PRODUCTO: LECHUGAS HIDROPONICAS
SUPERFICIE DEGRADADA
The RTD data obtained from the step input tracer tests were used to identify the effect of electrode’s slots orientation and volumetric flow rate on the reactor performance.
Plots of the dimensionless average residence time (E( )Q vs.
Q
) are useful for showing deviations from ideal plug-flow. In order to be able to sight such deviations, the generated RTD data is presented in the following set of figures as a tool for comparing how the slot orientation and volumetric flow rate influence the reactor’s behavior. This side-by-side comparison indicates not only that our reactor does not behave as an ideal PFR but also that it is sensible to changes in its electrodes configuration and rate of flow.Figure 21 and 22 show the E
( )
Q vs. Qcurves obtained for different slot orientation at the same flow rate in an 8-cell reactor.Figure 21. Comparison of the effect of slot orientation on RTD curves for an 8-cell reactor at Q= 0.5 L min-1.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 1 2 3 4 5 6 E( θ ) θ
Horizontal Slots, 0.5 l/min
48
For both 0.5 L min-1 and 1.0 L min-1, positioning the electrodes so that their openings were vertically oriented, produced a narrower peak. This is an indication of the fluid approximating plug-flow behavior under this plate arrangement. However, in the case of the lower flow rate and vertical slots, the sharpness of the peak and its earliness with respect to the mean residence time suggest some short- circuiting from inlet to outlet due to stagnant water. With horizontal slots the curve is more
symmetrical but with several decaying peaks along the tail suggesting the occurrence of internal
recirculation. At 1 L min -1, having the slots oriented horizontally and forcing the fluid to follow a vertical
upward-downward path makes the curve to spread out over the dimensionless time scale, which translates as slow internal circulation of the fluid.
Figure 22. Comparison of the effect of slot orientation on RTD curves for an 8-cell reactor at Q= 1.0 L min-1.
Figures 23 and 24 show a comparison of theE
( )
Q vs.Q
curves for the same slot orientation and different rates of flow.0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 1 2 3 4 5 6 E( θ ) θ
Horizontal Slots, 1.0 l/min
49
Figure 23. Comparison of the effect of volumetric flow rate on RTD curves for an 8-cell reactor with horizontal slots.
Figure 24. Comparison of the effect of volumetric flow rate on RTD curves for an 8-cell reactor with vertical slots.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 1 2 3 4 5 6 E( θ ) θ
Horizontal Slots, 0.5 l/min
Horizontal Slots, 1.0 l/min
0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 1 2 3 4 5 6 E( θ ) θ
Vertical Slots, 0.5 l/min
50
These curves show the effect of flow rate variation on the reactors RTDs. For both slot
orientations, having the fluid flowing at the lower velocity generates a narrower curve but also increases the amount and frequency of decaying peaks. Therefore, a flow rate of 0.5 L min-1 seems to generate problems of internal recirculation probably associated with the low speed of the fluid through the reactor. A wider curve symmetrical to the mean detention time is common for both fluid velocities when the slots are horizontal but even for the higher velocity, internal recirculation remains an issue.
If the reactor is operated with the slots in vertical position, the horizontal path of flow imposed to the fluid causes little fluid stagnation indicated by the early mean but it also minimizes internal recirculation and improves the overall flow behavior, especially at higher velocities. This is observed in the curve for vertical slots and 1.0 L min-1 in figure 24, which is nicely shaped and exhibits few signs of plug flow misbehavior.
Table 2 summarizes the calculated RTD parameters, which are presented in detail in the appendix, tables 8 through 11. These results clearly indicate that the plate’s slot orientation has a significant effect on the overall performance of this reactor. In general, by orienting the electrodes’ openings vertically, the reactor’s average residence time increases by 50% as compared to having the slots oriented horizontally. This effect will be exacerbated by the accumulation of electro-generated gas at the top of the cells when the slots are horizontal. Similar behavior was observed for both fluid velocities.
Table 2. Summary of RTD results for the 8-cell reactor operated under different configurations and volumetric flow rates.
A larger
t
translates into a larger effective volume, characteristic reactor’s length and ultimately a bigger effective electrode area, which means higher treatment efficiency. This is equivalent to add additional cells to the same reactor with horizontal slots.51
In this bench-scale reactor, the upward-downward path of flow resulting from plate’s slots horizontally oriented causes a significant loss of reactive volume. This lost or dead volume may be occupied by stagnant fluid or gas and remains as empty pockets along the reactor and throughout the entire operation.
The Peclet number, which gives an indication of the amount of dispersion, does not vary importantly among the explored configurations, but the lowest value resulted when operating the reactor with vertical slots and 1.0 L min-1. Electrocoagulation experiments and subsequent results were
obtained for this configuration.