed, with its effect on crystal agglomeration, was found to have the most impact on the agglomerated particle size. In general, the crystal agglomeration process consists of two consecutive steps: crystal adhesion and molecular growth.3 The first step involves crystal collisions due to fluid motion and the physical adhesion of crystals, and then the molecular growth step forms a growth layer on the adhered crystals and transforms them into particles of crystal agglomerates. The agitation speed * To whom correspondence should be addressed. Tel.: +82-31-2012576. Fax: +82-31-202-1946. E-mail:
[email protected]. † Dong-A University. ‡ Chungnam National University. § Kyunghee University.
and properties of the dispersion medium have already been shown to be important factors in the agglomeration of potassium sulfate. In addition, various other factors have also been suggested as influential in crystal agglomeration, including the feed concentration, supersaturation, crystallizer type, and feed input mode.4-9 However, in most previous studies, the crystallization has involved inorganic materials based on strong ionic bonding, such as calcium carbonate, potassium sulfate, aluminum hydroxide, yttrium oxalate, etc., where the agglomerated crystals are so strong that they can hardly be broken by an external force. As a result, the crystal agglomeration is predominantly determined by the crystal collisions, and the size in the product suspension invariably increases with increasing agitation speed. Nonetheless, in the crystallization of organic materials that form molecular crystals based on physical bonding, such as hydrogen bonding and van der Waals forces, the influence of an external force on the crystal agglomeration will likely be different, as the organic crystals are somewhat fragile. Accordingly, the present study examined the influence of the crystallization operating variables on agglomeration, including the adhesion and breakage of L-ornithine aspartate crystals produced using the drowning-out method. In addition, the role of supersaturation on crystal agglomeration was also investigated. The major crystallization operating factors considered were the agitation, feed flow rate, feed concentration, and drowning ratio. Experiments For the drowning-out crystallization, L-ornithine aspartate (pH 6.6, Brix 43%, purity greater than 99.9%) supplied by the Korea Research Institute of Bioscience and Biotechnology was used without further purification. ACS-grade methanol (Adlrich-Fluka Chemical Co., Milwaukee, WI) was used as the antisolvent for
10.1021/ie050831j CCC: $33.50 © 2006 American Chemical Society Published on Web 02/07/2006
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Figure 1. Schematic diagram of the experimental apparatus for the drowning-out crystallization of L-ornithine aspartate: 1, agitator; 2, impeller; 3, reactor; 4, three-way valve; 5, flow meter; 6, control valve; 7, bypass valve; 8, pump; 9, feed reservoir.
Table 1. Solubility of L-Ornithine Aspartate in Methanol/Water Binary Solutions mole fraction of MeOH
solubility (mol/L)
0.00 0.0472 0.100 0.160 0.229 0.309 0.401 0.510 0.641 0.801 1.00
3.20 2.513 2.148 1.322 0.586 0.261 5.29 × 10-2 1.61 × 10-2 4.65 × 10-3 9.38 × 10-5 1.21 × 10-4
the drowning-out agent. The L-ornithine aspartate solution concentration was adjusted using deionized water. Initially, pure methanol was loaded in the reactor, and then the L-ornithine aspartate solution was injected to crystallize L-ornithine aspartate, as shown in Figure 1. A standard Rushton mixing tank (working volume of 0.4 L) made of Pyrex glass was used as the crystallization reactor.10 To minimize the effect of the heterogeneous solid surfaces of the reactor on the crystallization, the reactor wall was carefully cleaned with distilled water and rinsed with pure methanol, and the impeller was also treated in the same manner after being finely polished with abrasive paper. The feed flow of the L-ornithine aspartate solution was monitored using a floating flow meter until the suspension in the reactor reached the working volume of the reactor. The volume ratio of the initially loaded methanol to the feed solution injected into the reactor was defined as the drowning ratio. Thus, at the fixed working volume of the reactor, an increase in the drowning ratio represented not only an increase in the methanol volume but also a corresponding decrease in the feed solution volume. In addition, the feed injection of the L-ornithine aspartate solution was stopped just after the suspension of the reactor reached its working volume. Here, the time period for the feed injection was defined as the feed time. At fixed feed volume, the feed time was reduced as the feed flow rate was increased. The crystallization conditions were modified in terms of the agitation speed, feed concentration, feed flow rate, and drowning ratio. As such, the impeller speed was varied from 500 to 1500 rpm, the feed concentration from 0.5 to 2.0 mol/L, the feed flow rate from 5 to 30 mL/min, and the drowning ratio from 2 to 5. The temperature of crystallization was fixed at 25.0 ( 0.1 °C. After crystallization, a sample of the product suspension in the reactor was quickly filtered using a vacuum, and then the
Figure 2. Typical morphology of L-ornithine aspartate crystal agglomerates produced by drowning-out crystallization with methanol: (a) 0.5 mol/L feed concentration, 4.0 drowning ratio, 500 rpm agitation speed, 10 mL/min feed flow rate; (b) 1.0 mol/L feed concentration, 2.0 drowning ratio, 500 rpm agitation speed, 10 mL/min feed flow rate; (c) 0.5 mol/L feed concentration, 4.0 drowning ratio, 1000 rpm agitation speed, 10 mL/min feed flow rate.
