Download file - Ayuntamiento de Zaragoza

As seen from Fig. 94, the tanks-in-series-model does not re-create the short circuiting streamand therefore gave a poor fit. The model of tanks in series connected in parallel with dead volumegenerates the best fitting. Thus, this model was used to quantify the short circuiting stream and deadvolume. At a total flow of 3.6 m 3 /s the reactor was characterized with 12.8% dead volume, 85.8%main active volume and 1.3% short circuiting volume. In a similar experiment with a higher flow rateof 4.7 m 3 /s, the reactor was characterized with 7.0% dead volume, 88.5% main volume and 4.5% shortcircuiting volume. It indicates that the short circuiting flow was increased at a higher flow rate.The CFD simulation was used to generate the RTD response for the first tank. Both RTDcurves, tracer experimental one and CFD calculated, fit well (Fig. 95). As shown in Fig. 95, a shorthigh initial peak (after approx. 3 min.) appears indicating short circuiting streams in the tank. Therather long ‘tail’ of RTD curves indicates areas with poor mixing in the tank (dead volume). Inaddition, the CFD modeling provides the velocity map inside the aeration tank (Fig. 96).FIG.95. Experimental RTD curve and RTD curve generated from CFD simulation in the first tank.FIG.96. Velocity map obtained by CFD simulation.The velocity map shows the main flow passing straight through the upper part of the tank; this ismost likely caused by the short circuiting stream creating the initial peak in the RTD-curve. Near thebottom the flow seems to be slowly transported back towards the entrance of the tank.This phenomenon is causing the ‘tail’ of tracer shown in the RTD-curve and indicating that thispart of the aeration tank is not ‘active’ (dead or stagnant zone). Thus, the six mixers installed topromote mixing in the tank, are obviously not able to make active the whole volume of the aerationtank.CFD simulation of aeration tank hydraulics was performed to evaluate different correctivemeasures for improving the process efficiency in aeration tank (reduce bypass flow and dead volume).RTD curves generated from CFD simulations of different corrective measures are plotted inFig.97. Figure 98 shows velocity maps obtained by CFD simulations for three different correctivemeasures.81