4.2 Effect of H+ concentration on Xs
Because the linear relationship between \(I_{3}^{-}\) concentration and its absorption is valid within a particular range, the H+ concentration should be selected appropriately to avoid excessive or too little I2 being generated. A wide range of H+ concentration from 1.2 mol/L to 6.0 mol/L has been tested. The experiment was conducted under the condition of the Reynolds number of 12564 with 1.0 mL injection. It can be seen from Figure 5 that the value of Xs firstly increases, then levels off with the increase of H+ concentration. At a higher H+ concentration, more time is needed for\(H_{2}BO_{3}^{-}\) to neutralize H+. However, the amount of \(H_{2}BO_{3}^{-}\) was kept at a constant level when changing the H+ concentration. Thus, the excessive H+ will lead to the occurrence of Reaction (2). It can be seen from the rate laws of (6) and (7) that Reaction (2) is sensitive to H+ due to the its higher rate order. Moreover, the chemical reaction rate of Reaction (2) is usually higher if measured based on the micromixing rate (Fournier et al. , 1996), which will be presented and discussed in Section 4.4. It can be seen that a large amount of I2 is generated, leading to a high value ofXs . It is worth mentioning that the continuous increase of H+ concentration does not give rise to the continuous increase of Xs . Due to the local excess of H+concentration, both Reactions (1) and (2) have been completed, which means that all reactants have achieved their maximum conversion. As a result, the I2 concentration will not increase any further. It can also be seen from Figure 5 that the most sensitive point for the UV detection corresponds to the H+concentration of 2.0 mol/L. This value has been chosen for all the subsequent experiments.
One can see from Figure 5 that the micromixing Xs in the LTC is better than that of the CTC at all ranges of the H+concentration, but such a difference is less noticeable at high H+ concentration. For the LTC, the rotational lobed inner cylinder generates a periodic variation of the gap size. The circumferential flow will experience an expansion and contraction, leading to the generation of the induced turbulent eddies from the surface of the concaved top of the inner cylinder and the enhanced turbulent eddy interactions. Such turbulent eddy interaction can effectively re-disperse the concentration field of reactants, promoting a better distribution of the reactants, less local H+accumulation and less local formation of I2. This is another piece of evidence to suggest that lobed geometry can effectively reduce the overall mixing time and improve the micromixing efficiency for the TC reactor.