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A new method for atomization of liquids have been developed. The method exploits the physical phenomenon of disintegration of thin liquid films by gas jets.
INNOVATIVE LIQUID ATOMIZATION TECHNOLOGY FOR DRYING PROCESSES M. Mezhericher1, I. Ladizhensky1, I. Etlin1, I. Borde2 1

Shamoon College of Engineering Basel 71, Beer Sheva 8428448, Israel, email: [email protected] 2

Ben-Gurion University of the Negev P.O. Box 653, Beer-Sheva, 84105, Israel, email: [email protected] Keywords: atomization, droplet, drying, liquid film, spray ABSTRACT A new method for atomization of liquids have been developed. The method exploits the physical phenomenon of disintegration of thin liquid films by gas jets into fine micron and submicron droplets. In several tested prototypes, the main characteristics of the generated spray have been experimentally measured by means of the laser diffraction technique under various conditions. The overall range of experimentally observed Sauter mean diameters of water droplets was between 2.66 μm and 7.53 μm for the atomizing air pressure varied between 2 and 5.5 bar, and the corresponding spray flow rates were measured between 1.0 - 3.8 l/h. The developed simple and low-cost liquid atomization technology can be useful in many drying applications involving sprays.

INTRODUCTION Micron and sub-micron droplets and functional particles suitable for delivery, controlled release, protection, or masking active ingredients is actively used in pharmaceutical, food, cosmetics, medicine, chemical and agricultural applications. Particle technologies often involve liquid atomization and spraying of droplets, which can be either subjected to drying (spray drying, spray freeze drying), or used for agglomeration/coating of core particles (spray-fluidized bed drying/coating), or subjected to chemical reaction at high temperatures (aerosol flow reactor), or droplets of low-boiling point solvent are dried under ambient conditions (supercritical fluid drying), or low-electrical conducting solutions are atomized (electro-spraying) (Chan and Kwok, 2011). Apart from the particles production, sprays of droplets, owing to their high surface to volume ratios, are widely utilized for fabrication of functional films, generation of medical aerosol dispersions for nasal and throaty drug delivery, injection of fuels in combustors and internal combustion engines, electrospray printing, cooling/heating of surfaces and fluid media, coating of surfaces, fragrance and moisture spreading, dust and contaminants removing from air and many other applications. Recent progress in engineering and science demonstrates stable trends of micronization and nanonization of production methods and growing demand for micro- and nano-products. This tendency is also observed in the field of atomization techniques where the efforts are concentrated

on obtaining submicron and nano-droplets (Abate et al., 2011; Booth et al., 2012; Kemp et al., 2013a; Kemp et al., 2013b; Li et al., 2010; Thiele et al., 2011). The known methods and devices for liquid atomization into droplets, sprays of droplets and fine mists can be generally subdivided into (Ashgriz, 2011; Booth et al., 2012; Drazil et al., 1978): nozzles; rotating discs; droplet generators; nebulizers for inhalers; analytical nebulizers; other atomizers - pintle injector, cross-flow liquid jet atomizer, impinging jet atomizer, splash plate atomizer, flash evaporation atomizer and rotating film atomizer. The major drawbacks of the aforementioned methods for production of submicron and nanodroplets is either demands of high atomization energy (Kemp et al., 2013a; Kemp et al., 2013b), or high temperature and boiling of liquid (Booth et al., 2012), or low liquid flow rates, or wide distribution of droplet sizes, or poor scalability, or low flexibility and controllability. Moreover, for sensitive materials like organics, pharmaceuticals and biologicals, influences of high temperatures, pressures, boiling, electric and magnetic/ electro-magnetic fields and ultrasonic waves can be harmful. In addition, specific costs of submicron and nano- atomization (expenses on atomization per effective atomization flow rate) are typically high. Fabrication of ordered particle microstructures, micro-encapsulation and coating of micro-particles is nowadays performed by many methods and is being extensively studied both theoretically and experimentally (Lin, 2012; Mittal, 2013). Many of the techniques for high throughput production, especially dealing with organic and pharmaceutical materials, utilize sprays of droplets and particles, and thus the revealed drawbacks of liquid atomization are presented in these technologies as well. Therefore, development of simple, low cost, low energy consuming, ecological, high-throughput way of generation of submicron and nano-droplets is required for further progress of technology and science. For particle technology, the new liquid-atomization method should be able to/compatible with production of particle microstructures, encapsulation and coating of microparticles.

