Isothermal drying of pore networks: influence of pore structure on drying kinetics [Elektronische Ressource] = Isotherme Trocknung von Porennetzwerken: Einfluss der Porenstruktur auf die Trocknungskinetik / von Anton Irawan

Informations

• Publié par: otto-von-guericke-universitat_magdeburg
• Publié le: 01 janvier 2006
• Nombre de lectures: 25
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

ISOTHERMAL DRYING OF PORE NETWORKS:
INFLUENCE OF PORE STRUCTURE ON DRYING KINETICS


(Isotherme Trocknung von Porennetzwerken:
Einfluss der Porenstruktur auf die Trocknungskinetik)




Dissertation
zur Erlangung des akademischen Grades



Doktoringenieur
(Dr.-Ing.)




von Anton Irawan, MEng
geboren am 01.10.1975 in Silungkang, Indonesien





genehmigt durch die Fakultät für Verfahrens- und Systemtechnik
der Otto-von-Guericke-Universität Magdeburg



Promotionskommission: Jun.-Prof. Dr. rer. nat. habil. Ulrich Tallarek (Vorsitz)
Prof. Dr.-Ing. habil. Eckehard Specht (Gutachter)
Dr. Thomas Metzger (Gutachter, Betreuer)
Prof. Dr.-Ing. habil. Evangelos Tsotsas (Betreuer)
Dr. rer. nat. habil. Peter Streitenberger (Mitglied)
Dr. Marc Prat, DR (Mitglied)


eingereicht am 31. Mai 2006

Promotionskolloquium am 10. Juli 2006













This work is dedicated with pride
to my parents...
...and with love to my wife and my son




















ii

Acknowledgements





This thesis is submitted to the Faculty of Process and System Engineering in fulfillment of the
requirements for the degree of Doctoringenieur (Dr.-Ing) at Otto-von-Guericke-University of
Magdeburg. This project was carried out at the chair of Thermal Process Engineering,
Institute of Process Engineering from March 2003 to May 2006.

First, I thank God for giving me the ability to finish this work.

I would like to thank Prof. Dr.-Ing. habil. Evangelos Tsotsas for giving me the opportunity
to live and study in Magdeburg, Germany, also for support, advice and guidance to my
project.

I would like to express my heartfelt thanks to Dr. Thomas Metzger for his effective
guidance, clear instructions, constructive ideas and valuable discussions in my project, also
for helping my family to live in Magdeburg.

I express many thanks to the DFG (Deutsche Forschungsgemeinschaft) for financial support
via the graduate school GK 828

“Micro-Macro Interactions in Structured Media and Particle Systems”.

I also thank Prof. Dr. Gerald Warnecke (speaker of graduate school) and Prof. Dr.-Ing.
Albrecht Bertram (ex-speaker) for supporting me in finishing my project.

I would also like to thank Prof. Patrick Perre (Nancy, France), Dr. Marc Prat (Toulouse,
France), Prof. Henk van As (Wageningen, Netherlands) and Dr. Pavel Capek (Praha,
Czech Republic) for nice and fruit discussion about pore networks in Magdeburg.

To other students in GK 828, especially to Thai Vu Hong, Vikranth Kumar Surasani, Ga
Sashikumar and Jitendra Kumar, I say many thanks for good discussion about my project
and for good environment to study and to learn about the culture of their countries. I also say
many thanks to the staff and members of the chair of Thermal Process Engineering, especially
to Dr. Milan Stakic and Suherman for fruitfull discussions.

Finally, I would like to thank my parents and family (my wife and son) for their love,
kindness, encouragement, and understanding. I dedicate this thesis, that I have worked very
hard for, to you.





