Generalized convection and power-law models to represent ... - USP

The best model fit was obtained with the combined PFR+CSTR model, whose parameters are presented in
Figure 4c. For laminar flow, parameter plug is actually the breakthrough time associated with the maximum
velocity at the centre of the tube. The difference between the convection and combined models resides on the
shape of the descending part or the curve. As can be noticed in Figure 2, the convection model better
represented this part of the curve.
The difference between the theoretical residence time and the residence time predicted by the combined
model can be seen in Figure 4d. This difference suggests that 19% of the internal volume of the exchanger is
associated with stagnation and recirculation regions.
Figure 5 brings the theoretical E-curve and the predictions from the adjusted RTD models for laminar flow in
the double-pipe heat exchanger with a flow rate of 15 L/h. It is clear that the proposed models could capture
the nonideality of the flow. The combined model provided the best fit, but the difference among the model
curves in Figure 5 is rather small. The curves in Figure 5 could be combined with the temperature
distribution along the exchanger in order to evaluate the distributed and integrated lethality in the heating
section of the exchanger [11].
CONCLUSIONS
The proposed models proved to be useful to represent RTD of non-laminar flow in tubular systems, which
are usually found in the continuous processing of liquid foods. The E-curve of these models can represent the
tail of tracer that is inherent in laminar flow. These models can be used in practice to represent the flow
patterns of tubular vessels and to diagnose deviations from non-ideal flow. Future testes will be conducted
using a glycerol/water mixture, which is a Newtonian viscous liquid, and a CMC solution, which is a pseudoplastic
viscous fluid.
ACKNOWLEDGMENTS
The authors wish to thank the São Paulo State Research Fund Agency (FAPESP 2009/07934-3) and the
National Council for Scientific and Technological Development (CNPq PIBIC) for financial support.
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