OBJECTIVES The main objective of the study was to investigate the feasibility of an idea of generation of sprays of fine droplets by disintegrating thin liquid films using gas jets.

EXPERIMENTAL SETUP A prototype of liquid-atomization device implementing the new invented method of fine droplets generation from thin liquid films disintegrated by gas jets has been constructed. The atomizer was a part of experimental setup shown in Figure 1. The main part of the device is an elastic rubber tube of 11 mm in diameter. The 2 mm walls of the tube were perforated with 0.6 mm orifices along the tube circumference at several axial positions. Two spatial configurations of the tube were tested: straight tube and tube bent into a ring (as given in Figure 1). The tube was horizontally oriented and partially submerged into a liquid (water, salt water): the lower portion of the tube was immersed in the liquid, and the upper portion of the tube was exposed to the environment. The tube was connected to a source of compressed air, and the supplied air was continuously discharging through the perforated orifices. The compressed air, released through the submerged lower part of the tube, formed ensembles of small bubbles, which came up to the liquid surface and created ensembles of thin spherical liquid films (estimated thicknesses is less than 500 nm (Geguzin, 1985)) in the vicinity of the tube upper part. Subsequently, the air discharged through the upper orifices broke and disintegrated these liquid films into ensembles of fine droplets (spray), see Figure 2a. A photo illustration of a prototype device in operation is shown in Figure 2b. All the media in experiments were kept at room temperature. 2

To determine the basic parameters of the atomized droplets, MALVERN Spraytec instrument was utilized to measure the droplet statistics by laser diffraction method. In the experimental studies, flow rate and pressure of the supplied compressed air as well as the number of perforated orifices in the tube walls were varied.

Air

Figure 1. Experimental setup. 1 – vessel, 2 – perforated tube (atomizer), 3 – air filter, 4 – air pressure regulator, 5 – MALVERN Sraytec device, 6 – laser acceptor, 7 – laser emitter, 8 – vent.

droplet gas jets

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(a)

tube

liquid

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Figure 2. New concept of liquid atomization using spherical thin liquid films. a – sketch of the new atomization method (Mezhericher et al., 2014), showing vertical cross-section of perforated tube partially submerged in liquid; b – photo of a prototype device in operation (video is available at https://sites.google.com/site/maxmezhome/research/atomization).

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RESULTS Figure 3 shows two examples of the droplet distributions in sprays obtained in the experimental runs. Here d10 and d32 are correspondingly the arithmetic and Sauter mean droplet diameters (Ashgriz, 2011). The overall range of experimentally observed Sauter mean diameters of water droplets was between 2.66 μm and 7.53 μm, depending on number of perforated orifices, their pitch, atomizing air pressure, level of water in vessel, tube material, tube wall thickness etc. • air pressure 1.5 bar • d = 7.53 μm (Sauter diam.)

• air pressure 3.5 bar • d = 2.66 μm (Sauter diam.)

