Effect of Droplet-Wake Phenomena on Mixing-Sensitive Pharmaceutical Reactions - Pharmaceutical Technology

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Effect of Droplet-Wake Phenomena on Mixing-Sensitive Pharmaceutical Reactions

Pharmaceutical Technology

Figure 6
Images of the droplets falling through the stagnant liquid were captured using a high-speed CCD camera that records images at the rate of 500 frames/s. The camera was set to span the entire 3-in. width of the column, and the images were captured after the droplets had fallen approximately 25 cm. Figure 6 shows snapshots of droplets falling through 10% and 30% glycerin solution. From the detailed analysis of all the snapshots (34), one can see that the droplets change shape continuously as they fall. The droplets change from a horizontally elongated cap shape to an elliptical shape and then back to a cap shape. When a droplet fell through the 10% glycerin solution, it was observed to have both path and shape oscillations. The droplets were always released in the center of the column. In the case of the 10% solution, the droplet clearly has moved off center, moving from one side of the column to the other in a zig-zag fashion as it falls, as illustrated in the schematic drawing. Although not visible in the snapshots, this clearly indicates the existence of vortex shedding, as shown by many groups (24).

As the viscosity of the continuous phase increased, the droplet size increased. The droplets became less elongated, and the shape oscillations were less pronounced as the viscosity increased. In addition, the droplet remained at the center of the column, which indicated a closed (steady) wake.

Figure 7
Figure 7 presents a set of snapshots with a time difference of 10 ms for a single droplet falling in a continuous phase with increasing viscosities (10, 30, and 50% glycerin solutions). These snapshots show that as the viscosity increases, the droplet falls at a slower speed, has less pronounced shape oscillations, and becomes more spherical in shape. Zig-zagging was observed only in the lowest viscosity (10%) case.

At the lowest viscosity, the vortex shedding provides good mixing. Because mixing is strong, when products are formed in the wake they are quickly convected downstream and a fresh supply of reactants is brought into the wake region. Thus, as was found with our computational example, the selectivity should be maximized when operating in the vortex-shedding regime because the reaction products do not remain in the wake and therefore, the potential for overreaction to the undesired product is minimized.

Figure 8
Experimental results are shown in Figure 8. The results shown are for 17 different experiments with the charge time of the iodine solution varying from 0.25 h to 2.0 h. When the amount of iodine that transfers phases exceeds the amount of l-tyrosine available for reaction, the initial molar charge ratio is less than one, and the selectivity rapidly decreases as the charge ratio decreases. This result is expected because these conditions favor the second, undesired reaction in our reaction network. If the initial amount of l-tyrosine available for reaction exceeds the amount of iodine that transfers phases, then the initial molar-charge ratio is greater than one, and the selectivity approaches 100%. When the initial molar-charge ratio is equal to one, however, then the effects of mixing can be observed. For this set of conditions, the selectivity at an initial molar charge ratio of one is approximately equal to 85 2%. As shown in Figure 8, the data are reproducible and minimal scatter is observed.


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