Batch Cooling Crystallization in Non-Isothermal Taylor Vortex Flow

Article pubs.acs.org/crystal

Batch Cooling Crystallization in Non-Isothermal Taylor Vortex Flow: Effective Method for Controlling Crystal Size Distribution Zhaohui Wu,† Do Hyun Kim,‡ and Woo-Sik Kim*,† †

Department of Chemical Engineering, Kyung Hee University Seocheon-Dong, Giheung-Gu, 446-701 Yongin-Si, Korea Department of Chemical Engineering, KAIST, 291 Daehak-Ro, Yusung-Gu, 341-41 Daejeon-Si, Korea

‡

S Supporting Information *

ABSTRACT: A non-isothermal Taylor vortex fluid motion was applied for effective control of the crystal size distribution (CSD) in batch cooling crystallization without seed crystals. The non-isothermal Taylor vortex fluid motion was generated using different cylinder temperatures, i.e., a hot inner cylinder and cold outer cylinder, in a Couette−Taylor (CT) crystallizer. Thus, an internal loop of heating dissolution of crystals on the inner cylinder and cooling recrystallization on the outer cylinder was created in the gap between the two cylinders by the Taylor vortex fluid motion. As a result, the crystal size distribution can be effectively controlled by adjusting the operating parameters, including the temperature difference between the inner and outer cylinders, rotation speed of the inner cylinder, and cooling rate in the CT crystallizer. When increasing the temperature difference, the mean crystal size becomes larger and the CSD becomes narrower. Meanwhile, increasing the rotation speed enlarges the mean crystal size and broadens the CSD. Conversely, a fast cooling rate reduces the mean crystal size and narrows the CSD. The mean crystal size and CSD in the non-isothermal CT crystallizer are 3−4 times larger and 30−40% narrower, respectively, when compared with those in the isothermal CT crystallizer and mixing tank crystallizer.

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INTRODUCTION The crystal size distribution (CSD) is an important property of crystal products in the fine chemical, pharmaceutical, and powder industries, as it influences the downstream processes, including the filtration, washing, and drying of the crystal suspension and determines the activity and functionality of fine chemicals and pharmaceuticals.1,2 Generally, the supersaturation profile during crystallization had a direct influence on the CSD. That is, a broad supersaturation profile could produce a crystal suspension with a broad size distribution. Thus, many techniques for controlling the CSD are based on manipulating the supersaturation profile via such operating parameters as the cooling rate, flow rate, and hydrodynamic flow motion during crystallization.3−5 As a result, a fast cooling rate was found to produce a small mean crystal size and narrow CSD in the batch cooling crystallization of C15, increasing the supersaturation level reduced the benzoic acid crystal size in a plug flow crystallizer, and a different hydrodynamic flow motion using a static mixer in a plug flow crystallizer caused a different mean crystal size and changes the coefficient of variation (CV) of flufenamic acid due to the different supersaturation profile in the crystallizer.6−8 The temperature swing, fines destruction, and seeding methods were proposed to design the supersaturation profile in cooling crystallization and thereby control the CSD.9−11 In the temperature swing method, the solution temperature in the crystallizer were periodically programmed to heat up and then cool down. During the heating period, the crystals were dissolved for fines destruction, and the remaining crystals then © XXXX American Chemical Society

grow during the cooling period. As a result, the initially small size and broad distribution of crystals was shifted to a large size and narrow distribution of crystals after several temperature swing cycles, as demonstrated by Takiyama et al.12 In their study, initial potash alum crystals with a mean crystal size of 227 μm and CV of 0.24 were shifted to a mean crystal size of 784 μm and CV of 0.157 when using the programmed temperature swing method, whereas they were changed to a mean crystal size of 266 μm and CV of 0.586 when using a simple linear cooling method. The CSD in the product suspension was also shown to vary significantly according to the initial crystal amount, initial CSD, temperature swing profile, and number of temperature swing cycles.10,11,13 However, the temperature swing cycles method could be applicable only to the batch operation and also require a long operation time, resulting in low productivity in the cooling crystallization.11,14−18 The fines destruction technique using an external fines dissolver was an effective way to control the CSD in a cooling crystallization process.8,19 That is, the small crystals were moved to the upper region of the crystallizer, while the large crystals were precipitated in the lower region of the crystallizer. The suspension in the upper region of the crystallizer was circulated to an external fines dissolver to destroy the fines crystals, and then fed back into the crystallizer for recrystallization. In this technique, the cutoff threshold of fine Received: May 31, 2016 Revised: November 4, 2016 Published: December 5, 2016 A

