Controlling the Size of Taurine Crystals in the Cooling Crystallization

Article pubs.acs.org/IECR

Controlling the Size of Taurine Crystals in the Cooling Crystallization Process Ruohui Lin,†,‡ Meng W. Woo,‡ Cordelia Selomulya,‡ Jianping Lu,§ and Xiao Dong Chen*,† †

Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen City, Fujian 361005, People’s Republic of China ‡ Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia § Jiangsu Yuanyang Pharmaceutical Co., Ltd., Zhitang Industrial Park, Changshu 215531, People’s Republic of China S Supporting Information *

ABSTRACT: Taurine crystals (TC) with high fluidity, anticaking, and antisticking properties are commercially required by manufacturers. Crystal size and shape are two main factors controlling these properties. This study investigated the size and shape of TCs produced by a cooling crystallization processing. The addition of activated carbon to decolor the product exhibited negligible effects on the size of TCs. Increasing the crystallization temperature resulted in the formation of small-size crystals, whereas TC size decreased as the stirring rate increased. A careful control of post high-speed stirring aging in a slowly stirred system enabled the preparation of larger crystals. The estimation of optimal crystallization time is crucial as lengthening the crystallization time might not effectively increase the overall size of the crystals. It was found that the shape of the TC was not affected by various operating conditions, delineating a similar inherent molecular arrangement of taurine molecules under the range of operating conditions tested. These findings contribute to the understanding of controlling properties of TCs for industrial manufacturing via the cooling crystallization method. other parameters in the batch crystallization.16−22 In particular, a controlled process with in situ solution concentration measurement and crystal number determination was reported to be able to adjust final mean sizes of glycine and L-glutamate crystals.16,17,21 However, taurine-related studies have mainly focused on the separation methods to extract taurine from biological samples, the chemical synthesis, and the physiological function.23−32 Few studies specifically manipulating the size and shape of TCs produced by cooling crystallization processing under various conditions are available.33,34 Therefore, the motivation of this work is to investigate the influence of crystallization parameters on the shape and size of TCs. In collaboration with an industrial partner, our study was based on an existing industrial cooling crystallization process. The industrial process employed to produce TCs is as follows: (1) Crude taurine solution is prepared at high temperatures and decolored with activated carbon (AC). (2) The solution is then filtered and cooled at low temperature. (3) The precipitated crystals obtained by centrifugation are dried at 90−95 °C for 3 h. The intended application of this work was to look at how operating conditions at each stage of the process will affect the shape and size of the precipitated taurine crystals.

1. INTRODUCTION Taurine (2-aminoethanesulfonic acid) is a colorless sulfurcontaining amino acid that crystallizes as tetragonal needles.1 Many studies have indicated taurine plays an important role in numerous physiological processes. The essential physiological processes include detoxification, development of the central nervous system and antioxidant activity, membrane stabilization, antitumor activity, and immunity.2−7 Moreover, it has been recently recognized to maintain an antihypertensive effect in rat.8 However, humans have a very limited ability to synthesize taurine, being largely dependent on exogenous sources, leading to higher vulnerability to taurine deficiency.9,10 Therefore, the addition of taurine is encouraged in infant formula, dietary supplements, and energy drinks.11−13 As the need for taurine has increased recently, industrial productions of taurine have expanded. Due to the shape or size of taurine crystals (TCs), however, the properties of industrial grade taurine crystals (for example, low fluidity, caking, and stickiness) are far from satisfactory, influencing the downstream process ability and the performance of the products. One way to overcome these drawbacks is to manipulate the size and shape of taurine crystals. Increasing the crystal size will lead to better flowability and reduction in caking in terms of bulk properties due to the decrease of the surface area per unit mass, which subsequently reduces the interparticle cohesive strength.14 Crystal shapes also influence those properties due to the changes of the surface contact between the crystals.14,15 To date, many papers have been published on the topic of controlling crystals’ size and shape for other amino acid crystals via cooling crystallization.16−22 The manipulations were conducted by adjusting seeding ratio, temperature profile, cooling rate, solution concentration, and © 2013 American Chemical Society

2. MATERIALS AND EXPERIMENTAL METHOD 2.1. Crude Taurine Solution Property and Cooling Crystallization. The industrial process described earlier was Received: Revised: Accepted: Published: 13449

January 8, 2013 May 10, 2013 August 12, 2013 August 12, 2013 dx.doi.org/10.1021/ie4000807 | Ind. Eng. Chem. Res. 2013, 52, 13449−13458

Industrial & Engineering Chemistry Research

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Figure 1. Translating the industrial cooling crystallization process into a laboratory scale.

