Effect of Carrier Gas Flow Behavior on Performance of Separation by Using Ultrasonic Atomization

Transcription

1 Effect of Carrier Gas Flow Behavior on Performance of Separation by Using Ultrasonic Atomization Yoshiyuki Bando 1, Keiji Yasuda 1, Akira Matsuoka 1 and Yasuhito Kawase 2 1. Department of Chemical Engineering, Nagoya University, Nagoya, Japan 2. Nippon Refine Company Limited, Tokyo, Japan Abstract: In order to improve the efficiency of the separation process by using the ultrasonic atomization, the effect of flow behavior of the carrier gas on the separation performance was investigated. The flow simulations of the carrier gas and the liquid droplets were performed by using the CFD software. After the flow field of the carrier gas was decided, the liquid droplets were introduced into the flow field and the trajectories of the droplets were calculated by using the Lagrangian approach. From the simulation results, the flow rate of the liquid accompanied by the carrier gas was calculated. The atomization vessel was rectangular (.3 m in length,.5 m in width and.3 m in height) and made from transparent polyvinyl chloride resin. The ultrasonic transducer (2.4 MHz) was attached at the center of the vessel bottom with the inclination angle of about 1 degrees with respect to the liquid surface. Nitrogen was used as the carrier gas. Water and ethanol aqueous solution were used as the liquid samples to be atomized. The flow direction against the inclined liquid fountain and the positions of inlet and outlet of the carrier gas were combined, and the flow rate of the was measured. For the ethanol aqueous solution, the concentration ratio was examined. The flow rate of the became higher when the inlet and outlet of the carrier gas were located at the lower position. The calculated results under some assumptions reproduced the tendency of the measured data. The concentration ratio of the ethanol aqueous solution became higher when the flow rate of the was lower. Keywords: Ultrasonic Atomization, Separation, Carrier Gas, CFD 1. INTRODUCTION When high frequency ultrasound is irradiated to the liquid surface, a liquid fountain is formed on the liquid surface and the liquid is atomized from the surface of the fountain. Capillary wave has been considered to cause the ultrasonic atomization. That is, the ultrasonic atomization arises from the rupture of the capillary surface wave [1, 2]. Lang [3] has derived the theoretical equation for the diameter of the atomized droplet as a function of the ultrasonic frequency, surface tension and density of liquid. Yano et al. [4] have reported that the ethanol rich droplets with nanometer size are generated by the ultrasonic atomization of ethanol aqueous solutions. Sato et al. [5] have reported the selective separation of ethanol from the aqueous solutions. Some researchers have used the ultrasonic atomization to the aqueous solutions of surfactants, and examined the separation and concentration of surfactants [6, 7]. It is considered that the solutes with interface activity are separated and concentrated to the atomized droplets. However, the mechanism of the separation is scarcely clarified. The separation and concentration by using the ultrasonic atomization have advantages, such as the easy operation, little phase change and non-thermal process. However, the flow rate of atomization and the separation and concentration efficiency are relatively low. From a viewpoint of the industrial application of the ultrasonic atomization to the separation and concentration, it is necessary to improve the performances of the separation and concentration. The flow rate of atomization increases with increasing power input to the ultrasonic transducer, but the high power input causes a short lifetime of the ultrasonic transducer. Therefore it is desired to enhance the flow rate and concentration performance by improving the other parameters. The authors [8-11] have examined the effects of apparatus and operation conditions on the performance of ultrasonic atomization and the concentration of solutes from the aqueous solutions. From these studies, we have suggested that the concentration efficiency is enhanced by collecting the smaller liquid droplets. In the process for the separation and concentration by using the ultrasonic atomization, the atomized liquid droplets are accompanied and collected by the carrier gas. Accordingly, the flow pattern of the carrier gas is considered to have a significant influence on the flow rate of the and the separation efficiency. In this study, the effect of flow behavior of the carrier gas on the performance of the separation by using the ultrasonic atomization was investigated. A computational fluid dynamics (CFD) approach was attempted to conduct 3D simulations of the flows of the carrier gas and atomized liquid droplets. For the flow of the liquid droplets, the Lagrangian method was employed to track the individual liquid droplets. In the experiment, the flow pattern of the carrier gas in the atomization vessel was changed, and the flow rate of the and the separation performance were investigated. Corresponding author: Y. Bando, bando@nuce.nagoya-u.ac.jp 878