DOI: 10.1021/acs.cgd.6b00821 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Taylor vortex fluid motion was formed in the gap of the crystallizer. However, when both cylinders were controlled at different temperatures, a non-isothermal Taylor vortex was generated in the gap of the crystallizer. That is, one cylinder was set at a hot temperature to heat the solution (Th), while the other cylinder was set at a cold temperature to cool the solution (Tc). Thus, in practice, two kinds of non-isothermal modes could be considered in a CT crystallizer, one where the inner cylinder was heated and the outer cylinder was cooled, and the other where the temperature settings were reversed. In the present study, the first mode of heating the inner cylinder and cooling the outer cylinder was adopted due to its higher efficiency, as demonstrated in our previous study.21 Plus, the heating and cooling temperatures of the cylinders in the non-isothermal mode were adjusted to create the same bulk solution temperature (Tb) as with the isothermal mode. The L-lysine solution was prepared by dissolving 900 g of raw dehydrate L-lysine in 1 L of deionized water at a saturated temperature of 43.0 °C. The isothermal CT crystallizer, which was preheated to 50.0 °C, was filled with the L-lysine solution without a seed crystal. At this point, both cylinder temperatures were equal at 50.0 °C. The crystallizer was then run at 50.0 °C for 20 min to completely dissolve any L-lysine crystals in the solution. Thereafter, the bulk solution was linearly cooled from 50.0 °C to a set bulk solution temperature (Tb = 28.0 °C) by decreasing the cylinder jacket temperatures. The bulk solution was then maintained at the set temperature for 6 h. For the isothermal mode, the temperature profiles of inner and outer cylinders were same with the cooing profile of the bulk solution, as shown in Figure 2. However, for the non-isothermal mode, the temperature

crystals of an internal crystal classifier was important to determine the CSD in the product. Ooshima’s lab developed a wall wetter/double-deck jacket (WWDJ) crystallizer to control the CSD.20 The main principle of this crystallizer was also based on consecutive cycles of the heating−dissolution of fines crystals and cooling−recrystallization. That is, fines destruction was induced in the upper heating region of the crystallizer, while recrystallization occurred in the lower cooling region of the crystallizer. Thus, the crystal suspension was internally circulated to the heated and cooled regions of the crystallizer to increase the mean crystal size and decrease the CSD. Yet, practically, this technique was also applied to the batch operation and required a long operation time over 25 h to improve the CSD from 44% to 35%. In our previous study using seed crystals, it was demonstrated that a non-isothermal Taylor vortex fluid motion was quite effective for controlling the crystal size distribution even at the saturated conditions. That is, the two different temperatures of the cylinders generated local nonequilibriums on the two cylinders, and then the toroidal circular fluid motion of a Taylor vortex automatically created an internal loop of heating−dissolution and cooling−recrystallization in the crystallizer. As a result, initially small and broad seed crystals (230 μm mean crystal size and 0.814 CV) were changed to large and narrow crystals (654.7 μm mean crystal size and 0.335 CV).21 Accordingly, the present study investigated the working mechanism of a non-isothermal Taylor vortex fluid motion for controlling the CSD in the batch cooling crystallization of Llysine without seed crystals. Plus, the influence of the operating parameters was also studied, including the temperature difference between the inner and outer cylinders, rotation speed of the inner cylinder, and cooling rate.

