Abstract
Liquid or gas-liquid stirred tank reactors with vapour generation are ubiquitous in many important industrial processes. The presence of the large volume of vapour may have considerable effects on liquid mixing and mass transfer performance. The aim of the present work has been the investigation of the liquid macro- and micro-mixing characteristics in these reactors. This objective is achieved through the examination of the differences in power draw characteristics, liquid mixing times and undesired byproduct distributions of mixing-sensitive reactions between hot sparged and boiling reactors, and ambient temperature systems. Experimental work was performed in a standard vessel and a tall tank reactor (H = 2T) under ungassed, gassed and boiling conditions. The power draw of up-pumping Lightnin A340 and Chemineer Maxflo (MFu) impellers and the liquid mixing times and micromixing achieved by a Chemineer CD-6 radial flow impeller were investigated in the standard vessel. The power draw characteristics and liquid macromixing rates obtained by three composite impellers (BT6+BT6+CD6, BT6+MFu+MFu and triple A340 impellers) were studied in the tall tank reactor. Experimental results show that the wide blade up-pumping impellers (A340 and MFu) can maintain a relatively high RPD even at high gas loadings and high impeller speeds. When multiple A340 impellers are used as a composite agitator, however, this combination has a greater power decrease upon gas sparging than the radial flow composite agitator (BT6+BT6+CD6) and the mixed flow impeller system (BT6+MFu+MFu) whether under cool gassed or hot sparged conditions. The effect of boiling on RPD is significant only when multiple agitators are operated at the extremes of low and high impeller speeds. Liquid mixing times in boiling and high gas loading systems were investigated through a conductivity technique. In single-impeller (CD-6) systems, gas sparging accelerates the liquid mixing. This can be attributed to the contribution of aeration to the potential energy. With the same shaft power input, boiling systems have the fastest liquid macromixing. This benefits from the work done by the expansion of vapour bubbles. Unlike in single impeller systems, the impeller geometry of composite impeller agitators has a significant effect on liquid mixing rates. Gas sparging has little influence on the liquid mixing times achieved by the radial flow agitator but reduces the rate of liquid mixing in both cool gassed or hot sparged systems agitated by the axial and mixed flow agitators. A compartment model has been used to explain this phenomenon. The effect of boiling on the liquid mixing time depends on the combination of impellers used in the agitator. A new test reaction system, suitable for the investigation of micromixing in boiling and hot sparged systems, is proposed. Experimental results have demonstrated that this reaction pair is easy to use and its product distribution is sensitive to liquid micromixing. Micromixing is apparently improved in boiling reactors. This could in part be attributed to the decrease of liquid viscosity and the contribution of rapid expansion and violent collapse of vapour bubbles, but much work still need to be done before a general conclusion could be drawn. Sparging gases into a hot liquid has little effect on the rate of micromixing, the reasons for which have been discussed. Feed location has a significant effect on the product distribution both in hot ungassed and boiling systems. This is consistent with the results in ambient temperature systems found in the literature.