Article pubs.acs.org/IECR
Ultrasound Assisted Cooling Crystallization of Sodium Acetate Ujwal N. Hatkar and Parag R. Gogate* Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 400019, India ABSTRACT: The present work deals with improvement in the cooling crystallization of sodium acetate using ultrasonic irradiations. Initially the effect of different operating parameters in the conventional approach such as seeding temperature, seed amount, and initial amount of dissolved sodium acetate has been investigated. It has been found that seeding is essential for the onset of crystallization, and the amount of the seed had a significant effect on the final crystal size distribution and average particle size. Also the seeding temperature affects the type of crystals generated with formation of trihydrate crystals at a temperature higher than 10 °C, whereas a seeding temperature of 10 °C results in formation of only anhydrous crystals. Experiments under ultrasonic irradiation revealed that the intentional seeding can be avoided using ultrasound provided the irradiation was applied as soon as the cooling is started. The average particle size was found to be dependent on the power dissipation and it decreased with an increase in the irradiation time and power of ultrasound. At low power dissipation levels, unimodal crystal size distribution was obtained. It has been established that the use of ultrasonic irradiation improves the crystallization operation in terms of the avoiding the requirement of intentional seeding and also the final mean crystal size and crystal size distribution can be controlled on the basis of the ultrasound operating parameters.
1. INTRODUCTION Batch cooling crystallization is widely practiced in fine chemical, pharmaceutical, and agrochemical industries. Seeding is very commonly used to initiate the process of cooling crystallization. However, the seeding time related to the instantaneous supersaturation level is crucial in deciding the final product characteristics. If seeding is applied too early in the overall crystallization operation, the solution may be under saturated and hence a part of the smallest seeds may dissolute, compared to larger seeds in an apparently saturated solution. Too-late seeding usually has no effect on the final product properties.1 Apart from seeding time, the amount of seed crystals and their size distribution have been found to have a significant effect on the final crystal size distribution.2,3 Overall the seeding conditions have a dramatic effect on the final product characteristics as also confirmed from various investigations for different organic and inorganic compounds.4−11 Ni and Liao6 investigated the polymorphic cooling crystallization of Lgluatamic acid in an oscillatory baffled crystallizer. The strategy of the seeding was found to strongly influence the final type of the product, that is, dominant formation of α crystals or the β crystals. It has been reported that at high concentration, β crystals were the dominant form for all cooling rates and mixing conditions. Seeding with the α crystal at this condition assisted the creation of both type of crystals. On the other hand, the α form was the leading polymorph at low concentration for all cooling rates. As the β crystal is the stable form that has the lowest energy state, the effect of seeding β crystals on the change of polymorph was much stronger than the opposite case. Similar dependence of the seeding procedure on crystallization of orthorhombic paracetamol from ethanolic solution has been reported.7 Monoclinic nucleation at high initial concentration (34% w/v), low seeding temperature (below 7 °C), and long seeding time (14.7 min) in addition to transformation of grown orthorhombic seeds have been reported. However, reproducible formation of the pure © 2012 American Chemical Society
orthorhombic form is possible at medium concentration (30% w/v), with an optimal crystal yield (60.9% w/w) corresponding to 0 °C seeding temperature. These studies clearly establish that any variation in the seeding conditions may cause great deviations on the final product properties and hence can lead to variations in the product quality between the different batches on a commercial scale of operation. A possible way to avoid the use of intentional seeding is to use ultrasonic irradiations as the cavitational effects generated in the medium can enhance the supersaturation level so as to induce the onset of the formation of crystals. The use of ultrasonic irradiations for controlling the crystallization operation has been generally referred to as sonocrystallization. In addition to the possible elimination of seeding, use of ultrasound can result in a dramatic reduction in induction time, and the metastable zone width as well as judicious application can give an excellent control over the final crystal size distribution.12 The use of ultrasound provides an effective way of improving crystal properties and process controllability, mainly by controlling the size distribution and the habit and morphology of the crystals. The main mechanism of the improvement due to the use of ultrasonic irradiations can be attributed to the cavitational effects in terms of hot spots and acoustic streaming. Cavitation appears to be particularly effective as a means of inducing nucleation, and there is evidence of dramatic improvements in reproducibility obtained through such sononucleation.12 Many efforts have been targeted to understand the exact mechanism by which the crystallization process is affected by ultrasound, but the precise mechanism behind the spectacular effects of sonocrystallization is yet unclear. Hem13 presented an overview of different possible reasons behind the improvement in Received: Revised: Accepted: Published: 12901
