Understanding and Optimising Sonication Conditions for Crystallisation Processes

Buenos Aires – 5 to 9 September, 2016 Acoustics for the 21st Century… PROCEEDINGS of the 22nd International Congress on Acoustics Sonochemistry and Sonoprocessing: Paper ICA2016-202 Understanding and optimising sonication conditions for crystallisation processes Nnamdi Ugochukwu(a), Madeleine Bussemaker(b), Judy Lee(c) (a)-(c) Chemical and Process Engineering University of Surrey, United Kingdom, [email protected] Abstract Crystallisation is a common technique adopted by many industries for separation and purification operations but it can suffer from unpredictable nucleation rates and inconsistencies in product qualities. To remediate these problems, ultrasound has been used to facilitate and control nucleation of crystals (sonocrystallisation), resulting in a more consistent and narrower size distribution of product. However, the mechanism behind sonocrystallization process is contentious and often reported is the continuous sonication of the system, which causes issues such as heating, probe surface erosion and cost of additional input energy. This study examines the influence of cavitation activity and different modes of pulse sonication on the crystallisation process. The results show a close association between cavitation activity, quantified in terms of sonoluminescence intensity, and the minimum crystal size obtained. In addition, the initial short sonication time 5 seconds proves to be effective for sonocrystallisation, showing results that are comparable to systems undergone 90seconds of sonication. Keywords: Acoustic cavitation, sonocrystallisation, sonoluminescence. 22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016 Acoustics for the 21st Century… Understanding and optimising sonication conditions for crystallisation processes 1 Introduction Antisolvent crystallisation is a common technique adopted by many industries for separation or purification operations[1]. This method allows processes to be conducted at a constant temperature, suitable for solutes that are heat sensitive, and is both convenient and economical. However, this approach also suffers from broad particle size distributions, asymmetrical crystal morphology and product purity[2]. Ultrasound has been shown to facilitate the nucleation of crystals and reduce the induction time in a controlled and reproducible way to generate narrower size distribution products[1-8]. However, crystallisation and ultrasound induced cavitation are both complex processes and when coupled together, this complexity is compounded. Ultrasound in liquids induces the cavitation, which is the creation, expansion and implosion of bubbles when a certain acoustic pressure threshold is exceeded. Upon collapse of these cavitation bubbles the bubble core can reach extreme temperatures (> 5000k) and pressures (> 1000atm). It has been shown that such cavitation event is particularly effective at inducing crystal nucleation in a controlled and reproducible way that provides a well-defined starting point for crystallisation processes[7,9,10], but the exact mechanism behind sonocrystallisation is still contentious[6,11]. In addition, a continuous sonication mode is applied throughout the crystallisation process and this can render the system energy intensive. Recently Kezia et al.[12] have shown that an initial sonication of 3-20 seconds were sufficient to reduce the induction time from 3.3 hours to 1.7 hours. This has significant energy saving implications. This investigation will examine the influence of short sonication times and at different stages during the crystallisation process. In addition, the size of crystals obtained under sonication is compared with the cavitation activity, quantified by sonoluminescence intensity. 2 Materials and methods 2.1 Materials Sodium chloride from Sigma-Aldrich (≥ 99% purity) and ethanol (100%) was purchased from Chem-Supply. High purity water was obtained from a Millipore system with resistivity of 18.2 MΩ/cm. 2.2 Methods Anti-solvent crystallisation experiments were performed in a glass cell with an ultrasound transducer located at the bottom. A Langevin type multi-frequency transducer of diameter 4.5 cm (resonance frequencies: 22 kHz, 44 kHz, 98 kHz and 139 kHz, Honda Electric) and a piezo-electric ceramic plate 2 22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016 Acoustics for the 21st Century… transducer of 5.5 cm (647 kHz and 1080 kHz, L-3 Communications ELAC Nautik GmbH) were used. The transducers were powered by a T&C Power Conversion, Inc. Amplifier (AG series). The actual power dissipated into the solution was determined using a standard calorimetric technique. The antisolvent crystallisation procedure consisted of 150 g of 