Figure 2. Olympus microscope images of TCs decoloration and nondecoloration: (a, a′) run 1, TCs; (b, b′) run 2, TCs with addition of AC.

supersaturation on final particle size and shape. In particular, the effects of cooling rate, stirring method or speed, and the presence of impurities on the final crystal size and shape were examined.35 A summary of tested operating conditions for each run is shown in the Supporting Information. 2.2. With and without Activated Carbon. To mimic the industrial practices in which crude taurine solution is prepared at high temperatures and decolored with AC, 20 wt % crude taurine solution was prepared by dissolution with water at

reproduced in a controlled laboratory-scale environment (Figure 1). Crude taurine (purity > 90%, Jiangsu Yuan Yang Co., China) solution was prepared by dissolving the crude taurine with water at high temperatures. This was then followed by precipitation at low temperatures to increase supersaturation of the solution. TCs were obtained by vacuum filtration and then dried in a vacuum oven (DZF-6020, Shanghai Jing Hong Laboratory Instrument Co., Ltd.) for 3 h (95 ± 2 °C). One of the main factors investigated in this work was the effect of 13450

dx.doi.org/10.1021/ie4000807 | Ind. Eng. Chem. Res. 2013, 52, 13449−13458


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Figure 3. Stereomicroscope images of TCs precipitated at different crystallization temperatures with 860 rpm stirring for 20 min: (a) run 3, σ = 7.11, 4 °C; (b) run 4, σ = 4.08, 10 °C; (c) run 5, σ = 2.37, 20 °C; (d) run 6, σ = 1.61, 30 °C; (e) run 7, σ = 0.80, 40 °C; (f) run 8, σ = 0.40, 50 °C; (g) run 9, σ = 0.13, 60 °C.

45 °C with the addition of AC (Shanghai Sinopharm Chemical Reagent Co., Ltd.) at the ratio of crude taurine solution, w/w 1:100. The solution was then filtered after 5 min of stirring. The solution was then immediately put into a water bath at 30 °C with 860 rpm stirring (DF-101s collector-type stirrer, China) for 20 min. The samples were labeled as runs 1 and 2. 2.3. Crystallization Temperatures. In aqueous phase crystallization, the level of supersaturation σ = (C − Ce)/Ce (Ce = concentration of the saturated solution) was taken as the thermodynamic driving force of the crystallization process.36 To assess the effect of the driving force on the shape and size of the taurine crystals, 30 wt % crude taurine solution was used across the runs tested. This was prepared by dissolving the crude taurine crystals at 80 °C. The solution prepared in this manner was also used in subsequent sections 2.4, 2.5, and 2.6 of

this paper. This solution was then immediately put into a water bath at 4, 10, 20, 30, 40, 50, or 60 °C with 860 rpm stirring for 20 min. The samples were labeled as runs 3−9. As the initial concentration of the taurine solution used was maintained constant for each run, different driving forces were induced by lowering the system to different temperatures. This was aimed to decrease the saturation concentration, and thus σ becomes positive.37 2.4. Crystallization Stirring Rate. Stirring rate is another experimental condition that affects the crystals size. The prepared solution of 30 wt % taurine as described earlier was immediately put into a water bath at 10 °C with different stirring rates for 20 min. Two pieces of equipment were employed to control the stirring rate: a magnetic stirrer (DF-101s collector-type stirrer, China) generates slow stirring of 200, 860, and 1000+ rpm, whereas a T25 high-performance 13451



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Figure 4. Results of the crystal length of TCs at different crystallization temperatures with 860 rpm stirring for 20 min: (a) run 3, σ = 7.11, 4 °C; (b) run 4, σ = 4.08, 10 °C; (c) run 5, σ = 2.37, 20 °C; (d) run 6, σ = 1.61, 30 °C; (e) run 7, σ = 0.80, 40 °C; (f) run 8, σ = 0.40, 50 °C; (g) run 9, σ = 0.13, 60 °C.

