Avaliação das condições hidrodinâmicas, de transferência de oxigênio e de cisalhamento em diferentes modelos e escalas de reatores pneumáticos
Abstract
Due to the high capacity of oxygen transfer and versatility, pneumatic reactors have been
constantly used in bioprocesses. However, aiming to expand the use of these bioreactors in the industry, as well as increase the understanding of the complex phenomena that occur in these devices, this thesis aimed to evaluate the hydrodynamic, oxygen transfer and shear conditions in three models of pneumatic reactors (bubble column, concentric-tube airlift and split-cylinder airlift) in the scales of 5 and 10 L, using as liquid phase four Newtonian fluids and eight non-Newtonian fluids, and five specific air flow rate (air of 1 to 5 vvm). Related to the
hydrodynamic were studied the global gas hold-up (g), the gas hold-ups in the riser (R) and
in the downcomer (D), liquid circulation time (tC), superficial liquid velocity in the riser (ULR)
and in the downcomer (ULD), and the percentage energy losses in the riser (%ER+%EFR), in the
downcomer (%ED+%EFD), and in the bottom (%EB) of airlift reactors. The values of g, R, D, ULR and ULD showed increasing behaviour with increase of air and decreasing behaviour with the kinematic liquid viscosity (L) and the rheologic properties (K e n), and observed the
opposite for tC. The higher values of g, R, D and tC were obtained for concentric-tube airlift
reactor (ACC) and scale of 10 L. With exception of ULR of Newtonian fluids, the others liquid
velocity tests resulted in higher values for split-cylinder airlift reactors (ASC) and scale of 10
L. This result was attributed to the greater driving force (R-D) to liquid circulation obtained in
the ASC reactors and the higher energy losses in the riser and in the downcomer observed in
the ACC reactors. In the bottom of the airlift reactors, the higher values of %EB were obtained
to the ASC reactor. To evaluate the mass transfer were studied, the average bubble diameter
(Db), the volumetric oxygen transfer coefficient (kLa) and the terms that compose the kLa, the
convective mass transfer coefficient (kL) and the specific interfacial area of mass transfer (aL).
With the increase of air, L, K and n, the air bubbles were predominantly coalescent in water,
presenting distorted shape, and non-coalescent with spherical/elliptical shape in the other
solutions. It was observed a similar behavior between the kLa and aL parameters, which were
directly proportional to the air and inversely proportional to the L, K and n. In water, the aL
values were lower than glycerol solutions due to the higher Db values observed in this liquid.
For the kL, it was observed a decreasing behaviour with the increase of the air in the most solutions. The magnitude of kL values was due mainly the oxygen difusivity in the liquid, and
the higher values were observed to the water, following by the non-Newtonian solutions. In
general, the higher values of the mass transfer parameters were obtained in the ACC reactor
and in the scale of 10 L. The proposed method to the estimate the average shear rate velocity
based on Kolmogorov’s theory of isotropic turbulence showed results consistent with the
literature relative to the behavior and magnitude of this variable, as well as the results obtained
by the analysis of the morphological changes of Streptomyces clavuligerus in two models of
airlift reactors and two aeration conditions. Were proposed correlations to predict all evaluated
parameters. Were obtained in all cases a good fit with the experimental data, with deviations
between the calculated and experimental values below 20%.