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.