Slug flow capillary microreactor hydrodynamic study

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African Physical Review (2007) 1 Special Issue (Microfluidics):0005
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Slug Flow Capillary Microreactor Hydrodynamic Study
David Fernández Rivas
1
, M. N. Kashid
2
, D. W. Agar
2
and S. Turek
3
1
Departamento de Ingeniería Nuclear,
Instituto Superior de Tecnología y Ciencias Aplicadas, InSTEC,
Quinta de los Molinos, Ciudad de la Habana, Cuba
2
Institute of Reaction Engineering, University of Dortmund, Dortmund, Germany
3
Institute for Applied Mathematics, University of Dortmund, Dortmund, Germany
1. Introduction
Micro-fluidic applications have served as interface
between the macro- and nano-world. Micro-scale
systems offer many advantages such as minimal
reagent consumption, complex chemical
waveforms, and significantly reduced analysis and
experimentation time (for example, an important
concept recently introduced was µTAS, the Micro
Total Analysis System for details see [14]). The
absence of inertial and turbulent effects in micro-
fluidic devices offers new horizons for physical,
chemical and biological applications. The small
dimensions give high surface-to-volume ratios,
small diffusion distances and easy temperature
profiling where needed, giving the opportunity to
manipulate substances in ways never imagined
before. Drops or slugs and their application in
micro- and nano-scales are very important in
various fields of present science. For instance, cell-
based assays [15], models for capillary blood
vessels for red cells infected with malaria [17], drug
delivery targeted at specific sites in the body for a
less invasive chemotherapy, miniature biosamples
preparations on fully automated biochips, for DNA
sampling are all known applications in medical and
genomic sectors. In chemical sciences, it has been
used in two-phase chemical reactions [11], [8], [7],
[3], [4], and elucidation and optimization of
nitration reaction demonstrated by Dumman, who
concluded that the capillary micro-reactor can be
used for quantitatively examining exothermic
liquid-liquid reaction systems [5], fast or dangerous
reactions, solvent extraction, substances separation
and so on. At the micro-scales, the problems
associated with the scaling-up for large scale
production by simply numbering-up are reduced.
This means that several micro-reactors can be used
to obtain the necessary products, instead of building
complicated and expensive plants. Mathematical
models describing the movement of drops, or in
general, multiphase flows developed so far, are not
able to predict or quantify properly all the important
particularities of this complex systems (capillary
micro-reactors). Hence a deeper knowledge of the
physical problem, say hydrodynamics transport, is
essential. This task requires powerful modeling
techniques. Consequently, we initiated the
hydrodynamic study of drops/slug movement
through capillaries. We focused on the application
of a slug flow micro-reactor model to match the
necessities and behavior described in [13] and [12].
2. Main results
2.1 Experimental results
Figure 1 shows the experimental setup and
illustrates how two immiscible liquids (aqueous and
organic) are introduced by continuously operating
high-precision piston pumps to a symmetric 1200
Y-piece mixing element made of Teflon (PTFE).
There is a complex flow that develops in each phase
and at the interface between both phases and our
objective is to gain insight of the flow pattern. In
this work, the focus was on the internal circulation
particularities within the slugs mainly in the
aqueous phase. Particle Imaging Velocimetry (PIV)
measurements were also conducted to gain insight
of the internal circulation inside a slug.

African Physical Review (2007) 1 Special Issue (Microfluidics):0005
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FIG.1: a) Experimental setup scheme,
b) Observed flow regimes in the capillary microreactor.
2.2 Computational modeling
The hydrodynamic flow pattern of a slug flow and
the evolution of complex interfaces was determined
using in-house, open-source, CFD code
FEATFLOW (levelset approach). This code uses an
implementation of surface tension effects in
interfacial flow combining two techniques: the
continuum surface force (CSF) method and a finite
element discretization together with the Laplace-
Beltrami operator [10], [9], [18], [16]. For the
discretization of the domain, the in-house
developed software DeViSoR 2.1 (Design and
Visualization Software Resource) was used [2]. A
structured two-dimensional coarse grid was built
with an increased density of nodes closer to the wall
boundaries in order to capture numerically, in a
more accurate way, the complex flow in the near
wall region (Fig. 2 a). The post-processing was
performed with GMV software [6], and for particle
tracing calculations, another in-house module,
gmvpt [1], was used
Among the main numerical results we have, the
velocity distributions can be plotted as color
gradients or velocity vectors. Detailed information
can be extracted where hydrodynamic conditions
are the most complicated, for example, at the
interface region, at the nose and back of the slug
(Fig.1). When considering mass transfer and
reactions in a micro-reactor, the transport of the
species within the phases will be determined
strongly on the hydrodynamic flow pattern; particle
tracing results shows how species can be distributed
as time passes by. Initially, a distribution of
particles is placed in the back part of the slug, and
as the slug moves to the right, the behaviour
described above is exactly obtained (Fig. 3).
Comparing the particle tracing numerical solution
with the experimental PIV measurements, it can be
seen that both results are qualitatively the same and
also coincide with the internal circulation expected
flow pattern.
3. Conclusions
The expected physical behavior, experimental
measurements and numerical results show excellent
agreement. The flow pattern occurring inside a two-
phase capillary micro-reactor was successfully
studied both experimentally and by numerical
modeling. The main hydrodynamic information
extracted from the numerical code and its
resemblance to the real physical system was shown.
The precise control of capillary multiphase flow
micro-reactors, from the hydrodynamics and
chemical reaction requirements, will be possible
once this model is validated and tuned.
References
[1] J. S. Acker and S. Turek, Postprocessing of
FEATFLOW Data with the Particle Tracing
Tool GMVPT version 1.2, Angewandte
Mathematik und Numerik (LS III),
Dortmund Universität, Germany, 2 (2000).
[2] C. Becker and D. Goeddeke, “DeViSoRGrid
2 User´s Manual”, Dortmund Universität,
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[3] Burns and Ramshaw, Lab. Chip J. (1) 1, 10
(2001).
[4] Burns and Ramshaw, Chem. Eng. Comm.
189, 1611 (2002) 1.
[5] Dumman et al., Catalysis Today 79-80, 433
(2003) 1.

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[6] GMV (General Mesh Viewer) user manual.
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FIG.2: a) Velocity vector plot of a single slug
b) Vector plot representing the internal circulation.
FIG.3: Particle tracing evolution at different times
inside a slug.