particles were redispersed in pure methanol to measure the particle size distribution (Mastersizer/E, Malvern Instruments, Malvern, U.K.) and observe the particle morphology (Leica SEM, Stereoscan 400, Leica Microsystems AG, Wetzlar, Germany). The solubility of L-ornithine aspartate in water/methanol mixtures was simply measured, as summarized in Table 1. Excess solute was added to the mixture solvent, and the remainder was filtered out to be weighed after gentle agitation of the mixture for 48 h at 25 °C. Results and Discussion The L-ornithine asparate crystallization was initiated by injecting the feed solution into the reactor containing the antisolvent (methanol), resulting in milky-white crystals. As shown in Figure 2, significant agglomeration was apparent based on tiny crystals around 1-3 µm in size, suggesting that the crystal size was predominantly influenced by the agglomeration of crystals in the course of the crystallization. The overall shape
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Figure 3. Agglomeration behavior of L-ornithine aspartate at various feed concentrations in drowning-out crystallization. The feed flow rate and drowning ratio were fixed at 10.0 mL/min and 4.0, respectively.
of the particles was always spherical, but the particle size varied with the crystallization conditions, i.e., agitation, drowning ratio, and feed concentration. In crystallization, crystal agglomeration is frequently encountered in an agitated reactor, as the crystals adhere through collisions during turbulent mixing. According to the agglomeration mechanism,3,9 the physically adhered crystals, called aggregates, then become strongly cemented particles, called agglomerates, by molecular growth in a supersaturated solution. Simultaneously, the adhered and agglomerated crystals can also be redispersed and broken by external forces, such as collision impact and turbulent shear force. Thus, if adhesion is promoted by agitation, the particle size of the product suspension is enlarged, whereas if breakage of the aggregates and agglomerates is increased, this results in a reduction of the particle size. In addition, a higher crystal population would promote better agglomeration through the greater chance of crystal collisions. At a fixed drowning ratio of 4.0, as shown in Figure 3, the mean particle size decreased monotonically as the agitation speed was increased, implying the amplification of breakage rather than adhesion with increased turbulent mixing of the suspension. In the case of L-ornithine aspartate, which forms molecular crystals through secondary bonding, it is reasonable to consider that the crystal agglomerates would be easily redispersed and broken by the external forces of collision impact and turbulent shear because of their low physical strength, thereby explaining why turbulent agitation effectively promoted breakage rather than adhesion of the crystals, as reflected in the mean agglomerate size profile relative to the agitation speed, which is also consistent with previous results.11 Such a trend of the mean particle size with respect to the agitation speed was clearly found at high feed concentration (1.6 mol/L) and diminished as the feed concentration was decreased. That is, at the high feed concentration of 1.6 mol/L, the high nucleation of tiny crystals was induced by a high supersaturation, providing a high chance of crystal collision and adhesion. Thus, the mean particle size reached 150 µm at 500 rpm and was sensitively reduced as the agitation speed was increased. However, at a lower feed concentration, a smaller
Figure 4. Agglomeration behavior of L-ornithine aspartate at various agitation speeds in drowning-out crystallization. The total amount of feed injected was fixed at 0.13 mol, and the feed flow rate was fixed at 10.0 mL/min.