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crystallization using ultrasound based on the various theoretical explanations involving the action of inertial cavitation bubbles, cooling effect, pressure effect, segregation effect, and evaporation effect. All these effects sound reasonable and may in fact act in a complementary fashion to each other, and it is still difficult to decipher the exact contribution of each effect. Wohlgemuth et al.14 investigated the mechanistic details by replacing the cavitation bubbles by gas bubbles to check if the bubble surface itself acts as a nucleation center or if the net cavitational effects are important. It has been reported that the overall process was influenced to a different extent by both gassing and ultrasound; ultrasound resulted in much better effectiveness as compared to only gassing. During cavitation bubble formation, latent heat for evaporation is taken from the surrounding liquid resulting in localized cooling which increases the supersaturation level, and also some part of liquid solvent is vaporized during the bubble formation and hence the concentration of solvent decreases locally. Sayan et al.15 investigated the effect of turbulence generated by ultrasound on the continuous crystallization process of potassium dihydrogen phosphate and reported that the supersaturation limit decreased with ultrasonic waves and the crystal morphology was modified with a decrease in the average crystal size. Another mechanism explaining the beneficial effects of ultrasound is based on the observation that the growth rate of a crystal in an ultrasonic field increases with the intensity of the ultrasound.16 Use of ultrasonic irradiations for improving the crystallization has been commonly applied in the case of antisolvent or reactive crystallization systems,17−19 whereas there have not been many studies reporting the application for cooling crystallization. In the present work, an approach has been used to improve the cooling crystallization by considering a case study of sodium acetate crystallization. The study is focused on implementing the use of ultrasound to control the crystal size distribution of sodium acetate anhydrate crystals crystallized by cooling crystallization initiated by seeding. The conventional operating conditions such as seed load, seeding temperature, and initial solution concentration have been optimized initially followed by investigations related to the effect of ultrasonic irradiations. The effect of ultrasound related parameters such as irradiation time and power dissipation on the crystal size has also been investigated
Figure 1. Experimental apparatus used for sonocrystallization using a horn: (A) flat bottom cylindrical glass reactor, (B) cooling container with ice, (C) ultrasonic horn, (D) magnetic stirrer, (E) magnetic needle, (F) ultrasonic generator, (G) conductivity meter probe, (H) thermocouple.
the solution, the reactor was immersed in a cooling ice-water bath. The ultrasonic horn used for investigating the effect of ultrasonic irradiations on the process of crystallization operates at 20 kHz and a power dissipation of 120 W and was procured from Dakshin Ultrasonics, Mumbai. The ultrasonic horn was fitted with a piezoelectric transducer with a tip diameter of 2.1 cm and was typically immersed 2 cm below the liquid level in the reactor. A conductivity meter was inserted into the solution below 1 cm with the help of metal stand, and the thermocouple was placed directly into solution. In the case of conventional approach, the only change in experimental configuration as compared to that used for sonocrystallization studies was that the ultrasonic horn and magnetic needle stirred were replaced with a pitched blade metal disk turbine in the reactor for complete mixing in the solution. A schematic representation of the experimental setup used in the conventional approach has been given in figure 2. 2.3. Methodology. Sodium acetate was dissolved in distilled water in a known quantity and heated to elevated temperatures (10 °C above the saturation temperature) to ensure that the crystals are completely dissolved in water. The solution is introduced in the glass reactor which is agitated by mechanical stirring in the conventional approach. The reactor is
2. MATERIALS AND METHODS 2.1. Materials. Anhydrous sodium acetate (99% Purity) and ethyl acetate was procured from SD Fine Chemicals Ltd., India. Distilled water was used as solvent, which was prepared in the laboratory by a glass distilled water unit procured from Borosil Glass Ltd., India. Conductivity meter (Spectralab Instruments Pvt. Ltd. Mumbai), digital thermocouple (T-Star, pt-100 DINMumbai) and Levia-Optical microscope with camera connected to a computer were used to monitor the different variables in the overall crystallization operation. 2.2. Experimental Setup. Figure 1 shows the schematic of the experimental setup used for investigations related to cooling crystallization of sodium acetate in the presence of ultrasonic irradiations. The setup consists of a flat bottom cylindrical glass reactor with a capacity of 150 mL. A magnetic stirrer was used for agitation because for the current experimental setup it was difficult to simultaneously introduce mechanical stirring and the ultrasonic horn; also mechanical agitation at higher speeds might interfere in the transmission of ultrasound. For cooling