samples were put into an ice−water bath at 10 °C for 10, 20, or 40 min with stirring of 200 rpm. Correspondingly, the samples were labeled as runs 18−20. 2.7. Particle Characterization. The length of the needlelike crystals was defined as the characteristic size of crystals and was analyzed using Image J software based on optical microscope and scanning electron microscopy (SEM) images. The average length of the taurine crystals (l ̅) was defined as l ̅ = (1/N) ∑i N= 1li = (l1 + l2 + ...lN)/N, where l is the length of taurine crystals and N is the total number of particles counted. In addition, the distribution of the crystal size was obtained from the same sampling used in the average size determination. Two optical microscopes were used to observe the precipitated TCs. One was an ordinary microscope (BX41TF, Olympus Corp.) and the pictures were analyzed using the software package cellSens Standard 1.2.1. The other one was a stereomicroscope (DTX Chongqing Optec Instrument Co., Ltd.), and the images were analyzed using the software package TS View 7.1.0.3. The difference between these two microscopes is that light is reflected from the surface of a crystal in the

disperser (IKA, German) generates high-speed stirring at 4000 and 7000 rpm. Correspondingly, the samples were labeled as runs 10−14. 2.5. High-Speed Stirring Post Aging. Sample solution used for post aging assessment was taken after high-speed stirring of 4000 rpm at 10 °C as described under section 2.4. The samples were put into an ice water bath at 10 °C for 20 min with 200 rpm stirring (high-speed stirring and then aging with slow stirring) or without stirring. The former is to assess how mixing affects the crystallization process during aging, whereas the latter examines the static aging crystal growth mechanism. In total, the crystallization time was 40 min. The samples with no aging time and only 20 min at 4000 rpm (high-speed stirring only) were set as the control benchmark. Correspondingly, the samples were labeled as runs 15−17. 2.6. Crystallization Time. Besides crystallization temperature, the crystallization time was also used as a controlled parameter in cooling crystallization. Therefore, the study of the effect of crystallization time on crystal size is proposed in this section. After the high-speed stirring of 4000 rpm at 10 °C, the 13452



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stereomicroscope, providing a silhouette of the particle, whereas light is transmitted through the crystal in the Olympus microscope, giving complete visualization of the particle structure. For SEM analysis, TC samples were prepared by fixing the samples onto an aluminum sample stub using doubleconducting carbon tape and observed with a Phenom Desktop SEM G1 under an accelerating voltage of 5 kV. A Hitachi s-4800 SEM was also used to observe the morphology of crystals, and it was operated at 10 kV. Samples were sputtercoated with 1 nm gold−palladium.

3. RESULTS AND DISCUSSION 3.1. With and without Activated Carbon. Typical microscope images of the TCs are shown in Figure 2. The images show no significant differences in either the shape or size between the TC samples with or without AC. The basic images show mostly small pieces of crystals with relatively long needle-like shape with a length of around 10−40 μm. As the solution is filtered after the AC treatment, the AC will not be able to participate in the precipitation process. The difference between an AC-treated and nontreated solution would be the presence of impurities in the solution. Small impurity particles might act as nucleation sites,38 or dissolved impurities might participate and disrupt the development of the crystal lattice, for example, as observed for lactose crystals.39 However, the results here show that these impurities have negligible effect on the nucleation or growth of the crystals. 3.2. Crystallization Temperature. As mentioned earlier, the crystallization temperature delineates the initial degree of supersaturation in the solution. Lower crystallization temperature results in a higher degree of supersaturation and vice versa. Stereomicroscope images of TCs at different crystallization temperatures with 860 rpm stirring after 20 min are shown in Figure 3. From Figure 3, the size gets bigger at higher temperature or lower initial supersaturation. However, no obvious changes could be observed in terms of the shape. This is in contrast to reported work in which the shape of taurine crystals changes from needle-like to rod-like under the condition of low supersaturation ratio (roughly σ < 1.67) in the antisolvent solution.34 However, in this work, regardless of the crystallization temperature or the level of initial supersaturation, the shape of TCs remained needle-like. Random length measurement of 70 crystals from each picture in Figure 3 was then undertaken to provide an analysis of the crystal size distribution. Figure 4 illustrates the size distribution obtained from each crystallization condition. The length at the peak of each distribution was collated and presented in Figure 5a. On top of confirming the trend discussed on the effect of supersaturation on the size of the crystals, an apparent crystallization “threshold” was observed. The trend shows that there is a slow increase in peak length from 4 to 30 °C with a plateau region at 30−50 °C and then a sudden jump in size at 60 °C in Figure 5a. Elucidating this trend, the pattern observed in Figure 5a can be divided into three distinct parts: a nucleation-dominated part from 4 to 30 °C (initial supersaturation > 1.5), a transitional period from 30 to 50 °C (1.5 > initial supersaturation > 0.5), and a crystal growth-dominated part from 50 to 60 °C (0.5 > initial supersaturation). For the nucleation-dominated part, at lower temperature, the nucleation comes out in a relatively short time, and higher supersaturation might have caused a higher number of crystal nuclei. Due to this higher initial precipitation, the concentration of taurine solution dropped

