(226b) Interaction Between Fluid Mixing and Chemical Reaction -Effect of Non-Uniform Chaotic Mixing On Concentration Distribution for Periodical Reaction System-

Interaction between Fluid Mixing and
Chemical Reaction -Effect of Non-Uniform Chaotic Mixing on Concentration
Distribution for Periodical Reaction System-

S. Hashimoto^*, Y.
Chikamochi, A. Nishimura

Division
of Chemical Engineering, Graduate School of Engineering Science, Osaka University,
1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japan.

^*corresponding
author and presenter: shunsuke@cheng.es.osaka-u.ac.jp, Phone & Fax:
+81-6-6850-6294 (S. Hashimoto)

Mixing in stirred tank reactors in a wide variety of tank sizes
and impeller shapes has been often utilized to homogenize different substances
and to conduct chemical reactions in industrial chemical processes. Recently in
various industrial processes, a wide range of operation for stirred tank is
required depending on purposes and conditions. "Mixing time" is essential as
one of index to evaluate the mixing process in stirred tank experimentally.
Various techniques for measuring mixing time have been used such as coloration,
decolorization, measurements of electric conductivity and temperature, and so
on. Coloration is most simple technique to observe the mixing state in a whole
tank, while it has a few demerits: it is difficult to detect the mixing state
in a central part of stirred tank and the required time for complete mixing.
Measurement of electric conductivity [1] enables us to detect delicate time
variation of conductivity, but only local information for mixing near the
sensor of conductivity can be obtained. Measurement of temperature has also
demerits that only local information for mixing near thermocouples can be
obtained, which is similar to measurement of electric conductivity. There are
additional problems such as the variation of fluid property due to temperature
change and thermal insulation for the measurement of temperature. It is
expected that tomography [2] can compensate these demerits for other techniques
efficiently in future, while its low resolution and high cost remain the key
issue for the evaluation of mixing state. Decolorization enables us to observe
easily the difference of mixing state at every position in stirred tank. In the
case of decolorization, it is easy to determine the completion time of mixing
because the termination of decolorization is directly equivalent to the completion
time of mixing. Consequently, decolorization is generally used as a simple
method for the measurement of mixing time. In the case of the visualization
using chemical reactions such as decolorization, however, the relation between
the rate of chemical reaction and that of convective mixing is quite important.

Based on general mixing theory, the completion time for mixing
should be defined as the time that is required to homogenize the concentration
of each fluid component all over the place in the system. In above general
techniques for the measurement of mixing time, fluid is judged to mixed
completely at the time that its concentration, conductivity, and temperature
are homogenized in stirred tank. The properties such as concentration,
conductivity, and temperature are considered to have one-to-one relationship
with substance quantity of fluid component (that is, concentration). Hence, it
is reasonable to use these properties as an index to evaluate mixing state. On
the other hand, there are unique cases containing fluid properties that are not
one-to-one relationship with concentration. For example, in the periodical
vibrational system by Beloousov-Zhabotinskii (BZ) reaction, fluid has not only
the property of concentration but also the properties of period and/or phase of
concentration oscillations, which depend on the dynamical mode of reaction and
do not have simple relationships with the concentration. In this case, it is
inadequate to judge the mixing completion based on only the homogeneity of concentration
of each component in stirred tank. In addition, final pattern by convective
mixing would potentially depend on only the structure of flow field. For
example, under low agitating Reynolds number (Re) conditions, the presence of segregated mixing regions from the
chaotic mixing regions (CMR) in the form of toroidal vortices above and below
an impeller, which is called as "isolated mixing region (IMR)" [3, 4], is well
known. Material exchange by diffusion is dominant at the interface between CMR
and IMR. In this case, even if the decolorization (homogenization of
concentration) in the whole region of stirred tank, IMR must remain there
essentially. Hence, it is necessary to evaluate mixing degree based on not only
the homogeneity of concentration but also various index.

The present study focused attention on reconsidering the
traditional concept of mixing that was based on the homogeneity of
concentration. In the present study, required time for complete mixing and
mixing patterns containing "partially mixing regions" such as IMR in CMR were
investigated by use of two chemical reactions: conventional decolorizing
reaction (with iodine and sodium thiosulfate) and periodical oscillating
reaction (BZ reaction). Based on experimental results, the body of matter that
was mixed by impeller agitation and the correspondence between chemical
reaction and convection were discussed with the view of "fluid-informatic"
engineering. In addition, the availability of periodical reaction for the
visualization of partially mixing regions was briefly discussed.

Figure 1
shows the simple summary of the present study. The periodical steady
color-variation in BZ reaction system remained after the sufficient time
required for the complete decolorization. The sequential spatial color-patterns
obtained in steady periodical variation process were similar to the
transitional color patterns observed in the decolorization process. The
color-patterns obtained in BZ periodical reaction were consistent with the
outline of partially mixing regions where the exchange of substance is
relatively slow in the vessel and they depended on Re. The phase of periodical concentration oscillation in each
partially mixing region was shifted with one another in spite of the same
period of oscillation. Whether the period and/or phase synchronize or not in
each partially mixing region would depend on the relative speed between the
exchange of substance and the synchronization of periodical oscillation of
concentration there.

Figure
1 The
experimental images obtained from the decolorizing experiment and the BZ
reaction experiment; (a)Re = 117 and
(b)Re = 780. Panel (c) shows the
color-pattern diagrams corresponding to these experimental images.

Keywords: Mixing;
Mixing Time; Visualisation; Fluid Mechanics; Nonlinear Dynamics; Periodical
Reaction

References

[1] Kramers H., Baars G.M. Knoll, W.H. (1935).
Chem. Eng. Sci, 2, 35-42.

[2] Kaminoyama M., Taguchi S., Misumi R.,
Nishi K. (2005). Chem. Eng. Sci., 60, 5513-5518.

[3] Bresler L., Shinbrot T., Metcalfe G.,
Ottino M.J. (1997). Chem. Eng. Sci, 52, 1623-1636.

[4]
Hashimoto S., Ito H. Inoue Y. (2009). Chem.
Eng. Sci., 64, 5173-5181.