Figure 2. Experimental apparatus used for cooling crystallization with mechanical stirring (A) flat bottom cylindrical glass reactor, (B) cooling container with ice, (C) electric motor. 12902
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placed in a cooling bath filled with ice to provide external cooling to the solution. It was found that, even if the saturated solution at 60 °C (27 g in 30 mL of distilled water as per Table 1, the solubility data has been established experimentally and
2.4. Analytical Method. Analytical method for determining the crystal size distribution involves the use of ImageJ software and Beckman coulter counter. It was observed under microscope that the crystal habit was polymorphic when the experiments were carried out using the conventional method based on mechanical agitation. The obtained crystals were needle shape as well as spherical shape, whereas only spherical crystals were obtained when experiments were carried out under ultrasonic irradiation conditions. Exactly 1 g of sodium acetate was taken from each sample and photographed using 10× optical zoom microscope. Different shapes of the crystals were filtered or extracted using the image analysis ImageJ software. The software has a provision to separate out the spherical crystals from all crystals with circularity between 0.8 and 1. Remaining are the needle shape crystals. The diameter of the spherical crystals is considered as length, whereas, for the needle shape crystals, ImageJ software gives the results by a comparison with the reference scale. Once the measurement is completed, EasyFit software was used to find the particle size distribution and the average particle size. Owing to the spherical nature of the crystals, a Beckman coulter counter was used to analyze the samples of experiments carried out under the influence of ultrasound. In the analysis process, the sodium acetate was mixed with ethyl acetate in which it is insoluble (exactly 1 g is mixed in 10 mL of ethyl acetate). Ethyl acetate was added to the coulter counter chamber and then the solution was added as soon as the instrument became ready to analyze the samples. The CSD and average particle size was obtained within set time of 1 min.
Table 1. Solubility Data for Sodium Acetate in Distilled Water solubility of sodium acetate in (g/100 g of distilled water) temperature (°C)
experimental
literature (Perry’s Handbook)
0 10 20 30 40 50 60
36.6 44.6 46.7 56.7 70 80 141
36.3 40.8 46.5 54.5 65.5 83 139
also verified from literature20) was cooled to 0 °C the crystallization process was not initiated and there was no appearance of any crystals on the surface. The idea of cooling the solution was to achieve nucleation without any seeding initially. While doing experiments, the focus was to find out the MZW for sodium acetate, hence, the saturated solution at 60 °C was cooled down to lower temperatures, but it was found that even after reducing the temperature up to 0 °C, there was no formation of crystals. Things were confirmed by performing multiple experiments, and based on these observations it was established that the sodium acetate supersaturated solution does not result in crystal formation even after an extended MZW. In a recent work,21 it has been reported that the degree of supercooling used in the ultrasonic assisted crystallization of sodium acetate ranged from 10 to 68 °C. To achieve the process of nucleation in the conventional approach based on stirring, a previously measured amount of sodium acetate was then added as seed and the appearance of crystals was observed, thus confirming the requirement of seeding for the crystallization in the conventional approach. After the formation of crystals in the reactor, the crystals were filtered using a filter paper placed on a Buckner funnel connected to a vacuum pump. After filtration, the crystals were dried in an oven and then analyzed to determine the crystal size distribution (CSD). In the case of ultrasonic irradiations, the methodology was similar and magnetic stirring was used for a lower degree of mixing along with the ultrasonic horn. It was also observed that seeding was not required and crystals began to appear in the system after a certain degree of cooling. In all the experimental method, proper care must be taken so as to avoid the possibility of unexpected nucleation based on the foreign particles or any other unexpected mechanisms such as sudden temperature differential. The unexpected nucleation can occur due to the presence of any foreign bodies, ranging from solid or liquid particles, gas bubbles, macromolecules etc. either in the nucleating systems or on the wall of crystallization vessels. Utmost care was taken during the experimental methodology so as to avoid presence of any foreign bodies and also to avoid vortex formation in the mechanical stirring. Proper washing of all the glass vessels was also performed so as to avoid any contamination. Care was also taken so as to include proper washing of the mechanical surfaces such as the ultrasonic horn, agitator, or the magnetic stirrer.