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EXPERIMENTAL SECTION

A Couette−Taylor (CT) crystallizer was used for the batch cooling crystallization of L-lysine, as shown in Figure 1. The CT crystallizer

Figure 2. Temperature profiles with different cooling rate in isothermal Couette−Taylor and mixing tank crystallizers. The inset presents the notations of the cooling profile for crystallization. profiles of inner and outer cylinders were differently controlled from that of the bulk solution, as described in detail later in the NonIsothermal Taylor Vortex section. The cooling rate of the bulk solution and the rotation speed of the inner cylinder were varied from 0.13 to 0.48 °C/min, and from 300 to 1000 rpm, respectively. In addition, for the non-isothermal mode, the temperature difference (ΔT = Th − Tc) between the two cylinders was varied from 5.1 to 13.4 °C while maintaining the bulk solution at the same set temperature of 28.0 °C. For achieving such a non-isothermal mode, the temperature of inner cylinder was higher than the bulk solution temperature, and the temperature of outer cylinder was lower than the bulk solution temperature. A similar batch cooling crystallization of L-lysine crystals was carried out in a standard mixing tank (MT) crystallizer with a volume of 500 mL. The MT crystallizer was equipped with a six-paddle impeller and four baffles for turbulent agitation. The agitation speed was varied from 500 to 2000 rpm. Plus, a cooling jacket was installed on the outer wall of the crystallizer for temperature control. The initial and set temperatures in the MT crystallizer were the same as those in the CT

Figure 1. Experimental setup for non-isothermal batch cooling crystallization using a Couette−Taylor crystallizer. consisted of inner and outer coaxial cylinders, of which the length was 30 cm and radii were 2.4 and 2.8 cm, respectively. The Taylor vortex fluid motion was induced in the gap of 0.4 cm between the inner and outer cylinders by the rotation of the inner cylinder. In the CT crystallizer, both cylinders were equipped with thermal (cooling/ heating) jackets for control of the cylinder temperatures. When both cylinders were controlled at the equal temperature, an isothermal B



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crystallizer at 50.0 and 28.0 °C, respectively. The cooling profile of the bulk solution in the MT crystallizer was also the same as that in the CT crystallizer. The temperatures of the inner and outer cylinders and bulk solution in the CT and MT crystallizers were all in situ monitored and recorded using the Labview program (National Instruments). Intermittently, suspension samples were taken from the sampling port of the CT crystallizer to analyze the CSD. A video microscope (IT system, Sometech, USA) was used to monitor the CSD of the crystal suspension and crystal shapes. Size measurements of over 500 crystals were counted in the microscopic images to analyze the CSD (Xojo Binary Project, Xojo Inc., USA).

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RESULTS AND DISCUSSION Isothermal Taylor Vortex. The influence of the cooling rate on the CSD in the cooling crystallization was investigated by using isothermal mode CT crystallizer. In the crystallization, the time taken to cool the initial solution temperature (Ti) to the set solution temperature (Ts) was noted as “ts”, while the time to the end of the crystallization was noted as “te” (Figure 2). The change in the CSD according to the crystallization time was also monitored, as shown in Figure 3. The mean crystal size in the CT crystallizer increased gradually and then leveled off after 3 h of crystallization time, whereas the coefficient of variation (CV) of the CSD showed little variation with the crystallization time. These profiles for the mean crystal size and CV of the CSD with the crystallization time were similar for all the cooling rates, indicating that the crystals were mostly formed during the cooling period (before te) and then grew without further nucleation until the supersaturation was depleted. However, the mean crystal size and CV of the CSD were significantly reduced when increasing the cooling rate, due to a higher nucleation rate with a higher cooling rate. (Figure 4) That is, nucleation was induced at a higher supersaturation (lower solution temperature) when increasing the cooling rate, thereby generating a higher population of crystals with a smaller size and narrower size distribution.4,22 This explanation was also confirmed by the induction temperature when varying the cooling rate (Figure 4a). The induction temperature was lowered when increasing the cooling rate, indicating a higher supersaturation for the induction of nucleation. As result, the 214 μm mean crystal size and 0.7 CV at a slow cooling rate of 0.13 °C/min were reduced to a 98 μm mean crystal size and 0.5 CV at a cooling rate of 0.48 °C/min. (Figure 4b). These crystallization results were also confirmed with microscopic images, as shown in Figure 5. The broad size range of crystals produced at a low cooling rate of 0.13 °C/min (Figure 5a) became small and uniform in size when increasing the cooling rate (Figure 5d). The influence of the Taylor vortex fluid motion on the CSD in the CT crystallizer was investigated and compared to the CSD in the MT crystallizer, as shown in Figure 6. When increasing the rotation speed of the inner cylinder in the CT crystallizer, the mean crystal size and CV of the CSD also increased. This dependency of the mean crystal size and CV on the rotation speed of the inner cylinder was due to the induction of nucleation depending on the Taylor vortex fluid motion. It is already known that the periodic Taylor vortex in a CT crystallizer effectively promotes nucleation.19,23 Thus, the induction of nucleation in the Taylor vortex occurred at a higher temperature (Figure 6c), resulting in a larger crystal size and broader CSD when increasing the rotation of the inner cylinder in the CT crystallizer.