2 2. CALCULATION In order to estimate the flow rate of the atomized liquid droplets accompanied to the outlet of the atomization vessel by the carrier gas, the flow simulations of the carrier gas and the liquid droplets were conducted by using a CFD software (FLUENT ver. 6.3). A sequence of flow simulations consisted of three parts of procedures. First, the flow field of the carrier gas was decided. Second, the liquid droplets were introduced in the flow field of the carrier gas and each droplet was tracked individually in the flow field. Third, the liquid droplets accompanied to the outlet without a collision to the wall of the vessel were counted, and the flow rate of the was estimated. In the simulation, no interaction between the liquid droplets was assumed. It was also assumed that the presence of the liquid droplets has no influence on the flow behavior of the carrier gas due to a small amount of droplets mass. To simulate the flow field for the various positions of inlet and outlet of the carrier gas, several computational domains were constructed. The computational domains have the geometry of the inside of the atomization vessel used in the experiment (.3 m in length,.5 m in width and.3 m in height), excepting the bulk liquid. Since the vibration of the liquid surface and the presence of the liquid fountain were neglected, the geometry of the domain was a simple rectangular shape. The geometry was divided into the finite number of volumes. For the flow simulation, the velocity of the carrier gas was given at the inlet. The flow field of the carrier gas was obtained by solving the conservation equations of mass and momentum in the computational domain. After the flow field of the carrier gas was decided, about 35 liquid droplets were introduced. The liquid droplets were put in the specified region determined from the observation of the ultrasonic atomization. As the physical properties of the liquid droplet, those of the liquid sample to be atomized were adopted, and the diameter of the droplets was assumed to be 5 µm. The equation for the rate of change in the droplet velocity was derived directly from Newton s second law of motion; F = m D (dv D / dt) (1) where F is the resultant force vector on the liquid droplet, v D and m D are the velocity vector and mass of the liquid droplet. The major component of the force acting on the liquid droplet is the drag force which is exerted on the droplet by the surrounding fluid. The drag force, F d, was expressed as follows; F d = π d D 2 ρ D v D v G (v D v G ) C d / 8 (2) where v G represents the local gas velocity vector, and d D and ρ D are the diameter and density of the liquid droplet. The drag coefficient, C d, is given by C d = a 1 + a 2 / Re + a 3 / Re 2 (3) where a 1, a 2 and a 3 are the constants given by Morsi and Alexander [12], which are applied to the smooth spherical particles over several ranges of the relative Reynolds number, Re. The Re is defined as follows; Re = ρ D d D v D v G / µ G (4) where µ G is the viscosity of the gas. In addition, the other forces, which account for the pressure gradients, buoyancy and added mass, act on the liquid droplet. In the simulation, these forces were included as well as the drag force. In tracking the individual liquid droplets, no interaction between liquid droplets was assumed. When the liquid droplet struck the wall of the vessel, the liquid droplet disappeared and was not accompanied to the outlet. From the simulation of trajectories of the liquid droplets, the number of the droplets was counted. The flow rate of the, R A,Calc., was obtained from the following equation;. R A,Calc. = R D (N A / N) (5) where N A and N are the numbers of the accompanied liquid droplets and introduced ones. R D is the generation rate of the liquid droplets. 3. EXPERIMENTAL Figure 1 shows the outline of the experimental apparatus. A vessel for the ultrasonic atomization was rectangular and the dimensions were.3 m in length,.5 m in width and.3 m in height. The vessel had six positions for the inlet and seven ones for the outlet as shown in Figure 1-(a). The diameter of the inlet and outlet was.13 m. The ultrasonic transducer of.2 m in diameter was attached to the center of the vessel bottom, and the inclination angle of the transducer with respect to the liquid surface was approximately 1 degrees. The frequency of ultrasound was 2.4 MHz and the input power was 2 W. Nitrogen was used as the carrier gas. From the cylinder the carrier gas was passed through a saturation column, and then fed to the atomization vessel. The flow rate was changed. In order to examine the effect of evaporation, in a part of the