3. RESULTS AND DISCUSSION 3.1. Cooling Crystallization of Sodium Acetate without Using Ultrasound. Initially, the cooling crystallization of sodium acetate was investigated under normal conditions to establish the effect of various parameters in the conventional mode. The different parameters like seeding temperature, seed quantity, amount of sodium acetate anhydrate dissolved initially, etc. have been varied for examining the effect on the crystals. The effect of change in the solvent system was also investigated using a water−ethanol mixture as solvent instead of pure distilled water. 3.1.1. Effect of Seeding Temperature. The effect of seeding temperature was investigated by adding seeds at three different initial temperatures during cooling (30, 20, and 10 °C). Experiments were carried out using a pitched blade metal disk turbine and the stirring speed was fixed at 450 rpm in each case. The other fixed parameters used in the study were 27 g of sodium acetate anhydrate dissolved in 30 mL of distilled water, an elevated temperature to which the solution was heated, 70 °C, to get a saturated solution, and a 1 g seed. As shown in Table 1 at 60 °C, the solubility of sodium acetate in distilled water is 27 g/30 mL of distilled water (reported data is based on the experimental results conducted in the laboratory and also verified using literature20). Thus, to ensure that all the sodium acetate is dissolved completely in water, the mixture was heated to 70 °C. A careful analysis of the obtained crystals by visual observation by naked-eye at these three different seeding temperatures revealed that the crystals obtained at 20 and 30 °C were of trihydrate form, whereas the anhydrous sodium acetate crystals were obtained only at a seeding temperature of 10 °C. Since the main objective of the work was to recrystallize anhydrous form of crystals, for all the further experiments, 10 °C was selected as the seeding temperature. 12903
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3.1.2. Effect of Change in the Solvent System. The effect of change in the solvent system was studied by replacing pure water with an ethanol−water mixture (70% water and 30% ethanol on volume basis). The reason behind moving to a dual solvent system was to investigate whether seeding can be avoided by this approach and hence some preliminary studies were carried out using this approach. Solubility of the sodium acetate in the mixed solvent is shown in Figure 3. 50 mL of
Figure 4. Size distribution of the initial seed used to initiate crystallization.
Table 2. Effect of Seed Mass on Average Particle Size of Sodium Acetate Crystals
Figure 3. Solubility curve for liquid mixture of 70% distilled water and 30% ethanol at different temperatures.