100 % ethanol (17.1 M) as the antisolvent and the addition of 15 g of 200 g/L (3.4 M) NaCl aqueous solution. The addition of the NaCl was performed by simply pouring the solution into the cell containing the antisolvent (addition time is less than 2 seconds). This procedure was performed independently at least three times to ensure reproducibility. The solution was allowed to crystallise for 90 seconds before it was filtered using a 0.2µm filter paper and dried in the oven. The size and morphology of the sodium chloride crystals were observed in an inverted Olympus IX71 wide field microscope fitted with a 10 x and 40 x objective lens, and a CCD camera. The mean size and the size distribution of the crystals were determined by measuring approximately 200 crystals using the software ImageJ. The cavitation activity for a given sonication frequency and power was quantified by measuring the total intensity of the sonoluminescence emitted by cavitating bubbles. This was achieved by using a photomultiplier tube (PMT; Hamamatsu end-on; responsive between 300 nm – 650 nm) and by placing the setup in a lightproof enclosure to minimise interference from background light. The signals detected by the PMT are displayed on a Tektronix oscilloscope and the amplitude of the equilibrium sonoluminescence intensities recorded in Volts (V). For all measurements, 200 ml of water was sonicated until the steady state sonoluminescence (SL) intensity is reached. A more detailed procedure is described in a previous report41. 3 Results and discussion It has been shown that the crystallisation time for the concentration used in this study is 90 seconds [7]. The size of the crystals during this 90 seconds is presented in Figure 1 and shows a steady increases in the crystal size until a maximum crystal size is reached at 70 seconds, which confirms that the crystallisation process is complete. 3 22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016 Acoustics for the 21st Century… Figure 1: Average sodium chloride crystal size as a function of crystallisation time. Often in the literature, continuous sonication throughout the crystallisation process is applied. The effect of different sonication power on the crystal size under continuous sonication of 90 seconds is shown in Figure 2. The average crystal size decreases sharply with increase power until a minimum size of approximately 4.4 µm is reached after a calorimetric power of 20 W where further increase in power had no significant effect on the crystal size. Figure 2: Average sodium chloride crystal size as a function of ultrasound calorimetric power. A continuous sonication time of 90 seconds was used. For all the data a frequency of 98 kHz was used. The effect of sonication time less than the full crystallisation time of 90 second were investigated and results indicates that a sonication of 5 seconds was sufficient to reduce the crystal size to the smallest size obtained and further sonication time beyond 5 seconds had little effect on the average crystal size. 4 22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016 Acoustics for the 21st Century… A 5 second burst of ultrasound was applied at various positions within the 90 second crystallisation period to probe the effect of ultrasound. It is clear from Figure 3 that ultrasound is effective at the start of supersaturation and less effective later during the crystallisation. Figure 3: Influence of pulsing 5 second ultrasound at various time points along the 90 second crystallisation time. The size of the crystals obtained at the end of the 90 crystallisation time in the absence of ultrasound is indicated by the line. The minimum crystal size obtained under different ultrasound power and frequency were plotted against the sonoluminescence intensity obtained under the same sonication conditions (Figure 4). The results show for each frequency the mean crystal size decreases with increasing sonoluminescence intensity, and there appears the minimum crystal size reached for a given frequency appears to decrease with increasing sonoluminescence intensity. 5 22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016 Acoustics for the 21st Century… Figure 4: Volume mean diameter as a function of sonoluminescence intensity obtained under different sonication power and frequency. The dotted line is drawn to illustrate the relationship between decreasing mean crystal diameter with increasing sonoluminescence intensity. The sonoluminescence intensity is proportional to the number of sonoluminescing bubbles as well as the collapse intensity of the cavitating bubble, and increasing frequency increases the number of bubbles but the collapse intensity decreases due to smaller bubbles

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