Figure 3. Cooling crystallization of L-lysine at various cooling rates in an isothermal Couette−Taylor crystallizer. (a) Profile of mean crystal size with crystallization time; (b) profile of CV with crystallization time. The crystallization conditions were fixed at a bulk solution temperature (Tb) of 28 °C and a rotation speed of the inner cylinder of 500 rpm.

A similar change in the mean crystal size and CV of the CSD according to the impeller speed in the MT crystallizer was also observed. However, there were significant differences in the crystallization between the CT and MT crystallizers due to the different fluid motions in each crystallizer: a periodic Taylor vortex in the CT crystallizer and a random turbulent eddy in the MT crystallizer. Apparently, a periodic Taylor vortex fluid motion was more effective for inducing nucleation than a random turbulent eddy motion at the same rotation speed.23 Thus, the induction temperature in the CT crystallizer was much higher than that in the MT crystallizer, resulting in a larger mean crystal size and broader CDS in the CT crystallizer than in the MT at the same rotation speed. Non-Isothermal Taylor Vortex. The non-isothermal Taylor vortex fluid motion was created by using different temperature settings for the inner and outer cylinders of the CT crystallizer. Initially, the bulk solution temperature, inner cylinder temperature, and outer cylinder temperature were all in equilibrium at 50 °C, and then cooled down to their set temperatures of Tb, Td, and Tr, respectively, at a constant C



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Figure 5. Microscope images of crystals obtained at te in an isothermal Couette−Taylor crystallizer. Cooling rates of (a) 0.13 °C/min, (b) 0.18 °C/min, (c) 0.26 °C/min, and (d) 0.48 °C/min. The crystallization conditions were fixed at a bulk solution temperature (Tb) of 28 °C and a rotation speed of the inner cylinder of 500 rpm.

addition, the temperature difference (ΔT) for the nonisothermal Taylor vortex flow was defined as the difference between the inner and outer cylinder temperatures in the CT crystallizer (Td − Tr). Figure 8 showed the influence of the temperature difference (ΔT = Td − Tr) on the mean crystal size and CV during the cooling crystallization. The rotation speed of the inner cylinder and cooling rate of the bulk solution were always fixed at 500 rpm and 0.26 °C/min, respectively. Plus, the bulk solution temperature (Tb) was always fixed at 28.0 °C, although the temperature difference (ΔT) varied. The mean crystal size increased monotonically, while the CV decreased gradually with the crystallization time. These profiles for the mean crystal size and CV according to the crystallization time were similar for all the temperature differences and were significantly amplified when increasing the temperature difference. This result was due to the internal dissolution−recrystallization cycle in the nonisothermal CT crystallizer. That is, as demonstrated in our previous study,21 the crystals in the bulk solution, including fine crystals, were dissolved on the heated inner cylinder, thereby increasing the bulk solution concentration. The toroidal fluid motion of the Taylor vortex then circulated the bulk solution into the cooled outer cylinder, causing recrystallization for crystal growth. As a result, this internal cycle of dissolution and recrystallization enlarged the crystal size and narrowed the CSD. Thus, when the temperature difference (ΔT) was 0, corresponding to the isothermal CT crystallizer, the mean crystal size and CV was hardly changed as no internal dissolution-recrystallization cycle was formed in the crystallizer. However, when the internal dissolution−recrystallization cycle was formed in the non-isothermal CT crystallizer (ΔT > 0), the mean crystal size increased significantly and the CV decreased markedly with the crystallization time. This non-isothermal effect was amplified when increasing the temperature difference (ΔT), as the driving forces for dissolution (ΔTd = Td − Tb) and recrystallization (ΔTr = Tb − Tr) increased, resulting in a higher efficiency for the internal dissolution-recrystallization cycle, as shown in the Supporting Information (Figure S1a). Thus, large size crystals (400 μm mean crystal size) and a narrow CSD (0.29 CV) in the crystal product were obtained with a