iii


Abstract
Drying is quite an old method to remove liquid from inside of a porous material such as
wood, food, paper, ceramics, building materials, textiles, granular products, pharmaceuticals
and electronic devices. The kinetics of this liquid removal depends on the material properties
of its solid phase as well as on pore structure. In order to optimize the drying process for good
product quality and energy considerations, theoretical modelling of the involved transport
phenomena is necessary. Essentially, there are two approaches of modelling, namely the
continuum and the discrete approach.
The traditional approach treats the partially saturated porous medium as a fictitious
continuum, and transport is described by effective parameters, which depend on saturation
and pore structure. The macroscopic conservation equations for mass and enthalpy are
obtained by volume averaging or homogenization, which both require a length scale
separation of pore phenomena and macroscopic variation of relevant state variables such as
moisture content. This requirement is not fulfilled in general, so that the second approach of
discrete modelling is an alternative. In this method, the porous medium is represented by a
pore network, and transport phenomena are directly investigated at the pore scale. Besides
modelling of drying kinetics, such network models might also be used to calculate the
effective parameters of the continuous model.
The aim of this project is to study the influence of pore structure on convective drying
behaviour by pore network modelling under isothermal conditions. To this purpose, a
literature pore network model has been extended to describe the influence of liquid viscosity
and lateral vapour transfer in the gas-side boundary layer. This model was applied to two- and
three-dimensional networks of different pore size distribution and pore space topology.
Four different two-dimensional network structures were investigated: the first has a mono-
modal pore size distribution; the remaining three all have a bimodal pore size distribution but
differ in the correlated spatial arrangement of micro and macro pores. All results are given as
drying curves and phase distributions during drying. It was found that, for favourable drying
with a long first drying period, a bimodal pore size distribution is essential and both micro
and macro pore phase must be continuous. For efficient evaporation from wet spots on the
network surface, the micro pores must additionally have a good spatial distribution. The role
of boundary layer thickness, and especially lateral vapour transfer in this boundary layer, was
systematically investigated and assessed by comparison with literature models.
The importance of liquid viscosity in comparison with capillary forces was studied by
variation of mean and standard deviation of monomodal pore size distributions. For broad
distributions, capillary pumping is effective, whereas for narrow distributions, the network
rather dries out with a receding front. It could also be shown that for bimodal distributions,
liquid viscosity is less significant for overall drying behaviour.
Concerning the influence of random generation of pore radii in the 50x50 networks, drying of
all network structures was investigated by the Monte Carlo method. Additionally, the merit of
periodic boundary conditions to increase effective network size was studied.
A limited number of three-dimensional network drying simulations has also been carried out
because they can more realistically describe certain transport phenomena in drying, like
capillary pumping, than modelling in only two dimensions.

iv Zusammenfassung
Die Trocknung ist ein altes Verfahren, um Flüssigkeit aus porösen Materialien zu trennen,
wie zum Beispiel Holz, Lebensmittel, Papier, Keramiken, Baustoffe, Textilien, granulare
Medien, Pharmazeutika und elektronische Bauelemente. Die Kinetik dieser Trennung hängt
ab von den Materialeigenschaften des Feststoffs sowie der Porenstruktur. Um den
Trocknungsprozess hinsichtlich der Produktqualität und des Energieverbrauchs zu optimieren,
ist die Modellierung der beteiligten Transportprozesse nötig. Im Wesentlichen gibt es hierzu
zwei Ansätze, nämlich die Kontinuums- und die diskrete Modellierung.
Der traditionelle Ansatz behandelt das teilgesättigte poröse Medium als fiktives Kontinuum,
und der Transport wird mittels effektiver Parameter beschrieben, welche von Sättigung und
Porenstruktur abhängen. Die makroskopischen Erhaltungsgleichungen für Masse und
Enthalpie werden durch Volumenmittelung oder Homogenisierung abgeleitet; beide
Methoden basieren auf einer Unabhängigkeit der Größenskalen von Phänomenen in einzelnen
Poren und der makroskopischen Variation relevanter Zustandsgrößen, wie der Feuchte. Diese
Bedingung ist jedoch nicht allgemein erfüllt, so dass der zweite Ansatz der diskreten
Modellierung eine Alternative darstellt. In dieser Methode wird das poröse Medium durch ein
Netzwerk aus Poren repräsentiert, und Transportphänomene werden direkt auf der Porenebene
beschrieben. Neben der Modellierung der Trocknungskinetik könnten Netzwerkmodelle auch
dazu dienen, die effektiven Parameter des Kontinuumsmodells zu berechnen.
Das Ziel dieser Arbeit ist es, den Einfluss der Porenstruktur auf das Verhalten bei der
Konvektionstrocknung mittels Porennetzwerkmodellierung unter isothermen Bedingungen zu
untersuchen. Zu diesem Ziel wurde ein Netzwerkmodell aus der Literatur um den Einfluss der
Reibungseffekte in der Flüssigkeit und die laterale Dampfdiffusion in der gasseitigen
Grenzschicht erweitert. Diese