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32

• d = 1.48 μm (arithmetic diam.) 10

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10

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1000

0.1

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10

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Figure 3. Examples of obtained droplet distributions in the sprays generated by different experimental configurations (abscissa – droplet diameter, μm; ordinate – probability density function, %) a, b – sprays with the smallest and the biggest mean droplet diameter, respectively

For the atomizing air pressure raised between 2 and 5.5 bar, the measured spray flow rates increased linearly for each studied tube specimen and the overall range of droplets flow rates was between 1.0 l/h and 3.8 l/h. Figure 4a shows an example of the obtained correlation between the air pressure and spray flow rate. At the same time, non-linear behavior of the corresponding Sauter mean droplet diameters was observed, as illustrated by Figure 4b (obtained for the same conditions as in Figure 4a): first, the mean diameters were gradually growing with air pressure increase up to 4 bars, then the droplet mean sizes were rapidly decreasing until 5 bars and after that slowly increasing at 5.5 bars. To explain the observed non-linear behavior, a theoretical dimensional analysis was performed. The study allowed us to establish that for disintegration of flat thin liquid film by a cross-flowing gas jet, the size of the obtained droplets depends on the following parameters:   dd µ ρ δ T = f  Re g −1 ; We g −1 ; Eu g ; l ; l ; l ; l ; θls  ,   µ g ρg d gj Tg d gj  

(1)

Therefore, the process of droplet formation by the proposed method is governed by gas inertia and pressure; gas and liquid viscosities, densities and temperatures; liquid surface tension and wetting phenomenon, and also by diameter of the gas jet. For constant temperature conditions, elevation of supplied air pressure increases the diameter of air jets coming out from elastic orifices located in the top unsubmerged part of the tube (see Figure 2a). This effect enlarges the size of generated droplets. On the other hand, increase in air pressure leads to increase in velocity of the air jets which disintegrate the liquid film; this influence tends to reduce the diameter of the generated droplets. Combined action of the two effects results in the non-linear behaviour of the droplet size, as illustrated by Figure 4b. To visualize details of the process of liquid film disintegration, numerical simulations of fragmentation of a flat thin liquid film into droplets by a continuously flowing gas jet coming at cross 4

direction have been conducted. The CFD software ANSYS FLUENT 15 was utilized. Figure 5 demonstrates the numerical predictions of fragmentation of 1 mm water film into droplets by compressed air supplied at pressure of 1 bar and room temperature via a nozzle. The captured snapshots, from left to right, show the progress of interaction between the air jet and the liquid film. First, the liquid film is ruptured by the jet, then the torn part of the film is broken into small droplets, and after that the liquid ligaments, left due to capillary effects in the film, are also shredded into tiny droplets by the oncoming air jet. These preliminary simulations demonstrate that theoretically the generated droplets can be substantially smaller than the thickness of the parental liquid film.

(a)

(b)

Figure 4. Spray flow rate (a) and Sauter diameter of spray droplets (b) for different values of atomizing air pressure. The experimental data are represented by dots and associated trendlines. The results were obtained for identical experimental configuration with straight atomizer tube; the values of spray flow rate are related to a tube of 1 m length.

In general, the undertaken experimental and theoretical studies have confirmed the feasibility of the original idea on generation of fine droplets from thin liquid films disintegrated by gas jets, and a patent application has been submitted by the authors (Mezhericher et al., 2014). Moreover, the obtained droplet characteristics have indicated that the proposed method of fine droplets generation has advantage over the known atomization technologies (Ashgriz, 2011; Kemp et al., 2013b; Li et al., 2010) in many parameters, like droplets mean size and size distribution combined with liquid flow rate and required atomizing gas pressure; simplicity of manufacturing, operation and control; versatility and low cost value; scalability, environmental friendliness and suitability for organic, biological and pharmaceutical materials.

environment

water film compressed air Figure 5. Disintegration of water film with thickness 1 mm into droplets by cross-flowing air jet supplied at 1 bar and room temperature. Snapshots from numerical simulation (time progress is shown from left to right, colors: water – red, air – blue). 5

It is worth mentioning that though ultrasonic atomization method is also suitable for generation of comparable small micron droplets (~ 5 μm (Ashgriz, 2011)), but ultrasonic waves can be harmful for biological and pharmaceutical liquids because they can change chemical environment by locally heating liquid until extreme temperatures and also producing undesirable chemical reactions (e.g., leading to appearance of hydrogen peroxide in water), changing by this the pH and chemical composition of the initial solution (Suslick, 1988). In contrast, the developed method does not affect the chemical parameters of the involved media.