Figure 4. Influence of cooling rate on crystallization of L-lysine in isothermal Couette−Taylor crystallizer. (a) Influence of the cooling rate on the induction temperature and corresponding supersaturation ratio (C/C*) and (b) influence of cooling rate on mean crystal size and CV at te. The crystallization conditions were fixed at a bulk solution temperature (Tb) of 28 °C and a rotation speed of the inner cylinder of 500 rpm.

cooling rate for cooling crystallization in the CT crystallizer. The inner cylinder temperature (Td) was set higher than the bulk solution temperature (Tb) for heating dissolution of crystals on the inner cylinder, while the outer cylinder temperature (Tr) was set lower than the bulk solution temperature (Tb) for cooling recrystallization on the outer cylinder, as depicted in Figure 7. In the present study, the cooling rate was represented by that of the bulk solution. Thus, the actual cooling rate of the inner cylinder was slower than the cooling rate of the bulk solution, while the cooling rate of the outer cylinder was faster than the cooling rate of the bulk solution. These different cooling rates were necessary to allow the bulk solution, inner cylinder, and outer cylinder to reach their set temperatures (Tb, Td, and Tr) at the same time of ts. Thereafter, the bulk solution temperature, inner cylinder temperature, and outer cylinder temperature were all maintained at their respective set values for 6 h (te). Thus, the cooling crystallization occurred primarily during the cooling period before ts. After ts, the crystallization continued based on the consumption of the remaining supersaturation until te. In D



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Figure 7. Temperature profiles for a non-isothermal Couette−Taylor vortex flow in a crystallizer.

Figure 6. Influence of rotation speed on mean crystal size and CV obtained at te in an isothermal Couette−Taylor crystallizer and a mixing tank crystallizer. (a) Variation of mean crystal size and CV with rotation speed at te, (b) variation of CV with rotation speed at te, (c) variation of induction temperature and supersaturation ratio (C/C*) with rotation speed in the isothermal Couette−Taylor and mixing tank crystallizers. The crystallization conditions were fixed at a bulk solution temperature (Tb) of 28 °C and a cooling rate of 0.26 °C/min.

Figure 8. Cooling crystallization of L-lysine at various temperature differences (ΔT) in a non-isothermal Couette−Taylor crystallizer. (a) Profile of mean crystal size with crystallization time. (b) Profile of CV with crystallization time. The crystallization conditions were fixed at a bulk solution temperature (Tb) of 28 °C, rotation speed of 500 rpm, and cooling rate of 0.26 °C/min.

temperature difference of 13.4 °C. It should be mentioned that according to our previous study,21 the mean crystal size became smaller and the CV becomes larger when the temperature

difference went above a certain criterion, due to extra nucleation during the recrystallization period of the internal cycle. A criterion of 1.35 was suggested for the temperature difference, as defined by the ratio of the dissolution driving E