population of tiny crystals was generated because of the lower supersaturation, thereby reducing the chance of crystal collision and adhesion and resulting in a smaller crystal agglomerate size insensitive to variations in agitation. Thus, at the feed concentration of 0.5 mol/L, the mean particle size became almost independent of the agitation speed. It should be mentioned that, in Figure 3, the mean particle size at the feed concentration of 0.5 mol/L appeared slightly enlarged by e10% with agitation speed. This result can be attributed to the turbulent eddy size relative to the particle size, dictating the acting mechanism of the turbulent fluid force on the particles. As in the present study at low feed concentration, where the particle size was closely comparable to the Kolmogorov microscale representing the turbulent eddy size, the particles flow along with the turbulent eddy stream, resulting in ineffective breakage and redispersion. Then, crystal agglomeration might be slightly more promoted by turbulent fluid motion rather than the redispersion and breakage of particles. The influence of the drowning ratio and feed concentration for a fixed total quantity of feed on the crystal agglomeration is displayed in Figure 4. In this case, the feed concentration, feed volume, and drowning ratio were automatically changed under a fixed molar quantity of L-ornithine aspartate fed into the reactor (0.13 mol). For example, the increment of feed concentration from 1.0 to 2.0 mol/L automatically brought about the decrement of the feed volume from 133 to 67 mL as well as the corresponding change of the drowning ratio from 2.0 to 5.0. Therefore, the increment of the feed concentration brought about a high supersaturation for crystal nucleation. At the same time, the corresponding increment of the drowning ratio from 2.0 to 5.0 also contributed to an enhancement of supersaturation resulting from the decreased solubility of L-ornithine aspartate in water/methanol mixtures from 0.00168 to 0.00032 mol/L. As a result, the increased feed concentration meant that a higher population of crystals was nucleated, thereby accelerating the crystal agglomeration to produce larger mean agglomerate sizes depending on the agitation. Also, the turbulent motion predominantly contributed to breakage of the crystal agglomerates, reducing the particle size as the agitation speed was increased.
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Figure 5. Dynamic behavior of crystal agglomeration during the drowningout crystallization of L-ornithine aspartate.
The transient behavior of the crystal agglomeration during the crystallization was investigated, as shown in Figure 5, using three sets of experimental conditions selected from those in Figure 4. Here, the feeding time (tF) is defined as the time period from the beginning of the feed injection until the solution in the reactor reached the working volume (0.4 L) at a constant feed flow rate of 10 mL/min. For example, at a drowning ratio of 2.0, the feed volume of the solution was 133 mL and the feeding time 13.3 min. At the low agitation speed of 500 rpm, the mean crystal size gradually grew during the crystallization, as the low external force from the agitation was ineffective in breaking and redispersing the crystals agglomerated by collision and adhesion. However, at the high agitation speed of 1500 rpm, the agglomeration of crystals was quickly limited by the high turbulent mixing, even though crystals were continuously nucleated during the feed injection. In agglomeration,9 a supersaturated solution is required to form particles (crystal agglomerates) via molecular growth, and better molecular growth provides stronger particles. Therefore, after the feed injection had been completed, the resulting suspension was further agitated to examine whether any additional crystal agglomeration would be generated without the creation of supersaturation. This was also expected to reveal the strength of the particles produced in the course of the crystallization. Here, product suspensions prepared by crystallization at 500 and 1000 rpm (with other crystallization conditions fixed) were further agitated at 1000 rpm without any feeding of the L-ornithine aspartate solution. As shown in Figure 6, the mean particle size did not increase, implying no crystal agglomeration without supersaturation. Instead, the mean size of particles crystallized at 500 rpm was markedly reduced from about 140 µm to an equilibrium size of 55 mm with increasing agitation time, as the weakly agglomerated particles formed in the crystallization were broken by the highly turbulent agitation (1000 rpm). However, the particles produced by the crystallization at 1000 rpm were strong enough to resist further turbulent agitation after the feeding time, making the mean particle size almost independent of the agitation time. Because the molecular growth in supersaturation is promoted by mass transfer around the crystals, high agitation during crystallization would assist in the formation of strong particles.
Figure 6. Crystal agglomeration of L-ornithine aspartate after feeding has stopped. The feed concentration and the drowning ratio were fixed at 1.6 mol/L and 4.0, respectively. The mean agglomerate sizes at just end of feeding (L0) of the particles produced at 500 and 1000 rpm were 138.8 and 113.1 µm, respectively.
Figure 7. Effect of feed flow rate on crystal agglomeration in the drowningout crystallization of L-ornithine aspartate. The drowning ratio was fixed at 4.0.
