Solid Mixing Assessment from a Simplified Radioactive Particle Tracking Technique
font-family:"Times New Roman","serif"'>Solid mixing assessment from a
simplified radioactive particle tracking technique
Salierno1, Mauricio Maestri1, Stella Piovano1,
Miryan Cassanello1, MarÃa Angélica Cardona2,3, Daniel
Hojman2, Héctor Somacal2,3 justify"> 0in;margin-left:0in;margin-bottom:.0001pt;text-align:justify">1Laboratorio de
Reactores y Sistemas para la Industria (LARSI), Dto. Industrias, FCEyN, UBA.
Int. Güiraldes 2620, C1428BGA C.A.B.A. 0in;margin-left:0in;margin-bottom:.0001pt;text-align:justify">2Laboratorio de
Diagnóstico por Radiaciones (LaDiR), Dto.de FÃsica, Centro Atómico
Constituyentes, Comisión Nacional de EnergÃa Atómica (CNEA). margin-left:0in;margin-bottom:.0001pt;text-align:justify;line-height:115%">3ECyT,
Universidad Nacional de San MartÃn, San MartÃn, Argentina
Fluidized
three-phase reactors are mainly used in petrochemical industries and for biotechnology.
Main advantages of these installations are the almost isothermal operation, the
low mass transfer limitations and the possibility of addition/removal of solids
without reactor shutdown (Shaikh and Al-Dahhan, 2013; MartÃnez et al., 2010;
Park and Fan, 2007). In the recent literature, an increasing number of examples
of biotechnological processes with suspended immobilized cells or enzymes are
appearing. Immobilized biocatalysts or microorganisms are used in order to
protect biocatalyst from high shear stress or to improve bioprocess
productivity (Milivojevic et al., 2012). Low density polysaccharide gels
particles, such as alginate or carrageenan beads, are generally used as the solid
phase since they provide high porosity and ease of entrapment, not requiring
drastic conditions (Perez-Bibins et al., 2014; Carvalho et al., 2003).
Assessment
of solid mixing and regions of hydrodynamic stress is important for the
three-phase bioreactor design and scale-up, and for the operation monitoring. Information
about the way substrates, media and cells are moving within the reactor is
crucial for choosing the operating conditions leading to good mixing, minimizing
the existence of watertight areas and checking if the levels of turbulence are
compatible with living organisms (Collignon et al., 2010). This type of
information is usually difficult to achieve by procedures that do not interfere
with the operation.
Radioactive particle
tracking (RPT) is a powerful technique that has successfully been applied to
non-invasively extract features of many multiphase reactors dynamics (Dudukovic
et al., 2002). For instance, solids mixing times can be readily inferred from
the radioactive tracer trajectory in three phase fluidized beds (Cassanello et
al., 1996). Since RPT generally requires calibration under the actual conditions, it is rather cumbersome to
implement within an industrial environment. Recently, it has been shown that
color:black'>relevant solid motion information
"Times New Roman","serif"'>can be attained by reconstructing a solid trajectory
using a set of axially aligned detectors (AAD). A coarse trajectory is
obtained, with
"Times New Roman","serif";color:black'>2N-1 possible axial positions, where N
is the number of detectors in the array. The
detectors scan simultaneously different column heights of the examined vessel
and the reconstruction considers that the detector located closer in axial
coordinate to the tracer is the one which records the largest number of counts.
Details of t
color:black'>he reconstruction methodology can be found in a previous
contribution (Salierno
et al., 2012)
color:black'>.
In this work, the
solid motion of alginate beads in a three-phase fluidized bed is examined using
AADs. A neutrally buoyant radioactive tracer, built by introducing a tiny piece
of gold inside a sphere of calcium alginate is used. The calcium concentration
is adjusted to compensate the gold mass and match the density of the other
fluidized alginate beads. The piece of gold is activated to 198Au (412
keV, t1/2 2.7d) in a nuclear reactor by slow neutron bombardment to
reach an activity of 0,030mCi, prior to introduce it in the alginate sphere.
Experiments are carried
out in an acrylic three-phase fluidized bed (1.2 m height and 0.1 m inner
diameter) provided with a gas distributor, mounted onto a structure jointly to
an array of fifteen axially aligned 2?x2? NaI(Tl) scintillation detectors.
Calcium alginate gel beads of 4mm mean diameter is the solid phase. The liquid,
a dilute solution of CaCl2 (0.05M), is in batch mode. The solid
holdup at rest is 10% and the solid density is 1.1 the liquid one. The gas
phase is air, fed from a compressor at different velocities between 0.03 and
0.11 m/s. The tracer path is followed for several hours with a counting
sampling period of 30 ms.
Figure 1 illustrates
the response of the fifteen detectors for a representative period of 60s, and
the corresponding reconstructed tracer trajectory, for two gas velocities. From
the tracer coarse trajectory, information is extracted by data mining
techniques. Solid mixing and turbulence features (solid mixing times, axial dispersion
coefficients, preferential solid residence axial regions, turbulent kinetic
energy, axial shear stresses) can be estimated from the analysis (Salierno et
al., 2013). In addition, global solid hold up distribution and flow regime
transition can also be assessed.
Complementarily to
the experiments with the tracer freely moving within the vessel, scanning
experiments are performed by locating a 2mCi 241Am (60keV, t1/2
432.7y) sealed source diametrically opposite to each AAD, for determining the
chordal liquid holdup at different column heights at the same explored
experimental conditions.
Figure 1 ? Signals
gathered by the axially aligned detectors (left) and reconstructed axial
trajectory (right) for two gas velocities.
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