Figure 5. Crystal yield and peak length of TCs at different temperatures and supersaturations with 860 rpm stirring for 20 min.

quickly and smaller size crystals of 125 μm were obtained. For the crystal growth-dominated part, low initial supersaturation might have caused less production of crystal nuclei. As less material was precipitated out in the initial period, the higher taurine concentration might have sustained crystal growth for a longer period, resulting in the formation of large crystal size of 325 μm. Apart from the crystal size, another factor that is of concern to the industry is the yield of the taurine crystals. As can be seen from Figure 5b, yield was higher at higher supersaturation and vice versa. At fixed crystallization duration, higher supersaturation will cause more crystal to precipitate, albeit dominated by nucleation and resulting in smaller crystals. On the other hand, lower supersaturation could have resulted in slower precipitation of mass. This can be another factor to consider in the operation of the actual industrial process in which there might be a trade-off between yield for a chosen operation time and acceptable size of the crystals. With the crystal size and yield taken into account, crystallization time at 10 °C was chosen as the condition for subsequent experiments. 3.3. Crystallization Stirring Rate. Due to the continuous production of nucleus or crystals, the concentration of taurine solution might not be homogeneous, with possibly lower concentration adjacent to the precipitated crystals. Thus, stirring is necessary as it helps to distribute the concentration within the solution.37 Stereomicroscope images of TCs at 10 °C with different stirring rates produced after 20 min of crystallization are shown in Figure 6. SEM images of TCs at 10 °C with highspeed stirring are shown in Figure 7. In terms of crystal size, as the stirring rate was increased, TC size became smaller; for example, the smallest crystals around 20−122 μm were obtained at the speed of 7000 rpm. High-speed stirring could have caused more attrition between the precipitated crystals, leading to such smaller sizes. For high-speed stirring at 7000 rpm, fragments (non-needle-like) were observed amidst distinct 13453



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Figure 6. Stereomicroscope images of TCs precipitated at 10 °C with different rates of stirring for 20 min: (a) run 10, 200 rpm; (b) run 11, 860 rpm; (c) run 12, 1000+ rpm; (d) run 13, 4000 rpm; (e) run 14, 7000 rpm.

Figure 7. SEM images of TCs precipitated at 10 °C with the following rates of stirring for 20 min: (a) Run 13: 4000 rpm and (b) Run 14: 7000 rpm.

reduce the effect on breakage due to the mechanical force caused by the stirrer. However, the effect of high-speed agitation on crystal size is not examined in detail in this study, but it will be explored in a future study.

needle-like crystals (Figure 7b). This clearly shows a significant breakage due to high-speed stirring. As the breakage is not avoided completely during the batch cooling crystallization process,17 4000 rpm was chosen as the optimal stirring speed to 13454



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Figure 8. SEM images of TCs with 4000 rpm of high shear for 20 min: (a, a′) run 15, high-speed stirring and then aging for another 20 min without stirring; (b, b′) run 16, high-speed stirring and then aging for another 20 min with slow stirring of 200 rpm; (c, c′) run17, high-speed stirring only.

at 10 °C, which could have resulted in decreasing the crystallization driving force. Therefore, this aspect has to be considered in the interpretation of the high-mixing results. Improvements were then adopted for the following trials on post aging under high-speed stirring at 10 °C. 3.4. High-Speed Stirring Post Aging. A very interesting observation was made when samples with the precipitated crystals under high-speed stirring were subsequently left to age without stirring. It was found that there was no significant crystal growth without stirring even when the crystals were left in the solution for 20 min (compare panels a and c of Figure 8). However, when there was stirring during the 20 min of aging, significant growth could be observed (compare panels b and c of Figure 8). This highlights the effect of the internal mass transfer mechanism within the solution, between the precipitated crystal and the mother liquor. Due to the precipitation of the taurine crystals, the concentration in the vicinity of the precipitated crystals would be locally lower when compared to the concentration in the bulk mother liquor. If stirring is not available, the diffusion of the dissolved taurine might not be

