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Evaporation) Kinetics of Fine Aerosol Particles (Droplets

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Evaporation) Kinetics of Fine Aerosol Particles (Droplets Experimental and Theoretical Characterization on the Sublimation (Evaporation) Kinetics of Fine Aerosol Particles (Droplets) Vom Fachbereich Maschinenbau und Verfahrenstechnik der Technischen Universitat¨ Kaiserslautern zur Erlangung des akademischen Grades Doktor-Ingenieur (Dr.-Ing.) genehmigte Dissertation von Ms. Sc. Haomin Ding aus Beijing/China Kaiserslautern 2008 D386 Dekan: Prof. Dr.-Ing. S. Ripperger Vorsitzender der Promotionskommission: Prof. Dr. rer. nat. R. Ulber Berichterstatter: Prof. Dr.-Ing. S. Ripperger Prof. Dr.-Ing. B. Sachweh Prof. Dr. J-F. Chen Tag der mundlichen¨ Prufung:¨ 18. Dezember 2008 Vorwort Die vorliegende Arbeit wurde in der Zeit von Juli 2005 bis Dezember 2008 an dem Lehrstuhl fur¨ mechanische Verfahrenstechnik der Technischen Universitat¨ Kaiserslautern durchgefuhrt.¨ Fur¨ das entgegengebrachte Vertrauen mochte¨ ich besonders Herrn Prof. Siegfried Ripperger danken, der mir die Moglichkeit¨ gegeben hat meine Doktorarbeit an seinem Lehrstuhl durchzufuhren.¨ Sein stetiges Interesse am Fortgang der Arbeit sowie die Bereitstellung des Arbeitsplatzes und des notigen¨ Freiraums fur¨ eigenstandiges¨ Forschen waren wichtige Bausteine zum Erfolg der Arbeit. Ebenso gilt mein herzlicher Dank Herrn Professor Bernd Sachweh, der mich durch seine konstruktiven Diskussionen und sein Vertrauen unterstutzt¨ hat. Herrn Professor JianFeng Chen mochte¨ ich fur¨ seine ideelle Unterstutzung¨ sowie fur¨ die Ubernahme¨ des Koreferates danken. Fur¨ die finazielle und materialle sowie die wissenschaftliche Forderung¨ der Arbeit be- danke ich mich bei der BASF SE, die durch die Projektkooperation mit der TU Kaiser- slautern meine Promotion ermoglicht¨ hat. Mein Dank gilt auch der Palas GmbH fur¨ die fachlichen Diskussionen und materialle Unterstutzung.¨ Mein Dank gilt auch allen Kollegen des Lehrstuhls fur¨ die schone¨ gemeinsame Zeit, die ich mit ihnen am Lehrstuhl verbringen durfte. Das hervorragende Arbeitsklima sowie die fachliche und personliche¨ Unterstutzung¨ ermoglichten¨ mir ein zielorientiertes und effektives Arbeiten. Zu guter Letzt danke ich meiner Familie fur¨ die personliche¨ und finanzielle Unterstutzung¨ wahrend¨ meines Studiums und der Promotion. Besonders inniger Dank gilt neben meiner Familie auch meinen Freunden. Sie gaben mir alle durch ihr Verstandnis¨ und ihren Zuspruch stets die notige¨ Sicherheit und Ruckhalt.¨ Kaiserslautern Haomin Ding i Abstract Sublimation (Evaporation) is widely used in different industrial applications. The impor- tant applications are the sublimation (evaporation) of small particles (solid and liquid), e.g., spray drying and fuel droplet evaporation. Since a few decades, sublimation tech- nology has been used widely together with aerosol technology. This combination is aiming to get various products with desired compositions and morphologies. It can be used in the fields of nanoparticles generation, particle coating through physical vapor deposition (PVD) and particle structuring. This doctoral thesis deals with the experimental and theoretical investigations of sub- limation (evaporation) kinetics of fine aerosol particles (droplets). The experimental study was conducted in a test plant including on-line control of the most important paramters, such as heating temperature, gas flow and pressure. On-line and in-line particle measurements (Optical sensor, APS) were employed. Relevant parameters in sublimation (evaporation) such as heating temperature, particle concentration and aerosol residence time were investigated. Polydispersed particles (droplets) were intro- duced into the test plant as precursor aerosols. Two kinds of materials were used as test materials, including inorganic particles of NH4Cl and organic particles of DEHS. NH4Cl particles with smooth surface and porous structure were put into the experiments, re- spectively. The influence of the particle morphology on the sublimation process was studied. Basing on the experiments, different theoretical models were developed. The simula- tion results under different parameters were compared with experimental results. The change of concentration of particles was specially discussed. The discussion was fo- cused on the relationship of the total particle concentration and the change of single ii particles with diverse initial diameters. The study of the sublimation kinetics of parti- cles with different morphologies and different specific surface areas was carried out. The factor of increased surface area on the sublimation process was taken into the simulation and the results were compared with experimental results. A sublimation (evaporation) kinetics was investigated in this thesis. Basing on the prop- erty of a material, such