Esper, Guenter J.

Huelsmann, Ramona

Kohlus, Reinhard

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Using an acoustic levitator to investigate thedrying kinetics and solids forming process ofindividual droplets during spray dryingRAMONA HUELSMANN1p , GUENTER J. ESPER1 and REINHARD KOHLUS21 Ramona Huelsmann, University of Applied Sciences Fulda, Germany2 University of Hohenheim, Hohenheim, GermanyORIGINAL RESEARCH PAPERReceived: August 19, 2020 • Accepted: October 11, 2020Published online: November 03, 2020© 2020 The Author(s)ABSTRACTSpray drying is a widely used process to turn slurries into dry powders and is especially important for ther-mally-sensitive materials, that are often found in the food or pharmaceutical industry. However, detailed insightinto the drying kinetics during spray drying is difficult to investigate due to the boundary conditions in a spraydrying tower. As a result, there is a lack of important information on the drying process and subsequentsolidification of individual droplets. In this context, an experimental setup for a droplet positioned in a sta-tionary ultrasonic field of an acoustic levitator is designed to enable a non-contacting measurement of thedrying kinetics and the subsequent solidification process. To generate a comparable situation like in a real spraydrying process, the droplet is positioned in an airflow, where air temperature, humidity, and velocity can beadjusted over wide range. Using an infrared camera to measure the surface temperature and a ComplementaryMetal Oxide Semiconductor (CMOS) camera for object recognition, the droplet can be observed continuouslyand drying kinetics of the droplet can be determined from the measured surface temperature and decreasingdroplet size. Result of a 10 wt.% aqueous micro particle TiO2 suspension are reported and show that theinvestigated method is a very valuable and fast tool to safely scale-up spray drying systems very close to realprocess conditions. Especially when only small sample amounts are available in an early development stage.KEYWORDSacoustic levitator, drying kineticspCorresponding author. E-mail: Ramona.Huelsmann@lt.hs-fulda.deProgress in Agricultural Engineering Sciences 16 (2020) 1, 41–49DOI: 10.1556/446.2020.00011Unauthenticated | Downloaded 10/06/22 06:59 AM UTCINTRODUCTIONIn food industry spray drying processes are widely used to produce dry free flowing powderstarting from a liquid formulation. Since this is a one stage process with high drying capacity ithas advantages compared to multi stage processes.In order to generate large surface areas for fast drying, the slurry is disintegrated in verysmall droplets which are sprayed into a hot gas stream using nozzles. During the free fallingperiod of the droplets there is a heat and mass exchange between the droplets and hot dryinggas. The drying velocity depends on the air conditions (temperature and rel. humidity) and onthe size and drying behavior of the product droplets. The latter is mostly responsibly for thelength of the spray tower.Although the drying kinetics of a product is a crucial parameter in this process, theexperimental setup to measure the drying kinetics in an environment which comes close to thedrying situation in a real spry dryer, is very difficult to establish. With the setup presented here,these investigations can be carried out on single droplets. In this way, preliminary tests for spraydrying can be carried out, which are of considerable importance for the research projectsinvolving thermally sensitive products (Breitenbach et al., 2016).In the recent years, acoustic levitation of single droplets has been developed. So showEberhardt and Neidhart (1999) the development of an ultrasonic levitator apparatus for samplepretreatment in microanalysis and trace analysis. Also, in the publication from Yarin et al.(1998), Kastner et al. (2001), Schiffter and Lee (2007) or Zaitone (2009) the Levitator assembly isdescribed. The focus here is on the influence of the ultrasonic field on the evaporation rate of thelevitated droplets. The thermographic observation of levitated droplets is investigated byTuckermann et al. (2005) and Wulsten and Lee (2008). They conclude that interpreting theevaporation rate of levitated droplets requires measuring the surface temperature.With none of these setups reported it is possible to measure the drying kinetics of productsin conditions similar to that existing in a spray dryer.In this work a setup is created for measuring the evaporation rate, which guarantees a veryaccurate measurement of the surface temperature via infrared thermography and the contour ofthe levitated droplet via a CMOS camera. The flow conditions with the newly created nozzle aresimilar to those in the spray dryer