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depending on the temperature difference. As shown in the Supporting Information (Figure S2), the bulk solution temperature (Tb), inner cylinder temperature (Td), and outer cylinder temperature (Tr) at the induction point of nucleation varied with the temperature difference in the CT crystallizer. The induction temperature of the outer cylinder decreased when increasing the temperature difference, although the induction temperatures of the bulk solution and inner cylinder both increased. This result implied that the induction of nucleation in the non-isothermal CT crystallizer was predominantly determined by the outer cylinder temperature. Also, the hot temperature of the inner cylinder might suppress the induction of nucleation at the cold temperature of the outer cylinder. Thus, the induction temperature of the outer cylinder in the non-isothermal CT crystallizer was always lower than that in the isothermal CT crystallizer. As a result, the induction of nucleation at a higher supersaturation produced smaller crystals and a narrower CSD when increasing the temperature difference. The effect of the cooling rate on the mean crystal size and CV in the non-isothermal CT crystallizer was shown in Figure 10. Similar to the results in the isothermal CT crystallizer, the mean crystal size and CV in the non-isothermal CT crystallizer were reduced when increasing the cooling rate (Figure 10a). These results were confirmed by the induction temperature when varying the cooling rate (Figure 10b). When increasing the cooling rate, the induction of nucleation occurred at a higher supersaturation, producing smaller sized crystals and a narrower CSD at ts (Supporting Information Figure S3). Thereafter, these crystals kept undergoing the dissolution− recrystallization cycle in the non-isothermal crystallizer until the end of the crystallization time (te). For example, when using a cooling rate of 0.48 °C/min, the mean crystal size was enlarged from 75 μm at ts to 350 μm at te, while the CV was improved from 0.43 at ts to 0.34 at te. It is interesting to note from Figures 4 and 10 that the mean crystal sizes and CVs at ts in the nonisothermal CT crystallizer were only slightly different from those in the isothermal CT crystallizer across the whole range of cooling rates. However, the mean crystal size at te in the nonisothermal CT crystallizer was 3 times larger than that in the isothermal CT crystallizer, while the CV at te in the nonisothermal CT crystallizer was about 30−40% lower than that in the isothermal CT crystallizer. Therefore, this demonstrates that the non-isothermal conditions in a CT crystallizer were highly effective for controlling the crystal size and CSD during cooling crystallization. Figure 11 showed the effect of the rotation speed on the mean crystal size and CV in the non-isothermal CT crystallizer. It was already well-known that a Taylor vortex fluid motion effectively promoted the induction of nucleation. Thus, the induction temperature increased when increasing the rotation speed of the inner cylinder (Supporting Information Figure S4), thereby increasing the mean crystal size and CV at ts, which were consistent with those in the isothermal CT crystallizer. By virtue of the internal dissolution−recrystallization cycle, the crystals at ts were about 3 times larger at te, while the CV at ts was 20−30% reduced at te. Of course, the dependency of the mean crystal size and CV at te was also the same as that at ts. The mean crystal size and CV produced in the nonisothermal and isothermal CT crystallizers and MT crystallizer were compared in Figure 12. The two different fluid motions, the Taylor vortex and random turbulent eddy that varied

force to the recrystallization driving force (ΔTd/ΔTr). In the present study, this ratio was always below 1.35, as shown in the Supporting Information (Figure S1b), implying recrystallization for crystal growth without any extra nucleation. The present results are consistent with our previous study. It was also interesting to find that the dependency of the mean crystal size at ts on the temperature difference differed from that of the mean crystal size at te, as shown in Figure 9a.

Figure 9. Influence of temperature difference (ΔT) on cooling crystallization of L-lysine in a non-isothermal Couette−Taylor crystallizer. (a) Influence of temperature difference (ΔT) on mean crystal size and CV with different temperature differences (ΔT) at ts. (b) Influence of temperature difference (ΔT) on mean crystal size and CV with different temperature differences (ΔT) at te. The crystallization conditions were fixed at a bulk solution temperature (Tb) of 28 °C, cooling rate of bulk solution of 0.26 °C/min, and rotation speed of 500 rpm.

That is, the mean crystal size at ts became smaller when increasing the temperature difference, whereas the mean crystal size at te became larger when increasing the temperature difference (Figure 9b). The increment of the mean crystal size at te according to the temperature difference was reasonable due to the efficiency of the internal dissolution−recrystallization cycle, as explained above. However, the trend of the mean crystal size at ts was originated from the induction temperature F



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Figure 10. Influence of cooling rate on mean crystal size and CV at crystallization time of te in a non-isothermal Couette−Taylor crystallizer. (b) Variation of induction temperature and corresponding supersaturation ratio (C/C*) with cooling rate. The crystallization conditions were fixed at a temperature difference (ΔT) of 8.8 ± 0.1 °C, bulk solution temperature (Tb) of 28 °C, and rotation speed of 500 rpm.