experiments, the carrier gas was fed directly to the vessel without passing through the saturation column. The flow pattern of the carrier gas was changed by varying the combination of inlet and outlet positions of the carrier gas. The flow direction was also changed, that is, the same or the opposite direction against the inclination of the liquid fountain. Water and ethanol aqueous solution were used as the liquid samples to be atomized. The liquid sample was poured in the atomization vessel. The vessel was placed on an electronic balance. The liquid height from the surface of the ultrasonic transducer was.3 m *). Under the various combinations of inlet and outlet positions, the carrier gas was flowed into the vessel at a given flow rate. After the carrier gas flow became stable, the ultrasonic irradiation was started. The change in sample weight with the ultrasonic irradiation time was measured. The initial content of the ethanol aqueous solution was 15 wt%. After the ultrasonic atomization, the ethanol content in the residual solution in the vessel was analyzed with a gas *) In the preliminary experiment where the liquid height was changed, the flow rate of had a maximum at the liquid height of about 3 mm. 879

3 Proceedings of International Symposium on EcoTopia Science 27, ISETS7 (27) (a) Atomization vessel (a) (b) (c) (d) (e) (f) (b) Experimental setup (4) (1) 1 mm N2 (1) Atomization vessel (3) Electronic balance Carrier gas flow (2) Fig. 2 Photos of ultrasonic atomization for water without carrier gas; Time: (a) s, (b).2 s, (c).4 s, (d).6 s, (e).8 s, (f).1 s (3) (2) Ultrasonic transducer (4) Saturation column Fig. 1 Outline of experimental apparatus; (a) Atomization vessel, (b) Experimental setup chromatography with a flame ionization detector (GC-9A, Shimadzu). The flow rate of the and the ethanol content in the were calculated from the following material balance; RA = (W W) / t (6) CA = (C W CR W) / (W W) (7) where W and C are the weight and ethanol content of the initial liquid, W and CR are the weight and ethanol content of the residual liquid after the ultrasonic atomization and t is the ultrasonic irradiation time. 4. RESULTS AND DISCUSSION Observation of ultrasonic atomization By using a high-speed video camera, the evolution process of the ultrasonic atomization was visualized. Figure 2 shows the photos of the temporal evolution of the ultrasonic atomization for water without the carrier gas. When high frequency ultrasound is irradiated upward from the bottom of the liquid layer, a liquid fountain starts to be formed as shown in Figure 2 (b). The fountain evolves and the liquid is atomized from the surface of the liquid fountain as shown in Figure 2 (c) (f). In the observation of the ultrasonic atomization, the generated liquid droplets were classified into three groups according to the size. That is, several micrometers, 88 several tens or hundreds micrometers and several millimeters. The liquid droplet with the diameter of several micrometers (noted just as liquid droplet ) is generated as a result of liquid pinching from the crests of the capillary waves due to the capillary instability. In Figure 2 (c) (f), these liquid droplets are observed to make the cloud of droplets. The liquid droplet with the diameter of several tens or hundreds of micrometers (noted as large liquid droplet ) can be generated due to the violent movement of the fountain surface. Although the presence of these large liquid droplets is hardly seen from a series of photos shown in Figure 2, their appearance was confirmed in the observation of moving images. The liquid droplet with the diameter of several millimeters (noted as liquid fragment ) is foamed by the detachment of the tip of the liquid fountain. In Figure 2 (e), this liquid fragment is seen at the upper right corner in the photo. In any photos, no liquid droplet exists around the base of the fountain. Not shown in these photos, it was observed that the height of the liquid fountain was about.6 m and the atomized liquid droplets reached up to about.13 m from the surface of the bulk liquid Flow behavior of carrier gas and trajectories of atomized liquid droplets (calculated results) By using the CFD software, the flow simulations of the carrier gas and atomized liquid droplets in the atomization vessel were conducted for the various positions of inlet and outlet of the carrier gas. Figure 3 shows the flow patterns of the carrier gas calculated for the various positions of the inlet and outlet. The velocity of the carrier gas based on the cross-sectional area of the inlet is 2.5 m/s. The carrier gas flows horizontally from the inlet and changes the

