Micro Impeller Mixing

Impellers of all types dominate conventional mixing numerous textbooks are available in this field (see e.g. [104,105]). This is typically carried out in a batchwise 

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M 21 ] [P 20] CFD simulation results for micro impeller mixing in an extended circular mixing chamber (width 2 mm, length 2 mm, depth 50 pm) is in line with experimental results yielded under similar condition. Mixing is complete within less than 1 min when using a 3 x 3 array of micro impellers at 600 rpm [32],... 

Figure 1.50 Liquid mixing time of micro-impeller mixing in a mixing chamber derived by plotting a mixing index yield by photometry versus time [32]...

M 21] [P 20] Mixing near a T-junction (channel 750 pm wide and 40 pm deep) is complete at a 0.17 pi min-1 flow rate when using a micro impeller at 120 rpm, as demonstrated by scanometric color index profiles providing line concentration profiles [32]. 

All of the power applied by a mixer to a fluid through the impeller appears as heat. The conversion of power to heat is through viscous shear and is approximately 2542 Btu/h/hp. Viscous shear is present in turbulent flow only at the micro-scale level. As a result, the power per unit volume is a major component of the phenomena of micro-scale mixing. At a 1-pm level, in fact, it doesn t matter what specific impeller design is used to supply the power. 

The ratio of the rms velocity fluctuation to the average velocity in the impeller zone is about 50 percent with many open impellers. If the rms velocity fluctuation is divided by the average velocity in the rest of the vessel, however, the ratio is on the order of 5 percent. This is also the level of rms velocity fluctuation to the mean velocity in pipeline flow. There are phenomena in micro-scale mixing that can occur in mixing tanks that do not occur in pipeline reactors. Whether this is good or bad depends upon the process requirements. 

Recently, one of the most practical results of these studies has been the ability to design pilot plant experiments (and, in many cases, plant-scale experiments) that can establish the sensitivity of process to macro-scale mixing variables (as a function of power, pumping capacity, impeller diameter, impeller tip speeds, and macro-scale shear rates) in contrast to micro-scale mixing variables (which are relative to power per unit volume, RMS velocity fluctuations, and some estimation of the size of the micro-scale eddies). 

The experimental results in Fig. 27 show the influence of the reactor system (see Fig. 28) on the disintegration of enzyme activity. It was found that the low-stress bladed impeller results in less activity loss than the propeller stirrer which causes much higher maximum energy dissipation ,. The gentle motion the blade impeller produces means that stress is so low that its disadvantage of worse micro mixing in NaOH (in comparison with the propeller) is more than compensated. 

External energy sources for active mixing are, for example, ultrasound [22], acoustic, bubble-induced vibrations [23,24], electrokinetic instabilities [25], periodic variation of flow rate [26-28], electrowetting induced merging of droplets [29], piezoelectric vibrating membranes [30], magneto-hydrodynamic action [31], small impellers [32], integrated micro valves/pumps [33] and many others, which are listed in detail in Section 1.2. 

The fluidfoil impellers are often called high efficiency impellers , but that is true only in terms of flow, and makes the assumption that flow is the main measure of mixing results. Flow is one measure, and in at least half of the mixing applications is a good measure of the performance fliat could be expected in aprocess. These impellers are low in efficiency in providing shear rates—either of the macro scale or the micro scale. 

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