4.1 Effect of feeding time on Xs
Mixing in the TC reactor involves all the scales from macro-scale, meso-scale to micro-scale. In order to better separate the influence of macromixing on micromixing and only observe the micromixing behavior, the feeding rate of acid solution should be controlled as low as possible as mentioned earlier. In our experiment, the injection of acid solution was maintained at a constant feeding rate, which means the feeding time should be controlled long enough.
Figure 4 shows the change of Xs with feeding time in the CTC and LTC, where Xs gradually decreases and reaches an almost constant value with little fluctuation. When taking a very fast injection, the value of Xs will be jointly controlled by both macro- and micro-mixing. Under such a condition, the local concentration gradient can be very high, as the acid plume cannot be dispersed well throughout the whole reactor scale (Baldyga & Bourne, 1999). Accordingly, this will lead to the local excess of H+ and a large value of Xs . However, this is mainly caused by poor dispersion rather than poor micromixing in the reactor, as the effect of macromixing is not eliminated. On the other hand, a fast injection leading to a random fluctuation will break the steady state of flow field. When more turbulent eddies are involved, the dynamic balance of acid engulfment with bulk reactants cannot be achieved. As local \(H_{2}BO_{3}^{-}\) is not enough to consume a large amount of H+, the excessive H+ will react with \(I^{-}\)and\(\text{\ IO}_{3}^{-}\), yielding a large value of Xs . With a slow injection of acid, the turbulence is less affected such that the acid has enough time to be dispersed evenly in a macro-scale in the reactor and a well-established environment for micromixing is obtained. Consequently, the UV measurement result given by chemical test reactions is free from the macromixing influence and is only dependent on the micromixing behavior. Thus, the feeding time of 120 s was chosen for all the subsequent experiments.