seed quantity (g)
average particle size (μm)
0.5 1 1.5
120.1 99 112
of sodium acetate as seed. The effect of seed mass on the crystal size distribution (CSD) has been represented in Figure 5. It can be seen from the figure that when 0.5 g of sodium acetate was added as seed the CSD obtained is bimodal and distribution is broad. The obtained CSD is unimodal and narrow when 1 g of sodium acetate was added as seed and similar unimodal nature but broad CSD was observed with 1.5 g as the seed mass. Thus it can be said that 1 g of seed yields optimum results in terms of crystal size and CSD. Similar results were obtained by Kubota et al.5 while investigating the cooling crystallization of potassium alum. Photographic analysis of the crystals using a microscope revealed a polymorphic habit, having spherical as well as needle shape crystals as shown in Figure 6. The size and mass of seed influences the crystal size distribution and the crystal growth rate. The initial surface area of the seeds increases with an increase in the quantity of seeds added to the solution. Larger surface competes for growth resulting in higher supersaturation and consequently bigger crystals were produced. Hence the average size of crystals is larger when 1.5 g sodium acetate was added as seed, compared to the crystals obtained with 1 g of seed mass. At high seed mass, unimodal grown seeds were obtained as products, regardless of the cooling mode (natural or controlled). It has been reported that slow cooling was not a necessary condition to stabilize batch crystallization with suppressed secondary nucleation. 3.1.4. Effect of Initial Amount of Dissolved Sodium Acetate (Effect of Initial Supersaturation Ratio). Initial supersaturation governs the final characteristics of crystals such as crystal habit and CSD.7 In the present work, initial supersturation was varied by varying the initial amount of sodium acetate dissolved to form the given solution. Experiments were carried out by dissolving anhydrous sodium acetate in quantities of 21, 24, and 27 g in distilled water. The seeding temperature was 10 °C, and seed mass used was 1 g. The molar
mixed solvent was taken and the initial amount of anhydrous sodium acetate dissolved was 27 g. The solution was agitated by using a pitched blade metal disk turbine. The change in the solvent system was deliberately made to reduce the solubility of sodium acetate in water by adding ethanol in which it is insoluble. The solubility of sodium acetate in distilled water at 10 °C is 36.7 g/100 mL while in a water−ethanol mixture (70% water and 30% ethanol on volume basis) the solubility is observed to be 25 g/mL. The aim of the change was to somehow avoid the intentional seeding which can bring the size and shape variation if the conditions are not properly optimized, as discussed in detail earlier. It has been observed that the change in the solvent resulted in no improvement and seeding was still essential to obtain crystals in the conventional approach based on the mechanical agitation. As it was observed that even after changing the solvent system nucleation was not seen in the absence of seeding, this approach was not investigated in any further detail. 3.1.3. Effect of Seed Mass. In the case of intentional seeding, the crystallization process is influenced by the way in which the seeds are introduced, seed size distribution, and seed loadings (mass). The effect of seed mass on the average particle size and size distribution was investigated by adding seeds in three different amounts (0.5, 1, and 1.5 g). Other parameters like seeding temperature and initial amount of anhydrous sodium acetate were kept constant as 10 °C and 27 g, respectively. The seed size distribution was also constant and was based on the standard sodium acetate available from the supplier. The seed size distribution as used in the present work has been given in Figure 4. The obtained results for the average particle size as a function of seed mass has been given in Table 2. It can be seen from the table that the average particle size of sodium acetate crystals crystallized by adding 1 g of sodium acetate was smaller than the average particle size obtained by adding 0.5 and 1.5 g 12904
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Figure 5. Effect of seed mass on CSD of sodium acetate under normal conditions: (a) 0.5 g, (b) 1 g, (c) 1.5 g.
Figure 6. Crystal habits of sodium acetate crystals observed under 10× optical zoom microscope: (a) 1 g seed mass; (b) 1.5 g seed mass.
saturation concentration at 10 °C is 5.44 mol/L. The molar concentrations when 21, 24, and 27 g of sodium acetate is dissolved in 30 mL of water are 8.53, 9.75, and 10.97 mol/L, respectively. If supersaturation is defined as molar supersaturation, which is the concentration difference between that of the supersaturated solution in which crystal is growing and the solution in equilibrium with the crystals; the corresponding initial supersaturations are 3.09, 4.31, and 5.53 mol/L. Table 3 shows the effect of an initial amount of sodium acetate (initial supersaturation) on the average particle size of the sodium acetate crystals. The average particle size was found to be only
Table 3. Effect of Initial Amount of Sodium Acetate on Average Particle Size sodium acetate dissolved initially (g)
average particle size (μ)
21 24 27
99 94.7 93.1
marginally affected by the initial amount of sodium acetate. There is a marginal decrease in particle size (within ±3%) which can be within limits of experimental errors. The effect of 12905
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Figure 7. Effect of initial amount of sodium acetate on CSD: (a) 21 g, (b) 24 g, (c) 27 g.