On the other hand, high-speed stirring also reduced the induction time in the experiment, that is, the time taken for crystals to appear during the process.36 The induction time was obtained by visual observation on the first sign of turbidity in the mother liquor. From the observation of the experiments, with slow stirring, the induction time was 30 s. By contrast, with high-speed stirring, the induction time is within 5 s. This can be elucidated by the nucleation process. The nucleation process conventionally consists of primary nucleation and secondary nucleation.36 In the primary nucleation, the initial formation of fine solids (nuclei) occurs from a supersaturated system, which does not initially contain crystalline matter; in the secondary nucleation, nuclei begin to form in the presence of crystalline matter in the supersaturated solution. In the current experiments, higher speed stirring could have caused more rigorous interaction or collision contact between the dissolved taurine materials, which would have led to higher probability in forming fine nuclei. It is noteworthy that the continuous heat generation caused by high-speed stirring made it difficult to maintain the solution 13455



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sufficient to “replenish” the lower concentration in the vicinity of the precipitated crystals. Therefore, it is very important for the industrial application to provide this internal mass transfer in their operation via stirring. 3.5. Crystallization Time. SEM images of TCs after highspeed stirring of 4000 rpm and post aging at 10 °C followed by stirring of 200 rpm for 10, 20, or 40 min are shown in Figure 9.

Figure 10. Statistical results of the crystal length of aging post highspeed stirring of 4000 rpm at 10 °C with stirring of 200 rpm for 10, 20, or 40 min: (a) run 18, 10 min; (b) run 19, 20 min; (c) run 20, 40 min.

the peak shifted to 51−75 μm. However, at 40 min, the peak shifted back to 26−50 μm. Therefore, there is no clear correlation of the peak length with crystallization time. The peak shifting back to the small scale indicates the smaller crystals grow faster than the large ones as the crystallization time lengthened. Although a “backward” shift was observed for the peak length, it is interesting that the general distribution tends to shift toward the larger sizes at extended crystallization time. The size distribution shifts from 1−150 μm at a crystallization time of 10 min to 1−200 μm at crystallization times of 20 and 40 min. This translated to a lower average length of 58.5 μm at 10 min and increases to 65.2 and 65.9 μm for the 20 and 40 min runs, respectively. This shift of peak length and broadened range indicated lengthening of the crystallization time encouraged further growth of the crystals. However, further analysis not shown here showed that the slight differences observed here might not be statistically significant. This implies that industrial operation with a longer crystallization time might not necessarily produce significantly larger crystals. As a result, estimation of the optimal crystallization time would be required to obtain acceptable crystal size and to avoid incurring excessive cost due to longer processing time.

Figure 9. SEM images of TCs obtained after high-speed stirring of 4000 rpm and postaging at 10 °C with stirring of 200 rpm for 10, 20, 40 min: (a) run 18, 10 min; (b) run 19, 20 min; (c) run 20, 40 min.

It was observed that the size of the TCs became bigger at longer stirring time. Random measurement of 80 crystals from each picture in Figure 9 was then undertaken, and the resultant size distributions are shown in Figure 10. Some interesting results can be found in terms of peak length and distribution. At a crystallization time of 10 min, the peak length falls into the range of 26−50 μm, and at the crystallization time of 20 min, 13456