4 (a) (a) (b) (b) (c) (c) 2.5 m/s Fig. 3 Flow pattern of carrier gas (calculated results); (a) H IN =.45 m, H OUT =.45 m, (b) H IN =.45 m, H OUT =.3 m, (c) H IN =.177 m, H OUT =.3 m direction near the opposite-side wall. While almost the carrier gas flows from the inlet to the outlet and forms the main flow, a part of the carrier gas forms the swirl flows above and/or below the main flow. Additionally, some small eddies are generated near the corners of the vessel. When the inlet position is the same, the flow pattern of the carrier gas, including the region of the swirl flow formation, is hardly affected by the outlet position. When the outlet position is the same, on the other hand, the flow pattern of the carrier gas is largely changed by the inlet position. Figure 4 shows the trajectories of liquid droplets calculated in the flow fields of the carrier gas shown in Figure 3. The liquid droplets were introduced to the region corresponding to the parallel flow. Not all accompanied droplets not accompanied droplets region where liquid droplets are introduced Fig. 4 Trajectories of liquid droplets (calculated results); (a) H IN =.45 m, H OUT =.45 m, (b) H IN =.45 m, H OUT =.3 m, (c) H IN =.177 m, H OUT =.3 m trajectories of the droplets are shown in the figure, but the trajectories of 5 droplets are shown. The trajectories of accompanied and not accompanied droplets are represented by the solid and dotted lines, respectively. For the lower inlet position, some liquid droplets are directly accompanied to the outlet by the main flow of the carrier gas. In addition, the liquid droplets which are initially accumulated in the swirl flow, deviate from the swirl flow, and flow to the outlet. For the higher inlet position, in contrast, most liquid droplets are accumulated into the swirl flow formed in the lower part of the vessel and as a result, fewer droplets are accompanied to the outlet. 881

5 RA 1 3 [kg/s] Key Flow direction Parallel Counter Sample: Water H IN =.45 m H OUT =.3 m ; The data when carrier gas is not passed through saturation column for parallel flow u G [m/s] Fig. 5 Effect of carrier gas velocity on flow rate of Effect of carrier gas flow behavior on flow rate of The flow direction, flow mode (inlet and outlet positions) and velocity of the carrier gas were changed. Water was used as the liquid, and the flow rate of the was measured. Figure 5 shows the effect of velocity of the carrier gas on the flow rate of the, R A, for the different flow directions of the carrier gas. The inlet and outlet are at the positions of.45 m and.3 m from the vessel bottom. (The outlet is at the upper wall of the vessel.) In every case, R A increases monotonously with increasing velocity of the carrier gas till 3.8 m/s, and no significant influence of the flow direction is found. However, when the carrier gas velocity is beyond 3.8 m/s, the increment in R A becomes small and R A for the parallel flow is higher than that for the counter one. This is because the carrier gas disturbs the formation of the liquid fountain. In particular, when the flow direction is the counter, the liquid fragments from the fountain often fall near the surface of the bulk liquid above the transducer. As a result, the formation of the liquid fountain becomes unstable and the atomization from the liquid fountain is lessened. When the carrier gas is fed without passing through the saturation column, R A becomes a little higher. This is because of the enhancement in evaporation from the bulk liquid and liquid droplets [9]. Figure 6 shows the effect of outlet position on the flow rate of the, R A. The velocity of the carrier gas at the inlet is 2.5 m/s. The outlet position is changed from.45 to.265 m high (located at the sidewall) and.3 m high (located at the upper wall) from the vessel bottom. (see Figure 1-(a)) In every case, R A decreases as the outlet position becomes higher. This is because the large liquid droplets are more difficult to be accompanied to the higher outlet. In addition, since the distance from the liquid fountain to the outlet is longer, much liquid droplets are adhered on the vessel wall. When the outlet position is relatively low, R A is lower for the parallel flow than for the counter one. The liquid fragments from the fountain fall near the outlet and interrupt accompanying the liquid droplets to the outlet. This interruption is more significant for the parallel flow RA 1 3 [kg/s] Key H IN [m] Flow direction.45 Parallel Counter.177 Parallel Sample: Water u G = 2.5 m/s H OUT [m] Fig. 6 Effect of outlet position on flow rate of RA 1 3 [kg/s] Key Flow direction Data Parallel Exp. Counter Exp. Parallel Calc. Sample: Water H OUT =.3 m u G = 2.5 m/s H IN [m] Fig. 7 Effect of inlet position on flow rate of than for the counter one. When the outlet position is higher, on the other hand, R A is higher for the parallel flow than for the counter one. This is because the region where the liquid droplets are generated is closer to the outlet for the former than for the latter. For the higher inlet position, although R A is lower than for the lower inlet position, the tendency of R A to the outlet position is the same. Figure 7 shows the effect of inlet position on the flow rate of the, R A. The calculated results for the parallel flow are also shown. The outlet is at the upper wall of the vessel. For both the flow directions of the carrier gas, R A becomes lower as the inlet position is higher. When the inlet position is relatively low (.45 m and.89 m from the vessel bottom), the main flow of the carrier gas strikes the liquid fountain and as a result, much liquid droplets are accompanied to the outlet. As the inlet position becomes higher, much liquid droplets are accumulated in the swirl flow formed below the inlet, and R A becomes lower. Although the calculated results are much fluctuated, they reproduce the tendency of the measured data. 882