Figure 8. Crystal habits of sodium acetate crystals observed under 10× optical zoom microscope (a) under normal conditions and (b) under sonication.
3.2. Effect of Ultrasound on Cooling Crystallization of Sodium Acetate. The effect of ultrasound on cooling crystallization was investigated using a 20 kHz vertical horn. The main ultrasonic parameters investigated in the work include the ultrasonic power and irradiation time, whereas the frequency of ultrasound could not be varied due to the fixed operating freuqency of the horn. 3.2.1. Crystal Habit Observed under Microscope. Images of sodium acetate crystals were captured using 10× optical zoom microscope to observe the habit of crystals in the case of normal operation and in the presence of ultrasound. The obtained crystal characteristics have been given in Figure 8. It
varying initial concentration on the CSD has been shown in Figure 7 and it can be seen that the CSD obtained in every case is unimodal. The distribution is evenly spread and uniform in all experiments. Thus it can be concluded that the initial supersaturation does not play a major role in deciding the final crystal morphology. It should be noted that the result may not be generalized and the obtained results might be specific to the product and the considered initial supersaturation levels under question. Al-Zoubi and Malamataris7 have reported a significant decrease in the average particle size with an increase in the initial concentration for the crystallization of orthorhombic paracetamol from ethanolic solution. 12906
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events in the system. Ultrasound results in turbulence and intense circulation of liquid (acoustic streaming) which helps in reducing the average size of the final particles generated. 3.2.4. Effect of Ultrasonic Power Dissipation. Experiments were also carried out to study the effect of ultrasonic power on the size of the sodium acetate crystals. Ultrasound was applied at a seeding temperature (10 °C) for 3 min for various ultrasonic power ranges between 27.7 and 62.5 W. It was observed that the average particle size decreased with an increase in the power as shown in Table 5. Again the decrease
has been observed that polymorphism was involved in the case of experiments without ultrasound (normal conditions) as shown in Figure 8a. Crystallization using ultrasound resulted in the formation of spherical crystals as shown in Figure 8b. Thus it can be concluded that the ultrasound can give control on polymorph formation in the case of sodium acetate crystals. 3.2.2. Use of Ultrasound to Avoid Intentional Seeding. As discussed earlier cooling crystallization of sodium acetate performed under normal conditions (mechanical agitation) required seeding to obtain the crystals of sodium acetate in the cooling process. The conductivity was monitored and found to decrease with a decrease in the temperature indicating the formation of cluster from ions, but crystals were not agglomerated unless some quantity of sodium acetate was added as a seed. The observation was confirmed by reducing the temperature to 0 °C. Experiments were then carried out using ultrasonic irradiation with all the other operating parameters being the same during crystallization. Initially 27 g of sodium acetate was dissolved in 30 mL of distilled water and heated to 70 °C, 10 °C above the saturation temperature of 60 °C. The solution was then taken into the reactor. In this case, ultrasonic irradiations with a frequency of 20 kHz and power of 62.5 W were applied from the moment when cooling was started. In the case of crystallization with ultrasound (applied as soon as the cooling is started), it was found that the crystals were formed at 14 °C without seeding, clearly indicating that the use of ultrasound can avoid intentional seeding. The efficacy of ultrasound irradiations in eliminating the need of intentional seeding can be explained on the basis of the controlling effects of ultrasonic irradiations. The main action of ultrasound passage into liquid medium is the consecutive formation and burst of cavitation bubbles that cause severe disturbance inside the solution. The cavitational