and work in the temperature range from ambient temperatureto 90 8C is possible.Within this project, the drying kinetics of different substance systems can be characterizedusing individual droplets such combining accurate determination of the drying behavior and thesmall product amount necessary.MATERIALS AND METHODSAn experimental setup for a droplet positioned in a stationary ultrasonic field of an acousticlevitator is designed to enable a non-contacting measurement of the drying kinetics and thesubsequent solidification process (Fig. 1).The ultrasonic levitator is comprised of an ultrasonic transducer and reflector. If US-frequencyand distance between transducer and reflector are set correctly, a stationary wave is established. Thedroplet can be positioned in one of the power nodes and is kept in place by the US-field.42 Progress in Agricultural Engineering Sciences 16 (2020) 1, 41–49Unauthenticated | Downloaded 10/06/22 06:59 AM UTCFig.1.ExperimentalsetupultrasoniclevitatorProgress in Agricultural Engineering Sciences 16 (2020) 1, 41–49 43Unauthenticated | Downloaded 10/06/22 06:59 AM UTCTo generate a comparable situation like in a real spray drying process, the reflector has beenconstructed as a nozzle, which is used to establish an airflow towards the droplet (Fig. 2). The airtemperature, humidity and velocity can be adjusted over wide range.Using an infrared camera to measure the surface temperature and a CMOS-camera forobject recognition, the droplet can be observed continuously. The data from the CMOS-cameraare used to determine the size of the droplet from which the volume and mass can be derived.The drying velocity in the constant rate period (CRP) can be assumed by the experimentallyobserved loss of mass in a given time range. During that period (constant rate period: CRP) thesurface temperature of the droplet remains at the wet bulb temperature. All the energy that istransferred from the hot gas stream to the droplet is used for evaporation. As soon as the dropletstops shrinking, a rise in surface temperature can be observed and is detected by the infraredsensitive camera (Fig. 3).During this period (falling rate period: FRP) the heat flow from the gas to the particle staysconstant but only a part of the energy is still used for drying. The rest is heating up the droplet.Fig. 2. US reflector modified as nozzle44 Progress in Agricultural Engineering Sciences 16 (2020) 1, 41–49Unauthenticated | Downloaded 10/06/22 06:59 AM UTCFig. 3. Determination of the drying kinetics from combined measurement of the volume reduction (moisture content) and the surface temperature ofthe productProgressinAgriculturalEngineeringSciences16(2020)1,41–4945Unauthenticated | Downloaded 10/06/22 06:59 AM UTCFrom the slope of the temperature rise and the thermodynamic properties of the slurry thedrying velocity in the FRP can be calculated.The drying air is controlled in terms of temperature, relative humidity and velocity. Thus, thecomplete characteristic drying kinetics of a single droplet in dependence of the major boundaryconditions in a spray dryer can be measured.To start the experiment, the complete setup is heated up to the desired temperature and thedroplet is positioned in one of the nodes, using a micro-syringe. After the experiment, theresulting particle is removed from the apparatus for further investigation (final moisture con-tent, strength, etc.)The product system under investigation is a titanium dioxide (TiO2) suspension. TiO2 issupplied by Merck KGaA (CAS-No. 13463-67-7). Fig. 4 shows the primary particle size dis-tribution (PSD). It is obvious that the PSD is a bi-modal distribution, where the bigger particleshave an average size of 45 mm and the smaller fraction has an average size of 2.5 mm. The largerparticles in the second peak are due to agglomeration.RESULTSTiO2 is suspended into water with an initial water content of 90%. The suspension is positionedin a node of the ultrasonic field. Here it displays a very stable levitation behavior. The resultingTiO2 particle have an almost perfect spherical shape.Fig. 5 shows a typical result for a drying experiment. Here the surface temperature andthe volume of droplet are displayed as a function of time. The volume of the droplet isreducing continuously the drying during the CRP, while the surface temperature staysconstant. In the case of the used TiO2 suspension ideal shrinkage can be observed. Thecritical point where the CRP is switching to the falling rate can easily be detected. Here theshrinkage comes to a stop, because the particle has reached its final size while the tem-perature starts to rise. Finally, the droplet reaches the surrounding air temperature and thedrying experiment is stopped.The removed particle (Fig. 6) shows a spherical shape.Fig. 4. Particle size distribution TiO2-suspension46 Progress in Agricultural Engineering Sciences 16 (2020) 1, 41–49Unauthenticated | Downloaded 10/06/22 06:59 AM UTCDISCUSSIONFig. 