INAUGURAL–DISSERTATION zur Erlangung der Doktorwürde der ...

Abstract
Spray drying is one of the most widely used drying techniques to convert liquid feed
into a dry powder. The modeling of spray flows and spray drying has been studied
for many years now, to determine the characteristics of the end products, e.g. particle
size, shape, density or porosity. So far, the simulation of polymer or sugar solution
spray drying has not been studied because drying behavior as well as properties are
unknown. Previous studies concentrated on the systems of milk, salt solution, colloids
or other materials for which the thermal and physical properties are well tabulated.
The present study deals with the modeling and simulation of polyvinylpyrrolidone
(PVP)/water and mannitol/water spray flows. PVP is a polymer, widely used as
a pharmaceutical excipient, and mainly manufactured by BASF under several patented
names, whereas mannitol is a sugar, which is used in dry powder inhalers and tablets.
Experimental studies have shown that the powder properties of PVP and mannitol are
significantly influenced by the drying conditions. The growing importance of PVP or
mannitol powders and the inability of existing studies to predict the effect of drying
conditions on the properties of the end product have prompted the development of a
new reliable model and numerical techniques.
Evaporating sprays have a continuous phase (gas) and a dispersed phase, which
consists of droplets of various sizes that may evaporate, coalesce, or breakup, as well
as have their own inertia and size-conditioned dynamics. A modeling approach which
is more commonly used is the Lagrangian description of the dispersed liquid phase.
This approach gives detailed information on the micro-level, but inclusion of droplet
coalescence and breakup increase computational complexity. Moreover, the Lagrangian
description coupled with the Eulerian equations for the gas phase, assuming a pointsource
approximation of the spray, is computationally expensive. As an alternative to
Lagrangian simulations, several Eulerian methods have been developed based on the
Williams’ spray equation. The Euler – Euler methods are computationally efficient
and independent of liquid mass loading in describing dense turbulent spray flows.
The objective of this thesis is the modeling and simulation of spray flows and
spray drying up to the onset of solid layer formation in an Euler – Euler framework.
The behavior of droplet distribution under various drying conditions in bi-component
evaporating spray flows is examined using, for the first time, direct quadrature method
of moments (DQMOM) in two dimensions. In DQMOM, the droplet size and velocity
distribution of the spray is modeled by approximating the number density function in
terms of joint radius and velocity. Transport equations of DQMOM account for droplet
evaporation, heating, drag, and droplet–droplet interactions.
At first, an evaporating water spray in nitrogen is modeled in one dimension (ax-
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