CONCLUSIONS The results of the performed studies demonstrate that the proposed methodology of application of thin liquid films for liquid atomization allows obtaining sprays with very fine droplets. The tested prototypes generated water droplets with Sauter diameters in range of 2.66 μm and 7.53 μm, whereas the spray flow rates were between 1.0 l/h and 3.8 l/h and atomizing air pressure varied between 2.0 - 5.5 bar. The new method is low cost, scalable, environmental friendlily, suitable for different liquids including biological and pharmaceutical materials, and thus can be perspective for many drying applications involving sprays, droplets and particles. As one of the examples, the novel method will allow intensification of spray drying processes. Further research in the field of the spray drying will be devoted to improvement of the quality of dried product.

NOTATION d Eu p Re T v We

diameter Euler number (= p/ρv2) pressure Reynolds number (=vd/ν) temperature velocity Weber number (=ρv2d/ σ)

m Pa ⁰C m/s -

Greek Symbols δ film thickness θ wetting angle µ dynamic viscosity ρ density σ liquid surface tension ν kinematic viscosity Subscripts d g j l s

m kg/ms kg/m3 N/m m2/s

droplet gas jet liquid solid

REFERENCES Abate, A.R., Thiele, J.W.P., Weitz, D.A., Holtze, C. and Windbergs, M. (2011), Spray drying techniques, Patent, USA. Ashgriz, N. (2011), Handbook of atomization and sprays, Springer, New York. 6

Booth, A., McIntosh, A.C., Beheshti, N., Walker, R., Larsson, L.U. and Copestake, A. (2012), Spray technologies inspired by bombardier beetle, in: Bhushan, B. (Ed.), Encyclopedia of Nanotechnology, Springer, New York. Chan, H.K. and Kwok, P.C.L., 2011. Production methods for nanodrug particles using the bottomup approach, Advanced Drug Delivery Reviews, Vol. 63(6), pp. 406-416. Drazil, R., Hanus, J., Drazilova, J., Hofman, J. and Dvorak, Z. (1978), Zarízení na postrikování plosných utvaru, zejména textilních pásu, cásticeni tekutin ve forme filmu, Patent, Czech Republic. Geguzin, Y.E. (1985), Bubbles (in Russian), Nauka, Moscow. Kemp, I.C., Hartwig, T., Hamilton, P., Wadley, R. and Bisten, A. (2013a), Spray drying of fine particles from organic solvents at small and large scales, Eurodrying 2013, Paris, France, 2-4 October 2013. Kemp, I.C., Wadley, R., Hartwig, T., Cocchini, U., See-Toh, Y., Gorringe, L., Fordham, K. and Ricard, F. (2013b), Experimental study of spray drying and atomization with a two-fluid nozzle to produce inhalable particles, Drying Technology, Vol. 31(8), pp. 930-941. Li, X.A., Anton, N., Arpagaus, C., Belleteix, F. and Vandanune, T.F. (2010), Nanoparticles by spray drying using innovative new technology: the Buchi Nano Spray Dryer B-90, Journal of Controlled Release, Vol. 147(2), pp. 304-310. Lin, Z. (2012), Evaporative self-assembly of ordered complex structures, World Scientific, Singapore. Mezhericher, M., Ladizhensky, I. and Etlin, I., 2014. Liquid-atomization method and device, in: Office, I.P. (Ed.), Israel. Mittal, V., 2013. Encapsulation nanotechnologies, Wiley, New York. Suslick, K.S. (1988), Ultrasound: its chemical, physical, and biological effects, VCH Publishers, New York. Thiele, J., Windbergs, M., Abate, A.R., Trebbin, M., Shum, H.C., Forster, S. and Weitz, D.A., (2011), Early development drug formulation on a chip: fabrication of nanoparticles using a microfluidic spray dryer, Lab on a Chip, Vol. 11(14), pp. 2362-2368.

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