Figure 11. Influence of rotation speed on mean crystal size and CV in a non-isothermal Couette−Taylor crystallizer. (a) Influence of rotation speed on mean crystal size and CV at crystallization time ts, and (b) influence of rotation speed on mean crystal size and CV at crystallization time te. The crystallization conditions were fixed at a temperature difference (ΔT) of 8.8 ± 0.1 °C, bulk solution temperature (Tb) of 28 °C, and cooling rate of bulk solution of 0.26 °C/min.

according to the rotation speed of the inner cylinder and impeller, respectively, were quantitatively characterized using the viscous energy dissipation.24,25 The mean crystal size and CV in the isothermal CT crystallizer and MT crystallizer correlated well with the viscous energy dissipation, implying that the intensity of the fluid motion was the most predominant factor affecting the crystal size and CSD during the cooling crystallization. However, the mean crystal size and CV in the non-isothermal CT crystallizer were quite different from those in the isothermal CT crystallizer and MT crystallizer. The mean crystal size in the non-isothermal CT crystallizer was 2 times larger than that in the isothermal CT crystallizer and MT crystallizer, and the CSD in the non-isothermal CT crystallizer was at least 30% narrower than that in the isothermal CT crystallizer and MT crystallizer. Therefore, this result clearly demonstrated that a non-isothermal Taylor vortex was much more effective for controlling the CSD during cooling crystallization than an isothermal Taylor vortex or random turbulent eddy.

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CONCLUSIONS A non-isothermal Taylor vortex fluid motion was shown to be highly effective for controlling the CSD during cooling crystallization due to the internal cycle of heating dissolution and cooling recrystallization on the two cylinders of the nonisothermal CT crystallizer, respectively. In the isothermal CT crystallizer, most crystallization occurred during the cooling period before ts and the CSD was mostly determined during this time. After the cooling period, the CSD was only slightly changed until the end of the crystallization (te). Thus, the CSD predominantly depended on the cooling rate and rotation speed of the inner cylinder. When the cooling rate was increased, the mean crystal size and CV of the CSD were reduced due to the induction of nucleation at a high supersaturation. Meanwhile, the mean crystal size and CV increased when increasing the rotation speed of the inner G



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appropriate for cooling crystallization, and simple and effective for practical application to control the CSD.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00821. Figure S1. Temperature conditions for non-isothermal Couette−Taylor crystallizer; Figure S2. Variation of bulk solution and heating and cooling surface temperature at induction point with temperature difference (ΔT) in Couette−Taylor crystallizer; Figure S3. Influence of cooling rate on mean crystal size and CV at crystallization time of ts in non-isothermal Couette− Taylor crystallizer; Figure S4. Variation of induction temperature and corresponding supersaturation (C/C*) with rotation speed in non-isothermal Couette−Taylor crystallizer (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +82-31-201-2970. Fax: +8231-273-2971. ORCID

Woo-Sik Kim: 0000-0001-6876-4726 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Engineering Research Center of the Excellence Program of the Korean Ministry of Science, ICT & Future Planning (MSIP)/National Research Foundation of Korea (NRF) (Grant NRF-2014R1A5A1009799)

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REFERENCES

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Figure 12. Comparison of cooling crystallization of L-lysine in three types of crystallizers, i.e., a mixing tank crystallizer, an isothermal CT crystallizer, and a non-isothermal CT crystallizer. Comparison of (a) mean crystal sizes and (b) CVs obtained in the three crystallizers. The crystallization conditions were fixed at a bulk solution temperature (Tb) of 28 °C, cooling rate of bulk solution of 0.26 °C/min, and temperature difference (ΔT) of 8.8 ± 0.1 °C (in the non-isothermal Couette−Taylor crystallizer).

cylinder, as the periodic Taylor vortex promoted the induction of nucleation at a low supersaturation. In the non-isothermal CT crystallizer, the temperature difference between the inner and outer cylinders was the most influential factor controlling the CDS. When increasing the temperature difference, the induction of nucleation occurred at a higher supersaturation, resulting in a smaller mean crystal size and higher CV of the CSD during the cooling period (ts). However, the mean crystal size at the end of the crystallization (te) was larger and the CV was smaller when increasing the temperature difference, due to the more efficient internal cycle of dissolution and recrystallization in the crystallizer. When compared with the crystallization in the isothermal CT crystallizer and MT crystallizer, the mean crystal size in the non-isothermal CT crystallizer was 3−4 times larger and the CSD 30−40% narrower. This non-isothermal technique with a CT crystallizer would be especially H



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Batch Cooling Crystallization in Non-Isothermal Taylor Vortex Flow

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