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Atomization

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Atomization of viscous and non-newtonian liquids by a coaxial, high-speed gas jet. Experiments and droplet size modelingIntroductionDescription of experimentExperimental setupDroplet size and velocity measurementsCharacterization of liquid rheologyAtomization modelRheological propertiesResults and discussionQualitative observationsDroplet size measurements and comparison with model predictionConclusionsAcknowledgementsReferencesAtomization of viscous and non-newtonian liquidsby a coaxial, high-speed gas jet. Experimentsand droplet size modelingA. Alisedaa,1, E.J. Hopfingera, J.C. Lasherasa,D.M. Kremerb,*, A. Berchiellib, E.K. ConnollybaDepartment of Mechanical and Aerospace Engineering, University of California, San Diego, 9500 Gilman Drive,La Jolla, CA 92093-0411, United StatesbOral Products Center of Emphasis, Pfizer, Inc., Global Research and Development, Groton/New London Laboratories,Eastern Point Road MS 8156-07, Groton, CT 06340, United StatesReceived 15 May 2007; received in revised form 22 August 2007AbstractThis paper describes a collaborative theoretical and experimental research effort to investigate both the atomizationdynamics of non-Newtonian liquids as well as the performance of coaxial atomizers utilized in pharmaceutical tablet coat-ing. In pharmaceutically relevant applications, the coating solutions being atomized are typically complex, non-Newtonianfluids which may contain polymers, surfactants and large concentrations of insoluble solids in suspension. The goal of thisinvestigation was to improve the understanding of the physical mechanism that leads to atomization of viscous and non-Newtonian fluids and to produce a validated theoretical model capable of making quantitative predictions of atomizer per-formance in pharmaceutical tablet coaters. The Rayleigh–Taylor model developed by Varga et al. has been extended toviscous and non-Newtonian fluids starting with the general dispersion relation obtained by Joseph et al. The theoreticalmodel is validated using droplet diameter data collected with a Phase Doppler Particle Analyzer for six fluids of increasingrheological complexity. The primary output from the model is the Sauter Mean Diameter of the atomized droplet distri-bution, which is shown to compare favorably with experimental data. Critical model parameters and plans for additionalresearch are also identified. 2007 Elsevier Ltd. All rights reserved.Keywords: Atomization; Modeling; Non-Newtonian; Pharmaceutical; Experiment0301-9322/$ - see front matter  2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.ijmultiphaseflow.2007.09.003*Corresponding author. Tel.: +1 860 686 2856; fax: +1 860 686 6509.E-mail address: douglas.m.kremer@pfizer.com (D.M. Kremer).1Current Address: Department of Mechanical Engineering, University of Washington, Stevens Way, Box 352600, Seattle, WA 98195,United States.Available online at www.sciencedirect.comInternational Journal of Multiphase Flow 34 (2008) 161–175www.elsevier.com/locate/ijmulflow1. IntroductionThe atomization of a liquid jet by a co-flowing, high-speed gas is a process of considerable practical interestin many industrial settings as wel l as being a fundamental research topic in multiphase flow. Although atom-ization processes are utilized frequently in industrial applications, the underlying physical mechanisms thatdetermine atomization characteristics are not fully understood. In particular, while the atomization of liquidsis utilized extensively in a variety of pharmaceutical manufacturing processes, a clear need remains for physics-based models to facilitate process understanding and scale-up. The role of atomization in pharmaceuticalmanufacturing can be organized into two broadly defined categories. One category of pharmaceutical manu-facturing processes utilizes atomization to alter the in vivo performance of the active pharmaceutical ingredient(API), often by mod ifying the bioavai lability of the API itself. A common manufacturing process of this typeis spray drying. During spray drying, API and other excipients are dissolved in solvents and the solution isatomized in a heated gas stream and dried to form powd ers (Masters, 1976). Research has shown that the sizedistribution of the atomized droplets coupled with the operating parameters of the spray dryer can influencethe morphology of the dried powder (Lin and Gentry, 2003). Additionally, scale-up of the spray drying pro-cess is notoriously difficult due to the inability of models to predict atomizer performance at different scales,especially for pharmaceutically relevant solutions (Kremer and Hancock, 2006; Oakley, 2004). Thus, scale-upof this process can result in unanticipated changes in the size and morphology of the dried powder which candeleteriously impact the downstream manufacturing steps necessary to produce the final dosage form.Another example of a pharmaceutical manufacturing operation in this category is spray congealing. In thisprocess, the API is mixed with waxes and atomized, normally via a rotary atomizer, with the goal of producingvery small particles containing encapsula ted API (Kawase and De, 1982; Mackaplow et al., 2006). Encapsu-lation can modify the release profile of the API or target dissolution of the encapsulated particle to specificregions of the gastrointestinal tract.In the other category of applications, atomization is utilized to modify the appearance or improve thein vivo performance of the final dosage form. The most common example of this type of process is tablet coat-ing, with a recent survey indicating that 55% of pharmaceutical tablets manufactured in 2006 were coated(IMS Midas Database, 2007). There are a number of reasons why such a large percentage of pharmaceuticaltablets are coated, which adds an additional unit operation to the manufacture of the final dosage form. Non-functional tablet coatings improve the appearance and handling of tablets and may protect against counter-feiting by improving brand recognition. Functional tablet coatings are applied to mask unpleasant taste oralter the tablet dissolution profile either by controlling the rate of dissolution, normally via semi -permeablemembrane coatings, or by protecting the tablet from the acidic environment of the stomach via enteric coat-ings. As is the case for spray drying, scale-up of the tablet coating process is difficult as the operation alsoinvolves several coupled physical processes occurring simultaneously.


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