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(13) Baum, M.; Weiss, M. The influence of a taurine containing drink on cardiac parameters before and after exercise measured by echocardiography. Amino Acids 2001, 20 (1), 75−82. (14) Schubert, H. Food particle technology. Part I: Properties of particles and particulate food systems. J. Food Eng. 1987, 6 (1), 1−32. (15) Kim, E. H. J.; Chen, X. D.; Pearce, D. Effect of surface composition on the flowability of industrial spray-dried dairy powders. Colloids Surf., B 2005, 46 (3), 182−187. (16) Doki, N.; Seki, H.; Takano, K.; Asatani, H.; Yokota, M.; Kubota, N. Process control of seeded batch cooling crystallization of the metastable α-form glycine using an in-situ ATR-FTIR spectrometer and an in-situ FBRM particle counter. Cryst. Growth Des. 2004, 4 (5), 949−953. (17) Doki, N.; Yokota, M.; Sasaki, S.; Kubota, N. Size distribution of needle-shape crystals of monosodium L-glutamate obtained by seeded batch cooling crystallization. J. Chem. Eng. Jpn. 2004, 37 (3), 436−442. (18) Grön, H.; Borissova, A.; Roberts, K. J. In-process ATR-FTIR spectroscopy for closed-loop supersaturation control of a batch crystallizer producing monosodium glutamate crystals of defined size. Ind. Eng. Chem. Res. 2003, 42 (1), 198−206. (19) Loï Mi Lung-Somarriba, B.; Moscosa-Santillan, M.; Porte, C.; Delacroix, A. Effect of seeded surface area on crystal size distribution in glycine batch cooling crystallization: a seeding methodology. J. Cryst. Growth 2004, 270 (3), 624−632. (20) Kee, N. C.; Tan, R. B.; Braatz, R. D. Selective crystallization of the metastable α-form of L-glutamic acid using concentration feedback control. Cryst. Growth Des. 2009, 9 (7), 3044−3051. (21) Doki, N.; Yokota, M.; Kido, K.; Sasaki, S.; Kubota, N. Reliable and selective crystallization of the metastable α-form glycine by seeding. Cryst. Growth Des. 2004, 4 (1), 103−107. (22) Patchigolla, K.; Wilkinson, D. Characterization of organic and inorganic chemicals formed by batch-cooling crystallization: shape and size. Ind. Eng. Chem. Res. 2008, 47 (3), 804−812. (23) Kendler, B. S. Taurine: an overview of its role in preventive medicine. Preventive Med. 1989, 18 (1), 79−100. (24) Gu, Y.; Shi, F.; Yang, H.; Deng, Y. Leaching separation of taurine and sodium sulfate solid mixture using ionic liquids. Sep. Purif. Technol. 2004, 35 (2), 153−159. (25) Mou, S. F.; Ding, X. J.; Liu, Y. J. Separation methods for taurine analysis in biological samples. J. Chromatogr., B: Anal. Technol. Biomed. Life Sci. 2002, 781 (1−2), 251−267. (26) White, A.; Fishman, J. B. The formation of taurine by the decarboxylation of cysteic acid. J. Biol. Chem. 1936, 116 (2), 457−461. (27) Bondareva, O.; Lopatik, D.; Kuvaeva, Z.; Vinokurova, L.; Markovich, M.; Prokopovich, I. Synthesis of taurine. Pharm. Chem. J. 2008, 42 (3), 142−144. (28) Hu, L.; Zhu, H.; Du, D. M.; Xu, J. Efficient synthesis of taurine and structurally diverse substituted taurines from aziridines. J. Org. Chem. 2007, 72 (12), 4543−4546. (29) Cortese, F. On the synthesis of taurine. J. Am. Chem. Soc. 1936, 58 (2), 191−192. (30) Norman, A. Separation of conjugated bile acids by partition chromatography. Bile acids and steroids 6. Acta Chem. Scand. 1953, 7 (10), 1413−1419. (31) Hsieh, H. H. Preparation of 2-aminoalkanesulfonic acid. U.S. Patent 4,444,694, April 24, 1984. (32) Bulychev, E. Y.; Rubanyak, N. Y. Commercial synthesis of 2aminoethanesulfonic acid (taurine). Pharm. Chem. J. 2013, 1−2. (33) Yang, C. Process for refining 2-aminoethanesulfonic acid from crude 2-aminoethane-sulfonic acid, 2-aminoethanesulfonic acid obtained there from and use thereof. Eur. Patent 2239251A1, 2010. (34) Takiyama, H.; Ito, K. Production of organic fine-crystalline particles by using the liquid-liquid interface in an emulsion. Chem. Eng. Technol. 2012, 35 (6), 991−994. (35) Bonnin-Paris, J.; Bostyn, S.; Havet, J. L.; Fauduet, H. Determination of the metastable zone width of glycine aqueous solutions for batch crystallizations. Chem. Eng. Commun. 2011, 198 (8), 1004−1017.

4. CONCLUSIONS Different operating conditions of the cooling crystallization process had little impact on the shape and structure of TCs but significantly affected the TC size. Addition of activated carbon into the mother liquid did not alter the size of TCs, whereas increasing the crystallization temperature increased the crystal size and vice versa. The phenomenon can be explained by different initial supersaturation resulting in contrasting nucleation and growth rates. TC size decreased as the stirring rate increased, whereas aging post high-speed stirring with slow stirring rate lengthened the crystals. On the basis of the conditions used in the experiments here, a longer crystallization time might not necessarily produce significantly larger TCs. This aspect has to be evaluated with respect to the additional cost associated with a longer operation time. The findings are useful to understand some effects of process conditions for the synthesis of TCs via cooling crystallization.

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ASSOCIATED CONTENT

S Supporting Information *

Summary of tested operating conditions for each run. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(Xiao Dong Chen) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Controlling the Size of Taurine Crystals in the Cooling Crystallization

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