At a fixed drowning ratio of 4.0, an increase of the feed flow rate resulted in a decrease in the mean particle size, as shown in Figure 7. Because the feeding time was defined as the time period of feed injection to fill the working volume of the reactor, the holding time of the suspension in the reactor during the feed injection decreased as the feed flow rate was increased. In this case, because the feed flow rate can simultaneously modify the supersaturation for crystal nucleation and the exposure time of the particles for agglomeration and breakage by turbulent impact, which were critical factors directly dictating the agglomeration process, it was hard to determine how much each factor contributed to the crystal agglomeration. At least, however, it can be inferred from the present results that, at a long holding time of low feed flow rate, an enhanced chance
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of collisions for crystal agglomeration apparently predominated over turbulent impact for particle breakage. Also, it can be marginally considered that a low supersaturation for a long feeding time was more beneficial to crystal agglomeration than a high supersaturation for a short feeding time. Thus, the mean particle size was found to increase when the feed flow rate was decreased. Conclusion In the drowning-out crystallization of L-ornithine aspartate, crystal agglomeration was identified as the major process determining the mean crystal size and was found to depend on the agitation speed, feed flow rate, feed concentration, and drowning ratio. Because agglomeration is dictated by crystal adhesion and breakage, it is directly controlled by the supersaturation and external forces of the agitated suspension. That is, a high supersaturation nucleated a high crystal population, increasing crystal adhesion in the suspension. Therefore, a high feed concentration and drowning ratio promoted crystal agglomeration, producing a high mean particle size for the induction of a high supersaturation. Meanwhile, the external force of turbulent fluid motion enhanced the breakage of the crystal agglomerates over the adhesion of crystals by collisions. Thus, particle size was reduced at increasing agitation speed. Such a trend of the particle size with the agitation speed was diminished as the feed concentration was decreased because of a low supersaturation. Finally, crystal agglomeration was found to require a state of supersaturation to cement the physically adhered crystals. Thus, even with continuous agitation, the crystals did not agglomerate without feed injection.
Acknowledgment The authors are grateful for research funding from KOSEF (R01-2005-000-10245-0). Literature Cited (1) Plasari, E.; Grison, P. H.; Villermax, J. Influence of Process Parameters on the Precipitation of Organic Nanoparticles by Drowningout. Trans. Inst. Chem. Eng. 1997, 75, 237. (2) Iyer, H. V.; Przybycien, T. M. Metal Affinity Protein Precipitation: Effect of Mixing, Protein Concentration and Modifier on Protein Fractionation. Biotechnol. Bioeng. 1995, 48, 324. (3) Macy, J. C.; Cournil, M. Using a Turbidometric Method to Study the Kinetics of Agglomeration of Potassium Sulfate in a Liquid Medium. Chem. Eng. Sci. 1991, 46, 693. (4) Wojcik, J. A.; Jones, A. G. Experimental Investigation into Dynamics and Stability of Continuous MSMPR Agglomerative Precipitation of CaCO3 Crystals. Trans. Inst. Chem. Eng. 1997, 75, 113. (5) Ilievski, D.; White, E. T. Agglomeration during Precipitation: Agglomeration Mechanism Identification for Al(OH)3 Crystals in Stirred Caustic Aluminate Solution. Chem. Eng. Sci. 1994, 49, 3227. (6) Ilievski, D.; White, E. T. Agglomeration during Precipitation: I Tracer Crystals for Al(OH)3 Precipitation. AIChE J. 1995, 41, 518. (7) Ilievski, D.; Hounslow, M. J. Agglomeration during Precipitation: II Mechanism Deduction from Tracer Data. AIChE J. 1995, 41, 525. (8) Zumstein, R. C.; Rouseau, R. W. Agglomeration of Copper Sulfate Pentahydrate Crystals within Well-Mixed Crystallizers. Chem. Eng. Sci. 1989, 44, 2159. (9) Sung, M. H.; Choi, I. S.; Kim, J. S.; Kim, W.-S. Agglomeration of Yttrium Oxalate Particles Produced by Reaction Precipitation in Semi-Batch Reactor. Chem. Eng. Sci. 2000, 55, 2173. (10) McCabe, W. L.; Smith, J. C.; Harriott, P. Unit Operations of Chemical Engineering, 4th ed.; McGraw-Hill: New York, 1985, p 208. (11) Shin, D. Y.; Kim, W.-S. Drowning-out Crystallization of L-Ornithine Aspartate in Turbulent Agitated Reactor. J. Chem. Eng. Jpn. 2002, 35, 1083.
ReceiVed for reView July 14, 2005 ReVised manuscript receiVed November 17, 2005 Accepted November 29, 2005 IE050831J