events occur at number of locations in the reactor. Therefore, the ultrasound may control the rate of nucleation as well as the number of nucleation events in the system and also reduce the induction time/ metastable zone width. Ultrasound also helps in altering the local supersaturation conditions favorably to yield crystals. Because of all these factors, comparatively higher nucleation events occur in the presence of ultrasonic irradiations resulting in the formation of crystals even in the absence of seeding. In a recent work, Seo et al.21 have also reported that ultrasound can induce nucleation in the system, confirming the obtained results in the present work. 3.2.3. Effect of Irradiation Time. Experiments were carried out to study the effect of irradiation time on the size of the sodium acetate crystals. Ultrasound was applied at seeding temperature (10 °C) for various time intervals at constant ultrasonic power of 62.5 W. It was observed that the average particle size decreases with an increase in the irradiation time as shown in Table 4. The obtained decrease in the particle size with an increase in the ultrasonic irradiation time can be attributed to the physical effects generated by the cavitational
Table 5. Effect of Ultrasonic Power on Average Particle Size for 3 min Irradiation Time
particle diameter (μm)
1 3 5
17.8 12.4 8.6
particle diameter (μm)
27.7 36.7 62.5
25.1 18 12.4
in the particle size can be attributed to more intense tubulence and liquid streaming obtained due to an increase in the power dissipation in the system.22,23 The obtained results are in good agreement with the results obtained by Abbas et al.24 for the effect of ultrasonic power on the size of sodium chloride crystals when crystallized by cooling crystallization. Park and Yeo17 have also reported similar results for the antisolvent crystallization of Roxithromycin assisted by ultrasonic irradiations. The crystal size distributions under varying power dissipation levels as depicted in Figure 9 revealed interesting facts. It can be seen that under lower power dissipation levels, unimodal distribution is observed but it changes to bimodal distribution at very high power dissipation. The average size is still lower at the higher power dissipation levels. Thus it appears that depending on the requirements of the final product specifications (strict requirement of a unimodal distribution); optimum ultrasonic power dissipation needs to be maintained to avoid a bimodal distribution of the crystals.
4. CONCLUSIONS In this work, cooling crystallization of sodium acetate from its aqueous solution to obtain anhydrous sodium acetate crystals has been investigated. Experiments under normal conditions revealed that seeding is absolutely essential and the seeding temperature is crucial when sodium acetate is crystallized by cooling crystallization. Anhydrous sodium acetate was obtained at a seeding temperature of 10 °C and the trihydrate form was obtained if the seeding was done at a higher temperature (at 20 or 30 °C). The average particle size and CSD was affected by the seeding quantity, while the initial amount of sodium acetate dissolved has only a marginal effect on CSD and average particle size. During sonocrystallization, it was found that intentional seeding can be avoided by using an ultrasound application from the beginning of the cooling. The use of ultrasound also aids in obtaining the correct polymorph of the crystals. It has been clearly established that the average particle size decreases with an increase in the irradiation time and ultrasonic power dissipation. Overall, the present work has clearly established the utility of ultrasonic irradiations for controlling the cooling crystallization of sodium acetate in terms of avoiding the seeding, formation of desired crystal shape, and achieving lower crystal size with control in terms of the irradiation time and power levels.
Table 4. Effect of Irradiation Time at 62.5 W Power Dissipation irradiation time (min)
US power (W)
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Figure 9. Effect of ultrasonic power dissipation on the crystal size distribution: (a) 62.5 W, (b) 36.7 W, (c) 27.7 W.
