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ABSTRACT: Stirred tank reactors are used for variety of applications at different scales of operation. The conventional impellers
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tend to develop regions having nonuniform energy dissipation rates in the stirred reactor. In this work, we propose a novel fractal
impeller, which helps in reducing such nonuniformities and help develop a uniform randomness throughout the reactor. The
impeller geometry is discussed in detail. Experimental measurements of the power consumption, mixing time, suspension quality,
and the ability for gas dispersion were carried out, and the performance is compared with the conventional impellers. The impeller is
seen to have a low power number, and it can generate a uniform suspension of particles even at relatively lower impeller speeds and
can efficiently disperse gas into liquid to yield relatively higher gas hold-up values. The Fourier analysis of the power consumption
time series data indicates that no specific prominent frequency events exist in the reactor, and the spectrum showed several
frequency events to exist in the reactor with almost identical prominence.Published: April 18, 2011
1. INTRODUCTION,4521
Stirred tank reactors (STR) form an integral component of
chemical, pharmaceutical, and the fermentation industry. These
types of reactors are in operation for last several decades and a
number of investigators have analyzed them in detail to optimize
the designs based on the power consumption, mass and heat
transfer, and the internal hydrodynamics. In the stirred reactors,
the energy is supplied in the form of a kinetic energy by rotating
the impeller at desired speed. STRs are largely used for (i)mixing
or blending of two miscible liquids, (ii) generation of dispersions
for gasliquid and liquidliquid reactions, (iii) keeping the
solid particles in suspension to facilitate the solidfluid contact
to achieve solid dissolution, (iv) crystallization, etc. The energy
requirement of these processes forms a significant part of the
total energy and contributes toward major expenses. Thus, the
efficiency of a stirred tank reactormainly depends on the impeller
design and its location in the stirred reactor. The variations in the
design and operational protocols make the fluid mechanics
prevailing in a STR complex, and hence the design procedures
have largely been empirical. However, over the years detailed
experiments and theoretical analyses have contributed to some
extent in making the operation of a STR more efficient.
1
In
general, the industrial inclination toward following the efficient
design procedures or using efficient impellers has increased,
while a large fraction of the industry still believes in stirring hard
to achieve desiredmixing with the same old generation impellers.
Typically (except for the highly viscous fluids), the system
operates in turbulent regime. Usually, the distribution of energy
dissipation is considerably heterogeneous. Thus for instance, for
a paddle mixer, 90% of the input energy is dissipated below the
impeller while the remaining 10% is dissipated above the
impeller.
2
Also, for a pitched blade downflow turbine (PBTD),
30% energy is dissipated in the impeller region, 57% below the
impeller and just 13% above the impeller.
3
Usually, the impeller
region is the most active zone of the reactor and also a region
yielding high transient shear gradients. Thus, uniform spatial
distribution of energy is difficult to achieve in the conventional
STRs and this implies that it is necessary to look for alternatives
that would make the entire reactor active in a hydrodynamically
similar manner. Also, for achieving uniform temperature
throughout the reactor while operating it at lower impeller speed