as molecular weight, molecular size and vapor pressure, the sublimation (evaporation) kinetics was described. The optimum sublimation (evapora- tion) conditions with respect to the material properties were advanced. A Phase Transi- tion Effect during the sublimation (evaporation) was found, which describes the increase of the large particles on the cost of small particles. A similar effect is observed in crystal suspension (called Ostwald ripening) but with another physical background. In order to meet the need of in-line particle measurement, a hot gas sensor (O.P.C.) was developed in this study, for measuring the particle size and the size distribution of an aerosol. With the newly developed measuring cell, the operating conditions of the aerosol could be increased up to 500 oC. iii Zusammenfassung: Charakterisierung der Sublimationskinetik (Verdampfungskinetik) von feinen Aerosolpartikeln durch Experimente und Modellierung Ziel der Arbeit Das Ziel der vorliegenden Arbeit ist Sublimations- bzw. Verdampfungsprozesse von einem feinen Aerosol wie z.B. Harnstoff zu beschreiben damit Scale-up von Apparaten abgesichert und industrielle Prozesse gestaltet werden konnen.¨ Die grundlegende Frage in diesem Zusammenhang ist das Verhalten der Partikeln wahrend¨ der Sublima- tion im Kollektiv. Diese Frage wird mit Experimenten und Simulationsrechnungen beant- wortet. Sublimations- bzw. Verdampfungsprozesse treten u.a. in folgenden Prozessen auf: 1. Herstellung von Nanopartikeln durch Sublimation/Kondensation; 2. Dunnschichttechnologien¨ durch physikalische Abscheidung eines Dampfes auf einer Tragerpartikel;¨ 3. Abtrennung von Verunreinigungen aus Partikeln; 4. Verdunstung von Kraftstofftropfen in Brennraumen.¨ iv Experimentelle Untersuchung Im Zuge des Forschungsvorhabens wurde eine Anlage aufgebaut, bei der Partikeln in einem Stromungsrohr¨ aufgeheizt wurden. Die Sublimation bzw. Verdampfung wurde an polydispersen Partikeln im kontinuierlichen Partikelstrom on-line untersucht. Der Gasvolumenstrom, die Temperatur und der Druck wurden on-line kontrolliert. Die Par- tikelgrossenverteilungen¨ wurden mit verschiedenen Messgeraten¨ on-line bzw. in-line gemessen. Als organisches Material wurden DEHS Tropfen in einem Stickstoff Strom verspruht¨ und als anorganisches Material wurden NH4Cl Partikeln in Luft dispergiert. An der Versuchsanlage wurden alle fur¨ den Sublimations- bzw. Verdampfungsprozess relevanten Parameter (Temperatur, Partikelgrossenverteilung,¨ Volumstrom, Partikelkon- zentration) jeweils am Ein- und Austritt gemessen und registiert. An Hand von Proben wurde die Partikelmorphologie untersucht. Folgender Zusammenhang wurde ermit- telt: • Die Erwarmung¨ der Partikeln im Gasstrom bestimmt in Verbindung mit dem Stof- faustausch wesentlich den Sublimations- bzw. Verdampfungsprozess. • Die Sublimations- bzw. Verdampfungskinetik ist wesentlich von dem Anstieg der Gasphasenkonzentration des jeweiligen Stoffes, die von der Partikelkonzentra- tion, der Partikelgrossenverteilung¨ und der Partikelmorphologie beeinflusst wird, abhangig.¨ • Bei der Untersuchung feiner Partikel darf die Dampfdruckerhohung¨ aufgrund der starker¨ gekrummten¨ Partikeloberflache¨ nicht vernachlassigt¨ werden (Kelvin Ef- fekt). • Die jeweiligen Stoffparameter (Dampfdruck, Diffusionskoeffizient, Verdampfungs- warme)¨ mussen¨ berucksichtigt¨ werden. Der Einfluss der Partikelmorphologie auf die Sublimation wurde durch Vergleiche von NH4Cl Partikeln mit glatter Oberflache¨ und entsprechenden Partikeln mit poroser¨ Agglo- meratstruktur untersucht. Im Temperaturbereich von 100oC bis 130oC wurde bei Par- tikeln mit glatter Oberflachenstruktur¨ ein Wachstum der gemittelten Partikeldurchmesser beobachtet. Die Anzahlkonzentration der kleinen Partikeln (dp < 1µm) nahm dabei v stark ab. Die Anzahlkonzentration der großen Partikeln blieb konstant. Im Temper- aturbereich von 100oC bis 130oC wurde bei porosen¨ Partikeln mit hoher spezifischer Oberflache¨ eine steigende Sublimationskinetik beobachtet. In dem entsprechenden Temperaturbereich nahm sowohl die Anzahlkonzentration der kleinen Partikeln als auch die Anzahlkonzentration der großen Partikeln ab. Mit weiter steigender Temperatur wurde bei beiden Partikelmorphologien die Partikelkonzentration insgesamt verringert. Bei der Verdampfung der DEHS Tropfen wurde eine starke Reduzierung aller Partikeln gemessen, obwohl DEHS aus der Literatur als langsam verdampfend bekannt ist. (75) (78) (79) Entwickelter Heissgassensor (HGS) Bei der Untersuchung von Aerosolen ist die Partikelmesstechnik unverzichtbar. Im Ver- gleich zu anderen Messtechniken haben optische Sensoren den großen Vorteil, die Partikeln beruhrungslos¨ unter den gegebenen chemischen und physikalischen Bedin- gungen zu vermessen. Die bisher zur Verfugung¨ stehenden optischen Messsysteme sind stark temperaturbegrenzt. Die maximale Gastemperatur
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