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(9) Nowee, S. M.; Abbas, A.; Romagnoli, J. A. Optimization in seeded cooling crystallization: A parameter estimation and dynamic optimization study. Chem. Eng. Process. 2007, 46, 1096−1106. (10) Huang, D.; Liu, W.; Zhao, S. K.; Shi, Y. Q.; Wang, Z. X.; Sun, Y. M. Quantitative design of seed load for solution cooling crystallization based on kinetic analysis. Chem. Eng. J. 2010, 156, 360−365. (11) Liu, J. J.; Ma, C. Y.; Hu, Y. D.; Wang, X. Z. Effect of seed loading and cooling rate on crystal size and shape distributionA study using morphological population balance. Comput. Chem. Eng. 2010, 34, 1945−1952. (12) Luque de Castro, M. D.; Priego-Capote, F. Ultrasound-assisted crystallization (sonocrystallization). Ultrason. Sonochem. 2007, 14, 717−724. (13) Hem, S. L. The effect of ultrasonic vibrations on crystallization processes. Ultrasonics 1967, 202−207. (14) Wohlgemuth, K.; Kordylla, A.; Ruether, F.; Schembecker, G. Experimental study of the effect of bubbles on nucleation during batch cooling crystallization. Chem. Eng. Sci. 2009, 64, 4155−4163. (15) Sayan, P.; Sargut, S.; Kiran, B. Effect of ultrasonic irradiation on crystallization kinetics of potassium dihydrogen phosphate. Ultrason. Sonochem. 2011, 18, 795−800. (16) Amara, N.; Ratsimba, B.; Wilhelm, A.; Delmas, H. Growth rate of potash alum crystals: comparison of silent and ultrasonic conditions. Ultrason. Sonochem. 2011, 11, 17−21. (17) Park, M.; Yeo, S. Anti-solvent crystallization of roxithromycin and the effect of ultrasound. Sep. Sci. Technol. 2010, 45, 1402−10. (18) Kitamura, M.; Sugimoto, M. Anti-solvent crystallization and transformation of thiazole derivative polymorphsI: Effect of addition rate and initial concentrations. J. Crystal Growth 2003, 257, 177−184. (19) Bund, R. K.; Pandit, A. B. Sonocrystallization: Effect on lactose recovery and crystal form. Ultrason. Sonochem. 2007, 14, 143−152. (20) Green, D. W.; Perry, R. H. Perry’s Chemical Engineering Handbook, 8th ed.; McGraw-Hill Professional: New York, 2007.
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Corresponding Author
*Tel: +91 22 33612024. Fax: +91 22 33611020. E-mail: pr.
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Louhi-Kultanen, M.; Karjalainen, M.; Rantanen, J.; Huhtanen, M.; Kallas, J. Crystallization of glycine with ultrasound. Int. J. Pharm. 2006, 320, 23−29. (2) Zhang, G.; Rohani, S. On-line optimal control of a seeded batch crystallizer. Chem. Eng. Sci. 2003, 58, 1887−1896. (3) Vollmer, U.; Raisch, J. Control of batch cooling crystallization processes based on orbital flatness. Int. J. Control 2003, 76, 1635− 1643. (4) Lacmann, M. R. Secondary nucleation of sodium chlorate studied with the aid of asymmetric crystallization. Chem. Eng. Technol. 1997, 20, 633−640. (5) Kubota, N.; Doki, N.; Yokota, M.; Sato, A. Seeding policy in batch cooling crystallization. Powder Technol. 2001, 121, 31−38. (6) Ni, X.; Liao, A. Effects of mixing, seeding, material of baffles and final temperature on solution crystallization of L-glutamic acid in oscillatory baffled crystallizer. Chem. Eng. J. 2010, 156, 226−233. (7) Al-Zoubi, N.; Malamataris, S. Effect of initial concentration and seeding procedure on crystallization of orthorhombic paracetamol from ethanolic solution. Int. J. Pharm. 2003, 260, 123−135. (8) Sarkar, D.; Rohani, S.; Jutan, A. Multi-objective optimization of seeded batch crystallization processes. Chem. Eng. J. 2006, 61, 5282− 5295. 12908
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Industrial & Engineering Chemistry Research
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(21) Seo, K.; Suzuki, S.; Kinoshita, T.; Hirasawa, I. Effect of ultrasonic irradiation on the crystallization of sodium acetate trihydrate utilized as heat storage material. Chem. Eng. Technol. 2012, 35 (6), 1013−16. (22) Kumar, A.; Gogate, P. R.; Pandit, A. B.; Delmas, H.; Wilhelm, A. M. Gas liquid mass transfer studies in sonochemical reactors. Ind. Eng. Chem. Res. 2004, 43, 1812−19. (23) Sutkar, V. S.; Gogate, P. R. Design aspects of sonochemical reactors: Techniques for understanding cavitational activity distribution and effect of operating parameters. Chem. Eng. J. 2009, 155, 26− 36. (24) Abbas, A.; Srour, M.; Tangc, P.; Tangc, H.; Chan, H.; Romagnoli, J. A. Sonocrystallization of sodium chloride particles for inhalation. Chem. Eng. Sci. 2007, 62, 2445−2453.
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dx.doi.org/10.1021/ie202220q | Ind. Eng. Chem. Res. 2012, 51, 12901−12909
