Micromixers flow rate

In the SFM the reactor is divided into three zones two feed zones fj and (2 and the bulk b (Figure 8.1). The feed zones exchange mass with each other and with the bulk as depicted with the flow rates mi 2, i,3 and 2,3 respectively, according to the time constants characteristic for micromixing and mesomix-ing. As imperfect mixing leads to gradients of the concentrations in the reactor, different supersaturation levels in different compartments govern the precipitation rates, especially the rapid nucleation process. 

In most experimental investigations involving parallel reactions, species B is in stoichiometric excess, so that fsi < f) 1. In this range, only Yfff is non-zero, and thus its value serves as a measure of the rate of micromixing. Ideally, in order to maximize the range of possible values of Cs = Cq s2Y2oo, the flow rates and inlet concentrations... 

Similar to the case of the investigation on micromixing, the impinging velocity cannot be adjusted and controlled directly, but is done by changing the rotary speed of the propellers, N. Prior to all the measurements the curve describing the relationship between (> and N was calibrated with the same method as that used in Ref. [110], and the results are shown in Fig. 11.2, in which the curve is, in turn, used for conversion between the rotary speed and the impinging velocity in the data treatment. The curve in Fig. 11.2 is essentially the same as that shown in Fig. 10.9 but with some differences in specific data. The existence of such differences is natural, because the shape of the propeller paddle and particularly the width of the gap between the paddle of the propeller and the drawing tube have a fundamental influence on the flow rate drawn by the propeller, while errors in mechanical manufacture are also unavoidable.. 

FIGURE 3.38 A micromixer based on distributive mixing. Flow visualization using fluorescein and rhodamine B at a total flow rate of 50 mL/min. Only 10 out of 16 ministreams are shown [466]. Reprinted with permission from the Royal Society of Chemistry. 

It is important to note that mixing efficiency strongly depends on flow rate and that each micromixer has its characteristic flow rate range for efficient mixing. Therefore, we have to choose an appropriate flow rate to achieve fast mixing. If the flow rate is fixed, we have to choose a suitable micromixer at that flow rate. 

Another important point that should be considered in industrial applications of microflow systems is an increase in the flow rate to increase productivity, because productivity depends on the flow rate as well as the cross-sectional area of the microchannel. Therefore, it is necessary to construct a system that avoids an increase in the pressure drop with increasing flow rate. The pressure drop in a system strongly depends on the structure and size of the micromixer used. As shown in Figure 10.7 the use of a T-shaped mixer leads to a small pressure drop, whereas the use of a... 

Figure 10.7 Plots of the pressure drop (DP) against flow rate (medium-scale reaction system). Effect of the structure of the micromixers. Copyright 2005 American Chemical Society...

Evidently, mixing efficiency improves strongly with miniaturization of the mixing elements for the caterpillar micromixer [27]. For all three mixers tested, mixing efficiency improves with increasing flow rate, which is due to more intense recirculation patterns and thus interfacial stretching. The slope is steeper for the smaller caterpillar micromixers, i.e., the 800-pm device shows a more pronounced increase of mixing efficiency with flow rate. 

At higher flow rates we observed a drastic rise in the backpressure of the micromixer, which eventually destroyed the silicon body of the mixer. This problem could be overcome by a new mixer-design with parallel channels (Fig. 15.10). The individual mixing structures (Fig. 15.11) were arranged in rows with six split-... 

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