5 exemplifies that the drying of a 10 wt.% aqueous TiO2 suspension occurs mainly in theCRP. The loss of size occurs continuously during the drying process and the shrinkage of thedroplet stops immediately when the particle solidifies. A time gap of ∼130 s can be observedbetween the end of the shrinkage and the onset of the temperature rise. This indicates an in-ternal structure with broad capillaries which results in a fast moisture transport to the particlessurface. In this case, the particle stays in the CRP regime.The steep slope of the temperature rise in the FRP regime indicates that no water content isleft inside dried particle. Only quite short FRP regimes are detected.Fig. 6 shows microscopy images (VHX-1000, Keyence, Japan) of obtained dried particles.The surfaces of the resulting particles in Fig. 6a and b show different topographies. This resultmay be traced back to the bi-modality of the PSD and a simultaneous sedimentation processduring the drying. Larger particles sediment faster to the bottom of the droplet, resulting in analtered particle surface.CONCLUSIONIn summary, the implementation of an acoustic levitator for drying kinetics experiments ofindividual droplets is shown. First experiments prove the applicability of the setup and yieldresults with high accuracy.Using a 10 wt.% aqueous TiO2 micro particle suspension, the different drying phases of thedrying process can be measured and identified. Additionally, details of the solidification processduring the drying can be observed. Therefore, the acoustic levitator proves to be an effective andFig. 5. Results for a TiO2 suspension at an air temperature of 50 8C and an air velocity of 1 m/sProgress in Agricultural Engineering Sciences 16 (2020) 1, 41–49 47Unauthenticated | Downloaded 10/06/22 06:59 AM UTCFig. 6. Dried TiO2 particle (a) top view, (b) bottom view, (c) cross section48ProgressinAgriculturalEngineeringSciences16(2020)1,41–49Unauthenticated | Downloaded 10/06/22 06:59 AM UTCaccurate tool to investigate the drying behavior of individual droplets. The investigated methodis a very valuable and fast tool to safely scale-up spray drying systems very close to real processconditions, especially when only small sample amounts are available in an early developmentstage.REFERENCESBreitenbach, J., Beermann, C., and Esper G. J. (2016). Comparison of different methods for microencap-sulation of probiotics, Progress in Agricultural Engineering Sciences, 12(1).Eberhardt, R. and Neidhart, B. (1999) Acoustic levitation device for sample pretreatment in microanalysisand trace analysis. SpringerLink (Online service).Kastner, O., Brenn, G., Rensink, D., and Tropea, C. (2001). The acoustic tube levitator – A novel device fordetermining the drying kinetics of single droplets. Chemical Engineering & Technology, 24(4).Schiffter, H. and Lee, G. (2007) Single-droplet evaporation kinetics and particle formation in an acousticlevitator. Part 1: evaporation of water microdroplets assessed using boundary-layer and acoustic levi-tation theories. Journal of pharmaceutical sciences, 96(9).Tuckermann, R., Bauerecker, S., and Cammenga, H. K. (2005). IR-thermography of evaporating acousti-cally levitated drops. International Journal of Thermophysics, 26(5).Wulsten, E. and Lee, G. (2008) Surface temperature of acoustically levitated water microdroplets measuredusing infra-red thermography. Chemical Engineering Science, 63(22).Yarin, A. L., Pfaffenlehner, M., and Tropea, C. (1998). On the acoustic levitation of droplets. Journal ofFluid Mechanics, 356.Zaitone, B. A. A. (2009). Drying of multiphase single droplets in ultrasonic levitator. http://tuprints.ulb.tu-darmstadt.de/1892/1/AL_ZAITONE_DISS.pdf.Open Access. This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0International License (https://creativecommons.org/licenses/by-nc/4.0/), which permits unrestricted use, distribution, andreproduction in any medium for non-commercial purposes, provided the original author and source are credited, a link to the CCLicense is provided, and changes – if any – are indicated.Progress in Agricultural Engineering Sciences 16 (2020) 1, 41–49 49Unauthenticated | Downloaded 10/06/22 06:59 AM UTC

Using an acoustic levitator to investigate the drying kinetics and solids forming process of individual droplets during spray drying

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Using an acoustic levitator to investigate the drying kinetics and solids forming process of individual droplets during spray drying

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