6 Table 1 Concentration characteristics by ultrasonic atomization for different flow directions and flow modes of carrier gas (C = 15 wt%, u G = 2.5 m/s) Case Flow H IN H OUT R A 1 3 C A direction [m] [m] [kg/s] [wt%] Parallel Counter Effect of carrier gas flow behavior on concentration performance of ethanol aqueous solution The ethanol aqueous solution was atomized. The initial content of ethanol was 15 wt% and the velocity at the inlet was 2.5 m/s. The flow rate of the accompanied liquid and the ethanol content in the were measured, and the concentration performance was investigated. Table 1 shows the concentration characteristics by the ultrasonic atomization for the different flow directions and flow modes of the carrier gas. Under the same conditions of the carrier gas, the flow rate of the for the ethanol aqueous solution is very similar to that for water. For both the flow directions of the carrier gas, the ethanol content in the accompanied liquid becomes high when the inlet and outlet positions are high. This is because the large liquid droplets, which include less ethanol [1], are more difficult to be accompanied for the higher inlet and outlet positions. When the inlet and outlet positions are high, the ethanol content in the is a little higher for the counter flow than for parallel one. As mentioned above, accompanying the large liquid droplets results in high flow rate of the accompanied liquid. So, the concentration ratio (= C A / C ) was calculated and plotted against the flow rate of the. The plot is shown in Figure 9. For both the flow directions, the concentration ratio increases with decreasing flow rate of. From these results, the control of the flow rate of the by changing the flow behavior of the carrier gas is found to be efficient for the enhancement in the concentration performance. 5. CONCLUSIONS In the rectangular vessel for ultrasonic atomization, the flow direction and the positions of inlet and outlet of the carrier gas were combined. The effect of flow behavior of the carrier gas on the flow pattern in the vessel and the separation performance were examined. The following results were obtained: 1. The flow pattern of the carrier gas in the vessel is largely affected by the inlet position of the carrier gas. The atomized liquid droplets are accumulated in the swirl flows formed by a part of the carrier gas flow. 2. The flow rate of the increases CA / C [-] Key Flow direction Parallel Counter Sample: Ethanol Aq. Soln. C = 15 wt% u G = 2.5 m/s R A 1 3 [kg/s] Fig. 9 Plot of concentration ratio against flow rate of for ethanol aqueous solution with increasing velocity of the carrier gas. When the gas velocity is too high, the increment in the flow rate becomes small. For the high gas velocity, the flow rate is higher for the parallel flow than for the counter one. The flow rate becomes higher as the positions of outlet and inlet of the carrier gas are lower. The calculated results reproduce the tendency of the measured data. 3. As the flow rate of the is lower, the concentration ratio becomes higher. The control of the flow rate of the by changing the flow behavior of the carrier gas is efficient for the enhancement in the concentration performance. NOMENCLATURE C A = ethanol content in [wt%] C = ethanol content in initial solution [wt%] C R = ethanol content in residual liquid [wt%] C d = drag coefficient [-] d D = diameter of liquid droplets [m] F d = drag force [N] H IN = height of inlet position [m] H OUT = height of outlet position [m] N A = number of droplets [-] N = number of introduced liquid droplets [-] R A = flow rate of [kg/s] R D = rate of liquid droplet generation [kg/s] Re = relative Reynolds number [-] t = ultrasonic irradiation time [s] u G = gas velocity based on cross-sectional area of inlet [m/s] v D = velocity of liquid droplets [m/s] v G = velocity of gas phase [m/s] W = weight of residual liquid [kg] W = initial weight of liquid [kg] µ G = viscosity of gas phase [Pa s] ρ D = density of liquid droplets [kg/m 3 ] REFERENCES 1. W. Gaertner, J. Acoust. Soc. Am., 26 (1954), pp D. Sindayihebura and L. Bolle, Atomization Sprays, 8 (1998), pp

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